Photocurrent Amplification in Bulk Heterojunction Organic Phototransistors with Different Donor–Acceptor Ratio

In this paper, the amplification of the photocurrent in organic phototransistors (OPTs) employing bulk heterojunctions (BHJs) with different donor–acceptor ratios has been studied. As a model system, poly[2‐methoxy‐5‐(3’,7’‐dimethyloctyloxy)‐p‐phenylene vinylene] (OC₁C₁₀‐PPV) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) are used as donor and acceptor, respectively, combined with Al contact. It is found that for OPTs the mobility of the majority carrier is the dominating factor for maximizing the photocurrent. This is different from organic photodiodes (OPDs) where balanced hole and electron mobilities are important. For OPDs using the same model system, the maximum photocurrent is obtained for a PCBM content of 80 wt.% while for OPTs, it retained its photocurrent from PCBM 80 wt.% up to PCBM 100 wt.%. In addition, BHJ transistors respond more rapidly to changes in the illumination intensity than transistors with a pure PCBM layer.