A. Laades

Physical and Technological Aspects of a-Si:H/c-Si Hetero-Junction Solar Cells

We report on the basic properties of a-Si:H/c-Si hetero-junctions, their effects on the recombination of excess carriers and its influence on the a-Si:H/c-Si hetero-junction solar cells. For this purpose we measured the gap state density distribution in thin a-Si:H layers, determined its dependence on deposition temperature and doping by an improved version of near UV-photoelectron emission spectroscopy. Furthermore, the Fermi level position in the a-Si:H and the valence band offset were directly measured. In combination with interface specific methods such as surface photovoltage analysis and our numerical simulation program AFORS-HET, we are able to find out the optimum in wafer pretreatment, doping and deposition temperature for efficient a-Si:H/c-Si solar cells without an i-type a-Si:H buffer layer. By a deposition at 210degC with an emitter doping of 2000 ppm of B2 H6 on a well cleaned pyramidal structured c-Si(p) wafer we reached 19.8 % certified efficiency

Wet Chemical Oxidation of Silicon Surfaces Prior to the Deposition of All-PECVD AlOx/a-SiNx Passivation Stacks for Silicon Solar Cells

Aluminum oxide (AlOx) is currently under intensive investigation for use in surface passivation schemes in solar cells. AlOx films contain negative charges and therefore generate an accumulation layer on p-type silicon surfaces, which is very favorable for the rear side of p-type silicon solar cells as well as the p+-emitter at the front side of n-type silicon solar cells. However, it has been reported that quality of an interfacial silicon sub-oxide layer (SiOx), which is usually observed during deposition of AlOx on Silicon, strongly impacts the silicon/AlOx interface passivation properties [1]. The present work demonstrates that a convenient way to control the interface is to form thin wet chemical oxides of high quality prior to the deposition of AlOx/a-SiNx:H stacks by the plasma enhanced chemical vapor deposition (PECVD).

Iron Gettering in CZ Silicon during the Industrial Solar Cell Process

We investigated the impact of using low quality feedstock such as recycled silicon and simplified pulling condition on the performance of CZ silicon solar cells. Groups of wafers carefully chosen from different ingots were analyzed after different solar cell process steps by minority carrier lifetime measurements, by measurements of the interstitial iron content and by measurements of the total impurity content using NAA. Our results show that the main electronic properties of the ingots, namely the carrier lifetime, interstitial iron content and base resistivity are strongly affected by feedstock quality. Surprisingly, high solar cell efficiencies were achieved using highly contaminated silicon. These positive results are due to the beneficial effect of impurity segregation gettering by phosphorous diffusion and aluminum alloying. Post-diffusion gettering by an additional annealing step was demonstrated to enhance the charge carrier lifetime.

Basic electronic properties and optimization of TCO/a-Si:H(n)/c-Si(p) hetero solar cells

We report on a detailed analysis of the basic electronic properties and the optimization of amorphous/crystalline silicon heterojunction solar cells (a-Si:H(n)/c-Si(p)). The gap states density of the ultrathin a-Si:H emitter on c-Si was determined by photoelectron yield spectroscopy. By varying the a-Si:H film thickness the valence band offset was determined to about 0.45 eV. The density of states at the a-Si:H/c-Si interface amounts to about 2/spl times/10/sup 11/cm/sup -2/eV/sup -1/ at midgap. This result was obtained by field dependent surface photovoltage measurements. In addition, photoluminescence measurements were performed to investigate the recombination at the a-Si:H/c-Si interface. To gain an optimized solar cell performance the deposition temperature of the a-Si:H and the gas phase doping concentration was varied. These optimizations lead to a maximum efficiency of 17.2% for a TCO/a-Si:H(n)/c-Si(p)/a-Si:H(p) solar cell fabricated using low temperature processes only.

Wet-Chemical Conditioning of H-Terminated Silicon Solar Cell Substrates Investigated by Surface Photovoltage Measurements

For further enhancement of solar energy conversion efficiency the passivation of silicon (Si) substrate surfaces and interfaces of Si-based solar cell devices is a decisive precondition to reduce recombination losses of photogenerated charge carriers. These losses are mainly controlled by surface charges, the density and the character of rechargeable interface states (Dit) [], which are induced by defects localised in a small interlayer extending over only few Å. Therefore, the application of fast non-destructive methods for characterization of the electronic interface properties directly during the technological process has received an increasing interest in recent years.

Association of FeB Pairs under Illumination

It has been found that association of iron-boron (FeB) pairs takes place during illumination of iron contaminated boron-doped silicon. The established FeB pair model is interpreted with respect to the quasi-Fermi level position, the stability condition of FeB pairs and the steady state reaction between interstitial iron, electrons and boron. At an excess charge carrier density of Δn = 5.4·1014 cm-3, still 5 % of the interstitial iron associates to FeB pairs.

Wet-Chemical Preparation of Textured Silicon Solar Cell Substrates: Surface Conditioning and Electronic Interface Properties

The dominance of crystalline silicon (Si) in photovoltaics can be ascribed partly to the extensive knowledge about this material, which has been accumulated in microelectronics technology. Methods to passivate Si interfaces, which were developed for microelectronic device technologies, have been extended to solar cell manufacturing in the past. These methods, however, have been optimised for polished substrates, and do not work so effective with textured surfaces, which commonly used in the fabrication of high efficiency Si solar cells to enhance anti-reflection properties.

On the Trade-Off between Industrially Feasible Silicon Surface Preconditioning Prior to Interface Passivation and Iron Contaminant Removal Effectiveness

We are investigating the effect of different wet chemical surface preconditioning sequences for silicon wafers prior to the deposition of aluminum oxide based passivation layers coated by plasma enhanced chemical vapor deposition. We are focusing on the development of a simple and industrially feasible preconditioning process to achieve a high level of interface passivation after the firing process applied to industrial solar cells. Our process optimization is monitored by characterizing the passivation quality before and after a firing process. We are also investigating the effectiveness of the removal of residual surface iron concentrations by the wet chemical process.

Improvement of Silicon Solar Cell Substrates by Wet-Chemical Oxidation Studied by Surface Photovoltage Measurements

The deposition of thin and ultra-thin layers requires extremely clean, smooth and defect-free Silicon (Si) substrate surfaces as starting point. The preparation-induced surface micro-roughness and surface coverage of the substrates often affect the initial layer growth, the morphology or the adhesion of deposited layers. Si device fabrication includes multiple wet cleaning and etching steps involving different oxidizing and etching solutions, which modify the surface electronic properties according to fixed charges and defect states present on the surface. Depending on the details of the device structure, surface conditioning methods have to be carefully optimized to achieve the desired electronic interface properties.