Elucidating the mechanism of perovskite surface passivation with organic molecules: the impact of π-conjugation length

Abstract

To further enhance the performance of perovskite solar cells (PSCs), a more comprehensive analysis of the mechanisms through which organic molecules induce defect passivation and enhance hole extraction is essential. In this study, we employ several organic molecules with varying π-conjugation lengths to examine how factors such as their molecular desorption, energy levels, and radical-cation stability affect defect passivation, hole extraction, and the overall PSC performance. Our results show that passivation molecules with extended π-conjugation suppress molecular desorption from the perovskite surfaces during overlayer spin-coating and improve energy-level alignment at interfaces, thereby enhancing PSC efficiency through improved defect passivation and hole extraction. Additionally, extended π-conjugation improves radical-cation stability, contributing to greater device durability. Among the defect passivation materials studied, 2-(3-ethlylamine)benzothieno[3,2-b]benzothiophene, hydroiodide (BTBTAI) can provide the most significant improvements in these factors, increasing the initial efficiency from 22.7% to 24.6% and raising the efficiency retention from 61% to 85% after 1,000 hours of continuous light illumination at 25 °C in formamidinium lead iodide-based PSCs. Reports on defect passivation from the perspectives of molecular desorption and cation stability are extremely limited. Therefore, these findings deepen the understanding of PSC operating mechanisms and offer valuable insights for developing design guidelines for future defect passivation materials with even higher device performance.