(333f) Formation and Degradation Dynamics of 2D-on-3D Perovskite Heterostructures

2D-on-3D (2D/3D) perovskite heterostructures present a promising strategy to realize efficient and stable photovoltaics by integrating the superior stability of 2D-metal halide perovskites (2D-MHPs) and excellent carrier properties of 2D-MHPs. However, their applicability in inverted solar cells is limited due to the quantum confinement of the 2D-layer, solvent incompatibilities that disrupt the underlying 3D layer and phase segregation under operation, hampering electron transport at the 2D/3D interface. Herein, solvent-dependent formation dynamics and structural evolution of 2D/3D heterostructures under extrinsic stressors are investigated via in situ X-ray scattering.

We revealed that solvent interaction with the 3D surface determines the formation sequence and spatial distribution of quasi-2D (n ≥ 2) phases. Isopropanol (IPA) reconstructs the perovskite into a PbI₂-rich surface, forming a strata with smaller n first, followed by a thinner substratum of larger n. In contrast, 2,2,2-Trifluoroethanol (TFE) preserves the 3D surface, promoting the formation of uniformly distributed larger n domains first, and smaller n last. Leveraging these insights, we used Dion–Jacobson perovskites with superior charge transport properties and structural robustness to fabricate 2D/3D heterostructures dominated by n ≥ 3 and engineer a favorable energy landscape for electron tunnelling. Inverted solar cells based on 3-Aminomethylpyridine and TFE achieve a champion power conversion efficiency (PCE) of 23.60%, with V_oc and FF of 1.19 V and 84.5%, respectively, and superior stabilities with t₉₄ of 960 h under thermal stress.

In addition, we examined the structural evolution of 2D/3D heterostructures with sharp interfaces under concurrent extrinsic stressors via in-situ GIWAXS (e.g. light, humidity, temperature, and atmosphere). Ruddlesden-Popper perovskites (RPPs) with butylammonium BA as ligand and methylammonium MA as small cation as capping layers undergo rapid phase segregation into non-perovskite MAPbI₃H₂O and BAPbI₃H₂O hydrates, eventually degrading into PbI₂. While no degradation of the underlying 3D-MHPs were observed, due to the protective effect of 2D-MHPs as sacrificial layers, noticeable reduction in carrier lifetime and PCE are observed in solar cells. Complementary confocal photoluminescence mapping and SEM were used to examine structural evolution of 2D-MHP crystals along different facets. To stabilise the crystal edges and increase lattice rigidity, we investigate A-site engineering of Dion-Jacobson perovskites (DJPs). Formamidinium substitution into phase-pure n = 3 DJP capping layers exhibit a dramatic reduction in phase segregation irrespective of extrinsic stressor with minimal changes in PCE. As the culmination of high phase stability and favourable band alignment, 2D/3D inverted solar cells with 3AMP-based DJPs as capping layers achieve a high PCE of 25.3% with high operational stability under combined illumination and temperature.