Quantification of Efficiency Losses in Cu(In,Ga)Se2 Solar Cells

In thin film technology, CuInxGa1-xSe2 (CIGS) solar cells are prominent due to their tunable bandgap, high efficiency and stability. Despite significant achievements in CIGS technology, a gap remains between the theoretically calculated efficiency and the experimentally measured device performance. For this reason, I developed a simulation framework in the first part of the thesis to calculate realistic efficiency limits for CIGS solar cells, which combines two different solar cell theories: the Shockley-Queisser and the Classical Diode Theory. To combine these two theories, I implemented photon recycling (PR), which refers to the reabsorption probability of incoming photons within the device itself, with one-dimensional drift-diffusion solvers that also include bandgap grading. Simulations were performed by varying limiting factors, including front and back grading, back surface, interface recombination velocity, electron and hole mobilities, Shockley-Read-Hall lifetime, and back reflection at the interface of the Mo/CIGS structure. I investigated under which circumstances photon recycling has a significant effect on device performance and study the optimal bandgap grading for different material properties. In the second part of the thesis, in order to quantify efficiency losses, I analyzed recombination losses using the transient photoluminescence method. I performed the experiments under different light intensities and used the differential lifetime method for CIGS solar cells to investigate the relation between the carrier density and the charge carrier lifetime. As a next step, I complemented experimental analysis with both analytical modelling and numerical simulations. This combined analysis emphasises the influence of carrier mobility on the charge carrier lifetime, and intensity dependence of radiative and non-radiative losses, which also includes the effect of capacitive charging-discharging on recombination dynamics.