Eur. Phys. J. Appl. Phys. 36, 307–311 (2007) DOI: 10.1051/epjap:2006159 T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS Out-door testing and long-term stability of plastic solar cells ⋆ E.A. Katz 1, 2, a , S. Gevorgyan 1 , M.S. Orynbayev 1 , and F.C. Krebs 3 1 Dept. of Solar Energy and Environmental Physics, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boker Campus 84990, Israel 2 Ilse-Katz Center for Meso- and Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel 3 The Danish Polymer Centre, RISØ National Laboratory, PO Box 49, 4000 Roskilde, Denmark Received: 25 July 2006 / Accepted: 30 October 2006 Published online: 10 January 2007 – c  EDP Sciences Abstract. In spite of the high potential of polymer photovoltaic (PV) cells, considerable improvement of their stability under operational conditions needs to be achieved. The few published data on the stability of such cells are devoted to accelerated indoor testing at elevated temperatures. The acceleration factor is undoubtedly dependent on the PV materials, the cell architecture and may vary with the degradation. We report preliminary results of long-term evaluation of PV performance of polymer cells under real sun operational conditions. The studied devices include three types of encapsulated polymer/fullerene cells which differed by the configuration and content of the photoactive layer: (1) bulk heterojunction of MEH-PPV:PCBM; (2) bulk heterojunction of P3HT:PCBM; (3) bilayer heterojunction P3CT-C60 . The MEHPPV-PCBM cell exhibited the fastest degradation. The degradation of PV performance of the P3HT-PCBM cell was much slower while the P3CT-C60 device was found to be the most stable. Effect of restoration of Isc and Voc was found when the P3HT-PCBM and P3CT-C60 cells were kept in the dark overnight. The first Isc and Voc measurements every morning yielded the highest values compared to those during the rest of a day. While Isc recovered only partly and exhibited significant degradation during a month, Voc values recovered completely every night and showed almost no reduction on a long-term time scale. PACS. 84.60.Jt Photoelectric conversion: solar cells and arrays – 82.50.Hp Processes caused by visible and UV light – 82.35.Cd Conducting polymers – 72.80.Rj Fullerenes and related materials 1 Introduction Over the last decade organic photovoltaics, based on sol- uble conjugated polymers, has been suggested as a low- cost alternative, combining key features of inorganic semi- conductors with the advantages of low-cost plastic-based technologies. One example would be plastic solar cells based on a bulk heterojunction (BHJ) between donor- type conjugated polymers and acceptor-type fullerenes (or fullerene derivatives) [1]. Although the achieved power conversion efficiencies of up to 5% [2] clearly demonstrate the high potential of polymer photovoltaics, considerable improvement of the cell stability under operational con- ditions needs to be achieved. It has been demonstrated already [3–5] that the degradation behavior of polymer cells involves a number of photochemical mechanisms in- cluding direct photooxidation of conjugated polymers, a photochemical reduction of the organic constituents by aluminium (from a back contact of the cells) and sub- ⋆ This paper has been presented at “ECHOS06”, Paris, 28–30 juin 2006. a e-mail: keugene@bgu.ac.il sequent chemical reaction between the organo-aluminium species and molecular oxygen, etc. However, the data on the stability of PV parameters of BHJ cells have been ob- tained by an accelerated indoor testing at elevated tem- peratures [4–8]. The acceleration factor is undoubtedly de- pendent on the PV materials, the cell architecture and may vary with the degradation. The published data on PV behavior of plastic cells under real sun out-door oper- ational conditions are very limited [9] and to the best of our knowledge there is no report on long-term recording of evaluation of the PV performance under such conditions. This paper reports preliminary results of long-term recording of PV performance of encapsulated plastic so- lar cells under real sun out-door operational conditions at Sede Boqer using the documented closeness of the noontime clear sky spectrum at that site to the standard AM1.5G spectrum [10]. 2 Experiment Three encapsulated polymer-fullerene solar cells with photoactive area of 10 cm 2 were produced in RISØ Article published by EDP Sciences and available at http://www.edpsciences.org/epjap or http://dx.doi.org/10.1051/epjap:2006159