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

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Vol. 7; no. 27; pp. 16285 - 16293
• Main Authors: Rajagopal, Adharsh, Stoddard, Ryan J, Hillhouse, Hugh W, Jen, Alex K.-Y
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 2019
• Subjects: Alloys; Anions; Bowing; Cations; Cesium; Charge distribution; Chemical effects; Composition; Interpolation; Lead; Lead base alloys; Linearity; Microstrain; Optoelectronics; Organic chemistry; Perovskites; Photoluminescence; Photonic band gaps; Photons; Photovoltaic cells; Photovoltaics; Solar cells; Spray coating; Tin; Tin base alloys
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/c9ta05308e

High quality small-bandgap hybrid perovskites (AMX 3 with M = Pb 1− x Sn x ) are pivotal for all-perovskite multi-junction photovoltaics. The bandgap of these alloys significantly deviates from the linear interpolation between the bandgaps of APbI 3 and ASnI 3 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 APb 1− x Sn x I 3 alloys reveal that the bandgap bowing is correlated with the extent of microstrain in their respective APbI 3 compounds. We discover that bandgap bowing in APb 1− x Sn x I 3 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 APb 1− x Sn x I 3 alloys. The optimal composition, (MA 0.24 FA 0.61 Cs 0.15 )(Pb 0.35 Sn 0.65 I 3 )I 3 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. Experimental insights regarding bandgap evolution in hybrid perovskite alloys and the optimal small-bandgap absorber composition desired for next-generation perovskite tandems.