III–V Solar Cells
The III–V semiconductor materials provide a range of opto-electronic properties well suited to bandgap engineering and high efficiency solar cells. The design process, III–V growth and fabrication methods are described for homogeneous and heterogeneous structures, and the magnitude of fundamental thermal and radiative losses for important III–V solar cell materials calculated. An analytical model is presented, analysing solar cell performance in detailed terms of processes in the space charge region and charge neutral layers of solar cells. The model formulates the solar cell radiative efficiency as a function of bias, providing a quantitative measure of how close devices come to the ideal efficiency limit. Single junction pin and record efficiency pn GaAs cells are analysed and their radiative efficiency quantified, concluding that radiatively dominated behaviour is reached in the more efficient np design. Tandem and triple junction III–V concepts are reviewed and efficiency limits placed in the context of achievable designs. Experimental data are modelled for both structures and the radiative efficiency quantified. The more radiatively efficient tandem design is found to be closer to its fundamental efficiency limit for a radiatively dominated dual junction structure, as a consequence of lower non-radiative recombination rates. The application of III–V materials to quantum confined structures is finally reviewed with specific regard to the quantum well solar cell and its demonstrated 90% radiative efficiency at high bias as a result of the lower bandgap undoped multiple quantum well region.