Ch. Hof

On the Way Towards High Efficiency Thin Film Silicon Solar Cells by the “Micromorph” Concept

Note: IMT-NE Number: 222 Reference PV-LAB-CONF-1996-008 Record created on 2009-02-10, modified on 2017-05-10

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 1016/cm3 could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 A/s for microcrystalline silicon could be achieved.

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...

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...

H2-Dilution vs. Buffer Layers for Increased Voc

Note: IMT-NE Number: 218 Reference PV-LAB-CONF-1996-011 Record created on 2009-02-10, modified on 2017-05-10

Influence of electric field distortion and i-layer quality on the collection function of drift-driven a-Si:H solar cells

Note: IMT-NE Number: 306, http://www.unine.ch/web_pvlab/Publications/PS_files/preprint_306.PDF Reference PV-LAB-ARTICLE-2000-003doi:10.1016/S0022-3093(99)00913-8 Record created on 2009-02-10, modified on 2017-05-10

Stability of a-Si:H Prepared by Hot-Wire and Glow Discharge Using H2 Dilution Evaluated by Pulsed Laser Degradation

Abstract The quality of intrinsic amorphous silicon films prepared by different deposition techniques was investigated. For very high frequency glow discharge, both the substrate temperature as well as the hydrogen dilution were varied. These layers were compared to hot wire material produced at comparable temperatures. To study the stability of these films, an optimised degradation method was developed in which a pulsed dye laser was used in combination with a monochromatic steady beam to achieve a relatively fast stabilisation of the light induced degradation. The film quality was monitored by the photoconductivity and by the ambipolar diffusion length from which the material parameter, μ0τ0, was extracted. Taking into account the transport properties after light soaking as well as the optical absorption we concluded that the hot wire material could lead to more stable solar cells in comparison with plasma enhanced chemical vapor deposition material.

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

Long term behaviour of passively heated or cooled a-Si:H modules

We compare the outdoor performance of single junction a-Si:H PV-modules which were mounted in three different ways. One was thermally well isolated against convection and radiation losses in order to reach maximum operating temperatures. A second one was fixed onto a radiator to keep its temperature as close as possible to the air temperature. A third one served as a reference and was mounted with an open back side. The operating temperature of the modules could be strongly influenced by the different mountings. Although a high operating temperature results in a reduced V/sub OC/, it turned out to be beneficial for effective energy conversion in the long term due to a strongly reduced Staebler-Wronski degradation.

High efficiency p-i-n solar cells with layer deposited by hot-wire technique

More stable amorphous silicon (a-Si:H) material obtained with the hot-wire technique requires high substrate temperatures (T/sub sub/>300/spl deg/C) and low gas pressures (p/sub dep//spl ap/10/sup -2/ mbar). The second condition implies that heat transfer between the heater and the substrate is mainly dominated by thermal radiation; therefore, T/sub sub/ is strongly affected by the emissivity /spl epsi//sub surf/ of the growth surface. Here, it is shown experimentally that /spl epsi//sub surf/ depends on the thickness of the growing a-Si:H layer; a significant variation in /spl epsi//sub surf/ during the deposition of the first 3000 /spl Aring/ has been observed. With the help of a new heating concept, we integrated an intrinsic layer deposited at constant T/sub sub/=270/spl deg/C into a p-i-n solar cells with initial efficiency of 8.7%.