Evaluation of AA-CVD deposited phase pure polymorphs of SnS for thin films solar cells

Abstract

Six different thin film solar cells consisting of either orthorhombic (α-SnS) or cubic (π-SnS) tin(II) sulfide absorber layers have been fabricated, characterized and evaluated. Absorber layers of either π-SnS or α-SnS were selectively deposited by temperature controlled Aerosol Assisted Chemical Vapor Deposition (AA-CVD) from a single source precursor. α-SnS and π-SnS layers were grown on molybdenum (Mo), Fluorine-doped Tin Oxide (FTO), and FTO coated with a thin amorphous-TiO_x layer (am-TiO_x-FTO), which were shown to have significant impact on the growth rate and morphology of the as deposited thin films. Phase pure α-SnS and π-SnS thin films were characterized by X-ray diffraction analysis (XRD) and Raman spectroscopy (514.5 nm). Furthermore, a series of PV devices with an active area of 0.1 cm² were subsequently fabricated using a CdS buffer layer, intrinsic ZnO (i-ZnO) as an insulator and Indium Tin Oxide (ITO) as a top contact. The highest solar conversion efficiency for the devices consisting of the α-SnS polymorph was achieved with Mo (η = 0.82%) or FTO (η = 0.88%) as the back contacts, with respective open-circuit voltages (V_oc) of 0.135 and 0.144 V, and short-circuit current densities (J_sc) of 12.96 and 12.78 mA cm⁻². For the devices containing the π-SnS polymorph, the highest efficiencies were obtained with the am-TiO_x-FTO (η = 0.41%) back contact, with a V_oc of 0.135 V, and J_sc of 5.40 mA cm⁻². We show that mild post-fabrication hot plate annealing can improve the J_sc, but can in most cases compromise the V_oc. The effect of sequential annealing was monitored by solar conversion efficiency and external quantum efficiency (EQE) measurements.