Quaternary chalcogenides have gained a lot of attention, especially kesterite-type semiconductors Cu₂ZnSn(S,Se)₄ (CZTSSe) which consist of earth abundant and non-toxic elements. These compounds are promising low-cost alternative absorbers for thin film solar cells due to their suitable criteria for photovoltaic applications, a record conversion efficiency of 13.5 % was reported recently [1]. In such high efficient devices the polycrystalline absorber layer shows an off-stoichiometric composition. Deviations from stoichiometry cause intrinsic point defects in the material (vacancies, anti-sites, interstitials), determining the electronic properties of the semiconductor significantly. It is agreed in literature that large band tailing observed in Cu-based kesterite-type semiconductors causes voltage losses limiting the efficiency of kesterite-based devices. The Cu/Zn disorder, which is always present in these compounds [2], is discussed as a possible reason for band tailing.

Structural characterization is conventionally based on X-ray diffraction, but isoelectronic cations, like Cu⁺ and Zn²⁺, are difficult to distinguish when analyzing diffraction data. But their neutron scattering lengths are different, thus their site occupations in the crystal structure can be determined by neutron diffraction. The experimental determination of the order parameter Q, which is a quantitative measure of the degree of Cu/Zn disorder [3], requires a differentiation between these cations as well.

Kesterite-type based thin film solar cell technologies are mainly based on thin absorber layers, which makes it quite hard to correlate the crystallographic structure (determined via neutron diffraction) to a photovoltaic performance of these materials. A promising alternative technology uses CZTSSe monograins (single crystals, 50-100 μm size) fixed in a polymer matrix to form a flexible solar cell.

We will present a detailed structural investigation of CZTSSe monograins based on neutron powder diffraction, examining the influence of small changes in their chemical composition on Cu/Zn disorder as well as determining the type and concentration of intrinsic point defects present. We present correlating trends between chemical composition, presence of secondary phases, Cu/Zn disorder, occuring intrinsic point defects as well as the optical bandgap (obtained by diffuse reflectance) of the CZTSSe monograins with the stability and efficiency of respective devices.

This work has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 952982 (Custom-Art). Part of this research used resources at the SNS, a DOE Office of Science User Facility operated by the ORNL. Authors gratefully acknowledge the PSI and SINQ for providing beamtime at the HRPT diffractometer.

[1] X. Xu et al., Adv. Energy Mater. (2021) 2102298

[2] S. Schorr et al., J. Phys.: Energy 2 (2020) 012002

[3] D. Toebbens et al., Phys. Stat. Sol. B 253 (10) (2016) 1890