III–V Nanowires for Solar Energy Harvesting—From Growth to Integration in Substrate-Free Devices

By marrying III–V material systems with a quasi-one dimensional geometry, III–V nanowires comprise an ideal structure for solar energy harvesting. Resonant light trapping and waveguiding effects in individual nanowires of subwavelength diameter permit enhanced optical absorption of solar radiation. Multiple scattering events and in-plane waveguiding within arrays of vertically-standing nanowires can further enhance light absorption. The nanowire geometry also presents the opportunity for large interfacial areas allowing efficient charge carrier separation, for example at core–shell junctions in GaAs nanowire solar cells, and at photoelectrode–electrolyte interfaces in (In)GaN nanowire photoelectrochemical water splitting devices. These efficiency improvements can be achieved despite the microscopic quantity of III–V material present in the nanowire arrays, using up to 10⁴ times less material compared to planar structures [1].<br/>To achieve the full potential of nanowires for solar cells and photoelectrochemical water splitting, a number of materials challenges must be addressed. Scalable approaches to growth are needed, for example III–nitride nanowire growth by metalorganic vapour phase epitaxy (MOVPE). We have investigated the self-assembly of GaN nanowires by MOVPE and found that nitridation of the growth substrate, and the formation of a thin AlN layer, leads to N-polar nanowire growth on sapphire substrates. A further challenge is the passivation of nanowire surfaces, which is essential for reducing non-radiative losses and reaching high open circuit voltages [2].<br/>In addition, nanowire arrays can be embedded in a flexible transparent polymer and removed from the substrate [3]. This has the dual advantages of creating an ultra-thin flexible device, and permitting immediate re-use of templated substrates in subsequent growth runs. We have identified the mechanical constraints that dictate the choice of embedding polymer and limit the geometric properties of the nanowire array.<br/>Together, these advances will enable free-standing membranes of nanowires optimised for solar photovoltaics and photoelectrochemical fuel generation.<br/>[1] L Gao; Y Cui; J Wang; A Cavalli; A Standing; TT Vu; MA Verheijen; JE Haverkort; EP Bakkers; PH Notten, Nano Lett. 14:3715-9 (2014)<br/>[2] N Jiang, HJ Joyce, P Parkinson, J Wong-Leung, HH Tan, C Jagadish, Front. Chem. 8: 1136 (2020)<br/>[3] SA Baig, JL Boland, DA Damry, HH Tan, C Jagadish, HJ Joyce, MB Johnston, Nano Lett. 17:2603-2610 (2017)