20Congreso Nacional 
Sociedad Mexicana de Ciencia de Superficies y Vacío A.C.

CRYSTALLINE SILICON BASED PHOTOVOLTAICS: TECHNOLOGY AND MARKET TRENDS

J.F. Nijs*+, J. Szlufcik, J. Poortmans and R.P. Mertens*
Interuniversity Micro-Electronics Center (IMEC),  Kapeldreef 75;  3001-Leuven,  Belgium
 + author for correspondance:  tel.+32-16-281284  ;  fax +32-16-281501
* Also Professor at Katholieke Universiteit Leuven, Electrotechnical Department, Belgium

  The worldwide PV-market amounted to app. 200 MWp in 1999 and crystalline silicon solar cells constitute about 80-85 % of this PV world market. In the crystalline segment the use of multi-crystalline (also named poly-crystalline) silicon is relatively increasing and the use of mono-crystalline silicon is relatively decreasing. A remaining issue in crystalline silicon (bulk) solar cells is the secured availability problem of cheap EG- or SG-Si feedstock.  Therefore for the longer term a lot of R&D –activities are performed on thinner Si- wafers, but also on thin film crystalline silicon layers on cheap substrates. The IMEC R&D-work tries to cover the whole spectrum from long term oriented research all the way to development of industrial solar cell technologies, ready for implementation in a production line

Industrial technologies
Screenprinting as metallization technique is used in more than  90 % of the worldwide crystalline silicon solar cell production, while the buried contact technology is only used by one producer on part of his mono-crystalline silicon solar cell production. Particularly the screenprinting process based on firing through/after PECVD SiNx antireflection layer has been found by IMEC and others to be extremely beneficial for efficiency improvement of industrial solar cells. This technology is also well suited for lower quality, defected substrates. Industrial efficiencies in excess of 16 % on large mono- and in excess of 15 % on large multicrystalline silicon substrates are readily obtained.

Advanced pilot line crystalline silicon solar cell technologies.
For isotropic texturing of multicrystalline substrates, many techniques, such as mechanical grooving, defect etching and reactive ion etching, are being tested by several groups.  An IMEC-proprietary acid solution gives isotropic surface structuring, which in combination with an antireflection coating decreases the surface reflection to a value significantly below that of randomly textured multicrystalline Si wafers. A large attention has been drawn lately on a very simple single-step selective diffusion from screenprinted P-dopant paste. Implementing these advanced pilot line processes leads to efficiencies of close to 18 % on large mono- and close to 17 % on large multicrystalline silicon solar cells.

Medium-term implementable crystalline silicon solar cell technologies
Rapid Thermal Processing is particularly attractive in solar cell applications due to reduced process time, reduced cross contamination and low thermal budget. Key process steps can all be done in optically heated systems with low thermal mass. Considerable progress has been made in applying screenprinting metallization to RTP emitters with high-enough surface concentration and junction depth .
One of the ways to simplify cell interconnection during module fabrication and also to deal in a better way with the requirements for building integration is to have both p- and n-type contacts at the back side of the wafer. A novel technology, called the (Front) Metallization Wrap Through (fMWT) cell, has recently been developed, where the busbars are moved to the backside of the cell with the fingers remaining on the front side and connected to the busbars via holes. A promising efficiency of close to 15 % has been obtained with Si3N4 ARC.  Another promising approach is Metal-Wrap-Around (MWA).

Thin-film crystalline Si solar cells
IMEC is mainly concentrating on CVD of thin polycrystalline silicon layers on cheap silicon-like and other type of substrates. Excellent efficiencies have been obtained on cells made in these active layers on top of p+ MG-substrates and p+ ribbons.

Acknowledgements
IMEC gratefully acknowledges partial financial support for their R&D-work from the European Commission (Joule-programme) and from the Flemish government.


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