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Abstract
Silicon quantum dot (Si QD) embedded in its dielectric matrix is regarded as a promising structure for the next generation photovoltaics with high efficiency due to its potential to tune the effective bandgap through quantum confinement which allows fabrication of optimized tandem devices. However, the properties of Si QD material should be further studied as the energy conversion efficiency of such Si QD based devices is relatively low so far. This thesis focuses on the optical characterization of Si QD materials and possible routes for the performance improvement of all Si tandem solar cells.
Two comprehensive and non-destructive methods to extract optical constants of Si QD materials have been studied in this thesis. For the investigation of transmittance (T)/reflectance (R) spectra, an improved “Hishikawa approach” which combines the advantages of global fitting and point by point fitting is realized. For the application of ellipsometry, a homogeneous mixture model with Cody-Lorentz and Gaussian combined oscillator is developed to simulate the optical properties of Si QD materials. In addition, the photoluminescence (PL) spectra of Si QD films is discussed and a model based on spontaneous emission and the size distribution of the QDs is developed to fit the PL spectrum. With this model, the QD size and its distribution can be analysed quantitatively using the PL spectra only saving the need of time consuming and destructive characterization methods such as transmission electron microscopy (TEM). The optical bandgap can be extracted naturally from this PL model.
The optical and electronic properties derived from the aforementioned approaches are then put into perspective by including them in the calculation of efficiencies in solar cells that could be fabricated with these films. The practical efficiency limit of Si QD material based double junction solar cell is studied. The simulation is based on material properties such as minority carrier lifetime and mobility rather than simple Shockley-Queisser assumptions. Simulation results show that the practical efficiency limit of a double junction cell (Si QD material with 1.6eV bandgap as top cell and 25% efficient c-Si PERL cell as the bottom cell) is around 32%. Based on this simulation, a roadmap to improve the performance of such tandem solar cells is outlined.