Non-toxic, earth-abundant 2% efficient Cu2SnS3 solar cell based on tetragonal films direct-coated from single metal-organic precursor solution

A process for deposition of tetragonal Cu2SnS3 (CTS) thin films from methanolic precursor solution of metal-thiourea complex by direct liquid coating (DLC) is described. For synthesis of CTS films, precursor films are first deposited by dip-coating and then thermolysed at 200 °C in air for 10 min. Formation of tetragonal Cu2SnS3 is confirmed by X-ray Diffraction (XRD). X-ray Photoelectron Spectroscopy (XPS) disclosed that Cu and Sn are in oxidation states +1 and +4, respectively. The films are smooth and homogenous with roughness (RMS) of 1–2 nm as revealed by atomic force microscope (AFM) and scanning electron microscopy (SEM). Optical studies show that the energy band gap (Eg) of Cu2SnS3 is 1.12 eV and absorption coefficient (α) is >105 cm−1. The films are p-type with electrical conductivity (σ) of 0.5 S/cm. The concentration and mobility of holes are about 1018 cm−3 and 1 cm2/V/s, respectively as determined from Hall measurement. The variation of conductivity in the temperature range 5 to 290 K can be explained by considering a combination of Mott variable range hopping, nearest neighbour hopping and thermionic emission over GB barriers as conduction mechanism. Non-toxic thin film solar cell (TFSC) of graphite/Cu2SnS3/ZnO/ITO/SLG are fabricated by DLC which had power conversion efficiency (PCE) of 2.10% with open circuit voltage, short circuit current and fill factor of 0.816 V, 6.14 mA/cm2, 0.42, respectively.