Berlin 2012 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 2: Focus Session: Structural Ordering and Electronic Transport I (jointly with CPP)

HL 2.7: Invited Talk

Monday, March 26, 2012, 11:45–12:15, ER 270

Charge transport and recombination in organic light-emitting diodes — •Paul Blom — TNO Holst Centre, Eindhoven, Netherlands

Charge transport and charge recombination are recognized as key ingredients in the performance of polymer light emitting diodes (PLEDs). We observe that in conjugated polymers the electron transport is limited by traps that are Gaussianly distributed in energy within the band gap. Remarkably, we show that the electron trap distribution is identical for a large variety of polymers, hinting at a common origin for electron traps. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin pre-factor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate. As a result, we show that the ideality factor in the diffusion regime of a bipolar diode is governed by the recombination of trapped electrons with free holes.