Photophysical Properties and Optoelectronic Device Applications of a Novel Naphthalene-Vinylene Type Conjugated Polymer Gerald Zerza,* ,† Birgit Ro 1 thler, and N. Serdar Sariciftci Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler UniVersity Linz, Altenbergerstrasse 69, A-4040 Linz, Austria Rafael Go ´ mez, Jose ´ L. Segura, and Nazario Martı ´n* ,‡ Departamento de Quı ´mica Orga ´ nica, Facultad de Quı ´mica, UniVersidad Complutense, E-28040 Madrid, Spain ReceiVed: June 16, 2000; In Final Form: December 1, 2000 We report on photoinduced charge transfer and energy transfer reactions using a new cyano-substituted naphthalene-vinylene type conjugated polymer. Optical absorption and emission spectra of thin films of mixtures of this polymer with the fullerene derivative 1-(3-(methoxycarbonyl)propyl)-1-phenyl-(6,6)C 61 (PCBM) exhibit several interesting features such as luminescence quenching and polaronic photoinduced absorption (PIA) of charges indicating photoinduced charge transfer. On the other hand this new polymer exhibits energy transfer phenomena in blends with MDMO-PPV (poly((2-methoxy-5-((3,7-dimethyloctyl)oxy)-1,4-phenylene)- vinylene)). Photovoltaic devices and light emitting diode (LED) devices have been fabricated and characterized using this novel conjugated polymer in composite with fullerenes and MDMO-PPV’s, respectively. 1. Introduction In the past two decades, research on conjugated, semicon- ducting polymers has experienced an increasing attraction in the scientific community. 1 Especially due to their application in light-emitting diodes (LED) 2-7 which was discovered around a decade ago and is today entering large scale commercialization as well as due to their potential for cost-effective photovoltaic cells, 8-12 these materials are more and more getting to the focus of industrial optoelectronic applications. In this context of optoelectronic device applications, special attention has been drawn to photoinduced charge generation, 13-25 charge transport, and charge recombination at interfaces. 26-28 Important for understanding the involved photophysics is also the question of the nature of the primary photoexcitations in conjugated polymers. 29 For an efficient light emission in electroluminescent devices, both electron and hole transport have to be balanced. Though most of the conventional conjugated polymers have a hole- conducting character, efficient electron-conducting polymers are rare. Cyano-substituted conjugated polymers have been widely investigated as candidates for electron conductors. The electron accepting character of the cyano side groups reduces the electron density on the polymer backbone and enhances the electron affinity of the polymer. Especially cyano-substituted phenylene- vinylene type polymers (CN-PPV) have been synthesized and utilized in LED devices. 5 In bilayer structures of CN-PPV with poly[phenylenevinylene] (PPV), injected electrons accumulate at the CN-PPV side of the interface where mobile holes injected into the PPV layer travel to and recombine at this interface with accumulated electrons giving rise to high electroluminescence quantum efficiencies. The enhanced electron affinity of these CN-PPV polymers has also been utilized in photovoltaic cells, where they are used as electron-conducting materials together with poly[(2-methoxy-5-((2′-ethylhexyl)oxy)-1,4-phenylene)- vinylene] (MEH-PPV) in bilayers or in blend films. 9, 30 Principally, both charge transfer as well as energy transfer may be observed depending on the relative energetic positions of the donor and acceptor components prior to photoexcitation. 28 Only minor changes in the substituents on the side chain can turn the balance from one process in favor of the other one. This opens up a possibility of chemical tailoring and “engineer- ing” of optoelectronic properties. In this work we present studies on the electronic properties and photoactivity of a new cyano-substituted naphthalene- vinylene type conjugated polymer. Due to the cyano groups it combines the interesting properties of an electron-accepting, n-conducting cable and a strongly luminescent material. The aim was to investigate besides the photophysical properties also possible applications of this conjugated polymer in optoelec- tronic devices, like polymer solar cells and LEDs. The chemical structure of the polymer 1 is shown in Figure 1. The repeating unit consists of two cyano-substituted naphthalene-vinylene units terminated with a naphthalene unit (2). Four alkyloxy side groups provide sufficient solubility in common organic solvents. The syntheses of polymer 1 and the corresponding monomer 2 have been described elsewhere. 31 Photoinduced energy and charge transfer was observed in the presence of fullerenes and other conjugated polymers, respec- tively. It is further shown that this new conjugated polymer exhibits redox properties acting both as electron donor and as electron acceptor. LED devices emitting in the blue-green spectral range with surprisingly low turn on voltages could be fabricated out of this polymer. 2. Experimental Section Poly[(2-methoxy-5-((3,7-dimethyloctyl)oxy)-1,4-phenylene)- vinylene] (MDMO-PPV) was delivered by Covion Inc. and † Tel: 0043-732-2468-8767. Fax: 0043-732-2468-8770. E-mail: Gerald.Zerza@jk.uni-linz.ac.at. ‡ E-mail: nazmar@eucmax.sim.ucm.es. 4099 J. Phys. Chem. B 2001, 105, 4099-4104 10.1021/jp0021852 CCC: $20.00 © 2001 American Chemical Society Published on Web 04/20/2001