Open Access Version

Abstract:
In the last years the demand for electrical energy has been increasing continuously due to an expanding world economy, which has to be satisfied by sustainable and renewable energy sources to preserve the environment. Especially photovoltaic solar cells feature a high potential due to the immense energy reaching earth by solar radiation. Unfortunately, today’s solar cell absorber materials often show certain disadvantages like high costs or the utilization of rare or environmentally harmful materials. A promising solar cell absorber are kesterites (Cu2ZnSnSxSe4−x), which only consist of earth abundant, non-toxic, and highly available elements, assuring availability in the future. This work reviews kesterites as a solar cell absorber by quantum chemical first- principles calculations to understand key factors for the still low efficiencies, and give insight on possible performance enhancing material modifications. Hereby Cu2ZnSnSxSe4−x alloys are utilized for band gap engineering to increase the efficiency, whereby varying material qualities due to different structural alloy patterns introduce small band gap fluctuations. Further varying material qualities are shown by disorders on the 2c and 2d Wyckoff positions in Cu2ZnSnS4, whose influence is shown by an analysis of the electronic structure with respect to different structural disorder patterns and proportions. The 2c/2d disorders are revealed to be one of the main reasons for the band gap fluctuations, which induce lower efficiencies, whereby the Cu2ZnSnSxSe4−x alloys only slightly contribute. For a large-scale energy production via kesterite solar cells, further improvements are required, like efficiencies beyond the Shockley–Queisser limit and a reduction of material costs, which can be introduced by nanostructuring. A step towards nanostructuring is taken by theoretically investigating Cu2ZnSnS4 surfaces and clusters, which simulate different forms of nanostructuring. By studying the stability of different lowindex surfaces via surface energies, an insight on structural stabilizing patterns is given, whereby the challenge of calculating surface energies for off-stoichiometric symmetric slabs is successfully addressed via an extrapolation scheme. Decreasing the nanostructure size further to finite clusters, a structural model is designed to simulate a realistic nanocrystal with a fixed bulk-like core and a relaxed surface. Both modeling schemes show a magnetization of the surface within the computational model. The Cu2ZnSnS4 clusters show a size-dependent fundamental gap and the Cu2ZnSnS4 surfaces feature surface states within the bulk band gap, which can be utilized for an increased energy harvest. The quantum chemical first-principles investigations show a main reason for the band gap fluctuations and an opportunity for an enhancement of the solar cell performance by nanostructuring. By combining these theoretical findings with experiments, a possible route for more efficient kesterite solar cells is indicated.