Generalized detailed balance theory of solar cells

The principle of detailed balance is the requirement that every microscopic process in a system must be in equilibrium with its inverse process, when the system itself is in thermodynamic equilibrium. This detailed balance principle has been of special importance for photovoltaics, since it allows the calculation of the limiting efficiency of a given solar cell by defining the only fundamental loss process as the radiative recombination of electron/hole pairs followed by the emission of a photon. In equilibrium, i. e. in the dark and without applied voltage, the absorbed and emitted photon flux must be equal due to detailed balance. This equality determines the radiative recombination from absorption and vice versa. While the classical theory of photovoltaic efficiency limits by Shockley and Queisser considers only one detailed balance pair, namely photogeneration and radiative recombination, the present work extends the detailed balance principle to any given process in the solar cell. Applying the detailed balance principle to the whole device leads to two major results, namely (i) a model that is compatible with the Shockley-Queisser efficiency limit for efficient particle transport, while still being able to describe non-ideal and non-linear solar cells, and (ii) an analytical relation between electroluminescent emission and photovoltaic action of a diode that is applied to a variety of different solar cells. This thesis presents several variations of a detailed balance model that are applicable to different types of solar cells. Any typical inorganic solar cell is a mainly bipolar device, meaning that the current is carried by electrons and holes. The detailed balance model for pn-type and pin-type bipolar solar cells is therefore the most basic incorporation of a detailed balance model. The only addition compared to the classical diode theory or compared to standard one-dimensional device simulators is the incorporation of photon recycling, making the model compatible with the Shockley-Queisser limit and the classical diode theory