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.