Compositional optimization of a 2D-3D heterojunction interface for 22.6% efficient and stable planar perovskite solar cells

The stacking of 2D perovskites on the top of 3D perovskites has been recognized as a promising interfacial treatment approach to improve the stability and efficiency of planar perovskite solar cells (PSCs). However, traditional 2D-3D perovskite heterojunctions obtained from the high-temperature annealing process still exhibit unsatisfactory charge transfer performance and interfacial voltage loss. Herein, we introduce isopentylammonium iodide (PNAI) as the large organic ammonium salt, and adjust the in situ grown processes of 2D perovskites by thermal treatments to form a multi-component capping layer composed of 2D perovskites with plenty of high n-value 2D phases (n >= 3, n is the number of inorganic layers) and residual PNAI molecules on 3D perovskites. Such an optimized composition for a 2D-3D perovskite heterojunction can remarkably improve the charge transfer performance, further suppress the interfacial ionic defects, and enlarge Fermi-level splitting, leading to a low bandgap-to-voltage loss (0.38 V). Consequently, this treatment strategy significantly improves the efficiency of planar PSCs to 22.62% with an outstanding open-circuit voltage of 1.16 V. Moreover, the unencapsulated PNAI-90 treated device stored under a relative humidity of 30 +/- 5% for 1000 h still retains 89% of its initial PCE. This work offers a new strategy to construct a robust 2D-3D heterojunction for planar PSCs.