ELSEVIER Synthetic Metals 102 (1999) 1147-1148 Dependence of the Hole-Injection Barrier on the Hole Conductor in Organic Light Emitting Diodes Based on Composites Markus Gross,David Mtiller, Christoph Brauchle, Klaus Meerholz* Institut fm Physikalische Chemie, Sophienstr. 11,80333 Munich, GERMANY Abstract Organic light-emitting diodes based on compositesusing different hole conductors were fabricated and characterized. In these devices the current flux is limited by the injection of holes into the semi-conductingpolymer layer through tunneling. The dam were evaluated using the common Fowler-Nordheim formalism. The barrier height was found to depend linearly on the oxidation potential of the hole conductor, but unexpectedly the slope is only 0.55 f 0.1. This result is explained by the non-polar nature of the internal interface be- tween lT0 and the conductive layer. Keywords: organic light emitting diodes, hole injection, injection barrier, Fowler-Nordheim tunneling 1. Introduction Organic light emitting diodes (OLED) have become a rapidly growing field of research due to their promising applications as large, flexible flat-panel displays and easy and inexpensive processibility [ 11. But many problems still remain such as short life time, low efficiency, or inconveniently high operating volt- ages. Only after solving them, commercialization will become possible. In recent years, research has concentrated on p-conjugated polymers, such as poly(p-phenylenevinylene) (PPV) and its de- rivative MEH-PPV, on the one hand [2] and on small semicon- ducting molecules like the chelate complex aluminum trisquino- late (AlQ3) on the other [3]. Both material classes possess good luminescence properties and have been thoroughly investigated. Our approach is based on polymer blends as the active lumines- cent layer, allowing a broader range of tunability of the OLED device. A disadvantage of OLEDs of this kind so far has been the rather large onset voltage compared with OLEDs based on con- jugated polymers or vacuum-deposited molecules [4]. The interfaces between the organic layer(s) and the contact electrodes play a central role for the physical understanding and optimization of OLED devices, Since these are internal inter- faces, their exploration is naturally difficult. In the case of con- ventional inorganic semiconductor devices, extensive studies of the electronic levels of contacts and semiconducting materials have been performed by surface-sensitivemeasurements such as UPS and XPS. By contrast, only very few such experiments have been presented in the past on organic systems [5, 61. This is partly due to the higher chemical reactivity of organic compounds compared with the inorganic analogues, leading to electrically insulating interfacial layers, interface diffusion, and morphologi- * kmeerhol@olymp.phys.chemie.uni-muenchende cal changes which dramatically influence the electronic proper- ties of the compounds. Electrochemical methods are also regarded as a tool to learn about the electronic levels (HOMO and LIMO) of the charge- transporting units. The offset between the electrochemical po- tential (typically given relative to a reference electrode such as Ag/AgCl) and the electronic work function I& (vs. the vacuum level) can be estimated from the work function of the standard hydrogen electrode (NHE) and the standard redox potential of Ag/AgCl to 4.45 + 0.2 eV [7]. In this paper, we will investigate the hole injection barrier of single-layer composite OLEDs using different hole conductors (Fig. 1) 2. Experimental All preparation steps were performed in an inert gas atmos- phere. One-layer OLEDs were fabricated by spincasting the ac- tive luminescent layer (EML) from chloroform solution (= 2000 rpm, 20 mg/ml) onto ITO-coated substrates (resistivity < 20 Wsq.). The thicknesswas typically about 100-120 mn. The EML consisted of 50 wt% of the hole transporting molecule and 50 wt% polycarbonate (PC) as the binder. Aluminum was evapo- rated as the cathode (= 100 nm). The diode area defined by the cathode was 0.09 cm’. The different OLED devices were then characterizedunder inert gas atmosphere by measuring the I-V characteristics. 3. Results and Discussion The current characteristicsof OLED devices is generally de- termined by the majority carriers, which are the holes in this case. A linear dependence between the thickness of the conduc- 0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01441-6