Bandgap engineering of Cu2ZnSn1-xGexS(e)4 by adjusting Sn-Ge ratios for almost full solar spectrum absorption

The substitution of Sn with Ge is one of the promising approaches to fabricate high-efficiency Cu2ZnSn1-xGexS(e)(4) (CZTGS(e)) thin-film solar cells, especially for the multijunction or bandgap-graded solar cells. However the evolution of structure and optical properties of CZTGS(e) in a wide composition range (0 <= x <= 1) hasn't been researched and elucidated systematically. In this paper, CZTGS(e) films were synthesized by selenizing or sulphidising oxide precursors, and their optical, componential and structural property were investigated by absorption spectroscopy, scanning electron microscopy and X-ray diffraction (XRD)/Raman, respectively. The insertion of germanium into the lattice of Cu2ZnSnS(e)(4) (CZTS(e)) is verified by relevant XRD peaks and Raman vibrational modes, as both of them shift towards higher diffraction angles and wave-numbers respectively. Besides, the spectrum of absorption revealed that the bandgap of CZTGS(e) can be widely adjusted between 0.96 and 2.0 eV. The preliminary solar cells show that the Voc increases with the increase of bandgap, while the Jsc exhibits opposite tendency. This can be expected to be applied in full solar spectrum absorption. (C) 2017 Elsevier B.V. All rights reserved.