Unraveling the Impact of Thickness on Active Layer Morphology and Device Performance of Semitransparent Organic Solar Cells: A Comprehensive Study

In organic solar cells (OSCs), the morphology of the active layer plays a significant role in exciton dissociation into charge carriers and their subsequent transportation and extraction. The morphology of the active layer depends upon the thickness, concentration, and pre-post treatments. In this study, we investigated the effect of thickness on the morphology of the active layer and the resulting photovoltaic performance of semitransparent (ST) bulk heterojunction OSCs. We used a blend of PBDB-T (polymer donor) and a well-known nonfullerene small-molecule acceptor ITIC as the active layer for fabricating opaque (OP) and ST OSCs. This study is performed under ambient conditions. To obtain active layer films with thicknesses ranging from 350 to 100 nm, the rotations per minute (rpm) of the spin coating was varied from 1000 to 4000. Through a comprehensive analysis, we identified the optimal rpm for achieving an efficient active layer morphology and improved device performance for both OP and ST devices. We have achieved overall power conversion efficiencies of 10.39 and 7.26% for optimized OP and ST OSCs, respectively. The effect of varying the rpm of the active layer on nonradiative voltage loss is also studied. The correlation between the rpm, active layer morphology, and the overall performance of the ST OSCs is elucidated by our findings, providing valuable insights into optimizing devices. The results of this study could potentially aid in the development of more effective photovoltaic devices.