CuIn1−xAlxSe2 Thin Films Grown by Co-Sputtering and Modified Selenization: Application in Flexible Solar Cells

Thin films of CuIn_1-xAl_xSe₂ (CIAS) were grown on the flexible 10 micrometer thin stainless steel substrates, by dc co-sputtering from the elemental cathodes, followed by annealing with modified selenization. CuInAl alloyed precursor films were selenized both by noble gas assisted Se vapor transport in a tubular furnace and vacuum evaporation of Se in an evaporation chamber. CIAS thin films were optimized for better adhesion. X-ray diffraction, scanning electron microscopy, and UV-visible absorption spectroscopy were used to characterize the selenized films. The composition of CIAS films was varied by substituting In with Al in CuInSe₂ (CIS) from 0 ≤ x ≤ 0.65 (x = Al/Al+In). Lattice parameters, average crystallite sizes, and compact density of the films, decreased when compared to CIS and (112) peak shifted to higher Bragg´s angle, upon Al incorporation. The dislocation density and strain were found to increase with Al doping. Solar cells with SS/Mo/CIAS/CdS/iZnO:AZnO/Al configuration were fabricated and were tested for current-voltage characteristics for various `x´ values, under Air Mass 1.5 Global one sun illumination. The best CIAS solar cell showed the efficiency of 6.8%, with x = 0.13, E_g = 1.17 eV, fill factor 45.04, and short circuit current density J_sc 30 mA/cm².