J. Daey Ouwens

Interpretation of the silicon‐hydrogen stretching doublet in a‐Si:H hydrogenated amorphous silicon

We studied the correlation between the infrared integrated absorption strengths of the wagging, bending and stretching modes for four hydrogenated amorphous silicon (a‐Si:H) series, deposited by rf glow discharge. The strength of these modes was varied by changing the deposition temperature while keeping other parameters fixed. For the first time it is shown that a straightforward correlation exists between these three modes. It is argued that the 2100‐cm−1 mode is due to SiH2 bonds and that there is no contribution of SiH species on the inner surface of voids. The vibrational spectrum of a‐Si:H can be well described by considering the vibrating dipole as a harmonic oscillator with an effective charge e*=0.44 electrons.

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.

Effect of hydrogen dilution on the bonding configurations in a-SiC:H

Abstract We have studied the effect of hydrogen dilution of the rf glow discharge gas mixture on the properties of amorphous silicon carbon a -Si 1−x C x :H films. The chemical bonding structure has been determined from infra-red (IR) absorption structural analysis. Elastic Recoil Detection (ERD) was used to determine the composition. It is shown that hydrogen dilution reduces the density of CC bonds rather than the density of CH n species.

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.

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.

Improving the stability of amorphous silicon tandem cells

We present an optimization procedure for a-SiC:H/a-Si:H tandem cells to limit light induced degradation effects. Optimum deposition conditions are combined with an optimum light trapping configuration, that has been determined by optical modelling. Upon light soaking, a remarkable light-induced behaviour is observed. The open circuit voltage (V/sub oc/) in the degraded state is higher than in the annealed state.