We report on III-nitride photovoltaic cells with external quantum efficiency as high as 63%. InxGa1−xN/GaN p-i-n double heterojunction solar cells are grown by metal-organic chemical vapor deposition on (0001) sapphire substrates with xIn=12%. A reciprocal space map of the epitaxial structure showed that the InGaN was coherently strained to the GaN buffer. The solar cells have a fill factor of 75%, short circuit current density of 4.2 mA/cm2, and open circuit voltage of 1.81 V under concentrated AM0 illumination. It was observed that the external quantum efficiency can be improved by optimizing the top contact grid.

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The use of InGaN photovoltaic devices as a top cell in a tandem solar cell has the potential to improve the power conversion efficiency of multi-junction devices. The effects of the InGaN top cell’s external quantum efficiency, voltage offset, and fill factor on the integrated III-nitride/non-III-nitride solar cell’s power conversion efficiency are presented. The results are summarized into the III-nitride device parameter requirements for top cell applications. The minimum acceptable area ratio between the III-nitride and non-III-nitride subcells in a 3- or 4-terminal device is also determined.

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 top cell to that of high-efficiency multi-junction non-III-nitride solar cells if the device components are properly designed and optimized. The proposed tandem solar cell is comprised of a III-nitride top cell bonded to a non-III-nitride, series-constrained, multi-junction subcell. The top cell is electrically isolated, but optically coupled to the underlying subcell. The use of a III-nitride top cell is potentially beneficial when the top junction of a stand-alone non-III-nitride subcell generates more photocurrent than the limiting current of the non-III-nitride subcell. Light producing this excess current can either be redirected to the III-nitride top cell through high energy photon absorption, redirected to the lower junctions through layer thickness optimization, or a combination of both, resulting in improved total efficiency. When the non-III-nitride cell's top junction is the limiting junction, the minimum power conversion efficiency that the III-nitride top cell must contribute should compensate for the spectrum filtered from the multi-junction subcell for this design to be useful. As the III-nitride absorption edge wavelength, k N , increases, the performance of the multi-junction subcell decreases due to spectral filtering. In the most common spectra of interest (AM1.5 G, AM1.5 D, and AM0), the technology to grow InGaN cells with k N < 520 nm is found to be sufficient for III-nitride top cell applications. The external quantum efficiency performance, however, of state-of-the-art InGaN solar cells still needs to be improved. 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. V C 2012 American Institute of Physics. [http://dx.

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