The challenge with high permittivity acceptors in organic solar cells: a case study with Y-series derivatives

Y-series acceptors have brought a paradigm shift in terms of power conversion efficiencies of organic solar cells in the last few years. Despite their high performance, these acceptors still exhibit substantial energy loss, stemming from their low-permittivity nature. To tackle the energy loss situation, we prepared modified Y-series acceptors with improved permittivities via an alternative synthetic route. The route yields a thienothiophene block, which can be easily functionalized further on. That way, two new acceptors (Y-thio-EG and Y-pyr-EG) were synthesized with permittivity-improving glycol side chains in the central pyrrol and outer thiophene substitution site. Both acceptors exhibit a significantly increased relative permittivity (4.73 and 5.24, respectively), compared to Y6 (2.39). When employed in binary bulk heterojunction solar cells with PM6 as polymer, however, both acceptors only reach efficiencies of 8.2% (Y-thio-EG) and 5.3% (Y-pyr-EG). We show by extensive morphology investigation (atomic force microscopy, transmission electron microscopy and X-ray scattering) that the glycol modification and side chain positioning have a strong influence on the crystallization behaviour of these acceptors. A thorough comparison of the acceptors blended with PM6 to PM6:Y6 absorber layers showed that differences in crystallinity and aggregation behaviour are present. However, the degree of the morphological change is likely not the main reason for the large efficiency drops of the solar cells. We hypothesize that the combination of an apolar donor with polar, glycol-modified acceptors yields a much more complex situation in the bulk heterojunction absorber layer, where further effects - such as interface dipole formation, energy level broadening and increased interface-state assisted recombination - are also expected to be highly relevant. Eventually we propose strategies to circumvent these issues in future permittivity modification research.