Calculation aided miscibility manipulation enables highly efficient polythiophene:nonfullerene photovoltaic cells

Abstract

Polythiophenes, with merits of low cost and high scalability of synthesis, have received growing interest in organic solar cells. To date, the best-performing polythiophene:nonfullerene solar cells exhibit typical power conversion efficiencies (PCEs) of 10%–12%, which is much lower than those employing PM6- and D18-type polymers. This inferior performance is mostly limited by the improper miscibility between polythiophene and acceptors. Efforts on engineering the molecular structure to systematically tune the miscibility are required. With the aid of group contribution calculations, the miscibility of polythiophene:nonfullerene blend system was finely tuned by varying the ratios of siloxane-terminated chains and alkyl chains in ester-substituted polythiophenes through random copolymerization. Based on a series of the polythiophene and nonfullerene acceptors, the detailed analysis of blend miscibility and performance reveals a surprising anticorrelation between the Flory-Huggins interaction parameter ( χ aa) and the optimal time of solvent vapor annealing for device performance across these systems. Primarily due to the slightly higher χ aa, the blend of PDCBT-Cl-Si5 and ITIC-Th1 results in a record-high PCE of 12.85% in polythiophene:nonfullerene solar cells. The results not only provide a calculation-guided approach for molecular design but also prove that precise control of the miscibility is an effective way to design high-performance polythiophene:nonfullerene blends and beyond.