Optical spectroscopy of carrier dynamics in semiconductor nanostructures

Most commercial optoelectronic devices are currently made of bulk semiconductors, whose fundamental optical properties feature limitations. In solar panels, which are mostly made of bulk Si, the high-energy photons are inefficiently converted into electricity due to fast thermalisation, and the low-energy photons are not absorbed. A possible way for improvement is down-scaling the material dimensions toward the material-specific exciton Bohr radius (for most materials between ~1-50nm), allowing quantum confinement effects to modify the properties. Another promising approach is doping the material with rare earth ions to which energy transfer can take place. Both routes are elaborated upon by investigations of the spectral and temporal characteristics of recombination and relaxation processes of excited carriers in nanostructures of Si, GaN and perovskites. Among others, the photoluminescence properties of Si nanocrystals as function of the excitation light intensity are studied. These results demonstrate that the radiative (photon) emission rate of Si nanocrystals can be enhanced upon laser-induced heating, which provides a possible avenue to enhance the optical faculty of Si and could also be relevant for the use of Si nanocrystals in some photovoltaic applications. The carrier multiplication process, which can enhance the efficiency of photovoltaic and electronic devices by generating multiple carriers upon the absorption of a single high-energy photon, is investigated and discussed for perovskites nanocrystals, which show promising characteristics for a wide range of applications.