Role of electron-phonon coupling and thermal expansion on band gaps, carrier mobility, and interfacial offsets in kesterite thin-film solar cells

TL;DR

This study investigates how electron-phonon interactions and thermal effects influence the electronic properties of kesterite solar cell materials, revealing insights into band alignment, mobility limits, and recombination processes affecting efficiency.

Contribution

It provides new calculations of band offsets, temperature effects on band alignment, and phonon-based mobility estimates for CZTS and CZTSe, addressing key efficiency bottlenecks.

Findings

Band offsets vary with temperature, affecting interface barriers.

Electron mobility is limited by phonon scattering, with electrons being faster.

Temperature increases can significantly alter band alignment and offsets.

Abstract

The efficiencies of solar cells based on kesterite Cu$_2$ZnSnS$_4$ (CZTS) and Cu$_2$ZnSnSe$_4$ (CZTSe) are limited by a low open-circuit voltage due to high rates of non-radiative electron-hole recombination. To probe the origin of this bottleneck, we calculate the band offset of CZTS(Se) with CdS, confirming a weak spike of 0.1 eV for CZTS/wurtzite-CdS and a strong spike of 0.4 eV for CZTSe/wurtzite-CdS. We also consider the effects of temperature on the band alignment, finding that increasing temperature significantly enhances the spike-type offset. We further resolve an outstanding discrepancy between measured and calculated phonon frequencies for the kesterites, and use these to estimate the upper limit of electron and hole mobilities based on optic phonon Fr\"ohlich scattering, which uncovers an intrinsic asymmetry with faster (minority carrier) electron mobility.