N. Beck

Mobility lifetime product - a Tool for Correlating a-Si:H Film Properties and Solar Cell Performances

The missing correlation between film characteristics and a‐Si:H‐based p‐i‐n solar cells is still a controversial subject. The authors present a new parameter μ0τ0, evaluated from steady‐state transport measurements on a‐Si:H layers, which can indeed relate film quality and cell performance as far as the latter is limited by the quality of the intrinsic 〈i〉 layer. Thereby, two specific features of the evaluated μ0τ0 product can explain its successful role as a quality parameter for a‐Si:H: First, the computation of μ0τ0 takes into account the effects of the prevailing dangling bond occupation, which is very different in uniform films as compared to the occupation profile prevailing through the i layer of a p‐i‐n solar cell; second, the evaluated μ0τ0 product combines information about band mobility and defect density; furthermore it avoids some of the well‐known pitfalls of usual deep defect density measurements such as constant photocurrent method and photothermal deflection spectroscopy. Experimental dat...

Structural properties and electronic transport in intrinsic microcrystalline silicon deposited by the VHF-GD technique

A series of microcrystalline samples was deposited by the very high frequency glow discharge (VHF-GD) technique, with various input powers while keeping all the other parameters of deposition constant. The goal was to correlate transport and structural properties and avoid as much as possible the problem of a variation of the Fermi level between the samples. The observed decrease of the photoconductivity and of the product mobility-lifetime of hole (as measured by time of flight, TOF) with the increase of the power was surprisingly not connected to the structural properties, which remain approximately unchanged, but with a surface contribution to the transport properties.

Ambipolar diffusion length and photoconductivity measurements on ‘‘midgap’’ hydrogenated microcrystalline silicon

Hydrogenated microcrystalline silicon (μc‐Si:H) deposited by VHF plasma‐enhanced chemical vapor deposition has recently been proven to be fully stable, with respect to light‐induced degradation, when adequately used in p‐i‐n solar cells. Stable solar cells efficiencies of 7.7% have been obtained with single‐junction cells, using ‘‘midgap’’ microcrystalline i‐layers, having an optical gap of around 1 eV. In the present paper, the electronic transport properties of such microcrystalline layers are determined, by the steady‐state photocarrier grating method (SSPG) and steady‐state photoconductivity measurements, in a coplanar configuration. The conditions for the validity of the procedure for determining the ambipolar diffusion length, Lamb, from SSPG measurements (as previously theoretically derived in the context of amorphous silicon) are carefully re‐examined and found to hold in these μc‐Si:H layers, taking certain additional precautions. Otherwise, e.g., the prevalence of the ‘‘lifetime’’ regime (as opp...

The “Micromorph” cell: a new way to high-efficiency-low-temperature crystalline silicon thin-film cell manufacturing?

Hydrogenated microcrystalline Silicon (μc-Si:H) produced by the VHF-GD (Very High Frequency Glow Discharge) process can be considered to be a new base material for thin-film crystalline silicon solar cells. The most striking feature of such cells, in contrast to conventional amorphous silicon technology, is their stability under light-soaking. With respect to crystalline silicon technology, their most striking advantage is their low process temperature (220 °C). The so called “micromorph” cell contains such a μc-Si:H based cell as bottom cell, whereas the top-cell consists of amorphous silicon. A stable efficiency of 10.7% (confirmed by ISE Freiburg) is reported in this paper. At present, all solar cell concepts based on thin-film crystalline silicon have a common problem to overcome: namely, too long manufacturing times. In order to help in solving this problem for the particular case of plasma-deposited μc-Si:H, results on combined argon/hydrogen dilution of the feedgas (silane) are presented. It is sho...

Transport Properties of Compensated µc-Si:H

Note: IMT-NE Number: 220 Reference PV-LAB-CONF-1996-015 Record created on 2009-02-10, modified on 2017-05-10

Fast Deposition of μc‐Si:H by Restrictive Dilution and Enhanced HF‐Power

Note: IMT-NE Number: 258 Reference PV-LAB-ARTICLE-1997-009 Record created on 2009-02-10, modified on 2017-05-10

The “Micromorph” Cell: a New Way to High-Efficiency-Low-Temperature Crystalline Silicon Thin-Film Cell Manufacturing?

Note: IMT-NE Number: 235 Reference PV-LAB-CONF-1997-006 Record created on 2009-02-10, modified on 2017-05-10

Electronic Transport and Structure of Microcrystalline Silicon Deposited by the VHF-GD Technique

Electronic transport parallel and perpendicular to growth direction has been studied in a series of microcrystalline silicon samples obtained by various dilutions of silane in hydrogen. It is clearly shown that the transport properties (dark conductivity, drift mobility, ambipolar diffusion length and photoconductivity) under dark and under illumination conditions are enhanced as the dilution is increased. Furthermore, these films exhibit no degradation upon light-soaking. X-Ray diffraction patterns of the samples and the transport properties, as well as a preferential orientation along the growth direction. A similar correlation is found with the shift of the Si-H stretching mode peak of the infrared spectra (IR). Because transport properties have been measured by different techniques (dark conductivity, ambipolar length and photoconductivity in the direction perpendicular to growth direction, drift mobility in the direction parallel to growth direction), no statement can be made about a possible anisotropy in transport, as it would be expected from the columnar shape of the crystallites.

Microcrystalline Silicon Solar Cells at Higher Deposition Rates by the VHF-GD

Note: IMT-NE Number: 240 Reference PV-LAB-CONF-1997-021 Record created on 2009-02-10, modified on 2017-05-10

Determination of the quality of a-Si:H films: "true" transport parameters

For the characterisation of a-Si:H layers, steady-state photoconductivity (SSPC) and steady-state photocarrier grating (SSPG) are currently used. But the /spl mu//spl tau/-products deduced from these measurements are a function of the prevailing dangling bond occupation in the film and, thus, are not a measure of material quality. In the present paper the authors introduce the product /spl mu//sup 0/ /spl tau//sup 0/, which is independent of dangling bond charge and which monitors material quality in terms of band mobility /spl mu//sup 0/ and capture time /spl tau//sup 0/, the latter being, in its turn, inversely proportional to dangling bond density. They present a simple evaluation procedure to deduce /spl mu//sup 0//spl tau//sup 0/ from simultaneously performed SSPC and SSPG measurements. In undoped, but slightly n-type a-Si:H, the quality of the material will typically be related to the ambipolar diffusion length rather than to photoconductivity. The application of this method to a series of undoped a-Si:H films and solar cells incorporating these materials exhibited a good correlation between film properties and cell performances.