Understanding the Morphology of High-Performance Solar Cells Based on a Low-Cost Polymer Donor

A low-cost and high-performance bulk heterojunction (BHJ) solar cell comprising an emerging polymer donor, poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)] (PTQ10), shows an efficiency of 12.7%. To improve the performance of the solar cells, a better understanding of the structure-property relationships of the PTQ10-based devices is crucial. Here, we fabricate PTQ10/nonfullerene and fullerene BHJ devices, including PTQ10/IDIC, PTQ10/ITIC, and PTQ10/PC₇₁BM, processed with or without thermal annealing and additive and provide detailed descriptions of the relationships between the morphology and performance. PTQ10 is found to be highly miscible with nonfullerene IDIC and ITIC acceptors and poorly miscible with fullerene PC₇₁BM acceptors. Thermal annealing promotes the crystallization of PTQ10 and phase separation of all PTQ10/IDIC, PTQ10/ITIC, and PTQ10/PC₇₁BM devices, leading to an increased power conversion efficiencies (PCEs) of the PTQ10/IDIC and PTQ10/ITIC devices but a decreased PCE of PTQ10/PC₇₁BM devices with 1,8-di-iodooctane (DIO) additive. Without thermal annealing, DIO greatly improves the morphology of PTQ10/PC₇₁BM, leading to a higher PCE. The results show that the degree of phase separation and ordering in the PTQ10-based devices significantly influences device performance. The morphology-property correlations demonstrated will assist in the rational design of these low-cost polymer donor-based solar cells to achieve even higher performance.