Encapsulation of flexible solar cells to improve their lifetime
Monitoring the change in resistance of a calcium test line (circled) as it reacts with water permeating through the barrier films allows determination of water vapour transmission rate.
One of the challenges faced when developing flexible solar cells is mitigating degradation of the cell caused by UV irradiation and the ingress of oxygen and water. Ingress is inhibited by flexible high barrier polymer films and edge sealants that are used to encapsulate the solar cells. At the CRC-P, we are evaluating the barrier properties of commercially available high barrier films and edge sealants to identify those with the best properties and, where necessary, to devise technologies to further improve their performance. The key to barrier property evaluation is to be able to measure extremely low oxygen transmission rates (OTRs) and water vapour transmission rates (WVTRs). Initial evaluation of film barrier properties by the CRC-P commences by OTRs and WVTRs using a MOCON permeation testing instrument at Monash University that allows measurements down to 0.0001 g/m2/day. However, more sensitive measurements are needed for ultrabarrier films and we have developed two highly sensitive WVTR measurement protocols, a calcium and a tritium test. The calcium test, which has been established at the University of Newcastle, exploits the electrical properties of a thin film of encapsulated calcium metal, and allows the WVTRs of flexible polymer-based barrier materials to be determined to a high level of accuracy and sensitivity. The optical properties of calcium metal also make it well suited as a rapid screening tool for new barrier materials. CRC-P researchers at ANSTO have developed a test to measure low WVTRs using a radioactive labelled water (HTO) tracer through the determination of the rate of tritium transfer through the film being tested. These three capabilities provide WVTR measurement accuracy across the range of measurement decades required for permeation measurement.
The enhancement of barrier film properties is also being investigated by CRC-P researchers at ANSTO, the University of Queensland and the University of Wollongong through the use of atomic layer deposition, nanoparticles and graphene on appropriate polymer substrates. The resulting polymer composite films also have potential applications other than as barrier films.
