(461f) Solvent-Mediated Formation of Quasi-2D Dion-Jacobson Heterostructures for Inverted Solar Cells over 23% Efficiency

Dimensional mixing by forming two-dimensional (2D) metal halide perovskites (MHPs) capping layers over 3D-MHPs (2D/3D) has emerged as a promising strategy to rapidly propel the power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) through concurrent defect passivation at the perovskite/transport layer interface and enhancement of PSCs durability. Typically, a layer of 2D-phase forming ligands (eg. PEAI, BAI) is deposited atop a 3D-MHP, leading to the formation of n = 1 or 2 2D-MHP phases on the 3D-MHP surface, facilitating electron-blocking. However, for inverted PSCs, the presence of 2D-MHPs hinders electron transport to the electron transport layer (ETL) due to the quantum-confined nature of 2D-MHPs, with mismatched conduction band minima (CBM) of the 2D and 3D-MHPs. It is possible to reduce the degree of quantum confinement and downshift the CBM of the 2D-MHPs by preferentially growing large n (n = 3, 4) 2D-MHPs and ligand engineering. A detailed understanding of processing and composition engineering is necessary to control the surface termination of 3D-MHPs and subsequent nucleation of 2D-MHPs with large n values to facilitate charge tunneling at the ETL.

In this work, we fabricated Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) 2D-MHPs on Cs_0.03(FA_0.90MA_0.10)_0.97PbI₃ 3D-MHPs, using different solvent combinations and ligands for the effective fabrication 2D/3D MHP films dominated by n ≥ 3 phases. A combination of scanning electron microscopy and confocal photoluminescence mapping reveals the critical influence of neat processing solvents on the underlying 3D-MHP films. Exposing the 3D-MHP film to conventionally used isopropanol (IPA) inflicts significant damage to the 3D-MHP film by FAI/MAI leaching and formation of excess crystalline PbI₂ on the perovskite surface, while 2, 2, 2-trifluoroethanol (TFE) leads to almost no damage to the 3D-MHP films, forming a relatively FAI/MAI-rich surface.

We then comprehensively studied the formation dynamics of ligand solutions containing 3-AMPY, 4-AMPY, and PEAI on 3D-MHPs via in-situ GIWAXS to understand the film formation during spin-coating. First, the incorporation of a trace quantity (0.5 vol%) of DMF and MAI in the ligand solution was found to afford more [PbI₆]^4-octahedra for the formation of the larger density of n ≥ 2 phases. Second, the progression of the 2D-phase formation is critically dependent on the choice of dominant solvent; Using a solvent mixture that significantly leaches the 3D-MHP (IPA: DMF) led to the formation of small n (n = 2) phases first, followed by the larger n (n = 3) last, while a relatively inert solvent (TFE: DMF) resulted in the nucleation of larger n (n = 4) phases first, followed by dimensional reduction to form smaller n. Lastly, in stark contrast to the DJ-capped 2D/3D films, while RP-phases conformed to the above trend in dimensional progression depending on the solvent choice, the phases eventually transform into a capping layer dominated by n = 2 phases, irrespective of which phase nucleated first over the spinning cycle. Mapping the evolution of the 2D/3D film during thermal annealing revealed that DJ-heterojunctions are more resilient to thermal-induced changes in composition in contrast to RP-heterojunctions which disproportionate into 2D/3D films dominated by n = 1.

Complementary transient absorption spectroscopy validates the increased density of n ≥ 3 phases, and time-resolved photoluminescence indicates a longer electron diffusion length of c.a. 4 µm for 3-AMPY based 2D/3D heterojunctions constructed using TFE: DMF solvent, benefitting from favourable energy alignment with the CBM of 3D-perovskite and decreased defect density. Leveraging these advantages enabled efficient inverted solar cells with a peak PCE of 23.2% and large V_oc of 1.19 V, with superior stability under the influence of ambient aging and thermal stress at 65°C.