Lifetime and reliability of polymer solar cells [electronic resource]

The power conversion efficiency of organic photovoltaic (OPV) cells has increased from 4-5% in 2005 to 8.3% in 2010. The goal of a 10% single junction OPV device seems attainable making the commercialization of OPV more realistic. With advances made on the efficiency front, the lifetime and reliability of OPV devices has come into focus. To date there has been considerable work done in understanding and quantifying the lifetime and degradation of bulk heterojunction solar cells (BHJs) based on poly-(para-phenylene-vinylene) (PPV) and poly(3-hexylthiophene) (P3HT) polymers. A comparison of OPV lifetime experimental results across different research groups has posed challenges due to the lack of standardized testing and reporting procedures; however, great strides have been made in this regard during the most recent International Summit on OPV Stability (ISOS-3). Modules based on P3HT/fullerene BHJs have shown lifetimes of 5,000 hours when state-of-the-art encapsulation with a glass-on-glass architecture is used. Assuming negligible degradation in the dark and 5.5 hours of one-sun intensity per day, 365 days per year, this translates into an operating lifetime approaching three years. More recently P3HT/PCBM devices utilizing an inverted architecture have been shown to retain more than 50% of their initial efficiency after 4,700 hours of continuous exposure to one-sun intensity at elevated temperatures and have exhibited a long shelf life when stored in the dark in ambient conditions. However, results to date have yet to show polymer based OPV lifetimes greater than 3-4 years. In my dissertation I present a detailed operating lifetime study of encapsulated solar cells comprised of poly[N-9'-hepta-decanyl-2,7-carbazole-alt-5,5-(4', 7'-di-2-thienyl-2', 1', 3'-benzothiadiazole) (PCDTBT) in BHJ composites with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC70BM). PCDTBT/PC70BM solar cells achieved an efficiency greater than 6%, making this one of a small number of polymers able to achieve this level of performance. I describe an experimental set-up that is capable of testing large numbers of solar cells simultaneously, holding each device at its maximum power point while controlling and monitoring the temperature and light intensity. Using this set-up we were able to compare the PCDTBT/PC70BM system with the well-studied P3HT/PCBM system and demonstrate a lifetime for PCDTBT devices that approaches 7 years, which is the longest reported operating lifetime for a polymer based solar cell. I will further present a systematic study of the burn-in degradation mechanism behind PCDTBT:PC70BM solar cells. I will show that a photochemical reaction in the photoactive layer creates states in the bandgap of PCDTBT. These sub-bandgap states increase the energetic disorder in the system, which reduces the FF, Voc and to a lesser extent Jsc. The photochemical reactions are shown to progress rapidly when first exposed to light but subsequently decrease in occurrence, which results in the stabilization of the Voc and FF.

Keywords: Materials Science and Engineering
Advisor: McGehee Michael
Dauskardt R. H. (Reinhold H.) advisor. Thesis advisor
Sellinger Alan advisor. Thesis advisor