Silicon nanostructures for photovoltaics

The basic motivation of this thesis is to explore a path beyond the 29% fundamental efficiency limit of crystalline silicon (c-Si) through a reduction of the spectral losses of single-junction solar cells. The overall concept envisages a c-Si tandem solar cell by implementing band gap engineered Si NCs as a high band gap absorber of the top solar cell. The first necessary step towards a Si NC solar cell is the demonstration of a high open-circuit voltage (higher than with bulk Si). The feasibility of a high open-circuit voltage is demonstrated in this thesis by means of a theoretical model in combination with experimental data for the optical properties. Experimental investigations were performed with Si nanocrystals (Si NCs) embedded in silicon dioxide (SiO2) or silicon carbide (SiC), both prepared according to a superlattice approach and high-temperatures thermal annealing (up to 1150°C). Calibrated absorption- and photoluminescence spectra of Si NCs in SiO2 are analyzed according to the generalized Planck law of radiation to determine the open-circuit voltage by optical means, i. e. without the need of selective contacts to the Si NC film. It is shown that the generalized Planck law is not valid to describe the luminescence of Si NCs embedded in SiO2, but the spectrum is rather a superposition of single luminescence centers which is broadened by the size distribution. Si NCs in a SiC matrix were investigated motivated by the poor electrical transport through the SiO2 matrix. An electrical characterization is presented and related to the structural properties. In previous Si NC solar cell test devices, either the performance was adversely affected by the fabrication processes, or the structure did not allow for a separation of the Si NC and substrate contributions to the measurement. To remedy this shortcoming, a novel, membrane-based device structure has been developed in the framework of this thesis. This device structure permits to decouple the high-temperature annealing needed to form Si NCs from the preparation of the selective contacts and thus enables for the first time an unambiguous characterization of Si NC films in a minority carrier device. Membrane-based p-i-n solar cells were prepared with Si NCs embedded in SiC as the absorber layer and doped amorphous silicon carbide (aSixC1x: H) and indium tin oxide (ITO) as selective contacts. Based on a detailed general device analysis, illumination-dependent IV measurements were employed to determine the fundamental transport and recombination properties, the effective mobility lifetime product, on device level.