Electroluminescence properties of poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4- phenylenevinylene]/tris(8-hydroxyquinoline)aluminum two-layer devices

Two-layer electroluminescing (EL) devices were constructed employing poly[2-(2′-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) and tris(8-hydroxyquinoline)aluminum (Alq₃). On the indium-tin oxide (ITO) glass anode MEH-PPV layer was first spin-coated, on which the Alq₃ layer and the aluminum (Al) anode were vapor-deposited sequentially. When the thickness of Alq₃ was less than that of MEH-PPV, the device showed the EL emission spectrum originated only from the latter. The devices, however, exhibited the combined EL spectra of MEH-PPV and Alq₃ when latter's layer was thicker than the former's layer. The external efficiency for light emission by the double-layer devices was improved by close to two orders of magnitude when compared with efficiency of the MEH-PPV single-layer device, when the thickness of the Alq₃ layer was slightly thicker than that of the MEH-PPV layer. The maximum efficiency obtained was about 0.02%. Current flow in the two-layer devices was limited effectively by the Alq₃ layer and it decreased steadily as the thickness of the Alq₃ layer was increased. The results were discussed on the basis of improved exciton formation resulting from accelerated and balanced carrier transport.