Luminescent poly(p-phenylenevinylene) hole-transport layers with adjustable solubility

The active part of present polymer light-emitting diodes (PLEDs) consists of only a single layer. Multilayer devices have the advantage that the electron and hole transport can be balanced and that the recombination can be removed from the metallic cathode, leading to highest efficiencies. A major problem for polymer-based multilayer devices is the solubility of the materials used; a multilayer can not be fabricated when a spin-cast layer dissolves in the solvent of the subsequent layer. We demonstrate the development of high mobility poly(p-phenylenevinylene) (PPV)-based hole-transport layers with tunable solubility by chemical modification. Enhanced charge-transport properties are achieved by using symmetrically substituted PPVs; copolymers of long and short side chains enable us to tune the solubility without loss of the enhanced charge transport. Dual-layer PLEDs, in which the holes are efficiently transported via this copolymer towards the luminescent layer, exhibit an enhanced efficiency at high voltages (> 10 V) and a strongly improved robustness against electrical breakdown