Materials for Photovoltaics: Overview, Generations, Recent Advancements and Future Prospects

*Muhammad Aamir Iqbal, Maria Malik, Wajeehah Shahid, Syed Zaheer Ud Din, Nadia Anwar, Mujtaba Ikram and Faryal Idrees*

## **Abstract**

As a consequence of rising concern about the impact of fossil fuel-based energy on global warming and climate change, photovoltaic cell technology has advanced significantly in recent years as a sustainable source of energy. To date, photovoltaic cells have been split into four generations, with the first two generations accounting for the majority of the current market. First generation of thin-film technologies is based on monocrystalline or polycrystalline silicon and gallium arsenide cells and includes well-known medium- or low-cost technologies with moderate yields, whereas, second generation includes devices with lower efficiency and manufacturing costs. Third generation is based on novel materials and has a wide range of design options, as well as expensive but highly efficient cells. However, fourth generation, also known as "inorganics-in-organics," combines the low cost and flexibility of polymer thin films with the durability of innovative inorganic nanostructures (metal nanoparticles or metal oxides) in organic-based nanomaterials (carbon nanotubes, graphene, and their derivatives). The aim of this chapter was to highlight the current state of photovoltaic cell technology in terms of manufacturing materials and efficiency by providing a comprehensive overview of the four generations as well as the relevance of graphene and its derivatives in solar cell applications.

**Keywords:** efficiency, generations, graphene, photovoltaics, polymers

### **1. Introduction**

The rapid growth of the world's community and industrial area is leading to the great need for energy while renewable energy sources consumption such as geothermal, biomass, wind, and photovoltaic sources are challenging the planet's survival having its adverse effects. From the above-listed energy sources, photovoltaics is the technology used for the conversion of sunlight into electrical power by means of semiconductor materials. By considering their history, in 1883, Fritts worked on photovoltaics applications for the first time [1]. In 1954, the p-n junction diode potential was discovered at Bells laboratory with the efficiency of 6% using silicon material [2], and the same work has also been reported to make heterojunction solar cells based on Cu2S/CdS [3]. These diodes work on the phenomenon of generated voltages when sunlight falls on them. In the 1960s, the photovoltaic system for the first time was employed in commercial applications for space solar cells to deliver the power for satellite applications [4], and silicon semiconductor materials have been reported to be widely used in photovoltaic technology [5]. Moreover, in spite of the extensive use of silicon semiconductor-based technology, it has a high cost, which is the main drawback of not using this technology for home-based device applications. To overcome these challenging problems, researchers have put their efforts to replace silicon-based solar cells technology with the one having superior results [6]. Several researches show numerous classifications of materials, such as organic, inorganic, and hybrid materials, to potentially replace silicon materials from existing solar cells technology [7].

### **2. Overview of solar cell technology**

Solar cells can be categorized according to their material composition whereas silicon-based semiconductors are dominant in the industrial share of photovoltaics, and despite considering the advantages of silicon material in photovoltaics, they lack some factors, such as very low absorbing power as well as needing almost 200–300 semiconducting material films to absorb the incident sunlight.

To overcome the lacking of silicon-based semiconductors, alternative technology was developed with solar cells having a micron-sized thickness, low cost but also low efficiencies [8], while thin sheet solar cells technology was also developed on flexible sheets. The advanced technologies are cadmium telluride, copper indium gallium selenide, and thin film-based technologies with reported efficiency of 20% [9, 10]. Some of the cadmium telluride-based solar cells companies have gained tremendous success at the commercial level with an efficiency of about 11% [11]. However, silicon-based single-junction and multi-junction solar cells have an extensive history of their commercial use including different active layers, such as microcrystalline silicon or hydrogenated amorphous silicon germanium with varying efficiencies from 8 to 12%, but their high cost and low efficiencies have limited the use of silicon material in solar cells and introduced the improved model.

Considering the mechanism of solar cells as shown in **Figure 1**, when sunlight falls on solar cell's surface, it reflects off from the surface. If the photon energy is lesser than materials bandgap energy then incident light is directly passed out from the semiconductor without being absorbed but if the photon energy is more than materials bandgap energy then the semiconductor material absorbs that energy and

**Figure 1.** *Schematic diagram of a solar cell [12].*

*Materials for Photovoltaics: Overview, Generations, Recent Advancements and Future Prospects DOI: http://dx.doi.org/10.5772/intechopen.101449*

#### **Figure 2.**

*Photon energy compared to the bandgap energy of a semiconductor [13].*

excites electrons from valance to conduction band and generates electron-hole pair as shown in **Figure 2**. The advanced solar cells contain layers, such as the contact layer and window layer, which reduce the reflection losses by increasing the absorption efficiency of solar cells, resulting in electron-hole pairs that are then separated and driven to their collection electrodes. In p-n junction, the charge separation occurs by diffusion method and is collected by electrodes that generate current. However, the recombination process hurdles the carriers to reaching out to the electrodes, therefore not all the charge carriers got captured [13].

#### **2.1 Approaches to light trapping**

The solar cell surface is structured to trap the incident light within the semiconductor that enhances absorption over multiple passes while light trapping is the foremost mechanism of advanced solar cells. Silicon-based solar cells have a pyramidal surface structure that allows light to reflect from the cell surface to the silicon layer. The considerations for thick and thin-film light trapping designs are different—for thick films, light trapping can be defined using ray optics, although thin films can be treated with wave optics [13].

#### *2.1.1 Ergodic limit*

A thick film light trapping mechanism can be demonstrated with ergodic behavior if the cell texture is fair enough to randomize the light direction within a solar cell. The total internal reflection between the semiconductor material and surrounding medium generates the refractive index contrast which enhances photon path length within the semiconductor material, and hence absorption capacity. However, some portion of light may leave the semiconductor through an exit cone of some angle [13].

#### *2.1.2 Thin films*

In the case of smooth thin films, various mechanisms are used to exhibit absorption, such as interface reflection and refraction, material thickness, and light coherence. If the incident light coherency is greater than the film thickness, then it may act as a Fabry–Perot cavity, which serves as a resonant absorber [14]. The Fresnel coefficients are used to calculate reflection and refraction at normal incidence angle with tracked phasors over multi-passes.

#### **3. Solar cell generations**

Photovoltaic cells are categorized into four main classes according to their modifications, and these classifications are called generations [15]. **Figure 3** depicts materials that comprise each generation while **Figure 4** represents a historical overview of efficiencies of solar cells.

## **3.1 First-generation photovoltaic solar cells**

The first-generation of photovoltaic solar cells is based on crystalline film technology, such as silicon and GaAs semiconductor materials. Silicon (Si) is the extensively used material for commercial purposes, and almost 90% of the photovoltaic solar cell industry is based on silicon-based materials [17], while GaAs is the oldest material that has been used for solar cells manufacturing owing to its higher efficiency. There are some advantages to use silicon material for photovoltaic solar cells manufacturing, such as:


#### **Figure 3.**

*Four generations of photovoltaic cells along with the materials that comprise each generation [15].*

*Materials for Photovoltaics: Overview, Generations, Recent Advancements and Future Prospects DOI: http://dx.doi.org/10.5772/intechopen.101449*

**Figure 4.** *A historical overview of solar cell efficiency [16].*

• Silicon-based photovoltaic solar cells are easily compatible with the siliconbased microelectronic sector, resulting in the creation of the most interesting technologies [17].

The schematic representation of a silicon-based solar cell is shown in **Figure 5**. The GaAs has specialized use in multi-junction cells comprising several semiconductor materials and photovoltaic solar cells [19] having the following special characteristics:


Hence, the first-generation photovoltaic solar cells can be further classified into monocrystalline, polycrystalline, and GaAs solar cells.

#### **3.2 Second-generation photovoltaic solar cells**

The second-generation photovoltaic solar cells have the main focus of cost minimization that was the main issue of first-generation photovoltaic solar cells, and this can be achieved using thin-film technologies by reducing the material quantity as well as

**Figure 5.**

*Schematic representation of a silicon-based solar cell [12].*

improving its quality. This modification is based on materials that showed good results in first-generation development and were extended to a-Si, c-Si, CdTe, and copper indium gallium selenide (CIGS) [15]. The advantageous factors of second-generation photovoltaics are listed below [20, 21];


However, some of the drawbacks are still present and listed below:


### **3.3 Third-generation photovoltaic solar cells**

Third-generation photovoltaics emerged from the gap left by second-generation technologies which required improved device efficiency via thin-layer deposition and intend to introduce novel materials with new techniques [22]. This sophisticated technology may be costly but the cost per watt peak would be decreased. As this technique is nontoxic and uses readily accessible materials, it is best suited for large-scale photovoltaic solar cell applications. These materials might be organic or

*Materials for Photovoltaics: Overview, Generations, Recent Advancements and Future Prospects DOI: http://dx.doi.org/10.5772/intechopen.101449*

nanostructured with high efficiency of more than 60% achieved by the use of different charge carrier collecting methods [23]. More emphasis has been placed on charge carrier mechanism, charge collecting, and energy capture improvements in thirdgeneration photovoltaic solar cells. The important technologies used in third-generation photovoltaic solar cells are—dye-sensitized solar cells (DSSCs), organic and polymeric solar cells, perovskite cells, quantum dot cells, and multi-junction cells. The considerable advantages of third-generation photovoltaic solar cells may include solution-processable technologies, efficient technologies for commercial production, mechanical toughness, and high efficiencies at higher temperatures. However, the important challenge of this generation is to reduce the cost of solar electricity.

#### **3.4 Fourth-generation photovoltaic solar cells**

Fourth-generation photovoltaic solar cells combine the benefits of previous generations, such as lower cost, flexibility, and high stability of third-generation nanomaterials, metal oxides, graphene, and carbon nanotubes. These characteristics will give improved solar cell devices with the needed low-cost manufacturing as well as durability and the usage of nanomaterials in solar devices will aid to increase charge dissociation and transportation inside the cells. Because of its amazing properties and allotropic forms appearing in all four dimensions, graphene is a potential solar cell material with great scientific hopes in fourth-generation technological accomplishments [12].

## **4. Graphene and graphene-doped solar cells**

Given the desire for renewable energy sources and their complete contributions to technological advancement, the demand for solar cells has increased with the added benefit of being low cost and simple to operate. Solar cells are not very efficient in general; it is the substance that makes them so. Recent developments in graphene-based solar cells boosted their efficiency about 20% by lowering the reflection of incident light. To improve the efficiency of solar cells, graphene may be doped in a variety of ways. Here is a thorough review of graphene-based solar cells and their doping variants that are being considered and researched across the world.

#### **4.1 Principles of graphene-based solar cells**

Graphene-based solar cells operate in the same way as conventional inorganic solar cells do today but the sole difference is that graphene or graphene-based materials replace inorganic components. Using graphene as a promising material in solar cells improves the adaptability along with tuneability of solar cells, while the number of graphene layers in a device and the doping effects are two essential characteristics that determine the efficiency of graphene-based devices. **Figure 6** depicts the schematic representation for inorganic and organic solar cells with graphene.

#### **4.2 Graphene-silicon solar cells**

Solar cells can also be implemented using carbon allotropes that are used for a wide range of applications and made them cost-effective. Some of the carbon allotropes show remarkable results while some are not much efficient because of the inability to tune their electronic properties and layer thickness. Graphene-based solar cells feature such efficient qualities, such as the capacity to modify layer thickness as per the requirement as well as the ability to tune properties based on the material combination. Graphene is used in solar cells with graphene-silicon combination with

**Figure 6.** *Schematic representation for (a) inorganic solar cells, and (b) organic solar cells with graphene [24].*

both p-type and n-type heterojunctions and as per reported literature, pure silicon solar cell's efficiency is better than any combination. However, graphene tuneability is much better in the case of hybrid solar cells and in the phase of advancement to improve its efficiency compared with pure silicon. According to available research, n-type heterojunctions create about 0.55 internal voltages for electron-hole pair separation while Schottky junctions generate 1.5% efficiency with a filler factor of 56%. As a result, depending on the dopant, efficiency can be increased and gold particles doped in graphene sheets showed a 40% increase in efficiency [25].

### **4.3 Graphene-polymer solar cells**

Graphene-polymer-based solar cells are gaining popularity in the market and these polymer materials may be organic and beneficial due to their simple production method, low cost, and easy tuneability. Graphene has been used in possible applications of coating, layering, and temperature annealing in electrodes as a hybrid material with a combination of organic-inorganic materials. The Fermi level of graphene and semiconductor sheets is closer for charge injection applications and they have strong energy characteristics. Graphene can be combined with polymers to produce a hybrid material with a reported bandgap of 3.6 V that prevents cathode to electrode electron transport. Graphene layers with a thickness of 2 nm are claimed to provide the best outcomes in terms of improved electrical resistance and electron transmission [26].

#### **4.4 Graphene-quantum dot solar cells**

Carbon allotropes such as graphene and carbon nanotubes when coupled with quantum dots can produce efficient solar cells and the graphene-quantum dot combination has potential applicable uses. They are synthesized using an electrophoretic and chemical bath deposition method which produces a layered pattern of graphene and quantum dots that have 18 layers of both materials. This combination is said to have 16% efficiency while graphene-carbon nanotubes have 7% efficiency. The combination of graphene and quantum dots provides a superior framework as well as rapid electron transport between graphene and quantum dots while limiting charge recombination [27].

#### **4.5 Graphene-tandem solar cells**

Tandem solar cells consist of more than two subcells combined together called multi-junction solar cells. According to studies, a single solar cell is almost 40% efficient but tandem solar cells based on graphene oxides are up to 86% efficient owing to their greater subcell combination, and as a result tandem configuration can improve the energy conversion of solar cells [28].

*Materials for Photovoltaics: Overview, Generations, Recent Advancements and Future Prospects DOI: http://dx.doi.org/10.5772/intechopen.101449*

#### **4.6 Graphene-perovskite solar cells**

Perovskite solar cells have an intriguing bandgap that results in remarkable absorption properties which eventually results in increased energy conversion efficiency of a solar cell and because of their standard structure, this type of solar cell can easily be tuned by changing the required material. These cells also feature a graphene layer which accounts for only 0.6 weight-percent of a cell and any other proportion reduces device efficiency. This cell arrangement is a little more complicated but it results in higher energy conversion efficiency when compared to non-graphene oxide layer solar cells. When graphene oxide materials are employed in this sort of solar cell and coupled with a light absorber they serve as a hole-conductor. Considering graphene oxide's other applications, it increases interface wettability on the surface of a perovskite solar cell and reduces the hole transporting layer contact angle. In addition, the C-C bonds of graphene sheets absorb the hole transport layer molecules which improve interfacial contacts of solar cells and results in better device performance [29].

#### **4.7 Graphene-organic solar cells**

Organic solar cells like inorganic ones play an important role in industrial applications and because of their endurance and low cost, organic devices are becoming more popular in technology. Major inorganic components in solar cells are being replaced by hybrid organic-inorganic components that exhibit superior physical and chemical characteristics as well as easy processability, easy availability, environmental friendliness, and most significantly, cost-effective synthesis and manufacturing. The major disadvantage of recent solar cells is their environmental impact; however organic components address this issue by offering stability against chemical deterioration, temperature, humidity, and other factors [30].

#### **4.8 Graphene bulk-heterojunction solar cells**

Graphene is a potential material used in various applications including electrodes, donor-acceptor layers as well as active layers because of its promising properties, such as conductivity, flexibility, and transparency, that make it an efficient material. Multi-junction solar cells also depend on graphene specificity such as its thickness, annealing temperature, doping concentration, and so on. Graphene heterojunctions employ graphene-based solar cells as well and they all rely on how graphene and its derivatives are mixed in these cells. Transparent electrodes, gallium arsenide (GaAs) solar cells, and photovoltaic layers are the primary components of graphene heterojunction solar cells [31].

### **5. Future prospects**

The photovoltaic solar cell is a fascinating research area in both academic and industrial fields with advances and breakthroughs occurring on a regular basis. Solar cells [32] are often manufactured utilizing silicon and inorganic materials which have significant limits in current uses. The development of improved organic graphenebased and graphene derivative-based materials, as well as narrow bandgap polymers, is revolutionizing the market by demonstrating their potential and perfect characteristics necessary for solar cell device structure applications. There has been tremendous development in the improvement of graphene-based solar cells and more work has to be done in the future to achieve further growth in this industry. Because of their capacity to enhance different characteristics, graphene-based solar cells are

customizable and adaptable to future limits in solar research. However, by enhancing existing solar cells and illuminating the characteristics of already employed nongraphene-based solar cells or developing a new variety of graphene photovoltaics, it is clear that graphene will play an important role in this intriguing analog [33].

## **6. Concluding remarks**

An overview of photovoltaic solar cells is provided in this chapter, along with illustrations of four generations as well as prospective applications of graphene and graphene-based materials. Briefly, the first-generation thin-film technology was based on monocrystalline or polycrystalline silicon cells and gallium arsenide while the second generation includes devices with lower efficiency and lower manufacturing costs. The third generation has novel materials and a wide range of design options and expensive but highly efficient cells; however, the fourth generation, also known as "inorganics-in-organics," combines the low cost/flexibility of polymer thin films with the durability of innovative inorganic nanostructures in organic-based nanomaterials. Recent developments in graphene-based solar cells boosted their efficiency about 20% by lowering the reflection of incident light.

## **Author details**

Muhammad Aamir Iqbal1 \*, Maria Malik<sup>2</sup> , Wajeehah Shahid3 , Syed Zaheer Ud Din4 , Nadia Anwar3,5, Mujtaba Ikram6 and Faryal Idrees7

1 School of Materials Science and Engineering, Zhejiang University, Hangzhou, China

2 Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan

3 Department of Physics, The University of Lahore, Lahore, Pakistan

4 International School of Optoelectronic Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, China

5 School of Materials Science and Engineering, Tsinghua University, Beijing, China

6 Institute of Chemical Engineering and Technology, University of the Punjab, Lahore, Pakistan

7 Department of Physics, University of the Punjab, Lahore, Pakistan

\*Address all correspondence to: maamir@zju.edu.cn

© 2022 The Author(s). Licensee IntechOpen. 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.

*Materials for Photovoltaics: Overview, Generations, Recent Advancements and Future Prospects DOI: http://dx.doi.org/10.5772/intechopen.101449*

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## **Chapter 3**

## Binary Semiconductors Thin Films Characterization for Solar Cells Applications

*Kenza Kamli and Zakaria Hadef*

## **Abstract**

The increasing development of technologies based on the thin films, imposed a high quality of these films. The crucial importance for all applications of thin films is related to the stability of their physical and morphological properties. Therefore, to optimize the performances of the thin films it is recommended to study carefully all their parameters in order to enhance the elaborated films. With this intention, various characterizations methods were developed and carried out to study the different qualities of thin films. In this chapter, we take an interest to the study of the characteristics of some binary semiconductors thin films elaborated by ultrasonic spray pyrolysis, and which are destined for solar cells applications. Several used characterizations techniques to the determination of the thin films properties will be given; namely: X-rays diffraction (XRD), Scanning Electron Microscopy (SEM), EDS (Energy Dispersive Spectroscopy), Hall effect and spectrophotometry will be discussed in detail.

**Keywords:** Thin films, ultrasonic spray, X-rays diffraction, SEM, EDS, spectrophotometry, Hall effect, Binary semiconductors

### **1. Introduction**

The materials have different properties, which are described by their structure, morphology and chemical composition. The determination of these properties and the study of the different characteristics of the given materials in order to their development is very necessary to achieve the requirements of the new technologies. Therefore, materials characterization is a fundamental process in the field of materials science.

While many characterization techniques have been practiced for centuries, such as basic optical microscopy, but new techniques and methodologies are constantly emerging. Detection ranges of the wide variety of instrumental analytical techniques can be summarized versus the probe sizes/resolution as shown in **Figure 1**.

This chapter is devoted to describe certain important characterization techniques used in general to study and develop the different characteristics of certain binary semiconductors thin films, which are used in solar cells, to get layers with high performances.

**Figure 1.** *Schematic of detection ranges versus probe sizes/resolution.*

## **2. X-ray diffraction technique**

The use of X-ray methods in the field of materials analysis is now entering its eighth decade. X-ray diffraction techniques are a very useful characterization tool to study, non-destructively, the crystallographic structure, chemical composition and physical properties of materials and thin films. It can also be used to measure various structural properties of these crystalline phases such as strain, grain size, phase composition, and defect structure.

#### **2.1 X-rays creation**

The processes of X-rays creation are based on the energy loss of the energetic electrons. **Figure 2a** shows the process of elastic and inelastic scattering where the deflection, or more specifically the acceleration during the deflection would always produce radiation.

Two routes including energy transfer between the incident electron and the electrons of the atom, exists. Both of these processes involve a primary ionization where a core electron is ejected from the atom. The ejected electron falls with an excess energy, which can be disposed as an X-ray or Auger electron [1]. The characteristic X-ray carries the full energy difference of the two electron states as shown in **Figure 2b**. Furthermore, an X-ray for a diffraction experiment is characterized by its wavelength, λ, but the energy, E, is typically more useful.

For XRD analysis, it is always required to use a coherent beam of monochromatic X-rays with a known wavelength [2]. That is why a right selection of metal anode and energy (i.e., a known wavelength) of accelerated electrons is very necessary.

#### **2.2 Principle of measurements of X-rays diffraction**

A crystal lattice consists of a regular arrangement of atoms, with layers of high atomic density existing throughout the crystal structure. Knowledge of how atoms are arranged into crystal structures and microstructures is the foundation on which we build our understanding of the synthesis, structure and properties of materials [1]. *Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

**Figure 2.** *Interaction processes between a high-energy electron and an atom.*

Planes of high atomic density means planes of high electron density. W.L. Bragg derived a simple equation treating diffraction as reflection from planes in the lattice.

$$\mathbf{n}\lambda = \mathbf{2d}\_{\text{hkl}} \sin \theta \tag{1}$$

where n is an integer, dhkl is the distance inter reticular separating the plans defined by the indices from Miller (h, k, l), θ the angle of incidence and thus of reflexion compared to these plans and finally λ is the wavelength of photons X.

In fact, dhkl does not represent the inter-reticular distance only, but it is defined too:


Bragg's Law express and interpret the interaction of X-rays with sample, which creates secondary "diffracted" beams generated in the form of cones of X-rays. These beams are related to interplanar spacings in the crystalline powder according to the Bragg's mathematical relation.

Consequently, a family of planes produces a diffraction peak only at a specific angle 2θ. Diffraction experiments are generally made at a fixed wavelength (as we have mentioned above), thus a measure of the diffraction angles will allow the associated d-spacings to be calculated.

**Figure 3** show the Bragg X-ray diffraction condition.

X-ray diffraction peaks are produced by constructive interference of monochromatic beam scattered from each set of lattice planes at specific angles. X-ray diffraction from crystalline solids occurs as a result of the interaction of X-rays with the electron charge distribution in the crystal lattice. The ordered nature of the electron charge distribution, which is distributed around atomic nuclei and is regularly arranged with translational periodicity, means that superposition of the scattered X-ray amplitudes will give rise to regions of constructive and destructive interference producing a diffraction pattern.

Bragg's equation tells us about the position of the diffraction peaks (in terms of θ), but tells us nothing about the intensity. The intensities of the diffraction peaks are determined by the arrangement of atoms in the entire crystal. These intensities can

**Figure 3.** *Bragg X-ray diffraction condition [3].*

be explained by the variation of the square of the structure factor according to the following equation [4]:

$$\left| \mathbf{I}\_{\text{hkl}} \mathbf{u} \| \mathbf{F}\_{\text{hkl}} \right|^2 / \mathbf{F}\_{\text{hkl}} = \sum\_{j=1}^{M} \mathbf{N}\_j \mathbf{f}\_j \exp\left[2\pi i \left(\mathbf{h} \mathbf{x}\_j + \mathbf{k} \mathbf{y}\_j + \mathbf{l} \mathbf{z}\_j\right)\right] \tag{2}$$

This factor, Fhkl, represent the sums of resulting scattering from all of the atoms in the unit cell to form a diffraction peak from the (hkl) planes of atoms.

Nj is the fraction of every equivalent position that is occupied by atom j.

The three factors: xj, yj and zj, which are the fractional coordinates, represents the atoms position in the atomic planes, and gives the first information about the amplitude of scattered light.

The other information of the amplitude of scattered light is given by the scattering factor fj, which quantifies the efficiency of X-ray scattering at any angle by the group of electrons in each atom.

#### **2.3 Sample preparation and diffractometer**

Sample preparation is usually the most critical factor influencing the quality of the analytical data. Preferably, the sample should exhibit a plane or flattened surface.

All conventional X-ray spectrometers comprise three basic parts: the primary source unit, the spectrometer itself and the measuring electronics. The acquisitions are generally carried out using a goniometer θ-2θ and by using a linear detector.

The diffraction pattern is collected by varying the incidence angle of the incoming X-ray beam by θ and the scattering angle by 2θ while measuring the scattered intensity I(2θ) as a function of the latter. Wide number of powder samples have been measured by using these tools, but it is also applied to the investigation of thin films.

Nowadays, CCD detector or scintillation are used in the novel generation of X-ray Diffractometer detector to record the angles and intensities of the diffracted beams with high resolution.

#### **2.4 X-ray diffraction applications**

X-ray Diffraction is considered as one of the most useful characterization techniques, because it is capable of providing general purpose qualitative and quantitative information on the presence of phases in an unknown mixture. This technique uses X-ray (or neutron) diffraction on powder or microcrystalline samples, where ideally every possible crystalline orientation is represented equally.

*Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

#### **Figure 4.**

*Information content of an idealized diffraction pattern [5].*

**Figure 4** shows the information we can get from an idealized diffraction pattern. A diffraction pattern contains a good deal of information of which three parameters are of special interest:


The three pieces of information can be used in particular to identify and quantify the contents of the sample, as well as to calculate the material's crystallite size and distribution, crystallinity, stress and strain.

The most traditional use of XRD can be summarized as shown below.

The diffraction pattern is like a fingerprint of the crystal structure. It is a powerful and rapid technique for identification of an unknown material.

### *2.4.1 Phases identification*

Among these applications, the identification of unknown crystals in a sample seems to be the most important. The idea is to match the positions and the intensities of the peaks in the observed diffraction pattern to a known pattern of peaks from a standard sample or from a calculation.

A single crystal specimen in a diffractometer would produce only one family of peaks in the diffraction pattern as given in **Figure 5**. According to this figure, we can resume the most common cases of diffraction peaks identification used in reality.

From **Figure 5**, we can distinguish three cases:


Nevertheless, a polycrystalline sample should contain thousands of crystallites as given in **Figure 5d**. Therefore, all possible diffraction peaks should be observed. For every set of planes, there will be a small percentage of crystallites that are properly oriented to diffract (the plane perpendicular bisects the incident and diffracted beams).

**Figure 5.** *Information content of an idealized diffraction pattern [5].*

#### *Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

In addition, the position and the form of the peaks could give us some important information about the lattice stresses.

As shown in **Figure 6**, it can be clearly noticed the difference between the stressed and none stressed peaks. Cause, in the case of a uniform strain we note a peak move with no shape changes, contrary to the non-uniform strain which produce with the peak displacement a peak broadens.

#### *2.4.2 Grain size*

The determination of the crystallite size is based on the calculation of the width with middle height of peak (Full Width at Half Maximum FWHM) expressed in radian, according as mentioned before, to the well-known Sherrer formula (**Figure 7**).

The identification of this parameter is as follow:

An increase in the width of the diffraction causes a decrease in the crystallite size. Destructive interference of the all diffracted beams will result a sharp peak. Complete destructive interference beside the Bragg angle is produced from taking diffraction of large number of planes. Thereby, broadened peak can be observed when the crystallites are of very small sizes in which there are not enough planes to produce complete destructive interference.

**Figure 6.** *Strain effect on the diffraction peak [6].*

**Figure 7.** *Determination of the FWHM [6].*

## **3. Scanning electron microscopy**

Since its first commercialization, Scanning Electron Microscope (SEM) shows a remarkable progress. This technique is now well known and used in many laboratories. A SEM can be utilized for high magnification imaging of almost all materials.

## **3.1 Principle**

The SEM instrument is very suitable for different kinds of investigations. It is possible to investigate for example the fiber in wood and paper, metal fracture surfaces, production defects in rubber and plastic ect. [7]. Therefore, to be able to interpret the different images and information's collected by using SEM it is essential to understand the principal of function of this tool.

The principle of operation is as follows:


For the observation of the PZT, a light metallization is necessary so to evacuate the loads.

#### **3.2 Description of the microscope**

A knowledgeable SEM operator should have a basic of contents, which can simplify at this point:


**Figure 8.** *Principe of electron beam/specimen interactions.*

*Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

#### **Figure 9.**

*Scheme of scanning electron microscope [8].*


#### **3.3 SEM applications**

The different applications of this technique can be summarized as follow:

## **4. Energy dispersive spectroscopy EDS**

Energy dispersive X-ray spectroscopy (EDS, EDX or EDXRF) is an analytical technique used for the elemental analysis or chemical characterization of a sample (**Figure 10**).

## **4.1 Principle**

The operating principle of EDS is based on the interaction between electromagnetic radiation and matter; the X-rays emitted by the matter in response will be collected and analyzed. This analysis is due to the fact that each element has its own specific atomic structure; this latter allows the characteristic X-rays of the element atomic structure to be identified uniquely from each other (**Figure 11**) [9].

**Figure 10.** *Energy dispersive spectroscopy (EDS).*

#### **Figure 11.**

*X-ray source region, with path of X-rays to the spectrometer.*

*Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

High energy beam of charged particles is focused into the sample being studied in order to stimulate the emission of characteristic X-rays from the specimen. Furthermore, an Energy Dispersive Spectrometer, EDS, is used in order to measure the number and energy of the X-rays emitted from a specimen. As the energy of the X-rays are characteristic of the difference in energy between the two shells, and of the atomic structure of the element from which they were emitted, this allows the elemental composition of the specimen to be measured [9].

#### **4.2 EDS applications**

The Energy-dispersive spectrometer is especially useful for qualitative analysis because a complete spectrum can be obtained very quickly, but is also used for the Quantitative analysis. This method permit to find what elements are present and their quantities in an 'unknown'specimen by identifying the lines in the X-ray spectrum using tables of energies or wavelengths (**Figure 12**) [10].

Six types of major artifacts may possibly be generated during the detecting process:


## **5. Spectrophotometry**

The spectrophotometer has well been called the workhorse of the modern laboratory. In particular, ultraviolet and visible spectrophotometry is the method of choice in most laboratories concerned with the identification and measurement of organic and inorganic compounds in a wide range of products and processes.

**Figure 12.** *EDX spectrum of (K,Na)NbO3.*

## **5.1 Principle**

Spectrophotometry is commonly divided in two spectroscopic analyses:


## **5.2 Description of the spectrophotometer**

The instrument used in ultraviolet–visible spectroscopy is called a UV/Vis spectrophotometer (**Figure 14**).

**Figure 13.** *Principle of operation UV–VIS spectrophotometer [11].*

*Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

**Figure 14.** *Schematic drawing of spectrophotometer basic construction [12].*

The minimum requirements of an instrument to study absorption spectra (a spectrophotometer) can be listed as below:


#### **5.3 Spectrophotometer applications**

Samples characterization by using UV/Vis spectrophotometry works by comparing the intensity light called reference (which represent the intensity of light before it passes through a sample), with the measured intensity of light passing through the sample. The ratio is called the transmittance (or absorbance), and is usually expressed as a percentage. Therefore by using a spectrometer we can measure Transmittance/Absorbance, and extract from it the following optical properties:


### **6. Hall Effect**

Hall Effect is a process in which a transverse electric field is developed in a solid material when the material carrying an electric current is placed in a magnetic field that is perpendicular to the current. Hall Effect was discovered by Edwin Herbert Hall in 1879.

Hall effect is used to determine if a substance is a semiconductor or an insulator. The nature of the charge carriers, resistivity and carrier's mobility can be measured.

#### **6.1 Principle and theory of Hall effect**

The principle of Hall Effect states that when a current-carrying conductor or a semiconductor is introduced to a perpendicular magnetic field, a voltage can be

**Figure 15.**

*Hall measurement HMS set up of CuO grown onto glass substrate [13].*

measured at the right angle to the current path. This effect of obtaining a measurable voltage is known as the Hall Effect (**Figure 15**).

When a conductive plate is connected to a circuit with a battery, then a current starts flowing. The charge carriers will follow a linear path from one end of the plate to the other end. The motion of charge carriers results in the production of magnetic fields. When a magnet is placed near the plate, the magnetic field of the charge carriers is distorted. This upsets the straight flow of the charge carriers. The force which upsets the direction of flow of charge carriers is known as Lorentz force.

Due to the distortion in the magnetic field of the charge carriers, the negatively charged electrons will be deflected to one side of the plate and positively charged holes to the other side. A potential difference, known as the Hall voltage will be generated between both sides of the plate which can be measured using a meter.

The Hall voltage represented as VH is given by the formula:

$$\mathbf{V\_H} = (\mathbf{I}\mathbf{B})/(\mathbf{qnd})\tag{3}$$

Here,

I: is the current flowing through the sensor;

B: is the magnetic Field Strength;

q: is the charge;

n: is the number of charge carriers per unit volume;

d: is the thickness of the sensor.

## **6.2 Hall coefficient**

The Hall Coefficient RH is mathematically expressed as.

$$\mathbf{R\_H} = \mathbf{E\_y}/(\mathbf{JB})\tag{4}$$

where: J is the current density of the carrier electron, Ey is the induced electric field and B is the magnetic strength. The hall coefficient is positive if the number of positive charges is more than the negative charges. Similarly, it is negative when electrons are more than holes.

## **7. Binary semiconductors layers' characterization**

## **7.1 Tin monosulfide SnS**

Tin monosulfide, SnS, seems to be one of the most important and the most studied binary semiconductor compounds [14]. Indeed, it can be used in many

#### *Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

technological devices [15]. It is specifically used in solar cells [15], as an absorber [16]. Stannous sulfide (SnS), is a very important candidate in photovoltaic application due to his high characteristics such as: direct band gap with an almost optimum value varying in the range 1.3–1.7 eV [17], high absorption coefficient (≈105 cm<sup>1</sup> ) [18] and the orthorhombic structure [15]. In addition, it has an orthorhombic structure [15].

By using a chemical ultrasonic spray deposition method (CUS) **[**13, 19–21**]**, we have elaborated two samples of SnS thin films on glass substrates. Solution of (0.07 M) molarity was prepared by mixing tin chloride SnCl2 and thiourea (SC (NH2)2) as sources of Sn and S respectively. The precursors were dissolved in methanol. The experimental details are given in **Table 1**.

X-ray diffraction patterns of deposited SnS thin films at different deposition time are shown in **Figure 16**.

The films were found to be polycrystalline with a relatively strong (120) peak. In addition, the other weak peaks contained in the diffraction pattern indicates that the prepared SnS films have the orthorhombic crystal structure according to the PDF Card No. 033–1375. Other additional peaks appearing at 28.73°, 32,72° and


#### **Table 1.**

*Experimental conditions used for the preparation of SnS thin films.*

**Figure 16.** *XRD patterns of SnS layers deposited at different deposition time [21].*

49,78° can be attributed to the SnS2 phase (PDF Card No. 031–1399) which was found to accompany the preparation of SnS [22].

The grain sizes and internal strains of the prepared films were calculated from the XRD data using the standard Debye–Scherer formula, and are summarized in **Table 2**.

The grain size calculated from the XRD diffraction spectrum varied from 16.54 nm to 19.07 nm when the deposition time was increased from 25 min. to 30 min. Similar values of the grain size have been found by other investigators **[**23, 24**]**. These sizes values indicate the nanocrystalline nature of the films.

Films morphology and chemical composition was carried out using scanning electron microscopy (SEM) and EDX technique. The typical SEM image of SnS thin film deposited during 30 min. is shown in **Figure 16**. The film is homogeneous, devoid of cracks and has a near stoichiometric ratio (Sn/S = 1.1). This smooth aspect of the obtained film (which is consistent with the XRD) can be related to the viscosity and the surface tension of methanol. Indeed, when using methanol as solvent, the droplets are more easily spread on the substrate surface [21]. Furthermore, we notice, the presence of Al, Cl, Si and O elements which are not expected to be in films and may originate from the glass substrates (**Figure 17**).

The optical band gap is calculated using the relation

$$(\alpha \mathbf{h} \boldsymbol{\nu})^2 = \mathbf{A} \left( \mathbf{h} \boldsymbol{\nu} - \mathbf{E}\_{\mathbb{g}} \right) \tag{5}$$

where A is a constant, Eg is the optical band gap, ν is the frequency of the incident photon and h is the Planck's constant. The plots of (αhν) <sup>2</sup> versus the photon energy hν for direct transition for the film deposited during 30 min. is shown in **Figure 18**. The band gap energy of this film is determined using the intercept of the tangent to the plot with the abscissa axis.

The obtained optical band gap values of the two SnS thin films are summarized in **Table 3**. We notice that the optical band gap of the prepared films decreases when the deposition time increases. This decrease of the Eg can be due the grain size increase [25].


**Table 2.**

*Grain sizes and internal strain of SnS films.*

**Figure 17.** *SEM images and EDX spectrum of SnS film prepared during 30 min [21].*

*Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

#### **Figure 18.**

*(A) Plot of (αhv)<sup>2</sup> vs. hν of SnS film. Inset: representation of the absorption coefficient α as a function of the wavelength. (B) Optical band gap of SnS films [21].*


#### **Table 3.**

*Electrical conductivity of of SnS films* **A** *and* **B** *and their deposition time.*

In **Table 3** we report the electrical properties of the as-deposited SnS thin films. Stannous sulfide is generally known to be a p type semiconductor [26]. Ionized tin vacancies leads to create the acceptor levels [27].

The atomic ratio obtained from the EDX analysis, conduction type and resistivity are indicated in **Table 3**. We note that SnS films have an p-type conductivity, with quasi-stoichiometric composition. Since the conductivity type of the SnS films are essentially controlled by the excess in Sn concentration in the compound [21]. In this case, the Sn atoms act as donor and lead to the n-type conduction. But due to the quasi-stoichiometric nature of our films, they exhibit a p-type conductivity. Furthermore, we note that the resistivity of the films decreases when the deposition time increases. This variation can be attribute to the grain size increase as indicated in **Table 2**. These results are consistent with the literature [18].

#### **7.2 Tin disulfide SnS2**

Tin disulfide (SnS2) was considered as one of very interesting tin sulfides semiconductors. SnS2 has been known for its potential applications in solar cells as well as electrical switchings [28]. This material belongs to IV–VI group of semiconductor compound with hexagonal crystal structure (a = 0.3648 nm, c = 0.5899 nm) [29]. It has a wide band gap energy (2.88 eV) [30], and n-type electrical conductivity with magnitude depending on the preparation methods.

SnS2 thin films were deposited onto ordinary glass substrates using the same method CUS as for SnS. The precursors used as sources of tin (Sn) and sulfur (S) are: (SnCl4: 2H2O) and (SC(NH2)2) respectively. Two different molarities of SnCl4 (MSn) were diluted with a fixed molarity (0.1 mol/l) of thiourea (MS) in methanol, in order to study their effect on SnS2 properties. These experimental conditions are summarized in **Table 4**.

**Figure 19** shows the X-ray diffraction patterns of the elaborated films. From the obtained spectra it can be noticed, that the films are polycrystalline with a preferred orientation along the (001) direction and fit well with hexagonal SnS2 structure according to the ASTM card number 23–0677. This indicates the presence of the pure hexagonal β-SnS2 phase [31]. In addition, it can be noticed that, the main peak intensity increases with increasing MSn molarity.

Structural properties of the two samples are given in **Table 5**.

It can be noticed that the grain size decreases when the molarity increases, contrary to the strain. This comportment is due to the fact that grains growth is


**Table 4.**

*Experimental conditions used for tin disulfide thin films elaborations.*

**Figure 19.**

*XRD patterns of SnS2 thin films deposited at different molarities [19].*


**Table 5.**

*Structural parameters of dominate phase of the prepared films.*

*Binary Semiconductors Thin Films Characterization for Solar Cells Applications DOI: http://dx.doi.org/10.5772/intechopen.100513*

**Figure 20.** *SEM image of as-synthesized SnS2 thin films [19].*

controlled by the strain in film network. Because the presence of internal strain in the film network cause a minimization in the grain growth driving forces, which prevent the grain size enlargement during the film formation and vice-versa.

Morphological study of typical SnS2 thin film (Msn = 0.07 mol/l), shows that the surface topography is dense and rough with an arbitrary distribution of the bubbles (**Figure 20**).

On the other hand, the measurement of films resistivity revealed that this latter decreases from 0.46x10<sup>3</sup> Ω.cm when the molarity increases and reaches its minimum value of 0.18 x10<sup>3</sup> Ω.cm for MSn = 0.07 mol/l.

According to the obtained results, for the two studied materials, with the different analysis techniques we can conclude that the good quality of the deposited films and the low fabrication cost of the used method can lead to solar cells whose costquality ratio is better than the solar cells which are fabricated by the other standard process.

## **Author details**

Kenza Kamli\* and Zakaria Hadef Department of Physics, Faculty of Sciences, University 20 Août 1955-Skikda, Skikda, Algeria

\*Address all correspondence to: kenza\_kamli@yahoo.fr; k.kamli@univ-skikda.dz

© 2021 The Author(s). Licensee IntechOpen. 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.

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[12] Basic UV/Visible Spectrophotometry.

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[18] E. Guneri, C. Ulutas, F. Kirmizigul, G. Altindemir, F. Gode, C. Gumus, 'Effect of deposition time on structural, electrical, and optical properties of SnS thin films deposited by chemical bath deposition', Applied Surface Science, 2010, 257, 1189–1195.

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## **Chapter 4**

## Kesterite Cu2ZnSnS4-xSex Thin Film Solar Cells

*Kaiwen Sun, Fangyang Liu and Xiaojing Hao*

### **Abstract**

Kesterite Cu2ZnSnS4-xSex (CZTS) is a promising thin film photovoltaic (PV) material with low cost and nontoxic constitute as well as decent PV properties, being regarded as a PV technology that is truly compatible with terawatt deployment. The kesterite CZTS thin film solar cell has experienced impressive development since its first report in 1996 with power conversion efficiencies (PCEs) of only 0.66% to current highest value of 13.0%, while the understanding of the material, device physics, and loss mechanism is increasingly demanded. This chapter will review the development history of kesterite technology, present the basic material properties, and summarize the loss mechanism and strategies to tackle these problems to date. This chapter will help researchers have brief background knowledge of kesterite CZTS technology and understand the future direction to further propel this new technology forward.

**Keywords:** kesterite, Cu2ZnSn(S,Se)4, CZTS, thin film solar cells, loss mechanism

#### **1. Introduction**

Thin film photovoltaic (PV) technologies such as Cu(In,Ga)Se2 (CIGS) and CdTe have already demonstrated more than 20% power conversion efficiency (PEC) [1, 2] and are at their commercial stage. However, considering the Restriction of Hazardous Substances Directive (RoHS) adopted in European Union [3] and the recent classification of critical raw materials (CRM) by the European Commission [4], emerging thin film PV technologies with RoHS-compliant and CRM-free constituents are increasingly desirable. Kesterite copper-zinc-tin-selenosulfide and related quaternary semiconductor represented by chemical formula Cu2ZnSnS4-xSex is generally accepted as one promising option for low-cost and nontoxic thin film PV.

The family of kesterite Cu2ZnSnS4-xSex includes pure sulfide Cu2ZnSnS4 (CZTS), pure selenide Cu2ZnSnSe4 (CZTSe), and selenosulfide Cu2ZnSn(S,Se)4 (CZTSSe) and other related compound semiconductors. Herein we abbreviate all Cu2ZnSnS4-xSex compounds as CZTS. The formation of CZTS is derived from cation mutations (cross-substitutions) in CuInS2 by replacing two In atoms with one Zn and one Sn [5, 6]. CZTS possess high absorption coefficient of over 104 cm−1, tunable band gap that can range from 1.0 to 1.5 eV to favorably match the solar spectrum, intrinsic p-type conductivity, and a three-dimensional symmetry of carrier transport [7–9]. These decent photovoltaic properties of CZTS have attracted considerable attention and enabled its rapid development in the last decades. The highest power conversion efficiencies (PCEs) of Cu2ZnSnS4, Cu2ZnSnSe4

and Cu2ZnSn(S,Se)4 have been set to 11%, 12.5%, and 13%, respectively [10–12], which represent the best PCE among the emerging RoHS-compliant and CRM-free inorganic thin-film PV technologies.

In this chapter, the development history and current status of CZTS PV technology will be briefly introduced. The basic physical and chemical properties of CZTS thin film will be described to help readers have a better understanding of the material. The device architecture and absorber processing will be touched; finally. The limiting factors as well as the perspective for future development of this technology will also be reviewed and addressed.

## **2. The development history of CZTS**

The CZTS single crystal was first grown by Nitsche, Sargent, and Wild in 1966 when they tried to prepare a serious of AI 2BIICIVX4-type quaternary chalcogenides using iodine vapor transport [13]. The photovoltaic effect of CZTS was exhibited for the first time in 1988 by Ito and Nakazawa on a heterojunction diode consisting of cadmium-tin-oxide transparent conductive layer and CZTS thin film on a stainless steel substrate. The open-circuit voltage was measured to be 165 mV under AM1.5 illumination [14]. The open-circuit voltage was increased to 250 mV after annealing the device in air, and short-circuit current of 0.1 mA/cm2 was achieved [15].

The first CZTS solar cell with PCE of 0.66% was reported by Katagiri et al. in 1996 at PVSEC-9, with the device structure of ZnO:Al/CdS/CZTS/Mo/soda lime glass (SLG) substrate [16]. The CZTS thin film was fabricated by vapor-phase sulfurization of E–B-evaporated precursors [17]. In 1997, Friedlmeier et al. reported the CZTS solar cell with PCE of 2.3% and open-circuit voltage of 570 mV based on thermal evaporation [18]. In 1999, the Katagiri group improved the PCE up to 2.63% [19], after that they did a lot of work on optimizing the CZTS thin film and pushed the efficiency to 5.74% until 2007 [19–24].

The CZTS solar cell started to gain intensive interest from academic and industry community from 2008 when its efficiency was further boosted to 6.7% by Katagiri et al. and when the CIGS solar cell became mature in its commercialization stage [25]. A lot of research institutes and solar cell manufactures such as Toyota, IBM, NREL (National Renewable Energy Laboratory), Solar Frontier, EMPA (Swiss Federal Laboratories for Materials Science and Technology), HZB (Helmholtz-Zentrum Berlin), ZSW (Centre for Solar Energy and Hydrogen Research Baden-Württemberg), UNSW (University of New South Wales), and so on involved in the development of this technology and significant advances have been achieved in the following decade [10, 26–33]. During the development period, Se incorporation has attracted significant attention and led to impressive progress in PCE [34–37]. Moreover, various fabrication methods including vacuum deposition process and non-vacuum process such as solution method, electrochemical deposition, etc., have been developed for fabricating the CZTS thin film [29, 37–40].

An important milestone in the development of CZTS solar cell is the 10% benchmark efficiency breakthrough achieved by IBM Thomas J. Watson Research Center in 2011 [34], which shows substantial commercial promise for the CZTS-based class of thin film PV materials. This breakthrough also established the leading position of IBM in CZTS PV technology research, represented by a serious of world record efficiencies [38, 39, 41]. The highest PCE for CZTS solar cell has been stagnant at 12.6% for more than 6 years since 2014 when IBM last updated their efficiency breakthrough [39]. Thanks to the better understanding of the CZTS material and the loss mechanism of the device, quite a few groups reported CZTS PCE close to

*Kesterite Cu2ZnSnS4-xSex Thin Film Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.101744*

the 12.6% record efficiency in recent years [11, 42–46]. The recently announced NREL Best Research-Cell Efficiency Chart included the newly refreshed CZTS record efficiency of 13% achieved by Xin et al. from NJUPT [12]. This small step breakthrough comes from great efforts in this research area and hopefully will bring more interests and confidence to the CZTS R&D community.

### **3. The physical and chemical properties of CZTS thin film**

The investigation and understanding of nature of the CZTS thin film are crucial for further developing this technology. Intensive studies have been conducted during the rapid development stage; therefore, the physical and chemical properties of CZTS have been well revealed.

#### **3.1 Crystal structure**

As we briefly introduced earlier, CZTS is derived from the cation mutations of CuInS2 (CIS), while both are originated from the binary II–VI semiconductors adopting the cubic zinc-blende (or hexagonal wurtzite) structure as shown in **Figure 1**.

Similarly, the structure of CZTS is also derived from ternary I-III-VI2 compounds. In general, as shown in **Figure 2**, chalcopyrite (CH) and CuAu-like (CA) structures are two fundamental I-III-VI2 structures that obey the octet rule [5, 47, 48]. Therefore, the quaternary CZTS is well known as two principal structures, kesterite (space group *I***4**, **Figure 2d**) and stannite-type (space group *I***42m, Figure 2e**), which are derived from CH structure and CA structure, respectively. Other primitive mixed CA structure (PMCA) (space group *I***42m, Figure 2f**) derived from CA structure has also been reported in CZTS [5, 47, 48]. As the most common structures discussed in literature are kesterite and stannite-type, we only focus on these two structures in this section. The two structures are closely related with the main difference of cation arrangement. Both structures are composed of a cubic close-packed lattice of S anions, with half of the tetrahedral interstices occupied by cations. Sn atoms occupy the same fixed positions in both structures, but the Cu and Zn atoms are in different position [49, 50]. In kesterite structure, cation layers of CuZn, CuSn, CuZn, CuZSn alternated at *z* = 0, ¼, ½, and ¾ respectively (**Figure 2d**), while in stannite structure, ZnSn and Cu2 layers alternate with each other (**Figure 2e**). The similarities of structure make it difficult to distinguish kesterite and stannite experimentally by employing common X-ray diffraction and

#### **Figure 1.**

*Schematic illustration of the origin of CZTS structure. Reproduced from [6] with permission from the Royal Society of Chemistry.*

**Figure 2.**

*The crystal structure of (a) zinc-blende ZnS, (b) chalcopyrite CuInS2, (c) CuAu-like CuInS2, (d) Kesteritetype Cu2ZnSnS4, (e) stannite-type Cu2ZnSnS4, and (f) PMCA-Cu2ZnSnS4. Reproduced from [47] with permission from the American Physical Society.*

Raman spectroscopy techniques [51, 52]. Only advanced techniques such as neutron powder diffraction analysis are capable to tell them apart [50, 53].

CZTS usually exists as kesterite-type structure, which is more stable thermodynamically than the stannite-type [54]. This is in agreement with the experimental observation [21, 50, 55, 56]. Plenty of theoretical studies have also confirmed that the kesterite-type structure is the ground state structure in CZTS [5, 47, 48, 57–59]. This is also the reason that CZTS is named after Kesterite because it crystallizes as kesterite structure. However, the energy difference between the kesterite and the

stannite-type structure is rather small [5, 47, 48, 57–59]. This indicates that kesterite structure should be formed under equilibrium growth conditions, but both phases may exist, especially when growth method and conditions are changed, it should be relatively easy to grow materials with mixed phases. This may also partly explain the existence of disorder structure with more random distribution of the Cu and Zn on the cation positions [60], which will be discussed in more details in the following sections.

## **3.2 Electronic band structure**

Kesterite CZTS has a similar tetrahedral bonds geometry as traditional group I**–**V, III**–**V, and II**–**VI semiconductors. It obeys Lewis' octet rule with eight electrons around each anion atom (S or Se); the four bonds of each anion therefore form together a close valence shell [48]. However, there are fundamental differences between the Cu-based quaternary compound and the group I**–**V, III**–**V, and II**–**VI binary semiconductors. First of all, the bonds in CZTS involve Cu-d–anion-p hybridized antibonding states. This weakens the bonds in CZTS. Second, Cu in CZTS has only one valence s-like electron while the group III**–**V (group II**–**VI) semiconductors have three (two) valence s-like electrons for their cations.

Several first principle studies have revealed the electronic band structure of CZTS [47, 48, 58, 59, 61, 62]. An example is as shown in **Figure 3**, the calculated band structures of kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 from different methods are presented. Overall, the kesterite CZTS materials are all direct-gap semiconductors. Different calculation methods generated slightly different values, for example, generalized gradient approximation (GGA) method calculated the band gap energy for Cu2ZnSnS4 and Cu2ZnSnSe4 as 1.56 eV and 1.05 eV, respectively. Corresponding band gap values from hybrid functional calculations (HSE06) are 1.47 and 0.90 eV, while the values from the GW0 calculation are 1.57, and 0.72 eV. These values are in agreement with other calculated data and available experimental measurement, converging to the results that Eg ≈ 1.5 eV in CZTS and Eg ≈ 1.0 eV in CZTSe [14, 24, 28, 64–72] to vary linearly as a function of the Se content x [73] with Eg = 1.47, 1.30, 1.17, 1.01, and 0.90 eV for x = 0, 1/4, 1/2, 3/4, and 1, using the HSE06 potential. This linear relationship also agrees with experimental results [66, 70, 71].

The density of states (DOS) of kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 can also be calculated as illustrated in **Figure 4** [63]. It is not surprising to see that Cu2ZnSnS4 and Cu2ZnSnSe4 show comparable DOS as they are in same tetrahedral bond geometries. The DOS of conduction bands in Cu2ZnSnS4 is ~0.5 eV higher than that of Cu2ZnSnSe4 because of large energy gap. It is common that in Cu-based chalcogenides including the quaternary (for example, CIS) and ternary compounds (for example, CZTS), the valence band maximum (VBM) is derived mainly from the hybridization of anion p and Cu d states because Cu has higher d orbital energy than Zn, Ga, In, and Sn [59, 74]. In CZTS, the valence p level of S is lower in energy than Se, thus the VBM of the sulfides is lower than that of the selenides. This difference is reduced by anion p–Cu d overlap (p-d hybridization) because the hybridization is stronger in the shorter Cu–S bond and pushes the antibonding VBM level of the sulfide up relative to that of the selenide. Therefore, the valence band offset between the Cu2ZnSnS4 and Cu2ZnSnSe4 is less than 0.2 eV. The DOS of the CZTS conduction band minimum (CBM) is primarily controlled by the Sn-s and anion p-like states due to the lower s orbital energy of Sn than the other cations [59, 74]. More importantly, in CZTS the lowest CB is separated from the higher energy bands and is therefore more localized in energy. Therefore, the CBM of CZTS is expected to vary depending on the alloying on the Sn-site with other group-IV elements

**Figure 3.**

*The electronic band structure of the kesterite structures of Cu2ZnSnS4 and Cu2ZnSnSe4 along four symmetry directions. The energy refers to the VBM (dashed lines). The spin–orbit interaction is included, but the index of the bands refers to spin-independent bands where c1 represents the lowest CB and v1 represents the topmost VB. Different line shapes represent different calculation methods [63]. Reproduced from [59] with permission from American Institute of Physics.*

#### **Figure 4.**

*Atomic and angular momentum resolved DOS of kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 presented with a 70 meV Lorentzian broadening. Reproduced from [59] with permission from American Institute of Physics.*

(e.g., Ge) due to this localized CB. The band gap energy can therefore be tailored by cation alloying for an optimized optical efficiency of the materials.

## **3.3 Composition variation and phase competition**

As a quaternary semiconductor, CZTS consists of three metals and up to two chalcogens, which provides a wide range for composition variation and secondary phase formation. However, high-efficiency CZTS thin film solar cells require singlephase kesterite absorber, and some secondary phases have been reported detrimental to the device performance. This section reviews the stable phase region of CZTS and possible secondary phases likely to be formed in the quaternary system.

The 3D CZTS quaternary phase diagram as shown in **Figure 5** can be deduced from Cu2S-ZnS-SnS2 pseudo-ternary diagram [77] and CuS-ZnS-SnS pseudoternary phase diagram [75] (selenide kesterite system also refers to Cu2SnSe3– SnSe2–ZnSe diagram [78]. In addition, the stability region of the CZTS in the atomic chemical potential space can be calculated as shown in **Figure 5** [76, 79]. All these diagrams indicate that volume of the stable CZTS region is small, and the slight deviation (maximum 1–2 at%) outside this space will cause the formation of different secondary phases [5, 80]. The narrow stable window also implies that the composition control and the chemical potential control are crucial for the growth of high-quality and single-phase CZTS absorber. The Zn content is particularly important because the stable region along the μZn axis is much narrower. Moreover, it is well accepted experimentally and theoretically that high-efficiency solar cells need CZTS absorber with Zn-rich and Cu-poor composition [8–10, 27, 28, 33, 67, 81]. This makes it more difficult to control the secondary phases formation.

According to the phase diagram and literature report, the most common secondary phases in the Cu-Zn-Sn-S/Se system are list in **Table 1** [92]. The influence of the secondary phases on the solar cell performance depends on their position in the film as well as their physical properties. For example, Cu2S(Se) phases in the final film may act as shunting path due to both the high conductivity and contact with front and back interfaces. However, Cu2S(Se) is also an important fluxing agent to promote lateral grain growth during the film growth [28]. Generally, If the secondary phase has a lower band gap than the CZTS absorber, it will limit the open-circuit

#### **Figure 5.**

*Left: CZTS quaternary phase diagram including the known phases. Reproduced from [75] with permission from American Institute of Physics. Right: The calculated chemical-potential stability diagram of Cu2ZnSnS4 in a 2D Cu-rich plane (the stable 3D region is inset) reproduced from [76] with permission from American Institute of Physics.*


#### **Table 1.**

*Most common secondary phases in the Cu-Zn-Sn-S/Se system.*

voltage of the solar cell. While secondary phases with higher band gaps than CZTS are less detrimental; however, they can block the transport when present in large amounts [93] or at least increase the series resistance [94]. ZnS(e) is the most likely secondary phase considering the phase diagram and especially when adapting the Zn-rich and Cu-poor condition. Fortunately, ZnS(e) is a large band gap compound and is expected to be rather benign if present in small amounts. It has even been reported that ZnS with similar crystalline structures to CZTS may passivate grain boundaries or heterojunction interface [81, 95, 96] by reducing strain and lowering recombination velocities at the grain interfaces. The tin compounds are unlikely to form because they are usually volatile [97] and will evaporate in most preparation conditions. As shown in **Table 1**, one unfavorable low band gap secondary phase in both Cu2ZnSnS4 and Cu2ZnSnSe4 system is the ternary Cu2SnS(e)3, which is also one reason for optimal composition range (Zn-rich and Cu-poor) found for the best solar cells. However, the non-homogeneous composition across the CZTS film under normal preparation conditions still provides the possibility to form such detrimental secondary phases [98]. Therefore, how to control the amount and position of the secondary phases in the CZTS absorber needs to be elaborately studied.

#### **3.4 Lattice defects**

The formation and properties of lattice defects are important parameters of semiconductor materials and are crucial to the function of photovoltaic devices, because they directly influence the generation, separation, and recombination of electron–hole pairs. The lattice defects (e.g., vacancies, interstitials, antisites) in kesterite CZTS system are complicated because of the increased number of component elements and the similar cation size as well as small chemical mismatch of Cu+ and Zn2+. In this section, the formation and ionization of the lattice defects in kesterite will be briefly reviewed. More importantly, the underlying mechanism of p-type conductivity, Zn-rich and Cu-poor condition growth condition, as well as limiting factors for device performance in kesterite CZTS solar cells from the perspective of lattice defects will be discussed.

The concentration of the lattice defects is determined by their formation energy. In **Figure 6**, the calculated formation energies of different defects are plotted as functions of the Fermi energy (0 means that the Fermi energy is at VBM, while 1.5 or 1.0 eV means that Fermi energy is at CBM). It is obvious that in both Cu2ZnSnS4 and Cu2ZnSnSe4, CuZn antisite is the lowest energy defects, which is different from the defect properties of their parent compounds (CuInSe2 or CuGaSe2) where the dominant defect is the Cu vacancy VCu [76, 79, 99]. In addition, the formation energies of most acceptor defects are lower than those of donor defects, explaining the intrinsic p-type conductivity observed in literature [21, 48, 64, 84, 100–110].

#### **Figure 6.**

*Calculated defect formation energy as a function of the Fermi energy at the thermodynamic chemical-potential point a (from Figure 5 right) for Cu2SnZnS4 (left) and Cu2SnZnSe4 (right). For each value of the Fermi energy, only the most stable charge state is plotted, with the filled circles (change of slope) representing a change in charge state (transition energy level). Reproduced from [76, 79] with permission from American Institute of Physics and American Physical Society.*

Another important parameter of lattice defects is their ionization (transition) levels, which determines if they can produce free carriers and contribute to the electrical conductivity. The calculated ionization levels of intrinsic defects in the band gap of Cu2ZnSnS4 and Cu2ZnSnSe4 are shown in **Figure 7**. First of all, in both cases, the dominant defect CuZn has an acceptor level (0.11 eV and 0.15 eV above VBM in Cu2ZnSnSe4 and in Cu2ZnSnS4, respectively) deeper than that of VCu. The relatively deep level of dominant antisite defect in CZTS is not favorable to the device performance because it will limit the open-circuit voltage. This also partially explains why Zn-rich and Cu-poor condition has normally been found to beneficial to solar cell efficiency because it could decrease the formation energy and enhance the population of shallow VCu. The other acceptor defects (e.g., CuSn, ZnSn, VZn, and VSn) have higher formation energy; therefore, they have negligible contribution to the p-type conductivity. However, they may act as recombination centers especially for those deep transition levels such as (4−/3-) and (3−/2-) in band gap as seen in **Figure 7**.

In addition to the point defects, various self-compensated defect clusters can be formed in CZTS due to the large amount of low-energy intrinsic defects. Defect compensation in ternary CIS is well known to have electrically benign character because the intrinsic defects undergo self-passivation through the formation of defect complexes such as [2VCu− + InCu 2+]. Therefore, it is also interesting to see if the same behavior can be observed in quaternary kesterites. CuZn and ZnCu are the lowest-energy acceptor and donor defects, respectively; therefore, antisite pair [CuZn<sup>−</sup> + ZnCu+ ] also shows extremely low formation energy. Fortunately, its impact on the electronic structure and optical properties is relatively weak. The detrimental defect clusters are those composed of deep level defects, such as SnZn, SnCu, CuSn, and Zni. For example, clusters with SnZn induce large conduction band edge downshift, which could limit the solar cell performance, because the induced states are deep and may trap photo-generated electrons from the high conduction band. [2CuZn + SnZn] clusters could present in high population if in single-phase CZTS (Cu/(Zn + Sn) and Zn/Sn ratios near 1) chemical potential conditions and be detrimental to the solar cell performance. The Zn-rich and Cu-poor condition could prevent the formation of [2CuZn + SnZn] clusters because its formation energy

#### **Figure 7.**

*The ionization levels of intrinsic defects in the band gaps of Cu2ZnSnS4 (top) and Cu2ZnSnSe4 (bottom). The red bars show the acceptor levels, and the blue bars show the donor levels, with the initial and final charge states labeled in parentheses. Reproduced from [79] with permission from American Physical Society.*

is very sensitive to the chemical potential of Zn. This again partly explains that the Zn-rich and Cu-poor condition is beneficial to solar cell efficiencies from the defect perspective.

#### **4. The path towards high-efficiency kesterite solar cell**

#### **4.1 Device architecture**

The typical device architecture (**Figure 8**) of kesterite-based solar cell inherits from its predecessor chalcopyrite CuInSe2 solar cells due to their similar optical and electronic properties. Metal Mo layer with thickness of 500 nm**–**1000 nm is usually deposited with sputtering on soda lime glass or other substrates as back contact. Then the kesterite CZTS absorber layer is deposited on top of the Mo layer. The p-n junction is formed by p-type CZTS and the following deposited n-type buffer layer. Typical n-type buffer layer in kesterite and chalcopyrite solar cell is 50 nm**–**100 nm thick CdS layer usually by chemical bath deposition. Alternative buffer materials such as ZnSnO, Zn(O, S), ZnCdS, etc., have been explored, especially in pure sulfide Cu2ZnSnS4 to tackle the unfavorable band alignment found at p-n junction. Next, a 50 nm–100 nm high-resistive intrinsic ZnO (i-ZnO) is deposited.

*Kesterite Cu2ZnSnS4-xSex Thin Film Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.101744*

**Figure 8.**

*Schematic device structure of a typical kesterite Cu2ZnSnS4 thin film solar cell.*

Subsequently, the device structure is completed by deposition of 200 nm**–**500 nm thick Al-doped ZnO (AZO) or indium tin oxide (ITO) transparent conducting oxide (TCO) layer as the front contact. Ni/Al metal contacts are deposited on the TCO layer for improved current collection. Antireflection coating such as MgF2 is often deposited on top of the cell to reduce the reflection loss.

Various deposition methods have been investigated for producing high-quality CZTS layer during the development of this technology. These deposition methods are broadly classified as vacuum-based and non-vacuum-based techniques. The vacuum-based methods are usually considered easy to be expanded to commercial scale because of the precise process control. All physical vapor deposition techniques including thermal evaporation, E-beam evaporation [17], sputtering [10, 33, 81, 111], pulsed laser deposition [112] fall into this category. The non-vacuum-based methods are always regarded as low-cost, high-throughput techniques and feasible in roll-to-roll production. These methods usually involve chemical or solution process, such as electrochemical deposition [40], nanoparticle-based synthesis [113, 114], sol–gel spin coating [44, 115], chemical bath deposition (CBD) [116], successive ion layer adsorption and reaction (SILAR) [117], screen printing [118], etc.

#### **4.2 Loss mechanism and solutions**

## *4.2.1 Open-circuit voltage (VOC)*

In kesterite CZTS solar cells, it has been widely accepted that VOC loss accounts for the majority (more than 50% [119]) of the efficiency gap between the best performance device and the theoretical limit (i.e., Shockley–Queisser radiative limit [120]). VOC loss is determined by the recombination path in the device. The dominant recombination can occur in the bulk of the absorber in the quasi-neutral zone as band-to-band recombination, via defects, or in the space charge region. Another important recombination path can be located at the heterojunction interface between the buffer/window layer and absorber.

Therefore, the absorber problems discussed in **Section 3** such as abundant point defects and defect clusters (i.e., cation disordering), composition variation with narrow stable region are believed to contribute to the recombination. The Cu and Zn substitution with low energy causes a large population of antisite defects such as CuZn and ZnCu and related defects complexes. Consequently, severe electrostatic potential fluctuation and associated band tailing can be observed [121]. In addition, the microinhomogeneities in composition, nonuniform strain, as well as formation of secondary phases lead to band gap fluctuations, which are also detrimental to

VOC. Moreover, acceptor-like CuZn defects at the interface will cause Fermi level pinning to a low energy level [122], thus reducing the band bending in the absorber and weakening the electric field. To alleviate the recombination occurring in absorber layer, great efforts have been made over recent years. One solution that has been intensively investigated is cation substitutions because the introduction of larger size cations could result in better cationic ordering, thereby reducing the defects formed because of the Cu and Zn substitution. Using Ag as a substitute for Cu [115], Cd as a substitute for Zn [123, 124], and Ge substitute for Sn [125] are popular choices as they are picked from the same cation groups. As shown in **Figure 9**, Promising progress has been made in this direction both experimentally and theoretically, demonstrating several remarkable efficiencies and mechanism of the defect emerging with cation substitution. Another possible solution to tackle the cationic disorder and related band tailing, as well as to activate the shallower defect is deliberate control of the synthesis condition, for example, post annealing the CZTS absorber within the critical temperature range of 200*–*250°C and reasonable time range of 1*–*4 hours will improve the CZTS lattice ordering and increase the band gap [126, 127]. Control of the precursor fabrication process such as the metal stack order, the valence states of the Sn source, has also been reported effective in obtaining less defective absorber and corresponding high-efficiency device [128, 129]. Modification of the sulfurization or selenization condition could also suppress the formation of detrimental intrinsic defects and defect clusters by creating a desirable local chemical environment [11].

In term of the interface recombination path, it is mainly caused by the unpassivated charge defects at the heterojunction and/or the undesirable band alignment between the CZTS absorber and conventional CdS buffer layer. The band alignment problem is more prominent in high band gap pure sulfide Cu2ZnSnS4 because of its higher conduction band position. A serious of alternative wide band gap buffer layer materials including ZnCdS [33], Zn(O,S) [130, 131] and ZnSnO [132, 133] have been reported effective in mitigating the band alignment issues. The charge defects at the interface can be reduced by introducing ultrathin layer that has better lattice match with CZTS [10, 81] or alloying Ag near the heterojunction interface to from intrinsic or weak n-type (Cu,Ag)2ZnSn(S,Se)4 [115, 134]. Dielectric layers have also been investigated for passivating the interface defects, thereby suppressing the interface recombination [111, 135].

#### *4.2.2 Short-circuit current density (JSC)*

While the VOC loss contributes dominant performance loss in kestrite solar cells, JSC also represents a significant limiting factor for efficiency increases. The JSC loss is mainly caused by two reasons: one is the light or photons that is reflected or absorbed by layers above the CZTS (such as the buffer or window layers as shown in **Figure 8**), which cannot generate electron–hole pairs. The other is the low carrier collection efficiency due to the short diffusion length or narrow depletion width. The first problem can be addressed by introducing antireflection coating and optimizing the optical designs of the top layers above CZTS to increase the fraction of incident light reaching CZTS. The solution has been proven feasible when applying wide band gap buffer layer [111, 133], reducing the thickness and roughness of the top layers [119]. The typical External Quantum Efficiency (EQE) curve of CZTS usually shows relative lack of long wavelength response [9], which might be related to the low carrier lifetime. This low lifetime could also be the results of high defect density in the absorber layer or could simply be a consequence of high recombination loss at the back contact or at the front interface. Therefore, the solutions for suppressing the formation of detrimental defects disused in VOC loss above are also

*Kesterite Cu2ZnSnS4-xSex Thin Film Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.101744*

#### **Figure 9.**

**(***a***)** *The band diagrams of CZTSSe and Ag-graded (Cu1 − xAgx)2ZnSn(S,Se)4 (CAZTSSe) solar cells under AM 1.5G 100 mW cm***2** *illumination and the J–V curves of CAZTSSe solar cells with different Ag composition gradients from the CdS interface to the Mo substrate.* **(***b***)** *J–V curves of CZTS and champion Cu2ZnxCd1–xSnS4 (CZCTS) devices and absorption coefficients and Urbach energy (EU) obtained from PDS measurement.* **(***c***)** *J–V curves for the champion CZTSSe and CZTGeSSe (30% Ge) solar cells. Reproduced from [115, 124, 125] with permission from Royal Society of Chemistry, American Chemical Society and John Wiley and Sons.*

good for the JSC improvement In addition, band gap grading has also been developed in CZTS to increase the EQE by controlling the [S]/[S + Se] ratio at the surface and back surface [136].

#### *4.2.3 Fill factor (FF)*

FF is another significant parameter deficit in kesterite solar cell, which is mainly limited by the series resistance (RS) in the device. Careful study has been conducted to identify the origin of the RS, which implies that nonohmic back contact contributes greatly [137]. The nonohmic back contact could be resulted from Schottky barrier formation and/or secondary phase formation at the CZTS/Mo back interface. It is especially severe in high band gap CZTS because of the concentration decrease. Furthermore, the presence of low band gap secondary phases in CZTS could act as shunting pathway, which limits the FF and is therefore undesirable in the device as

well. Deliberate investigation and control of the chemical reaction at the back interface have been proposed to modify the back interface microstructure and improve the device performance including FF [138, 139]. Introducing barrier layer at the CZTS/Mo interface has also been attempted in improving the back interface quality [140, 141].

## **5. Conclusion**

Kesterite CZTS material is an emerging and promising PV technology, which could have the opportunity to realize low-cost and high-volume thin film solar cell production. After undergoing a rapid development in last decades, the technology has attracted increasing attention and demonstrated high potential in high efficiency. Experimental and theoretical studies reveal that the quaternary material exits in kesterite structure, similar to the chalcogenide CuInS2. It has a narrow stable region regarding the chemical potential, which makes the formation of secondary phases very easy. The intrinsic defects characteristic in the CZTS is complex where the deep level antisites defects and defect cluster could be prevalent. These unique properties make it struggling in achieving high-efficiency kesterite solar cells. The sensitive defects environment causes undesirable band tailing, electrostatic potential fluctuations, etc., which become recombination path and limit the open-circuit voltage, as well as other PV performance. The formation of secondary phases can lead to high serious resistance, shunting path depends on the band gap of the phase, which will also limit the device performance. In addition, the heterojunction interface and the Mo/CZTS back interface could also contribute to the performance loss because of the unfavorable band alignment, unpassivated defects, or detrimental reaction during high-temperature annealing. The solutions and approaches for tackling these loss mechanisms have been reviewed at the end of the chapter. Finally, in order to build a successful kesterite CZTS technology, combining the advanced approaches summarized in this chapter with further exploring the material synthesis and device physics would pave a path for higher efficiency.

## **Acknowledgements**

This work was supported by the National Key R&D Program of China (No. 2018YFE0203400), the Science and Technology Innovation Program of Hunan Province (No. 2020RC2005), Australian Renewable Energy Agency (ARENA, 2017/RND006), and International Postdoctoral Exchange Fellowship Program (YJ20200207). X.H. acknowledges Australian Research Council (ARC) Future Fellowship (FT190100756). K.S. acknowledges the support from Australian Government through the Australian Renewable Energy Agency (ARENA) and the Australian Centre of Advanced Photovoltaics (ACAP, Grant No.1-SRI001).

## **Conflict of interest**

The authors declare no conflict of interest.

*Kesterite Cu2ZnSnS4-xSex Thin Film Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.101744*

## **Author details**

Kaiwen Sun1,2\*, Fangyang Liu1 \* and Xiaojing Hao2 \*

1 School of Metallurgy and Environment, Central South University, Changsha, China

2 Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, Australia

\*Address all correspondence to: kaiwen.sun@unsw.edu.au, liufangyang@csu.edu.cn and xj.hao@unsw.edu.au

© 2021 The Author(s). Licensee IntechOpen. 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.

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## **Chapter 5**
