Versatile Hole Selective Molecules Containing a Series of Heteroatoms as Self-Assembled Monolayers for Efficient p-i-n Perovskite and Organic Solar Cells

Abstract

Inverted type perovskite solar cells (PSCs) have recently emerged as a major focus in academic and industrial photovoltaic research. Their multiple advantages over conventional PSCs include easy processing, hysteresis-free behavior, high stability, and compatibility for tandem applications. However, the maximum power conversion efficiency (PCE) of inverted PSCs still lags behind those of conventional PSCs because suitable charge-selective materials for inverted PSCs are limited. In this study, excellent hole-selective materials for inverted PSCs are introduced. A series of tricyclic aromatic rings containing O, S, or Se, respectively, as a core heteroatom, along with a phosphonic acid anchor, form a self-assembled monolayer (SAM) that directly contacts the perovskite absorber. The influence of heteroatoms in the aromatic structure on the molecular energetics and operating characteristics of the corresponding inverted PSCs is investigated using complementary experimental techniques as well as density functional theory (DFT) calculations. It is found that all of the SAMs formed an energetically well-aligned interface with the perovskite absorber. The interaction energy between the Se-containing SAM and perovskite absorber is the strongest among the series and it reduces the interfacial defect density, in turn leading to an extended charge carrier lifetime. As a result, PSCs incorporating the Se-containing SAM achieves a PCE of 22.73% and retains ≈96% of their initial efficiency after a maximum power point tracking test of 500 h without encapsulation under ambient conditions. All of the SAMs are then employed in organic solar cells (OSCs). Again, the Se-containing SAM-based OSCs demonstrates the highest PCE of 17.9% among the three molecular SAM-based OSCs. This study demonstrates the great potential for precisely engineered SAMs for use in high-performance solar cells.