Issue 27, 2019

On understanding bandgap bowing and optoelectronic quality in Pb–Sn alloy hybrid perovskites

Abstract

High quality small-bandgap hybrid perovskites (AMX3 with M = Pb1−xSnx) are pivotal for all-perovskite multi-junction photovoltaics. The bandgap of these alloys significantly deviates from the linear interpolation between the bandgaps of APbI3 and ASnI3 for all A-site cations examined thus far. This non-linearity of the bandgap with composition is referred to as bandgap bowing. Here, we explore a wide-range of A-site compositions to understand bandgap bowing and identify the optimal Pb–Sn alloy composition. Optical and structural investigations of different APb1−xSnxI3 alloys reveal that the bandgap bowing is correlated with the extent of microstrain in their respective APbI3 compounds. We discover that bandgap bowing in APb1−xSnxI3 alloys is primarily due to local structural relaxation effects (changes in bond angles and lengths) that result from the size, shape, and charge distribution of the cations on the A-site, and that these effects are intimately coupled with chemical effects (intermixing of atomic orbitals) that result from changes in the M-site. The choice of X-site also impacts bandgap bowing because of the X-site anions' influence on local structural relaxation and chemical effects. Further, we extend these results to provide a general rationale for the origin and modulation of bandgap bowing in HP alloys. Subsequently, using high-throughput combinational spray coating and photoluminescence analysis, we find that ternary combinations of methylammonium (MA), formamidinium (FA), and cesium (Cs) are beneficial to improve the optoelectronic quality of APb1−xSnxI3 alloys. The optimal composition, (MA0.24FA0.61Cs0.15)(Pb0.35Sn0.65I3)I3 has a desirable low bandgap (1.23 eV) and high optoelectronic quality (achieving 86% of the detailed balance limit quasi-Fermi level splitting). This study provides valuable insights regarding bandgap evolution in HP alloys and the optimal small-bandgap absorber composition desired for next-generation HP tandems.

Graphical abstract: On understanding bandgap bowing and optoelectronic quality in Pb–Sn alloy hybrid perovskites

Supplementary files

Article information

Article type
Paper
Submitted
20 May 2019
Accepted
10 Jun 2019
First published
10 Jun 2019

J. Mater. Chem. A, 2019,7, 16285-16293

Author version available

On understanding bandgap bowing and optoelectronic quality in Pb–Sn alloy hybrid perovskites

A. Rajagopal, R. J. Stoddard, H. W. Hillhouse and A. K.-Y. Jen, J. Mater. Chem. A, 2019, 7, 16285 DOI: 10.1039/C9TA05308E

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