Ž . Synthetic Metals 111–112 2000 75–78 www.elsevier.comrlocatersynmet Optical and electrical properties of substituted 2,5-diphenyl-1,3,4-oxadiazoles Y. Kaminorz ) , B. Schulz, L. Brehmer UniÕersity of Potsdam, Institute of Physics, Physics of Condensed Matter, Am Neuen Palais 10, 14469 Potsdam, Germany Abstract Ž . New substituted 2,5-diphenyl-1,3,4-oxadiazoles are reported as luminescent materials in light emitting diodes LEDs . The investi- gated new oxadiazoles show efficient blue and green emission in single layer devices. The combination with a hole transporting and red Ž . emitting polythiophene led to a white emission with higher quantum efficiency QE . q 2000 Elsevier Science S.A. All rights reserved. Keywords: Optical property; Electrical property; Substituted 2,5-diphenyl-1,3,4-oxadiazoles; Quantum efficiency 1. Motivation Oxadiazoles are known for their high thermal and hy- wx drolytic stability 1 . The electron acceptor properties of the oxadiazole ring makes these substances usable as Ž . electron transport material in light emitting diodes LEDs w x 2–5 . Some aromatic substituted 1,3,4-oxadiazoles show Ž . intensive photoluminescence PL and also electrolumines- Ž . cence EL in combination with an additional hole or w x electron transport layer 6–9 . In the present paper, new substituted 2,5-diphenyl- 1,3,4-oxadiazoles are reported as EL materials. The oxadi- azoles were investigated as single layer devices. By modi- fication of the chemical structure as well as the blending with an additional hole transporting polythiophene, the emission properties of the LED were optimized. 2. Experimental Ž The substituted 2,5-diphenyl-1,3,4-oxadiazoles Fig. 1a . and Table 1 as well as the blends with a polythiophene Ž . Fig. 1b in a PMMA matrix were spin coated onto an ITO Ž . glass substrate 18.3 VrI, enhanced quality at 600 to 800 rpm. Using a 5 mgrml chloroform solution, homoge- neous and pin hole free films with different thicknesses Ž . Table 1 were obtained. On top of the emission layer, an opaque aluminum or calcium electrode has been deposited ) Corresponding author. Ž . E-mail address: yvette@rz.uni-potsdam.de Y. Kaminorz . through a shadow mask. The active area of the device was 2 = 2 mm 2 . 3. Results New 2,5-diphenyl-1,3,4-oxadiazole derivatives were synthesized having excellent film forming properties due to their solubility in common organic solvents and dimin- ished tendency to crystallize. Spectroscopic investigations have shown, that strong PL of films could be achieved by using a substituent with strong electron donor properties Ž . diethylamino or dimethylamino groups . Therefore, the investigations of EL were focused mainly on those sub- stances. OXA-10, substituted with diethylamino and amino group with both groups having electron donor properties, show Ž . PL and EL in the blue range Fig. 2a . The OXA-10 LED exhibited a very low external quan- Ž . tum efficiency QE . Because no remarkable change in the slope of current was observed, poor hole transport capabili- Ž . ties are assumed Fig. 2b . To improve QE, an oxadiazole w x with comparable mobility for electrons and holes 11,12 , OXA-12, was chosen, symmetrically substituted with di- ethylamino groups. The optical properties were almost Ž . unchanged compared to OXA-10 Fig. 3a . The improved hole transport capability of OXA-12 can be seen in the I Ž . Ž . V and L V curves. Up to a voltage of 5 V, the current is determined by electrons. At about 5 V, holes are also injected, which results in a start of emission of blue light. The external QE of the ITOrOXA-12rAl device was 0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0379-6779 99 00360-4