Theory of Contact-limited Solar Cells

The International Panel on Climate Change has made it clear that drastic action is required in order to prevent warming of global average temperatures from reaching 1.5-2.0 °C above the pre-industrial average. If this is to be achieved, global power generation from photovoltaics will need to increase by more than an order of magnitude. Most of the dominant and upcoming photovoltaic technologies, including silicon and metal-halide perovskites, are limited more by their contacts than recombination in the bulk of the absorber. While we have a very good understanding of how bulk recombination limits the efficiency of a solar cell, we do not completely understand how contact processes determine the efficiency of a solar cell. This work attempts to fill this gap in the literature by considering a solar cell model that is completely dominated by solar photon generation and contact recombination. The partial currents of electrons and holes at both contacts to an intrinsic absorber are assumed to be linearly proportional to the excess carrier density at the contacts. By linking the currents across the device with the continuity equation, assuming the quasi-Fermi levels are approximately flat, and adding the partial currents at each contact, an expression for the current-voltage behavior can be algebraically calculated. The resulting analytic expression provides useful qualitative and quantitative insights into how the four equilibrium exchange current densities, which determine the rate of electron and hole extraction at both contacts, shape the current-voltage curve. In particular, it demonstrates that the features of the curve depend on the relative rate at which a particular carrier (electron or hole) is collected at one contact vs. the other. The model provides a unified explanation for non-ideal contact related behavior seen in the literature, such as S-shaped curves and dark/light crossover (i.e. failure of superposition). The work will be insightful for researchers investigating technologies with yet-to-be optimized contacts. This dissertation includes both previously published/unpublished co-authored material