Interfacial Engineering Suppresses Voltage Losses in Fully Solution‐Processed, Symmetric Organic Photovoltaics

ABSTRACT Fully solution‐processed organic photovoltaic (OPV) devices hold great promise for enabling low‐cost, flexible, and transparent solar energy technologies. However, their performance remains limited by voltage losses arising from interfacial energy level misalignment, particularly in devices employing solution‐processed electrodes. Here, we elucidate the primary mechanism responsible for these losses in fully solution‐processed OPV devices that utilize high‐conductivity poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as both the anode and the cathode. We show that amphiphilic surfactant molecules in the PEDOT:PSS formulation used for the top anode self‐assemble into a stable negative dipole layer at the top anode interface, inducing a Schottky barrier that weakens the built‐in electric field and enhances non‐radiative voltage losses. To overcome this limitation, we introduce a simple yet effective interfacial modification strategy that employs polyethylenimine ethoxylated (PEIE), a common cathode interfacial material, as a top anode interlayer. As a result, fully solution‐processed devices incorporating PEIE as both the cathode and anode interlayers achieve internal quantum efficiencies and non‐radiative voltage losses comparable to, and stability superior to, those of conventional indium tin oxide (ITO)/metal‐based devices. This work represents the first demonstration of symmetric, ITO‐free, and metal‐free OPV devices, offering scalable and transparent architecture for next‐generation, low‐cost OPV technologies.