Insight Into Designing High-Performance Polythiophenes for Reduced Urbach Energy and Nonradiative Recombination in Organic Solar Cells

Abstract

In the pursuit of high-efficiency polythiophene (PT) organic solar cells (OSCs), a critical challenge is the reduction of nonradiative recombination. This study comprehensively explores polydithienylthiazolothiazole (PTTz)-based PT terpolymers: PTTz-Tz and PTTz-TzT, in which it is demonstrate that molecular structure alterations greatly influence the aggregation kinetics and orientation of these polymers. Specifically, PTTz-TzT achieves rapid ordering aggregation during spin coating, effectively suppressing excessive polymer aggregation and facilitating appropriate phase separation upon mixing with the acceptor. Meanwhile, PTTz-TzT inherently adopts a face-on orientation, resulting in more structured pi-pi stacking in the vertical direction after acceptor integration, compared to the intrinsic edge-on orientation of PTTz-Tz. These factors collectively contribute to lower Urbach energy and a substantial reduction of nonradiative recombination in PTTz-TzT-based OSCs, culminating in a high photovoltaic conversion efficiency (PCE) exceeding 16%. Furthermore, a prominent PCE of 19.11% is obtained by PTTz-TzT via ternary blend strategy, which is among the highest values reported for the OSCs. This investigation underscores the significance of aggregation kinetics and orientation in PT-based polymers, especially regarding Urbach energy and nonradiative recombination, and offers novel insights for designing high-performance polythiophene donors. A reduction of the nonradiative recombination in polythiophene-based organic solar cells from the control of aggregation kinetics and orientation is achieved. By backbone design, PTTz-TzT with short drying time and face-on orientation exhibit reduced Urbach energy and nonradiative recombination in solar cells. PCEs higher than 16% in binary and 19% in ternary are achieved. image