Understanding of Imine Substitution in Wide-Bandgap Polymer Donor-Induced Efficiency Enhancement in All-Polymer Solar Cells

Abstract
All-polymer solar cells (all-PSCs) are proven to possess outstanding thermal and mechanical stabilities. However, concurrently achieving appropriate phase-separated pattern, efficient charge transportation, and adequate charge transfer between donor and acceptor components is still a challenge, and thus, only a few polymer-polymer bulk heterojunction (BHJ) blends have yielded BHJ device power conversion efficiency (PCE) values of >8%. Generally, polymer backbone substitutions may have a direct influence on the device performance. Thus, this report examines a set of wide bandgap polymer donor analogues composed of thienothiophene (TT) or thiazolothiazole (TTz) motif, and their all-PSC device performance with N2200. Results show that all-PSCs based on the imine-substituted derivative PBDT-TTz exhibit PCE values as high as 8.4%, which largely outperform the analogue PBDT-TT-based ones with PCEs of only 0.7%. This work reveals that the imine substitution in polymer backbones of PBDT-TTz not only increases the ionization potential (IP) and electron affinity (EA), narrows the optical gap (Eopt), but also has significantly impacts on the BHJ film morphologies. PBDT-TTz:N2200 BHJ blends present better miscibility, suppressed phase separation, much stronger crystallinity, and face-on ordering, which contribute to efficient exciton dissociation, charge transportation, and therefore, high-efficiency in all-PSCs. This study demonstrates that the imine-substituted polymers composed of TTz motif, which can be easily synthesized through a facile two-step procedure, are a promising class of wide-bandgap polymer donors for efficient all-PSCs.

Citation
Cao, Z., Chen, J., Liu, S., Qin, M., Jia, T., Zhao, J., … Cao, Y. (2019). Understanding of Imine Substitution in Wide-Bandgap Polymer Donor-Induced Efficiency Enhancement in All-Polymer Solar Cells. Chemistry of Materials, 31(20), 8533–8542. doi:10.1021/acs.chemmater.9b03570

Acknowledgements
The authors thank Dawei Wang (Sun Yat-Sen University),Yuexing Zhang (Hubei University), Xin Song (King Abdullah University of Science & Technology (KAUST)), Lingling Shui (SCNU), Jinwei Gao (SCNU), and Wei Wei (SCNU) for help in XRD, DFT calculation, PESA, contact angle, thin film thickness, and PL measurements, respectively. The authors thank SCNU Analysis & Testing Center for technical support.

Publisher
American Chemical Society (ACS)

Journal
Chemistry of Materials

DOI
10.1021/acs.chemmater.9b03570

Additional Links
https://pubs.acs.org/doi/10.1021/acs.chemmater.9b03570

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