Thiophene Expanded Self‐Assembled Monolayer as Hole Transport Layer for Organic Solar Cells with Efficiency of 20.78%

Abstract Carbazole‐derived self‐assembled monolayers (SAMs) as hole transport layers (HTLs) have achieved groundbreaking progress of device efficiency in perovskite and organic photovoltaics. Expanding the π‐conjugation of carbazole is an effective approach to enhance the molecular dipole moment and facilitate charge extraction of SAMs. However, this strategy tends to cause poor solubility and excessive self‐aggregation of SAMs. In this work, two highly efficient SAMs are developed by substituting a non‐fused thiophene unit on 3,6‐position of carbazole, namely (2‐(3,6‐di(thiophen‐3‐yl)‐9H‐carbazol‐9‐yl)ethyl)phosphonate (2PAThCz) and diethyl (4‐(3,6‐di(thiophen‐3‐yl)‐9H‐carbazol‐9‐yl)butyl)phosphonate (4PAThCz). The introduction of thiophene can completely alter the molecular packing behavior of SAM, promoting more compact π–π stacking and increasing dipole moment, which enhances hole transport. Furthermore, the long spacer length on 4PAThCz enable to help it achieves excellent solubility, inhibit self‐aggregation, and strengthen the molecular orderliness. As a result, an impressive efficiency of 20.78% (certified as 20.45%) is achieved for single‐junction organic solar cells.