M.B. Von Der Linden

Defect and Band Gap Engineering of Amorphous Silicon Solar Cells

This paper demonstrates that the incorporation of an unoptimized, wide band gap a-SiC:H layer near the p-type emitter layer in addition to a graded bandgap ”buffer” layer, leads to improved fill factors and open circuit voltages, in spite of the increased number of recombination sites at the p/i heterojunction. The as deposited as a function of a-SiC:H thickness shows an optimum of 10.5 % at a thickness of 10 – 20 A. We have further improved this type of cell by incorporating a reverse carbon graded p-type layer and have thus achieved efficiencies in excess of 11.0 %. The cells are all amorphous and do not comprise antireflective coatings or enhanced back reflectors. A new defect engineering scheme to accomplish enhanced stabilized efficiencies of amorphous silicon solar cells is also proposed here.

Apparent “gettering” of the Staebler-Wronski effect in amorphous silicon solar cells

The stability behaviour of intrinsic amorphous silicon materials incorporated in a p+-i-n+ solar cell structure is considerably different from that observed by electrical characterization methods in individual thin films. This is due to the fundamental difference in Fermi-level position in a single layer compared to the situation occuring in devices. We have employed the differences in the re-equilibration behaviour that have been observed in various intrinsic materials when the Fermi-level is shifted towards the valence band edge, in order to design a cell with a new profiled i-layer which would possess an improved electric field distribution after light soaking compared to cells with a constant i-layer. The contribution of the interface region to the stabilized conversion efficiency is greatly improved, whereas the first 50 nm of the cell structure remain unchanged. Thus, it appears that the Staebler-Wronski effect is gettered away from the junction, much like the impurity gettering concept in crystalline solar cells.

Stability of amorphous silicon materials incorporated in solar cells and intrinsic layer profiling for enhanced stabilized performance

Abstract This paper shows that the study of bulk intrinsic materials in itself can not be used as a guideline for stability enhancement that is obtainable in p + -i-n + solar cells. Good stability observed in a single layer by typical characterization methods does not simply lead to more stable solar cells upon implementation of this layer in the p + -i-n + structure. We attribute this lack of correlation to the significant and fundamental difference in the position of the Fermi level in a single layer during Staebler-Wronski re-equilibration compared to the situation occurring in devices. We propose and demonstrate a latent defect engineering approach leading to enhanced stabilized efficiency in amorphous silicon solar cells.

Further study of the determination of the density of gap states by thermally stimulated conductivity

To determine the density of gap states by the thermally stimulated conductivity (TSC) technique, values for the μτ-product have to be deduced. For this purpose a new method, i.e. photoconductivity matching thermostimulated conductivity (PMTSC) is introduced. Using this method, the DOS and μτ-product of different a-Si:H films, deposited by RF and VHF GD, are presented. For higher electric fields the position of the TSC peaks near ≈ 100 and 300 K shifts to lower temperatures and the attempt-to-escape frequency veff increases.

Effects of Electrode Spacing and Hydrogen Dilution on a-SiC:H and a-Si:H Layers

A series of hydrogenated amorphous silicon carbide (a-Si 1–x C x :H) films was deposited by rf glow discharge deposition using various pressures, electrode spacings and hydrogen dilution ratios. We found that improvement of the structure by hydrogen dilution is more effective when a large electrode spacing is applied. In the case of undiluted a-SiC:H, the product of pressure and electrode spacing appears to be the important parameter. Dilution causes an increase of the photoconductivity. The band gap decreases but increases again for highly diluted samples. A striking result is that the Fourier transform infra-red spectroscopy (FTIR) bands assigned to CH X and SiH x increase upon dilution when a small electrode spacing is applied, although the hydrogen content is reduced. It is shown that this is due to an increase of the density of the films and to an increase of the amount of carbon built into the bulk instead of into voids. The combination of decreasing hydrogen content, void fraction and increasing amount of carbon atoms into the bulk explains the behaviour of the photoconductivity and band gap as a function of H 2 dilution.

Inhomogeneities in PECVD deposited a-Si:H films induced by a spacing between substrate and substrate holder

Abstract Homogeneous deposition of a -Si:H films on glass substrates is routinely done by plasma deposition. A major cause of non-uniformity is the presence of a spacing between substrate and (grounded) electrode to which it is mounted. We have observed a reduced deposition rate if a gap exists between the glass substrate and the metal electrode. The amount of deposition-rate reduction scales with the size of this gap. A reduction of 50% of the initial deposition rate is measured in the case of a gap of 2-mm thickness, for a 65 MHz SiH 4 /H 2 plasma at 0.35 mbar. In addition, the material quality is affected. Filling the gap with a dielectric (Corning 7059 glass) leads to a smaller reduction, i.e., of 20%. These effects are explained by using the equivalent electrical circuit of the plasma reactor system: an extra capacitor representing the gap is added. For a conductive substrate no deposition-rate reduction is observed, which supports the electrical origin of the effect.

Progress in inexpensive a-Si:H/a-Si:H tandem module technology

This paper describes a research and technology development project within the framework of the EU JOULE II program in order to implement a-Si:H/a-Si:H tandem solar cell structures developed in the laboratory in the manufacturing procedure for large area integrated modules at NAPS France. In less than 20 test runs utilizing a stacked a-Si:H/a-Si:H tandem solar cell structure, the authors have obtained current-matched PV panels (30 cm/spl times/90 cm) with an initial power output equal to conventional single junction panels. Stability tests performed outdoor showed that tandem modules are indeed more stable than their single junction counterparts. This is accomplished without adding production steps with respect to single junction module production. Therefore the tandem fabrication concept will lead to a drastic reduction in the cost per peak Watt, even at small production levels.

The Density of States in a-Si:C:H Revealed by Electrophotography

Electrophotographic dark decay measurements have been used to determine the surface density of states (SDOS) of a-Si:C:H. Injection of trapped charge from these deep states into the conduction band governs the dark discharge of a photoconductor, provided bulk generation and bulk space charge are negligible. It is found that the SDOS profiles peak around 0.60 eV below the conduction band for materials with different carbon concentration. This observation implies that the energy position of these states is fixed with respect to the conduction band edge, even though the optical band gap of these materials increases with increasing carbon concentration. The nature of these states may be ascribed to D − states, whose density is strongly enhanced by filling D ° states when the material is charged negatively. Furthermore, we observed that the SDOS around 0.60 eV below the conduction band edge is approximately the same for materials with up to 8 at.% carbon. From temperature dependent measurements a value of 2·10 8 s −1 was obtained for the attempt-to-escape frequency.