Publication date: 11th March 2026
Perovskite-inspired materials (PIMs) have been proposed as alternatives to lead-halide perovskites (LHPs) for different optoelectronic applications [1], such as indoor photovoltaics (IPVs) [2]. In addition to the inspiration by the extraordinary properties of LHPs, search for alternative PIMs is motivated by the stability issues and the controversial effect of lead-toxicity on the sustainability of LHPs [1]. From the application point of view, IPVs present an interesting opportunity to improve the energy efficiency of buildings by using the ambient lighting to power small electronic appliances. Further, wide-bandgap perovskites and PIMs have shown promising results for efficient indoor light conversion [2].
To provide insights on fundamental optical and optoelectronic properties of Bi-based PIMs (Bi-PIMs), we measured the optical constants (that is, complex refractive index spectra) of two promising Bi-PIMs, namely Cu2AgBiI6 (CABI) and Cs3Bi2I6Br3 (CBI), as thin films via spectroscopic ellipsometry [3]. From the data, we extracted the bandgaps of 1.98 to 2.09 eV for CABI and 2.33 to 2.43 eV for CBI, respectively, where the ranges represent variances among the different methods that were applied (that is, Tauc plot, Elliot’s model, and critical point fitting). High absorption coefficients (>105 cm-1) were observed. Notably, spectra for both the real and imaginary parts of the refractive indices (which are required for device simulations) were simultaneously determined.
The obtained optical constant spectra were applied to simulate a reference device using these Bi-PIMs as the absorber for IPV application under the irradiance of white light-emitting diode (WLED) with varying color temperature and illuminance level. Theoretical limit for the power conversion efficiency of CABI device based on the measured optical constants exceeded 40% under 4000 K WLED irradiance. Further research on the electronic properties, for example, the origin of observed below bandgap absorption, and electrical device operation are suggested to help closing the gap between the theoretical and experimental (5-6% [4]) efficiencies.
We thank the LMD group at the University of Turku for discussions about ellipsometry, Joaquin Valdez Garcia for discussions about AFM, and Ermei Mäkilä for discussions about SEM. A.K. thanks the University of Turku Graduate School (UTUGS), Jenny and Antti Wihuri Foundation and Lieto Savings Bank Foundation (KesPV project) for funding his doctoral research. M.H. thanks SUSMAT profiling funding (Research Council of Finland and University of Turku). G.K.G., S.T., and P.V. thank the SPOT-IT project founded by the CET Partnership, the Clean and Energy Transition Partnership under the 2022 CET Partnership joint call for research proposal, cofounded by the European Commission (GA 101069750) and with the founding of the organizations detailed on https://cetpartnership.eu/funding-agencies-and-call-modules. S.Y. and K.M. thank Circular Materials Bioeconomy Network funded by Ministry of Education and Culture, Finland (CIMANET, Decision No. VN/3137/2024-OKM-6). P.V. acknowledges funding from Research Council of Finland, Decision No. 347772. The work is part of the Research Council of Finland Flagship Programme, Photonics Research and Innovation (PREIN), Decision No. 346511. This study was conducted using MARI infrastructure at the University of Turku.
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