C. Beneking

Solution of the ZnO/p contact problem in a-Si:H solar cells

Abstract This paper addresses the problem of preparing a good p-layer contact with zinc oxide as TCO. Our approach was to deposit pin cells with different p-layer recipes on ZnO coated SnO2:F and on uncoated SnOi2:F in one run, in order to obtain a direct comparison of the interface properties of the two TCO materials under the condition of as equal as possible surface morphology. The pin cells prepared on the ZnO surface exhibit a lower fill factor (FF). Our experiments demonstrate that the hydrogen interaction with the ZnO surface plays the most decisive role for the ZnO/p contact. We explain the observed effects using a band diagram of the ZnO/p interface and show that the accumulation layer at the ZnO surface — caused by atomic hydrogen in the plasma — is responsible for the low FF in pin cells. Based on this model the contact problem is solved by introducing a μc-n-Si intralayer between ZnO and p-layer resulting in an identical high FF on both ZnO and SnO2 substrates.

Interfaces in a-Si:H Solar cell structures

Abstract The performance of amorphous silicon based solar cells depends on the tailored properties of the various layer materials making up the cell structure as well as on the properties and on the design of the interface regions between the layers. The electronic properties related to the various interfaces are markedly influenced by the Fermi level position within these regions, and by structural properties and chemical compositions resulting from the preparation conditions. Results are presented for the p/i and the TCO/p interfaces and discussed with respect to device performance. Further examples of interface effects are described which are related to chemical reactions and hydrogen diffusion in the course of sample preparation.

Novel light-trapping schemes involving planar junctions and diffuse rear reflectors for thin-film silicon-based solar cells

Optical optimization of planar junction amorphous and microcrystalline silicon-based thin-film solar cells offers improved conversion efficiency. Planar junction solar cells with diffuse and selective angle rear reflectors were examined in detail. The refractive index of the diffuser material needs to be as close to that of silicon as possible. Insulating diffuse rear reflectors must be used in conjunction with conductive rear-window layers of the appropriate band gap, conductivity and refractive index. The refractive index of the aear window layer should be at least as large as that of the diffuser material. Rear windows comprised of ZnS, CdS and ZnSe (as well as other II–VI semiconductors and semiconductor alloys) used with a TiO2 diffuse reflector could increase the current of optically thin silicon solar cells beyond that possible using rough TCO/glass substrates. Selective angle reflectors must also be comprised of either the solar cell semiconductor itself (not possible in many cases) or comprised of a material having properties similar to those of the conducting rear windows used with the diffuse reflectors.

Material Basis of Highly Stable a-Si:H Solar Cells

We achieved a stabilized efficiency of 9.2 % after only 8 % relative degradation for an a-Si:H/a-Si:H stacked cell with the top-cell i-layer prepared at 140 °C using a high hydrogen dilution of the silane process gas. From a comprehensive characterization of p-i-n cells and the corresponding i-layer material prepared at 140 °C and 190 °C substrate temperature with different hydrogen dilutions, we conclude that the performance of these pin cells strongly correlates with the material properties of the corresponding i-layers. High fill factors after light soaking are reflected in a good microstructure, high photo-conductivity, and relatively low defect density. Whereas the initial V oc is limited by interface recombination, volume recombination dominates the forward-dark current after light soaking. The stabilized Voc as well as the short-circuit current densities correlate with the optical bandgap of the i-layer.

Spectral response modelling of a-Si:H solar cells using accurate light absorption profiles

An optical model calculating the generated carrier profile in a-Si:H solar cells deposited on hazy and nonhazy transparent conducting oxide (TCO) has been integrated into a numerical transport and recombination program. A comparison of simulated and measured reflectances of pin structures opens up a possibility to check the prepared layer thicknesses and the optical device properties. Furthermore, the examination of the current gain employing a ZnO/metal back contact reveals the importance of using accurate light absorption profiles for spectral response modelling and device characterisation of pin solar cells.

Optoelectronic Properties of Thin Amorphous and Micro-Crystalline p-Type Films Developed for Amorphous Silicon-Based Solar Cells

VIIF-PECVD at 110 MI-z was used to deposit micro-crystalline p-layers on glass substrates for detailed analysis and onto ZnO coated substrates for incorporation into p-i-n solar cell structures. Solar cell and film analysis confirmed that the films incorporated into the solar cells contained significant crystalline silicon volume fractions despite being only 30 nm thick. The p-i-n solar cells employing a micro-crystalline silicon p-layer deposited on ZnO coated substrates had series resistances, fill factors and V oc similar to those of the reference solar cells deposited onto SnO 2 coated substrates and having optimized amorphous silicon-carbon p-layers. The short circuit current of the micro-crystalline p-layer case was 10 percent lower than that of the reference cell indicating that further optimization is required.

Measured and Simulated Temperature Dependence of A-Si:H Solar Cell Parameters

In contrast to the successful application of analytic equations to the current-voltage behavior of crystalline silicon solar cells in the dark and under AM1.5 illumination, the description of a-Si:H solar cells parameters requires device modelling concepts taking the full set of semiconductor equations into account. This in particular holds for the explanation of the temperature dependence (225--400K) of experimentally determined a-Si:H p-i-n solar cell parameters. Device modelling calculations show that the observed decrease of the short circuit current at AM1.5 with lower T is much more effected by the additional charge trapped in the tail states and recharging of defect states than by the broadening of the gap. The induced electric field distortion blocks the extraction of photo generated holes. The open circuit voltage V{sub oc} increases with lower T which is caused by the same trapping effect.

Improvement in stabilized efficiency of a-Si:H solar cells through optimized p/i-interface layers

This paper focuses on the relation between p/i-interface layer properties and the light stability of the corresponding solar cells. In a series of cells containing different p/i-interface structures large differences in relative degradation were found. These differences are explained by a redistribution of the electric field due to the insertion of different p/i-interface layers leading to different collection from the i-layer volume in the course of i-layer degradation. Based on this hypothesis, the authors developed an optimized design of the p/i-interface region, which increases the initial efficiency without introducing additional degradation. a-Si:H/a-Si:H stacked cells including this new design exhibit only 12% degradation after 300 hours of one Sun light-soaking. A stabilized efficiency of 9% was achieved.