Numerical simulation of high-efficiency CIGS-based solar cells

Abstract

This thesis aims to quantitatively identify and study, through numerical simulations, the properties and the main factors that limit the performance of the state-of-the-art CIGS solar cells, and develop new solutions to increase the efficiency of this type of solar cells. As properties of the absorber material the content of gallium (GGI) and copper (CGI) inside the CIGS were analyzed, finding the best grading profile and the optimum CGI to increase the efficiency of the cells. Depending on the CIGS properties, also temperature dependent J-V characteristics, in particular the roll-over effect, and the admittance spectroscopy, focusing on the capacitance step, were studied.

In the second part of this work the attention was shifted on the limiting factors of the solar cells, both non-radiative recombinations and optical losses were analyzed. For recombinations: bulk traps, interface traps at CIGS/Buffer surface and the presence of grain boundaries were taken into account; in all of these cases the behavior of the efficiency varying the carriers lifetime was studied. From an optical point of view the optical losses for both the record cells (EMPA and ZSW) were studied, observinging that the layer which causes the highest optical loss is the window one (AZnO).

The last part of the manuscript was dedicated to the possible solutions to increase the efficiency of the thin-film CIGS solar cells. Starting from electrical solutions the introduction of surface passivation and point contacts at the CdS/CIGS interface was studied, a detailed analysis was performed on the geometry of point contact introducing the chemical passivation and the field effect passivation. It was showed that a positive charge in the order of 1∙1012 cm-2 inside the passivation can improve the efficiency of the cell and it helps to relaxing the requirements on the point contact geometry. The same solution at the back of the cell was simulated, so back-side point contacts were taken into account in order to reduce back-surface recombinations and increase the photo-generated carrier collection. Also in this case both chemical and field-effect passivation were analyzed. As optical solution the introduction of a back reflector was analyzed and it was observed that the best material which can be used like mirror is the Aluminum. The substitution of the buffer material was also studied, replacing CdS with Zn(O,S) and CdZnS.