Numerical Modeling and Optimization of InxGa1?xN Silicon Multi-Junction Tandem Solar Cell

This thesis studied silicon based InxGa1−xN multi-junction tandem solar cells. To investigate the the multi-junction tandem solar cells, we developed a simulation model by using 1D Poisson and drift-diffusion solver with an assumed effective tunneling layer. As we know, if we want to calculate the current-voltage curve, we have to calculate the
tunneling probability between the heavily doped n layer and p layer. However the 1D Poisson and drift-diffusion solver does not include the function to handle the tunneling current. Therefore, we assume that there is a virtual tunneling layer between the heavily doped n layer and p layer by changing the tunneling layer potential bandgap,
recombination coefficient to explain influences of defect state level to the tunneling probability.
Besides, we propose the concept of electron/hole blocking layer for the photovoltaic. We discusses the effect of electron/hole blocking layer on the photovoltaic performance of the single junction solar cells. The study shows that with a pure electron blocking on the p-type doping Si and a pure hole blocking layer on n-type doing, it is possible
to enhance the open circuit voltage and short circuit current. Therefore, the Ga2O3 and TiO2 materials are chosen as the electron and hole blocking layer. The result shows that the open circuit voltage increases from 0.65 eV to 0.80 eV, and the short circuit current increases from 35.1 mA/cm2 to 35.9 mA/cm2 , where the power efficiency can increase from 21.9 % to 27.6 %.
Finally, we use the above studies to investigate the performance of the silicon based InxGa1−xN multi-junction tandem solar cells. We find the optimized condition of the In composition and layer thickness of crystalline Si for the highest efficiency double-junction and triple-junction solar cells. After that, we analyzes the polarization effect on
of the silicon based InxGa1−xN tandem junction solar cells with different top layer. Under the different InxGa1−xN growth face, the polarization charge at the heterojunction will induce different polarization field which can reduce or enhance the ability of the photo-generated holes collection. Making a great difference to the efficiency of the InxGa1−xN tandem junction solar cell. In our simulation, the graded
InxGa1−xN tandem junction solar cells growth on the N-face can be the electron blocking layer which enhance the performance of the solar cells. The efficiency increases from 29.6% to 33.4%.