The performance of organic solar cells is heavily influenced by the active layer morphology, which ideally provides a large interface area between the donor and the acceptor material, while providing continuous pathways in each phase to the respective electrodes. Donors such as D18 or PM6 are known to form network-like structures within bulk heterojunctions, yet detailed insights into these networks beyond surface studies remain limited. In this work, we present a comprehensive investigation of D18:L8-BO active layers using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Elemental ratio mapping results in sufficient contrast of donor and acceptor domains to provide insight into donor network structures in bulk heterojunctions.

Using this approach, we investigate the morphological evolution induced by systematic donor dilution and its consequences for solar cell performance. Starting from the optimal donor–acceptor weight ratio of 1:1.5, enabling power conversion efficiencies (PCEs) of up to 16.4% without additives, the donor fraction is progressively reduced. While decreasing the donor content from 40% to 20% leads to a pronounced efficiency loss (11.3% PCE), further dilution to 11% results in no further significant loss in device performance (10% PCE), with nearly constant exciton dissociation and charge collection efficiencies over 60% - despite substantially increased transmission in the visible part of the solar spectrum. EELS analysis shows that the robustness in device performance arises from the donor–acceptor interfacial regions remaining largely preserved, while donor dilution primarily reduces the diameter of fibrillar donor domains, thereby maintaining efficient charge generation and extraction. Decreasing the donor content leads to increasingly large acceptor domains, which significantly influence the external quantum efficiency (EQE) as well as photoluminescence characteristics across the different blend ratios. Additionally, a noticeable reduction in shunt resistance is observed for devices with strongly reduced donor content.

Overall, this study demonstrates that the D18:L8-BO system exhibits a high tolerance toward donor dilution, with reasonably high efficiencies maintained despite significant optical and morphological changes. Understanding donor network formation and its impact on charge generation, collection, and electrical losses provides valuable insights for optimizing organic solar cells toward high efficiency and enhanced visible light transmission, enabling new opportunities for semi-transparent photovoltaic applications. [1]