Photovoltaic response of natural Kesterite crystals

The photoelectrochemical performance of naturally occurring single and multicrystalline Kesterite or Cu2(Zn,Fe)SnS4 (CZTS) is presented. Elemental analyses of the crystals indicate high sulfur to metal atomic ratios and the presence of 12.3-14.5 at% of iron presumably in the 2+ metal site replacing zinc in the lattice. Raman spectroscopy indicates the possible presence of a Cu2SnS3 impurity phase. Photocurrent spectroscopy measurements give a bandgap of 1.61-1.63 eV, higher than most reported values for pure Cu2ZnSnS4 films. Mott-Schottky measurements show that these crystals have a high doping density of about 1019 cm−3, that reduces to about 1017 cm−3 after a brief etch in 10% KCN (aq.) solution. Although still low, the carrier diffusion length is increased by the removal of surface defects. This work demonstrates the ability to simply and easily evaluate the photovoltaic potential for a plethora of naturally occurring semiconducting materials.