Photoconductivity and Photoconductive Gain in Organic Bulk Heterojunction Materials

A comprehensive study on photoconductive gain in organic bulk heterojunction (OBHJ) materials is reported. High photoconductive gains in OBHJs occur in operating regions where the device characteristics are dominated by photomodulated contact injection effects. The photomodulated injection current contributes to the total device current by augmenting the photocurrent resulting from photoabsorption in the device. An experimentally confirmed simulation of bulk photoconduction in lateral P3HT:PC₇₁BM bulk heterojunction blend devices is presented, and it clarifies the origins and operating regimes in these devices. High gains occur at lower light intensities, where the total device current is dominated by photomodulated contact injection rather than bulk photoconduction effects. At higher light intensities, the total device current is dominated by bulk photoconduction effects, where the photocurrent results from light absorption in the bulk material. In agreement with previous research, gains also increase with higher applied biases. The inclusion of field-dependent mobilities plays a critical role in accurately modeling this relationship between applied voltage bias and gain.