Relaxation Dynamics of Photoexcited Excitons in Rubrene Single Crystals Using Femtosecond Absorption Spectroscopy

Tao, Shoichi; Ohtani, Naoki; Uchida, Ryota; Miyamoto, T.; Matsui, Yoshiki; Yada, H.; Uemura, H.; Matsuzaki, Hiroyuki; Uemura, Tomomasa; Takeya, Jun; Okamoto, Hiroshi

doi:10.1103/physrevlett.109.097403

Cited by 22 publications

(39 citation statements)

References 27 publications

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“…We do not rule out the contribution of exciton states with different symmetry (N or L, transition from higher excited states) in amorphous rubrene, because the difference between the first and the second peak is 0.16 eV rather than the carbon double bond vibration of 0.18 eV. This picture is complicated by Tao et al, 18 who showed the existence of a self-trapped state with a depth of 35 meV in rubrene crystals arising from coupled modes of molecular deformations with phenyl side groups. It is possible that these self-trapped excitons exist in amorphous rubrene as well.…”

Section: A Steady State Photophysicsmentioning

confidence: 90%

Competition between polaron pair formation and singlet fission observed in amorphous rubrene films

et al. 2013

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In this paper, we investigate excited state dynamics in amorphous rubrene vacuum sublimed films. We report the direct observation of singlet fission in amorphous rubrene films. We have determined the fission rate to be >2.5 × 10 12 s −1 . Simultaneously, we observe strong polaron pair absorption and propose that polaron pair formation could be competing with singlet fission. Another possible conclusion from our experiments could be that two triplets from singlet fission might arise via polaron pairs. In either case, polaron pairs play an important role in singlet fission in an amorphous rubrene film. We also observe that triplets created by singlet fission fuse to regenerate a singlet, giving delayed fluorescence (DF) scaling linearly with initial laser energy (i.e., one singlet gives two triplets and two triplets give back one singlet). This is a strong evidence of S n 1 → 2T 1 . We did not observe substantial temperature dependence of DF decay curve shape, indicating that triplet migration in amorphous rubrene films is not hopping limited and that triplets undergo fusion before their migration.

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Section: A Steady State Photophysicsmentioning

confidence: 90%

Competition between polaron pair formation and singlet fission observed in amorphous rubrene films

et al. 2013

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“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] However, our theoretical understanding of these processes is still lacking in many cases. At the most basic level, one may describe the energy transport as a quantum diffusion process occurring in a system that is influenced by both static disorder and thermal fluctuations.…”

Section: Introductionmentioning

confidence: 99%

Coherent quantum transport in disordered systems: A unified polaron treatment of hopping and band-like transport

Lee

Moix

Cao

2015

The Journal of Chemical Physics

103

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Quantum transport in disordered systems is studied using a polaron-based master equation. The polaron approach is capable of bridging the results from the coherent band-like transport regime governed by the Redfield equation to incoherent hopping transport in the classical regime. A non-monotonic dependence of the diffusion coefficient is observed both as a function of temperature and system-phonon coupling strength. In the band-like transport regime, the diffusion coefficient is shown to be linearly proportional to the system-phonon coupling strength and vanishes at zero coupling due to Anderson localization. In the opposite classical hopping regime, we correctly recover the dynamics described by the Fermi's Golden Rule and establish that the scaling of the diffusion coefficient depends on the phonon bath relaxation time. In both the hopping and band-like transport regimes, it is demonstrated that at low temperature, the zero-point fluctuations of the bath lead to non-zero transport rates and hence a finite diffusion constant. Application to rubrene and other organic semiconductor materials shows a good agreement with experimental mobility data. C 2015 AIP Publishing LLC. [http://dx

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“…Tao et al 25 studied the relaxation dynamics of photoexcited excitons in rubrene crystal by use of temperaturedependent transient absorption spectroscopy. They show that within 100 fs free excitons relax to self-trapped excitons via coherent oscillation.…”

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confidence: 99%

Fluorescence from rubrene single crystals: Interplay of singlet fission and energy trapping

Zhang

Kloc

et al. 2013

Phys. Rev. B

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We have studied fluorescence in rubrene single crystals by use of fluorescence up-conversion, fluorescence anisotropy, and temperature-dependent time-resolved fluorescence techniques. Thermally activated singlet fission was demonstrated to play an important role in the quenching of two intrinsic fluorescence bands, 565 and 610 nm. At low temperatures, singlet fission is suppressed while another process, namely energy trapping, becomes pronounced. The 650 nm fluorescence originates from the hole trap states located 0.27 eV above the valence band. Rubrene (5,6,11, has attracted increasing attention in recent years due to its applications in organic light emitting diodes (OLEDs) 1 and organic field effect transistors (OFETs). 2 Rubrene single crystals are well known for their high hole mobility (up to 40 cm 2 /V s) 3,4 and photoconductivity. 5 Observations of singlet fission 6,7 and long exciton diffusion lengths 8 makes it potential to improve the efficiency of organic photovoltaics. Although there has been much done on the applications, some fundamental questions such as the origin of the fluorescence in rubrene single crystals remain controversial. In particular, in the case of steady-state fluorescence spectra, both experimental results and interpretations contradict depending on experimental conditions, e.g., the polarization of the excitation beam, the crystal quality, and the degree of photo-oxidation.Rubrene crystals grown by physical vapor transport (PVT) are orthorhombic, with space group Cmca [Figs. 1(a)-1(d)]. 9 The reported steady-state fluorescence spectra of rubrene single crystals can be summarized into three types [Fig. 1(e)]:(1) 565 nm type. For incident light polarized parallel to the c axis, a strong fluorescence band centered at 565 nm dominates the fluorescence spectrum, which is explained to be due to the M-polarized transition 10 or c-polarized emission. 11 When the excitation polarization is parallel to the ab facet, the 565 nm fluorescence band disappears, and instead, the fluorescence spectra are present in two other types: (2) 610 nm type. The fluorescence spectrum centered at 610 nm is usually considered as the fluorescence spectra from pristine rubrene. 12 (3) 650 nm type. Another type of fluorescence spectrum under b polarized excitation is centered at around 650 nm. It is disputed to be due to either the oxidation related defects, 10 the band gap states, 13 or the amorphous phase embedded in the crystal. 12 In this work, we discuss the origin of the steady-state fluorescence in rubrene single crystals by stating the important role of singlet fission and energy trapping. The fluorescence kinetics observed was on a picosecond time scale due to the quenching by singlet fission. Thus the majority (99.9%) of the intrinsic fluorescence is quenched due to singlet fission. At low temperatures, energy trapping is observed while the thermally activated singlet fission is suppressed. The hole trap states located 0.27 eV above the valence band produce the 650 nm fluorescence band. Based on the fl...

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Relaxation Dynamics of Photoexcited Excitons in Rubrene Single Crystals Using Femtosecond Absorption Spectroscopy

Cited by 22 publications

References 27 publications

Competition between polaron pair formation and singlet fission observed in amorphous rubrene films

Competition between polaron pair formation and singlet fission observed in amorphous rubrene films

Coherent quantum transport in disordered systems: A unified polaron treatment of hopping and band-like transport

Fluorescence from rubrene single crystals: Interplay of singlet fission and energy trapping

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