P3HT Molecular Weight Determines the Performance of P3HT:O-IDTBR Solar Cells

Large-scale production of organic solar modules requires low-cost and reliable materials with reproducible batch-to-batch properties. In case of polymers, their (photo)physical properties depend strongly on the polymers’ molecular weight. Here, we study the impact of the molecular weight of the donor polymer poly(3-hexylthiophene) (P3HT) on the photophysics in blends with a recently-developed rhodanine-endcapped indacenodithiophene non-fullerene acceptor (IDTBR), a bulk heterojunction system that can potentially fulfill the aforementioned criteria for large-scale production. We find that the power conversion efficiency (PCE) increases when the weight-average molecular weight (MW) is increased from 17kDa (PCE: 4.0%) to 34 kDa (PCE: 6.6%), while a further increase in MW leads to a reduced PCE of 4.4%. We demonstrate that the charge generation efficiency, as estimated from time-delayed collection field (TDCF) experiments, varies with the P3HT molecular weight and is the reason for the differences in photocurrent and device performance. Our findings provide insight into the fundamental photophysical reasons of the molecular weight dependence of the power conversion efficiency, which has to be taken into account when using polymer-based non-fullerene acceptor blends in solar cell devices and modules.