Stepwise co-sensitization as a useful tool for enhancement of power conversion efficiency of dye-sensitized solar cells: The case of an unsymmetrical porphyrin dyad and a metal-free organic dye

A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H-2[Porph] (ZnP-H2P), where Zn[Porph] and H-2[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycinemoiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO2 electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H2P/DC) and D (DC/ZnP-H2P). The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H2P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J(sc) = 11.72 mA/cm(2)), open circuit voltage (V-oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J-V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J(sc) = 13.45 mA/cm(2), V-oc = 0.76V, and FF = 0.75), when a formic acid treated TiO2 ZnP-H2P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J(sc) value (= 13.45 mA/cm(2)), which resulted from an increased dye loading, and an enhanced V-oc value (= 0.76 V), attributed to an upward shift and increased of electron density in the TiO2 CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO2 photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (tau(d)), and longer electron lifetime (tau(e)). (C) 2014 Elsevier B. V. All rights reserved.