Fast Wetting of a Fullerene Capping Layer Improves the Efficiency and Scalability of Perovskite Solar Cells.

Fullerene derivatives, especially [6,6]-phenyl-C-butyric acid methyl ester (PCBM), have been widely applied as electron transport layers of inverted planar heterojunction perovskite solar cells (PSCs). However, the solution-processed PCBM capping layer suffers from limited surface wetting which hinders the improvement in efficiency and scalability of PSCs. Herein, we develop a facile hybrid solvent strategy that enables very fast wetting of the PCBM capping layer atop of the perovskite surface, leading to an improved interfacial contact and electron transport. The significantly enhanced wettability of the PCBM solution fulfilled through blending isopropyl alcohol into the commonly used chlorobenzene (CB) is attributed to the reduced surface tension while retaining viscosity. As a result, the electron mobility and electric conductivity of the PCBM capping layer increase by around two times, and the PSC devices exhibit the highest power conversion efficiency (PCE) of 19.92%, which is improved by ∼18% relative to that of the control device (16.78%). Importantly, this strategy is also applicable for other alcohols (ethanol and methanol) and CB blends. Moreover, the fast wetting approach enables us to deposit the PCBM capping layer using a facile drop-casting method, affording comparable PCEs to those obtained by the conventional spin-coating method, which is not achievable by using the conventional single solvent. This fast wetting PCBM capping layer also contributes to efficiency improvement of large-area (1 cm) devices. These advances hold great potential for other scalable deposition methods such as blade-coating and slot-die coating.