Ribbon Silicon Solar Cells

Progress in state-of-the-art solar cell processing has allowed the use of highly defected crystalline silicon wafers in industrial type solar cell production without a major reduction in cell efficiency. One prerequisite for obtaining 

Ti/Pd/Ag front contact thick film back contact 

Fabrication of solar cells using ribbon silicon wafers has to be adapted to the material needs to reach satisfactory conversion efficiencies. As for all multicrystalline silicon wafers, material quality should be improved during cell processing to cope with the defect structure present in the as-grown material. The implementation of gettering and hydrogenation steps into the solar cell process is, therefore, crucial, as the efficiency obtainable for solar cells from a given material is important for cost-effectiveness. 

When considering efficiencies, two types of cell processes have to be distinguished, namely lab-type (small area cells, determination of materials potential, disregarding fabrication costs) and industrial-type processing (large area cells, transferable into mass production). 

Both EFG and SR wafers are fabricated commercially and solar cells have been processed on a large scale, industrial base since 1994 (EFG) and 2001 (SR), respectively. Progress for SR and EFG record efficiencies have developed in parallel, again demonstrating their comparable material quality (Fig. 7.8). 

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The same is true for minority carrier lifetime. Typical as-grown lifetimes in ribbon silicon are lower than in cast or CZ doped wafers. Ribbon silicon wafers therefore depend upon solar cell processing steps to improve their electrical characteristics during the solar cell process (gettering and passivation). Thus, monitoring the minority carrier lifetime and the respective diffusion length during the solar cell process has proven to be a valuable instrument in the development of the ribbon materials. 

As nearly all the developed ribbon silicon growth techniques result in multicrystalline material, crystal defects play a major role in the solar cell efficiencies obtained on ribbon silicon wafers. There is a general trend for higher defect concentrations with faster ribbon growth, but even within wafers of one growth technique, there is an inhomogeneous defect distribution. In this section, we will introduce the known relevant defects for the three materials under closer examination (EFG, SR and RGS) and their impact for solar cell processing. Especially, interaction between different types of defects must be taken into account to understand the behaviour of the different ribbon silicon materials within the cell process. 

Solar cells from ribbon silicon wafers are more cost-effective, when efficiencies are in the same range as for cells from costlier wafers originating from ingots. Record efficiencies for EFG and SR cells in the range of 18% are comparable to the best cells fabricated from multi-crystalline wafers from ingots, when lab-type processes of the same complexity are used. The same is true for... 

Current tendencies for multi-crystalline wafer based solar cells are heading towards thinner and larger wafers. Ribbon silicon based wafer technologies have to deal with these developments in the future to maintain their cost effectiveness. As thin EFG and RGS wafers have already been produced on a laboratory scale with thicknesses of <200 fim, their industrial application remains a topic for ongoing research. 

For solar cell applications, thin film LPE is economically viable only if it is combined with a low-cost multicrystalline Si substrate (high-throughput silicon ribbons, upgraded metallurgical grade silicon MG-Si) or with a foreign substrate (glass, ceramic, metallic sheet... see section 9-7). 

Ribbon (Photovoltaic) Cells - A type of solar photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material. 

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