A biomolecule-assisted, cost-efficient route for growing tunable CuInS2 films for green energy application

Abstract

CuInS₂ has become a popular and promising candidate as an absorber material in photovoltaic devices and photo-electrochemical cells. Here we report the successful L-cysteine-assisted growth of homogeneous, nanostructured CuInS₂ thin films deposited on fluorine-doped tin oxide coated glass. In contrast to existing synthesis routes using thioacetamide as a sulfur source, our method offers the advantage of being environmentally friendly and non-toxic. We found that L-cysteine is able to reduce the Cu²⁺ ions of the precursor CuSO₄ to Cu⁺ in aqueous solution, thus enabling the formation of CuInS₂ thin films. By varying the concentration of the reaction solution during the solvothermal synthesis route, we obtained CuInS₂ films with different thicknesses and compositions, investigated with the help of energy dispersive X-ray spectroscopy with scanning and transmission electron microscopy. X-ray and electron diffraction experiments of the films prove the synthesis of a pure CuInS₂ phase with the chalcopyrite structure except for the highest L-cysteine concentration where additional In₂S₃ forms. UV-Vis absorption spectroscopy reveals absorption over the whole visible spectrum. The calculated band gap using the UV-Vis data ranges between 1.4 and 1.5 eV, and can be adjusted by changing crystal size and chemical composition. This offers a promising route towards tuning the optical and transport properties. First dye degradation experiments show promising activity under solar illumination.