Thin film solar cells based on Cu (In,Ga)Se2 (CIGS) and earth abundant Cu2ZnSnS4 (CZTS) photo-absorbers utilize chemical bath deposited (CBD) CdS as heterojunction layer to attain high efficiency. Toxicity of CdS is a deterrent both in deployment and in production. As CBD is indispensable for high efficient cells, circumventing visible absorbance by thinner CdS deposition has homogeneity issue. This led to search for alternate sulfides, Zn and In with O and OH species (InX(OH,S)Y , ZnS(O,OH)) which proved less efficient than CdS. Though SnO2 as indium-tin oxide (ITO) is used in solar cells, Sn-sulfides have received little attention. Difficulty lies in concurrent deposition of sulfides with Sn(II) and Sn(IV) valence states as SnS is p-type (Eg=1.2-1.4 eV) and SnS2 is an n-type (Eg =2.44 eV) semiconductor. We investigated the CBD method and modified it to suppress SnS formation and also achieved controlled oxygen induction to form a new Sn disulfide-oxide (SnS2-XOX) buffer layer. Addition of oxygen have provided phase stability and also increased visible transmission as SnO2 has wider 3.6 eV bandgap. Using x-ray photoelectron spectroscopy (XPS), we investigated the bonding state of O and by Raman spectroscopy ascertained the effect of deposition conditions on SnS and SnS2 phase evolution. Combined with the optical band gap analysis, this study led to evaluation of SnS2-XOX as a heterojunction layer for solar cells.
The CBD growth of SnS2 films was accomplished with SnCl2 and thioacetamide as precursors for Sn and S, respectively in varied constituent ratio and forming Sn complex with tartaric acid to control Sn to S reaction. With added organic constituent having OH affinity groups, selective formation of Sn(OH)4 was initiated to add oxide phase by decomposition in creating new SnS2-XOX buffer layer. Such compound formation was confirmed by deconvolution of Sn 3d5/2 XPS line consistent with the binding energies at 486.6 and 487.3 eV corresponding to SnS2 and SnO2 phases and further affirmed by oxygen O1s XPS line analysis. Consistency of the characteristic Raman peak at 312±2 cm-1 from A1g vibration modes of SnS2 with decrease and or absence of 94 cm-1 Raman line from SnS with reduction in S-precursor indicated that it was possible to obtain single phase SnS2 in preparation of final SnS2-XOX phase in which x-variation is dependent on organic additive concentration. Optical analysis is consistent with direct band gap energy indicating possible band edges at 2.43 and 3.1 eV which is attributed to oxygen incorporation and thus higher transmittance for visible photons reaching absorber- junction. The fibrous and porous morphology of SnS2 buffer layer in contrast with compact morphology of the SnS2-XOX phase is highly suited for a conformal coating over p-absorber for increased surface coverage without limiting the transparency, a desired attribute for a heterojunction layer. This paper will report result of these investigations.