Enhanced Quasi‐Fermi Level Splitting of Perovskite Solar Cells by Universal Dual‐Functional Polymer

Abstract Perovskite solar cells (PSCs) have attracted extensive attention due to their higher power conversion efficiency (PCE) and simple fabrication process. However, the open‐circuit voltage (V_OC) loss remains a significant impediment to enhance device performance. Here, a facile strategy to boost the V_OC to 95.5% of the Shockley‐Queisser (S‐Q) limit through the introduction of a universal multifunctional polymer additive is demonstrated. This additive effectively passivates the cation and anion defects simultaneously, thereby leading to the transformation from the strong n‐type to weak n‐type of perovskite films. Benefitting from the energy level alignment and the suppression of bulk non‐radiative recombination, the quasi‐Fermi level splitting (QFLS) is enhanced. Consequently, the champion devices with 1.59 eV‐based perovskite reach the highest V_OC value of 1.24 V and a PCE of 23.86%. Furthermore, this strategy boosts the V_OC by at least 0.07 V across five different perovskite systems, a PCE of 25.04% is achieved for 1.57 eV‐based PSCs, and the corresponding module (14 cm²) also obtained a high PCE of 21.95%. This work provides an effective and universal strategy to promote the V_OC approach to the detailed balance theoretical limit.