Transport of carriers in organic light-emitting devices fabricated with a p-phenylenevinylene-derivative copolymer

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Abstract
The majority carriers of polymer-based light-emitting diodes are often considered to be holes. However, we demonstrate that the majority carriers of p-phenylenevinylene-copolymer-based devices, which show very high emission efficiency, are electrons. This conclusion is attained by comparing the current-voltage characteristics of these devices with those of electron-only and hole-only devices. The current due to electrons is proportional to the fourth power of the voltage. This result is interpreted in terms of the space-charge-limited conduction of electrons with field-dependent carrier mobility. The transport of holes in the hole-only devices is also attributed to the same mechanism. The mobility of holes under the normal operational conditions of the devices is estimated to be lower than that of electrons by an order of magnitude. This large difference in the mobility leads to carrier recombination near the interface between the copolymer and the anode, and thus to the high emission efficiency of the light-emitting devices.