Growth and optical properties of III-V semiconductor nanowires: : Studies relevant for solar cells

Sammanfattning: This thesis deals with epitaxial growth and optical properties of semiconductor nanowires with implications and insights about materials for solar cells. The chapters leading up to the papers give a broader background and an introduction to the field and include nanowires and their synthesis, semiconductor properties, solar cell operational principles, light interaction with a nanowire array, and optical characterization of such arrays.The nanowires were grown from gold seed particles using metal organic vapor phase epitaxy. Growth of ternary GaInP nanowires has been developed with a triethylgallium precursor that has not been commonly used before for this material structure in nanowires. We have achieved high yield and wide composition range nanowires with high control, which will be a crucial element for development of tandem solar cells where a high band-gap GaInP nanowire array could be the top cell.In terms of optical properties, lifetimes and carrier dynamics are important parameters for optoelectronic devices, including solar cells. We have investigated surface passivation of nanowires by capping GaAs nanowires with in situ grown shells, at the same time evaluating the possibility of measuring time-resolved photoluminescence signal of as-grown nanowires, even when the substrate is made of the same material. We have identified that depending on doping levels in the substrate and the nanowires, excitation wavelength can be chosen to separate nanowire signal from the substrate signal. Moreover, we have preliminary proposed a simple way to extract doping, which needs to be tested more extensively in the future. Such measurements of as-grown arrays could provide a fast and completely non-destructive characterization method for solar cell materials and allow further processing of the devices.Further, we have investigated reflectance and transmittance of flexible nanowire arrays embedded in a transparent polymer. Such flexible membranes could be interesting as flexible solar cells on their own, or could be incorporated on top of a lower band-gap material, for example silicon, to create a tandem solar cell. We have identified two potential issues with such structures. First of all, gold can absorb a significant fraction of the incoming light. The gold particles can be etched away, which recovers transmittance for the long wavelengths. However, a resonant reflectance peak is then observed. Through our work, we have identified that in-plane array modes arise in nanowire arrays embedded in a polymer that lead to resonant reflectance or absorptance in weakly absorbing materials. Such effect would be detrimental for transmitting long wavelengths to the bottom cell. Thus, we have investigated how these resonances depend on geometry in order to give guidelines for controlling this effect.