**1. Introduction**

In recent years, researchers of state-of-the-art electronics utilizing organic semiconducting materials have succeeded in advancing various devices, such as organic light-emitting diodes (OLEDs), photovoltaic cells, organic thin-film transistors (OTFTs), and sensors, among others [1–10]. Among these, intensive efforts in OLEDs have led to high brightness, efficiency, and full-color electroluminescent (EL) emissions for various light-emitting optoelectronic devices [7–10]. The advantages of such OLEDs over conventional liquid crystal displays (LCDs) are well known, especially for high-quality displays in terms of their viewing angle, response time, thickness, and contrast ratio [11]. For instance, small OLED displays are constructed on an array of thin-film transistor (TFT) switches, allowing precise control of the states of the pixels [12–14]. In such active-matrix OLEDs (AM-OLEDs), the OLED is driven in the current mode; thus, at least two TFTs, in this case a switching TFT to select a pixel and a driving TFT to operate the OLED, are required, as shown

in **Figure 1(a)** [12, 13]. Perhaps unexpectedly, however, the complexity of such pixel circuit designs with their sophisticated procedures has led to a significantly limited light-emitting area and aperture ratio (the light-emitting area as a fraction of the total area of the device, typical aperture ratios: 25–34%) [13–15], introducing severe problems associated with limited device performance and limited display sizes for AM-OLEDs. Besides these issues, fundamental factors related to the architecture of the OLED itself, such as exciton quenching and photon loss, also still limit the efficiency and brightness of these devices.

To overcome some of the limitations of (AM-)OLEDs, research on different structures and materials is currently yielding new developments [15–30]. Among these, organic light-emitting transistors (OLETs), such as static-inductiontransistor OLETs (SIT-OLETs) [17, 18], metal-insulator-semiconductor OLETs (MIS-OLETs) [19], lateral-type OLETs [20–29], and vertical-type OLETs (VOLETs) [30], have been devised by integrating the capability of the OLED to generate EL light with the switching functionality of a field-effect transistor (FET) into a single device structure. In these OLETs, the current that flows through emissive semiconductor channel layers can be controlled by the gate voltage, which can also change the EL emission brightness state from the dark off- to the bright on-state. The on-state implies that holes and electrons injected into the channel layer form excitons that recombine radiatively to generate EL light [17–30]. These OLETs are of key interest; not only do they provide a novel device architecture to investigate fundamental optoelectronic properties related to charge carrier injection, transport, and radiative exciton recombination processes in organic semiconducting materials, at the same time OLETs can also be used to develop highly integrated organic optoelectronic devices such as highly bright and efficient light sources, optical communication systems, and electrically driven organic lasers [21–30].

In principle, the luminance from OLETs can be modulated by the gate voltage without any additional driving devices; thus, displays using OLETs have the advantage of greatly reducing both the number of TFTs and the circuit complexity (**Figure 1(b)**), thereby providing an effective means of increasing the aperture ratio [29]. Hence, OLETs could be a key part of the development of next-generation AM display technology [29]. Indeed, a proof-of-principle device was recently developed using carbon nanotubes (CNTs, **Figure 2**), delivering a CNT-based vertical-type OLET (CNT-VOLET) [31–34]. In the CNT-VOLET, a dilute network of CNTs is used as a source electrode, leading to several improvements, such as a high on/off ratio, attributed to the gate-bias-induced modulation of the lateral (or horizontal) Schottky barrier height [31, 32]. Nevertheless, the improvement of the effective aperture ratio (*AR*eff, the percentage ratio of the current efficiency of a surface emitting OLET to that of a control ITO-base OLED at a given luminance) in the

#### **Figure 1.**

*(a) A conventional two TFTs + one capacitor AM-OLED pixel circuit diagram with a switching TFT and driving TFT. (b) A simple AM-OLET pixel circuit of an integrated OLET and a switching TFT.*

**145**

*Vertical-Type Organic Light-Emitting Transistors with High Effective Aperture Ratios*

CNT-VOLET is still limited to less than those of control OLEDs, and its parasitic power consumption (*PPC*, percentage of power dissipated across the driving transistor element of the device not contributing to light generation) requires further reductions [32]. Moreover, this attempt resulted in a complicated source structure, and the production of porous CNT network sources with smooth and homogeneous surfaces was problematic due to the aggregation of CNTs [35]; significant obstacles thus remain with regard to the limited reproducibility of these devices. Thus, the goal of high-performance and reliable OLETs with high *AR*eff and low *PPC* values

*The structure of carbon nanotube (CNT), single-layer graphene (SLG), and bilayer graphene (BLG).*

In this chapter, for the VOLET, the use of a nonporous, homogeneous, smooth, and easily processable graphene layer is described as the source contact, together with an emissive channel layer. Here, the graphene is a two-dimensional (2D) material in the form of a single atomic layer of carbon with a hexagonal lattice structure

sionality of graphene to CNTs [36, 37], the optoelectric properties of a VOLET based on graphene have not yet been fully characterized. Herein, the fabrication and characterization are described for a simple VOLET with a single-layer graphene (SLG) source contact (Gr-VOLET), capable of efficiently modulating device perfor-

voltage. The Gr-VOLETs with doped SLG sources with FeCl3 are demonstrated to exhibit greatly improved device performance, especially in their enhanced current efficiency and *AR*eff values of more than 150% of those of a control OLED, even at

est ever published for OLETs, and their low *PPC*s make them all the more attractive. Moreover, such high device performance has also been successfully confirmed even

*Preparation of substrates*: The transparent VOLET substrate used here was prepared

sheet resistance) on a glass substrate, and a sputter-

with a pre-patterned bottom gate electrode, consisting of a transparent ITO layer

deposited aluminum oxide (Al2O3) top layer (400 nm) as a gate dielectric over the ITO gate (glass/ITO/Al2O3). The VOLET substrate used was then cleaned with alcohol, followed by a UV treatment (5 min), prior to the fabrication of the devices. *Synthesis and transfer of SLG*: The procedure used for transferring the chemicalvapor-deposition (CVD)-grown graphene onto a target substrate [40–45], in this case an FET substrate, a VOLET substrate, or a glass substrate, is described below. The first

for micro-VOLET pixels fabricated by an inkjet-patterning technique [39].

mance levels with high luminance on/off ratios (~104

**2. 2D material electrode: doped CVD graphene**

**2.1 Preparation of SLG source electrodes**

(80-nm-thick, 30 ohm square<sup>−</sup><sup>1</sup>

high EL luminance levels exceeding 500 cd m<sup>−</sup><sup>2</sup>

configuration (**Figure 2**) [36–38]. Despite the similar low dimen-

) upon the application of a gate

. These figures are among the high-

*DOI: http://dx.doi.org/10.5772/intechopen.92833*

remains a considerable challenge.

bonded in the sp2

**Figure 2.**

*Vertical-Type Organic Light-Emitting Transistors with High Effective Aperture Ratios DOI: http://dx.doi.org/10.5772/intechopen.92833*

**Figure 2.**

*Liquid Crystals and Display Technology*

efficiency and brightness of these devices.

in **Figure 1(a)** [12, 13]. Perhaps unexpectedly, however, the complexity of such pixel circuit designs with their sophisticated procedures has led to a significantly limited light-emitting area and aperture ratio (the light-emitting area as a fraction of the total area of the device, typical aperture ratios: 25–34%) [13–15], introducing severe problems associated with limited device performance and limited display sizes for AM-OLEDs. Besides these issues, fundamental factors related to the architecture of the OLED itself, such as exciton quenching and photon loss, also still limit the

To overcome some of the limitations of (AM-)OLEDs, research on different structures and materials is currently yielding new developments [15–30]. Among these, organic light-emitting transistors (OLETs), such as static-inductiontransistor OLETs (SIT-OLETs) [17, 18], metal-insulator-semiconductor OLETs (MIS-OLETs) [19], lateral-type OLETs [20–29], and vertical-type OLETs (VOLETs) [30], have been devised by integrating the capability of the OLED to generate EL light with the switching functionality of a field-effect transistor (FET) into a single device structure. In these OLETs, the current that flows through emissive semiconductor channel layers can be controlled by the gate voltage, which can also change the EL emission brightness state from the dark off- to the bright on-state. The on-state implies that holes and electrons injected into the channel layer form excitons that recombine radiatively to generate EL light [17–30]. These OLETs are of key interest; not only do they provide a novel device architecture to investigate fundamental optoelectronic properties related to charge carrier injection, transport, and radiative exciton recombination processes in organic semiconducting materials, at the same time OLETs can also be used to develop highly integrated organic optoelectronic devices such as highly bright and efficient light sources, optical communication systems, and electrically driven organic lasers [21–30]. In principle, the luminance from OLETs can be modulated by the gate voltage without any additional driving devices; thus, displays using OLETs have the advantage of greatly reducing both the number of TFTs and the circuit complexity (**Figure 1(b)**), thereby providing an effective means of increasing the aperture ratio [29]. Hence, OLETs could be a key part of the development of next-generation AM display technology [29]. Indeed, a proof-of-principle device was recently developed using carbon nanotubes (CNTs, **Figure 2**), delivering a CNT-based vertical-type OLET (CNT-VOLET) [31–34]. In the CNT-VOLET, a dilute network of CNTs is used as a source electrode, leading to several improvements, such as a high on/off ratio, attributed to the gate-bias-induced modulation of the lateral (or horizontal) Schottky barrier height [31, 32]. Nevertheless, the improvement of the effective aperture ratio (*AR*eff, the percentage ratio of the current efficiency of a surface emitting OLET to that of a control ITO-base OLED at a given luminance) in the

*(a) A conventional two TFTs + one capacitor AM-OLED pixel circuit diagram with a switching TFT and driving TFT. (b) A simple AM-OLET pixel circuit of an integrated OLET and a switching TFT.*

**144**

**Figure 1.**

*The structure of carbon nanotube (CNT), single-layer graphene (SLG), and bilayer graphene (BLG).*

CNT-VOLET is still limited to less than those of control OLEDs, and its parasitic power consumption (*PPC*, percentage of power dissipated across the driving transistor element of the device not contributing to light generation) requires further reductions [32]. Moreover, this attempt resulted in a complicated source structure, and the production of porous CNT network sources with smooth and homogeneous surfaces was problematic due to the aggregation of CNTs [35]; significant obstacles thus remain with regard to the limited reproducibility of these devices. Thus, the goal of high-performance and reliable OLETs with high *AR*eff and low *PPC* values remains a considerable challenge.

In this chapter, for the VOLET, the use of a nonporous, homogeneous, smooth, and easily processable graphene layer is described as the source contact, together with an emissive channel layer. Here, the graphene is a two-dimensional (2D) material in the form of a single atomic layer of carbon with a hexagonal lattice structure bonded in the sp2 configuration (**Figure 2**) [36–38]. Despite the similar low dimensionality of graphene to CNTs [36, 37], the optoelectric properties of a VOLET based on graphene have not yet been fully characterized. Herein, the fabrication and characterization are described for a simple VOLET with a single-layer graphene (SLG) source contact (Gr-VOLET), capable of efficiently modulating device performance levels with high luminance on/off ratios (~104 ) upon the application of a gate voltage. The Gr-VOLETs with doped SLG sources with FeCl3 are demonstrated to exhibit greatly improved device performance, especially in their enhanced current efficiency and *AR*eff values of more than 150% of those of a control OLED, even at high EL luminance levels exceeding 500 cd m<sup>−</sup><sup>2</sup> . These figures are among the highest ever published for OLETs, and their low *PPC*s make them all the more attractive. Moreover, such high device performance has also been successfully confirmed even for micro-VOLET pixels fabricated by an inkjet-patterning technique [39].
