Solar cell encapsulation

Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulants, Annual Report," DOE/JPL - 954527-79-10, Sprlngborn Laboratories, Inc., June 1979. Pluddemann... 

Electronic applications for Bayer s Desmopan thermoplastic PU film include a solar cell encapsulation process. In November 2005, Bayer revealed that its new manufacturing process for the innovative carbon nanotubes had reduced their manufacturing cost. Bayer has also introduced formable PC films which incorporate EMI/RFI shielding. This is obtained by printing them with conductive ink or by dispersing the ink into the cap layer of the sheet. 

Ethylene vinyl acetate, commonly referenced as EVA, is a rubberlike polymer that sometimes can be processed on either rubber or plastics processing equipment. EVA has good clarity and can be compounded with peroxide curatives and sometimes blowing agents (for foam) for use in athletic shoes (for absorbing shock), ski boots, hose, tubes, wire insulation, solar cell encapsulation, and medical applications. 

In addition to the thin-film solar cells, this system is suitable for use in other flexible electronics, such as a flexible, transparent, and light-weight encapsulation of organic light-emitting diode (OLED) displays and lighting. 

Although photovoltaic conversion is nonpolluting, environmental, health, and safety aspects must be considered, especially with regard to harmful emission and waste products resulting from the production of the solar cell modules. It has been shown that, with proper encapsulation and a proactive recycling program, it should be possible to minimize environmental concerns. 

Reliable stability data of the p-i-n solar cell itself are not easily obtained, especially for non-encapsulated cells or modules. One of these tests e.g. for EN/IEC 61646 certification of modules is the so-called damp-heat test (85°C, 85% humidity, up to lOOOh). Recent studies were performed by Stiebig et al. [50, 51] exposing different types of cells to harsh conditions. One of the most important results was the excellent stability of silicon thin film solar cells. Remarkably, this is also valid for small area modules even without encapsulation [52]. This is of high interest because costs and efforts for module encapsulation strongly depend on the inherent stability of the solar cells. As a more detailed treatment of this subject is beyond the scope of this chapter, the reader is referred to the original papers [50,51]. 

These devices are usually encapsulated in plastic. Si diode photocells have an expected lifetime of 40 years. In late 2001, with batteries to provide power at night, desert climates can get solar power for about 0.08 per kWh (kilowatt-hour) using Si solar cells, batteries, and electronic inverters. By contrast, nuclear and hydroelectric power plants can provide power at 0,015 to 0.03 per kWh. Solar power is already cheaper than internal combustion... 

While progress towards more efficient polymer solar cells has been steady and directions for future work are mapped out, there are few reports of cell performance under continuous illumination. Kroon et al. (2002) studied the response of encapsulated MDMO-PPV cells, similar to that illustrated in Fig. 10.23(a), subjected to thermal stress and continuous illumination. A 30-50% decrease in efficiency was observed for cells stored in the dark at... 

Usually, organic solar cells are encapsulated to prevent the degradation induced by oxygen and/or moisture. The protected devices present much longer lifetimes than bare devices [11,12]. To fulfill a very good encapsulation, glass substrate or... 

Other encapsulation applications of polymers for specific designs Include soil, ultraviolet, and abrasion-resistant front covers. The cover can serve as a transparent structural superstrata. Substrate support designs require a hard, durable front cover film to protect the relatively soft pottant from mechanical damages and excess soil accumulation. A polymeric front cover must be low In cost, highly transparent, and weather resistant to compete with glass. For applications out of the optical path between the sun and the solar cells (adhesives. Insulation, edge seals, gaskets) requirements for polymeric use In encapsulation are the same as for other applications. 

Solar cell modules must undergo substantial reductions in cost in order to become economically attractive as practical devices for the terrestrial production of electricity. Part of the cost reductions must be realized by the encapsulation materials which are used to package, protect, and support the solar cells, electrical interconnects, and other ancillary components. As many of the encapsulation materials are polymeric, cost reductions necessitate the use of low-cost polymers. This article describes the current status of low-cost polymers being developed or Identified for encapsulation application, requirements for polymeric encapsulation materials, and evolving theories and test results of antlsolllng technology. 

The analyses for structural adequacy Identified that the thermal expansion or wind deflection of photovoltaic modules can result In the development of mechanical stresses In the encapsulated solar cells sufficient to cause cell breakage. The thermal stresses are developed from differences In the thermal expansion properties of the load carrying panel, and the solar cells. However, the analysis Interestingly Identlfed that the solar cell stresses from either thermal expansion differences or wind deflection can be reduced by Increasing the thickness t of the pottant, or by using pottants with lower Young s Modulus E. 

Cuddihy, E.F., Baum, B., and Willis, P., "Low-Cost Encapsulation Materials for Terrestrial Solar Cell Modules", Solar Energy, Vol. 22, p. 389 (1979). 

Encapsulants are necessary for electrical Isolation of the photovoltaic circuit. They also provide mechanical protection for the solar cell wafers and corrosion protection for the metal contacts and circuit interconnect system over the 20-year design life of a photovoltaic array. The required components Include the solar cell circuit, the rigid or structural member, the pottant, and the outer cover/insulator. Surface modifications may be needed to develop strong, stable bonds at the Interfaces in the composite. If the module is to be framed, edge sealants may also be required. The functions of the Individual components and the performance requirements as they are now known are described. Costs are ccmipared where possible and candidate materials identified. 

Encapsulant Delamination - Solar Cell Cracking - Structural Fatigue... 

Laboratories, Low-Cost Encapsulation Materials for Terrestrial Solar Cell Modules, JPL Internal Document No. 5101-78, Pasadena, California, September 1978. 

---