Maximal Visible Light Energy Transfer to Ultrathin Semiconductor Films Enabled by Dispersion Control

Increasing light absorption in an ultrathin semiconductor is critical for developing thin-film photovoltaic devices. Here, it is shown that a maximal absorption of visible light is possible through controlling the dispersion of thin-film materials. The ideal dispersion relation is determined for the permittivity of a thin film placed on a reflector with a dielectric spacer, and it is explained how the ideal dispersion relation can be realized for semiconductor materials possessing bandgaps. To experimentally verify dispersion control and maximal absorption, the permittivity of lead selenide (PbSe) thin film is tailored by controlling its polycrystallinity through the sputtering conditions. The measured reflectance of a dispersion-controlled PbSe film (9 nm) deposited on an SiO2 (48 nm)/Al substrate shows a record level of absorbance for PbSe film of 88% taken as an average over the entire visible spectrum. This value is close to the theoretical maximum value of 95%. Overall, the dispersion control scheme offers promising avenues for thin-film solar cell research.