CPM Seminar
Barrierless electron-hole recombination and exciton
regeneration at semiconductor polymer heterojunctions
Carlos Silva
Université de Montréal
The combination of electron- and hole-transporting organic semiconductors
can lead to highly efficient organic light-emitting diodes (LEDs). The
heterojunction between the compounds is usually designed so that the
offsets between valence and conduction band edges (HOMO and LUMO levels)
are of 'type II' and electrons and holes accumulate on opposite sides
of the heterojunction. Understanding the electron-hole capture and
subsequent electronic dynamics at the heterojunction is crucial for
device optimization, but also to develop the semiconductor science of
conjugated polymers. Typically, it is described as a two-step process
involving injection of one of the charges over the band-offset barrier
(via thermionic injection or tunneling) and subsequent formation of
an intramolecular exciton. We have contrasted this picture with a new,
barrier-free process. In this model, charge capture occurs directly across
the interface and the neutral state formed has partly intramolecular
and charge-transfer character and is localized at the interface. Such
states are termed exciplexes in donor-acceptor dimers in solution and
are characterized by featureless, red-shifted emission spectra and
long radiative lifetimes. The barrier-free capture into the exciplex is
possible because, due to its charge-transfer character and lower energy,
it is readily accessible from the charge-separated state. Following
charge capture, the exciplex either emits or transfers endothermically
towards the bulk exciton leading to efficient exciton electroluminescence.
Because there is no need for field-assisted charge injection across the
interface, the barrier-free capture mechanism makes possible very low
turn-on voltages of less than 2 V even in polymer blend LEDs where high
fields at the interfaces are unlikely, and results in very high device
brightness (100 cd/m2 at 2.1 V) and efficiencies (above 19 lm/W for
green emission). We demonstrate unambiguously that the only source of
bulk excitons during electrical excitation with moderate driving voltages
is endothermic energy transfer from exciplex states which are generated
via barrier-free electron-hole capture at the heterojunction.
Thursday, October 13th 2005, 15:30
Ernest Rutherford Physics Building, R.E. Bell Conference Room (room 103)
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