Discovering Novel Low-Bandgap Halide Perovskites for Solar Cells Using a Computational and Experimental Approach

To pass the theoretical maximum efficiency of single-junction solar cells, there have been numerous efforts in developing tandem solar cells, which allow for the absorption of a broader band of the sun’s emission spectrum. Halide perovskites are particularly of interest for tandem devices [1] because of their impressive electrical and optical properties as well as the sheer tunability of their bandgaps and defect behavior via composition and structure engineering. In this work, we use a combination of first principles simulations and rational experimental synthesis and characterization to discover some promising new low bandgap (&lt; 1.1 eV) perovskites for use as bottom cells in tandem devices. Using previously established predictive machine learning models [2], we navigate the massive chemical space of ABX3 perovskite alloys considering various inorganic and organic species at different cation sites, to determine a few hundred compositions predicted to be stable and display narrow bandgaps. Next, we perform density functional theory (DFT) computations on selected candidates and calculate their bulk formation energy and decomposition energy using the PBEsol functional. In addition, we calculate their electronic band structures using the hybrid HSE06 functional [3]. Along with this approach, we consider a novel class of double layered perovskite materials with the general formula A4M(II)M2(III)X12 or A4M(IV)M2(II)X12, which are promising for achieving low bandgaps (as seen for Cs4Cu(II)Sb2(III)Cl12, for instance) [4,5]. As double perovskites incorporate multiple B site elements, they provide additional opportunities to find low-bandgap perovskites. Based on our calculations, we arrive at multiple promising ABX3 3D perovskite alloys and layered perovskite compositions, which are experimentally tested by spin-coating of thin films. From the film studies, Tauc plots, photoluminescence measurements, and thermal stability measurements (ex. thermogravimetric analysis) are carried out. These measurements help determine the experimental bandgap, thermal stability, and an indication of the defects in the films. In the end, multiple compounds are identified as promising candidates for further investigation and use in solar cells.<br/><br/><b>References</b><br/>[1] T. Miyasaka et al., Chem. Rev. 2019, 119, 5, 3036–3103<br/>[2] J. Yang et al., J. Chem. Phys. 160, 064114 (2024).<br/>[3] J. Heyd et al., J. Chem. Phys. 118, 18, 8207-8215 (2003).<br/>[4] Alsonso J. C., et al. (2017). J. Am. Chem. Soc. 2017, 139, 27, 9116–9119.<br/>[5] Feng. C., et al. Chem. Mater. 2020, 32, 1, 424–429.