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 μm), 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 (η) is higher for electrodes having higher loading of N719 dye. For single N719 dye loading, the highest η of 4.5% is observed for OA-treated TiO₂; while upon co-sensitization the highest η of ∼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 η 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.