PUB - Publikationen an der Universität Bielefeld

In this thesis the influence of metallic nanoparticles on the absorption of hydrogenated amorphous silicon (a-Si:H) thin film solar cell devices is investigated. Small metal nanostructures with lateral dimensions well below 100 nm accompany strong absorption and large electric field amplitudes in their vicinity. This is caused by the localized surface plasmon (LSP) resonances that are excited upon interaction of light with the nanostructures. By combining silver nanoparticles (Ag NPs) with amorphous silicon photovoltaic devices the influence of the enhanced fields on the amorphous silicon absorption is investigated. In the presence of Ag NPs an enhanced optical absorption is measured, assigned to the LSP resonances. The Ag NPs are incorporated in different configurations in direct contact to the active a-Si:H layer of thin film devices. Irrespective of the device configuration an external quantum efficiency (EQE) signal is observed for photon energies below the bandgap of a-Si:H. States must be present that allow transitions for sub-bandgap energies. By a variation of the Ag NP position within the applied devices the role of interface states is evaluated. It turns out that defects in the a-Si:H material are responsible, that are created by the presence of the NPs and the internal surfaces. The according defect levels energetically lie within the a-Si:H bandgap. The exposure of defects to the strong fields in direct vicinity of the resonant absorbing NPs enable high transition rates from the defect levels to the conduction band. According to this mechanism, a model is proposed that in addition incorporates a charge compensation and transport process. Thermal escape provides the completion of transitions from the inner gap states to the nearest band edge (Fig. 5.11). No direct contact to the TCO (transparent conductive oxide) is necessary to provide carriers for charge compensation. The applied devices are a demonstration of the impurity photovoltaic (IPV) effect in a-Si:H. The defect states contributing to the observed transitions are broadly distributed in the band gap. Most dominant transitions belong to states at a typical distance of 0.15 eV from the valence band edge, while also states deeper in the gap with a distance of up to 0.5 eV contribute. Since these transitions are related to the enhanced fields caused by the LSP resonances, transitions are most dominantly observed when they overlap with the LSP resonance energy. Influencing the LSP resonance position by using different NP sizes also influences the position of dominant transitions. The observed signal in EQE measurements is shifted to longer wavelengths in agreement with a shifted LSP absorption. The energetic position of the Fermi level (EF) determines the occupation of inner gap defect states. When EF is shifted downwards by a variation of the doping concentration in the NP environment, the sub-bandgap response decreases. This is related to inner gap defect states that become unoccupied. Occupied defect levels near the valence band are necessary for the generation process. The decreasing tendency could also be caused by recombination processes related to the introduced doping impurities or due to limitations of the charge transport. For microcrystalline silicon (μc-Si:H) devices a similar EQE enhancement is found in the presence of Ag NPs for near or sub-bandgap light. With increasing Raman crystallinity of the host material, i.e. with decreasing bandgap, the IPV induced signal is shifted towards lower energies. The IPV effect is therefore not limited to the a-Si:H phase. The exposure of Ag NPs to the atmosphere decreases the measured EQE enhancement. This could be related to the formation of an oxide shell that changes the field strength at the defect location. The approach will likely not be feasible for the enhancement of solar cell performance. Parasitic absorption in the NPs decreases the EQE in the high absorption regime of a-Si:H. These losses are not compensated by the advantages in the sub-bandgap regime. However, the effect can be of interest for near infrared detector applications. Especially due to the enormous enhancement factors that are possible with these systems. Depending on the used photosensitive device structure, enhancement factors between 40 and 200-500 are achieved. The investigations show the feasibility of a controlled large area deposition of metallic nanostructures. This is definitely of interest for a variety of future applications that utilize plasmonic effects, either in photovoltaic or other thin film electronic devices.