In order to predict outdoor module lifetimes, appropriate accelerated ageing tests are needed to make useful extrapolations to realistic outdoor conditions and to identify possible degradation mechanisms.
A systematic investigation of intrinsic chemical stability was carried out on devices specially designed for high power applications . For this purpose, accelerated ageing test procedures were developed for nc-DSCs and it turned out that, to first order, a separation can be made between the effects of visible light soaking, UV illumination and thermal treatment on long term stability.
• Visible light soaking alone is not a dominant stress factor, which means that the dye (a ruthenium bipyridyl complex) used in these tests is surprisingly stable [7,18,19].
• UV light exposure often leads to strong degradation due to loss of iodine in the electrolyte. A dramatic improvement in stability can be obtained by using surface additives like Mgl2 and Cal2 in the electrolyte .
• Thermal stress appears to be one of the most critical factors determining the long term stability of nc-DSCs and is strongly related to the chemical composition and purity of electrolyte solvents and additives . Continuous (1 000 h at 85°C) and periodic (—40 to 85°C, 200 cycles) thermal tests’ according to IEC norms appear promising since no leakage of electrolyte solvent was observed, but with 30-40% loss in efficiency still critical.
• Several Surlyn-sealed cells were exposed to outdoor conditions over a period of one year. Major failures only occurred due to imperfect sealing and ageing effects on the electrical contacts. The results of the best performing cells are shown in Fig. 7.9 and it can be seen that the efficiency remains remarkably constant after one year of outdoor exposure.
Fig. 7.9. Efficiency data for two test cells (active area 4 cm2) exposed to outdoor conditions for 1 year. The efficiencies of the cells were measured from time to time with an indoor solar simulator. The dye used is Ru(NCS)2(2,2′-bipyridyl-4,4′- dicarboxylate)2 (N719). 0.6 M hexylmethylimidazolium iodide (HMII), 0.1 M Lil, 0.05 M І2, 0.5 M tert-butylpyridin (TBP) in propionitril was used as the electrolyte. Surlyn 1702 (Dupont) was used as the sealant. The cells were placed under a window to protect them from rain. Test location INAP, Gelsenkirchen, Germany
Reference can be made to the standard test conditions required for thin film and crystalline silicon modules (IEC 1646:1996 and IEC 1215:1993). It is obvious that before nc-DSCs can be commercialized on a large scale, IEC 1646 has to be fulfilled in the current or a somewhat modified form. The ultimate answer as to whether nc-DSCs can be produced with lifetimes > 20 years cannot be given at this stage. However, on the basis of present results and knowledge, stability can certainly be improved by a better understanding of the degradation mechanisms and chemical balancing of the electrolyte components.