Electrochemical codeposition of 3-methylthiophene (3MT) and bithiophene (BT) was carried out to form polymer films on the surface of indium-tin-oxide glass, and tris(8-hydroxyquinolato) aluminium(III) (Alq(3)) based organic light emitting diodes (OLEDs) were fabricated with or without the codeposited polymer film being used as hole transport layer (HTL). It was found that poly(3-methylthiophene) (PMT) provided much lower turn-on oxidation potential than polybithiophene did, and a small amount of BT in the monomers solution improved significantly the compactness of PMT films. These features made the codeposit film excellent as HTL in the Alq(3) based OLEDs. A combination of a low percentage of BT in the growth solution (molar ratio of BT to 3MT was 1: 1000) and a moderate film thickness (the polymerization charge was 8 mC cm(-2)) gave OLEDs with lower driving voltages and greater brightness. (c) 2006 Elsevier B.V. All rights reserved.