Polyelectrolyte Layer as a Work Function Modifier for Transparent Electrodes Enabling Efficient Perovskite Solar Cells With Superior Low‐Light Performance

Polyelectrolytes represent a low‐cost and effective platform for tailoring charge extraction interfaces in inverted perovskite solar cells (PSCs). In this work, we introduce a protonated polyethyleneimine (PEI) layer (pH = 2) between the ITO electrode and the PTAA hole transport layer to increase the apparent work function of ITO by 0.066 eV and reduce contact resistance and the work‐function mismatch with PTAA from 0.170 to 0.104 eV. By incorporating low concentrations of PEI (0.3–1 mg/mL), we significantly increased the open‐circuit voltage up to 1.143 V and achieved a peak power conversion efficiency of 23.45%. The PEI‐modified devices maintain superior photovoltaic performance not only under standard AM 1.5 G illumination, but also at low sunlight intensities (30 and 50 mW/cm 2 ) and under indoor illumination conditions. These results highlight the potential of protonated polyelectrolyte interlayers for developing efficient and versatile PSCs suitable for both outdoor and ambient‐light applications.