Accelerated stability testing of organic photovoltaic materials using concentrated sunlight
Ma’or Gabay
Department of Chemistry, Ben Gurion University of the Negev, Be’er Sheva, Israel
Organic photovoltaics (OPV) have been suggested as a low-cost, lightweight, flexible alternative to inorganic photovoltaics, with energy conversion efficiencies exceeding 8%. The greatest challenge in the development of OPV devices is combining high efficiency, processability and stability. Recently OPV cells with life-time of 1 year and longer have been demonstrated. This achievement raises the need for accelerated stability tests. A strict requirement for such tests is that the accelerated degradation rate can be related to the standard condition degradation rate. Heat is the conventional approach, by which acceleration factors in excess of 20 are reachable; however this approach neglects photo-induced degradation mechanisms. We use a novel system developed at BGU where solar radiation is concentrated outdoors into an optical fiber which guides the concentrated sunlight indoors onto the sample being tested. This system allows, for the first time, to use actual sunlight to irradiate samples with intensity of up to 800 suns. The goals of this study are (1) to study OPV material stability (lifetime) under concentrated sunlight and determine the dependence of the degradation rate on irradiance level, and (2) to study degradation mechanisms under concentrated sunlight and relate them to those at 1-sun conditions, to determine the validity of stability tests under concentrated sunlight to the materials’ actual working conditions.
Figure 1: Degradation after exposure to concentrated sunlight (20 & 100 suns) for different times.
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We used high intensity irradiation to accelerate the degradation of poly(3-hexylthiophene): phenyl-C61-butyric acid methylester (P3HT:PCBM) bulk heterojunction films. The treated samples were studied using UV-vis, FTIR and Raman spectroscopies. The addition of PCBM was found to significantly reduce the degradation rate of P3HT. Heating was found to induce morphological changes which improve photon absorption and compete with degradation processes, but also accelerate photo-bleaching. The photon flux was found to be a significant factor affecting the degradation rate. Different acceleration factors were determined for different sunlight concentration ranges.
