Two-dimensional eclipsed arrangement hybrid perovskites for tunable energy level alignments and photovoltaics

Excellent long-term durability and moisture tolerance, low cost synthesis routes, and the possibility of flexible tuning of electronic properties have meant that the two-dimensional (2D) hybrid halide perovskites have recently attracted much attention, and have been successfully used as light absorbers in efficient photovoltaic devices. 2D hybrid halides perovskites which feature long chain alkylammonium cations, however, are generally suggested for light emitting diode (LED) applications, due to their strong photoluminescence (PL), but none have been applied thus far for photovoltaic (PV) applications. Additionally, it has been shown that luminescence quenching occurs in the Pb-based analogues, (AEQT)PbX4 (X = Cl, Br, I), however, energy transfer and charge separation between organic and inorganic components of the structures are still not fully understood. Herein, we investigate the geometrical, electronic and optical properties of the semiconducting 2D perovskites (AEQT)BX4 (B = Pb, Sn; X = Cl, Br, I), using relativistic hybrid density functional theory calculations. We demonstrate that unlike the traditional 2D perovskites, the choice of the organic ammonium cation has a considerable effect on the carrier transport properties. Our calculations reveal that optical transitions between the organic and inorganic components is disallowed. The electronic structures of the series are flexibly tailored by different halides and metal cations, with band gaps ranging from 2.06 to 2.68 eV. Energy level alignments greatly hinder the electron-hole recombination in (AEQT)PbCl4, (AEQT)PbBr4, (AEQT)PbI4 and (AEQT)SnBr4, and thereby should enhance their PL efficiencies. With a moderate fundamental band gap (2.06 eV) and strong direct valence band to conduction band transition, (AEQT)SnI4 is the only composition that shows intense and broad optical absorption, and as expected displays a high spectroscopic limited maximum efficiency (SLME) of 20.8%. Our results indicate the (AEQT)SnI4 is a stable and efficient light-absorbing material for application as a top absorber of the tandem solar cell.