Early-Effect like Behavior in Space Charge Regions of Organic Bulk-Heterojunction Photodiodes
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Early-Effect like Behavior in Space Charge Regions of Organic Bulk-Heterojunction Photodiodes Ali Bilge Guvenc1, Cengiz Ozkan2, 3, and Mihrimah Ozkan1 1 Department of Electrical Engineering, University of California Riverside, Riverside, CA 92521, U.S.A. 2 Department of Mechanical Engineering, University of California Riverside, Riverside, CA 92521, U.S.A. 3 Material Science and Engineering Program, University of California Riverside, Riverside, CA 92521, U.S.A.
ABSTRACT The space charge region width of the Schottky barrier that forms on the interface between aluminum and organic semiconductor polymer of bulk-heterojunction organic photodiodes has been investigated according to reverse voltage bias over the device and the capacitance-voltage characteristics. Here, we investigated the space charge region widths according to incident light power. Comparison of the mathematical models and experimental data measured under different light power indicate that effect of light on the space charge region of photodiodes is similar to the effect of base-emitter voltage on the space charge region of base-emitter junction in bipolar junction transistors.
INTRODUCTION The space charge region (SCR) width of the Schottky barrier (SB) forms on the interface between aluminum and organic semiconductor polymer of bulk-heterojunction (BH) organic photodiodes (OPD) based on poly(3-hexylthiophene) (P3HT) : [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend, have been investigated according to reverse voltage bias (RVB) over the OPD. In this paper, we focused on the effect of incident light power (ILP) on these SCRs and I-V characteristics of the devices. Comparison of the models and experimental data measured under different ILPs indicates a dependency of SCR to the ILP. The most commonly used structure for the BH OPDs is the indium tin oxide (ITO)/poly(ethylene-dioxythiophene):poly(styrenesulphonate) (PEDOT:PSS)/P3HT:PCBM/Aluminum.1 During the fabrication process, P3HT:PCBM layer exposes to oxygen and moisture because of remaining water molecules at the interfaces. As previous studies show that P3HT can easily be p-type doped under these circumstances2 and doping concentration is directly related to the oxygen concentration in P3HT.3,4 The doping makes it possible to form SBs at the metal contacts of the devices. These barriers can be detected by Mott-Schottky characteristics.5 The SCRs on SBs forming only in the semiconductor side so these barriers behave as single junction Schottky diodes.5 Here we studied the effect of ILP on
the SCRs of the SBs, the I-V characteristics and performance of the devices and built an I-V model for the devices that involves these effects. EXPERIMENT
Figure 1. (a) Comparison of I – V models given in the equations (1), (3) and (6) with the experimental data, inset: log10(I)-V under AM1.5G - 1 Sun incident light. (b) Mott-Schottky Characteristic, inset (left): SB C – V measurement (98 Hz) data for 200 nm P3HT:PCBM thickness, inset (right): Calculation result of SCR width For keeping the effects of d
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