Light absorption and radiative recombination in thin-film solar cells

Solar cells and light emitting diodes are generally the same kind of device. Whereas solar cells are optimized for light absorption, light emitting diodes are optimized for light emission, i. e. radiative recombination. Both processes are present in each of these devices. The electroluminescence depends on the transport of injected charge carriers and radiative recombination, whereas the external quantum efficiency originates from light absorption and the extraction of photo generated charge carriers. According to Donolato and Rau, the external quantum efficiency and the luminescence are connected by the reciprocity relation. However, the reciprocity relation only holds under certain circumstances. Whereas these circumstances are given in defect-free solar cells made from crystalline silicon for instance, the situation can be different in thin-film devices. The physics in thin-film devices can be affected by localized inter-band defect states, which also affect the reciprocity relation. These states are found in thin-film chalcopyrite Cu(In, Ga)Se2 np heterojunction, hydrogenated microcrystalline silicon pin, and hydrogenated amorphous silicon pin devices as investigated in this thesis. This thesis is structured within this sequence, studying systems with increasing concentrations of defect states in their band gap to investigate these circumstances, where the reciprocity relation still holds.