Phase-Stabilized 2D/3D Heterobilayers via Lattice Matching for Efficient and Stable Inverted Solar Cells

State-of-the-art perovskite solar cells (PSCs) often incorporate a thin capping layer of 2D-perovskites atop 3D-perovskites (2D/3D heterostructures) for efficient and stable photovoltaics. However, their applicability in inverted PSCs mandates careful regulation of the layer thickness n of the 2D-perovskite, with larger nweakening the quantum-confinement effect and facilitating electron transport across the 2D/3D interface. Meeting this requirement is frustrated by two major hurdles. First, forming 2D-perovskite capping layers with larger n (n > 2) enforces the usage of the volatile and chemically unstable methylammonium MA⁺ in the interlayer space, limiting the operational stability. While formamidinium FA is an intuitive substitute, its larger atomic radius poses imposes significant lattice strain within a quasi-2D perovskite lattice increasing the electron-phonon coupling and compromising the carrier properties. Second, conventional approaches involve the deposition of a thin-film layer of 2D-perovskite forming ligands atop the 3D-perovskite, offering very limited control over the phase-purity, layer thickness and structural phase of the 2D-capping layer. As such, the current state-of-the-art in 2D/3D PSCs reflects an underexplored chemical space in terms of MA-free 2D-perovskites and a lack of synthetic control over solution-processed 2D/3D heterostructures.

We hypothesised that the major hurdle towards incorporation of FA in n > 2 quasi-2D perovskites is ascribed to the large disparity in bond angles and Pb-Pb distances between the pure-2D and pure-FAPbI₃ perovskite lattices. Towards this end, we identified a family of 2D-perovskite forming ligands that have appropriate lattice matching, successfully synthesising a library of phase-pure (single n) FA-rich (75% FA) and pure-FA substituted Ruddlesden-Popper (RPP) and Dion-Jacobson (DJP) perovskite single-crystals with layer thicknesses up to n = 4. We transferred the composition of the 2D-perovskite single-crystals into capping layers atop 3D-perovskite films by dissolving the 2D-perovskite crystals in a solvent orthogonal to the 3D-layer, forming solution-processed 2D/3D hetero-bilayer films (HBs). To verify our material design and their translatability into operationally stable devices under ISOS conditions, we performed detailed accelerated aging studies on HBs exposed to combined light, heat (85°C) and humidity (>90%). Despite their widespread deployment, HBs with 2D-capping layers based on MA⁺undergo rapid phase-segregation into non-perovskites, while those rich in FA⁺ display superior phase-stability. In particular, the FA-rich DJPs exhibited excellent phase stability with no degradation at all, with potential as a barrier layer for the underlying 3D-perovskite. We leveraged the favourable structural stability and superior carrier properties of the DJP HBs to fabricate inverted PSCs with 25.33% PCE while retaining 95% initial PCE over 1200 h under MPP tracking (1-sun illumination) at 65°C.