Co-sensitization of metal free organic dyes in flexible dye sensitized solar cells

Abstract A metal-free organic dye (JH-1) and an unsymmetrical squaraine dye (SQ2) were used for co-sensitization of a flexible TiO 2 electrode in order to obtain a broad spectral response in the visible light regions. Because of enhanced light absorption, the performance of the flexible plastic dye-sensitized solar cells (DSSCs) was enhanced. The dye concentration of co-sensitized TiO 2 film was higher than that of individual dye-sensitized TiO 2 film, leading to improved photovoltaic performance with the panchromatic light harvesting of JH-1 and SQ2 (350–720 nm). Optimum molar ratio of individual dye, dye concentration and adsorption kinetics onto the TiO 2 photoelectrode, and ideal thickness of HS-TiO 2 layer have been investigated. A flexible DSSC co-sensitized with the molar ratio of 60:40 (JH-1:SQ2) and TiO 2 film thickness of 6 μm yielded a high photocurrent density of 12.32 mA cm −2 , an open circuit voltage of 0.754 V, a fill factor of 0.68, and a power conversion efficiency of 6.31% under 1 sun illumination (100 mW cm −2 ). Interestingly, the co-sensitizers did not compete with each other for the absorption. Both the open circuit photovoltage and the photocurrent density were unexpectedly increased with co-sensitization which means that enhancement in photocurrents can be achieved without sacrificing the open circuit photovoltage for the co-sensitized solar cells, once the electron recombination and dye aggregation are retarded by the co-sensitization.