Introduction of Siloxane-Terminated Side Chains into Semiconducting Polymers To Tune Phase Separation with Nonfullerene Acceptor for Polymer Solar Cells

Qian Wang; Zelong Hu; Zhuhao Wu; Yanrui Lin; Lianjie Zhang; Linlin Liu; Yuguang Ma; Yong Cao; Junwu Chen

doi:10.1021/acsami.9b18963

Introduction of Siloxane-Terminated Side Chains into Semiconducting Polymers To Tune Phase Separation with Nonfullerene Acceptor for Polymer Solar Cells

ACS Appl Mater Interfaces. 2020 Jan 29;12(4):4659-4672. doi: 10.1021/acsami.9b18963. Epub 2020 Jan 14.

Authors

Qian Wang¹, Zelong Hu¹, Zhuhao Wu¹, Yanrui Lin¹, Lianjie Zhang¹, Linlin Liu¹, Yuguang Ma¹, Yong Cao¹, Junwu Chen¹

Affiliation

¹ Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices , South China University of Technology , Guangzhou 510640 , P. R. China.

PMID: 31898451
DOI: 10.1021/acsami.9b18963

Abstract

In this work, five PTB7-Th-based conjugated polymers (PTB7-Th, PTBSi20, PTBSi25, PTBSi33, and PTBSi100) with different contents of siloxane-terminated pentyl side chain were synthesized, and properties of corresponding blend films with narrow band gap nonfullerene IEICO-4F acceptor were extensively investigated. According to the contact angle testing, the PTB7-Th with 100% alkyl side chain and PTBSi100 100% siloxane-terminated side chain on the benzodithiophene unit showed surface energy values of 40.04 and 34.52 mJ/m², respectively. The results demonstrate that relative to alkyl side chain in PTB7-Th, the siloxane-terminated side chain could effectively reduce the surface energy of a resulting polymer. Based on Flory-Huggins interaction parameter estimations, the miscibility between the polymer and IEICO-4F would vary in an order of PTB7-Th > PTBSi20 > PTBSi25 > PTBSi33 > PTBSi100, suggesting that siloxane-terminated side chain would afford a tunable driving force for phase separation. Transmission electron microscopy and Raman mapping could confirm large bulk domains inside the PTBSi100:IEICO-4F blend film. In polymer solar cells, the blend film of the PTBSi100 with the lowest miscibility to IEICO-4F showed an undesirable power conversion efficiency (PCE) of 8.52%, which was significantly lower than that of 11.23% for PTB7-Th, suggesting that too large phase separation driving force is not beneficial for the device performance. Side-chain random copolymers PTBSi20, PTBSi25, and PTBSi33 for fine tuning could display PCEs of 11.94, 12.61, and 11.80%, respectively, all higher than that of PTB7-Th. Our results not only reveal the big surface energy difference between the siloxane-terminated side chain and the common alkyl side chain but also provide a guideline for side chain engineering of conjugated polymer donors with tunable morphology and optimal matching with a nonfullerene acceptor.

Keywords: conjugated polymer; non-fullerene solar cells; side-chain random copolymer; siloxane-terminated side chain; tunable morphology.