Evaluation of AA-CVD deposited phase pure polymorphs of SnS for thin films solar cells†
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-TiOx layer (am-TiOx-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 (Voc) of 0.135 and 0.144 V, and short-circuit current densities (Jsc) 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-TiOx-FTO (η = 0.41%) back contact, with a Voc of 0.135 V, and Jsc of 5.40 mA cm−2. We show that mild post-fabrication hot plate annealing can improve the Jsc, but can in most cases compromise the Voc. The effect of sequential annealing was monitored by solar conversion efficiency and external quantum efficiency (EQE) measurements.