Production process, photovoltaic modules

Vacuum lamination of terrestrial photovoltaic modules is a new production process requiring special equipment and a significant material development effort. Equipment design studies resulted in improved control and lower costs when using a double-chamber vacuum laminator. Application testing of new materials and primers showed the feasibility of two different back sheet materials, one encapsulant, two new primers for polymers and one primer for metals. 

So far the physical realization of the heterojunction and its optimization in photovoltaic efficiency have been discussed. The production process of a photovoltaic generator and its economical use have to satisfy a momber of additional criteria which involve a) technological procedures for economic cell and module fabrication allowing large scale production and b) material evaluation for substrates, contacts and encapsulation. These criteria result in a variety of fabrication methods, structures of cells and modules, and materials. 

In the early 1970s, the first companies to apply low cost, mass production techniques to photovoltaics, a technology that had previously been considered an exotic aerospace technology, emerged. These techniques included the use of electroplated and screen printed metal paste electrical conductors, reflow soldered ribbon interconnects, and by 1977, low cost, automobile windshield-style, laminated module constmction. Such processes benefitted from a substantial existing industrial infrastmcture, and have become virtually ubiquitous in the present PV industry. 

The photovoltaics industry could expand rapidly if the efficiency of polycrystalline modules could be increased to 15 percent, if these modules could be built with assurance of reliability over a 10- to 20-year period, and if they could be manufactured for 100 or less per square meter. Solar energy research has been largely directed toward only one of these issues efficiency. All research aimed at reducing manufacturing costs has been done in industry and has been largely empirical. Almost no fundamental engineering research has been done on either the laboratory scale or the pilot plant scale for cost-effective processes for the production of photoconverters. 

The standard device comprises a thin CdS buffer layer as described above. It is believed that market acceptance of chalcopyrite-based photovoltaics could be improved by introducing a Cd-free buffer layer. There may also be cost benefits in view of the cost associated with (occupational) safety, handling of toxic waste in production, and recycling of modules at their end of life. Research has identified Cd-free materials well suited for alternative buffer layers. They can be deposited by CBD in analogy to the standard CdS buffer layers or by other processes. In particular, dry processes are attractive because they offer a better compatibility with the other process steps used for the remainder of the module. Ultimately, the best solution would be to omit the buffer layer altogether in favor of a direct chalcopyrite-sputiered/MOCVI) ZnO junction. Here we will limit the discussion to the state of these latter direct junctions and to ZnO-based buffer layers (Table 9.1). Results achieved with other materials can be found in the literature [67,68]. 

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