Progresses of Molecularly Doped Semitransparent Organic Solar Cells

Developing the semitransparent solar cells is crucial to large-scale solar energy utilization in modern city. The photoactive layer of organic solar cells (OSCs) is composed of the donor and acceptor materials with complementary absorptions, where reducing the donor content is facile to enhance the visible-light transmittance. However, the donor dilution approach inevitably inhibits the photo-charge generation and collection, which leads to the critical trade-off between optical transparency and power conversion efficiency (PCE) in semitransparent OSCs (ST-OSCs). Herein, we briefly review the recent progresses on this issue, especially focusing on the molecularly doped ST-OSCs. We try to clarify two pivotal questions: (1) How to achieve effective molecular doping in organic photoactive layer? (2) How does molecular doping optimize photovoltaic process in the suboptimal blend film morphology? Following by the two questions, the basic knowledge of molecular doping is introduced, including doping mechanism (oxidation/reduction dopants and acid/base dopants) and representative P-type and N-type molecular dopants. Next, effective doping requires precise control over the distribution of dopants within the active layer, namely P-type dopants in the donor material and N-type dopants in the acceptor material. The concept of sequential doping processes and using dopant’s carrier, like solid solvent for dopants are explored to enhance electrical performance. In response to the reduction in donor content and the obstruction of hole transport, molecular doping improves in two aspects: trap filling and built-in electric field profiling, which enhances the fill factor (FF) and optimizes charge collection. As the donor content further decreases below 10%, exciton loss by failing to diffuse to the heterojunction dominates the PCE decay. At this situation, molecular doping can mitigate the exciton loss by enhancing the local electric field and inducing entropy gain at heterojunction. At the end of this review, the future directions of molecularly doped ST-OSCs are briefly discussed, including the doping efficiency, bipolar doping, and high doping concentration achievement.