Steric hindrance of organotin compounds in controlling the batch-to-batch variance of photovoltaic polymer donors

Abstract

Owing to its advantages of mild reaction conditions and a single reaction system, Stille coupling has become the main method of developing high-performance photovoltaic polymers. However, Stille coupling polycondensation, following a step-growth polymerization mechanism, still presents challenges in controlling the molecular weights of the polymers, leading to significant batch-to-batch variance. Herein, a strategy based on steric effects was applied to reduce molecular weight fluctuations using the large steric groups of organotin compounds to increase the difficulty of forming the transmetalation transition state. Consequently, we conducted competition experiments with small molecules and synthesized three polymers (PDF-1, PDF-2, and PDF-3) using BDF-based organotin compounds with varying steric hindrance. Theoretical calculations proved that the steric hindrance of organotin compounds significantly influenced the transition state in the transmetalation process. Device measurements revealed that the larger steric hindrance of organostannides could produce polymers with concentrated molecular weights, resulting in only a slight change in the PCEs. Although excessive steric hindrance could affect the photovoltaic properties, leading to lower PCEs, appropriate steric control of organostannides could yield polymer donors with high performance and low batch-to-batch variance. Therefore, this work provides guidelines for developing polymers with minimal batch-to-batch variance.