Achieving an Efficiency Exceeding 10% for Fullerene‐based Polymer Solar Cells Employing a Thick Active Layer via Tuning Molecular Weight

Abstract Recently, the influence of molecular weight (M_n) on the performance of polymer solar cells (PSCs) is widely investigated. However, the dependence of optimal thickness of active layer for PSCs on M_n is not reported yet, which is vital to the solution printing technology. In this work, the effect of M_n on the efficiency and especially optimal thickness of the active layer for PBTIBDTT‐S‐based PSCs is systematically studied. The device efficiency improves significantly as the M_n increases from 12 to 38 kDa, and a remarkable efficiency of 10.1% is achieved, which is among the top efficiencies of wide‐bandgap polymer:fullerene PSCs. Furthermore, the optimal thickness of the active layer is also greatly increased from 62 to 210 nm with increased M_n. Therefore, a device employing a thick (>200 nm) active layer with power conversion efficiency exceeding 10% is achieved by manipulating M_n. This exciting result is attributed to both the improved crystallinity, thus hole mobility, and preferable polymer orientation, thus morphology of active layer. These findings, for the first time, highlight the significant impact of M_n on the optimal thickness of active layer for PSCs and provide a facile way to further improve the performance of PSCs employing a thick active layer.