Berlin 2005 – scientific programme

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SYOO: Organic Optoelectronics and Photonics

SYOO 3: Organic Optoelectronics and Photonics I

SYOO 3.1: Invited Talk

Monday, March 7, 2005, 14:00–14:45, TU HE101

Excited states at heterojunctions between polymeric semiconductors — •Richard H. Friend — Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, UK

Much of the important device physics of molecular or polymeric semiconductor structures is controlled by heterojunctions between different semiconductors. Band-edge offsets at these heterojunctions can be engineered through the selection of the two semiconductors, and are generally chosen to provide type II heterojunctions.

For operation in photovoltaic mode, a diode requires a heterojunction with band-edge offsets larger than the exciton binding energy (typically 0.5 eV), so that an exciton present at the heterojunction will be ionisated across the heterojunction. For operation as a light-emitting diode, the band-edge offsets must then be set so that the bound exciton is stable against charge separation.

Close to the threshold for exciton ionisation at the heterojunction, we find that the intrachain exciton can be localised at the heterojunction, by acquiring some degree of charge-transfer character, though still retaining a substantial binding energy. With respect to the bulk exciton, the exciplex is red-shifted and its radiative lifetime is strongly increased. The barrier for thermal excitation of the exciplex to allow it to move away from the heterojunction can be chosen to be small, so that this process can give efficient bulk exciton emission at room temperature, and efficient operation of LEDs.