Electrochemistry in reverse biased dye solar cells and dye/electrolyte degradation mechanisms

Mismatched or shadowed individual cells in a module can operate in the reverse bias (RB) regime. We investigate and identify key mechanisms for RB operation and degradation in dye solar cells (DSCs). Current-voltage characteristics in RB are sensitive to the type of dye utilised and to TiCl(4) substrate treatment. Subjecting the cell to a RB of 0.4 V over 740 h has little effect on conversion efficiency whereas a significant lowering is observed for the harsher stress tests at 0.6 V and by forcing a constant current equal to its I(SC). For more prolonged reverse biases at I(SC) (>740 h), we show that depletion of [I(3)(-)] inside the DSC can lead the reverse bias potentials across the cells to considerably increase in time. Electrochemical impedance measurements show that the overpotentials at the counter electrodes (CEs) can eventually reach values high enough to cause hydrogen evolution. Clear evidence of gas bubbles forming inside a complete dye solar cell under reverse bias stress, leading to severe device degradation, is presented. We also show that reactions of iodine with water present in the electrolyte can play an important role in [I(3)(-)] depletion and in the formation of hydrogen at the Pt CE.