New advances in multi-junction solar cells
InGaN offers great potential in developing high-efficiency multi-junction photovoltaic (PV) devices due to its strong absorption properties and wide range of bandgaps. However, its conversion efficiencies because of different technological factors are still not high enough to be competitive in the market. Within SOLARIN (Solar cells based on InGaN nanostructures on silicon), scientists followed new approaches on InGaN solar cell design that can lead to obtaining multi-junction cells that cover the entire solar spectrum and offer high conversion efficiencies. Important considerations such as stability under high amounts of solar radiation and compatibility with silicon technology were fully integrated into the project. Research evolved from fundamental studies on single InGaN-on-silicon layers to the fabrication and characterisation of devices integrating InGaN in the p-doped region. Scientists used plasma-assisted molecular beam epitaxy to control InGaN layer growth, obtaining high-quality InGaN films with high In content (30 %). InGaN was grown on a GaN buffer on a transparent substrate: sapphire. However, one can use the same method for silicon substrates. When synthesising films with high In content, film thickness and composition depend not only on the growth conditions, but also on the strain state imposed by the underlayers. Scientists reported interplay between stress relaxation and In content in the thick InGaN layers. The team also developed a double p-type contact design based on nickel and gold, thereby overcoming concerns related to transparency and carrier collection. While the newly developed solar cells are still way off from record-setting ones, work was innovative since neither surface treatment nor top or back coating were used. SOLARIN sought to make sure that Europe stays at the forefront of high-efficiency PVs. Wide-band InGaN solar cells are a major enabler for an increasing array of technologies that span building-integrated PVs, solar-powered electric fences, solar-powered water pumps, rural electrification with PV systems, water treatment systems, and space- and solar-powered vehicles.
Keywords
Multi-junction, solar cells, InGaN, photovoltaic, silicon, solar-powered