A strategy to stabilize kesterite CZTS for high-performance solar cells

Cu₂ZnSnS_4-xSe_x (CZTS) is an important semiconductor with significant potential for applications in the next generation of solar cells. CZTS has an optimal band gap (∼1.5 eV) and contains no expensive or toxic elements. However, CZTS-based solar cells suffer from low efficiency because of poor crystal quality, which is partly caused by secondary phase formation during synthesis. We use density functional theory+U calculations to systematically investigate the stabilities of three CZTS phases: kesterite, stannite, and wurtzite. In agreement with previous experiment and theory, we confirm that these three phases have very similar formation energies. This finding is consistent with the known difficulties in synthesizing pure kesterite CZTS, the phase that is desirable for photovoltaic applications. To overcome this problem, we characterize surfaces and interfaces of CZTS and are able to identify certain "beneficial surfaces" that could be exploited to potentially provide extra stability for the kesterite phase. We propose the zinc blende ZnS (001) surface as a substrate to induce formation of these beneficial surfaces and to stabilize the kesterite phase, thereby serving as an effective crystallization template for the fabrication of high-performance CZTS solar cells.