Out-door testing and long-term stability of plastic solar cells

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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.