Optical absorption spectra of Cu₂ZnSn(S,Se)₄ thin-film solar cells by Fourier transform photocurrent spectroscopy

Abstract

We demonstrated the potential of Fourier transform photocurrent spectroscopy (FTPS) in investigating the optical absorption and quality of a Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorber within the cell structure. We fabricated thin-film solar cell structures with a CZTSSe absorber, which was prepared by a nanoparticle coating technique and a postannealing process. The optical absorption spectra of CZTSSe absorbers prepared at different annealing temperatures were measured by FTPS. We successfully evaluated the optical band-gap energy from the spectra. An exponential tail was found in the spectra. Urbach energy increased with increasing annealing temperature. The deterioration of photovoltaic performance with increasing Urbach energy might be due to potential fluctuation rather than to structural randomness.

Emerging Chalcogenide Thin Films for Solar Energy Harvesting Devices

“…It is interesting to note that the inclusion of a reduced effective band gap due to band gap fluctuations (that lead to sub-band-gap absorption) was found to be essential in drift-diffusion simulations (using wxAMPS, SCAPS, and Sentaurus) to reproduce the experimentally measured IV characteristics. − Urbach tails in kesterites have been characterized using photothermal deflection spectroscopy and Fourier transform photocurrent spectroscopy. E U values of 53 meV and ∼30 meV were reported for CZTS and Se-rich kesterites, respectively. − More importantly, it was shown that the presence of both band gap fluctuations and an Urbach tail is needed to fit the experimentally measured absorption coefficient (Figure ). These reports also showed an exponential tail in the absorption coefficient over 4 orders of magnitude spanning an energy range of ∼0.3–0.4 eV below the band gap, revealing the presence of deep sub-band states in kesterites (Figure ).…”

“…(b) Experimentally measured IQE showing exponential absorption tails in CZTSSe absorbers annealed at various temperatures (band gaps of these absorbers range from 1.28 to 1.32 eV). Reproduced with permission from refs and . Copyright 2018 IOP Publishing and 2018 Elsevier.…”