Insights into mechanical compression and the enhancement in performance by Mg(OH)2 coating in flexible dye sensitized solar cells

The engineering of flexible dye sensitized solar cells (DSCs) by mechanical compression is one of the methods that allow low temperature processing of these devices. However, suppressing the high temperature sintering process also significantly reduces the performance of the cells. In our previous work [ J. Phys. Chem. C , 2012, 116 , 1211], we have attempted to improve flexible DSC performance by coating the porous TiO 2 photoanode with an electrodeposited Mg(OH) 2 layer. In that work, we have obtained one of the highest photovoltages reported to date in flexible DSCs (847 mV). In order to gain more insights into the reasons for both poorer performance of compressed cells and the origin of the voltage enhancement achieved by the Mg(OH) 2 coating, here we present an in-depth study by means of electrochemical impedance spectroscopy, Mott-Schottky plots analysis and open-circuit voltage decays. The existence of a shunt resistance in the mechanically compressed cells is revealed, causing an additional drawback to the poor inter-particle necking. By introducing the Mg(OH) 2 coating the recombination in the cell becomes significantly reduced, being the key reason which is responsible for the higher photovoltage. Additionally, the coating and the compression cause modifications in the surface states and in the nature of the interfaces with the electrolyte. This induces TiO 2 conduction band displacements and shifts of the relative position of the modified states that influence the performance. An impedance analysis of compressed flexible dye-sensitized solar cells reveals new features and explains the origin of the high photovoltage induced by Mg(OH) 2 coating.