Photovoltaic solar cells
GaN-based photovoltaic solar cells have been discussed for quite some time. When alloying In, in principal the whole visible solar spectrum plus near-UV and IR can be accessed for charge-carrier generation. Several concepts have been tested, mostly based on nanowires  or quantum wells [181, 182]. QWs have the benefit of possible strained growth but the disadvantage of a low absorption volume. Bulk layers are difficult to grow without relaxation. Nevertheless, it is especially interesting to grow photovoltaic solar cell layers on low-cost substrates. For this, silicon is the ideal cell material, and in principle a simple tandem cell can be realized when growing InGaN with In contents above 40%, which is expected to yield a conversion efficiency above 30% .
First, tests of direct InGaN growth on Si(111), which should result in a low- contact resistance for p-Si/n-InGaN [41, 42], revealed that even at low growth temperatures below 700°C, meltback etching occurs. This effect can be reduced by applying a high In content AlInN seeding layer which suppresses a Ga-Si reaction. Nevertheless, InGaN growth is still highly columnar and In droplet formation difficult to avoid (Fig. 3.18). Taking into account the low In precursor efficiency of today’s MOVPE growth systems, this approach requires higher precursor efficiency to be competitive with other photovoltaic solar cell approaches.
Until now only InGaN-based cells based on GaN buffer layers or InGaN QWs have been realized. A GaN/Si tandem cell, however, was demonstrated
Fig. 3.18. TEM image of an InGaN layer with approximately 40% In on a thin AlInN seeding layer with In-content estimated above 40%. InGaN grows highly columnar with faceted tips. (TEM by A. Mogilatenko, HU-Berlin.) by Reichertz et al. , showing that in principal such cells are achievable, but in this case with additional UV illumination to generate electron hole pairs in the GaN top cell.