Porphyrin oriented self-assembled nanostructures for efficient exciton dissociation in high-performing organic photovoltaics† M. Vasilopoulou, * a D. G. Georgiadou, a A. M. Douvas, a A. Soultati, a V. Constantoudis, a D. Davazoglou, a S. Gardelis, a L. C. Palilis, b M. Fakis, b S. Kennou, c T. Lazarides, d A. G. Coutsolelos d and P. Argitis a Herein we report on enhanced organic solar cell performance through the incorporation of cathode interfacial layers consisting of self-organized porphyrin nanostructures with a face-on configuration. In particular, a water/methanol-soluble porphyrin molecule, the free base meso-tetrakis(1- methylpyridinium-4-yl)porphyrin chloride, is employed as a novel cathode interlayer in bulk heterojunction organic photovoltaics. It is demonstrated that the self-organization of this porphyrin compound into aggregates in which molecules adopt a face-to-face orientation parallel to the organic semiconducting substrate induces a large local interfacial electric field that results in a significant enhancement of exciton dissociation. Consequently, enhanced photocurrent and open circuit voltage were obtained resulting in overall device efficiency improvement in organic photovoltaics based on bulk heterojunction mixtures of different polymeric donors and fullerene acceptors, regardless of the specific combination of donor–acceptor employed. To highlight the impact of molecular orientation a second porphyrin compound, the Zn-metallated meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, was also studied and it was found that it forms aggregates with an edge-to-edge molecular configuration inducing a smaller increase in the device performance. 1. Introduction Organic photovoltaic (OPV) cells hold great economic potential as they may lead to a new generation of consumer devices that can be processed at low cost on large areas, have light weight and conform to exible substrates. 1–3 Until now, high efficien- cies of 7–10% have been realized in OPVs based on polymer donor–fullerene acceptor bulk heterojunctions (BHJs), mainly through the optimization of the bandgap and the highest occupied molecular orbital (HOMO) level of the semiconduct- ing polymer and the lowest unoccupied molecular orbital (LUMO) level of the fullerene acceptor. 4–6 However, further improvements are needed to enhance the efficiency towards the goal of 10% and, thus, to render their mass production and practical applications feasible. Because efficient operation of OPVs relies on the efficient separation and collection of pho- togenerated carriers, approaches to further increase their effi- ciency should aim towards recovery of energy losses caused by the interfacial energy level misalignments and the recombina- tion of excitons. 7–9 Especially, non-radiative exciton recombi- nation reduces (a) the charge concentration and thus, the corresponding photocurrent and (b) the quasi-Fermi energy difference (i.e. chemical potential) between electrons and holes, resulting in the lowering of the open circuit voltage (V oc ). This process is usually invoked to explain the hitherto observed relatively low V oc in most OPVs. 10,11 To date the most successful strategies to circumvent interfacial energetic and exciton recombination losses include, respectively, a plethora of alter- native conducting materials implemented in most cases as high mobility electron transport layers in combination with an air- stable metal cathode, 12–17 and the incorporation of ultra thin layers based on ferroelectric insulators to amplify the local electric eld and, thus, promote exciton dissociation and elec- tron–hole separation. 9,18,19 Yet, the obtained V oc values remain below their theoretical limits (i.e. the difference between the E HOMO of the donor and the E LUMO of the acceptor). 8 On the other hand, planar organic molecules with unusual electronic and optical properties, such as porphyrins and phthalocyanines, have been intensively investigated due to their unique structure, based on two-dimensional conjugated cores, a Department of Microelectronics, Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems (IAMPPNM), National Center for Scientic Research “Demokritos”, 153 10 Aghia Paraskevi Attikis, Athens, Greece. E-mail: mariva@imel.demokritos.gr b Department of Physics, University of Patras, 26500 Patras, Greece c Department of Chemical Engineering, University of Patras, 26500 Patras, Greece d Laboratory of Bioinorganic Chemistry, Chemistry Department, University of Crete, Voutes Campus, 71003 Heraklion, Crete, Greece † Electronic supplementary information (ESI) available: Materials and methods, ESI text, Fig. S1–S9. See DOI: 10.1039/c3ta13107f Cite this: J. Mater. Chem. A, 2014, 2, 182 Received 7th August 2013 Accepted 17th October 2013 DOI: 10.1039/c3ta13107f www.rsc.org/MaterialsA 182 | J. Mater. Chem. A, 2014, 2, 182–192 This journal is © The Royal Society of Chemistry 2014 Journal of Materials Chemistry A PAPER