Semiconductor India PHOTOVOLTAICS
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-BC (industrial interdigitated back- such thin wafers. With the traditional wire Epi-cell concept: Epitaxial thin film solar
contact) solar cell concept: Removing the sawing technique, it is unlikely that the cells are realised by growing a thin high-
front surface metal grid has many wafer thickness will reach values much quality active layer of silicon (20µm) on top
advantages: (1) it results in a higher below 100 µm, and the kerf loss (lost Si of low cost highly doped crystalline silicon
effective semiconductor area and thus has during the sawing process) will remain wafers (e.g. from metallurgic grade silicon
the potential for increased cell efficiencies; substantial. Such ultra thin wafers, or better, or from scrap material). The epitaxial layer
(2) it potentially allows for significant cost foils, can however be realised using a lift- is deposited by chemical vapour deposition.
reductions in module assembly since all off technique. IMEC recently presented a
external contacts are brought to a single completely new wafering method for the This epi cell approach has the advantage
surface; (3) it provides an added value from production of 50µm-thick crystalline silicon that the solar cell production process is very
an architectural point of view since the wafers. It is a lift off process that requires similar to that of conventional ‘bulk’ silicon
visual disturbance caused by the presence of only the use of a screen printer and a belt solar cells. Therefore, compared to any
the busbars and the tabbing is displaced to furnace; no ion implanted or porous layer is other thin film cell concept, it will be
the rear of the module. In the past, IMEC needed. On a thick substrate a layer is relatively easy to implement it in the
has developed the metallisation wrap deposited with mismatched thermal existing production lines. In this way, this
through solar cell, a technology that is now expansion coefficient with respect to the approach constitutes the bridge between
licensed to its spin-off Photovoltech. The substrate (for instance a metal layer). Upon bulk silicon solar cells and thin film solar
present activities on back contacted solar cooling, the differential contraction induces cells (on non silicon substrate).
cells are on the one hand specific issues a large stress field which is released by the
linked to module manufacturing of back initiation and the propagation of a crack The major drawback for the industrial
contacted solar cells, and on the other hand parallel to the surface. competitiveness of epitaxial thin film silicon
the development of new back-contacted solar cells is their moderate efficiency
solar cell concepts, with emphasis on the Thin film silicon solar cells compared to conventional bulk silicon solar
use of process techniques that are easily Another drastic approach to reduce the cells: about 11 to 12%. Open-circuit voltage
transferable to production. silicon content in solar cells is to reduce the and fill factor of these cells can obtain
active layer thickness to 20µm or less. similar levels compared to bulk silicon solar
Ultra thin bulk Si solar cell (U-cell These thin crystalline silicon layers are to cells but, due to the optically thin active
concept): In the future, thin wafer solar cells be supported by a low-cost carrier which layer (20µm compared to 200µm), light is
could be as thin as 40µm. A major can be either low cost silicon or a non lost in the low quality substrate from the
challenge to get to that point is to produce silicon substrate. moment it is transmitted from the epi layer
IMEC’s roadmap for teh continued innovation of photovoltaic manufacturing
Autumn 2008 |
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