Optical Probing of Polarons and Triplet Excitons in Conjugated Polymer Devices
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Optical Probing of Polarons And Triplet Excitons In Conjugated Polymer Devices Anoop S. Dhoot and Neil C. Greenham Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, United Kingdom ABSTRACT Polarons and triplet excitons in conjugated polymers exhibit sub-gap absorptions which allow them to be identified spectroscopically. We have used quasi-steady-state induced absorption techniques on working polymer light-emitting diodes to study charge carriers and triplet excitons in devices. We identify absorptions due to charges, and at low temperatures can also resolve features due to triplet excitons. From the magnitude of the absorption features, we study the charge and triplet densities as a function of applied voltage. We obtain a value for the triplet generation rate at low temperatures and an estimate of the singlet exciton formation probability. Analysis of the triplet lifetime as a function of charge density reveals the presence of triplet-polaron interactions and we obtain a rate constant for this triplet annihilation process. INTRODUCTION Understanding the behavior of charged and neutral excitations in conjugated polymer devices is of both theoretical and practical interest. As polymer light-emitting diodes (LEDs) advance towards applications [1], the nature of the charged and neutral states involved in device operation is not yet fully understood. In a polymer LED, positive and negative charges are injected at opposite electrodes and migrate towards each other under the influence of an applied electric field. They may then capture each other to form either a singlet or triplet exciton. Singlet excitons can decay radiatively or non-radiatively whereas triplet excitons decay non-radiatively. According to a simple spin statistical recombination model, 25% of the excitons initially formed will be singlets. However recent electroluminescence [2,3] and magnetic resonance experiments [4] suggest that more singlets are formed. Excitations in π-conjugated polymers differ greatly from conventional semiconductor excited states. Electrical injection of charges onto the semiconducting polymer chain leads to self-localisation due to strong charge-lattice coupling. The resulting polaronic charges produce states lying within the energy gap and the optical transitions between these sub-gap states can be used to identify the presence of charges. Charged excitations may be singly-charged polarons or doubly-charged bipolarons. Polarons are expected to show two sub-bandgap optical transitions while bipolarons show one. The triplet exciton is detected by an optical transition to a higherlying triplet state, typically near 1.4 eV in π-conjugated polymers. Optical spectroscopy on working LEDs [5-9] provides a means to study charged and neutral excited states. Here, we use this technique to study recombination processes in polymer LEDs, and to investigate decay mechanisms for triplet excitons.
C1.6.1
EXPERIMENT The experimental apparatus is shown in Figure 1. A freshly-made device was placed in a cryostat equipped with sapphire wi
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