Crystallographic Structure and Point Defects vs. Efficiency and Stability in Cu₂ZnSn(S,Se)₄ Monograin Solar Cells (S/(S+Se)=0.8)

In recent years quaternary chalcogenides have gained a lot of attention, especially the kesterite-type semiconductor compounds Cu₂ZnSn(S,Se)₄ (CZTSSe) which consist mostly of earth abundant and non-toxic elements. These compounds are a promising low-cost alternative absorber material for thin film solar cells due to their suitable criteria for photovoltaic applications: <i>p</i>-type semiconductor behavior, direct band-gap between 1.0-1.5 eV and absorption coefficient&gt;10⁴ cm^-1[1]. The record conversion efficiency of 13.5% reported for a CZTSSe based thin film solar cell was reached when the polycrystalline absorber layer exhibits an off-stoichiometric composition [2]. Deviations from stoichiometry cause intrinsic point defects (vacancies, anti-sites, interstitials), which determine the electronic properties of the semiconductor significantly [3]. 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 (Cu_Zn and Zn_Cu anti-sites in Cu-Zn planes at z=¼ and ¾), which is always present in these compounds [4,5], is discussed as a possible reason for band tailing. Conventional structural characterization is done with X-ray diffraction, but in the case of isoelectronic cations, like Cu⁺ and Zn²⁺, they are difficult to distinguish. Nevertheless, the neutron scattering lengths of Cu and Zn are different; by neutron diffraction we can distinguish between Cu⁺ and Zn²⁺ site occupation in the crystal structure [4,5]. Kesterite-type based thin film solar cell technologies are mainly based on polycrystalline absorber layers, which makes it quite hard to correlate the crystallographic structure (determined via neutron diffraction) to the photovoltaic performance of these materials. A promising low-cost alternative technology uses CZTSSe monograins (single crystals of 50-100 μm size) which are fixed in a polymer matrix to form a flexible solar cell [6].<br/>An in-depth analysis of neutron diffraction data provides information on the cation distribution in the crystal structure allowing the determination of the type and concentration of intrinsic point defects including a distinction between Cu and Zn [5]. On the other hand, neutron diffraction requires large sample volumes, thus kesterite monograins offer the unique possibility to correlate structural disorder in kesterite-type absorbers with device performance parameters.<br/>We will present a detailed structural investigation of CZTSSe monograins with S/(S+Se)=0.8 based on neutron powder diffraction experiments, examining the influence of small changes in the chemical composition on the Cu/Zn disorder. We will show a correlation between all of the obtained knowledge on the chemical composition, the presence and types of secondary phases, the Cu/Zn disorder, the intrinsic point defects, the optical bandgap obtained from diffuse reflectance with the stability and efficiency of the respective devices. We will compare the “optimal” composition area of monograins with S/(S+Se)=0.8 to previously studied S/(S+Se)=0.6.<br/>This work has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no.952982. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the ORNL. The authors gratefully acknowledge the PSI and the SINQ for providing us beamtime at the HRPT diffractometer and Australia's Nuclear Science and Technology Organisation (ANSTO) for providing us beamtime at the ECHIDNA diffractometer.<br/>[1] S. Delbos, EPJ Photovoltaics 2012; 3: 35004.<br/>[2] J. Zhou et al., Nano Energy 89 (2021) 106405.<br/>[3] S. Schorr et al., in: Crystallography in Materials Science, ed. by S. Schorr and C. Weidenthaler, De Gruyter, 2021.<br/>[4] S. Schorr, Solar Energy Materials & Solar Cells 2011; 95: 1482-1488<br/>[5] G. Gurieva et. al., J Appl Phys 2018; 123: 161519/1-12<br/>[6] www.crystalsol.com