%0 Thesis
%A Müller, Thomas Christian Mathias
%T Light Absorption and Radiative Recombination in Thin-Film Solar Cells
%V 272
%I RWTH Aachen
%V Dr.
%C Jülich
%M FZJ-2015-04874
%@ 978-3-95806-068-5
%B Schriften des Forschungszentrums Jülich Reihe Energie & Umwelt / Energy & Environment
%P ii, 146 S.
%D 2015
%Z RWTH Aachen, Diss., 2015
%X 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)Se$_{2}$ 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. The requirements of the reciprocity relation are investigated with temperature and charge carrier injection dependent experiments as well as comprehensive simulations. Since the electroluminescence is affected by series resistance, in most cases it is complemented with photoluminescence, where charge carrier transport is negligible. An expanded Fourier transform infrared spectroscopy setup is used to perform the luminescence experiments, and the external quantum efficiency is measured with a constant photocurrent method setup. The simulations use a commercial one dimensional numerical device simulator, which solves the optics and the semiconductor equations. A self-developed program, which uses full band diagrams of the onedimensional device simulator as input parameters, yields the luminescence and external quantum efficiency calculations. The luminescence in Cu(In,Ga)Se$_{2}$ originates from transitions between the bands and localized band-tail states. Since the band-tail densities of states are rather steep, the almost unshifted luminescencespectra are compatible to the external quantum efficiency in terms of the reciprocity relation. Around room temperature, the radiative ideality factor, which is determined from luminescence/voltage characteristics by fitting a common diode law, is found to be close to unity. However, temperature dependent experiments and simulations show that the reciprocity relation only holds if the thermal energy is higher than the characteristic energy of the band-tail densities of states. Transitions between band-tail states yield the luminescence in hydrogenated microcrystalline silicon. However, the reciprocity relation between the luminescence and the external quantum efficiency only holds in a small spectral range, since the band-tails are broad, i.e. their characteristic energies are higher than the thermal energy, even at room temperature. This yields blue shifted luminescence spectra with increasing charge carrier injection and a temperature dependent radiative ideality factor, which exceeds unity and increases with decreasing temperature. However, detailed simulations show that the reciprocity relation holds if the device under luminescence conditions is close to thermal equilibrium. These conditions yield very low luminescence intensities, which are not accessible within experiments. The luminescence in hydrogenated amorphous silicon originates from transitions between localized band-tails, which are even broader than in hydrogenated microcrystalline silicon. Additionally, around room temperature and/or at low injection, transitions between band and neutral amphoteric mid-gap defect states contribute to the luminescence beside the tail-to-tail luminescence. The band-to-defect luminescence spectra do not shift under different charge carrier injection levels, since the occupation of mid-gap defect states is almost constant, even if the charge carrier injection is moderate. This yields a radiative ideality factor of around two. However, the presence of additionally Stokes-shifted tail-to-tail luminescence, where the luminescence looses photon energy to the lattice, completely hampers the validity of the reciprocity relation within experiments. Only simulations can show that the reciprocity relation holds if the device is almost in thermal equilibrium at extreme low charge carrier injection.
%F PUB:(DE-HGF)11 ; PUB:(DE-HGF)3
%9 Dissertation / PhD ThesisBook
%U https://juser.fz-juelich.de/record/202690