Growth and Characterization of Thin Film Nanocrystalline Silicon Materials and Solar Cells

Abstract

The need for electrical energy is growing fast as a result of the expanding world population and economic activities. On top of this the energy need of each individual is also growing. At present the growth in energy demand is not matched by the growth in energy generation because of insufficient energy production. This energy gap therefore needs to be bridged. In addition, most conventional means of energy generation are not environmentally-friendly and in turn affect human lives. Solar energy is one of the alternatives for renewable energy generation. It can be extracted directly in electrical form from solar radiation using photovoltaic (PV) solar cells or solar concentrators. The PV market is dominated by crystalline-silicon based solar cells. However, thin-film silicon solar cells are becoming increasingly important, because they are deposited at relatively low temperatures and as such offer the possibility to produce flexible and light-weight solar panels. The latter can be applied on for instance the roofs of factory buildings. Thin film nanocrystalline silicon (nc-Si:H) is an important material for application in thin-film silicon solar cells. It has been mainly developed because unlike amorphous silicon (a-Si:H) it is stable against light-induced degradation, and because with this material the solar cells have an extended spectral response up to the infrared wavelength region. Because of this extended response nc-Si:H films are used in combination with a-Si:H to form multi-junction solar cells, leading to an increased solar-cell efficiency. The focus of this thesis is on the development and characterization of nc-Si:H layers and solar cells. In chapter 3 we present results of research on how the deposition parameters during the growth affect nc-Si:H material properties and device performance when using radio frequency plasma enhanced chemical vapor deposition. Particular attention is paid to p-type and intrinsic films and their application in nc-Si:H solar cells. For the p-layer development, the effects of deposition power, the substrate roughness, and doping are investigated. Intrinsic layer investigation focused on the effect of substrate temperature, deposition power, deposition pressure, and the gas-flow ratio. Intrinsic layers deposited at the amorphous-to-nanocrystalline transition during growth are investigated in detail. At this transition nc-Si:H films with favorable properties for application in thin film solar cells can be made. Within our growth regime, this transition shows high sensitivity to the deposition parameters hence narrowing the process window. We show that materials deposited at the amorphous-to-nanocrystalline transition, but at different deposition settings, can have similar crystalline mass fraction while showing different electrical properties. In chapter 4 we present how seed layers are used to enhance crystal nucleation at the onset of nc-Si:H growth. By this approach, uniform and rapid evolution of the crystalline mass fraction as a function of thickness is obtained. Our results show a possibility to grow thin-film nc-Si:H without the usual amorphous incubation layer. A depth profile Raman technique that enables the ex-situ investigation of the crystalline mass fraction depth profile in nc-Si:H films is presented. This approach reveals different growth phases in the development of nc-Si:H. From transmission electron microscopy (TEM) analyses, we observe that crystal sizes are not affected by seed layers. However, horizontal cracks are observed to dominate the early growth of nc-Si:H in p-i-n solar cells and this effect is reduced upon seeding. For the n-i-p cells the appearance of these cracks is not affected by seeding. X-ray diffraction (XRD) results indicate that the use of seed layers does not affect the crystal size, but affects the preferential orientation of the crystals. Solar-cell external parameters show that seeding of p-i-n solar cells leads to an increase in solar cell efficiency, mainly due to increase in the short-circuit current density. The investigation of seeding on the crystallinity development is further extended in chapter 5. Here, we show that different substrates have different profile for the development of the crystalline mass fraction. For the three substrates investigated, we found a gradual development of the crystalline mass fraction, starting from the amorphous incubation layer. By means of seeding, rapid nucleation is observed as indicated by the high value of the crystalline mass fraction from the onset of growth. The effect of the substrate is reduced as all three substrates show a similar development profile of the crystalline mass fraction upon seeding. In the last chapter of this thesis, the possibility to use Raman spectroscopy to determine the preferred crystal orientation in nc-Si:H is demonstrated. The preferred orientation of crystals in nc-Si:H can give insight into the film growth mechanism and is often determined from transmission electron microscopy and x-ray diffraction. The method presented in this thesis is based on the fact that molecular vibrations in films under polarized light give rise to polarization-dependent Raman scattering intensity, depending on the grain crystal orientation of the irradiated material. This approach has been tested on a series of nc-Si:H samples and the results comparable with x-ray diffraction results.