Arizona State University and University of California Santa Barbara (UCSB) in the USA have compared indium gallium nitride (InGaN) solar cells produced using non-polar, semi-polar and polar substrates [Xuanqi Huang et al, Appl. Phys. Lett., vol110, p161105, 2017]. The non-polar m-plane devices showed the best overall photovoltaic (PV) performance.
InGaN solar cells should be able to convert photons of energy spanning the solar range, with InN having an energy gap of 0.7eV (infrared) and GaN 3.4eV (ultraviolet). In practice, charge polarization of the III-nitride bond leads to strong electric fields in heterostructures arising from spontaneous and strain effects. Growing III-nitride structures in non-polar or semi-polar directions can eliminate or at least reduce these fields and improve performance.
http://www.semiconductor-today.com/news_items/2017/may/asu_100517.shtml