Precisely Controlling the Position of Bromine on the End Group Enables Well‐Regular Polymer Acceptors for All‐Polymer Solar Cells with Efficiencies over 15

• Published in: Advanced materials (Weinheim) 2020-12, Vol.32 (48), p.e2005942-n/a
• Main Authors: Luo, Zhenghui, Liu, Tao, Ma, Ruijie, Xiao, Yiqun, Zhan, Lingling, Zhang, Guangye, Sun, Huiliang, Ni, Fan, Chai, Gaoda, Wang, Junwei, Zhong, Cheng, Zou, Yang, Guo, Xugang, Lu, Xinhui, Chen, Hongzheng, Yan, He, Yang, Chuluo
• Format: Article
• Language: English
• Subjects: Absorption; all‐polymer solar cells; Bromination; Bromine; Charge transport; Electron mobility; Electron transport; Energy conversion efficiency; isomeric end groups; Isomers; Materials science; Morphology; Photovoltaic cells; polymer acceptors; Polymerization; Polymers; power conversion efficiency; small‐molecule acceptors; Solar cells; Terpolymers
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202005942

Recent advances in the development of polymerized A–D–A‐type small‐molecule acceptors (SMAs) have promoted the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs) over 13%. However, the monomer of an SMA typically consists of a mixture of three isomers due to the regio‐isomeric brominated end groups (IC‐Br(in) and IC‐Br(out)). In this work, the two isomeric end groups are successfully separated, the regioisomeric issue is solved, and three polymer acceptors, named PY‐IT, PY‐OT, and PY‐IOT, are developed, where PY‐IOT is a random terpolymer with the same ratio of the two acceptors. Interestingly, from PY‐OT, PY‐IOT to PY‐IT, the absorption edge gradually redshifts and electron mobility progressively increases. Theory calculation indicates that the LUMOs are distributed on the entire molecular backbone of PY‐IT, contributing to the enhanced electron transport. Consequently, the PM6:PY‐IT system achieves an excellent PCE of 15.05%, significantly higher than those for PY‐OT (10.04%) and PY‐IOT (12.12%). Morphological and device characterization reveals that the highest PCE for the PY‐IT‐based device is the fruit of enhanced absorption, more balanced charge transport, and favorable morphology. This work demonstrates that the site of polymerization on SMAs strongly affects device performance, offering insights into the development of efficient polymer acceptors for all‐PSCs. Two well‐regular polymer acceptors (PY‐IT and PY‐OT) with different polymerization sites are developed. For comparison, a random ternary copolymer (PY‐IOT) with the same ratio of the two acceptors is synthesized. All‐polymer solar cells (PSCs) based on PM6:PY‐IT achieve an excellent PCE of 15.05%, which is significantly higher than those based on PY‐OT (10.04%) and PY‐IOT (12.12%).