From identification of electrolyte degradation rates to lifetime estimations in dye solar cells with iodine and cobalt redox couples

Abstract

From identification of electrolyte degradation rates to lifetime estimations in dye solar cells with iodine and cobalt redox couples Degradation of dye solar cells is a major obstacle in their commercialization. Here we look into how much information on the degradation routes and rates one can extract from accessible measurements. Specifically we focus on tracking the color of the cell since all the main components of a dye solar cell have a specific color, and their color changes with degradation. Furthermore we look into extracting the degradation coefficients based on the specific color changes. One of the most vulnerable components of a dye solar cell is the electrolyte. Here we investigate the effect of two most interesting electrolyte compositions: 1) conventional iodine based electrolyte, which to date dominates the stability records of dye solar cells, and 2) cobalt complex electrolyte, which enables record high efficiencies. UV light is known to be highly detrimental as it destroys charge carriers-typically, a UV filter is recommended, but is it enough to prevent the loss of charge carriers? Here expectedly applying a UV filter improved the performance as the cells without a filter had only 4 ± 1 % of the initial efficiency left after a 1,000 hour light soaking test, whereas those with a filter maintained 90 ± 20 % of their initial efficiency. Applying a UV filter only hindered the loss of the charge carriers, but did not eliminate their degradation. From the color changes of the electrolyte, we could identify the degradation coefficient for these electrolytes. This analysis resulted in a highly relevant discovery: the loss rate of the charge carriers in iodine electrolyte was approximately double compared to cobalt electrolyte. Furthermore we could provide indicative estimates of future lifetimes of cells, which could be highly important in improving the lifetime of dye solar cells.

Extreme sensitivity of dye solar cells to UV‐induced degradation

“…The increase of R D for cells under filtered UV began as a slow, steady increase for the first 1000 hours in the samples under full‐spectrum illumination but accelerated from that point onward. The increase of diffusion resistance is often connected with the loss of charge carriers in the electrolyte, since it is inversely proportional to the concentration of charge carriers in the electrolyte 5,21 . In the case of iodine electrolyte that was used in this study, is the current limiting charge carrier and its loss is generally the reason behind increased diffusion resistance 22 …”

“…The loss of charge carriers can be studied in more detail by tracking the color changes in the electrolyte of the cells 3,5,21 . Since tri‐iodide gives the electrolyte its distinct yellow color, the loss of tri‐iodide is observable as bleaching of the electrolyte.…”

“…For dye solar cells (DSC), 2 the lowered performance is commonly caused by the degradation of charge carriers in their electrolyte 3 . This process is called bleaching due to the changes it causes in the electrolyte color, turning conventional iodine‐based electrolyte from yellow to transparent, and it has been shown to be greatly accelerated by exposure to UV illumination 3–5 . Therefore, DSCs require UV protection to improve stability, 5 which is often accomplished with a UV cutoff filter, that is, a film that is placed between the light source and the photoactive area of the DSC 6–8 …”

Nanocellulose and Nanochitin Cryogels Improve the Efficiency of Dye Solar Cells

“…In a conventional light soaking experiment with a UV filter, the reference cells used here exceeded a lifetime of 1000 h and had an expected lifetime of several thousand hours. 36 Initial DSC Photocurrent. The initial photovoltaic measurements showed significant variability of I SC for the different cell types ( Table 1).…”

“…The challenge is that, in usual testing with visible light or full spectrum with UV filters, the testing is expected to take several thousand hours (i.e., several months). 21 One option to accelerate the testing would be to utilize extremely high overall light intensity (≫1 Sun), 22 which does work for individual devices but does not scale easily for large series of devices. Instead, we opted to test the cells in harsh UV light to accelerate the degradation.…”