The greatest challenge in the development of organic photovoltaics (OPV) is combining high efficiency and stability. While operational lifetime of Si solar cells is in the range of 25 years, the operational OPV lifetime under full solar illumination (1 sun = 100 mW/cm²) only recently reached a few years. Degradation mechanisms in OPV are complex and include a variety of processes: photo-bleaching of the conjugated polymers, trap generation in polymer and fullerene moieties of the photoactive layer, degradation of the hole and electron transport layer, ion migration from the electrodes, morphological changes, etc. These processes are almost inseparable as they can be instigated by the same factors: exposure of the device to light, heat, oxygen or/and water.

Recently [1-3] concentrated sunlight was suggested for accelerated studies of light-induced degradation of OPV. Concentrated light degradation tests with single layers of various polymers demonstrated an acceleration of their photo-bleaching rate by a factor > 100 under illumination of 200 suns [2]. However, in all reported experiments it was impossible to independently control the light intensity and sample temperature. The sample temperature increase due to absorption of concentrated light is a fundamental problem in the context of OPV accelerated degradation experiments and it becomes more problematic with increased light intensities ( 1000 suns and higher).

Here we demonstrate the first realization of an experimental methodology that allows independent control of light intensity and the sample temperature during the exposure.

Experimental results for accelerated degradation of encapsulated films of P3HT:PCBM blends and complete bulk heterojunction OPV cells are reported. Sunlight concentration was varied from 1 to 4800 suns. Experiments with well encapsulated PCBM-P3HT blends up to 4800 suns and dose up to 3600 sun·hours (equivalent to 1.6 years of exposure under ‘1 sun’ operational conditions) revealed no P3HT photobleaching and slight degradation in PCBM absorption. However, similar treatment of OPV cells resulted in their significant degradation due to deterioration of charge collection. Underlying mechanisms for such effect are discussed.   

1. Tromholt, T., Katz, E. A., Hirsch, B., Vossier, A., Krebs, F. C.. Effects of concentrated sunlight on organic photovoltaics. Appl. Phys. Lett. 2010, 96, 073501. 2. Tromholt, T., Manceau, M., Helgesen, M., Carle, J.E., Krebs, F.C. Degradation of semiconducting polymers by concentrated sunlight. Sol. Energy Mater. Sol. Cells 2011, 95, 1308-1314. 3. Manor, A., Katz, E.A., Tromholt, T., Krebs F. C.. Electrical and photo-induced degradation of ZnO layers in organic photovoltaics. Adv. Energy Mater. 2011, 1, 836-843.