Hydroxyl-functionalized molecular engineering mitigates 2D phase barriers for efficient wide-bandgap and all-perovskite tandem solar cells

Abstract

All-perovskite tandem solar cells (TSCs) demonstrate exceptional potential to overcome the single-junction efficiency limit through enhanced photon harvesting across the solar spectrum and suppressed thermalization effects, achieving theoretical power conversion efficiencies surpassing 44%. Wide-bandgap perovskites solar cells (WBG PSCs) are crucial for tandem photovoltaics, and have witnessed exponential progress during the last decade. However, these devices suffer from severe open-circuit voltage (V_OC) deficits, primarily due to interfacial recombination and carrier transport losses. A major contributor to these losses is the uncontrolled formation of insulating two-dimensional (2D) perovskite phases during surface passivation. Here, we introduce 4-hydroxyphenylethyl ammonium iodide (p-OHPEAI) as a multifunctional molecular additive to address this critical trade-off. Unlike conventional phenethyl ammonium iodide (PEAI), which forms the insulating 2D phase and the invert electric field by vertical molecular orientation that impedes charge extraction, the hydroxyl group (-OH) in p-OHPEAI enables parallel molecular adsorption on perovskite surfaces via synergistic interactions between amino (-NH₃) and -OH groups. This configuration effectively eliminates the formation of insulating 2D perovskite phase, passivates undercoordinated halide and lead vacancies, reducing non-radiative recombination. Additionally, the polarity of p-OHPEAI generates a dipole moment at the perovskite/electron transport layer (ETL) interface, optimizing energy-level alignment and facilitating electron extraction. By incorporating p-OHPEAI into 1.77 eV WBG PSCs, we achieved a remarkable V_OC of 1.344 V, corresponding to a minimal voltage deficit of 0.426 V, which is among the lowest reported V_OC-deficit values for the inverted WBG PSCs with bandgaps ranging from 1.75 to 1.80 eV. The optimized device delivered a power conversion efficiency (PCE) of 19.24%, demonstrating superior performance compared to conventional PEAI-passivated cells. When integrated into all-perovskite TSCs, this strategy enabled a champion PCE of 28.50% (with a certified efficiency of 28.19%). Furthermore, the devices exhibited excellent operational stability, maintaining over 90% of their initial efficiency after 350 h of continuous illumination, highlighting the robustness of the hydroxyl-driven passivation approach. The introduction of hydroxyl groups in passivation molecules provides a versatile strategy to balance defect suppression and charge transport, bridging the gap between high voltage and efficient carrier extraction.