A Polymeric Two‐in‐One Electron Transport Layer and Transparent Electrode for Efficient Indoor All‐Organic Solar Cells

Abstract Transparent electrodes (TEs) are vital in optoelectronic devices, enabling the interaction of light and charges. While indium tin oxide (ITO) has traditionally served as a benchmark TE, its high cost prompts the exploration of alternatives to optimize electrode characteristics and improve device efficiencies. Conducting polymers, which combine polymer advantages with metal‐like conductivity, emerge as a promising solution for TEs. This work introduces a two‐in‐one electron transport layer (ETL) and TE based on films of polyethylenimine ethoxylated (PEIE)‐modified poly(benzodifurandione) (PBFDO). These PEIE‐modified PBFDO layers exhibit a unique combination of properties, including low sheet resistance (130 Ω sq−1), low work function (4.2 eV), and high optical transparency (>85% in the UV–vis‐NIR range). In contrast to commonly used poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the doping level of PBFDO remains unaffected by the PEIE treatment, as verified through UV–vis‐NIR absorption and X‐ray photoelectron spectroscopy measurements. When employed as a two‐in‐one ETL/TE in organic solar cells, the PEIE‐modified PBFDO electrode exhibits performance comparable to conventional ITO electrodes. Moreover, this work demonstrates all‐organic solar cells with record‐high power conversion efficiencies of >15.1% under indoor lighting conditions. These findings hold promise for the development of fully printed, all‐organic optoelectronic devices.