Self‐Assembled Interlayer Enables High‐performance Organic Photovoltaics with Power Conversion Efficiency Exceeding 20%

Abstract Interfacial layers (ILs) are prerequisites to form the selective charge transport for high‐performance organic photovoltaics (OPVs) but mostly result in considerable parasitic absorption loss. Trimming the ILs down to a mono‐molecular level via the self‐assembled monolayer is an effective strategy to mitigate parasitic absorption loss. However, such a strategy suffers from inferior electrical contact with low surface coverage on rough surfaces and poor producibility. To address these issues, here, the self‐assembled interlayer (SAI) strategy is developed, which involves a thin layer of 2–6 nm to form a full coverage on the substrate via both covalent and van der Waals bonds by using a self‐assembled molecule of 2‐(9H‐carbazol‐9‐yl) (2PACz). Via the facile spin coating without further rinsing and annealing process, it not only optimizes the electrical and optical properties of OPVs, which enables a world‐record efficiency of 20.17% (19.79% certified) but also simplifies the tedious processing procedure. Moreover, the SAI strategy is especially useful in improving the absorbing selectivity for semi‐transparent OPVs, which enables a record light utilization efficiency of 5.34%. This work provides an effective strategy of SAI to optimize the optical and electrical properties of OPVs for high‐performance and solar window applications.