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

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.