Publikationen / Ergebnisse
(Englisch)
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The central subject of this project phase was to continue research on the "micromorph" solar cell concept: a new solar cell concept based on the combination of microcrystalline and amorphous solar cells in a tandem configuration and pioneered by IMT in 1994; the general goal was to demonstrate the potential of this new type of thin-film silicon solar cells. Hereby, different subtasks have been studied in order to obtain a better understanding of the individual problems that need to be solved in order to fabricate, in an economically acceptable way, high-efficiency "micromorph" tandem solar cells. While in the precedent project phase ('94-96) IMT was basically the only group doing active and successful research on microcrystalline silicon solar cells, there has been during the last three years the establishment of an "internationalcommunity" working intensively on microcrystalline silicon and on "micromorph" tandem solar cells. On the other hand, many research groups have also started to try out the VHF-GD (Very High Frequency Glow-Discharge) deposition technique, an original deposition method that was introduced by IMT in 1986. The VHF technique is indeed convincing more and more groups, because of its potential for high deposition rates both, for microcrystalline as well as amorphous silicon and because of its usefulness in the growth of high-quality microcrystalline silicon layers. The success of microcrystalline silicon as low-band gap material goes today so far, that different groups (e.g. Canon Corp.) have now substituted their former research activities on silicon-germanium alloys by work on microcrystalline silicon. Thus, the justification of thisproject is certainly given: It is today clear that the concepts proposed by IMT have a high potential for use in the next generation of low-cost solar cells.Within this period we achieved the following individual results which are all related to the micromorph concept:1.) TEM (Transmission electron microscopy) investigations have revealed that microcrystalline silicon (_c-Si:H) does not constitute a single standard microcrystalline material but is, on the contrary, a semiconductor with a large variety of different microstructures or microtextures. These forms of microstructure depend essentially on the applied deposition conditions and on the properties of the substrates and underlying layers.2.) Systematic studies on different microcrystalline films have shown that the observed absorption enhancement for VHF-deposited _c-Si:H with respect to crystalline wafer silicon is mainly due to the as-grown surface texture of plasma-deposited _c-Si:H films. To obtain even higher effective absorption one needs to improve the surface roughness, while still keeping the sub-bandgap absorption (defect absorption) low.3.) Electronic transport measurements suggest that the behaviour of the mobilityxlifetime products (_ô-products) in _c-Si:H material is similar to that of _ ô-products in amorphous silicon (a-Si:H). We therefore conclude that electronic transport in _c-Si:H layers is governed by the amorphous phase present at the grain boundaries.4.) Intensive deposition studies on microcrystalline single-junction devices have led to an improvement of the open circuit voltage Voc to values clearly above 500 mV. Thereby, a p-i-n cell with an efficiency of 8.5 % (Voc = 531 mV, FF = 69.8 %, Jsc = 22.9 mA/cm2) could be achieved; in the case of n-i-p _c-Si:H cells we have reached, for single-junction cells, an efficiency of 7.8 %.5) Deposition rates for individual microcrystalline silicon layers with reasonable quality could be increased up to 16 A/s, reasonable n-i-p solar cells ( ?Å = 6.9%) could be fabricated at deposition rates as high as 10 A/s.6.) For high-rate deposition of amorphous silicon layers, with reasonable stability and good ?Ê?Ñ-products, both DC deposition (as used by BP-Solarex Inc.) and VHF deposition (as introduced by IMT) give excellent results, provided the hydrogen dilution ratio used during deposition is carefully optimised. Traditional RF deposition at 13.56 MHz gives clearly lower deposition rates. Thereby, the most important deposition parameter that controls the quality of the amorphous silicon obtained is the substrate temperature: With higher temperatures the ?Ê0?Ñ0-product in the light-soaked state increases.7.) It is possible to obtain with hot-wire (HW) deposition individual intrinsic () a-Si:H layers with excellent quality (high value of the ?Ê?Ñ-product in the light-soaked state), it is also possible by combining this deposition method with VHF-GD to achieve high deposition rates (26 A/s) for individual intrinsic ?Êc-Si:H layers. However, hot-wire deposition leads to additional technical problems when fabricating full p-i-n or n-i-p solar cells. Because of lack of time and resources, IMT has abandoned investigation on hot-wire deposition.8.) Applying the hydrogen dilution technique to the deposition of the intrinsic () layer within p-i-n single-junction amorphous silicon solar cells leads, indeed, to an improved stability of these cells. Combining hydrogen dilution with increased substrate temperatures leads to higher short-circuit currents but lower open circuit voltages.9.) By CO2 and H2 treatments of the i-p interface of n-i-p a-Si:H solar cells an improvement of both the open circuit voltage and the fill factor FF could be achieved. On flat stainless steel substrates Voc-values over 900 mV and FF-values of up to 74 % have been obtained.10.) Besides our sputtered ITO and ZnO layer deposition technology we have built up an inhouse LP-CVD ZnO technology which is a high-rate deposition process and which allows us to directly prepare rough TCO layers. Our work here was concentrated sofar mainly on the building up of the process system and on achieving process reproducibility.11.) Patterning techniques for p-i-n based solar cells have been developed based on both the pad printing technique and on the laser scribing method. Well-defined areas of the test cells are required for the short-circuit current determination and the verification of the efficiencies of our p-i-n type solar cell. A substantial effort was undertaken by us to reequip and install a laser scribing system (?É = 1064 nm) borrowed from the firm Phototronics-Solartechnik GmbH (D). This laser system is at present used to scribe the front ZnO layers of our micromorph cell modules. For scribing the silicon layers, a Nd:YAG laser (?É = 532 nm) originally acquired for light-induced degradation is used at present (low pulse frequency of only 20 Hz.). Using both laser wavelengths it now becomes possible to apply the well-established integrated seriesconnection method also to our micromorph p-i-n devices.12.) Our micromorph p-i-n-p-i-n tandem cells could be improved so as to obtain stabilized efficiencies of 11.0 to 11.6 % for a cell area of 0.25 cm2. A discrepancy observed between indoor simulator and outdoor (clear sky condition) characterisations may be due to a loss in the blue part of the spectrum present in our "artificial light source" (solar simulator). A larger number of micromorph cells with well-defined cell areas are needed so as to obtain full efficiency verification by the Fraunhof ISE (Institut fur solare Energiesysteme) Freiburg.13.) A first micromorph mini-module (23.6 cm2) was fabricated, thereby applying the integrated electric series connection concept, a concept already well-established for amorphous silicon modules. An initial module aperture efficiency of 9.2 % could thereby be achieved, the relatively high value obtained thereby for the module FF (70 %) proves that the laser scribing technique used here is indeed applicable not only for amorphous silicon, but also for our thicker micromorph tandem cells. The application of the integrated electrical series connection concept based on laser-scribing can be considered to be a key factor for cost reduction in PV.14.) Infrastructure: Over the past 3 years the photovoltaics and thin-film silicon laboratory of IMT has undergone very important developments. Until 1996 exclusively test solar cells of small areas (< 1 cm2) could be realised on research-type equipment. Now, however, under the influence of more application-oriented projects and by stepwise addition of substantial new equipment, this has considerably changed: IMT will soon own and operate a comprehensive technological infrastructure allowing the development of amorphous, microcrystalline and micromorph solar cells from small test cells for research upto 30x30cm size modules for testing and demonstration.It is to be wished that in Switzerland or at least elsewhere in Europe there will soon be an industrial company that takes serious interest in the micromorph concept - as is already happening at present in Japan. Pilot production of micromorph tandem cells would be the next step before mass production can be envisaged. This step is planned in Japan (Kaneka Corp.). All efforts have to be undertaken in Switzerland/Europe to keep up our present leading position in this sector.
Auftragnehmer/Contractant/Contraente/Contractor:
Autorschaft/Auteurs/Autori/Authors:
Shah,Arvind
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Schlussbericht
(Englisch)
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The three-year project (2000-2002) described in this report was concerned with specific issues that have to be addressed in order to ensure Industrialization of the ?micromorph? (microcrystalline/amorphous) thin-film silicon solar cell concept introduced in 1994 by IMT Neuchâtel:1) Increase of microcrystalline silicon deposition rate, with corresponding fabrication of entire solar cells; here, novel compact VHF (Very High Frequency), electrodes wereintroduced in a part of our deposition systems, ensuring, thereby, higher deposition rates, as well as greater compatibility with the ?plasmabox?-type electrodes used in the KAItype industrial production systems of UNAXIS; furthermore, first results were obtained on high-rate deposition in the so-called HPD (high-pressure depletion) regime.2) Improvement of the performances of nip-type single-junction microcrystalline silicon solar cells; a record (stable) efficiency value of 9.2 % could be obtained for such a smallsize solar cell.3) Investigation of the microstructure of microcrystalline silicon solar cells; establishment of a relationship between cell microstructure, as seen by TEM (transmission electron microscopy) and, also, by Raman spectroscopy, on one hand and the open-circuit voltage value of entire solar cells, on the other hand. This relationship is valid for both nip-type and pin-type solar cells. Furthermore, insight was gained into the complex growth mechanisms of microcrystalline layers and cells on various substrates.4) Improvement of pin- and nip-type amorphous silicon solar cells. Here, pin-type single-junction amorphous silicon solar cells with a stabilised efficiency of over 9% could be obtained, thanks to the excellent light trapping obtained with our inhouse LP-CVD (low-pressure chemical vapour deposition) zinc oxide. In the case of nip-type solar cells, the accent has been on 2 separate issues:a) Improvement of stabilized efficiency AND of optical absorption thanks to the use of ?moderately high temperatures? (300 to 350 degrees) for the plasma deposition of theintrinsic layer; b) Deposition of amorphous silicon layers at lower temperatures (190 degrees) on very low-cost substrates, like PET. These particular investigations on nip-solar cells are not yet completed.5) Specific studies on transparent conductive oxides (TCOs), and on light trapping (solar cell current enhancement) obtained with various TCOs and, in general with various types of back reflectors and various substrate materials. Here, the main accent has been on the development of LP-CVD ZnO on transparent glass substrates, for pin-type solar cells, and on the preparation of a larger area (30cm x 30cm) deposition system for the same. A further accent was on the development of rough silver layers, useable as textured back reflectors, on opaque substrates, in the case of nip-type solar cells.6) Basic considerations on the necessary spectral ranges for light trapping, both for amorphous silicon and for microcrystalline silicon solar cells of typical thickness werecarried out. It was shown that the spectral ranges involved in the 2 cases are far from being the same.7) Implementation of entire micromorph tandem solar cells, both in the pin- and in the nipconfiguration, as well as realization of pin-type micromorph minimodules with monolithic series connections. Schemes to improve the efficiency, such as the use of an intermediate reflector between the top cell and the bottom cell were studied. The problem of current matching of micromorph tandem solar cells was addressed.A considerable part (20 to 30%) of IMT?s project resources was invested, during the project period 2000-2002, in improving the infrastructure base (equipment/set-ups) of the thin-film solar cell laboratory: in materials and cell characterization, in laser scribing and, specially, in plasma-assisted deposition. This was absolutely necessary because we at IMT Neuchâtel do not have the financial resources for purchasing expensive commercial equipment, as our main competitors in Japan and Germany do.Finally, let us also mention that IMT Neuchâtel has now established a close collaboration with two Industrial firms:a) UNAXIS A.G., Balzers (FL), andb) VHF-Technologies, Le Locle.The collaboration involves extensive license agreements, based on intellectual property (IP) obtained by IMT Neuchâtel within the whole series of its research projects financed by the Swiss Office of Energy.These newly established Industrial collaborations should lead within the next few years toa) availability of professional mass production equipment from a Swiss manufacturer for the production of very low-cost thin-film silicon modules on glass substrates,b) low-cost flexible PV modules on plastic substrates based on amorphous silicon and produced in the Neuchâtel region (Le Locle, Yverdon).A substantial part of IMT?s efforts during the project period 2000-2002 was, therefore, devoted to building up these collaborations and to giving direct support to the firms involved. Investment from the Private (Industrial) sector) is, on the other hand, also very substantial; the following figures are being mentioned: a) a sum of the order of 15 to 20 Million Swiss Francs is envisaged (including investments already consented/planned) for during the period 2003-2005,b) a sum of approx 3 Million Swiss Francs has up to now already been invested, for the period 2000-2002 and corresponding additional amounts (of the same order ofmagnitude) are planned for the next few years, i.e. for the period 2003-2005.
Auftragnehmer/Contractant/Contraente/Contractor:
Institut de Microtechnique de Neuchâtel
Autorschaft/Auteurs/Autori/Authors:
Shah,Arvind
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