| Zdroj: |
Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich Reihe Energie & Umwelt / Energy & Environment 259, vi, 184 S. (2015). = RWTH Aachen, Diss., 2015 |
| Popis: |
Investigations on the relation between the growth rate, material quality, and device grade condition for intrinsic microcrystalline silicon is presented in this thesis. Hydrogenated microcrystalline silicon deposited by plasma enhanced chemical vapor deposition is a widely used material for the absorber layer of the bottom solar cell in silicon thin-film tandem solar cells. Microcrystalline silicon is a mixed phase material consisting of crystal grains, amorphous phase, grain boundaries, and voids. To guarantee sufficient light absorption absorber layer thicknesses of more than 1 μm to 3 μm are required for the absorber layer of the bottom solar cell. The increase of the deposition rate for intrinsic microcrystalline silicon is one essential point for cost reduction in the mass production of thin-film solar cells. The combination of excitation frequencies in the very high frequency range altogether with the application of the high pressure depletion regime enabled to reach deposition rates up to 2.8 nm/s for optimal phase mixture material, which is until today considered to be of device grade quality. According to conductivity, electron spin resonance, and Raman measurements the quality properties of the material deposited at high deposition rates is similar to reference material deposited at low deposition rates. Nevertheless this material showed to be susceptible to oxygen uptake, which was shown to occur along the grain boundaries. Furthermore a decrease in crystal grain size with a simultaneous increase in tensile stress was observed by X-ray diffraction and Raman measurements, respectively. Thickness dependent Raman measurements showed a decrease in incubation layer thickness with increasing deposition rate. The investigations performed by X-ray diffraction and thickness dependent Raman measurements were supported by investigations performed with transmission electron microscopy. With this work it was found that the present criteria to classify microcrystalline silicon being of device grade quality should be extended for deposition rates beyond 1nm/s. In addition to the measures describing the optimal phase mixture quantities describing the materials microstructure, the tendency for oxygen uptake, and the mechanical stress should be taken into account. The device performance of microcrystalline thin-film single junction as well as of amorphous / microcrystalline thin-film tandem solar was observed to decrease with increasing deposition rate. The decrease in device performance was shown to be either related to inferior material quality of the microcrystalline absorber layer with increasing deposition rate and to an impairment of the pi-interface of the microcrystalline (sub) solar cell. Simulations on the impact of ions in matter showed that a damage of the pi-interface by ion bombardment is unlikely. As possible sources for the impairment ofthe pi-interface a variation of the nucleation conditions and structural inhomogeneities at the substrate/film interface are discussed. Despite the decrease in device performance of the amorphous / microcrystalline thin-film tandem solar cells calculations showed that the output of deposition systems in produced Watt per hour can be increased by more than a factor of two. An increase in system output leads to a decrease in costs per produced unit and can lead to a decrease in initial investment costs. |
