Quasi-2D Ruddlesden–Popper perovskites with tunable wide bandgaps and phosphonium as additives for interface defect passivation in tandem solar cell design

Abstract

Quasi-two-dimensional (Q2D) additive engineering has recently been demonstrated as an effective strategy for defect control and property enhancement in narrow-band-gap absorbers for high-efficiency perovskite tandem solar cells (Nat. Energy, 2022, 7, 642–651). In this study, we design a series of Q2D Ruddlesden–Popper perovskites incorporating phosphonium-based 2D additives, specifically [Ph(CH₂)₂PH₃]₂PbI₄ (abbreviated as PEP₂PbI₄), layer-stacked with nMAPbI₃ as a wide-band-gap absorber for perovskite tandem solar cell devices. First-principles calculations reveal that Q2D perovskites modified with PEP cations exhibit an approximately 0.2 eV wider bandgap compared to their phenylethylammonium (PEA) counterparts with the same layer thickness n. Certain thickness sequences are able to generate the energy funneling effect, resulting in the directional migration of carriers(Adv. Mater., 2020, 32, 1906571). The larger terminal PEP cations introduce longer distances and more sharply tilt angles relative to the contact surface of bulk perovskite component nMAPbI₃, leading to distinct lattice matching properties and enhanced defect passivation when interfaced with charge transfer layers (CTLs). Additionally, PEP-functionalized Q2D perovskites demonstrate higher band offsets and larger work functions when interfaced with indium tin oxide (ITO) hole transport layers (HTLs), improving carrier extraction. Furthermore, the incorporation of PEP cations results in higher defect formation energies, effectively suppressing interface defect generation at the ITO contact. These findings highlight the potential of Q2D additive engineering to enhance both the stability and efficiency of wide-band-gap perovskites, offering a promising avenue for advancing perovskite-based tandem solar cells.