Unravelling phase-dependent electronic dimensionality and optoelectronic properties in lead-free layered A3B2X9 perovskites for photovoltaic applications

Abstract

All-inorganic lead-free layered A₃B₂X₉ perovskites have recently emerged as potential candidates for the development of high-performance perovskite-based optoelectronic and photovoltaic devices. However, there is still a lack of a deep understanding of the phase stability mechanism and optoelectronic properties of A₃B₂X₉ perovskites, which severely limits their device applications. Herein, using density functional theory calculations, we report the phase stability, electronic dimensionality, and optoelectronic properties of fifteen A₃B₂X₉ perovskites with two different structures: hexagonal (H) and trigonal (T) phases. Our calculation results indicate that the phase stability of A₃B₂X₉ perovskites originates from a delicate competition between the B–X bonding interactions and interlayer BX₆ octahedral coupling. Moreover, the T-phase perovskites exhibit smaller bandgaps, lower carrier effective masses, and higher absorption coefficients than H-phase ones due to higher electronic dimensionality. Benefiting from optimal bandgaps and higher absorption efficiency, T-phase perovskites show a higher power conversion efficiency (PCE) and the maximum PCE of T-phase Cs₃B₂I₉ (B = Sb, Bi) reaches ∼20%. Owing to weak interlayer interactions, two-dimensional T-phase perovskites can also be exfoliated from their bulk and they exhibit layer-dependent electronic and optoelectronic properties due to the quantum confinement effect. These findings offer an in-depth understanding of phase-dependent electronic dimensionality and optoelectronic properties of A₃B₂X₉ perovskites, highlighting their huge potential for use in integrated photovoltaic devices.