Article pubs.acs.org/JPCC
Singlet and Triplet Carrier Dynamics in Rubrene Single Crystal Xiaoming Wen,*,†,‡ Pyng Yu,† Chi-Tsu Yuan,† Xiaoqian Ma,† and Jau Tang*,† †
Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan The Australia Center for Advanced Photovoltaics, University of New South Wales, Sydney, NSW 2052, Australia
‡
ABSTRACT: We systematically investigate the singlet and triplet carrier dynamics in rubrene single crystal in the time scale from femtosecond (fs) up to microsecond (μs) using time-resolved and temperature-dependent photoluminescence (PL) techniques. Five PL bands were observed between 1.60 and 2.14 eV, and they are assigned to exciton peak, zero phonon intrinsic transition, and vibronic replicas, respectively. The ultrafast carrier scattering results in a broad distribution of hot carriers with a time constant as fast as 100 fs. The singlet fission and phonon scattering lead to 6.2 and 47 ps relaxation, respectively. The radiative recombination of singlet carriers exhibits wavelength-dependent lifetimes. The delayed fluorescence was observed on the microsecond time scale. The temperature-dependent and excitation intensity-dependent lifetime measurements confirmed that the delayed fluorescence is due to triplet−triplet annihilation that can be well described by rate equations of the triplet population.
I. INTRODUCTION Organic molecular crystals are recently attracting considerable research interest due to their great potential applications in photovoltaics and photonics.1−7 Rubrene is one of the most attractive organic molecular semiconductors because of its record-high charge carrier mobility, very large micrometer-scale exciton diffusion length, and unique photophysical properties.8−12 It has been shown that triplet excitons in highly ordered, ultrapure molecular crystal of rubrene can diffuse over a distance of 2−8 μm, much larger than the normal diffusion distance of 10−50 nm.1,13 Moreover, singlet fission was also observed in rubrene single crystal.14 These indicate rubrene is a very promising organic semiconductor for photovoltaic and photonic applications.15,16 Steady-state and time-resolved photoluminescence (PL) of organic molecular semiconductors can provide unique insight into the nature and dynamics of carriers, and thus it is of critical importance for fundamental understanding and practical applications.9,11,17−19 Steady-state PL and the carrier dynamics of rubrene have been extensively investigated. Recently, Irkhin et al. systematically investigated the PL of rubrene single crystals in various excitation and geometric conditions.7 It has been shown that the rubrene single crystal has a large optical anisotropyand that the experimental conditions have strong influence on the fluorescence spectra.11,19,20 Despite the large amount of work carried out, however, carrier dynamics in rubrene has not fully been understood and PL origin is debatable,7,9,21,22 mostly because the processes are sensitive to defects and impurities in the crystalline phase.9,23,24 Herein, we systematically investigate the carrier dynamics on the time scale from subpicosecond to microsecond by timeresolved and temperature-dependent techniques, such as © XXXX American Chemical Society
ultrafast upconversion PL and time-correlated single photon counting (TCSPC). We elucidate the dynamic processes occurring in rubrene single crystal, such as carrier scattering, singlet fission, defect/surface trapping, singlet carrier recombination, and triplet−triplet annihilation (TTA).
II. EXPERIMENTAL DETAILS High-purity rubrene single crystal used in this work was grown using an optimized physical vapor transport technique, by evaporating rubrene powder at 300 °C in a stream of ultrahighpurity argon, as described previously.1,25 Ultrafast photoluminescence experiments were performed on a Fluomax upconversion fluorimeter (IB Photonics), similar to that described previously.26 The excitation pulses are of 100 fs duration and 80 MHz repetition from a Ti:Sapphire laser. The second harmonic then is generated at 400 nm in a BBO crystal and is used as excitation. The excitation power is attenuated, and the focus spot has a size of 200 μm to keep the excitation fluence lower than 50 nJ/cm2. The fluorescence was mixed in a BBO crystal with a variable delayed gating pulse (800 nm, 100 fs) to generate a sum-frequency signal. The signal was collected into a dual-monochromator and detected by photon counting system. The time and spectral resolutions of the system were determined as 140 fs and 2 nm, respectively. Steady-state PL was measured in a MicroHR spectrometer (Horiba) with excitation at 406 nm, and spectra were recorded by a cooled CCD (SynapseTM). For temperature-dependent experiments, the sample was placed in a cryostat (ST500) with controllable Received: May 11, 2013 Revised: August 4, 2013
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dx.doi.org/10.1021/jp404666w | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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temperature between 77 and 300 K. The nanosecond lifetimes were measured using the TCSPC technique on a Microtime200 system (Picoquant) with excitation of 467 nm. The direct detection technique27 was applied to measure microsecond lifetimes using a photomultiplier tube (PMT, R928). An ultrashort 100 fs pulse was used as excitation from an optical parametric amplifier (OPA, TOPAS) pumped by a Ti:sapphire laser-seeded regenerative amplifier at 1 kHz repetition. The excitation wavelength is tuned at 400 nm to match the absorption. The evolution of fluorescence was recorded by an oscilloscope (Tektronix MSO4032). In each experiment, the laser beam is perpendicular onto the ab facet, the largest asgrown surface of a rubrene single crystal, and PL was collected from the ab facet.7 Figure 2. Fluorescence at 77 K and corresponding 5-Gaussian fitting.
III. RESULTS AND DISCUSSION Figure 1 shows the steady-state fluorescence of rubrene as a function of temperature from 77 to 300 K. At each temperature,
Table 1. Peak Energy and Bandwidth of Band-I to Band-V from 5-Gaussian Fitting in Rubrene Single Crystal at 77 K band
peak (nm/eV)
separation (ΔmeV)
bandwidth (nm/meV)
band-I band-II band-III band-IV band-V
580.6/2.136 604.3/2.052 651.4/1.904 706.9/1.754 772.9/1.604
83.48 148.52 149.51 149.69
19.7/72.57 21.5/72.92 30.4/88.83 36.9/91.02 70.4/146.18
band-IV exhibit slightly increased bandwidths, 89 and 91 meV. In comparison, band-V has a much larger bandwidth of 150 meV. This may suggest that band-V has a strong interaction with surface/defect states. Figure 3 compares their wavelength shifts and bandwidths as a function of temperature. Upon increasing temperature from
Figure 1. Fluorescence of rubrene single crystal as a function of temperature between 77 and 300 K.
the high-energy shoulder at 2.22 eV is very weak, which indicates the perfect PL measurement from the ab facet and the sample is high in quality.7 The shoulder was attributed to an artifact created by leakage of the strong c-polarized luminescence typical of rubrene and scattering by surface imperfections that was confirmed to give rise to the appearance of a strong PL band centered around 2.22 eV.7 At 77 K, five peaks can unambiguously observed, and the highest peak locates at 1.90 eV. The PL spectrum can be well fitted by five Gaussian functions, as shown in Figure 2 and the fitting parameters in Table 1. The five peaks locate at 2.14, 2.05, 1.90, 1.75, and 1.60 eV, respectively. For a simple description, the five peaks were hereafter referred to as band-I to band-V from the blue to the red. It should be noted that band-II to band-V evidently forms a progression with the same separation of 149 meV that corresponds to the C−C stretch vibration mode,19,20 consistent with the observation of Raman measurement of 144 meV.28 The progression is well consistent with the report by Irkhin et al., in which the progression was assigned to the intrinsic PL emitted by the ab facet of rubrene single crystal.7 Band-II can be ascribed to the transition of zero-phonon, followed by the vibronic replicas separated by 149 meV.7,11 In contrast, the separation between band-I and band-II is 83 meV. Band-I can be attributed to the exciton peak.17 The bandwidth of band-I is the same as that of band-II, 72 meV. Band-III and
Figure 3. (a) The wavelength shifting and (b) bandwidths of band-I to band-V as a function of temperature. B
dx.doi.org/10.1021/jp404666w | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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77 to 300 K, a small blue shift, 3−6 nm, was observed in bandIII to band-V, while band-II displays a complicated variation and eventually a small red shift. Temperature-dependent energy shifts have been extensively studied in semiconductors and their nanostructures. The red shift was generally observed, and it was ascribed to the renormalization of band energy by exciton− phonon interaction. On the other band, for organic semiconductors, spatial distortion of the phenyl can result in an increase of the band gap and thus the blue shift of the fluorescence.29−31 Here, we speculate the blue shift is due to the larger contribution of molecular distortion than the exciton−phonon scattering. It should be emphasized that band-I exhibits a distinct temperature effect that emission energy obviously red shifts and bandwidth distinctly increases with increasing temperature. The PL intensity of band-I quickly decreases upon increasing temperature; at 180 K, this band is too weak to be observed. In contrast, each of the other bands exhibits a weak temperature effect, and the wavelength does not evidently shift. At 300 K, these bands can be clearly observed and therefore fitted by 4-Gaussian function. The bandwidths of band-III to band-IV increase upon increasing temperature. For rubrene, the spatial stretch of phenyl and enhancement of phonon scattering can lead to bandwidth broadening.31 It should note that band-I exhibits evident red shifting and bandwidth broadening with increasing temperature, which in turn supports that band-I is intrinsically ascribed to exciton peak. The similar red shifting and bandwidth broadening have extensively been observed in semiconductor quantum dots (QDs) and were ascribed to electron/exciton−phonon interaction.27,32 For band-V, the much broader bandwidth and complicated variation with temperature may suggest that it is relevant to the surface states. With increasing temperature the PL intensity monotonously decreases, which is attributed to the thermal activated nonradiative trapping by defect/impurities.32,33 At low temperatures, the nonradiative channel is not thermally activated; therefore, the carriers/excitons can radiatively emit photons. Once temperature increases, the nonradiative channels become thermally activated, such as trapping by surface/defect/ionized impurity states, as expressed by τNR = τ0 exp(Ea/KBT), where Ea is activation energy. The quantum efficiency can be expressed as η = (1 + τR/τNR)−1. The thermally activated nonradiative channel results in a decrease of fluorescence intensity and thus quantum efficiency, as expressed by I(T) = I0/[1 + (τR/τ0) exp(−Ea/KBT)]. As a consequence, PL lifetime decreases. We acquired the activation energies from the Arrhenius plot (Figure 4), 26.4, 27.4, 29.5, and 30.3 eV corresponding to bandII to band-V. It should be noted that the similar activation energies may suggest the similar mechanism of defect or surface trapping in these bands. Band-I disappears around 180 K; therefore, there are no sufficient points to complete the Arrhenius plot. To obtain the insight into carrier dynamics, we measured the temperature-dependent lifetimes on the microsecond time scale at various wavelengths, corresponding to band-II to band-V. Basically, similar evolution was observed in these bands. The PL evolution at 650 nm (band-III) from 77 to 300 K was shown in Figure 5. The PL evolution does not evidently vary from 77 to 300 K. The delayed fluorescence was reported in rubrene on the microsecond time scale, and the mechanism was suggested as triplet−triplet annihilation,34 rather than thermal activated delayed fluorescence.35 For such a delayed fluorescence, the dynamics was determined by triplet−triplet
Figure 4. Arrhenius plot of PL intensity for band-IV at 707 nm and fitting curve. Activation energy of 29.5 meV was deduced.
Figure 5. Fluorescence evolution of band-III (650 nm) as a function of temperature on the microsecond time scale. Offsets are applied to avoid overlapping.
annihilation and therefore quadratically relevant to the population of the triplet state. The PL evolution was also measured on the nanosecond− microsecond time scale using TCSPC, as shown in Figure 6. A very fast decay was observed. This fast component is beyond the resolution of the TCSPC system, 100 ps. A slightly slow component with lifetime of a few nanoseconds then followed. The lifetimes of band-II to band-V were determined using two exponential fittings, and the slower time constants were 11.8, 8.59, 7.33, and 5.07 ns for band-II to band-V. In addition, a very slow component was observed on the microsecond scale, as shown in Figure 6b. This component should be ascribed to the delayed fluorescence, similar to the observation by direct detection technique in Figure 5. The decay rates were found to be similar at various wavelengths between 570 and 700 nm. Furthermore, we observed the evolution of delayed fluorescence as a function of excitation fluence. To exclude the effect of the prompt fluorescence, we focus on the delayed time scale, >100 ns. As shown in Figure 7a, the evolution significantly accelerates with increasing fluence. This clearly confirms the delayed fluorescence arose from triplet−triplet annihilation. To gain further insight, the PL evolution was measured by ultrafast up-conversion on the femtosecond−picosecond time scale. Figure 8 shows the evolutions at 550, 600, and 650 nm at an excitation of 400 nm. The PL intensity observed at wavelengths of 550, 600, and 650 nm occurred within the resolution of the system (100 fs), suggesting that the excited C
dx.doi.org/10.1021/jp404666w | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 7. (a) The evolutions of delayed fluorescence as a function of excitation fluence (I0 = 10 W/cm2) at room temperature. A fitting was displayed for 8I0 fluence and the residual was shown. (b) The simulation for the evolutions at various fluence by using eq 1.
Figure 6. Fluorescence evolutions of rubrene single crystal at wavelengths of 570, 600, 650, and 700 nm on (a) short time scale and (b) long time scale. The short component is similar to the response function (dashed), which suggests a very fast component
