E. Conrad

Electrical transport mechanisms in a-Si:H/c-Si heterojunction solar cells

We present temperature-dependent measurements of I-V curves in the dark and under illumination in order to elucidate the dominant transport mechanisms in amorphous silicon-crystalline silicon (a-Si:H/c-Si) heterojunction solar cells. ZnO:Al/(p)a-Si:H/(n)c-Si/(n+)a-Si:H cells are compared with inversely doped structures and the impact of thin undoped a-Si:H buffer layers on charge carrier transport is explored. The solar cell I-V curves are analyzed employing a generalized two-diode model which allows fitting of the experimental data for a broad range of samples. The results obtained from the fitting are discussed using prevalent transport models under consideration of auxiliary data from constant-final-state-yield photoelectron spectroscopy, surface photovoltage, and minority carrier lifetime measurements. Thus, an in-depth understanding of the device characteristics is developed in terms of the electronic properties of the interfaces and thin films forming the heterojunction. It is shown that dark I-V cu...

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

Optimization of Interface Properties in a-Si:H/c-Si Heterojunction Solar Cells

We report on the optimization of hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells which were completely processed at temperatures below 230degC. Efficient solar cells based on both n-type and on p-type c-Si substrates were performed. In contrast to the approach from Sanyo no additional a-Si:H(i) buffer layer was used. Instead the conditions of the amorphous silicon preparation by conventional plasma enhanced chemical vapor deposition (PECVD) were optimized and several wet- or plasma chemical treatments were applied to improve the interface properties. The highest efficiencies so far are 17.4 % on p-type c-Si wafers and 19.8 % on n-type c-Si wafers

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.

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.