The last two decades have seen some major breakthrough in developing III-nitride semiconductor materials and devices, leading to high efficient III-nitride based emitters for solid state lighting represented by InGaN based blue emitters. This also led to the award of the Nobel Prize in 2014. III-nitride semiconductors (GaN and its alloys such as InGaN) exhibit a wide range of bandgap from 3.4 eV of GaN to 0.69 eV of InN, incorporating the whole visible spectral range and thus matching the solar spectrum very well. As a result, III-nitrides can be employed for the fabrication of solar energy devices potentially with high conversion efficiency. Solar-powered water splitting would be the most promising approach towards the solar energy conversion into renewable and storable energy. However, so far there are only a limited number of reports using III-nitrides. My research projects concentrated on a number of prototypes of GaN based photoelectrodes featuring nanostructures which have been designed and then fabricated, leading to a step-change in conversion efficiency.