Microwave-induced band gap attenuation of novel manganese zinc aluminum sulfide kesterite nanomaterials for optimal photovoltaics

Abstract

Recently, kesterite (copper zinc tin sulfide, CZTS) have emerged as a promising alternative active layer to cadmium telluride and copper indium gallium selenide for low-cost thin films for solar cell applications due to its high absorption coefficient, direct band gap, natural abundance, and low toxicity. The photovoltaic power conversion efficiency achieved with CZTS is still very low when compared to silicon, thereby hindering its commercialization. Developing new materials with improved optical and photovoltaic properties is, therefore, encouraged. Here, we report for the first time the microwave-assisted solvothermal synthesis of novel manganese zinc aluminum sulfide using chlorides, sulfates, nitrates and a mixture of chlorides, nitrates, and sulfates in ratio 1:1:1 as cation precursor salts. The influence of microwave reaction temperatures from 100 to 190 °C for 5–60 min were investigated on the crystal structure, morphology, composition, optical, and electrochemical properties. It has been found out that the microwave reaction temperature and time have potential effect on the overall reaction. The optimum temperature and time were found to be 190 °C for 5 min and 150 °C for 20 min for our studies. The obtained powder samples were analyzed using X-ray diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy, high-resolution scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, ultraviolet-visible, cyclic voltammetry, and electrochemical impedance spectroscopy. Structural characterization using X-ray diffraction revealed that the samples synthesized with chlorides, nitrates, and mixtures of the precursor salts were all amorphous while samples synthesized with sulfate precursor salts have a kesterite crystal structure. Raman spectroscopy shows major intense peaks between 326 and 331 cm for all the nanomaterials which could be assigned to the Raman A mode of kesterite CZTS. The morphological features revealed spherical shapes with little agglomeration for all the synthesized materials except for that synthesized with sulfate precursor salts at 190 °C revealing a rod-like morphology with a width of 0.780–1.200 μm, a length of 1.90–2.5 μm and attached to the surface are agglomerated nanospheres. Small-angle X-ray spectroscopy revealed a core-shell shape for most of the nanomaterials with an average particle size between 10 and 100 nm. Electrochemical impedance spectroscopy revealed that electron transfer is faster in the nanomaterials synthesized with the chloride precursor salts at both temperatures studied. The band gap falls within the range of 1.2–1.7 eV. The nanomaterials presented in this study possess the desirable properties for application in photoelectrochemical and solar cells.