Cu2ZnSnS4 absorber layers grown by Pulsed Laser Deposition assisted by a reactive sulfur beam

Abstract

Thin film solar cells based on the p-type semiconductor Cu2ZnSnS4 (CZTS) have emerged as next-generation photovoltaics, as an alternative to CdTe and CuGainSe2 (CIGS), the two leading commercial thin film solar absorber materials, due to the low-cost materials, earth-abundant constituents and non-toxic elements together with a high absorption coefficient and an optimal band gap of 1.5 eV. By using pulsed laser deposition (PLD), a non-equilibrium processing technique suitable for depositing high-quality films with complex stoichiometry, a metallic target made of Cu2ZnSn is ablated in vacuum. At the same time, a reactive sulfur beam, which enhances the chemical reactivity of sulfur, is directed towards the substrate in order to incorporate sulfur into the metallic film and reduce its losses in vacuum. The material is deposited onto Mo-coated soda-lime-glasses in the temperature range from 150 ºC to 550 ºC and the deposition process lasts 80 min. For the characterization of the films, Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) are used. First, it is analyzed the dependence of the laser fluence on the elemental composition of the metallic films. Then, it is studied the elemental composition, the morphology, the crystallinity and the quality of the deposited CZTS absorber layers as a function of the growth temperature. Another point of this work is the examination of the influence of the reactive sulfur on the morphology of the films and finally, how the post-annealing of CZTS films deposited at different temperatures affects their properties. The Raman spectra show an increase of the intensity and a sharpness of the main peak associated to kesterite CZTS up to a substrate temperature of 450 ºC. Above the deposition temperature of 450 ºC, the film decomposes into multiple secondary phases. EDX analyses depict the optimal composition for solar cells in all the samples below 450 ºC, SEM pictures indicate the dependence of the grain size on temperature and photoluminescence reveals the improvement of the films after annealing.