# Tailored Monolayer Co‐Assembly for Enhanced Efficiency and Stability in Inverted Perovskite Solar Cells

## TL;DR

This paper introduces a new co-assembled monolayer that improves the efficiency and stability of perovskite solar cells.

## Contribution

A novel co-assembly strategy using Cys and S-2PACz is introduced to enhance interfacial properties in solar cells.

## Key findings

- The co-assembled Cys-S-2PACz monolayer improves molecular packing and ordering.
- The strategy reduces interfacial defects and non-radiative recombination in solar cells.
- PSCs with Cys-S-2PACz show higher efficiency and better thermal and operational stability.

## Abstract

Self‐assembled monolayers (SAMs) are extensively employed as hole‐selective interlayers in inverted perovskite solar cells (PSCs), yet their incomplete coverage and poor molecular ordering often induce interfacial defects and exacerbate non‐radiative recombination. To overcome these limitations, we developed a bulky methylthio‐functionalized carbazole‐based SAM, (2‐(3,6‐bis(bis(4‐(methylthio)phenyl)amino)‐9H‐carbazol‐9‐yl)ethyl)phosphonic acid (S‐2PACz), and implemented a sequential co‐assembly strategy utilizing cysteine (Cys). In this tailored architecture, Cys is pre‐anchored via  carboxyl groups, followed by the competitive adsorption of S‐2PACz via phosphonic acid anchoring, yielding a mixed Cys‐S‐2PACz monolayer. The size complementarity and cooperative anchoring facilitated denser molecular packing and improved ordering, while synergistic methylthio and thiol functionalities jointly passivated interfacial defects. Consequently, this strategy enhanced perovskite crystallization, optimized energy‐level alignment, and facilitated efficient hole extraction. Ultimately, PSCs incorporating Cys‐S‐2PACz achieved higher power conversion efficiency, reduced recombination losses, and demonstrated superior thermal, operational, and outdoor stability compared with devices based on single‐component SAMs. This work establishes competitive co‐assembly as an effective interfacial engineering strategy, providing a scalable route to tailored SAM architectures for high‐performance and durable p–i–n PSCs.

This work presents a co‐assembled monolayer, Cys‐S‐2PACz, constructed via sequential deposition. The size‐complementary pairing and competitive anchoring between Cys and S‐2PACz enable dense and highly ordered molecular packing. Meanwhile, the synergistic passivation from the methylthio and thiol groups effectively suppresses interfacial recombination, delivering enhanced efficiency and stability in inverted perovskite solar cells.