INTERCONNECTMATERIALS
Reliable adhesives
Every aspect of manufacturing is under the microscope seeking methods to reduce costs and improve output. Rich Wells and Mark Francis of Engineered Conductive Materials discuss how reliability of adhesives for interconnects is one area that manufacturers can look at to improve their cost and technical output.
W
ith the solar industry focusing on cost reduction and efficiency
improvements, the reliability of the cell interconnect is often taken for granted. The c Si technology has relied on solder as the cell interconnect, but typically the thin film industry is precluded from high temperature solders because of material temperature limitations.
Electrically conductive adhesives (ECAs) have emerged as the popular choice for interconnecting solar cells with tin and tin silver coated ribbon in the thin film industry. ECAs can be formulated to exhibit rubber like flexibility that reduces stress in the bond line, improves thermal cycling performance, mitigates the risk of delamination, and increases the overall peel strength of the assembly. Cell interconnects must also exhibit exceptional humidity resistance. Protecting non noble metals from corrosion has been an elusive task for ECAs, since high humidity conditioning can induce corrosion resulting in electrical resistance increases at the adhesive ribbon interface.
Stable electrical performance after 1000 hours at 85°C/85%RH on tin and tin silver coated ribbon has been demonstrated. ECAs with stable contact resistance to nonnoble metals with rubber like flexibility offer a robust cell interconnect for the thin film industry for both rigid and flexible solar panels.
Approaching grid parity with low cost solar energy solutions is the impetus of established large companies as well as numerous start up companies. Crystalline silicon (c Si) technology has been utilized for solar energy for several years. However in the last decade, thin film technology has emerged as a low cost alternative offering potentially lower material and manufacturing costs. [1]. In addition, thin film solar technology enables new applications where flexibility, low weight, and lower profiles can be realised.
Research in thin film has focused on cell level aspects such as cell efficiency, manufacturing costs, TCO optimizations, and deposition techniques. Of equal importance is the reliability of the cell interconnect. Because thin film technology requires lower processing temperatures, the historical soldering approach has been displaced with electrically conductive adhesive (ECA). For example, the thin film technology CIGS (Copper Indium, Gallium, and Selenium) should not undergo high temperature exposure to maximize efficiency [2].
Since the solar industry has been assuring customers reliability for up to 25 years, engineers are faced with a challenge of utilizing accelerated aging tests to evaluate failures with materials such as the interconnect. Since thin film technology is still in its infancy, long term data is unavailable. In addition to the IEC requirements for module testing, additional accelerated testing can be employed to test the limits of an ECA and provide additional certainty for the long term reliability of solar modules.
Solder Replace History
ECAs were identified as potential solder replacements for surface mounting of electronic components in the 1990s. Benefits such as lower process temperatures, improved mechanical fatigue resistance, and elimination of lead content from the tin lead solder were driving a potential switch to ECAs [3, 4]. However, two difficult reliability requirements became apparent.
First, the adhesive is required to pass a drop/impact test. In order to pass the test, the ECA needed to possess dampening or low modulus characteristics to dissipate the stress from impact. Rigid, high strength adhesives would typically delaminate upon impact. Second, the adhesive must maintain a stable electrical resistance after damp heat conditioning which is typically conducted at 85°C/85%RH. Since many electronic
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www.solar-pv-management.com Issue III 2011
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