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

Abstract

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-H2[Porph] (ZnP-H2P), where Zn[Porph] and H2[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycine moiety, 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 (Jsc = 11.72 mA/cm2), open circuit voltage (Voc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (JV 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 Jsc = 13.45 mA/cm2Voc = 0.76 V, and FF = 0.75), when a formic acid treated TiO2ZnP-H2P co-sensitized photoanode was employed (device E).

The increased PCE value of device E has been attributed to an enhanced Jsc value (=13.45 mA/cm2), which resulted from an increased dye loading, and an enhanced Voc 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 (τd), and longer electron lifetime (τe).

Citation

Sharma, G.D., Zervaki, G.E., Angaridis, P.A., Vatikioti, A., Gupta, K.S.V., Gayathri, T., Nagarjuna, P., Singh, S.P., Chandrasekharam, M., Banthiya, A. and Bhanuprakash, K., 2014. 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. Organic Electronics, 15(7), pp.1324-1337.

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Categories: Solar & Photovoltaics

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