Degradation Mechanisms Investigation for Long-term Thermal Stability of Dye-Sensitized Solar Cells

This paper describes a systematic study on exploring and understanding degradation mechanisms for long-term thermal stability of dye-sensitized solar cells (DSSCs). A 1080-hour thermal stability testing of DSSCs was conducted. Two different temperature conditions were selected to deeply explore the degradation mechanisms of DSSCs performance. One temperature condition is at 25°C, the other is under a temperature cycle from -20°C to 25°C. DSSCs were sensitized by bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2'-bipyridine-4-carboxylic acid, 4'-carboxylate)ruthenium(II) (N719). The cells maintained ca. 80% of its initial overall power conversion efficiency (η) after 1080-hour aging. It is found that the decrease of η was ascribed to the decrease of short-circuit current density (JSC) while the open-circuit photovoltage and the fill factor increased with time; and the decrease rate of JSC for the cell under the temperature cycle from -20°C to 25°C was slower, compared with the cell at 25°C. From the physical point of view, the reason for the drop in JSC was analyzed. Several possible degradation mechanisms, which are stability of the dye, aging of the photoelectrode film, change of the electrolyte components, and degradation of the counter electrode, were discussed systematically. Particularly, the long-term stability of N719 adsorbed on the TiO2 electrode permeated in the electrolyte was investigated by using UV-visible absorption spectrum and resonance Raman scattering. Density functional theory calculations were applied to illustrate the deterioration of N719 which caused the decrease of the light harvesting efficiency of photoelectrode and subsequently decreased JSC. Moreover, high temperature will accelerate the deterioration. Additionally, the change of the electron transport/transfer between the interfaces in DSSCs, which was caused by aging of the photoelectrode film, change of the electrolyte components, and degradation of the counter electrode, was analyzed with electrochemical impedance spectroscopy (EIS). The analysis result of EIS shows that the change of the electron collection efficiency of photoelectrode with time was not large. As a conclusion, the long-term stability of N719 is found to be the main reason for DSSCs degradation.