Rational Design of Low Band Gap Polymers for Efficient Solar Cells with High Open-Circuit Voltage: The Profound Effect of Me and Cl Substituents with a Similar van Der Waals Radius.

Generally, low band gap material-based photovoltaic devices have reduced open circuit voltage (), and realizing the trade-off between the low band gap ( < 1.6 eV) and high (>0.9 V) could be critical to give efficient polymer solar cells, especially for high-performance semitransparent PSCs and tandem solar cells. Although lots of efforts have been made to address the issue, most results have not been gratifying. In this work, the polymer PTBTz-Cl based on the chlorination method and efficient thiazole-induced strategy was designed and synthesized, aiming at the deep HOMO energy level, and the enhanced backbone planarity caused by the weak noncovalent Cl···S interaction. In addition, the methyl-substituted polymer PTBTz-Me was constructed as the reference due to the similar van der Waals radius of the side chain (CH: 0.20 nm vs Cl: 0.18 nm). Encouragingly, in comparison with that of PTBTz-2, the newly synthesized polymers exhibit the red-shifted absorption spectra ranging from 300 to 770 nm, with an obviously reduced of ∼1.6 eV. However, the function of Cl and Me substituents is different. Compared to the polymer PTBTz-Me, PTBTz-Cl exhibits a lower HOMO value, stronger crystallinity, and more compact intramolecular interactions. Consequently, the polymer PTBTz-Cl exhibits excellent photovoltaic performance with a notable of 0.94 V and a power conversion efficiency of 10.35%, which is ∼11% higher than the 9.12% efficiency based on PTBTz-Me, and is also one of the highest values among polymer/fullerene solar cells. Moreover, a smaller photo energy loss () of 0.64 eV is achieved, which is rare among the current high-performance polymer systems.