Preface

Chapter 7 **Valley Polarized Single Photon Source Based on Transition Metal Dichalcogenides Quantum Dots 111**

Chapter 8 **Widely Tunable Quantum-Dot Source Around 3 μm 145**

Giuseppe Leo

**VI** Contents

Alice Bernard, Marco Ravaro, Jean-Michel Gerard, Michel Krakowski, Olivier Parillaud, Bruno Gérard, Ivan Favero and

Fanyao Qu, Alexandre Cavalheiro Dias, Antonio Luciano de Almeida Fonseca, Marco Cezar Barbosa Fernandes and Xiangmu Kong

> This book was motivated by the desire others and we have had to further the evolution of the light emitting diode (LED) technology. It was also aimed to provide a review of recent developments in the quantum dot (QD)-based organic light emitting diode (OLED) technol‐ ogy and its application.

> In the beginning, the book provides a general introduction and then gives some information on the light emitting diode (LED) technology. In further chapters, the technological aspects of the light emitting diode (LED) manufacturing were covered by the authors. Subsequently, the book tried to explore the challenges of quantum dot–based light emitting diode (QD-LED) technology. Consequently, the recent progress in fabrication techniques and construc‐ tion of QD-LEDs was noticed in one of the chapters, and the other one focuses on the recent development in the application of QD-LEDs.

> The embedded CdS(Se) and PbS(Se) quantum dots with high room-temperature quantum efficiency in the fluorine phosphate glass matrix were evaluated in one of the chapters. The authors of this chapter focus on how these fluorine phosphate glasses were used in light emitting diodes (LEDs) and present the main advantages of applying these glasses in the LED structure.

> The recent progress on newly emerging perovskite light-emitting diodes based on organicinorganic hybrid perovskites (CH3NH3PbX3, X=Cl, Br, I) and inorganic perovskite cesium lead halide (CePbX3, X=Cl, Br, I) quantum dots was also considered in another chapter of this book. In the other chapter, the critical role of quantum dots to achieve full color con‐ version or white light generation in solid-state light sources was described in detail. This book provides a complete picture of the field for advanced undergraduates, postgradu‐ ates, and researchers.

> > **Morteza Sasani Ghamsari** Photonics and Quantum Technologies Research School NSTRI, Tehran, Iran

**Organic Light Emitting Diodes**

**Provisional chapter**

#### **Introductory Chapter: Quantum-Dots Based Organic Light-Emitting Diodes - The State-of-the-Art Light-Emitting Diodes - The State-of-the-Art**

**Introductory Chapter: Quantum-Dots Based Organic** 

DOI: 10.5772/intechopen.69744

Morteza Sasani Ghamsari Morteza Sasani Ghamsari Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69744

## **1. Promising QD-OLED technology**

Organic light-emitting diode (OLED) is a light source in which a thin layer of organic material placed between two conductors that can emit light of specific color by applying electrical current. To accelerate the commercialization of organic light-emitting diodes (OLEDs) have enhanced due to their capabilities as new generation displays and lighting sources. In comparison with other display technology, OLED displays are thin, efficient, flexible and transparent. In addition, they have better contrast, higher brightness, fuller viewing angle, a wider color range, much faster refresh rates and consuming lower power, thinner, very durable and also they can operate in a broader temperature range. But they have some disadvantages too. The cost of OLED displays is still too high. They have limited lifespan and suffer from permanent image retention. If these limitations can be overcome, OLED displays can find faster growth **opportunities** in specific **display technologies**.

In recent years, the display industry is rapidly interested in the world to combine **s**emiconductor nanocrystals or quantum dots (QDs) with organic light-emitting diode. Semiconductor nanocrystals or quantum dots (QDs) have considerable potential in assisting OLED displays and lighting to overcome their technological barriers. They are a type of nanomaterials which exhibit good optical characteristic. One of the important properties of QDs is that the absorption and photoluminescence (PL) spectra can be adjusted by their size. Consequently, the pure and tunable spectra of QDs can be conventionally obtained. Therefore, QDs can be used in photovoltaic devices, sensors and light-emitting diodes (LEDs). The smaller dimension of the QD results in shorter wavelength light emission and longer wavelength of light emitted will cause by bigger QDs. Therefore, it was believed that the next generation of OLED can be introduced by employment of this type of photonics material in manufacturing process of OLED.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Researchers all around the world try to improve this field and they found that quantum dots (QDs) can be helpful. Excellent color-rendering properties and high luminous efficiency (LE) of OLED are obtained when the combination of colloidal QDs with light-emitting diodes (LEDs) was done. QD-LEDs are the important part of the next generation of solid-state lighting and display technologies due to their great color saturation features, tunable wavelength and narrow full-width at half-maximum (FWHM). Nevertheless many developments have recently been made in display and lighting board technology and this ability was specially developed to build a very thin flexible and transparent displays; scientists have found that using some extra thin layers between two conductors and choosing appropriate materials can be helpful for better performance of QD-LEDs [1, 2]. High brightness, flexibility, efficiency with long lifetime and low processing cost of QD-LEDs makes them different from LCDs, OLEDs and plasma displays.

QD-LEDs can produce dark blacks and whiter whites and create higher brightness than OLEDs. In addition, a wider and more true-to-life color palette will be possible in QD-LEDs than in OLEDs. Because, an improvement in operation lifetime of OLED devices can be achieved by using quantum dots. Finally yet importantly, QD-LEDs are cheaper than OLEDs. Nowadays, researches try to improve efficiency of passing light through the quantum dot crystals, which can help the QD-LEDs performance [3]. QD-LEDs have some unique features such as better color accuracy, higher brightness, more stable performance and lower cost. Also, they are solution processable and suitable for wet processing techniques.

**Colloidal nanocrystal**-based **LEDs** can be classified into two main categories. In the first type of configuration, QD-doped material applied over the emitter chip and optically excited by a conventional epitaxial blue or UV LED. In the second category, the QDs themselves are directly excited by the passing of current through a QD-containing film. In QD-OLEDs, a layer of quantum dots is placed between layers of electron-transporting and hole-transporting organic materials. By applying the electric field, electrons and holes move into the quantum dot layer, recombine in QDs and emitting photons. The full width at half the maximum value of the QDs shows the width of the spectrum of emitted photons. In that case, a thin emissive layer is sandwiched between a hole-transport layer (HTL) and an electron-transport layer (ETL) to capture electrons and holes in the small region. Organic materials are used as the ETLs and HTLs, until now. The electron-hole recombination generally occurs near the cathode because organic electroluminescent material transport holes faster than electrons. Therefore, the exciton produced will be quenched. In order to prevent this event, a hole-blocking layer will be used.

#### **Author details**

Morteza Sasani Ghamsari

Address all correspondence to: msghamsari@yahoo.com

Nuclear Science and Technology Research Institute, Tehran, Iran

#### **References**

Researchers all around the world try to improve this field and they found that quantum dots (QDs) can be helpful. Excellent color-rendering properties and high luminous efficiency (LE) of OLED are obtained when the combination of colloidal QDs with light-emitting diodes (LEDs) was done. QD-LEDs are the important part of the next generation of solid-state lighting and display technologies due to their great color saturation features, tunable wavelength and narrow full-width at half-maximum (FWHM). Nevertheless many developments have recently been made in display and lighting board technology and this ability was specially developed to build a very thin flexible and transparent displays; scientists have found that using some extra thin layers between two conductors and choosing appropriate materials can be helpful for better performance of QD-LEDs [1, 2]. High brightness, flexibility, efficiency with long lifetime and low processing cost of QD-LEDs makes them different from LCDs,

QD-LEDs can produce dark blacks and whiter whites and create higher brightness than OLEDs. In addition, a wider and more true-to-life color palette will be possible in QD-LEDs than in OLEDs. Because, an improvement in operation lifetime of OLED devices can be achieved by using quantum dots. Finally yet importantly, QD-LEDs are cheaper than OLEDs. Nowadays, researches try to improve efficiency of passing light through the quantum dot crystals, which can help the QD-LEDs performance [3]. QD-LEDs have some unique features such as better color accuracy, higher brightness, more stable performance and lower cost. Also, they are solu-

**Colloidal nanocrystal**-based **LEDs** can be classified into two main categories. In the first type of configuration, QD-doped material applied over the emitter chip and optically excited by a conventional epitaxial blue or UV LED. In the second category, the QDs themselves are directly excited by the passing of current through a QD-containing film. In QD-OLEDs, a layer of quantum dots is placed between layers of electron-transporting and hole-transporting organic materials. By applying the electric field, electrons and holes move into the quantum dot layer, recombine in QDs and emitting photons. The full width at half the maximum value of the QDs shows the width of the spectrum of emitted photons. In that case, a thin emissive layer is sandwiched between a hole-transport layer (HTL) and an electron-transport layer (ETL) to capture electrons and holes in the small region. Organic materials are used as the ETLs and HTLs, until now. The electron-hole recombination generally occurs near the cathode because organic electroluminescent material transport holes faster than electrons. Therefore, the exciton produced will be quenched. In order to prevent this event, a hole-blocking layer

OLEDs and plasma displays.

4 Quantum-dot Based Light-emitting Diodes

will be used.

**Author details**

Morteza Sasani Ghamsari

tion processable and suitable for wet processing techniques.

Address all correspondence to: msghamsari@yahoo.com

Nuclear Science and Technology Research Institute, Tehran, Iran


**Provisional chapter**

#### **Recent Developments in Applications of Quantum-Dot Based Light-Emitting Diodes Based Light-Emitting Diodes**

**Recent Developments in Applications of Quantum-Dot** 

DOI: 10.5772/intechopen.69177

Anca Armăşelu Anca Armăşelu Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69177

#### **Abstract**

Quantum dot-based light-emitting diodes (QD-LEDs) represent a form of light-emitting technology and are regarded like a next generation of display technology after the organic light-emitting diodes (OLEDs) display. QD-LEDs are different from liquid crystal displays (LCDs), OLEDs, and plasma displays due to the fact that QD-LEDs present an ideal blend of high brightness, efficiency with long lifetime, flexibility, and low-processing cost of organic LEDs. So, QD-LEDs show theoretical performance limits which surpass all other display technologies. The goal of this chapter is, firstly, to provide a historical prospective study of QD-LEDs applications in display and lighting technologies, secondly, to present the most recent improvements in this field, and finally, to discuss about some current directions in QD-LEDs research that concentrate on the realization of the next-generation displays and high-quality lighting with superior color gamut, higher efficiency, and high color rendering index.

**Keywords:** quantum dots, quantum dot-based light-emitting diodes, display technology, lighting technology

#### **1. Introduction**

Quantum dots (QDs) have attracted interest in the fields of optical applications such as quantum computing, biological, and chemical applications.

In contradistinction to the traditional fluorophores, QDs have unique optical and electronic features, which comprise high quantum yields, high molar extinction coefficients, large effective Stokes shifts [1, 2], broad excitation profiles, narrow/symmetric emission spectra, high resistance to reactive oxygen-mediated photobleaching [2, 3], and are against metabolic degradation [4, 5].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

QDs are fluorophore nanocrystals whose excitation and emission is basically distinct than classical organic fluorophores [2]. The unique properties of QDs appear almost exclusively due to the size regime in which they exist [6]. QDs obey the quantum mechanical principle of three-dimensional confinement of the charge carriers (electrons, holes) that determine novel quantum phenomena and tunable optical properties, which are sensitive to the size, shape, and material composition of the QDs [7].

QDs have an intrinsic energy bandgap that decides required wavelength of radiation absorption and emission spectra. The bandgap energy increases with the decrease in the dimension of the QD [8]. The color of the light which a QD emits is directly connected to its size; the bigger dots cause longer wavelengths, lower frequencies, and redder light while the smallest dots produce shorter wavelengths, higher frequencies, and bluer light [9–11]. This dimension dependence permits the modulating of the bandgap energy by varying the size of the QD [6, 12].

The wavelength of fluorescence depends on the bandgap, and consequently, it is determined by the dimension of the QD [6, 12]. The ability to adjust and to control the size of the QDs is relevant and advantageous for many new applications.

Due to their highly tunable properties, QDs have an abundance of applications in a diversity of fields. QD could offer a choice for commercial applications such as display technology. QDs are also brighter than a competing technology that is known as organic light-emitting diode (OLED) displays and could eventually make OLED displays outdated [9]. An OLED is fabricated with organic compounds that light up when fed electricity [13].

An important drawback of the OLED technology is the lack of trust and cheap patterning methods for various color pixels [14]. Because OLEDs are composed of small molecule organics, they are not consistent with the classical lithographical patterning techniques that necessitate exposure to solvents which completely deteriorate the structures of OLED [14]. QDs are the reliable solutions for flat-panel TV screens, digital cameras, mobile phones, and personal gaming equipments because QDs could assist large, flexible displays and would not deteriorate as easily as OLEDs [15].

According to the synthetic method, the QDs can be categorized into epitaxial and colloidal QDs. Colloidal QDs are made through chemical synthesis and are composed of a small inorganic semiconductor core, an inorganic semiconductor shell with a broader bandgap, and a coating of organic passivation ligands [16, 17]. The optical properties of the original QDs, which contain a core, can be improved by using the coating of higher bandgap materials or passivation of the exterior part of the core [16, 18]. QD technology is used to filter light from light-emitting diodes (LEDs) to backlit liquid crystal displays (LCDs). With the recent enhancements introduced by the usage of the QDs to backlighting technology, LED/LCD TVs are much better today than they were just few years ago [13].

The use of QDs for the improvement of the LED backlighting leads to the enhancement of the useful light throughput and to the providing of a better color gamut [15]. This QD technology system sends the light from a blue LED through the medium of a tube filled with red and green QDs, so that at the other end of the tube, this blend of blue, green, and red light incurs less absorption of unwished colors by the color filters behind the LCD screen [13, 19]. The first TV manufacturer that presented the achievement of this new type of technology called Triluminos quantum dot display technology was Sony in 2013 [13].

A classic light-emitting diode (LED) is made of some materials that are chosen to emit the required color light and are arranged in layers in a structure named a device stack. The dimension of the total width of this device stack is around 10 μm [21].

#### **2. Evolution of quantum dot-based light-emitting diodes**

QDs are fluorophore nanocrystals whose excitation and emission is basically distinct than classical organic fluorophores [2]. The unique properties of QDs appear almost exclusively due to the size regime in which they exist [6]. QDs obey the quantum mechanical principle of three-dimensional confinement of the charge carriers (electrons, holes) that determine novel quantum phenomena and tunable optical properties, which are sensitive to the size, shape,

QDs have an intrinsic energy bandgap that decides required wavelength of radiation absorption and emission spectra. The bandgap energy increases with the decrease in the dimension of the QD [8]. The color of the light which a QD emits is directly connected to its size; the bigger dots cause longer wavelengths, lower frequencies, and redder light while the smallest dots produce shorter wavelengths, higher frequencies, and bluer light [9–11]. This dimension dependence permits the modulating of the bandgap energy by varying the size of the QD [6, 12].

The wavelength of fluorescence depends on the bandgap, and consequently, it is determined by the dimension of the QD [6, 12]. The ability to adjust and to control the size of the QDs is

Due to their highly tunable properties, QDs have an abundance of applications in a diversity of fields. QD could offer a choice for commercial applications such as display technology. QDs are also brighter than a competing technology that is known as organic light-emitting diode (OLED) displays and could eventually make OLED displays outdated [9]. An OLED is

An important drawback of the OLED technology is the lack of trust and cheap patterning methods for various color pixels [14]. Because OLEDs are composed of small molecule organics, they are not consistent with the classical lithographical patterning techniques that necessitate exposure to solvents which completely deteriorate the structures of OLED [14]. QDs are the reliable solutions for flat-panel TV screens, digital cameras, mobile phones, and personal gaming equipments because QDs could assist large, flexible displays and would not deterio-

According to the synthetic method, the QDs can be categorized into epitaxial and colloidal QDs. Colloidal QDs are made through chemical synthesis and are composed of a small inorganic semiconductor core, an inorganic semiconductor shell with a broader bandgap, and a coating of organic passivation ligands [16, 17]. The optical properties of the original QDs, which contain a core, can be improved by using the coating of higher bandgap materials or passivation of the exterior part of the core [16, 18]. QD technology is used to filter light from light-emitting diodes (LEDs) to backlit liquid crystal displays (LCDs). With the recent enhancements introduced by the usage of the QDs to backlighting technology, LED/LCD TVs

The use of QDs for the improvement of the LED backlighting leads to the enhancement of the useful light throughput and to the providing of a better color gamut [15]. This QD technology system sends the light from a blue LED through the medium of a tube filled with red and green QDs, so that at the other end of the tube, this blend of blue, green, and red light incurs less absorption of unwished colors by the color filters behind the LCD screen [13, 19]. The

fabricated with organic compounds that light up when fed electricity [13].

and material composition of the QDs [7].

8 Quantum-dot Based Light-emitting Diodes

rate as easily as OLEDs [15].

relevant and advantageous for many new applications.

are much better today than they were just few years ago [13].

Due to the multiple advantages of using QDs and their applications in optoelectronic instruments like LEDs, the scientists have created quantum dot-based light-emitting diode (QD-LED) with the improved efficiency and flexibility. QD-LED represents the following generation's display technology after OLED displays [20–22]. QD-LEDs are a form of lightemitting technology for creating large-area displays that could have applications for TVs, cell phones, and digital cameras [15, 20–22].

The structure of QD-LED is analogous to the fundamental design of OLED, with the difference that the light emitting is the QDs, such as cadmium selenide (CdSe) nanocrystals [20, 21].

A classical QD-LED is composed of three layers: one inner layer of QDs as an emissive layer, one outer layer that transports electrons, and one outer layer that transports holes. After applying an electric field on the outside layers, electrons and holes shift in the layer of QD, where they are captured by QD and recombine, emitting photons [22]. Due to the multiple advantages of using the colloidal QDs, the colloidal QDs are a promising way for making QD-LEDs. A great effect of an increased recombination efficiency is obtained by constructing an emissive layer in a single layer of QDs, so that the electrons and holes may be moved directly from the surfaces of electron-transport layer and hole-transport layer [15, 20, 21]. For the definition of the performance of a QD-LED is used the external quantum efficiency (EQE), which is the term that designates the number of photons emitted from the device per electron. For the methodical progress of QD chemistries and active layer designs as well as new device architectures for high-performance QD-LEDs, it is important to discover and to analyze the fundamental causes of inefficiency and to suggest potential solutions [23].

The investigation of the efficiency of the light generation process in the QD-LEDs is an important criterion for achieving high-performance QD-LEDs [24–26].

In 1907, the British scientist named Henry Joseph Round reported light emission from a crystal detector; thus, the idea of light-emitting diode was introduced [27, 28]. In 1927, the Russian researcher Oleg Vladimirovich Losev published a paper about the first light-emitting diode [27, 29].

In 1955, Rubin Braunstein, who worked at Radio Corporation of America, discovered that some common diodes emit infrared light when connected to a current [30, 31]. Also, Rubin Braunstein and Egon Loebner reported in 1958 a green LED which was realized from a lead antimonde/germanium alloy [32]. In 1962, the researcher Nick Holonyak Jr., who is called as "the father of the light-emitting diode," created the first practical visible-spectrum (red) GaAsP LED at General Electric Company in New York [33]. In 1964, IBM introduced the using of LEDs on circuit boards in an early mainframe computer [31].

Thomas P. Pearsall reported the first high-brightness and high efficiency LED in 1976, for utilization with fiber optics in telecommunications [34]. Akasaki et al. [35] made the first blue LED in 1992 based on GaN with efficiency of 1%.

Shuji Nakamura realized the first high-brightness blue LED in 1979 at Nichia Corporation laboratory, but it was too expensive for commercial use until 1994 [36, 37]. Then, Shuji Nakamura was awarded the 2006 Millennium Technology Prize for the development of a white LED [38]. The possibility of color displays with blue, red, and green LEDs was advantageously accomplished [39]. Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura have won the Nobel Prize in Physics in 2014 for the invention of the blue LED [40].

An actual impediment for the development of the performance of the LED is an insufficient understanding of the contribution of some extrinsic elements, such as non-radiative recombination at surface defects versus intrinsic processes, such as multicarrier Auger recombination [41].

In recent years, there have been a lot of research to enhance the quality of LEDs, so recently were introduced QD-LEDs that have the attractive features which correlate the excited state dynamics of structurally engineered QDs with their emissive performance inside LED.

In this regard, some important results of these efforts are the latest demonstrations of QD-LEDs with achievement surpassing that of fluorescent OLEDs that were seriously investigated for a minimum of two decades [26, 41, 42].

QD-LEDs not only reduce the consumption of energy but also show high color purity. Studies reported that QD-LEDs exhibit the ability to be more than twice as power efficient than OLEDs at the same color purity [43, 44]. QD-LEDs have the advantages of foldability and their wide application for next-generation electronic displays and optical communication technology [45].

QD-LEDs exhibit pure and saturated emission colors with narrow bandwidth. In QD-LEDs, the emission color is powerfully directed by the dimension of the used QD due to the confinement effects [44]. It has been proven that QD-LEDs present a 30–40% luminance efficiency advantage above OLEDs for the same color point [43, 44].

QD-LEDs offer several promising features, such as size-dependent emission wavelength, narrow emission spectrum, high efficiency, flexibility, and low-processing cost of organic lightemitting device [43]. The luminaire manufactural cost is diminished due to the capability to imprint large-area QD-LEDs on ultra-thin transparent or flexible substrates [44].

Alivisatos and his colleagues realized the first QD-LED in 1994 [46]. This kind of QD-LED is consisted of a common bilayer structure including an indium tin oxide (ITO) anode, plain CdSe QDs, a *p*-paraphenylene vinylene (PPV) layer, and Mg cathode. Another paper from two research groups at MIT reported a single-layer CdSe-QD-LED with the nanocrystals embedded into an organic polymer matrix [47]. These devices exhibited a very small value of EQEs of 0.001–0.01% [47, 48] and 0.0005% [47, 49].

In superior performance QD-LEDs, the utilized QDs are core-shell type with some structure gradient from core to shell and with various capping ligands or mixed emitting coats [16, 49, 50]. The researchers found different types of QD-LEDs which are divided into four categories of devices based on their design [16, 49, 50]: type I (QD-LEDs with polymer charge transport coats), type II (QD-LEDs with organic small molecule charge transport coats), type III (QD-LEDs which are composed of inorganic charge transport coats), and type IV (QD-LEDs which contain an inorganic metal oxide semiconductor as the electron transport layer-ETLand an organic semiconductor as the hole transport layer-HTL). In 2002, it was discovered the first type II QD-LED, which was made up of a single monolayer of QDs, was sandwiched between two organic thin films [51].

as "the father of the light-emitting diode," created the first practical visible-spectrum (red) GaAsP LED at General Electric Company in New York [33]. In 1964, IBM introduced the using

Thomas P. Pearsall reported the first high-brightness and high efficiency LED in 1976, for utilization with fiber optics in telecommunications [34]. Akasaki et al. [35] made the first blue

Shuji Nakamura realized the first high-brightness blue LED in 1979 at Nichia Corporation laboratory, but it was too expensive for commercial use until 1994 [36, 37]. Then, Shuji Nakamura was awarded the 2006 Millennium Technology Prize for the development of a white LED [38]. The possibility of color displays with blue, red, and green LEDs was advantageously accomplished [39]. Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura have won the Nobel Prize

An actual impediment for the development of the performance of the LED is an insufficient understanding of the contribution of some extrinsic elements, such as non-radiative recombination at surface defects versus intrinsic processes, such as multicarrier Auger recombination [41]. In recent years, there have been a lot of research to enhance the quality of LEDs, so recently were introduced QD-LEDs that have the attractive features which correlate the excited state dynamics of structurally engineered QDs with their emissive performance inside LED.

In this regard, some important results of these efforts are the latest demonstrations of QD-LEDs with achievement surpassing that of fluorescent OLEDs that were seriously investigated for a

QD-LEDs not only reduce the consumption of energy but also show high color purity. Studies reported that QD-LEDs exhibit the ability to be more than twice as power efficient than OLEDs at the same color purity [43, 44]. QD-LEDs have the advantages of foldability and their wide application for next-generation electronic displays and optical communication

QD-LEDs exhibit pure and saturated emission colors with narrow bandwidth. In QD-LEDs, the emission color is powerfully directed by the dimension of the used QD due to the confinement effects [44]. It has been proven that QD-LEDs present a 30–40% luminance efficiency

QD-LEDs offer several promising features, such as size-dependent emission wavelength, narrow emission spectrum, high efficiency, flexibility, and low-processing cost of organic lightemitting device [43]. The luminaire manufactural cost is diminished due to the capability to

Alivisatos and his colleagues realized the first QD-LED in 1994 [46]. This kind of QD-LED is consisted of a common bilayer structure including an indium tin oxide (ITO) anode, plain CdSe QDs, a *p*-paraphenylene vinylene (PPV) layer, and Mg cathode. Another paper from two research groups at MIT reported a single-layer CdSe-QD-LED with the nanocrystals embedded into an organic polymer matrix [47]. These devices exhibited a very small value of

imprint large-area QD-LEDs on ultra-thin transparent or flexible substrates [44].

of LEDs on circuit boards in an early mainframe computer [31].

LED in 1992 based on GaN with efficiency of 1%.

10 Quantum-dot Based Light-emitting Diodes

in Physics in 2014 for the invention of the blue LED [40].

advantage above OLEDs for the same color point [43, 44].

EQEs of 0.001–0.01% [47, 48] and 0.0005% [47, 49].

minimum of two decades [26, 41, 42].

technology [45].

A drawback to be mentioned to the previously indicated study [51] is the utilization of the organic charge support layers in QD-LEDs that creates an unwanted contribution to the light emission of the LED. This undesired emission perturbs the color purity when a saturated monochromatic emission is wanted. Several authors have shown that in certain cases the perturbing emission could be eliminated [51–53] and in other cases, it could be utilized to build an efficacious white QD-LED [54, 55]. The researchers established that the highest efficiency devices with the best construction architecture are QD-LEDs from type IV class. This type of QD-LED is a hybrid tool which comprises an inorganic metal oxide semiconductor such as the ETL and an organic semiconductor such as the HTL.

The usage of the inorganic coat produces substantial advantages for device stability in air [56, 57] and maximum current density, whereas the use of the organic coat provides a great tunability and an easy processing [16].

In 2007, Anikeeva et al. [58] reported an efficacious spectrally broad electroluminescent QD-LED with spectral emission tunable across the Comission Internationale de l'Eclairage (CIE) color space. More exactly, the authors describe in this paper LEDs with a broad spectral emission which is produced by electroluminescence from a mixed monolayer of red, green, and blue emitting colloidal QDs in a hybrid organic/inorganic QD-LED. Concurrent electroluminescence of numerous color QDs leads to the evolution of tunable LED colors such as white QD-LEDs [58]. The number of QDs colors which can be utilized in a single device is practically boundless; thus, it is possible to obtain higher color rendering and imitate the solar color temperature with the help of QD-LEDs.

Yang and coworkers [59, 60] demonstrated a full range of blue, green, and red quantum dotbased light-emitting devices exhibiting EQEs above 10%. These devices showed low turn-on voltages and saturated pure colors. It has been reported that the values of the lifetimes for the green and red devices are greater than 90,000 and 300,000 h, respectively.

NanoPhotonica is a company that offers advanced and original nanomaterials and manufacturing methods which assure for electronic displays to show a high resolution, pure, bright colors, and an improved efficiency for an important low cost of production. In 2015, NanoPhotonica exhibited enhanced efficiency for blue and green QD-LEDs [60, 61].

The contribution here reported that by using a promising design strategy for QD synthesis and device fabrication methods, a high value of 21% EQE for the green QD-LEDs and a value of 11.2% EQE for the blue QD-LEDs were discovered. The value of 21% EQE in the case of the green QD-LED is the greatest recorded efficiency of any color QD-LED and is the same value as in the case of vacuum-deposited red and green OLEDs utilized in AMOLED display technology which is commercially available [49].

In the following section, the current status of the applications of QD-LEDs and a summary of the issues concerning the limiting of the applicability of QD-LEDs are discussed.

With the aim of achieving a well-designed QD-LED appropriate for general lighting applications, two relevant criteria such as color rendering index (CRI) and the correlated color temperature (CCT) must be discussed in this case. The CCT represents a measure of the light source color appearance described by the vicinity chromaticity coordinates at the blackbody's locus as a one number rather than the two needed to specify a chromaticity [62, 63]. CRI is defined like the measurement of how the colors look under a light source in comparison with sunlight.

QD-LEDs have already exhibited important advantages in general lighting with higher efficiency and better rendering ability [48, 64, 65]. QD-LEDs have enjoyed a lot of attention as promising devices for next-generation displays. Regarding the usage of QD-LEDs in the area of the display technology, one of the most significant parameters for the characterization of the display devices is color gamut [63]. In this field of applications of QD-LEDs the chromaticity diagrams and color gamut standards in order to measure the purity of the color are utilized.

In this regard, it is noteworthy that the International Commission on Illumination (CIE) created the chromaticity diagram named CIE 1931 color space. QD-LED device represents a competitive backlight resolving for the next-generation LCDs because unlike traditional backlight solutions, the QD backlight provides a larger color gamut with a value more than 115% National Television System Committee (NTSC) in CIE 1931 color space and with a value more than 140% NTSC in CIE 1976 [62, 65]. These evaluation criteria listed above for QD-LEDs devices should be considered to create attractive next-generation display applications and superior quality lighting applications with better color gamut, higher efficiency, and high CRI [62, 65].

## **3. Actual applications of quantum dot-based light-emitting diodes**

QD-LEDs have proved impressive outcomes for medical field, lighting, and display applications.

#### **3.1. Quantum dot-based light-emitting diodes for phototherapy**

In this part, an important and new application of the use of QD-LEDs in phototherapy is summarized [66]. Phototherapy or light therapy consists of the dermal exposure to light for the treatment of the various medical disorders. Light therapy is used to treat the skin disorders (psoriasis, acne vulgaris, eczema, skin cancer, wound healing), neonatal jaundice, circadian rhythm disorders, and tumors. The researchers [66] have created two medical dressings for phototherapy, each of the two medical dressings include an occlusive layer and translucent layer. For one medical dressing, QD-LEDs chips are designed within the occlusive layer and covered with a translucent layer, so as to furnish a characteristic wavelength of light for utilization in phototherapy. The second device, which was discovered by the same authors [66], consists of an occlusive layer and a translucent layer with QDs that are enclosed in a layer or both layers.

#### **3.2. Quantum dots as electroluminescent light sources**

of 11.2% EQE for the blue QD-LEDs were discovered. The value of 21% EQE in the case of the green QD-LED is the greatest recorded efficiency of any color QD-LED and is the same value as in the case of vacuum-deposited red and green OLEDs utilized in AMOLED display

In the following section, the current status of the applications of QD-LEDs and a summary of

With the aim of achieving a well-designed QD-LED appropriate for general lighting applications, two relevant criteria such as color rendering index (CRI) and the correlated color temperature (CCT) must be discussed in this case. The CCT represents a measure of the light source color appearance described by the vicinity chromaticity coordinates at the blackbody's locus as a one number rather than the two needed to specify a chromaticity [62, 63]. CRI is defined like the measurement of how the colors look under a light source in comparison with

QD-LEDs have already exhibited important advantages in general lighting with higher efficiency and better rendering ability [48, 64, 65]. QD-LEDs have enjoyed a lot of attention as promising devices for next-generation displays. Regarding the usage of QD-LEDs in the area of the display technology, one of the most significant parameters for the characterization of the display devices is color gamut [63]. In this field of applications of QD-LEDs the chromaticity diagrams and color gamut standards in order to measure the purity of the color are utilized. In this regard, it is noteworthy that the International Commission on Illumination (CIE) created the chromaticity diagram named CIE 1931 color space. QD-LED device represents a competitive backlight resolving for the next-generation LCDs because unlike traditional backlight solutions, the QD backlight provides a larger color gamut with a value more than 115% National Television System Committee (NTSC) in CIE 1931 color space and with a value more than 140% NTSC in CIE 1976 [62, 65]. These evaluation criteria listed above for QD-LEDs devices should be considered to create attractive next-generation display applications and superior quality lighting applications with better color gamut, higher efficiency, and high CRI

**3. Actual applications of quantum dot-based light-emitting diodes**

**3.1. Quantum dot-based light-emitting diodes for phototherapy**

QD-LEDs have proved impressive outcomes for medical field, lighting, and display

In this part, an important and new application of the use of QD-LEDs in phototherapy is summarized [66]. Phototherapy or light therapy consists of the dermal exposure to light for the treatment of the various medical disorders. Light therapy is used to treat the skin disorders (psoriasis, acne vulgaris, eczema, skin cancer, wound healing), neonatal jaundice, circadian rhythm disorders, and tumors. The researchers [66] have created two medical dressings for phototherapy,

the issues concerning the limiting of the applicability of QD-LEDs are discussed.

technology which is commercially available [49].

12 Quantum-dot Based Light-emitting Diodes

sunlight.

[62, 65].

applications.

Solid-state lighting (SSL) represents another field in energy and environmental sustainability, in which QDs can have a remarkable price and a great advantage compared to the actual state of the art. Therefore, QDs are considered valuable materials for displays that are suitable for the incorporation into SSL technologies as downconversion phosphors or electroluminescent phosphors [49]. In last years, much research has been achieved on QD-LEDs as the optimal choice for SSL applications [49, 67]. The greatest assets of QD-LEDs as electroluminescent light sources for SSL technologies are their low cost, high efficiency production compatibility, flexible and versatile form factors, and the capacity to be a light source spread over a large area than a point light source. There are some performance products of this kind which become a commercial reality. For example, two UK companies [64] named Cumbria LED lighting company Marl and Manchester-based Nanoco (Nanoco is the global leader in the evolution and the fabrication of the cadmium-free quantum dots-CFQD) performed the world's first CFQD quantum dot LED lighting product, the Orion QD. QDs that are produced by Nanoco are cadmium-free, absorb light in a broad wavelength series, comprising blue, and emit at a color conditioned by their dimension. In this case, light is only converted to where it is needed.

This type of product finds its application in horticulture because the nanoparticle luminaires produce light comprising blue and red but not green that corresponds to the absorption of the chlorophyll, with none of the light it reflects [49, 68]. Another high-quality example is represented by a device from Zylight, namely the multiple award-winning F8 100 LED Fresnel which is the next generation of Fresnel light and encloses QDs into its fixture [49, 69]. More exactly, in compliance with Zylight, the F8 includes a certain mix of QDs with traditional phosphor and exhibits a value of the CR) of up to 97 and a quality of light adjusted only by traditional sunlight and incandescent bulbs [69, 70]. It is expected that with the progress of the research in the field of electroluminescent QD-LEDs, these devices bring substantial improvements for the SSL industry.

The recent studies conducted on the LED base next-generation lighting show remarkable results in the area of lighting for the energy saving for the world warming prevention. In this regard, Song [71] realized a multifunctional LED light source as investigation about the lighting installation using the QD. Due to the fact that artificial light is utilized in different areas such as the increase of the efficiency lighting, the rise of the growth of plant, and the prevention of the disease, LED is a great choice to be extensively used as artificial light in factory plant. Moreover, this LED mentioned before is an energy saving-device and diminishes the emissions of the greenhouse gas. The author presented the achievement [71] of a manufactured LED lamp by the use of QDs as phosphor, in order to be applied like an illumination technology for plant growth used in a multi-wavelength QD-LED device.

In 2013, Pickett et al. [72] realized an invention that has to do with the QD-LEDs useful for plant, algae, and bacterial growth applications. To overcome the disadvantages of using LEDs, the inventors have suggested the utilization of QD-LEDs as the optimal choice. Thus, the researchers presented in their work that QD-LEDs utilized a primary light source which is a solid-state LED, with blue or UV light, and a secondary light source (which downconverts the primary light) that comprises one or more QD components. In their paper, Pickett et al. [72] described that for the optimization of the plant growth in the agricultural and horticultural field, for the improvement of the process of the growth of algae, and for the stimulation of the photosynthetic bacterial growth in bioremediation goals, the QD-LED lighting systems are utilized. In contrast to the solid-state LED lighting, the QD-LED lighting system offers a less costly choice, emits less heat which could harm the plants and other photosynthetic organisms, can offer a greater light intensity, emits light at wavelengths better geared toward the promotion of the growth of bacteria and algae, thereby minimizing energy losses, and exhibits a high-energy efficiency. Thus, the value of the energy efficiency of QD-LEDs used in this case is in the range of 30–70 lm/W, in contrast to 10–18 lm/W for incandescent bulbs and 35–60 lm/W for fluorescent lamps. Another important advantage of the use of the QD-LED lighting systems is that due to the easy wavelength adjustment of QDs, the emission wavelength of the QD-LED can be simply changed to correspond to a diversity of various photosynthetic bacteria. The QD-LED systems described in this invention can be utilized in a wide variety of applications.

There are two kinds of QD-LEDs, and the dissimilates between these two kinds of QD-LEDs are that the first category is based on photo-excited QDs (photoluminescence QD-LEDs) and the second category of QD-LEDs relies on electro-excited QDs (electroluminescence QD-LEDs) [62]. The most frequently utilized type of QD-LEDs in applications is the photoluminescence QD-LEDs.

Klimov and colleagues [41, 73, 74] from the Nanotechnology and Advanced Spectroscopy Team at Los Alamos National Laboratory reported some substantial advances in the domain of the applications of QD-LEDs. Klimov believes that QD-LEDs can probably offer many benefits over traditional lighting technologies, such as incandescent bulbs, particularly in the fields of efficiency, functioning lifetime, and the color quality of the emitted light. Advanced investigations, made by Klimov and his team from Los Alamos, allow that less wasteful fluorescent light sources quickly substitute the incandescent bulbs, which are known for the conversion of only 10% of electricity into light and the loss of 90% of it to heat.

The researchers conducted some spectroscopic studies on the QD-LEDs [41, 73, 74] and have shown that the so-called Auger recombination effect has heavily influenced both LED efficiency and the onset of efficiency roll-off at high currents. The team of researchers established two methods that diminished this issue by using hetero-structured quantum dots.

In the last years, there has been much study in the field of LEDs and photovoltaic solar cells (PV SC). Though OLEDs have the guarantee to overcome the traditional LEDs in performance, OLED materials and manufactural processes of them are not sufficiently advanced to offer this economically [75]. By the evolving a hybrid tool, the efficiency can be increased and the manufactural price can potentially be reduced because this hybrid material system is compatible with inexpensive fabrication procedure such as solution processing and rollto-roll deposition and with patterning methods, enabling multicolor light sources to be prepared on the same substrate by replacing the emissive colloidal QD coating [14, 75].

In 2013, Pickett et al. [72] realized an invention that has to do with the QD-LEDs useful for plant, algae, and bacterial growth applications. To overcome the disadvantages of using LEDs, the inventors have suggested the utilization of QD-LEDs as the optimal choice. Thus, the researchers presented in their work that QD-LEDs utilized a primary light source which is a solid-state LED, with blue or UV light, and a secondary light source (which downconverts the primary light) that comprises one or more QD components. In their paper, Pickett et al. [72] described that for the optimization of the plant growth in the agricultural and horticultural field, for the improvement of the process of the growth of algae, and for the stimulation of the photosynthetic bacterial growth in bioremediation goals, the QD-LED lighting systems are utilized. In contrast to the solid-state LED lighting, the QD-LED lighting system offers a less costly choice, emits less heat which could harm the plants and other photosynthetic organisms, can offer a greater light intensity, emits light at wavelengths better geared toward the promotion of the growth of bacteria and algae, thereby minimizing energy losses, and exhibits a high-energy efficiency. Thus, the value of the energy efficiency of QD-LEDs used in this case is in the range of 30–70 lm/W, in contrast to 10–18 lm/W for incandescent bulbs and 35–60 lm/W for fluorescent lamps. Another important advantage of the use of the QD-LED lighting systems is that due to the easy wavelength adjustment of QDs, the emission wavelength of the QD-LED can be simply changed to correspond to a diversity of various photosynthetic bacteria. The QD-LED systems described in this invention can be utilized in a wide

There are two kinds of QD-LEDs, and the dissimilates between these two kinds of QD-LEDs are that the first category is based on photo-excited QDs (photoluminescence QD-LEDs) and the second category of QD-LEDs relies on electro-excited QDs (electroluminescence QD-LEDs) [62]. The most frequently utilized type of QD-LEDs in applications is the photolu-

Klimov and colleagues [41, 73, 74] from the Nanotechnology and Advanced Spectroscopy Team at Los Alamos National Laboratory reported some substantial advances in the domain of the applications of QD-LEDs. Klimov believes that QD-LEDs can probably offer many benefits over traditional lighting technologies, such as incandescent bulbs, particularly in the fields of efficiency, functioning lifetime, and the color quality of the emitted light. Advanced investigations, made by Klimov and his team from Los Alamos, allow that less wasteful fluorescent light sources quickly substitute the incandescent bulbs, which are known for the con-

The researchers conducted some spectroscopic studies on the QD-LEDs [41, 73, 74] and have shown that the so-called Auger recombination effect has heavily influenced both LED efficiency and the onset of efficiency roll-off at high currents. The team of researchers established

In the last years, there has been much study in the field of LEDs and photovoltaic solar cells (PV SC). Though OLEDs have the guarantee to overcome the traditional LEDs in performance, OLED materials and manufactural processes of them are not sufficiently advanced to offer this economically [75]. By the evolving a hybrid tool, the efficiency can be increased

version of only 10% of electricity into light and the loss of 90% of it to heat.

two methods that diminished this issue by using hetero-structured quantum dots.

variety of applications.

14 Quantum-dot Based Light-emitting Diodes

minescence QD-LEDs.

For example, McCreary developed a hybrid device by combining QDs with conjugated polymers to create a QD-LED. The motivation why this design was chosen is to be able to inkjet print the entire tool, at least the polymer and QD layers [75]. The researcher proposed a structure of the hybrid device which is of type ITO/PEDOT/CdSe QD/Au and is shown in **Figure 1(A)** [75]. In **Figure 1(B)** the energy bandgap structure for the same tool is presented [75].

**Figure 1.** The tool structure of a hybrid LED. (A) Three-dimensional description of the proposed QD-LED [75]. (B) An energy bandgap scheme of the proposed QD-LED and the suggested materials that were used to construct this type of QD-LED [75].

Also, the same author explained the manufacture of a hybrid LED with the structure ITO/ PEDOT: PSS/PVK/CdSe QD/Alq3/Al [75]. In order to ease proper hole transport and adequate QD coating, the researcher used a PVK/QD composite solution to make a monolayer layer of QDs using phase separation of the solutes in solution.

This physical modeling of hybrid QD-LEDs of this type, such as those mentioned before [75], makes them applicable to a diversity of hybrid organic QD optoelectronics tools like LEDs, solar cells, photodetectors, and chemical sensors.

#### **3.3. Quantum dot-based light-emitting diodes for near-field scanning optical microscopy**

QD-LEDs manufactured on silicon have the potential to be used in nanophotonics, optical micro/nanoelectromechanical systems (MEMS/NEMS), and micrototal analysis systems for real-time biomedical screening [21, 76]. QD-LEDs are of considerable interest for new optoelectronic applications such as that which comprise near-field microscopy beyond the diffraction limit, MEMS-based medical endoscopes for sub-cellular imaging, and compact light-on-chip biosensor and biochips [21].

In his paper, Zhu has investigated the case of a new thin film LED device utilizing nanocrystalline silicon QDs as an emission layer and metal oxide as charge transport layers [77]. Silicon (Si) is notably less costly in comparison with materials like germanium or gallium that are applied for commercial SSL devices and is relatively non-toxic as compared to heavy metal like Cd or Pb. The author developed a thin film LED structure which is based on colloidal silicon nanocrystals using nickel oxide (NiO) and zinc oxide (ZnO) as charge transport layers. The tool that was reported by the researcher is depicted in **Figure 2** [77]. ITO represents the anode, and Aluminum (Al) acts as the cathode. The light is produced when electrons and holes radiatively combine in the silicon nanocrystals (ncSi).

**Figure 2.** ncSiLED structure with metal oxide charge transport layers [77].

Zhu provided an alternative to organic charge transport layers in OLEDs, demonstrating that metal oxide transport layers based on NiO and ZnO are electrically more conductive than organic charge transport layers regularly found in OLEDs.

Also, the same author explained the manufacture of a hybrid LED with the structure ITO/ PEDOT: PSS/PVK/CdSe QD/Alq3/Al [75]. In order to ease proper hole transport and adequate QD coating, the researcher used a PVK/QD composite solution to make a monolayer layer of

This physical modeling of hybrid QD-LEDs of this type, such as those mentioned before [75], makes them applicable to a diversity of hybrid organic QD optoelectronics tools like LEDs,

**3.3. Quantum dot-based light-emitting diodes for near-field scanning optical microscopy**

QD-LEDs manufactured on silicon have the potential to be used in nanophotonics, optical micro/nanoelectromechanical systems (MEMS/NEMS), and micrototal analysis systems for real-time biomedical screening [21, 76]. QD-LEDs are of considerable interest for new optoelectronic applications such as that which comprise near-field microscopy beyond the diffraction limit, MEMS-based medical endoscopes for sub-cellular imaging, and compact

In his paper, Zhu has investigated the case of a new thin film LED device utilizing nanocrystalline silicon QDs as an emission layer and metal oxide as charge transport layers [77]. Silicon (Si) is notably less costly in comparison with materials like germanium or gallium that are applied for commercial SSL devices and is relatively non-toxic as compared to heavy metal like Cd or Pb. The author developed a thin film LED structure which is based on colloidal silicon nanocrystals using nickel oxide (NiO) and zinc oxide (ZnO) as charge transport layers. The tool that was reported by the researcher is depicted in **Figure 2** [77]. ITO represents the anode, and Aluminum (Al) acts as the cathode. The light is produced when electrons and

QDs using phase separation of the solutes in solution.

solar cells, photodetectors, and chemical sensors.

16 Quantum-dot Based Light-emitting Diodes

light-on-chip biosensor and biochips [21].

holes radiatively combine in the silicon nanocrystals (ncSi).

**Figure 2.** ncSiLED structure with metal oxide charge transport layers [77].

Nanocrystalline silicon QDs exhibited tunable luminescent colors. In this chapter, the author showed that nanocrystalline silicon QDs represent a feasible, truly benign, and ecological substitute for the heavy metal (Cd, Pb) QDs, without altering in any way the optical properties [77].

There are a lot of studies that demonstrated a number of procedures of creating QD-LEDs through microcontact printing of QDs on a micromachined silicon probe and resulting in this way a novel generation of highly integrated nano-scale optical fluorescent microscopy [76, 78]. The use of this innovative technology permits to detect the variation of sub-cellular characteristics and to measure the absorption at various wavelengths upon the near-field lighting of individual tumor cells with the aim of the identification of the cancer developmental phase [76].

Another important work in the same area of applications of QD-LEDs relates to the nearfield scanning optical microscopy (NSOM) areas of modern investigation with a QD-LED incorporated at the tip of a scanning probe. Hoshino et al. [79] proved near-field fluorescence excitation and imaging using a QD-LED integrated at the tip of a scanning probe. Also, because QD-LEDs have unique properties like well-controlled emission wavelength and narrow bandwidth, they represent a great choice as excitation sources for fluorescence imaging.

In other works [80, 81], the same researchers declared a microcontact printing method which is used to obtain some patterned QD-LEDs on flat silicon substrates. It has been observed that this method is very profitable owing to the connection of the methodology to build siliconbased electronics and MEMS. Hoshino and his team of researchers [79] proved a fluorescence imaging technique, showing that the sensitivity of fluorescence intensity to the QD-LED-QD specimen distance was evaluated down to 50 nm in order. This procedure might be expanded for the unique molecular order measurements [79, 82].

Zhang et al. [83] described a method for the development of a new near-field scanning probe with sub-diffraction-limit resolution by producing a nanometer-sized light source on a patterned probe tip and for the use of the probe in order to detect the molecular signatures of the tumors of the breast. In this project, the authors presented scanning fluorescence imaging with a nano-scale light-emitting diode incorporated at the tip of a silicon microprobe. More accurately, the researchers constructed and studied the QD-LED for bioimaging applications and examined the fluorescence imaging with the on-probe nano-scale QD-LED. It has been shown that QD-LEDs function like near-field excitation sources to enlighten fluorescently labeled cancer cells such as breast cancer cells named MDA435, prostate cancer cells called PC3, and circulating tumor cells in blood. In this chapter [81], a QD-LED at the tip of a micromachined silicon scanning probe was built; the very small size of the QD-LED makes it usable mainly for sub-wavelength optical measurements like in the case of NSOM measurements.

#### **4. Conclusions**

The area of QD-LED technology has made immense strides in the past 10 years and the demand for lower price and higher efficiency devices with raised functionality will continue to lead the novelty.

Among all the emerging display and lighting technologies briefly considered in this chapter, QD-LED technologies are by far the most nascent. Like technology, it is a direct challenge to OLEDs.

In this chapter, the recent developments in applications of QD-LEDs and some QD-LEDs qualities in display and lighting applications including their color tunability, durability, and high luminescence efficiency have been reviewed and discussed.

In this work, unique features of QD-LEDs applications for fundamental research and industry which will indubitably spectacularly bring novel design possibilities for the next-generation displays and solid-state lighting in the years to come are presented.

#### **Author details**

Anca Armăşelu

Address all correspondence to: anca\_armaselu@yahoo.com

Department of Electrical Engineering and Applied Physics, Faculty of Electrical Engineering and Computer Science, Transilvania University of Brasov, Brasov, Romania

#### **References**


[6] Melville J. Optical properties of quantum dots [final paper]. UC Berkeley College of Chemistry; 2015. Available from: https://www.ocf.berkeley.edu/~jmlvll/lab-reports/ quantumDots/quantumDots.pdf

**4. Conclusions**

18 Quantum-dot Based Light-emitting Diodes

to lead the novelty.

**Author details**

Anca Armăşelu

**References**

10.1155/2009/815734

1177/0192623307310950

10.1038/nbt767

OLEDs.

The area of QD-LED technology has made immense strides in the past 10 years and the demand for lower price and higher efficiency devices with raised functionality will continue

Among all the emerging display and lighting technologies briefly considered in this chapter, QD-LED technologies are by far the most nascent. Like technology, it is a direct challenge to

In this chapter, the recent developments in applications of QD-LEDs and some QD-LEDs qualities in display and lighting applications including their color tunability, durability, and

In this work, unique features of QD-LEDs applications for fundamental research and industry which will indubitably spectacularly bring novel design possibilities for the next-generation

Department of Electrical Engineering and Applied Physics, Faculty of Electrical Engineering

[1] Mazumder S, Dey R, Mitra MK, Mukherjee S, Das GC. Review: Biofunctionalized quantum dots in biology and medicine. Journal of Nanomaterials. 2009;**2009**:1-17. DOI:

[2] Deerinck TJ. The application of fluorescent quantum dots to confocal, multiphoton and electron microscopic imaging. Toxicologic Pathology. 2008;**36**(1):112-116. DOI: 10.

[3] Medintz IL, Uyeda HT, Goldman ER, Mattoussi H. Quantum dot bioconjugates for imaging, labelling and sensing. Nature Materials. 2005;**4**:435-446. DOI: 10.1038/nmat1390 [4] Jaiswal JK, Mattoussi H, Mauro JM, Simon SM. Long-term multiple color imaging of live cells using quantum dot bioconjugates. Nature Biotechnology. 2003;**21**(1):47-51. DOI:

[5] Ballou B, Lagerholm BC, Ernst LA, Bruchez MP, Waggoner AS. Noninvasive imaging of quantum dots in mice. Bioconjugate Chemistry. 2004;**15**(1):79-86. DOI: 10.1021/bc03453y

and Computer Science, Transilvania University of Brasov, Brasov, Romania

high luminescence efficiency have been reviewed and discussed.

displays and solid-state lighting in the years to come are presented.

Address all correspondence to: anca\_armaselu@yahoo.com


[35] Akasaki I, Amano H, Itoh K, Koide N, Manabe K. GaN-based ultraviolet/blue light emitting devices. Inst. Phys. Conf. Ser. 1992;**129**:851-856

[20] NanoPhotonica [Internet]. 2015. Available from: http://nanophotonica.net/technology/

[21] Hussain Z. Introduction to Nanobiotechnology [Internet]. 2015. Available from: http://

[22] Zyga L. Quantum Dot LED Approaches Theoretical Maximum Efficiency [Internet]. 2013. Available from: https://phys.org/news/2013-05-quantum-dot-approaches-theoret-

[23] Bozyigit D, Wood V. Challenges and solutions for high-efficiency quantum dot-based

[24] Anikeeva PO, Madigan CF, Halpert JE, Bawendi MG, Bulović V. Electronic and excitonic processes in light-emitting devices based on organic materials and colloidal devices based on organic materials and colloidal quantum dots. Physical Review B.

[25] Wood V, Panzer MJ, Halpert JE, Caruge J-M, Bawendi MG, Bulović V. Selection of metal oxide charge transport layers for colloidal quantum dot LEDs. ACS Nano.

[26] Mashford BS, Stevenson M, Popovic Z, Hamilton C, Zhou Z, Breen C, Steckel J, Bulović V, Bawendi M, Coe-Sullivan S, Kazlas PT. High-efficiency quantum-dot light emitting devices with enhance charge injection. Nature Photonics. 2013;**7**:407-412. DOI: 10.1038/

[27] Wac Lighting. Responsible Lighting. LED/OLED: Technical Training and Applications [Internet]. 2013. Available from: http://iie.ciapr.org/actividades/seminarios/2009/Presen-

[29] Losev OV. Luminous carborundum (silicon carbide) detector and detection with crys-

[30] Braunstein R. Radiative transitions in semiconductors. Physical Review. 1955;**99**:1

[31] Paisnik K, Rang G, Rang T. Life-time characterization of LEDs. Estonian Journal of

[32] Braunstein R, Loebner EE. Semiconductor device for generating modulated radiation.

[34] Pearsall TP, Miller BI, Capik KJ, Bachmann KJ. Efficient lattice-matched double-hetero-

In1−*<sup>x</sup>* As*<sup>y</sup>* P1−*<sup>y</sup>* ) junctions.

. Applied Physics Letters. 1976;**28**(9):499-

www.slideshare.net/zohaibkhan404/quantum-dot-light-emitting-diode

LEDs. MRS Bulletin. 2013;**38**:731-736. DOI: 10.1557/mrs.2013.180

2008;**78**(8):085434-085441. DOI: 10.1103/PhysRevB.78.085434

[28] Round HJ. A note on carborundum. Electrical World. 1907;**19**:309-310

tals. Telegrafiya i Telefoniya bez Provodov. 1927;**44**:485-494

Engineering. 2011;**17**(3):241-251. DOI: 10.3176/eng.2011.3.05

RCA Corp, assignee, U.S. Patent 3102201 [Issued: 27 August 1963]

Applied Physics Letters. 1962;**1**(4):82-83. DOI: 10.1063/1.1753706

structure LED's at 1.1. μm from Ga*<sup>x</sup>*

501. DOI: 10.1063/1.88831

[33] Holonyak N Jr, Bevaqua SF. Coherent (visible) light emission from Ga(As1-xPx

2009;**3**(11):3581-3586. DOI:10.1021/nn901074r

toggle-id-2

ical-maximum.html

20 Quantum-dot Based Light-emitting Diodes

nphoton.2013.70

892-1893

tacion LED-OLED.pdf


nanocrystal quantum dot luminophores. Optics Express. 2010;**18**(1):340-347. DOI: 10.1364/ OE.18000340


[62] Xie B, Hu R, Luo X. Quantum dots-converted light-emitting diodes packaging for lighting and display: Status and perspectives. Journal of Electronic Packaging. 2016; **138**(2):020803-020803-13. DOI: 10.1115/1.4033143

nanocrystal quantum dot luminophores. Optics Express. 2010;**18**(1):340-347. DOI: 10.1364/

[49] Kitai A, editor. Materials for Solid State Lighting and Displays. Chichester: Wiley; 2017.

[50] Supran GJ. QLEDs for display and solid-state lighting. MRS Bulletin. 2013;**3819**:703-711.

[51] Coe S, Woo W-K, Bawendi M, Bulović V. Electroluminescence from single monolayers of nanocrystals in molecular organic devices. Nature. 2002;**420**(6917):8100-8803. DOI:

[52] Tan Z, Zhang F, Zhu T, Xu T. Bright and color-saturated emission from blue light-emitting diodes based on solution-processed colloidal nanocrystal quantum dots. Nano

[53] Zhu T, Shanmugasundaram K, Price SC, Ruzyllo J, Zhang F, Xu I, Mohney SE, Zhang Q, Wang AY. Mist fabrication of light emitting diodes with colloidal nanocrystal quantum

[54] Li Y, Rizzo A, Mazzeo M, Carbone L, Manna L, Cingolani R, Gigli G. White organic light-emitting devices with CdSe/ZnS quantum dots as a red emitter. Journal of Applied

[55] Chen J, Zhao D, Li C, Xu F, Lei W, Sun L, Nathan A, Sun XW. All solution processed

[56] Klein M. Quantum Dots for LED Displays [Internet]. 2014. Available from: http://www. materialsforenergytypepad.com/materials/2014/02/quantum-dots-for-led-displays.html

[57] Kumar B, Campbell SA, Ruden PP. Modeling charge transport in quantum dot light emitting devices with NiO and ZnO transport layers and Si quantum dots. Journal of

[58] Anikeeva PO, Halpert JE, Bawendi MG, Bulović V. Electroluminescence from a mixed red-green-blue colloidal quantum dot monolayer. Nano Letters. 2007;**7**(8):2196-2200

[59] Yang Y, Zheng Y, Cao W, Titov A, Hyvonen J, Manders JR, Xue J, Holloway PH, Qian L. High-efficiency light-emitting devices based on quantum dots with tailored nanostruc-

[60] NanoPhotonica [Internet]. 2015. Available from: http://www.nanophotonica.net/about/

[61] Qin W, Yang Z, Jiang Y, Lam JWY, Liang G, Kwok HS, Tang BZ. Construction of efficient deep blue aggregation—induced emission luminogen from triphenylethene for nondoped organic light-emitting diodes. Chemistry of Materials. 2015;**27**(11):3892-3901.

tures. Nature Photonics. 2015;**9**(4):259-266. DOI: 10.1038/nphoton.2015.36

as blue emitters.

stable white quantum dot light-emitting diodes with hybrid ZnO@TiO<sup>2</sup>

dots. Applied Physics Letters. 2008;**92**(2):023111-023113. DOI: 10.1063/1.2834734

OE.18000340

22 Quantum-dot Based Light-emitting Diodes

384 p. ISBN: 978-1-119-14058-0

Letters. 2007;**7**(12):3803-3807. DOI: 10.1021/nl07230s

Physics. 2005;**97**(11):113501. DOI: 10.1063/1.1921341

Scientific Reports. 2014;**4**(4085):1-6. DOI: 10.1038/srep04085

Applied Physics. 2013;**114**(044507):1-6. DOI: 10.1063/1.4816680

DOI: 10.1021/acs.chemmater.5b00568

DOI: 10.1557/mrs.2013.181

10.1038/nature01217


**Provisional chapter**

#### **Quantum Dot-Based Light Emitting Diodes (QDLEDs): New Progress New Progress**

**Quantum Dot-Based Light Emitting Diodes (QDLEDs):** 

DOI: 10.5772/intechopen.69014

Neda Heydari, Seyed Mohammad Bagher Ghorashi, Wooje Han and Hyung-Ho Park Ghorashi, Wooje Han and Hyung-Ho Park Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Neda Heydari, Seyed Mohammad Bagher

http://dx.doi.org/10.5772/intechopen.69014

#### **Abstract**

[76] Gopal A, Hoshino K, Zhang JXJ. Quantum dots light emitting devices on MEMS: Microcontact printing, near-field imaging, and early cancer detection. In: Proceedings of SPIE, International Symposium on Photoelectronic Detection and Imaging 2011: Sensor and Micromachined Optical Device Technologies; Vol. 8191; 2011. p. 819106. DOI: 10.1117/12.901089. Available from: http://proceedings.spiedigitallibrary.org/proceeding.

[77] Zhu J. Nanocrystalline silicon quantum dot light emitting diodes using metal oxide charge transport layers [thesis]. University of Toronto; 2012. Available from: https:// tspace.library.utoronto.ca/bitstream/1807/42432/6/Zhu\_Jiayuan\_201211\_MASc\_thesis.

[78] Hoshino K, Gopal A, Zhang X. Contact printing of quantum dot light emitting diode on silicon probe tip. CLEO/QELS: 2010 Laser Science to Photonic Applications; IEEE; 2010. INSPEC Accession Number: 11428025. DOI: 10.1364/CLEO.2010.CTuNN4. Available

[79] Hoshino K, Gopal A, Glaz MS, Vanden Bout DA, Zhang X. Nanoscale fluorescence imaging with quantum dot near-field electroluminescence. Applied Physics Letters.

[80] Gopal A, Hoshino K, Kim S, Zhang X. Multi-color colloidal quantum dot based light emitting diodes micropatterned on silicon. Nanotechnology. 2009;**20**(23):235201-235201-

[81] Gopal A, Hoshino K, Zhan X. Photolithographic patterning of subwavelength top emitting colloidal quantum dot based inorganic light emitting diodes on silicon. Applied

[82] Hoshino K, Turner TC, Kim S, Gopal A, Zhang X. Single molecular stamping of sub-10-nm colloidal quantum dot array. Langmuir. 2008;**24**(23):13804-13808. DOI: 10.1021/

[83] Zhang XJ, Ferrari M, Cheng M-C. EPDT: Nano-scale light emitting diode on silicon cantilever for near-field microscopy of nanovectors biodistribution in tissues and living cells. Austin, TX, United States: University of Texas. 2011. Available from: http://grant-

Physics Letters. 2010;**96**(13):131109-131109-3. DOI: 10.1063/1.3373832

from: http://ieeexplore.ieee.org/document/5500224/

2012;**101**(4):043118-043118-5. DOI: 10.1063/1.4739235

9. DOI: 10.1088/0957-4484/20/23/235201

ome.com/ghrant/NSF/ECCS-0725886

aspx?articleid=1270250

24 Quantum-dot Based Light-emitting Diodes

pdf

la802936h

In recent years, the display industry has progressed rapidly. One of the most important developments is the ability to build flexible, transparent and very thin displays by organic light emitting diode (OLED). Researchers working on this field try to improve this area more and more. It is shown that quantum dot (QD) can be helpful in this approach. In this chapter, writers try to consider all the studies performed in recent years about quantum dot-based light emitting diodes (QDLEDs) and conclude how this nanoparticle can improve performance of QDLEDs. In fact, the existence of quantum dots in QDLEDs can cause an excellent improvement in their efficiency and lifetime resulted from using improved active layer by colloidal nanocrystals. Finally, the recent progresses on the quantum dot-based light emitting diodes are reviewed in this chapter, and an important outlook into challenges ahead is prepared.

**Keywords:** quantum dot, organic light emitting diode, efficiency, lifetime, active layer

#### **1. Introduction**

Due to increased population and consumption of more energy, the people of Earth are faced with a serious shortage of energy resources. Therefore, the primary concern of researchers and manufacturers is closely linked to energy consumption. In recent years, a lot of researches are conducted to achieve efficient and low-energy light sources. Inorganic light emitting diode (LED) and organic light emitting diode (OLED) have been introduced as a result of these efforts to achieve solid-state light sources [1–6]. The outdoor application is one of the important markets for LED lighting. For year 2015, the assessment of the total outdoor lighting market was \$6.5 billion USD with LEDs. The outdoor lighting market is expected to grow with

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

growth rate about 4% from 2015 to 2021 [7]. LEDs were used in many applications such as television backlight units and illuminated signs. The US Department of Energy has reported that the achievements to the expected developments in LED technology would save 300 TW per hour of electricity [8]. It means that a remarkable strategy needs to be developed for the simple design and better material to reduce the cost of fabrication. According to Stephanie Pruitt report, the packaged LED profits hit 15.4 billion dollars in 2014 and will grow to 22.1 billion dollars in 2019 [9].

#### **2. Why OLEDs?**

In recent years, the display industry and lighting panels have been changed. Many researchers are interested in using polymers and organic molecules as emissive layers in these devices to improve their characteristics. One of the most important developments related to OLED technology provides the ability to build flexible (can be deposited onto substrate like plastic), transparent and very thin displays and components. Simply an organic light emitting diode is constructed with a thin film of organic (carbon-based) put between a conductive cathode (electron injection site) and a conductive anode (electron removal site) considering that at least one of the electrodes should be transparent. This thin film is called emitter, which is electroluminescent; it emits light when excited by an electrical current. These organic matters have conductivity levels between insulating and conductive; therefore, they are considered as organic semiconductors. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in organic semiconductors are similar to the conduction and valence bands in inorganic semiconductors. The performance of OLEDs reaches important goals in display technology. In addition, OLEDs have many advantages over both LCDs and LEDs such as thinner, lighter, more flexible and brighter substrate. Moreover, OLEDs unlike the LCDs do not need a backlight and filters; thus, they are very thin, and their construction will be easier and reliable. Due to low energy consumption, OLEDs will be an important advantage for cellphones, which are battery-operated devices. Another important feature of the OLED is the solution-based emitting materials used in their structure. It can be possible to fabricate them into large area by a spin-coating method, which is low-cost fabrication techniques [10]. Also, OLEDs can be produced into large, thin sheet which makes them an interesting choice for industry. In addition, changing information in this technology is in real time, which is faster than LCDs.

The ability of a light source to reveal the colours of objects compared to a natural light source is called colour rendering index (CRI). This parameter is the most important advantage of an OLED in comparison with LED. Consequently, OLEDs have attracted a lot of attention due to light weight and high image quality. These features lead to a wide range of applications in industry, particularly in manufacturing flexible screens and full-colour light emitting pages. But there are still some problems like sensitivity to water vapour. Also, the production costs should be reduced more. Technology of OLED has much room for continuous progress in future. On the other hand, the fabrication process of small-molecule OLEDs is too expensive because thermal deposition with high vacuum is required. However, polymer-based OLEDs (POLEDs) are the good substitute due to their solution process, which makes them more cost-effective. In fact, straightforward way of fabrication is the necessary factor for the low-cost electronic devices. Another advantage of POLEDs is their lower power consumption in comparison with traditional option. Therefore, many researchers are interested in developing POLED technology. Bottom-emitting conventional, bottom-emitting inverted, top-emitting conventional and topemitting inverted are the four different architectures of POLEDs. Bottom-emitting inverted and top-emitting inverted can increase operational lifetime and reduce the fabrication and operating cost of the device. In addition, top-emitting conventional and top-emitting inverted can increase light out-coupling efficiency.

#### **3. A brief review of OLED development**

growth rate about 4% from 2015 to 2021 [7]. LEDs were used in many applications such as television backlight units and illuminated signs. The US Department of Energy has reported that the achievements to the expected developments in LED technology would save 300 TW per hour of electricity [8]. It means that a remarkable strategy needs to be developed for the simple design and better material to reduce the cost of fabrication. According to Stephanie Pruitt report, the packaged LED profits hit 15.4 billion dollars in 2014 and will grow to 22.1

In recent years, the display industry and lighting panels have been changed. Many researchers are interested in using polymers and organic molecules as emissive layers in these devices to improve their characteristics. One of the most important developments related to OLED technology provides the ability to build flexible (can be deposited onto substrate like plastic), transparent and very thin displays and components. Simply an organic light emitting diode is constructed with a thin film of organic (carbon-based) put between a conductive cathode (electron injection site) and a conductive anode (electron removal site) considering that at least one of the electrodes should be transparent. This thin film is called emitter, which is electroluminescent; it emits light when excited by an electrical current. These organic matters have conductivity levels between insulating and conductive; therefore, they are considered as organic semiconductors. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in organic semiconductors are similar to the conduction and valence bands in inorganic semiconductors. The performance of OLEDs reaches important goals in display technology. In addition, OLEDs have many advantages over both LCDs and LEDs such as thinner, lighter, more flexible and brighter substrate. Moreover, OLEDs unlike the LCDs do not need a backlight and filters; thus, they are very thin, and their construction will be easier and reliable. Due to low energy consumption, OLEDs will be an important advantage for cellphones, which are battery-operated devices. Another important feature of the OLED is the solution-based emitting materials used in their structure. It can be possible to fabricate them into large area by a spin-coating method, which is low-cost fabrication techniques [10]. Also, OLEDs can be produced into large, thin sheet which makes them an interesting choice for industry. In addition, changing information in this technology is in

The ability of a light source to reveal the colours of objects compared to a natural light source is called colour rendering index (CRI). This parameter is the most important advantage of an OLED in comparison with LED. Consequently, OLEDs have attracted a lot of attention due to light weight and high image quality. These features lead to a wide range of applications in industry, particularly in manufacturing flexible screens and full-colour light emitting pages. But there are still some problems like sensitivity to water vapour. Also, the production costs should be reduced more. Technology of OLED has much room for continuous progress in future. On the other hand, the fabrication process of small-molecule OLEDs is too expensive because thermal deposition with high vacuum is required. However, polymer-based OLEDs (POLEDs) are

billion dollars in 2019 [9].

26 Quantum-dot Based Light-emitting Diodes

real time, which is faster than LCDs.

**2. Why OLEDs?**

The first OLED was manufactured in 1987 by Tong in Kodak company [11]. He realized when an electric current is applied to the molecules of the organic material, this material emits green light. This was the first idea about OLEDs. In the first OLED, the structure was built by an indium tin oxide (ITO)/aromatic diamine/8-hydroxyquinoline aluminium (Alq3)/Mg-Al metal electrode. Up to now, the most organic components used in OLEDs are poly(para-phenylenevinylene) (PPV) [12], polyvinylcarbazole (PVK) [13] and aluminium-tris-(8-hydroxyquinoline) (Alq3) [14, 15]. To commercialize the OLEDs, several aspects must be improved. Therefore, during two decades, a lot of efforts have been made to achieve high performance of OLED devices. For example, the ability of charge injection, charge transport and emission of different layers of OLEDs are three important factors in their performance. To improve these factors, much effort has been devoted by researchers. They have tried to find better anode and cathode materials. They have also attempted to synthesize new materials' high emissivity. Therefore, development in synthesis process and application of electron transport materials, modification of surface in hole injection layer and electron injection layer, using high mobility materials in hole and electron transport layer (ETL), doping the high efficiency emitter dopants in emission layers and reducing the barrier to charge carrier injection by increasing the doping level of materials, was received [16, 17].

To achieve high-brightness display, high electron mobility is necessary in electron transport materials. For enhancement of charge injection, scientists try to use different cathode, and simultaneously, they have tried different surface treatments of ITO [18]. It is well known that employing electroluminescence material with high mobility is required for low-power consumption. On the other hand, the voltage can be decreased by doping, but rapid dopant diffusion can create the quenching centres in the emissive layers, which result in reduction of efficiency. Balancing of electron and holes will increase the efficiency of device that can be achieved by controlling the mobility of the transport layers. Therefore, an increase in the exciton recombination probability and control of the carrier accumulation needs to be adjusted for improving the current and power efficiencies by aligning the bands at the interface between the emitting layer (EML) and ETL [19]. Water/oxygen permeability is another that factor must be noticed. Moreover, encapsulation with a barium oxide (BaO) or calcium oxide (CaO) is used in OLEDs, and an acceptable level of water/oxygen permeability is achieved [20]. As mentioned above, the architecture of OLEDs is one of the parameters that need to improve the performance of organic light emitting diode. So far, different structures of the OLEDs are investigated. Scientists have tried to improve the performance and stability of these devices by substituting of alternative material in different layers of OLEDs. For example, carbon nanotubes [21, 22], graphene [23, 24], metal nanomeshes [25, 26], thin metal films [27, 28] and metal nanowires [29, 30] are employed instead of ITO up to now. In addition, Burns et al. have investigated the effect of thermal annealing super yellow emissive layer on efficiency of OLEDs [31]. By annealing of the emissive layer at 50°C, the external quantum efficiency (EQE) of this device reached a maximum of 4.09%.

#### **4. Looking to the future: outlook**

OLEDs have been commercialized in tablet, smart watches and smart phones up to now, and they are stable devices with good efficiency. But they still need to achieve more improvements. Higher efficiency, better stability and being more environmentally friendly are some important factors that researchers are trying to improve them. Samsung has manufactured its mobile displays by red, green and blue OLED subpixels, and LG used white emitting OLED material with WRGB colour filters for its TVs. OLED display includes red, green and blue pixels. The most critical issue is the blue gap in OLED materials. Nowadays, display industries use fluorescent materials for blue colour, but the use of fluorescent materials involves with an increase in power consumption. Therefore, new approaches should be introduced in the technology of the OLED display. From technological point of view, the fluorescence, phosphorescence and thermally activated delayed fluorescence (TADF) are three mechanisms to harvest excitons in OLEDs and considered for improvement of their performance. Highperformance and low-cost OLEDs are available after discovery of metal-free organic emitters with thermally activated delayed fluorescence (TADF). There are two kinds of TADF emitters named organic and metal-organic. The maximum external quantum efficiency of these OLEDs has been reached to 25% up to now [32]. The efficiency of TADF OLEDs is comparable with phosphorescent OLEDs. The most value of TADF OLED lifetime is reported over 10,000 h. Carbazole [33, 34] or arylamine-type donors [35, 36] are the main organic TADF emitters that are reported up to date. The excited-state lifetime or emission decay time of materials is the important problem that should be solved to commercialize TADF OLEDs. It needs more developments in this field.

#### **5. Structure of OLEDs**

Structure of OLED is another factor that can be employed to improve its characteristics. At the device level, each OLED pixel is a p-n junction that emits lights. Top emitting and bottom emitting are two main configurations of the OLEDs. Up to now, top emitting is a common structure that has been used to increase efficiency and the light output of the device by the display industry. When the current source creates potential difference in OLED circuit, a variable voltage between 2 and 10 V is applied between the cathode and the anode, and the flow of electrons from the cathode to the anode is established. After a while the negative charge density in the electron transport layer and the positive charge density in the hole transport layer will increase. Today, some thin layers are used between two conductive layers in order to achieve better performance of OLEDs. The half-life of the device can be reduced by high voltage. Therefore, highly conductive transmission layer is used to reduce injection barriers and achieve low-voltage operation in modern OLEDs. The basic and typical structure of an organic light emitting diode has been shown in **Figure 1**.

mentioned above, the architecture of OLEDs is one of the parameters that need to improve the performance of organic light emitting diode. So far, different structures of the OLEDs are investigated. Scientists have tried to improve the performance and stability of these devices by substituting of alternative material in different layers of OLEDs. For example, carbon nanotubes [21, 22], graphene [23, 24], metal nanomeshes [25, 26], thin metal films [27, 28] and metal nanowires [29, 30] are employed instead of ITO up to now. In addition, Burns et al. have investigated the effect of thermal annealing super yellow emissive layer on efficiency of OLEDs [31]. By annealing of the emissive layer at 50°C, the external quantum efficiency (EQE)

OLEDs have been commercialized in tablet, smart watches and smart phones up to now, and they are stable devices with good efficiency. But they still need to achieve more improvements. Higher efficiency, better stability and being more environmentally friendly are some important factors that researchers are trying to improve them. Samsung has manufactured its mobile displays by red, green and blue OLED subpixels, and LG used white emitting OLED material with WRGB colour filters for its TVs. OLED display includes red, green and blue pixels. The most critical issue is the blue gap in OLED materials. Nowadays, display industries use fluorescent materials for blue colour, but the use of fluorescent materials involves with an increase in power consumption. Therefore, new approaches should be introduced in the technology of the OLED display. From technological point of view, the fluorescence, phosphorescence and thermally activated delayed fluorescence (TADF) are three mechanisms to harvest excitons in OLEDs and considered for improvement of their performance. Highperformance and low-cost OLEDs are available after discovery of metal-free organic emitters with thermally activated delayed fluorescence (TADF). There are two kinds of TADF emitters named organic and metal-organic. The maximum external quantum efficiency of these OLEDs has been reached to 25% up to now [32]. The efficiency of TADF OLEDs is comparable with phosphorescent OLEDs. The most value of TADF OLED lifetime is reported over 10,000 h. Carbazole [33, 34] or arylamine-type donors [35, 36] are the main organic TADF emitters that are reported up to date. The excited-state lifetime or emission decay time of materials is the important problem that should be solved to commercialize TADF OLEDs. It needs more

Structure of OLED is another factor that can be employed to improve its characteristics. At the device level, each OLED pixel is a p-n junction that emits lights. Top emitting and bottom emitting are two main configurations of the OLEDs. Up to now, top emitting is a common structure that has been used to increase efficiency and the light output of the device by the display industry. When the current source creates potential difference in OLED circuit, a variable

of this device reached a maximum of 4.09%.

28 Quantum-dot Based Light-emitting Diodes

**4. Looking to the future: outlook**

developments in this field.

**5. Structure of OLEDs**

The anode is positive compared to the cathode, so the electrons flow from the cathode to the anode. The electrons injected into the cathode are placed in the LUMO level of the organic layer, and they also will withdraw in the HOMO level of the organic layer. Holes arrive from the hole transport layer to the HOMO level. The energy level of the emissive layer should be less than the hole transport layer in order that the injection of the electrons from electron transport layer to LUMO level of the emissive layer to be possible. To penetration the holes into emissive layer, this layer also should have higher HOMO level than HOMO level of the hole transport layer.

Electrostatic forces bring the electrons and holes towards each other and form the excitons in a singlet/triplet ratio of 1:3. It happens near the emissive layer. **Figure 2** shows the population of emitter states by energy transfer from singlet and triplet excitons. It is important to note that the holes are more mobile than electrons in organic semiconductors and arrive to electron transport layer faster. The destruction of this excited state led to radiation in the visible region. If the active layer is phosphorescent, non-radiative triplet excitons may be emitted. The frequency of this radiation depends on the difference between HOMO and LUMO levels of these materials. Because the holes must be logged in the HOMO level of the organic material in emissive layer with energy levels about 5–6 eV, anode with high work function is required till holes will be able to effectively enter to the organic material. Also anode should be transparent in order for the produced photons to be visible. ITO is often used as anode material because it is transparent compared to visible light. In addition, the injection of holes into the HOMO level of the organic layer will be possible due to its high work function.

**Figure 1.** The structure of (a) basic and (b) typical organic light emitting diode.

**Figure 2.** Population of emitter states by energy transfer from singlet and triplet excitons.

To inject electrons into the LUMO level of the organic layer, the work function of the cathode should be low. Calcium and magnesium are two metals used as cathode due to their low work function. But the drawback is that these metals are sensitive to moisture and therefore will reduce the lifetime of the device. To solve this problem, aluminium or various alloys such as Mg/Ag as a cathode are used as cathode [37–39]. **Figure 3** shows the evolution of OLED device structure. The electron and hole transport layers could help to high-speed movement of electrons and holes to meet each other in the emission layer. Electron transport layer prevents the penetration of hole into cathode, and in contrast hole transport layer prevents the penetration of electrons to the anode. The electrons and holes recombine with each other in

**Figure 3.** Evolution of OLED device structure.

the middle of emissive layer. The material of the hole transport and the electron transport layers (electron and hole blocking layers) depends on the characteristics of the charge and the values of the HOMO and LUMO levels. Usually PEDOT:PSS is used as a conductive layer in which its HOMO level is between the work function of the ITO and HOMO level of the commonly used polymers in order to reduce the energy barrier of the injecting holes.

#### **6. Different types of the OLEDs**

To date, various types of the organic light emitting diodes are presented. The six main types of the OLEDs are passive-matrix OLED (PMOLED), active-matrix OLED (AMOLED), transparent OLED, top-emitting OLED, foldable OLED and white OLED. Each of these types has different kinds of use. PMOLED consists of cathode, organic layers and anode. The manufacture of this type of OLED is easy. They have high-power consumption; therefore, they are effective for small screens. Passive-matrix addressed displays are attractive, as the device construction is relatively simple. AMOLED is composed of cathode, organic molecules and anode layers. The anode layer is established on a thin film transistor (TFT) and formed a matrix. Because of the use of less power, they are suitable for large-sized displays. Substrate, cathode and anode are transparent in the transparent OLED. Top emitting OLED consists of an opacity substrate, and it is suitable for active-matrix design. Foldable OLED has a flexible substrate made of metal or plastic that is very lightweight and durable and can be used in clothes with OLED display. White OLED emits white light that is brighter, more uniform and more efficient than fluorescent lights.

#### **7. Important features of OLEDs**

To inject electrons into the LUMO level of the organic layer, the work function of the cathode should be low. Calcium and magnesium are two metals used as cathode due to their low work function. But the drawback is that these metals are sensitive to moisture and therefore will reduce the lifetime of the device. To solve this problem, aluminium or various alloys such as Mg/Ag as a cathode are used as cathode [37–39]. **Figure 3** shows the evolution of OLED device structure. The electron and hole transport layers could help to high-speed movement of electrons and holes to meet each other in the emission layer. Electron transport layer prevents the penetration of hole into cathode, and in contrast hole transport layer prevents the penetration of electrons to the anode. The electrons and holes recombine with each other in

**Figure 2.** Population of emitter states by energy transfer from singlet and triplet excitons.

**Figure 3.** Evolution of OLED device structure.

30 Quantum-dot Based Light-emitting Diodes

Quantum yields and lifetime are the two important characteristics of an OLED that researchers are trying to improve by different structures and techniques all around the world. Optimizing the balanced charge is an important issue for the lifetime of the device. Also choosing the right ingredients in the manufacture of layers can be useful in improving the performance of the device. OLEDs have the limited peak emission so that the highest peak luminance of OLEDs is at most 500–600 nits. But this value for LCD TVs is about 1800 nits. The amount of the light emitted divided by the amount of the injected current into the piece is called quantum yields of an OLED. High yield achievement and suitable coordinates of colour for display applications are the other important features of an OLED. External quantum efficiency (EQE) can be explained in following formula:

$$\text{External quantum efficiency} \left( \eta\_{\text{tQt}} \right) = \gamma \chi \cdot \eta\_{\text{t1}} \eta\_{\text{cc}} \tag{1}$$

*γ* = recombination efficiency of holes and electrons; *χ* = fraction of excitons with spin allowed optical transitions, created in emissive layer; *η*PL = photoluminescent efficiency of the emitter; *η*OC = fraction of emitted photons that are coupled out of the device (1/2*n*<sup>2</sup> ); *n* = refractive index of the substrate (glass).

Electrons and holes which meet each other in active layer create different states; about 25% of the excitons are in the singlet states, and the rest of them are in the triplet states. Therefore, the maximum internal efficiency of the OLEDs based on fluorescent molecules is around 25%. It can be possible to improve the efficiency of OLEDs by enhancing spinorbit coupling and enable emission from the formally forbidden triplet state with the use of phosphors. Totally, display devices are typically assessed through a number of characterization measurements that include colour coordinates (perceived colour), current density (A/cm) versus voltage, luminance (a measure of brightness in cd/m2 ) versus voltage, current efficiency (cd/A) versus luminance, power efficiency (lm/W) versus luminance and lifetime (a measure of the stability of the device). Kim and his colleagues in 2014 have shown that approximately 35.6% EQE can be reached by using iridium compounds (HICs). This efficiency is one of the highest external quantum efficiencies achieved to date in the red OLEDs [40]. OLED has already been commercialized; LG company has commercialized the OLED-based TVs. OLEDs are emissive displays, which means they create their own light at each pixel, like CRTs and PDPs. But as mentioned, the possibility of degradation in the presence of moisture and oxygen is the big problem needed to be considered. There are several damaging processes in these materials such as thermal instability, optical and chemical oxidation of the active layer and penetration of the metal from electrodes [41]. So it should be a process for encapsulating structure to protect it from the influence of moisture and oxygen. Also, by replacing the organic materials to inorganic structures, there will be the possibility of a better stability. The best results have been achieved up to date related on the usage of quantum dots based on cadmium. Employing nanoparticles such as oxides and semiconductors to form composite materials is one of the available solutions to improve the stability of these devices [42]. Not only adding nanoparticle can increase the stability of the film, but also it will be possible to control the optical properties by adjusting their size. Thus this would be an appropriate way for optoelectronic applications. For example, the gap between energy levels (luminescence colour) of semiconductor increases by reducing the particle size.

#### **8. A brief review of quantum dot-based light emitting diodes (QDLEDs)**

Being cost-effective, much more brightness and more efficient as well as more stable devices made of environmentally sustainable materials are the most important factors that led to the development of the lighting industry. A new candidate for improvement of display industry is emissive layer based on quantum dots (QDs). Quantum dot technology is a novel innovation to help this industry. This technology has also applications in many other markets such as solar cells, biomedical, instrumentation, quantum computers and more. Quantum dot technology seems to offer the biggest colour gamut of the various approaches today. Quantum dots have three key elements to their structure. Core, shell and ligand are the three main properties of the structure of the QDs. The core adsorbs and re-emits the light. The shell layer is responsible to confine the emission and passivate defects in the structure. The ligand layer provides more stability. The addition of barrier layers is required to protect QDs from oxygen, water and heat. Quantum dot-based light emitting diodes (QDLEDs) are a new form of light emitting technology based on nanoparticle, and their structures are similar to the OLED technology. Although, in this technology, a layer of quantum dots is placed between electron and hole-transporting layers, like sandwiched structure. Electrons and holes are accumulated in the quantum dot layer by an applied electric field. Then, they will recombine and emit narrow spectrum of photons. For example, FWHM for Cd based is 25–35 nm and 40–50 nm for Cd-free QDs.

Electrons and holes which meet each other in active layer create different states; about 25% of the excitons are in the singlet states, and the rest of them are in the triplet states. Therefore, the maximum internal efficiency of the OLEDs based on fluorescent molecules is around 25%. It can be possible to improve the efficiency of OLEDs by enhancing spinorbit coupling and enable emission from the formally forbidden triplet state with the use of phosphors. Totally, display devices are typically assessed through a number of characterization measurements that include colour coordinates (perceived colour), current den-

current efficiency (cd/A) versus luminance, power efficiency (lm/W) versus luminance and lifetime (a measure of the stability of the device). Kim and his colleagues in 2014 have shown that approximately 35.6% EQE can be reached by using iridium compounds (HICs). This efficiency is one of the highest external quantum efficiencies achieved to date in the red OLEDs [40]. OLED has already been commercialized; LG company has commercialized the OLED-based TVs. OLEDs are emissive displays, which means they create their own light at each pixel, like CRTs and PDPs. But as mentioned, the possibility of degradation in the presence of moisture and oxygen is the big problem needed to be considered. There are several damaging processes in these materials such as thermal instability, optical and chemical oxidation of the active layer and penetration of the metal from electrodes [41]. So it should be a process for encapsulating structure to protect it from the influence of moisture and oxygen. Also, by replacing the organic materials to inorganic structures, there will be the possibility of a better stability. The best results have been achieved up to date related on the usage of quantum dots based on cadmium. Employing nanoparticles such as oxides and semiconductors to form composite materials is one of the available solutions to improve the stability of these devices [42]. Not only adding nanoparticle can increase the stability of the film, but also it will be possible to control the optical properties by adjusting their size. Thus this would be an appropriate way for optoelectronic applications. For example, the gap between energy levels (luminescence colour) of semiconductor increases by reducing

**8. A brief review of quantum dot-based light emitting diodes (QDLEDs)**

Being cost-effective, much more brightness and more efficient as well as more stable devices made of environmentally sustainable materials are the most important factors that led to the development of the lighting industry. A new candidate for improvement of display industry is emissive layer based on quantum dots (QDs). Quantum dot technology is a novel innovation to help this industry. This technology has also applications in many other markets such as solar cells, biomedical, instrumentation, quantum computers and more. Quantum dot technology seems to offer the biggest colour gamut of the various approaches today. Quantum dots have three key elements to their structure. Core, shell and ligand are the three main properties of the structure of the QDs. The core adsorbs and re-emits the light. The shell layer is responsible to confine the emission and passivate defects in the structure. The ligand layer provides more stability. The addition of barrier layers is required to protect QDs from

) versus voltage,

sity (A/cm) versus voltage, luminance (a measure of brightness in cd/m2

the particle size.

32 Quantum-dot Based Light-emitting Diodes

The efficiency of QDLEDs is still lower than OLEDs. But the pure emission colour, the easier tenability of colour emission by adjusting the particle size and their lower emitter cost make them interesting subject for researchers as well as artisans. Conducted researches improve the quantum efficiency of QDLEDs more than two order of magnitude up to now. A bounded electron and hole inside the QD can recombine and emit a photon that has energy equal to the gap between the highest occupied and lowest unoccupied states. In 1994, the first structure of organic light emitting diodes based on quantum dots is studied. This structure consists of a layer of CdSe quantum dots and the polymeric electron transport layer, which are placed between two electrodes [43]. Due to low mobility of organic semiconductor, QDLED had low performance, and the threshold voltage was as large as 4 V. Recently, a new colloidal quantum dot-based light emitting diode (QDLED) is reported with improved external quantum efficiencies (EQE) by applying the organic CIM/LiF/Al cathode [44]. QLEDs with this new structure increase the EQE about 25% comparing to the bare Al devices. Therefore, using an organic cathode interfacial material can result in better device performance, including the brightness, EQE and CE. In this proposed device, the peaks of EQE and CE were 8.5% and over 29 cd/A, respectively. This improvement is because of balanced electron/hole injection due to the presence of the organic CIM. The balancing of the carriers is hard, because most quantum dots are considered in n-type materials. So the current efficiency will be low in these devices. The p-type conductivity and hole injection barriers of the organic hole transport layer are necessary to improve the efficiency of QDLEDs. Further attempts are aimed to optimize charge injection, to transport, to improve stability of material and to control chemical and physical phenomena at the interface. Also, an all solution-processed QDLED with an inverted structure is investigated by Castan and his coworkers [45]. They demonstrated that the optimized amount of PTE in the PEDOT:PSS can balance the charge in the device. The red, green and blue devices using this structure have maximum luminance about 12.510, 32.370 and 249 cd/m2 and turn-on voltages of 2.8, 3.6 and 3.6 V, respectively. Because of the process used for the fabrication of this device, it is very promising in the future of display industry.

In addition, highly bright and efficient blue QDLEDs have been reported by employing ZnCdSe core/multishell QDs as emitters [46]. The efficiency and brightness were improved by doping poly vinyl(N-carbazole) (PVK) in the emissive layer. It balances the charge injection because of the lower HOMO level, which causes the reduction of potential barrier at the interface of QDs and hole transport layer. This blue QDLEDs show a high efficiency (EQE > 8%), and the peak of efficiency happens at the luminance about 1000 cd/m2 . In 2007, Xie et al. found that the inorganic core oxidizes through their lifetime. So they suggested growing the shell materials on the surface of the core to passivate the inorganic core [47, 48]. They could improve stability by growing a ZnS shell around the InP core. The properties of QDs strongly depend on their shell and their compounds. The cluster diameter is a significant factor in determination of bandgap in structure of the QDs. The emission band will be narrower, while the diameter of the cluster gets smaller. Also the thickness of shell is important in increasing the maximum amount of the PL efficiency. According to the result of Bera and his colleagues' research, the thicker shell layer, the lower amount of photoluminescence quantum yield (PLQY) [49]. The main reason of this phenomenon is that the misfit dislocations (sites of non-radiative recombination) are formed when the shell layer is thick. Higher quantum efficiency will be available by minimizing these sites in QDs. In addition, matching the energy levels of the shell and core should be considered. Confining the excitons within the QDs is possible by selecting proper material of shell with wider bandgap to create an appropriate potential barrier around the QD.

Colloidal CdSe/ZnS (core-shell QDs) have high quantum yield and high photo stability at room temperature. So they are good choice in lighting industry, and many researchers have investigated them [50]. To prevent the light scattering, the particle size should be smaller than one-tenth of the visible light's wavelength [51]. On the other hand, large particles tend to accumulate that tarnish the composite film. The fluorescence properties of the QDs can be affected by the ability of QDs to aggregation. Accumulation effect can drastically reduce the quantum efficiency. Recent researches have demonstrated that the repulsive force between the molecular chains of polymers can prevent the accumulation of nanoparticles, so the compound of the polymer quantum dot can improve this problem. Up to now colloidal nanoparticles of cadmium sulphides, cadmium silicon and lead sulphides are used in organic light emitting diodes. These QDLEDs emit green light potentially [52–56]. On the other hand, these kinds of QDs have toxicological properties so it is environmentally restricted and not to be able to be a commercial material in this field. New Cd-free quantum dots should be introduced to commercialize the QDLED technology. ZnO cores with a MgO shell, InP-based dots and CuInS2 are three new materials that need more studies to be performed by scientists [57, 58]. In 2015, Du et al. studied a stable photoluminescence QDLEDs based on hydrophilic CdTe QD. Inorganic nanocomposite CdTe quantum dots were prepared with two rotary steam and freeze-drying methods. Because of adhesion, flexibility and transparency, silica gel can be coated on the surface of UV light emitting diode and form photoluminescence QDLEDs. This new photoluminescence QDLED is sustainable and cost-effective. Also it is easy to operate and environmentally non-toxic [59]. Recently, Kim' group has studied a multiple structure of QDLED based on InP quantum dots. Current efficiency and brightness in this structure are reported to be 1 cd/A and 530 cd/m2 , respectively. As mentioned, the best results in improving the stability of organic light emitting diodes are based on Cd QDs. InP quantum dots are replaced with Cd QDs in this study because of the environmental risks of cadmium [60]. In addition, the interface trap states are very effective on the performance of the device. In 2016, Koh et al. investigated these traps in the presence of TCNQ between charge transfer layer and quantum dots. With the introduction of TCNQ, the electroluminescent efficiency (EL) in QDLED has been improved by increasing the charge injection into the QD layer [61].

ZnO nanowires are perfect single crystals, which increases the mean free path of carriers transmitted on them. High density of QDs layers is prepared by ZnO nanowires which caused much more brightness of QDLEDs. These ZnO nanorods allow high density for QD and provide brighter LED-based display. This structure emits light that is very similar to the sun's light, and energy transfer efficiency of this structure is measured and equal to 17% [62]. The efficiencies equal to 10.7 and 14.5% are obtained by combining electron transfer layer and hole transfer layer and using the Cd QD for blue and green QDLED, respectively. However, the efficiency of red QDLED has reached to 20% [63]. Furthermore, the red QDLED with reverse multiple structures and excellent performance with an external quantum efficiency of 18% has been reported [64]. Up to now the external quantum efficiencies of QDLEDs based on Cd QD obtained are 10.7% [65] and 14.5% [66] for blue and green QDLEDs, respectively, while red QDLED efficiency is 20.5% [67]. Of course, the higher efficiencies are obtained in OLEDs without the use of quantum dots. In many applications, nanoparticles are imported into the polymer to give a special feature. For example, nanoparticles improve stability of the host material, because they act as energy absorbers to reduce the structural defects of organic materials. The benefits of using nanoparticles are high stability, narrow emission spectrum and feasible use in the polymer structure and the formation of thin film layers. Dark details, image sticking, peak luminance, colour gamut, colour volume, efficiency and lifetime of QDLEDs are so much better than OLEDs. However, black level, haloing, viewing angle and being eco-friendly are the advantages of OLEDs comparing to QDLEDs. QDLEDs are the energy efficient and have tuneable colour display. They deliver about 35% more luminous efficiency in comparison with OLEDs at the same colour point. Also, power efficiency of QDLEDs can be twice more than OLEDs at the same colour purity. The last but not least advantage of QDLEDs over OLEDs is low-cost manufacture. They can be printed in large area on thin flexible substrates, and they are also solution processable [68]. QDs have very narrow emission spectra, but their absorption spectra are broad. Factually, they absorb all wavelengths higher than their bandgap and convert them into a single colour. This narrow spectrum will improve colour saturation in QDLEDs compared to OLEDs. In addition, QDLEDs can be more power efficient due to good colour coordinate and luminous efficiency.

shell materials on the surface of the core to passivate the inorganic core [47, 48]. They could improve stability by growing a ZnS shell around the InP core. The properties of QDs strongly depend on their shell and their compounds. The cluster diameter is a significant factor in determination of bandgap in structure of the QDs. The emission band will be narrower, while the diameter of the cluster gets smaller. Also the thickness of shell is important in increasing the maximum amount of the PL efficiency. According to the result of Bera and his colleagues' research, the thicker shell layer, the lower amount of photoluminescence quantum yield (PLQY) [49]. The main reason of this phenomenon is that the misfit dislocations (sites of non-radiative recombination) are formed when the shell layer is thick. Higher quantum efficiency will be available by minimizing these sites in QDs. In addition, matching the energy levels of the shell and core should be considered. Confining the excitons within the QDs is possible by selecting proper material of shell with wider bandgap to create an appropriate

Colloidal CdSe/ZnS (core-shell QDs) have high quantum yield and high photo stability at room temperature. So they are good choice in lighting industry, and many researchers have investigated them [50]. To prevent the light scattering, the particle size should be smaller than one-tenth of the visible light's wavelength [51]. On the other hand, large particles tend to accumulate that tarnish the composite film. The fluorescence properties of the QDs can be affected by the ability of QDs to aggregation. Accumulation effect can drastically reduce the quantum efficiency. Recent researches have demonstrated that the repulsive force between the molecular chains of polymers can prevent the accumulation of nanoparticles, so the compound of the polymer quantum dot can improve this problem. Up to now colloidal nanoparticles of cadmium sulphides, cadmium silicon and lead sulphides are used in organic light emitting diodes. These QDLEDs emit green light potentially [52–56]. On the other hand, these kinds of QDs have toxicological properties so it is environmentally restricted and not to be able to be a commercial material in this field. New Cd-free quantum dots should be introduced to commercialize the QDLED technology. ZnO cores with a MgO shell, InP-based dots and CuInS2 are three new materials that need more studies to be performed by scientists [57, 58]. In 2015, Du et al. studied a stable photoluminescence QDLEDs based on hydrophilic CdTe QD. Inorganic nanocomposite CdTe quantum dots were prepared with two rotary steam and freeze-drying methods. Because of adhesion, flexibility and transparency, silica gel can be coated on the surface of UV light emitting diode and form photoluminescence QDLEDs. This new photoluminescence QDLED is sustainable and cost-effective. Also it is easy to operate and environmentally non-toxic [59]. Recently, Kim' group has studied a multiple structure of QDLED based on InP quantum dots. Current efficiency and brightness in

results in improving the stability of organic light emitting diodes are based on Cd QDs. InP quantum dots are replaced with Cd QDs in this study because of the environmental risks of cadmium [60]. In addition, the interface trap states are very effective on the performance of the device. In 2016, Koh et al. investigated these traps in the presence of TCNQ between charge transfer layer and quantum dots. With the introduction of TCNQ, the electroluminescent efficiency (EL) in QDLED has been improved by increasing the charge injection into the

, respectively. As mentioned, the best

potential barrier around the QD.

34 Quantum-dot Based Light-emitting Diodes

this structure are reported to be 1 cd/A and 530 cd/m2

QD layer [61].

QDs can be used in solar cells as well as LEDs due to their broad excitation band and narrow emission spectra. The tunable colour of QDLEDs will be provided by controlling the quantum dot size [69]. For example, cadmium selenide quantum dots can emit optical wavelength in the range of 470–640 nm by varying the size of 2–8 nanometres. The size of the QD can make unique physical properties in QDLEDs because the electrons in a nanocrystal exhibit quantum mechanical effects. The quantum confinement phenomenon occurred in nanocrystal will lead to discrete energy levels. The bandgap energy of a QD is inversely proportional to its size; therefore the emission from a QD will be colour tuneable. At present, the best OLEDs can have a quantum efficiency of up to 33%, which is much higher than that of QDLEDs [70]. Defects in the crystal create some non-radiative electron-hole recombinations that are the main reason of the low quantum efficiency. Although, the PL efficiencies of QDs are high, still the EQEs in these devices are low mainly due to poor charge carrier injection into the QD layers [71]. QDLEDs will be a good choice for the future of LEDs due to their colour stability, easily tunable colour and long lifetime. In recent years, due to all the advantages of the QDLEDs mentioned above, many research groups have worked on QDLEDs [72–75], and the efficiency of this type of light emitting diodes has improved in subsequent researches [76–80].

#### **9. Features of QDLEDs**

QDLEDs are characterized by their total width at half maximum (FWHM). Moreover, having a high quantum yield and high charge transfer coefficient are two important features of the emissive layer [81]. FWHM is examined in these devices, and entirely these structures have a small FWHM. This value of a single QD size should be very small. However, the extension of the FWHM is unavoidable because there will be different sizes of the QDs. Because the size of the nanoparticles determines the wavelength of radiation and particles with similar size will be commensurate with the radiation intensity, radiation spectrum shows QD size distribution directly [82]. As mentioned above, the increase in FWHM shows that there is more diversity of QDs that can be caused by the reformation of QD result in exposure to UV and heat. When photons of UV are absorbed by colloidal quantum dots, the heat caused by losses stoke remains near to QDs and resizes QDs. Changes of FWHM will be more in higher currents. The use of semiconductor nanoparticles with narrow size distribution and narrow-band radiation leads to emit white light with low CRI. And this is because the CRI depends on the size and distribution of colloidal nanoparticles. In this way, we have developed a procedure for preparation of CdS colloidal nanocrystals. The emission spectrum of synthesized sample was shown in **Figure 4**. As can be found from this figure, FWHM of emission spectrum is reduced

**Figure 4.** Emission spectrum of CdS colloidal sample.

to 10 nm, which is very small [83]. The prepared sample displays a strong and narrow green emission peak centred at 519 nm that has not been reported before, and it is longer than the onset of absorption of ∼512 nm for bulk CdS. Several weak emission peaks appeared at wavelengths 490, 506, 521 and 543 nm, too. These two important characteristics of the prepared sample are due to the strong band-edge emission of CdS nanocrystals. **Figure 4** shows the PL spectrum of CdS nanoparticles excited by wavelength of 190 nm.

#### **10. Different types of QDLEDs**

colour stability, easily tunable colour and long lifetime. In recent years, due to all the advantages of the QDLEDs mentioned above, many research groups have worked on QDLEDs [72–75], and the efficiency of this type of light emitting diodes has improved in subsequent

QDLEDs are characterized by their total width at half maximum (FWHM). Moreover, having a high quantum yield and high charge transfer coefficient are two important features of the emissive layer [81]. FWHM is examined in these devices, and entirely these structures have a small FWHM. This value of a single QD size should be very small. However, the extension of the FWHM is unavoidable because there will be different sizes of the QDs. Because the size of the nanoparticles determines the wavelength of radiation and particles with similar size will be commensurate with the radiation intensity, radiation spectrum shows QD size distribution directly [82]. As mentioned above, the increase in FWHM shows that there is more diversity of QDs that can be caused by the reformation of QD result in exposure to UV and heat. When photons of UV are absorbed by colloidal quantum dots, the heat caused by losses stoke remains near to QDs and resizes QDs. Changes of FWHM will be more in higher currents. The use of semiconductor nanoparticles with narrow size distribution and narrow-band radiation leads to emit white light with low CRI. And this is because the CRI depends on the size and distribution of colloidal nanoparticles. In this way, we have developed a procedure for preparation of CdS colloidal nanocrystals. The emission spectrum of synthesized sample was shown in **Figure 4**. As can be found from this figure, FWHM of emission spectrum is reduced

**518 nm**

**10 nm**

**400 450 500 550 600 650 700**

**FWHM**

**Wavelength (nm)**

**Intensity (a.u.)**

**Figure 4.** Emission spectrum of CdS colloidal sample.

researches [76–80].

**9. Features of QDLEDs**

36 Quantum-dot Based Light-emitting Diodes

QDs are applied in three types of OLEDs. PLEDs which their emissive layer is based on polymers, fluorescent small molecules and PHOLEDs which are the organo–metallic phosphorescent small molecules. Phase separation and contact printing are two major fabrication techniques for manufacturing of QDLED. **Table 1** shows common materials and QDs used in QDLEDs and OLEDs. Emission wavelength of QDs can be controlled by its size or composition.

QDs are very impressed by the environment (humidity and oxygen), because of the small size of the QDs. As can found from **Table 1**, ZnO is one of the semiconductors that can be used as electron transport material. Recently, we have developed a procedure for preparation of high mobility nanostructured thin indium-doped ZnO film [84, 85]. **Figure 5(a)** shows the scanning electron microscopy (SEM) of nanostructured thin ZnO film, and the X-ray diffraction (XRD) has been depicted in **Figure 5(b)**. It can be seen that there are three sharp diffraction peaks approximately at 30°, 33° and 35° that correspond to (1 0 0), (1 0 1) and (0 0 2).


**Table 1.** Energy levels of some common hole and electron transport materials used in OLEDs and typical QDs.

**Figure 5.** (a) A scanning electron microscopy (SEM) image and (b) the X-ray diffraction (XRD) pattern of QD.

#### **11. Conclusions and challenge ahead**

It is expected to see much more progresses in the lighting industry particularly QDLEDs in the near future. Optimizing the efficiency of devices can help to improve the performance of QDLEDs. Many researchers try to do their best in this respect such as Shen and his group [86]. They have suggested a new high efficiency QDLED. Anikeeva et al. try to increase efficiency by using materials with high PLs in the red, green and blue regions of the visible spectrum [87]. Despite all efforts they made, improving the efficiency of blue QDLEDs seems to be challenging because of that the blue QDs and used electron and hole-transporting materials have low spectral overlap with each other. They expect that using wide bandgap hole and electron transporting organic materials improves the efficiency of the blue QDLEDs due to better exciton energy transfer and direct charge injection into the blue QDs. Hybrid devices that incorporate emissive layers using different types of emissive materials can play a big role in the future of QDLEDs. They could be made by a blue emitting TADF layer, a green phosphorescent layer and a red QD layer.

In conclusion, to improve the performance of the QDLEDs:


There are a number of requirements that must be met in order for quantum dots to be integrated into the LED device and replace phosphor-based solutions. For one, the quantum dots should be stable in air, and moisture and the colour performance must be stable. Another problem faced to the development of the quantum dot materials is self-quenching. The quantum dots are designed to absorb light in one wavelength range and re-emit in another. The efforts of the researchers to create such a display are still in progress. QDLEDs promise to introduce very high contrast device, but with lower power than other technologies existed up to now. In addition, the lifetime of QDLEDs is another feature that needs more attention.

#### **Acknowledgements**

This chapter was supported by a National Research Foundation of Korea (NRF) grant funded by the South Korean government (MSIP 2015R1A2A1A15054541) and by the third stage of Brain Korea 21 Plus Project in 2016.

#### **Author details**

**11. Conclusions and challenge ahead**

Band gap (eV)

Grain size (nm) (AFM)

(a) (b)

Used precursor

38 Quantum-dot Based Light-emitting Diodes

phorescent layer and a red QD layer.

**1.** Improve the structure of QDLEDs. **2.** Improve manufacturing techniques.

**4.** Structural differences of quantum dots.

In conclusion, to improve the performance of the QDLEDs:

**3.** Choose a suitable material for the injection and transfer layers.

It is expected to see much more progresses in the lighting industry particularly QDLEDs in the near future. Optimizing the efficiency of devices can help to improve the performance of QDLEDs. Many researchers try to do their best in this respect such as Shen and his group [86]. They have suggested a new high efficiency QDLED. Anikeeva et al. try to increase efficiency by using materials with high PLs in the red, green and blue regions of the visible spectrum [87]. Despite all efforts they made, improving the efficiency of blue QDLEDs seems to be challenging because of that the blue QDs and used electron and hole-transporting materials have low spectral overlap with each other. They expect that using wide bandgap hole and electron transporting organic materials improves the efficiency of the blue QDLEDs due to better exciton energy transfer and direct charge injection into the blue QDs. Hybrid devices that incorporate emissive layers using different types of emissive materials can play a big role in the future of QDLEDs. They could be made by a blue emitting TADF layer, a green phos-

> RMS (nm)

IZO 3.27 38 28 4.1 2.2 x 10-3 4.4 x 1020 6.49

**Figure 5.** (a) A scanning electron microscopy (SEM) image and (b) the X-ray diffraction (XRD) pattern of QD.

Resistivity (Ω.cm)

**Intensity (a.u.)**

Grain size (nm) (FESEM) **10 20 30 40 50 60**

**2**θ**(degree)**

Carrier concentration n (cm-3)

Mobility μ(cm2/Vs)

**(002)**

**(101)**

**(100)**

Neda Heydari<sup>1</sup> \*, Seyed Mohammad Bagher Ghorashi2 , Wooje Han3 and Hyung-Ho Park3 \*

\*Address all correspondence to: nheydari.ph88@gmail.com and hhpark@yonsei.ac.kr

1 Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran

2 Department of Physics, Faculty of Physics, University of Kashan, Kashan, Iran

3 Department of Materials Science and Engineering, Yonsei University, Seodaemun-gu, Seoul, Korea

#### **References**


[20] Lewis J, Material challenge for flexible organic devices. Materials Today. 2006;**9**(4):38-45

[5] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Applied Physics Letters.

[6] Bing LY. On thermal structure optimization of a power LED lighting. Procedia Engi-

[7] Vijay SH A Healthy Future Forecast for the Outdoor LED Luminaire Lighting Market,

[8] Bardsley N. Solid-State Lighting Research and Development: Multi-Year Program Plan provided for Lighting Research and Development Building Technologies Program: Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy; April

[9] Meadows C. LEDs magazine special report: SIL and the LED show bring out the social

[10] Li Y, Rizzo A, Cingolani R, Gigli G. White-light-emitting diodes using semiconductor

[11] Tang CW, VanSlyke SA, Chen CH. Electroluminescence of doped organic thin films.

[12] Zhen-Gang L, Zhi-Jian C, Qi-Huang G. Reduction of concentration quenching in a nondoped DCM organic light-emitting diode. Chinese Physics Letters. 2005;**22**(6):1536 [13] Mizoguchi SK, Santos G, Andrade AM, Fonseca FJ, Pereira L, Iha NYM. Luminous efficiency enhancement of PVK based OLEDs with fac-[ClRe (CO) 3 (bpy)]. Synthetic

[14] Kwong CY, et al. Efficiency and stability of different tris(8-hydroxyquinoline) aluminum (Alq3) derivatives in OLED applications. Material Science and Engineering B.

[15] Rosselli FP, et al. Experimental and theoretical investigation of tris-(8-hydroxy-quinolinate) aluminum (Alq3) photo degradation. Organic Electronics. 2009;**10**(8):1417-1423

[16] Gebeyehu D. Highly efficient p-i-n type organic light-emitting diodes using doping of the transport and emission layers. Ethiopian Journal of Science and Technology.

[17] Di D, Yang L, Richter JM, Meraldi L, Altamimi RM, Alyamani AY, … Friend RH. Efficient triplet exciton fusion in molecularly doped polymer light-emitting diodes. Advanced

[18] Santos ER, Moraes JIBD, Takahashi CM, Sonnenberg V, Burini EC, Yoshida S., … Hui WS. Low cost UV-Ozone reactor mounted for treatment of electrode anodes used in

[19] Ho S, Chen Y, Liu S, Peng C, Zhao D, So F. Interface effect on efficiency loss in organic light emitting diodes with solution processed emitting layers. Advanced Materials

P-OLEDs devices. Polímeros, (AHEAD). 2016;**26**(3):236-241

1987;**51**(12):913-915

40 Quantum-dot Based Light-emitting Diodes

2012

neering. 2012;**29**:2765-2769

Strategies unlimited; September 2016

(media) butterflies. LEDs Magazine. March 2015

Journal of Applied Physics. 1989;**65**(9):3610-3616

Metals. 2011;**161**(17):1972-1975

Materials. 2017;**29**(13):1605987

Interfaces. 2016;**3**(19):1600320

2005;**116**(1):75-81

2014;**7**(1):37-48

nanocrystals. Microchimica Acta. 2007;**159**(3-4): 207-215


[53] Hua F, Swihart MT, Ruckenstein E. Efficient surface grafting of luminescent silicon quantum dots by photoinitiated hydrosilylation. Langmuir. 2005;**21**(13):6054-6062

[37] Evans RC, Douglas P, Winscom, CJ. Coordination complexes exhibiting room-temperature phosphorescence: Evaluation of their suitability as triplet emitters in organic light emit-

[38] Geffroy B, Le Roy P, Prat C. Organic light-emitting diode (OLED) technology: Materials,

[39] Kietzke T. Recent advances in organic solar cells. Advances in OptoElectronics.

[40] Kim KH, Lee S, Moon CK, Kim SY, Park YS, Lee JH, … Kim JJ. Phosphorescent dyebased supramolecules for high-efficiency organic light-emitting diodes. Nature Commu-

[41] McElvain J, Antoniadis H, Hueschen MR, Miller JN, Roitman DM, Sheats JR, Moon RL. Formation and growth of black spots in organic light-emitting diodes. Journal of

[42] Kickelbick G. Concepts for the incorporation of inorganic building blocks into organic

[43] Colvin VL, Schlamp MC, Paul Alivisatos A. Light-emitting diodes made from cadmium

[44] Ding T, Yang X, Ke L, Liu Y, Tan WY, Wang N, … Sun XW. Improved quantum dot lightemitting diodes with a cathode interfacial layer. Organic Electronics. 2016;**32**:89-93 [45] Castan A, Kim HM, Jang J. All-solution-processed inverted quantum-dot light-emitting

[46] Wang L, Chen T, Lin Q, Shen H, Wang A, Wang H, … Li LS. High-performance azure blue quantum dot light-emitting diodes via doping PVK in emitting layer. Organic

[47] Xie R, Battaglia D, Peng X. Colloidal InP nanocrystals as efficient emitters covering blue to near-infrared. Journal of the American Chemical Society. 2007;**129**(50):15432-15433

[48] Lim J, Bae WK, Lee D, Nam MK, Jung J, Lee C, … Lee S. InP@ ZnSeS, core@ composition gradient shell quantum dots with enhanced stability. Chemistry of Materials.

[49] Bera D, Qian L, Tseng TK, Holloway PH. Quantum dots and their multimodal applica-

[50] Nguyen HT, Pham TN, Koh KH, Lee S. Fabrication and characterization of CdSe/ZnS

[51] Althues H, Henle J, Kaskel S. Functional inorganic nanofillers for transparent polymers.

[52] Ghosh B, Sakka Y, Shirahata N. Efficient green-luminescent germanium nanocrystals.

quantum-dot LEDs. Physic Status Solidi (a). 2012;**209**(6):1163-1167

polymers on a nanoscale. Progress in Polymer Science. 2003;**28**(1):83-114

selenide nanocrystals and a semiconducting polymer. 1994:354-357

diodes. ACS Applied Materials & Interfaces. 2014;**6**(4):2508-2515

devices and display technologies. Polymer International. 2006;**55**(6):572-582

ting diodes. Coordination Chemistry Reviews. 2006;**250**(15):2093-2126

2008;**2007**:40285

42 Quantum-dot Based Light-emitting Diodes

nications. 2014;**5**:4769

Electronics. 2016;**37**:280-286

2011;**23**(20):4459-4463

tions: A review. Materials. 2010;**3**(4):2260-2345

Chemical Society Reviews. 2007;**36**(9):1454-1465

Journal of Materials Chemistry A. 2013;**1**(11):3747-3751

Applied Physics. 1996;**80**(10):6002-6007


[84] Alamdari S, Jafar Tafreshi M, Sasani Ghamsari M. The effects of Indium precursors on the structural, optical and electrical properties of nanostructured thin ZnO films. Material Letters. 2017;**197**(15):94-97

[68] Anikeeva PO, Madigan CF, Coe-Sullivan SA, Steckel JS, Bawendi MG, Bulović V. Photoluminescence of CdSe/ZnS core/shell quantum dots enhanced by energy transfer

[69] Schreuder MA, Xiao K, Ivanov IN, Weiss SM, Rosenthal SJ. White light-emitting diodes based on ultrasmall CdSe nanocrystal electroluminescence. Nano Letters.

[70] Chang HW, Lee J, Hofmann S, Hyun Kim Y, Müller-Meskamp L, Lüssem B, … Gather MC. Nano-particle based scattering layers for optical efficiency enhancement of organic lightemitting diodes and organic solar cells. Journal of Applied Physics. 2013;**113**(20):204502

[72] Anikeeva PO, Halpert JE, Bawendi MG, Bulović V. Electroluminescence from a mixed red– green– blue colloidal quantum dot monolayer. Nano Letters. 2007;**7**(8):2196-2200

[73] Cho KS, Lee EK, Joo WJ, Jang E, Kim TH, Lee SJ, … Kim JM. High-performance crosslinked colloidal quantum-dot light-emitting diodes. Nature Photonics. 2009;**3**(6):341-345

[74] Coe S, Woo WK, Bawendi M, Bulović V. Electroluminescence from single monolayers of

[75] Sun Q, Wang YA, Li LS, Wang D, Zhu T, Xu J, … Li Y. Bright, multicoloured light-emit-

[76] Caruge JM, Halpert JE, Bulović V, Bawendi MG. NiO as an inorganic hole-transporting layer in quantum-dot light-emitting devices. Nano Letters. 2006;**6**(12):2991-2994

[77] Zhao J, Bardecker JA, Munro AM, Liu MS, Niu Y, Ding IK, … Ginger DS. Efficient CdSe/ CdS quantum dot light-emitting diodes using a thermally polymerized hole transport

[78] Kamat PV. Boosting the efficiency of quantum dot sensitized solar cells through modulation of interfacial charge transfer. Accounts of Chemical Research. 2012;**45**(11):1906-1915

[79] Matras-Postolek K, Bogdal D. Polymer nanocomposites for electro-optics: Perspectives on processing technologies, material characterization, and future application. In Polymer

[80] Fojtik A, Henglein A. Surface chemistry of luminescent colloidal silicon nanoparticles.

[81] Wood V, Panzer MJ, Halpert JE, Caruge JM, Bawendi MG, Bulovic V. Selection of metal oxide charge transport layers for colloidal quantum dot LEDs. ACS Nano.

[82] Hsu SC, Chen YH, Tu ZY, Han HV, Lin SL, Chen TM, … Lin CC. Highly stable and efficient hybrid quantum dot light-emitting diodes. IEEE Photonics Journal. 2015;**7**(5):1-10

[83] Sasani Ghamsari M, Sasani Ghamsari AH. CdS colloidal nanocrystals with narrow green emission. Journal of Nanophotonics. 2016;**10**(2):026007/doi: 10.1117/1.JNP.10.026007

Characterization. Berlin Heidelberg: Springer; 2010. 221-282

The Journal of Physical Chemistry B. 2006;**110**(5):1994-1998

nanocrystals in molecular organic devices. Nature. 2002;**420**(6917):800-803

ting diodes based on quantum dots. Nature Photonics. 2007;**1**(12:717-722

layer. Nano Letters. 2006;**6**(3):463-467

2009;**3**(11):3581-3586

from a phosphorescent donor. Chemical Physics Letters. 2006;**424**(1):120-125

[71] Zyga L. Quantum dot LEDs get brighter, more efficient. Phys Org. April 2012;1-3

2010;**10**(2):573-576

44 Quantum-dot Based Light-emitting Diodes


**Provisional chapter**
