Letter pubs.acs.org/JPCL
Nonlinear Density Dependence of Singlet Fission Rate in Tetracene Films Bo Zhang,† Chunfeng Zhang,*,† Rui Wang,† Zhanao Tan,‡ Yunlong Liu,† Wei Guo,† Xiaoling Zhai,† Yi Cao,† Xiaoyong Wang,† and Min Xiao*,†,§ †
National Laboratory of Solid State Microstructures & School of Physics, Nanjing University, Nanjing 210093, China New and Renewable Energy of Beijing Key Laboratory, School of Renewable Energy, North China Electric Power University, Beijing 102206, China § Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, United States ‡
S Supporting Information *
ABSTRACT: Singlet fission holds the potential to dramatically improve the efficiency of solar energy conversion by creating two triplet excitons from one photoexcited singlet exciton in organic semiconductors. It is generally assumed that the singlet-fission rate is linearly dependent on the exciton density. Here we experimentally show that the rate of singlet fission has a nonlinear dependence on the density of photoexcited singlet excitons in tetracene films with small crystalline grains. We disentangle the spectrotemporal features of singlet and triplet dynamics from ultrafast spectroscopic data with the algorithm of singular value decomposition. The correlation between their temporal dynamics indicates a superlinear dependence of fission rate on the density of singlet excitons, which may arise from excitonic interactions.
SECTION: Spectroscopy, Photochemistry, and Excited States inglet fission (SF) in organic semiconductors is a spinallowed process that creates two triplet excitons (2T1) from one photoexcited singlet exciton (S1).1−7 This process has been proposed to beat the Shockley−Queisser theoretical limit8 in single-junction solar cells.9,10 Very recently, its promising potential has been evidenced by the rapid development of SF-sensitized solar cells and photodetectors.11−15 However, the exact physical mechanism for SF, particularly in a prototypical material of tetracene, is still under intensive debate.3,4,7,16−19 Some new perspectives inspired by recent experiments have put forward interesting concepts such as quantum coherence,20,21 intermolecular interaction,17,22 and electronic coupling,23,24 in addition to the conventional charge-transfer mechanism.5 The difficulty in elucidating the SF process in tetracene is partially caused by the endothermic nature (ES1 − 2ET1 < 0).1,3,16,18,19,25−29 Thermal activation was originally proposed for compensating the energy uphill.1,3,27−29 This argument was supported by an ultrafast transient absorption (TA) study, in which Thorsmølle et al. observed a significant temperature dependence of the triplet signal in tetracene single crystals.3 However, opposite conclusions have been drawn in recent studies on tetracene films.18,19 In particular, with a similar TA experiment, Wilson et al. reported that the SF process is actually independent of temperature.19 Recently, coherent electronic coupling has been considered,18,20 which predicts a temperature-independent process for the initial stage of SF in crystalline tetracene.18 The energy uphill is overcome through a
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© XXXX American Chemical Society
direct access to an intermediate triplet-pair state by coherent electronic coupling.18 However, some theoretical and experimental works have identified a primary role of charge-transfer process rather than the contribution from coherent coupling.7,24,30 To test these outstanding controversies, we have systematically examined the dynamics of SF process in tetracene films with broadband ultrafast TA spectroscopy. In order to accurately evaluate SF, we extract the spectrotemporal features of singlet and triplet excitons from the TA data using the algorithm of singular value decomposition (SVD).The correlation between the dynamics of singlet and triplet excitons shows compelling evidence that, instead of a constant value, the rate constant for SF is strongly dependent on the singlet exciton density in tetracene films with small grain sizes. We conjecture that in addition to the conventional fission process, photogenerated singlet excitons in tetracene films may undergo another pathway of SF (the nonlinear part) arising from excitonic interaction. The steady-state absorption and emission spectra of a tetracene sample show clear features of crystalline tetracene (Figure S1, Supporting Information). We first confirm the existence of SF in the sample with the time-resolved Received: August 16, 2014 Accepted: September 26, 2014
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dx.doi.org/10.1021/jz501736y | J. Phys. Chem. Lett. 2014, 5, 3462−3467
The Journal of Physical Chemistry Letters
Letter
fluorescence (TRFL) spectra. Recently, Burdett and Bardeen have thoroughly reexamined the quantum beats in the delayed fluorescence of crystalline tetracene,31,32 which has been regarded as a fingerprint of SF.6,32,33 A typical TRFL trace of the sample is shown in Figure 1a. By subtracting the
small crystalline grains. We characterize the morphology of the film by atomic force microscopy. As shown in Figure 2b, the diameters of grains are about ∼100 nm, which is about 5 times smaller than that in the previous study.4 More trapping centers may be introduced due to over 2 orders of magnitude enhancement in surface-to-volume ratio in the sample. It may strongly affect the exciton dynamics in polyacene materials.2,3 The measured ΔT/T in the 1.3−2.6 eV spectral region within a temporal window of 3 ns monitors the dynamics of the SF process (Figure 3a). The excitation power is set to be ∼5
Figure 1. (a) A typical TRFL trace of the tetracene film. The red line is the curve fitting to multiexponential decay function. Inset shows the oscillations obtained by subtracting the multiexponential decay component in the TRFL trace. (b) The Fourier transform of the extracted oscillations with peaks at 1.07, 1.84, and 2.95 GHz.
multiexponential decay component, the oscillations emerge as shown in the inset of Figure 1a. The Fourier transformation of the oscillatory components shows three peaks at 1.07, 1.84, and 2.95 GHz in the frequency domain (Figure 1b). These values are in good agreement with the theoretical prediction of energy separations between manifolds of the triplet-pair states,32 indicating the presence of triplet pairs generated from SF in the sample. The exciton dynamics in crystalline tetracene may be strongly affected by many body interactions such as the singlet−singlet annihilation (SSA) and triple−triplet annihilation (TTA) when the density of excitons is high.16,25 We conduct power-dependent TA experiments to determine the linear power regime. The pump−probe traces probed at 2.34 eV with different power excitation at 3.1 eV are presented in Figure 2a. No significant power dependence has been observed with exciton fluence
