Copper zinc tin sulfide (CZTS) is a quaternary semiconducting compound which has received increasing interest since the late 2000s for applications in thin film solar cells. The class of related materials includes other I2-II-IV-VI4 such as copper zinc tin selenide (CZTSe) and the sulfur-selenium alloy CZTSSe. CZTS offers favorable optical and electronic properties similar to CIGS (copper indium gallium selenide), making it well suited for use as a thin-film solar cell absorber layer, but unlike CIGS (or other thin films such as CdTe), CZTS is composed of only abundant and non-toxic elements. Concerns with the price and availability of indium in CIGS and tellurium in CdTe, as well as toxicity of cadmium have been a large motivator to search for alternative thin film solar cell materials. The power conversion efficiency of CZTS is still considerably lower than CIGS and CdTe, with laboratory cell records of 11.0 % for CZTS and 12.6 % for CZTSSe .
Crystal structure
CZTS is a I2-II-IV-VI4 quaternary compound. From the chalcopyrite CIGS structure, one can obtain CZTS by substituting the trivalent In/Ga with a bivalent Zn and IV-valent Sn which forms in the kesterite structure.
Some literature reports have identified CZTS in the related stannite structure, but conditions under which a stannite structure may occur are not yet clear. First-principle calculations show that the crystal energy is only 2.86 meV/atom higher for the stannite than kesterite structure suggesting that both forms can coexist. Structural determination (via techniques like X-ray diffraction) is hindered by disorder of the Cu-Zn cations, which are the most common defect as predicted by theoretical calculations and confirmed by neutron scattering. The near random ordering of Cu and Zn may lead to misidentification of the structure. Theoretical calculations predict the disorder of the Cu-Zn cations to lead to potential fluctuations in the CZTS and could therefore the cause for the large open circuit voltage deficit, the main bottle neck of state-of-the-art CZTS devices. The disorder can be reduced by temperature treatments. However, other temperature treatments alone do not seem to be able to yield highly ordered CZTS. Other strategies need to be developed to reduce this defect, such as tuning of the CZTS composition.
Material properties
Carrier concentrations and absorption coefficient of CZTS are similar to CIGS. Other properties such as carrier lifetime (and related diffusion length) are low (below 9 ns) for CZTS. This low carrier lifetime may be due to high density of active defects or recombination at grain boundaries. Defect formation in CZTS is prevalent due to low defect formation energies of zinc-copper antisite defects and copper vacancies. These defects create 'effective' charge in the crystal structure, which is stabilized by the aggregation of different defects that compensate for the charge disparity to become effectively neutral. As a result, electron-trapping states are formed, which enables recombination. Having deep-level defect states lowers the open-circuit voltage and the conversion efficiency of a CZTS solar cell.
Many secondary phases are possible in quaternary compounds like CZTS and their presence can affect the solar cell performance. Secondary phases can provide shunting current paths through the solar cell or act as recombination centers, both degrading solar cell performance. From the literature it appears that all secondary phases have a detrimental effect on CZTS performance, and many of them are both hard to detect and commonly present. Common phases include ZnS, SnS, CuS, and Cu2SnS3. Identification of these phases is challenging by traditional methods like X-ray diffraction (XRD) due to the peak overlap of ZnS and Cu2SnS3 with CZTS. Other methods like Raman scattering are being explored to help characterize CZTS.
Fabrication
CZTS has been prepared by a variety of vacuum and non-vacuum techniques. They mostly mirror what has been successful with CIGS, although the optimal fabrication conditions may differ. Methods can be broadly categorized as vacuum deposition vs. non-vacuum and single-step vs. sulfization/selenization reaction methods. Vacuum-based methods are dominant in the current CIGS industry, but in the past decade there has been increasing interest and progress in non-vacuum processes owing to their potential lower capital costs and flexibility to coat large areas.