Optical pump-probe spectroscopy of photocarriers in rubrene single crystals

Abstract

The photocarrier dynamics of rubrene, an organic molecular semiconductor, are investigated by transient absorption spectroscopy over a wide energy range (0.1−2.2 eV). By photoirradiation, we observed three kinds of absorption bands in the infrared region, which are attributable to induced absorptions of excitons, free carriers, and polarons. The results show that for photoirradiation with energies higher than ∼2.7 eV (∼0.4 eV higher than the lowest exciton energy), free carriers are photogenerated just after the photoirradiation. Some of those free carriers are stabilized as polarons and the others are relaxed to excitons. For the resonant excitation to the lowest-exciton, polarons can also be generated from the excitons. The temperature dependence of the decay dynamics of polarons reveals that polaron mobility increases with decreasing temperature, indicating bandlike transport of large polarons. Below 80 K, photocarriers are trapped by defects or impurities and their decay dynamics are dominated by the escape time from traps.

Triplet exciton dynamics in rubrene single crystals

“…This PL is an observable that is proportional to the number of singlet excitons created per unit time, independently of the precise value and nature of the singlet-exciton lifetime, and of the mechanism with which an individual singlet exciton ultimately leads to some radiated PL. In this context, any complexity in the decay mechanisms of the singlet excitons 18,19 does not affect our results. Further research will be needed to understand in general the relative role of the possible relaxation processes of photoexcited states, such as radiative recombination, dissociation into free carriers, or fission into triplets.…”

Temperature dependence of exciton and charge carrier dynamics in organic thin films

“…Therefore, ultrafast spectroscopy would be needed to determine early time evolution 90,91 of excitonic and charged states and refine the physical picture of exciton and charge carrier dynamics in these materials. The exciplex decays radiatively with a lifetime τ exci , has a slow nonradiative recombination channel (dashed-line cross), and can dissociate into free charge carriers (η exci ) under applied electric field at room temperature.…”

Nanosecond pump and probe observation of bimolecular exciton effects in rubrene single crystals

“…2,3 Singlet fission in rubrene single crystals has not been consistently described yet. Tao et al 4 have observed a $100 ps fast decay of a photoinduced infrared absorption band which they assigned to singlet excitons, but they interpreted the decay as caused by exciton dissociation, not fission. Furube et al 5 observed transient absorption kinetics in the near infrared which they interpreted as singlet exciton fission into triplets with a time constant of 10 ps.…”

“…The spatial dependence of the first Fourier component of the photoinduced grating must have the same form as (1) qðt; yÞ ¼qðtÞ½1 þ mðtÞ cosðk g yÞ; (4) whereqðtÞ is the initial average density of (as yet unspecified) excited states in the crystal, which may be a function of depth, and mð0Þ ¼ m 0 , and we do not explicitly write the slow decrease in the z-direction. Local quadratic recombination as described by dqðt; yÞ=dt ¼ Àcqðt; yÞ 2 implies…”

“…With the exception of Ref. 4, transient absorption measurements have been largely interpreted as a singlet-to-triplet fission process that occurs on a time scale on the order of $10 ps, sometimes even in nominally amorphous films. [5][6][7][8] However, Piland et al 9 measured photoluminescence transients in rubrene films that they interpreted as indicative of a fission time of 2 ns.…”