Publication date: 2nd November 2020
We report on the origin on the non-linear band gap dependence of n=1 Ruddlesden-Popper perovskites of the form A2B1xB21-xX4, where A = methylammonium (MA) or phenethylammonium (PEA), B1 = lead (Pb), B2 = tin (Sn) and X = iodine (I), with a view to partially remove lead from 2D perovskites for use as the active layer in photovoltaics, the passivating surface layer for 3D perovskite photovoltaics or light emitting diodes (LEDs). Density Functional Theory (DFT) calculations with and without the use of spin-orbit coupling (SoC) are studied for x = 0, 0.25, 0.5, 0.75 and 1 for all 16 permutations of A2B1xB21-xX4 to study the effects of the short range ordering of B1 and B2 atoms. A partial density of states (PDOS) analysis is used to understand the chemical behaviour of the valence band maximum (VBM) and conduction band minimum (CBM). Partial density of states (PDOS) results show that the energy mismatch of Pb s and Sn s states in the VBM contributes to the non-linearity of the system. The SoC term affects the linear gradient, and is shown to strongly affect the behaviour of the CBM, resulting in much lower calculated effective electron masses. The short range ordering of Pb and Sn is shown to affect the band gap significantly in A2B10.5B20.5X4 where the permutations are most significant. This research allows for the design and analysis of mixed site 2D perovskite materials, which can be extended to n > 1 systems as well as other A, B1, B2 and X combinations.
The authors acknowledge the EPSRC for funding and support.
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