Eur. Phys. J. E (2005) 18: 21-28
https://doi.org/10.1140/epje/i2005-10026-6

Original Article

Transport and recombination in organic light-emitting diodes studied by electrically detected magnetic resonance

¹ Departamento de Fısica e Matemática, FFCLRP-USP, Av. Bandeirantes 3900, 14040-901, Ribeir ao Preto, Brazil
² Laboratory for Functional Polymers, EMPA, Überlandstrasse 129, CH-8600, Dübendorf, Switzerland
³ Laboratoire d'optoélectronique des matériaux moléculaire, IMX-STI-EPFL, CH-1015, Lausanne, Switzerland

^* e-mail: cfograeff@ffclrp.usp.br

Received: 28 January 2005
Accepted: 29 June 2005
Published online: 20 September 2005

Abstract

We have used electrically detected magnetic resonance (EDMR) to study a series of multilayer organic devices based on aluminum (III) 8-hydroxyquinoline (Alq₃). These devices were designed to identify the microscopic origin of different spin-dependent processes, i.e. hopping and exciton formation. The EDMR signal in organic light-emitting diodes (OLEDs) based on Alq₃ is only observed when the device is electroluminescent and is assigned to spin-dependent exciton formation. It can be decomposed in at least two Gaussians: one with peak-to-peak line ( ΔH _PP) of 1.6 mT and another with ΔH _PP of 2.0 to 3.4 mT, depending on bias and temperature. The g-factors of the two components are barely distinguishable and close to 2.003. The broad line is attributed to the resonance in Alq₃ anions, while the other line is attributed to cationic states. These attributions are supported by line shape and its electrical-field dependence of unipolar Alq₃-based diodes, where hopping process related to dication and dianion formation is observed. In these unipolar devices, it is shown that the signal coming from spin-dependent hopping occurs close to organic semiconductor/metal interfaces. The sign of the magnetic-resonance-induced conductivity change is dominated by charge injection rather than charge mobility. Our results indicate that the probability of singlet exciton formation in our OLEDs is smaller than 25%.