Exciton and Defect Photoluminescence Signatures in Single Crystal Rubrene
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0965-S13-08
Exciton and Defect Photoluminescence Signatures in Single Crystal Rubrene Oleg Mitrofanov1, David V. Lang2, Christian Kloc1, Theo Siegrist1, Woo-Young So2, Michael A. Sergent1, and Arthur P. Ramirez1 1 Device Physics, Bell Labs, Lucent Technologies, 600 Mountain Ave, Murray Hill, NJ, 07974 2 Columbia University, New York, NY, 10027
ABSTRACT Radiative recombination processes provide valuable information about exciton dynamics and allow detection of defects in rubrene crystals. We demonstrate that the photoluminescence spectra of crystalline rubrene reflect exciton dissociation through oxygen-related defects in addition to the direct exciton recombination. The defect-assisted exciton dissociation results in a well-defined photoluminescence band. These defects play an important role in charge transport. Dark- and photo-conductivity is higher in rubrene crystals with a large density of the defects. The observations strongly suggest that the oxygen-related defect forms a bandgap state and acts as an acceptor center in crystalline rubrene. INTRODUCTION Despite a large number of investigations, understanding of the fundamental processes governing charge transport remains a central issue for organic-based devices such as transistors, photovoltaic cells, and light emitting diodes. The role of bandgap states in electronic and optical characteristics is largely unexplored, nevertheless their impact is likely to be significant [1, 2]. Even small densities of impurities and defects (~10-16 cm-3) can substantially change transport characteristics of highly resistive organic semiconductors. Rubrene (C42H28) in single crystal form has emerged as an important aromatic molecular solid, showing the highest field effect transistor (FET) mobility to date among organics (20 cm2/Vs) [3]. In addition, several studies suggest that band-like transport is operative [3-6]. Electronic properties in rubrene are highly sensitive to both atmospheric environment as well as material treatment. Recent studies indicate that the presence of ambient oxygen affects the conductivity of crystalline rubrene [6-7]. Furthermore, chemical analysis of naturally oxidized crystalline rubrene shows that the relative concentration of rubrene peroxide (C42H28O2) can be as much as ~1% at a depth of 50 nm [8], a length scale greater than a typical FET channel depth. Such a high impurity concentration will likely have a significant effect on charge transport, particularly if the impurity forms a bandgap state. We demonstrate the formation of a bandgap state in oxidized rubrene crystals and address the impact of the state on charge transport and exciton recombination.
EXPERIMENT Rubrene single crystals were grown by the vapor transport method in a stream of hydrogen. Selected crystals were either deliberately oxidized in an oxygen atmosphere for 14 hours at 100oC or placed in vacuum shortly after the growth. The latter will be referred to as asgrown crystals. The oxidized crystals were sealed in a quartz capsule containing oxygen at normal pressure. PL c
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