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Reassessing the Significance of Reduced Aggregation and Crystallinity of Naphthalene Diimide-Based Copolymer Acceptors in All-Polymer Solar Cells

Published

Author(s)

Eliot Gann, Christopher McNeill, Doan Vu, Martyn Jervic, Chao Wang, Lars Thomsen, Mats Andersson

Abstract

Terpolymerization is a facile and effective strategy to control the aggregation and crystallinity of semiconducting polymers which has been exploited to improve the photovoltaic performance of all-polymer solar cells (all-PSCs). Applying this strategy to the well-studied n-type polymer acceptor PNDI2OD-T2, different amounts of 3-n-octylthiophene (OT) are used to partially replace the bithiophene (T2) unit, resulting in three newly-synthesized terpolymer acceptors PNDI-OTx where x = 5%, 10%, or 15%. Another copolymer, namely PNDI2OD-C8T2, consisting of naphthalene diimide (NDI) copolymerised with 3-n-octylbithiophene (C8T2) is also synthesized for comparison. It is found that the solution aggregation and thin film crystallinity of PNDI-OTx are systematically tuned by varying x, evidenced by temperature-dependent UV-Vis and grazing incidence wide angle X-ray scattering (GIWAXS) measurements. PNDI2OD-C8T2 is also found to have reduced solution aggregation and thin film crystallinity relative to P(NDI2OD-T2). However, the photovoltaic performance of all-PSCs based on J71:PNDI-OTx and J71:PNDI2OD-C8T2 blends are much lower than that of the reference J71:PNDI2OD-T2 system. Extensive morphological studies suggest that reduced aggregation and crystallinity do not guarantee a more favourable blend morphology, with coarser phase separation found in J71:PNDI-OTx and J71:PNDI2OD-C8T2 blends compared to J71:PNDI2OD-T2 blends. The results here challenge the common view that reduced aggregation and crystallinity are the key parameters in controlling the morphology for enabling high-performing all-PSCs. Instead, enhanced compatibility between donor and acceptor to generate a favourable morphology with well-balanced electron and hole mobilities are important underlying factors necessary for enhancement of the photovoltaic performance of terpolymer acceptors in all-PCSs.
Citation
ACS Applied Polymer Materials

Keywords

all-polymer solar cells, terpolymer acceptors, crystallinity, aggregation, miscibility.

Citation

Gann, E. , McNeill, C. , Vu, D. , Jervic, M. , Wang, C. , Thomsen, L. and Andersson, M. (2022), Reassessing the Significance of Reduced Aggregation and Crystallinity of Naphthalene Diimide-Based Copolymer Acceptors in All-Polymer Solar Cells, ACS Applied Polymer Materials, [online], https://doi.org/10.1021/acsapm.1c01822, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=932876 (Accessed February 29, 2024)
Created March 28, 2022, Updated November 29, 2022