Optimization of PDTS-DTffBT-Based Solar Cell Performance through Control of Polymer Molecular Weight

Abstract

Herein, we investigated the effect of molecular weight (MW) on the efficiency of PDTS-DTffBT based polymer solar cells (PSCs). PDTS-DTffBTs with three different MWs were synthesized by controlling the polymerization conditions. The performance of PSCs improved significantly as the number-average molecular weight (Mn) increased from 9 to 36 kDa. Combined with UV–vis absorption and electrochemical cyclic voltammetry measurements, the absorption properties and frontier orbital energy levels of the polymers were estimated, indicating the red-shifted light absorption and up-shifted highest occupied molecular orbital (HOMO) energy level when MW increased. PDTS-DTffBT with high MW also provided increased charge mobility, smoother film surface, and reduced domain size in morphology of the PDTS-DTffBT:PC71BM active layer. The performance of PDTS-DTffBT based PSCs was improved owing to these MW related properties, and both short circuit current density (JSC) and power conversion efficiency (PCE) went up significantly with increasing MW. The best PCE of 6.40% was achieved by the devices based on the PDTS-DTffBT with a Mn value of 36 kDa.