Investigation of Organic-Organic Interfaces by Time-Resolved Photocurrent, Electrochemical, And Photoemission Techniques
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ABSTRACT Hole transporting properties and energy barriers at organic-organic interfaces relevant to electrophotographic and organic electroluminescent (EL) devices are described. Three wellknown hole transporting molecules, 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC), N,N'diphenyl-N,N'-bis(1 -naphthyl)-(1,1 '-biphenyl)-4,4'-diamine (NPB), and N,N,N',N'-tetrakis(4tolyl)-(1,1 '-biphenyl)-4,4'-diamine (TTB) are used in this study. The ionization potentials (IP) and oxidation potentials (Eox) of these materials are determined by photoemission spectroscopy and electrochemical measurements, from which a conversion formula is obtained (IP - 4.5 eV + eEox). Hole transport across organic-organic interfaces is investigated by time-of-flight transient photocurrent techniques. The efficiencies of hole injection are consistent with the energy barriers, when present, at these interfaces.
INTRODUCTION Amorphous organic solids are used in the electrophotography industryI and have potential use in display, optoelectronics, and microelectronic industries. Recently, much attention has been given to organic electronic devices such as organic EL displays, '3 photorefractive polymers, 4 and thin film transistors. 5 One of the less well understood properties of these organic electronic devices is the dynamics of charge injection between organic layers, 6' 7 which is critical to the operation of the devices. Hole injection at an organic-organic interface with favorable energy levels is shown in Fig. 1(a). In principle, holes arriving at the interface should be readily transported across. In contrast, in Fig. 1(b), hole injection is expected to be impeded at the interface. Here, we present a study of transient hole injection dynamics and the effects of energetic barriers in organic bilayers.
EXPERIMENT A typical bilayer cell consists of a 300 A Se hole-generating layer coated on a poly(ethylene terephthalate) substrate pre-coated with a 300 A Ni layer, an organic bilayer from sequential vacuum sublimation onto the Se, and finally a 300 A Au layer deposited onto the free surface (Fig. 1(c)). Thickness of each layer is monitored by a quartz crystal, which is calibrated by ellipsometry, profilometry, and absorption spectroscopy. Photocurrents are measured with conventional time-of-flight photocurrent techniques.' Molecular structures of NPB, TAPC, and TTB are shown in Fig. 2. We denote, by HTLI HTL2, a bilayer with holes moving left to right through the cell (Fig. 1(c)). We prepare four types of bilayers: TAPCITTB, NPBITAPC, NPBITTB, and TTBjNPB. 689
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
HTL1
Ultraviolet photoemission spectroscopy (UPS) is performed in an analysis chamber with a base pressure -4 x 10l Torr. We prepare single layers of NPB or TAPC on indium tin oxide (ITO)-coated glass and a bilayer of TAPC and NPB on a silver substrate in a deposition chamber. The deposition chamber (-8 x 10-10 Torr) connects to the analysis chamber via a turntable transport chamber (-1 x 10l° Torr). Redox potentials a
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