High transmittance n-type tin-oxysulfide Sn(OxS2- x) heterojunction layer grown from surfactant mediated chemical bath deposition for thin film solar cells

Highlights: • Tin oxysulfide thin films with controlled oxygen alloying in SnS₂ are deposited. • Sn(O_xS_2-x) compare to SnS₂ films show low absorption coefficient and larger bandgap. • Sn(O_xS_2-x) offers Cd-free alternative heterojunction layer in thin film solar cells. Highly transparent n-type polycrystalline SnS₂ and Sn(O_xS_2-x) thin films were deposited by a modified chemical bath deposition technique using an ionic surfactant t-Octylphenoxy polyethoxy ethanol (abbreviated Tx) activator. X-ray photoelectron spectra (XPS) analysis of the films shows the presence of Sn3d_5/2 (486.65 eV) and S2p_3/2 (161.7 eV) XPS lines signifying the Sn⁴⁺-S²⁻ bonding confirming single phase SnS₂ film formation with the addition of < 5 v/v% Tx. With the addition of Tx in higher ≥ 5 v/v% Tx concentration, a strong O1s (531.1 eV) XPS peak attributed to the lattice oxygen matched with the asymmetrical Sn3d_5/2 (486.65 and 487.31 eV) XPS peaks confirm the growth of Sn-oxysulfide [Sn(O_xS_2-x)] film. The Raman spectral analysis based on single intense 314 cm⁻¹ peak suggests the monophasic SnS₂ film growth is promoted by Tx addition and complete suppression of the SnS and Sn₂S₃ phases. Grazing incidence X-ray diffraction show formation of highly crystalline SnS₂ film in the hexagonal phase with preferred [100] orientation. Scanning electron micrograph images show uniform lateral growth apparently aided by homogeneous substrate surface nucleation for films deposited with Tx added in the chemiplating solution. The mechanism of SnS₂ and Sn(O_xS_2-x) film growth is discussed in detail based on the reactions which involve controlled dissociation Sn(OH)₄ into SnO alongside the SnS₂ formation. The photoluminescence (PL) spectrum of deposited films showed a dominant PL peak at ~492 nm attributed to the direct excitonic transitions. The optical transmittance spectra of the Sn(O_xS_2-x) films are suggestive of sub-bandgap absorption region characterized by gradually decreasing mid-gap states from 2.60 to 2.49 eV and gradually increasing direct bandgap from 3.16 to 3.34 eV indicating increase in oxygen incorporation with increasing concentration of Tx in chemiplating solution. The high optical bandgap and low absorption coefficient $3.2-5.8\times {10}^{3}c{m}^{-1}$ in the 900–400 nm range of the Sn(O_xS_2-x) films is highly suitable for application as an n-type heterojunction layer in thin film solar cells.