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

Abstract

CuInS 2 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 CuInS 2 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 Cu 2+ ions of the precursor CuSO 4 to Cu + in aqueous solution, thus enabling the formation of CuInS 2 thin films. By varying the concentration of the reaction solution during the solvothermal synthesis route, we obtained CuInS 2 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 CuInS 2 phase with the chalcopyrite structure except for the highest L-cysteine concentration where additional In 2 S 3 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.

Facile and Robust Solvothermal Synthesis of Nanocrystalline CuInS2 Thin Films

“…Out of high-resolution HR TEM images, the crystal size and structure was extracted and compared to the one obtained by XRD. The results of the (S)TEM measurements on film_S [ 32 , 38 ] and film_48 h are shown in Figure 4 , a TEM image and according diffraction pattern taken from film_3 h is shown in the supporting information, Figure S5 , but also discussed in the following.…”

“…The FTO glass substrates (Sigma-Aldrich) were cut into pieces of 15 mm × 20 mm × 2 mm, cleaned ultrasonically in dilute nitric acid, double-distilled water, acetone and ethanol for 5 min each prior to synthesis. The films were grown with our recently reported synthesis strategy using l -cysteine as sulfur source [ 32 ], which is based on the method published by Peng et al [ 28 ] and our group [ 29 ] where thioacetamide was used.…”

“…That means that for a variation of the reaction time, the reaction temperature was kept constant at 150 °C and for a variation of the reaction temperature the reaction time was always 18 h. The ratio between the precursors was kept at Cu:In:S 1:1:2.5. The film grown at this concentration/ratio and at 150 °C for 18 h has been published before [ 32 ] and will be referred to as film_S (standard reaction conditions). The other films are named according to their variation in time or temperature as film_time or film_temperature.…”

“…However, this synthesis route involves the carcinogenic substance thioacetamide as sulfur source [ 31 ]. Therefore, we recently changed the sulfur source to the natural amino acid l -cysteine to achieve a complete non-toxic, green synthesis pathway towards CuInS 2 films on FTO substrates [ 32 ]. There, we varied the concentration and ratio of the used precursor salts to investigate the influence on sample morphology and properties, while keeping the reaction conditions constant (150 °C, 18 h).…”

“…There, we varied the concentration and ratio of the used precursor salts to investigate the influence on sample morphology and properties, while keeping the reaction conditions constant (150 °C, 18 h). In short, at high sulfur ratios, an additional nanoflake layer of In 2 S 3 on top of a compact CuInS 2 film was observed [ 32 ]. On reviewing literature, it becomes clear that it is possible to synthesize CuInS 2 thin films in a wide variety of reaction conditions and with many possible precursor salts and solvents; e.g., Peng et al [ 28 ] used CuSO 4 , InCl 3 and thioacetamide in ethanol at 160 °C for 12 h to produce pure CuInS 2 .…”