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 G.D. Sharma a,⇑ , G.E. Zervaki b , P.A. Angaridis b , A. Vatikioti b , K.S.V. Gupta c , T. Gayathri c , P. Nagarjuna c , Surya Prakash Singh c,⇑ , M. Chandrasekharam c , Ajita Banthiya c , K. Bhanuprakash c , A. Petrou b , A.G. Coutsolelos b,⇑ a R & D Center for Engineering and Science, JEC Group of Colleges, JEC Campus, Kukas, Jaipur 303101, Rajasthan, India b Laboratory of Bioinorganic Chemistry, Department of Chemistry, University of Crete, Voutes Campus, P.O. Box 2208, 71003 Heraklion, Crete, Greece c Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500 007, India article info Article history: Received 10 December 2013 Received in revised form 21 March 2014 Accepted 21 March 2014 Available online 5 April 2014 Keywords: Co-sensitization Dye sensitized solar cells Formic acid treated TiO 2 photoanode Power conversion efficiency 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-H 2 [Porph] (ZnP-H 2 P), 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 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 absorp- tion features and frontier orbital energy levels that are appropriate for use in DSSCs. Follow- ing a stepwise co-sensitization procedure, by immersing the TiO 2 electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H 2 P/DC) and D (DC/ZnP-H 2 P).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-H 2 P 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 perfor- mance of device C was further improved up to 7.68% (with J sc = 13.45 mA/cm 2 , V oc = 0.76 V, and FF = 0.75), when a formic acid treated TiO 2 ZnP-H 2 P 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 TiO 2 CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO 2 photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (s d ), and longer electron lifetime (s e ). Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2014.03.033 1566-1199/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding authors. Tel.: +30 2810545045 (A.G. Coutsolelos). E-mail addresses: gdsharma273@gmail.com (G.D. Sharma), spsin- gh@iict.res.in (S.P. Singh), coutsole@chemistry.uoc.gr (A.G. Coutsolelos). Organic Electronics 15 (2014) 1324–1337 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel