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 latterʹs layer was thicker than the formerʹs 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.