Intermediate band solar cell: Proof of concept

Despite years of extensive efforts, so far, there has been no unambiguous demonstration of the intermediate or multiband solar cell. Using unique properties of GaNAs highly mismatched alloys, we have designed, fabricated and tested an intermediate band photovolataic device. The device was grown on n-type GaAs substrate using metalorganic chemical vapor deposition with hydrazine as the N source. It consists of an active GaN_xAs_1-x film embedded in AlGaAs layers for blocking the charge transport between the intermediate band and the charge collecting contacts. The device was characterized using a variety of methods, including photomodulated reflection, electroluminescence and PV characteristics measurements. Combination of all the measurements provides an unambiguous evidence for three optical transitions between the conduction, the valence and the intermediate band. The corresponding transition energies of 0.89 eV, 1.15 eV and 1.99 eV are in a good agreement with the predictions of the band anticrossing model of the electronic band structure of GaN_xAs_1-x with x=0.024. Measurements of the external quantum efficiency indicate an onset of the increased charge collection for the photon energies corresponding to the optical transitions from the valence to the intermediate and from the valence to the conduction band. In addition, I/V measurements show an increased open circuit voltage indicating that the charge separation is determined by the largest energy gap between the conduction and the valence band. The measurements of the electroluminescence clearly indicate the presence of depletion regions next to the charge transport blocking layers.