29-01-2013, 02:31 PM
PLDMR study of rubrene and oxygen-doped rubrene films and powders
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Abstract
A comprehensive photoluminescence (PL)-detected magnetic resonance (PLDMR)
study of various vacuum-sealed 5,6,11,12-tetraphenyl-tetracene (rubrene) films and
powders is described. Three PLDMR features are observed and analyzed: (i) A
negative (PL-quenching) triplet exciton (TE) resonance at T > 50K, due to reduced
spin-dependent fusion of geminate TE pairs to singlet excitons (SEs). (ii) A positive
(PL-enhancing) triplet resonance at T < 50K. This resonance is suspected to result
from reduced quenching of SEs by a reduced population of polarons and nongeminate
TEs, the latter due to the spin-dependent annihilation of TEs by polarons. (iii) A
negative (PL-quenching) spin 1/2 (polaron) resonance, believed to be due to enhanced
formation of trions, i.e., bipolarons stabilized by a countercharge, at oxygen centers.
As single crystal thin films of oxygen-doped rubrene exhibit exceptionally high roomtemperature
carrier mobility, the relation between this negative resonance and the
transport properties is also discussed.
Introduction
Among π-conjugated materials, tetracenes and their derivatives, notably 5,6,11,12-
tetraphenyl-tetracene (rubrene) (Fig. 7-1), are unusual in their electronic structure:
The energy of their low lying triplet exciton (TE) state ETE is approximately one half
that of the low-lying singlet exciton (SE) state ESE [1]. Consequently, in neat rubrene
films or powders SEs efficiently fission to pairs of TEs, reducing the
photoluminescence (PL) quantum yield ηPL of such films and powders to ~10%, from
100% in dilute solutions. At the same time, rubrene films and powders also exhibit a
relatively strong delayed fluorescence due to relatively efficient fusion of such TEs
(back) to SEs.
Rubrene is also an attractive dopant in fluorescent OLEDs: When doped into
either N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPD)
hole transport layers or tris(8-hydroxyquinoline) Al (Alq3) electron transport layers, it
enhances the efficiency and stability of the devices [2,3]. In the former, this is
suspected to result from an increased glass transition temperature; in the latter, from
efficient trapping of holes, which chemically destabilize Alq3 [4]. When doped either
into green Alq3 or blue 4,4’-bis(2,2’-diphenylvinyl)-1,1’-biphenyl (DPVBi) OLEDs, it
shifts the emission to orange-red; in the latter case it enables extremely bright and
efficient (relative to other fluorescent devices) white OLEDs [3].
Results and discussion
a) The PL spectra
Fig. 7-2(a) shows the PL spectra of fresh (nominally oxygen free) rubrene
films at various temperatures. The spectra are in reasonable agreement with those
obtained by Mitrofanov et al. [8], and clearly show the three main bands, band I is at
570 nm (2.18 eV), band II is at 590 nm (2.10 eV) and band III is at 608 nm (2.04 eV) .
Compared with Band I and III, Band II is weaker and it starts to disappear when
T >60 K. When T >180 K, three individual bands merge together to be a broad band.
The PL intensity decreases as the temperature T increases from 20 to 300 K, in
agreement with Mitrofanov et al. [8].
Fig. 7-2(b) shows the spectra of aged (unintentionally oxygen-doped) rubrene
powders at various temperatures. An additional PL band I’ red shifted by about 40
meV from the band I is clearly seen at 60 K.