A NASA publication documented the use of Master Bond EP65HT-1 for use in organic photovoltaic devices.1 These thin film organic solar cells are lightweight, and their use in space-based applications is of particular interest. Devices used on the external portions of spacecraft encounter wide-temperature shifts as well as the vacuum of space. To meet the requirements of this target application, Master Bond EP65HT-1 provided superb service temperature range, rapid-cure, and compliance with NASA outgassing requirements when constructing their experimental devices. The choice of sealant is critical for maintaining the integrity of the device and protecting the sensitive contents from both terrestrial and space environments.
The NASA researchers sought to create an experimental device using fullerene-containing polymers to improve the performance of organic thin-film photovoltaic cells. Fullerenes are highly conductive and assist with efficient electron transfer due to their spherical symmetry. These devices operate based on the mechanism of photoinduced charge separation and utilize conductive polymers. In photovoltaic devices, adsorption of a photon of suitable energy by the light-absorbing material results in excitation of an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). This results in the generation of a positive charge (hole) and a negative charge (electron) called an exciton. A bicontinuous network of conductive polymers possessing donor and acceptor characteristics allow for the presence of a heterojunction to enable transport of the hole and electron to the cathode and anode, respectively, resulting in the generation of electric current.
As fullerenes possess limited solubility in conductive polymers, soluble derivatives called [6,6] PCBM and [5,6 PCBM] were used for the acceptor phase while MEH-PPV was used for the donor phase.1 The cathode comprised either calcium (Ca) or aluminum (Al) while indium tin oxide (ITO) served as the anode. The device was constructed by spin-coating a solution of donor and acceptor-type polymers in chlorobenzene or chloroform onto ITO coated glass fitted with wire electrodes. Polymer film thickness ranges from 600 to 1000Å. Construction was finalized by thermally evaporating metal contacts onto the polymer film surface with subsequent encapsulation in glass. Finally, Master Bond EP65HT-1 was used to seal the device to provide protection against air and other environmental factors encountered during the testing. Device configurations of ITO/Polymer/Ca/Al and ITO/Polymer/LiF/Al were constructed. The anodic and cathodic material provide an internal field within the device to drive holes to the ITO/polymer interface and electrons to the Ca/Polymer interface, respectively. The devices were evaluated for efficiency and open-circuit voltage.
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1 Bailey, S. G., Harris, J. D., Hepp, A. F., et al. Thin-Film Organic-Based Solar Cells for Space Power. NASA/TM-2002-211833. IECEC-2002-20154.