Chemical structure and processing solvent of cathode interlayer materials affect organic solar cells performance

Interlayers in organic solar cells (OSCs) are crucial for efficient charge carrier transport and extraction. Recent research has introduced cathode interlayer (CIL) materials, which are soluble in polar, amphiphilic, and non-polar solvents. However, studies on how these solvents affect device performance, particularly stability under various conditions, remain limited. In this work, we investigate the effects of the chemical structure of a recently synthesized perylene diimide (PDI) CIL material, F-PDIN-EH, and its eco-friendly polar (methanol), amphiphilic (1-butanol), and non-polar (heptane) processing solvents, on device performance, compared to PDINO, a widely studied PDI-based CIL material. This is one of the first investigations into the effect of non-polar CIL processing solvents on device stability. OSC devices with F-PDIN-EH yield comparable efficiency to PDINO-based devices but are consistently less stable, irrespective of the solvent. Notably, the heptane-processed F-PIN-EH-based devices exhibit the lowest stability. We investigated the degradation mechanisms in the device and the interfaces through an in-depth study using TPV, TPC, extracted charge carrier density, and light intensity dependence of J_sc and V_oc. Further studies are conducted using absorption spectroscopy, FTIR, and mobility measurements to ascertain the source of degradation. The loss in performance over time, especially in the heptane-processed F-PIN-EH-based devices, is mainly due to increased surface recombination and imbalanced charge mobility. This study provides valuable insights into the dependency of device performance on the chemical structure and processing solvents of CIL materials. It also highlights challenges for sustainable, greener OSCs.