Temperature-dependent singlet exciton fission observed in amorphous
rubrene films
Jing Li, Zhonghai Chen, Qiaoming Zhang, Zuhong Xiong
⇑
, Yong Zhang
⇑
School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China
article info
Article history:
Received 25 April 2015
Received in revised form 7 July 2015
Accepted 15 July 2015
Keywords:
Singlet fission
Time-resolved fluorescence decay
Magnetic field effect
abstract
The steady-state/transient fluorescence spectroscopy was used to demonstrate that the dynamics of sin-
glet exciton fission in amorphous rubrene were temperature-dependent (50–300 K). Based on the tradi-
tional three-state model of singlet fission, time-resolved fluorescence decay curves measured at different
temperatures could be well fitted by using a set of rate equations. The variations of specific rate constants
were consistent with the conventional Arrhenius-type, thermally activated process. Additionally, the
magnetic field effect of photoluminescence was apparently suppressed at low temperatures. All these
findings offer clear evidence that the amorphous rubrene solid undergoes thermally activated singlet
exciton fission due to the endothermic nature of fission process in rubrene.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
Singlet exciton fission observed in organic solid is a
spin-allowed process in which an excited high energy singlet exci-
ton (S
1
) shares its half of energy with another molecule at ground
state (S
0
), and subsequently both of them convert into a pair of low
energy triplet excitons (T
1
). In the past few years, singlet fission
became a scientific hotspot in the field of physics, chemistry, and
organic electronics (see Ref. [1,2] for recent reviews). Generally,
singlet fission can be regarded as a carrier multiplication process
in which two electron-hole pairs are created. If all of the charges
can be efficiently collected, the photocurrent of device will be
effectively enhanced. Therefore, it was suggested that organic
molecules with fission property can be used as a new type of sen-
sitizer to improve the quantum efficiency of organic photovoltaic
devices [3–8].
Generally, the energies of S
1
and T
1
states must fulfill the require-
ment of E(S
1
) 2E(T
1
) for fission process to occur. For instance,
E(S
1
) 2E(T
1
) = 0.11 eV in pentacene [9], E(S
1
) 2E(T
1
)=0.18 eV
in tetracene [10,11], and E(S
1
) 2E(T
1
)=0.05 eV in rubrene [12].
Ultrafast fission processes have been observed in these materials.
It was found by Thorsmølle et al. that there was no temperature
dependence in the fission process of pentacene since E(S
1
)>2E(T
1
)
in pentacene [9]. However, Wilson et al. demonstrated that singlet
fission in tetracene crystal was also independent of temperature
although E(S
1
)<2E(T
1
) in tetracene [13]. The above two results con-
stitute an important scientific puzzle that whether or not singlet fis-
sion should be temperature-independent for exothermic case and
temperature-dependent if it is endothermic. Although researchers
have made a lot of effort [9,14–16], more explicit experimental
results are still required in order to elucidate this basic question.
In this work, highly efficient fission material, i.e.,
5,6,11,12-tetraphenyltetracene (rubrene) was deposited by using
thermal evaporation, forming amorphous rubrene films. Steady
state/transient photoluminescence (PL) and magneto-
photoluminescence (MPL) were measured in a wide range of tem-
perature. In both PL and MPL measurements, we found that singlet
fission in amorphous rubrene was temperature-dependent. For
instance, at temperature of 300 K, the transition rate in singlet fis-
sion was fitted to be 0.5 ns
1
. However, the fission rate was almost
completely suppressed at temperatures lower than 50 K, confirming
that singlet fission in amorphous rubrene was a thermally activated
process. This is in line with the endothermic nature of singlet fission
in rubrene material.
2. Experimental
Rubrene films of 100 nm thick were thermally evaporated on
the glass substrates at high vacuum about 1 10
6
Pa. It is gener-
ally found by different researchers that the obtained rubrene films
usually exhibit amorphous morphology [16–18]. Park et al.
observed the growth of island-like crystalline domains only after
annealing the films at higher-than-room temperatures [18].
However, mixed amorphous/crystalline phases probably appear
http://dx.doi.org/10.1016/j.orgel.2015.07.035
1566-1199/Ó 2015 Elsevier B.V. All rights reserved.
⇑
Corresponding authors.
E-mail addresses: zhxiong@swu.edu.cn (Z. Xiong), yzh6127@swu.edu.cn (Y.
Zhang).
Organic Electronics 26 (2015) 213–217
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