Generalized detailed balance theory of solar cells

From the detailed balance between absorbed and emitted radiation follows the efficiency limit of Shockley and Queisser (SQ-limit). In the framework of the SQ-theory, the exact working principle and the transport mechanism in the solar cell are irrelevant. The efficiency is only determined by the absorptance and emissivity of the material, which are equal according to Kirchhoff’s law. This abstraction from all internal properties of the solar cell makes the SQ-theory general, however, it also implies a fundamental difference between the SQ-theory and typical device simulators, which base on the solution of the continuity equations for charge carrier transport. The present dissertation presents a model, which is compatible with the SQ-theory and which includes the internal details of the device like e.g. charge carrier transport. The simplest version of the model solves the continuity equations for electrons and holes as well as the Poisson equation and allows thereby to simulate inorganic pn and pin-junction solar cells. This model is used for instance to calculate the mobility dependent efficiency limit of SiO2/Si superlattice absorbers. To allow simulations of organic solar cells, the model has to include the continuity equation for excitons, which are relevant in organic materials due to the higher exciton binding energy. The special geometry of organic solar cells, which usually consist of a bulk heterojunction, is also considered. The model for organic polymer-fullerene solar cells allows both the reproduction of experimental data on current/voltage and quantum efficiency measurements and the prediction of the device behaviour as a function of mobility, band offsets and the geometry of the phase separation. The final version of the model also includes the effect of multiple exciton generation, meaning the creation of multiple electron/hole pairs or excitons from a single high energy photon. The model for multiple exciton generating absorber layers allows the identification of possible bottlenecks as well as ...