A CdTe Thin Film Module Factory with a Novel Process
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1165-M07-02
A CdTe Thin Film Module Factory with a Novel Process N. Romeo1,3, A. Bosio1,3, A. Romeo2,3, S. Mazzamuto1,3 1 University of Parma, INFM-Physics Department, V.le G.P. Usberti, 7/a – 43100 Parma, Italy. 2 University of Verona, Physics Department, Cà Vignal 2 – Strada delle Grazie – 37134 Verona, Italy. 3 Arendi Spa c/o Euroenergy Group S.r.l, Via G. Alessi, 12 – 20020 Lainate (Mi), Italy. ABSTRACT The CdTe thin film module technology can be considered mature for large scale application. First Solar is now capable of producing more than 400MW/year of CdTe modules and it is still increasing its production. First Solar also announced that they can get a production cost close to 1$/W. A further cost reduction will render this kind of energy production competitive with the energy obtained from fossil fuels. In our laboratory we developed a novel dry process that could further reduce the production cost. This process is now being used by a new company (Arendi Spa) in a line that will produce ~15-18MW/year. Half of the line has been already built and the complete line should be ready in July 2009. The module production will start at the end of 2009. INTRODUCTION CdTe exhibits optimal characteristics as a thin film photovoltaic material. Its forbidden gap is 1.45eV, very close to the theoretical maximum of solar energy photovoltaic conversion and it is direct, that means that only a few microns of the material are needed to absorb 90% of the solar spectrum. Its phase diagram is very simple and it grows stoichiometric in vacuum at a substrate temperature higher than 300°C, since any excess element, due to its high vapour pressure, re-evaporates. The only drawback is that, being a polycrystalline thin film, it contains a great number of grain boundaries. For this reason, it cannot be doped, since external impurities, segregating into the grain boundaries, can cause a short circuit in the finished solar cell. Fortunately, if CdTe is deposited at high substrate temperature, namely ≥500°C, for example by “Closed Space Sublimation” (CSS), it grows naturally p-type with a carrier number greater than 1014cm-3. Besides, the defects at the grain boundaries can be, for the most part, removed by annealing CdTe for 10-20min at 400°C in an atmosphere containing Cl. The treatment with Cl can also produce VCd-Cl complexes that can contribute to increase the p-type conductivity. Among thin film photovoltaic materials, CdTe is considered the most suitable for a large scale production since scalable techniques such as sputtering and CSS are used in the preparation process. For this reason, several new companies have been constituted during last years (see Table I). Among these, only two companies, namely First Solar and Antec Solar are in production, but First Solar has increased its production very fast, going, in few years, from 30MW/year to 600MW/year. Besides, First Solar has announced that they were able to reduce their production cost down to 1.08 $/WP. The highest efficiency (16.5%) CdTe solar cell has been fabricated by NRE
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