Influence of Substrate on the Microstructure of Microcrystalline Silicon Layers and Cells

Julien Claude Bailat, Arvind Shah
2002
Article

Résumé

Hydrogenated microcrystalline silicon i-layers were deposited with a fixed silane concentration on various substrates and incorporated into n-i-p solar cells. 'Average crystallinity' and detailed microstructure of layers and devices were evaluated by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The role of the substrate is thereby shown to be very critical; a change in the substrate can cause a transition from amorphous to microcrystalline growth in micrometer-thick layers. When crystalline growth occurs, the layer microstructure is depth-dependent. © 2002 Elsevier Science B.V. All rights reserved.

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A series of nip-type microcrystalline silicon (μc-Si:H) single-junction solar cells has been studied by electrical characterisation, by transmission electron microscopy (TEM) and by Raman spectroscopy using 514 and 633 nm excitation light and both top- and bottom-illumination. Thereby, a Raman crystallinity factor indicative of crystalline volume fraction is introduced and applied to the interface regions, i.e. to the mixed amorphous-microcrystalline layers at the top and at the bottom of entire cells. Results are compared with TEM observations for one of the solar cells. Similar Raman and electrical investigations have been conducted also on pin-type μc-Si:H single-junction solar cells. Experimental data show that for all nip and pin μc-Si:H solar cells, the open-circuit voltage linearly decreases as the average of the Raman crystallinity factors for top and bottom interface regions increases. © 2003 Elsevier B.V. All rights reserved.

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High Rate Growth of Microcrystalline Silicon by VHF-GD at High Pressure

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Microcrystalline silicon growth using very high frequency-glow discharge PECVD has been studied under conditions of high pressure and high VHF-power conditions. Hereby, the influence of the total gas flow and the silane concentration on the deposition rate has been investigated. Deposition rates of over 25 Å/s have been achieved at relatively low total gas flows of 100 sccm. These high-rate films show device-grade quality with respect to subband gap absorption and microcrystalline structure. Dark conductivity measurements reveal midgap character and transmission electron microscopy investigations confirm a highly crystalline microstructure from the bottom to the top of the μc-Si:H films. These high-rate μc-Si:H layers are very interesting candidates for solar cell and other devices. © 2002 Elsevier Science B.V. All rights reserved.

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