The direct measurement of energy barrier height at metal/ polyfluorene derivatives interface by internal photoemission s
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The direct measurement of energy barrier height at metal/ polyfluorene derivatives interface by internal photoemission spectroscopy Eiji Itoh and Shinya Takaishi Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano, 380-8553, Japan ABSTRACT We have investigated the barrier height for electron injection at the cathode / polyfluorene derivatives interface by the internal photoemission (IPE) spectroscopy techniques using the “electron only device” structure consisting of TiO2, electron transporting polyimide interlayer (IL), and polyfluorene derivatives. We also estimated the barrier height by the current analysis based on the Schottky thermal emission current model, and it coincides well to the threshold energy of IPE result only when the energy is lower than 1.1eV. The measured barrier height obtained by IPE linearly increases with both the work-function of cathode materials. However, the slope parameter becomes less than 1 (~0.6) for poly (9,9-dioctylfluorene) (F8) probably due to the interfacial gap states. On the other hand, the slope parameter becomes very small (~0.18) for the poly (9,9-dioctylfluorene)-co- benzo- thiadiazole) (F8BT) probably due to the electron pinning at the cathode/ acceptor interface. INTRODUCTION The interface properties of metal/ insulator and metal/ semiconductor attract an unprecedented attention in the electronic thin film devices, such as organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs). In general, the barrier height for electrons is assumed as the difference of the lowest unoccupied molecular orbital (LUMO) of organic material, and the work functions of the cathode. There are several methods to estimate the energy diagram of organic materials, such as electrochemical measurement, ultra-violet photoemission spectroscopy (UPS)[1-5], inverse photoemission spectroscopy (IPES)[6]. The recent progress in UPS enabled us the estimation of electronic structures of the occupied states with the order of meV. On the other hand, the reported values of work functions and LUMO levels have strong variation depending on the measuring method. Especially, the resolution of IPES for estimating the LUMO in solid states is the order of 0.2-0.5eV. Moreover, the surface states, interfacial polarization and the interfacial dipole moment also modify the barrier height seriously [1-6]. Therefore, the direct measurement technique for the barrier height at the electrode/ organic interface is required under the device operation. The internal photoemission (IPE) spectroscopy provides the most straightforward way to characterize the relative energies of electron states at interfaces of insulators and wide-gap semi-conductor from the Fermi level of metals by measuring the spectral onset of electron/hole photoemission with the high resolution of ~meV [7-10]. However, in general, it is difficult for OLEDs and solar cells to reduce the leakage current less than 1nA/cm2 under the high electric field of 105 –106 V/cm, while the typical
measured IPE current is only the or
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