Organic solar cells are highly promising for sustainable energy conversion with power conversion efficiencies already surpassing 19%. [1] One approach to further increase the performance of organic photovoltaics is to reduce the substantial energy loss in organic solar cells stemming from the low‑permittivity nature of the organic absorbers by applying high-permittivity (high-ε) active layer materials. [2] However, despite the increase in permittivity, many of the high-ε materials typically reveal lower efficiencies, which is generally explained with a non-ideal bulk heterojunction morphology.

In this work, we introduced polar sulfone side chains to perylene-based non-fullerene acceptors with the aim of increasing their dielectric permittivity. The novel acceptors PMI-[F-OS] and PMI-[C-OS], exhibited a permittivity increase by 56% (ε_r from 1.86 to 2.90) and by 44% (ε_r from 1.91 to 2.76) at 10⁵ Hz and high solubility in THF resulting from the sulfone modification. [3] This provides the possibility of a layer-by-layer processing using only non-halogenated solvents (o-xylene for the donor polymer PTQ10 and THF for the acceptor). By applying the resource-saving design of experiment (DoE) method, a still underrepresented approach in solar cell research, for the optimization of the processing parameters of the solar cells, the PCE maximum could be determined after performing only a comparably small number of experiments. The optimized solar cells revealed a PCE of 5.5% with a high V_OC of 1.3 V, surpassing the efficiency of solar cells containing the alkylated PMI-analog [4] of PMI-[F-OS], with the additional benefit of the non-halogenated solvent processing.

These findings clearly demonstrate that layer-by-layer processing can prevent efficiency losses typically stemming from unfavorable donor–acceptor phase separation in bulk heterojunction organic solar cells due to the distinctly changed surface free energies and solubilities of high-ε materials.

Furthermore, we extended this approach to the popular Y-series acceptors and synthesized Y6-derivatives with flexible glycol side chains. In one compound the functionalization was done on the outer thiophene position, the second acceptor bears glycol chains on the pyrrole unit. These compounds exhibit significantly higher permittivities (4.73 and 5.24) compared to Y6 (2.39), while maintaining the beneficial properties of this acceptor family. Besides a detailed morphology investigation of blends of the high-ε Y6-derivatives with PM6, we also present first results of their photovoltaic properties.