Triazine-Bridged Porphyrin Triad as Electron Donor for Solution- Processed Bulk Hetero-Junction Organic Solar Cells Ganesh D. Sharma, †, * Galateia E. Zervaki, § Panagiotis A. Angaridis, § Theophanis N. Kitsopoulos, ‡ and Athanassios G. Coutsolelos* ,§ † R&D Centre for Engineering and Science, JEC Group of Colleges, Jaipur Engineering College Campus, Kukas, Jaipur, Raj 303101, India ‡ Department of Chemistry University of Crete and IESL-FORTH, P.O Box 1527, 71110 Heraklion, Crete, Greece § Department of Chemistry, Laboratory of Bioinorganic Chemistry, University of Crete, Voutes Campus, P.O. Box 2208, 71003 Heraklion, Crete, Greece ABSTRACT: In this report we describe the use of a novel porphyrin triad (PPT) consisting of two zinc-metalated porphyrin units and one free-base porphyrin unit covalently linked through their peripheral amino-phenyl groups to a central s-triazine unit, in combination with PC 70 BM ([6,6]-phenyl C 70 butyric acid methyl ester), as electron donor and electron acceptors, respectively, for the fabrication of small-molecule based, solution-processed, bulk heterojunction (BHJ) organic solar cells. Photoluminescence studies of PPT:PC 70 BM blend films indicated that charge transfer is possible from PPT to PC 70 BM molecules. The solution- processed BHJ organic solar cell with the PPT:PC 70 BM blend in 1:1 weight ratio, processed from THF, was found to exhibit an overall power conversion efficiency (PCE) of 2.85%. When the BHJ active layer of PPT:PC 70 BM was processed from a 5% v/v mixture of 1-chloronaphathalene (CN) in THF, the PCE of the solar cell was increased up to 3.93%. This was attributed to the enhancement of the short circuit current J sc of the solar cell, which was ascribed to a stronger and broader incident photon to current efficiency (IPCE) response and to the higher degree of crystallinity of the active layer of the latter solar cell. The different surface morphologies of the two differently processed active layers result in different electron transport kinetics, and, as shown by electrochemical impedance spectra (EIS) and relaxation time measurements, the device with the active layer with the higher degree of crystallinity results in faster charge transfer process and more efficient exciton dissociation at the PPT/PC 70 BM interface. 1. INTRODUCTION The need for reduction of the overall cost of solar power production directed a considerable amount of research toward the development of unconventional solar cells based on earth abundant materials. A very interesting and promising technology that resulted from these efforts is organic photovoltaic devices. 1-4 These are based on low-cost, semi- conducting organic materials and offer the advantage of lightweight and large-area solar cell devices, using roll-to-roll processing on flexible substrates. 5 At present, the most efficient architecture of organic solar cell devices is based on a solution- processed bulk heterojunction (BHJ) active layer, which usually consists of a blend of a photoactive, conjugated polymer as electron donor, and functionalized fullerene molecules as electron acceptors, 6-9 sandwiched between two electrodes. Upon absorption of photons by the electron donor, electron- hole pairs (excitons) are formed; these diffuse to the donor/ acceptor interface, separate into free holes and free electrons (as electrons fall from the donor conduction band to the acceptor conduction band), which are finally directed to the corresponding electrodes, generating current. Solar cells of this type have been reported to result in power conversion efficiencies (PCEs) that exceed 10% at laboratory scale. 10,11 Recently, small organic molecules have attracted considerable attention as photoactive electron donors alternative to organic polymers, owing to their potential advantages in terms of defined molecular structure, definite molecular weight, high purity, ease of purification, and good batch-to-batch reprodu- cibility. 12-28 A variety of small, organic molecules have been designed and utilized for this purpose, resulting in solar cells with PCE values in the range of 5-7%. 29-32 A particular type of compounds that have been reported to result in highly efficient organic solar cell are π-conjugated molecules with a donor- acceptor (D-A) molecular structure. 33-35 In these π-delocalized systems the inherent internal charge transfer that follows light absorption leads to very efficient electron-hole separation. Received: January 4, 2014 Revised: February 18, 2014 Published: February 21, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 5968 dx.doi.org/10.1021/jp500090h | J. Phys. Chem. C 2014, 118, 5968-5977