The integration of III-nitride and non-III-nitride materials for tandem solar cell applications can improve the efficiency of the photovoltaic device due to the added power contributed by the III-nitride (III-N) top cell to that of high-efficiency multijunction non-III-nitride (non-III-N) solar cells if the device components are properly designed and optimized. The tandem solar cell is comprised of a III-N top cell wafer bonded to a non-III-N subcell. The top cell is electrically isolated but optically coupled to the underlying subcell resulting in an unconstrained tandem cell design between the III-N top cell and the non-III-N subcell. The underlying subcell is a series-connected multijunction cell. The use of a III-N top cell is potentially beneficial when the top junction of a stand-alone non-III-N cell generates more photocurrent than the limiting current of the non-III-N subcell. The high energy photon absorption by the III-N top cell and the use of layer thickness optimization to adjust the amount of light absorbed by the junctions in the subcell result in less thermalization loss and a higher limiting current for the non-III-N subcell. This consequently results in improved total efficiency. When the non-III-N cell's top junction is the limiting junction, the minimum power conversion efficiency that the III-N top cell must contribute should compensate for the spectrum filtered from the multijunction subcell for this design to be useful.

As the absorption edge wavelength, lambdaN, of the III-N top cell increases, the performance of the multijunction subcell decreases due to spectral filtering. In the most common spectra of interest (AM1.5G, AM1.5D and AM0), the technology to grow InGaN cells with lambdaN < 520 nm is found to be sufficient for III-N top cell applications.

The device requirements for the InGaN top cell applications in terms of minimum required external quantum efficiency, maximum tolerable voltage offset and maximum tolerable degradation in fill factor and power conversion efficiency are presented. The minimum tolerable area ratio between the III-N and non-III-N subcells in a 3- or 4- terminal device is also determined. Furthermore, the effects of surface/interface reflections are also presented. The management of these reflection issues determines the feasibility of the integrated III-nitride/non-III-nitride design to improve overall cell efficiency.

Finally, the process for integrating a III-N and a non-III-N cell as well as preliminary results of a wafer-bonded InGaN/GaAs tandem solar cell are presented.