Intermediate Band Solar Cells
Intermediate band (IB) solar cells is a relatively new solar cell concept, that have efficiency limits that are 50% higher than for conventional solar cells, but so far very attempts have been made to realize the concept. The Solar cell physics group is involved in both theoretical studies of such cells and in fabrication of new materials for the realization of these devices.
The most important results from the theoretical studies is that the partial filling of the intermediate band can be achieved through photo-filling. For high light exposure the efficiency is similar for a photo-filled and prefilled (e.g. due to doping) IB solar cell [Ref. Strandberg and Reenaas, J. Appl. Phys. 105, 124512 (2009)]. The theoretical investigations have also included development of a drift-diffusion model for photo-filled IB solar cells, studies of how thermalization of the population in the IB affects the cell performance for a finite IB width and finally, how the IB solar cell can perform better if spectrally selective filters are being used.
For the ralization of IB solar cells the group has focussed on materials that utilized quantum confinement in quantum dots (QDs) for the introduction of an intermediate band in the bandgap. The material system studied has been InAs QDs in an AlGaAs matrix. The QDs are fabricated using molecular beam epitaxy (MBE). The focus has been on optimizing the growth parameters to achieve as high QD density as possible. Densities above 1011 cm-2 were acheieved with is in the range of what is needed. The realization of devices based on these materials is in progress.