Solar cell thermodynamics including multiple impact ionization and concentration of radiation

The simultaneous effect of impact ionization and the concentration of radiation on solar cell performance is analysed. For maximum solar concentration and an infinite number of impact ionizations one obtains a maximum cell efficiency (0.845). This corresponds to an optimum reduced driving force qV / E g = 0.882, given a ratio of T s / T p ~300/6000 ( q , V and E g are electron electric charge, voltage and bandgap energy, respectively, while T s and T p are ambient and sun temperature, respectively). With full concentration the optimum reduced driving force equals the Carnot factor 1- T c / T p exactly ( T c is cell temperature). In the case of no illumination, there is a negative driving force. This is related to the coefficient of performance of a Carnot refrigeration engine. In the limit of an infinite number of impact ionizations, the Carnot factor occurs again, this time as an upper bound to the optimum reduced driving force.