Chemical beam epitaxy of dilute nitrides for intermediate band solar cells

Efficient utilization of the full solar spectrum extending from near infrared to ultraviolet is one of the primary challenges for solar power conversion technologies. Intermediate band solar cells are one type of third generation photovoltaic devices in which an increase of the power conversion efficiency is achieved through the absorption of low energy photons while preserving a large band gap that determines the open circuit voltage. The ability to absorb photons from different parts of the solar spectrum originates from the presence of an intermediate energy band located within the band gap of the material. This intermediate band, acting as a stepping stone allows the absorption of low energy photons to transfer electrons from the valence band to the conduction band by a sequential two photons absorption process. Using the unique features of the electronic band structure of highly mismatched alloys GaAs1-xNx we have implemented a single junction intermediate band solar cell. Dilute nitride GaAs1-xNx highly mismatched alloy with low mole fraction of N is a prototypical intermediate band semiconductor with a well-defined energy band separated from the conduction band. The device demonstrates an optical activity of three energy bands that absorb the crucial part of the solar spectrum and convert it into electrical current. Currently, using chemical beam epitaxy we have fabricated intermediate band solar cell structures. The structures were characterized by a variety of structural and optical methods to optimize their properties for intermediate band photovoltaic devices. The results are in a good agreement with the predictions of the band anticrossing model for the electronic band structure of dilute GaAsN alloys.