Co-sensitization of N719 and RhCL dyes on carboxylic acid treated TiO2 for enhancement of light harvesting and reduced recombination

We report herein the findings from a systematic study conducted on the effect of co-sensitization of two dyes i.e. N719 and rhodamine 19 perchlorate (RhCL) onto TiO2 electrodes (thickness: 4 mu m), which were treated by three different carboxylic acids namely, formic acid (FA), oxalic acid (OA) and citric acid (CA). Co-sensitization was carried out by first loading the carboxylic acid treated TiO2 electrodes with RhCL dye followed by N719. The amount of dye loading was estimated from the UV/vis spectroscopy and the results show that: (i) the amount of individual dye loading (RhCL or N719) onto carboxylic acid treated TiO2 electrodes is, in general, higher than that onto untreated TiO2 electrodes; (ii) the amount of individual dye loading is highest for OA-treated and lowest for CA-treated TiO2 electrodes; and (iii) the co-sensitization leads to highest loading of N719 onto FA-treated and lowest on CA-treated TiO2 electrodes. The results of dye sensitized solar cells fabricated using these acid treated TiO2 electrodes revealed that the efficiency (eta) is higher for electrodes having higher loading of N719 dye. For single N719 dye loading, the highest eta of 4.5% is observed for OA-treated TiO2; while upon co-sensitization the highest eta of similar to 5.3% is observed for FA treated TiO2. A detailed analyses of Fourier transform infrared spectroscopy (FTIR), UV-Visible, impendence, incident photon to current efficiency (IPCE) results show that eta enhancement occurs due to the following factors: (i) increased short circuit current density (J(SC)) owing to high N719 dye loading which enhances light harvesting; (ii) improved IPCE; (iii) increased open circuit voltage (V-OC) due to an upward shift of the conduction band edge (CBE) and quasi Fermi level; and (iv) suppressed back electron transfer. (C) 2013 Elsevier B.V. All rights reserved.