Achieving excellent charge balance and transport in low-donor bulk heterojunctions for high-performance semitransparent organic photovoltaics

The trade-off between the average visible transmittance (AVT) and power conversion efficiency (PCE) is a great issue in achieving high-performance semitransparent organic photovoltaics (ST-OPVs). Although realizing a lower donor ratio in the bulk-heterojunction (BHJ) is a promising strategy because it markedly reduces absorption in the visible range, this strategy is counterbalanced by a deteriorated hole transport capability in the BHJ. Here, to simultaneously achieve high PCE and high AVT, we develop hole transport layer (HTL)-free ST-OPVs by directly incorporating a self-assembled hole-selective small molecule, [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz), as an additive into the photoactive blend. A small amount of Me-4PACz in the blend forms a self-organized HTL on top of the indium tin oxide electrode, serving as a hole-selective layer and reducing parasitic absorption losses. More importantly, the high miscibility of Me-4PACz leads to a substantial residence throughout the BHJ layer, thereby suppressing trap-assisted nonradiative recombination. This approach results in optimized ST-OPVs with a high PCE of 10.70% and an AVT of 37.53%, achieving an impressive light utilization efficiency of 4.01% at a donor/acceptor ratio of 1 : 5. We believe our work provides an effective strategy for realizing high-performance ST-OPVs.