ARTICLE pubs.acs.org/JPCC
Metal-Diffusion-Induced Interface Dipole: Correlating Metal OxideOrganic Chemical Interaction and Interface Electronic States Kihyon Hong,† Kisoo Kim,† Sungjun Kim,† Soo Young Kim,‡ and Jong-Lam Lee*,† †
Division of Advanced Materials Science and Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 790-784, Korea ‡ School of Chemical Engineering and Materials Science, Chung-Ang University, 221 Heukseok-dong, Dongjak-gu, Seoul, 156-756, Korea
bS Supporting Information ABSTRACT:
The effects of metal oxide diffusion on the interface dipole (ID) energy at a metal oxide (SnO2)/organic semiconductor (copper phthalocyanine, CuPc) interface were studied. In situ synchrotron radiation photoelectron spectroscopy and ultraviolet photoemission spectroscopy studies showed that the ID energy for SnO2-on-CuPc (0.65 eV) was higher by 0.15 eV than that of CuPc-on-SnO2 (0.50 eV). When SnO2 deposited on a CuPc layer, hot Sn atoms release enough condensation energy to disrupt the weakly bonded CuPc and diffuse through the surface. The diffused Sn atoms made a chemical reaction with nitrogen atoms in CuPc molecules and made organo-metallic compounds, Sn2CuPc, resulting in the generation of gap states at the former lowest unoccupied molecular orbital. This observation explains why the ID and hole injection barrier at SnO2-on-CuPc are larger than those at the CuPc-on-SnO2 interface. Organic light-emitting diodes with a SnO2-on-CuPc interface showed a lower luminous efficiency (2.63 cd/A) than that of the device with the CuPc-on-SnO2 interface (5.26 cd/A), and this result indicates that ID tuning at SnO2CuPc interfaces by adjusting the metal diffusion can be readily applicable.
1. INTRODUCTION In semiconductors that feature a metal layer and an organic semiconductor (OS), the properties of the interface between the metal and the OS have an important influence on the injection characteristics of charge carriers.15 The injection of carriers from metal contacts to OS is central to the operation voltage, contact resistivity, efficiency, and stability of organic-based devices, such as organic light-emitting diodes (OLEDs), organic thin-film transistors, and organic photovoltaics.68 Deposition of metal on the organic layer causes metal atoms to diffuse into the organic layer.9 Quantifying the metal diffusion at the metal-onorganic interface is important in estimating the interface states and energy band structure, which provide a starting point for understanding the charge injection process. Various analyses and measurements have been performed to elucidate the metal diffusion into organic molecules at the metalon-organic interface. A scanning electron microscopy study showed that the Au deposition can cause metal penetration into r 2011 American Chemical Society
the pentacene molecules, which destroys their stacking geometry and increases the contact resistivity of pentacene/Au.10 An X-ray photoemission spectroscopy study showed that the deposited Au atoms interdiffuse into copper phthalocyanine (CuPc) molecules and form Au clusters in the CuPc layer.11 Diffusion of Mg atoms and formation of a Mg-tris(8-hydroxyquinolinato) aluminum (Alq3) organo-metallic complex at the Alq3/Mg interface have been proven using ultraviolet photoemission spectroscopy (UPS).12 An X-ray absorption study showed that a ferromagnetic metal, Fe, deposited on CuPc makes a chemical reaction and charge carrier traps into CuPc molecules.13 However, these results cannot explain the effect of metal diffusion on the change of the interface dipole (ID) and the charge carrier injection barrier (CIB). The ID has an important Received: June 20, 2011 Revised: October 11, 2011 Published: October 13, 2011 23107
dx.doi.org/10.1021/jp2057783 | J. Phys. Chem. C 2011, 115, 23107–23112
The Journal of Physical Chemistry C
Figure 1. (a) Schematic diagram and (b) energy band structure of the diffused SnO2-on-CuPc interface. (c) Schematic outline and (d) energy band structure of the abrupt CuPc-on-SnO2 interface. Δ indicates the interface dipole energy at each interface.
influence on charge injection properties and device performance in terms of low operation voltage, high quantum efficiency, high stability, and so on.14 For green-emitting OLEDs, copper phthalocyanine (CuPc) has been used as the hole injection layer (HIL) and Au as the anode.15,16 In the normal OLED configuration, the CuPc is coated on a Au layer and forms an abrupt Au/CuPc interface. However, in the inverted OLED configuration, Au is deposited on the CuPc layer. Because of the high melting point of Au (1337 K), the deposition of the Au layer causes metal atoms to diffuse into the organic layer.1719 This diffusion of Au deforms the CuPc/Au interface during the deposition process and reduces the hole injection efficiency in the anode region. Thus, IOLEDs have inferior characteristics, such as large current leakage, low luminous efficiency (
