Comparative Analysis of Organic Thin Film Transistor Structures for Flexible E-Paper and AMOLED Displays

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1 Comparative Analysis of Organic Thin Film Transistor Structures for Flexible E-Paper and AMOLED Displays Linrun Feng, Xiaoli Xu and Xiaojun Guo ECS Trans. 2011, Volume 37, Issue 1, Pages doi: / alerting service Receive free alerts when new articles cite this article - sign up in the box at the top right corner of the article or click here To subscribe to ECS Transactions go to: ECS - The Electrochemical Society

2 / The Electrochemical Society Comparative Analysis of Organic Thin Film Transistor Structures for Flexible E-Paper and AMOLED Displays Linrun FENG, Xiaoli XU and Xiaojun GUO Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, , China Organic thin-film transistors (OTFTs) have attracted considerable attention in applications for driving flexible e-paper and active matrix organic light-emitting diode (AMOLED) displays. In the systems, the pixel electrode, which connects the bottom electrode of the display media to the drain or source of the switch OTFT in the e-paper display, and the driving OTFT in the AMOLED display, will form parasitic effects with the OTFTs. Through numerical device simulations, it is found that, in the bottomcontact bottom-gate (BCBG) structure OTFT backplane, the presence of the pixel electrode may result in a shift of the transfer characteristics and a significant decrease of the output impedance. Although the DC electrical characteristics of the bottom-contact top-gate (BCTG) OTFTs are not affected by the presence of the pixel electrode, the BCTG structure has a larger parasitic capacitance, which can cause a higher feed-through voltage for performing switching in the e-paper displays. Introduction Owning to their excellent intrinsic flexibility and capability of being manufactured by cost-effective solution or printing processes at a low temperature, organic thin-film transistors (OTFTs) have attracted considerable attention in applications for driving flexible e-paper and active matrix organic light-emitting diode (AMOLED) displays. In the last decade, great efforts have been paid to develop high carrier mobility, chemically and physically stable organic semiconductor materials, which can now meet general requirements of driving e-paper and AMOLED displays. It has also been well proved that the device structure can significantly affect the OTFT s electrical performance (1, 2). Generally, the top-contact (TC) structure OTFTs can form better contacts between the metal electrode and the semiconductor layer than the bottom-contact (BC) ones to provide more efficient charge injection (3). However, bottom-contact structures are still more popularly used in circuit integration of OTFTs for display backplanes because of the process difficulty of making source/drain metal contacts onto the organic semiconductor layer with precise patterning (4-6).With respect of the relative locations of the gate electrode and the source/drain electrodes, BC OTFTs can be realized in two configurations: BC top gate (BCTG, also named inverted staggered structure), and BC bottom gate (BCBG, also named coplanar structure). Both structures have been widely employed in applications of display backplanes depending on process integration preferences (7, 8). As shown in Fig. 1, in e-paper display applications, the OTFT performs as a switch, while in AMOLED displays, two types of OTFTs are required for switching and current 105

pixel until the display is refreshed. The driving OTFTs in AMOLED displays are needed to provide uniform and stable current, as analog current sinks or current sources, to drive the OLEDs (9, 10).

To integrate the OTFT backplane with the front plane of display media for a completed display system, the pixel electrode, which works as the bottom electrode of the front plane, is connected to the

Since the pixel electrode is directly on top of the OTFTs separated by an interlayer dielectric (ILD) layer of a certain thickness, the resulted parasitic capacitance between the pixel electrode and

3 driving functions, respectively. The switching OTFTs in both displays need to have a high enough on-off current ratio to charge the pixel to the operational voltage within the line selection period and to hold the charge on the pixel until the display is refreshed. The driving OTFTs in AMOLED displays are needed to provide uniform and stable current, as analog current sinks or current sources, to drive the OLEDs (9, 10). Figure 1. Typical pixel circuits for the e-paper display and the AMOLED display. To integrate the OTFT backplane with the front plane of display media for a completed display system, the pixel electrode, which works as the bottom electrode of the front plane, is connected to the drain or source of the switching OTFT in the e-paper display, and the driving OTFT in the AMOLED display. Since the pixel electrode is directly on top of the OTFTs separated by an interlayer dielectric (ILD) layer of a certain thickness, the resulted parasitic capacitance between the pixel electrode and the intrinsic part of the OTFTs may affect their functions of switching or current driving. In this work, the parasitic effects caused by pixel electrode in both BCBG and BCTG OTFT backplanes will be carefully investigated and compared. Simulation Methods Two-dimensional numerical simulations were performed using the commercial software Atlas vended by SILVACO in this study to exclude any influence of process induced effects (11). Although originally developed for silicon and inorganic devices, Atlas allows user-defined semiconductor materials and has been proved to be a useful tool for studying device physics of OTFTs (1). The BCBG and BCTG device structures used in the simulations are given in Fig. 2, with the pixel electrode being electrically connected to the drain electrode. The devices have a 50 nm thick channel with the length of 10 μm, 100 nm thick source/drain electrodes with the length of 10 μm, and a gate insulator of 300 nm with the dielectric constant of 4.0. The thickness of the interlayer dielectric ( ) between the pixel electrode and the intrinsic part of the OTFTs is varied, and the dielectric constant is also set to be 4.0. The channel width is 1 μm. The usual values of pentacene (energy gap of 2.5 ev, ionization potential of 5eV and dielectric constant of 4.0) have been used for the organic semiconductors as the channel 106

(12). The field-dependent hole mobility is described by the Poole-Frenkel model, which can be expressed as: μ E/ E0 = μ0e [1] where µ 0, the low field mobility, is set to be 0.

4 (12). The field-dependent hole mobility is described by the Poole-Frenkel model, which can be expressed as: μ E/ E0 = μ0e [1] where µ 0, the low field mobility, is set to be cm 2 /V S, E is the electrical field and E 0 is a characteristics parameter equal to V/cm. The effective density of the states (N V ) is set to be cm -3. Simulations based on these models and parameters have been proved to be able to get the results well fitting with the experimental data (12). Neither bulk semiconductor trap states nor interfacial trap states have been included in the simulations, which will not affect the qualitatively comparative study in this work. Figure 2. Schematic of the device structures used in the simulation: bottom-contact bottom-gate (BCBG) structure with the pixel electrode and bottom-contact top-gate (BCTG) structure with the pixel electrode. By varying, the switching and current driving performance of BCBG and BCTG OTFTs are fully investigated and compared for applications in e-paper and AMOLED displays. Switching Performance Results and Discussions The simulated transfer characteristics ( - ) of both BCBG and BCTG OTFTs at different are given in Fig. 3. For the BCBG OTFT, it s obviously observed from Fig.3 that the presence of the pixel electrode results in a shift of the transfer characteristics to the positive as the drain bias is increased. The magnitude of the shift decreases with the increase of the. The transfer characteristics of the BCTG OTFT are not affected by the presence of the pixel electrode, as shown in Fig. 3, which is attributed to the electrical shielding of the channel from the effects of the pixel electrode by the top gate. 107

5 t 10-9 ILD 10-9 = 500nm t 10-9 ILD 10-9 = 5000nm Figure 3. Simulated transfer characteristics of the OTFTs in BCBG structure and BCTG structure, without the pixel electrode, and with the pixel electrode in the cases of interlayer dielectric thickness ( ) of 500 nm and 5000 nm respectively. In the BCBG structures, the pixel electrode covers the channel region and acts as the second gate, which makes the device similar to a double-gate transistor. As a drain bias is added, an additional conductive channel will be formed near the interface between the ILD layer and the organic semiconductor layer, as shown in Fig. 4, and become more conductive with the increase of the drain bias. Therefore, there is a shift of the transfer characteristics and the drain current at the same gate bias voltage is increased. The phenomena have also been experimentally demonstrated in double-gate OTFTs, where the presence of the second gate with a certain voltage bias can cause a shift of the threshold voltage (13, 14). The increase of will reduce the influence of the pixel electrode, and can thus bring a smaller shift of the - characteristics. 108

6 Additional conductive channel induced by the pixel electrode bias Figure 4. Schematic diagrams of the formed conductive channels in the BCBG structure OTFTs: without the pixel electrode and with the pixel electrode. Both the gate and drain biases are -8V When the OTFT works as a switch in e-paper or AMOLED displays, a high enough on-off current ratio should be met. For the BCTG structure, the presence of the pixel electrode does not cause any changes to the - electrical characteristics, so no additional design consideration is needed. But for the BCBG structure, according to the results in Fig. 3, a wider gate voltage swing must be used to enable the device to be turned on and off as required. The exact value of the voltage swing depends on how much the shift of the - characteristics is, which is a function of both the maximum input data voltage and. For the application of the OTFT as a switch, a small feed-through voltage is also important. The feed-through voltage (ΔV ft ) can be expressed as: Δ V = V C /( C + C ) [2] ft gw gd gd s where V gw is the gate voltage swing, C gd is the gate-to-drain parasitic capacitance and C S is the storage capacitance (as shown in Fig. 1). Based on [2], a larger C S can be designed to reduce ΔV ft, which, however, is limited by the pixel area and the strict requirement of the fast charging time. Therefore, as seen from equation [2], it s vital to minimize the C gd in the design to reduce ΔV ft. The simulated characteristics of gate-to-drain parasitic capacitance C gd as a function of V gd are given in Fig. 5, the drain electrode is zero biased, for both BCBG and BCTG structures with different values of. In the whole operation regimes, the BCBG structure owns smaller C gd than the BCTG one. For the BCTG structure, additional capacitance is formed between the pixel electrode and the gate electrode, therefore a larger C gd is induced, and increases with the decrease of. When the OTFTs are operated in the ON state with a negative gate bias, C gd in the BCBG structure does not change with the presence of the pixel electrode and 109

7 the variations of, since the pixel electrode is shielded from the gate by the conductive channel. When the OTFTs are turned off by a positive gate bias, C gd of both structures increases as the decreases. For the BCBG structure, since there is no conductive channel formed in the OFF state, an additional capacitance is induced between the pixel and gate electrode through the multi-layer dielectric composed of the ILD layer, organic semiconductor layer and the gate insulator layer. C gd (ff/μm) = 300 nm = 1000 nm V gd C gd (ff/μm) = 300 nm = 1000 nm V gd Figure 5. The simulated gate-to-drain parasitic capacitance (C gd ) as a function of V gd for BCBG structure and BCTG structure OTFTs, without the pixel electrode, and with a pixel electrode in the cases of interlayer dielectric thickness ( ) of 300, 500, 1000 and 5000 nm. In a summary for this part, to act as a switch in e-paper or AMOLED displays, the BCTG OTFT structure owns the advantage of no shift of - characteristics with the presence of the pixel electrode, but induces a larger C gd. To reduce the parasitic capacitance, a thicker ILD layer is required. For the BCBG OTFT structure, the C gd is much smaller, but the design needs to increase the gate voltage swing considering the shift of the - characteristics with the increase of the drain bias. Current Driving Performance When the OTFT works as a current source or sink in the AMOLED display, a high output impedance in the saturation regime is needed to provide a stable current without being affected by variations of the drain bias. As stated in the above, for the BCTG structures, since the channel is shielded from the pixel electrode by the gate, the output impedance is not affected by the pixel electrode. The following will mainly discuss the case for the BCBG structure. Fig. 6 shows the effects of the pixel electrode on the output characteristics of the BCBG OTFT at different. The output impedance is degraded with the presence of the pixel electrode, due to the additional channel formed at the interface between the ILD layer and the organic semiconductor layer, as already illustrated in Fig. 4. Even when the is increased to 5000 nm, there is still a significant decrease of the output impedance compared to that without the pixel electrode. 110

8 = 300 nm = 1000 nm = 300 nm = 1000 nm Figure 6. The simulated output characteristics of the BCBG structure OTFT without the pixel electrode, and with the pixel electrode in the cases of interlayer dielectric thickness ( ) of 300, 500, 1000 and 5000 nm: =-4V and =-8V Therefore, for the current driving application in the AMOLED display, the BCBG structure suffers the degraded output impedance due to the presence of the pixel electrode, while the BCTG structure does not have this issue. To use the BCBG OTFT for highperformance AMOLED displays, a very thick ILD layer is required to effectively suppress the parasitic effects, which, however, may increase the process difficulty to form the via hole connecting the pixel electrode and the drain electrode of the OTFT. Conclusions In this work, a comparative analysis of BCBG and BCTG OTFT structures for e- paper and AMOLED displays has been carried out. In the BCBG structure backplane, the presence of the pixel electrode results in a shift of the transfer characteristics and degradation of the output impedance of the OTFTs. When the device works as a switch in the e-paper display, a wider gate voltage swing can be designed to compensate the shift of the transfer characteristics. But for AMOLED drive OTFT applications, a very thick ILD layer is needed to effectively suppress the parasitic effects induced by degradation of the output impedance, which, however, might increase the process difficulty. In the BCTG structure backplane, the electric characteristics of the OTFTs are not affected by the presence of the pixel electrode, but a larger parasitic capacitance C gd can cause a higher feed-through voltage for performing switching in the e-paper displays, which needs to be considered in the design. 111

9 Acknowledgments The work is supported by The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Program for New Century Excellent Talents (NCET) in University in China, and the NSFC of China (Grant No , ). References 1. C. H. Shim, et al., IEEE Trans. Electron Devices, 57, 195 (2010) 2. D. J. Gundlach, et al., Journal of Applied Physics, 100, (2006) 3. I. G. Hill, Appl. Phys. Lett., 87, (2005) 4. J. Yuan, et al., Appl. Phys. Lett., 82, 3967 (2003) 5. M. Mizukami, et al., IEEE Electron Device Letters 27, 249 (2006) 6. H. Yan, et al., Appl. Phys. Lett., 87, (2005) 7. S. E. Burns, et al., Journal of the SID, 13(7), 583 (2005) 8. I. Yagi, et al., Journal of the SID 16(1), 15 (2008) 9. R. A. Street, Adv. Mater., 21, 2007 (2009) 10. G. Gelinck, et al., Adv. Mater., 22, 1 (2010) 11. ATLAS User s Manual, Silvaco Int. Inc., Santa Clara, CA, (2005) 12. A. Bolognesi, et al., IEEE Trans. Electron Devices 51, 1997, (2004) 13. G. H. Gelinck, et al., Appl. Phys. Lett., 87, (2005) 14. S. Iba, et al., Appl. Phys. Lett., 87, (2005) 112

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