Design and fabrication of new wide-band and wide-angle solar cell absorbing layer

Abstract

Cu–Zn– Sn– S (CZTS) is an ideal material for absorbing layer due to its high optical absorption coefficient and moderate bandgap. But, optical absorption and carrier recombination in thin films may lead to electron losses. Thus, introduction of radial PN junction nanostructures into thin film solar cells make light and current to travel along different paths. In this study, CZTS films were prepared by magnetron sputtering and crystalized at sputtering pressure of 3 mTorr, sputtering power of 120 W, and substrate temperature of 300 °C. The composition of CZTS thin films was closed to the optimal stoichiometry to be used as a solar cell absorption layer. These CZTS films were single-phase and vulcanized at 550 °C. A CZTS absorbing layer with a nanowire array structure had higher absorptivity in visible light region. The wideband optical absorptivity was also significantly higher than that of a planar thin film. The nanowire structure was simulated by the finite-difference time-domain method (FDTD), and its maximum absorption (with good size) reached up to 90% in 300 to ~ 2000 nm wide waveband. The mask was done after determining the ratio of the spreading agent (water to ethanol as 1:2), the surfactant concentration (4% wt.), the equilibrium time (2 min), and other parameters. The best results for normal incidence were evaluated as: D = 0.4 μm, h = 2 μm and f ≈ 0.8. According to calculations, the well-designed nanowires, in the visible light region, have an absorption rate of up to 90%.