Characterization and simulation of three-dimensional solid-state solar cells

Introduction : The newest PV technology that uses less material and cheaper materials and deposition methods (in order to lower the cost even more) resulted in extremely thin absorber solar cells, or ETA-cells. Such ETA-layers need to be applied to a nano-structured substrate. Dye-sensitized solar cells (DSSC’s) already fully use this potential but encounter a lot of long-term stability problems. In the cells studied in this thesis, both the organic absorber dye and the liquid electrolyte of the DSSC are replaced by a solid-state semiconductor and are called 3D-cells. The nanostructured substrate is nanoporous titanium dioxide (TiO2). Objective : Charge carrier generation and their transport to the contacts is separated in 3D-cells. This seems to be an advantage with respect to the one-dimensional standard and thin film technology, where charge generation and transport take place in the same materials. Our and other’s nanostructured solid-state cells however, do not seem to benefit from this three-dimensional feature like DSSC’s do. The question is why? Interface recombination : Different kind of nano- and microstructured 3D-cells are characterized and analyzed. In particular a dynamic model is presented which enables us to quantify the influence of recombination in these cells. Interface recombination turns out to be a crucial element in the determination of the final performance of a 3D-cell. Diffusion models : Detailed effective medium models are set-up which describe both steady-state and dynamic response of the cells. In one of the models, the cell is treated as an infinite network of flat band unit cells (i.e. there is no electric field on a microscopic level). In the other, an analytic diffusion model is set up with boundary conditions specific for the treated cell. Both models assume diffusion as the main driving force for electron transport; charge separation occurs due to the difference in electron negativity of the materials. Results : The diffusion coefficients that are found are in the same range as those found for DSSC’s. The main difference is in the lifetime of the electrons which is 104 times smaller for 3D-cells than for DSSC’s. Both analytic approximations of and full simulations with the analytic diffusion model revealed that the diffusion length is much smaller than the cell thickness, and also than the absorption depth of the light. In that case, the characteristic time constant associated with the break frequency of the optoelectronic photocurrent transfer function is equal to half the electron life time, the characteristic time constant associated with the break frequency of the optoelectronic photovoltage transfer function is equal to the electron life time. The quantum response is equal to the product of the electron diffusion length and the absorption coefficient, which in our case indeed yields a quantum response that is much smaller than one. At last, the analytic diffusion model turned out to represent the nanostructured solar cell in the same way as the proposed infinite network of flat-band unit cells