Chirality Specific Triplet Exciton Dynamics in Highly Enriched (6,5

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Chirality Specific Triplet Exciton Dynamics in Highly Enriched (6,5) and (7,5) Carbon Nanotube Networks Abasi Abudulimu,† Florian Spaeth,‡ Imge Namal,‡ Tobias Hertel,‡ and Larry Lüer*,† †

IMDEA Nanociencia, C/Faraday 9, 28049 Cantoblanco, Madrid, Spain Institute of Physical and Theoretical Chemistry, Julius-Maximilian University Würzburg, Germany

‡

S Supporting Information *

ABSTRACT: Single-walled carbon nanotubes (SWNTs) show high aspect ratio, thermal and chemical stability as well as charge mobility, and therefore appear ideally suited for improving charge extraction in organic photovoltaic (OPV) devices. Since typical charge extraction times in OPV devices are in the microsecond range, the interplay of the desired charged states with long-lived neutral states in SWNTs such as triplet excitons becomes important. Triplet excitons have recently been investigated in (6,5) SWNTs with an optical yield close to 32%. Here, we present transient absorption (TA) dynamics of (6,5)- and (7,5)-rich SWNT networks from the femtosecond to microsecond time scale. Comparing our TA spectra to results from a recent spectroelectrochemical study allows us to distinguish between contributions from charged and neutral photoexcitations. For long pump−probe delay times we identify an excess photobleach which we use as a chirality specific probe for the density of triplet excitons. We show that triplet energy transfer occurs between the (6,5) and the (7,5) chirality with a transfer time of about 70 ps. In contrast, no evidence of triplet exciton transfer is observed between (7,5) and (8,4) tubes. We find that the triplet yield is reduced at higher excitation densities, which points to singlet excitons as precursors for triplet states.

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INTRODUCTION

The overall efficiency of OPV devices depends on a sequence of elementary processes with typical time scales ranging from femtoseconds (charge generation, geminate recombination) to microseconds (charge extraction, trapping). Charge generation in OPV materials occurs predominantly at the donor−acceptor interface. While SWNTs have been shown to act as donors against C6010 or acceptors against P3OT,4 we note that also the ability of SWNT to create charges intrinsically might add to the overall device efficiency.11 Singlet excitons as precursors for free charges have been characterized with respect to their size and mobility,12 the latter strongly depends on the purity of the tubes.13−15 Very efficient intertube transfer of singlet excitons has been demonstrated in the spatial16 and temporal17 domains, which is not necessarily beneficial for an OPV device, since it can be accompanied by efficient energy trapping in larger diameter tubes, highlighting the need for precise chirality control in OPV devices. Charge generation has been shown at low11 as well as high excitation energies,18,19 and a trion state as optical probe for trapped charges on SWNTs has been demonstrated.20 Charge transport has been shown to involve charge transfer toward low-bandgap tubes by the observation of electroluminescence mainly from low-bandgap tubes21 again pointing to the need for precise chirality control.

Single-walled carbon nanotubes (SWNTs) are formally graphene sheets rolled up along a roll-up vector (n, m). Certain combinations of n and m (chiralities) yield quasi-onedimensional semiconductors with high aspect ratios giving them striking properties such as high charge carrier mobilities1 and mechanical as well as chemical stability. Moreover, their excitonic resonances can be tuned in energy to cover much of the range of solar emission.2 Such properties have stimulated fundamental investigations and development toward their use in organic photovoltaic (OPV) devices, where they contribute to the overall device efficiency as optical absorbers, electron acceptors, donors or as selective charge extraction agents.3−6 However, record efficiencies of OPV devices containing SWNTs remain far below SWNT free ones,7 which has been ascribed to a lack of knowledge and control of the nanostructure.8 To maximize the photocurrent density Gong et al.7 utilized polychiral SWNTs as electron donor material and the PC71BM fullerene as acceptor material, and with optimized device geometry they achieved NREL certified record power conversion efficiency (PCE) of 3.1% over device area of ≈1 mm2. Shastry et al.,9 the same group, also reported a PCE of 2.31% on a similar device but with increased device area of 6 mm2 by incorporating a solvent additive which allowed optimization of the active layer morphology. © 2016 American Chemical Society

Received: April 18, 2016 Revised: July 30, 2016 Published: August 1, 2016 19778

DOI: 10.1021/acs.jpcc.6b03923 J. Phys. Chem. C 2016, 120, 19778−19784

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The Journal of Physical Chemistry C

custom built Python program, using open source packages: Matplotlib, PyQt4, Scipy, and Pyserial. Nanomicrosecond Spectroscopy. In order to obtain TA spectra in the nanosecond to microsecond domain, we used the same femtosecond probe pulse in the same setup as described above, but replaced the femtosecond pump pulse by a 300 ps 532 nm pulsed laser, and controlled the pump−probe delays electronically via a BNC (Model 575 Pulse/Delay Generator). The pump beam had a slightly larger spot size on the sample (about 300 μm in diameter). In both femtosecond and nanosecond measurements, the samples were kept in a cryostat (Oxford OPTISTAT) under vacuum condition. Sample Preparation. The first step of the SWNT film preparation is based on chirality-specific SWNT extraction techniques by polyfluorenes reported in literature.26−28 For (6,5) SWNTs, 7.5 mg of CoMoCAT SG-65 nanotube material (SouthWest NanoTechnologies), 15 mg of poly[(9,9-di-noctylfluorenyl-2,7-diyl)-alt-co-(6,6′-{2,2′-bipyridine})] (PFOBPy, American Dye Source) and 15 mL of toluene were added to a 50 mL centrifugation vial. In the case of (7,5) SWNTs, 27 mg of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO, Sigma-Aldrich) were used instead of PFO-BPy. The mixtures were sonicated for 8 h in an ice bath by an ultrasonic disruptor equipped with a 5 mm microtip (Branson Sonifier II W-450; duty cycle, 50%; output control, 4). The resulting dispersions were centrifuged for 3 min in a benchtop centrifuge (Heraeus Biofuge 15) at 14000 rpm (≈20000 × g), the supernatants were collected and vacuum filtered through 0.01 μm pore size mixed cellulose ester filter membranes (Millipore VCWP02500). The SWNT-loaded filter membranes were placed into Petri dishes with the SWNTs pointing upward and acetone was added, which caused the membranes to dissolve. The Petri dishes were moved in eccentric circles until SWNT films detached from the filter residues. The films were transferred into fresh acetone baths by thin glass rods and left for 10 min. Then, the SWNT films were immersed in acetone filled 1.5 mL glass vials and the acetone was removed by glass pipettes. Solvent residues were evaporated in a nitrogen stream, 200 μL of chlorobenzene was added into each vial, and the vials were sealed with caps. The SWNTs were redispersed in the vials for 2 h with the ultrasonic disruptor now being equipped with a cup horn (duty cycle, 100%; output control, 4). The resulting dispersions were again centrifuged for 3 min at approximately 20000 × g and the supernatant was collected. By using the absorption cross section for (6,5) SWNTs from ref 29 and applying it also to (7,5) SWNTs, we estimate a polyfluorene-to-SWNT mass ratio smaller than 4:1 in both samples. The final films of (6,5) and (7,5) rich samples were obtained by drop casting 100 μL fractions of the dispersions, which had an optical density (OD) of approximately 30, onto clean glass substrates, resulting in an OD of the majority SWNT species’ S1 peak maximum of approximately 0.7 and 0.8. By means of spectral modeling, we determined the relative abundance of the chiralities. We found (see Figure S1 in Supporting Information) that the (6,5) rich sample contains about 91% of (6,5) tubes and about 2% of (7,5) tubes, while the (7,5) rich sample contains about 75% of (7,5) tubes and about 12% of (6,5) tubes.

Recently, the Zeeman splitting of triplet states in SWNTs has been observed experimentally by optically detected magnetic resonance spectroscopy.22 This was also used to determine a photoinduced triplet yield of 32%.23 Hence, the role of triplet states in the overall photovoltaic process must be considered. If triplet formation is a loss process competing with singlet exciton dissociation, then an understanding of this process is needed in order to reduce triplet yields. However, it is also possible that triplets, due to their enhanced lifetime as compared to singlet excitons, enhance charge carrier generation.24 In this case, the dynamics of triplet states must be assessed, and this assessment must be done specifically for the dominant chiralities present in the samples in order to characterize intertube transfer and on-tube mobility of triplet excitons. Such a study can only be done in the time domain and has been inhibited so far by the lack of clear optical probes for the triplet excitons in SWNT. In this work, we perform transient absorption (TA) spectroscopy of the SWNT samples rich in either (6,5) or (7,5) tubes. By comparing the positive and negative TA features with spectroelectrochemical data taken on similar samples, we are able to determine quantitatively the contribution of both charged states and triplet excitons to the transient photobleach (PB), thus yielding an optical probe for the time-resolved concentration of triplet states on each chirality. By global analysis of the data taken from the different samples, we demonstrate the occurrence of triplet exciton transfer only between certain chiralities, and we address the question whether there is on-chain mobility of triplet excitons. By varying the pump intensity, we obtain an indication about the generation mechanism of triplets on SWNTs.

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EXPERIMENTAL SECTION Femtosecond Spectroscopy. Femtosecond TA spectroscopy measurements were carried out on a probe wavelength scale ranging from 850 to 1600 nm and from 100 fs to 400 ps, at different excitation intensities. Optical pulses centered at 775 nm were generated from a Ti:sapphire laser (Clark-MXR, CPA2101) driven at 1 kHz by a regenerative amplifier, and split into two parts. One fraction passed through an optical delay line, which was controlled via a mechanical translation stage, and directed to a sapphire plate in order to obtain a white light continuum, which we used as the probe beam. One part of the probe beam was focused onto the sample (about 134 μm spot size) with a spherical mirror. After passing through the sample, the probe beam was focused onto the slit of a prism spectrometer (Entwicklungsbüro Stresing GmbH), which consisted of a dual channel CCD array (2 × 256 pixels, VISenhanced InGaAs, Hamamatsu Photonics Inc.). The other part of the probe beam was used as a reference to reduce laser fluctuation induced noise. The second part of the fundamental 775 nm pulses was sent to a second harmonic generator to achieve the pump pulse centered at 387 nm, and chopped at 500 Hz. Then the pump beam was focused onto the sample (about 260 μm spot size) to overlap with the probe beam, and blocked after the sample. Intensities of both pump and probe beams were controlled via neutral density filters. In this work, all the measurements were carried out on the magic angle (otherwise stated), by setting the polarization angle of both pump and probe beams with 2/λ plates. Data acquisition and the global and target analysis of transient absorption spectra25 (for details see Supporting Information), were conducted on a

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RESULTS AND DISCUSSION In Figure 1, we show differential absorption (ΔA) spectra for the (6,5) rich and (7,5) rich samples (left and right columns, respectively) at various pump−probe delay times t, as given in 19779

DOI: 10.1021/acs.jpcc.6b03923 J. Phys. Chem. C 2016, 120, 19778−19784

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bands at 1.06 and 1.27 eV, and to a concomitant reduction of the exciton resonance at 1.24 eV such that the overall spectral weight remains unchanged, at least for doping levels below about 0.5 e−/nm.46 Transferring these results to our pump− probe experiment, we expect that the spectral weight of PB should match the sum of the spectral weights of PAT and PAC, if PB is exclusively due to charged states. Hereby, we assume that pump-induced photodoping causes the same kind of charged states as those obtained by electrochemical doping. On the basis of this reasoning, we can obtain a first estimate for the relative contribution of neutral photoexcitations rN(t) to the total photoexcitation density by comparing the total spectral integral with respect to the probe energy ω, to the spectral weight of the bleach signals rN (t ) ≤

Figure 1. Transient absorption spectra of (6,5) rich and (7,5) rich sample (left and right column, respectively) on short and long picosecond time scale (upper and lower row, respectively). Arrows next to PA and PB bands indicate the direction of evolution with increasing pump−probe delay. PB, PAT, PAE, and PAC represent the photobleach, photoinduced absorption (PA) of trions, PA of singlet excitons, and PA of charges, respectively.

∫ω ΔA(t , ω) dω/∫ω 1/2 × (ΔA(t , ω) − |ΔA(t , ω)|) dω

(1)

where the equal sign holds for negligible spectral overlap between negative and positive photoinduced bands (see Supporting Information for the derivation of eq 1). In Figure 2, we show the relative contribution of neutral excited states rN(t), calculated according to eq 1, for the (6,5)

the legend. The dominant spectral features in Figure 1 are wellknown:12,30,31 negative ΔA bands are caused by the transient photobleach (PB) of the S1 excitonic transitions of the most abundant chiralities in the sample, as can be shown by the spectral coincidence of the PB bands with the respective ground state absorption bands (see Figure S1 in Supporting Information). Broad positive ΔA bands on the low and high energy side of the main PB are caused by the photoinduced absorption (PA) of singlet excitons and charged states, here referred to as PAE and PAC, respectively.20,32 A relatively sharp PAT band at 1.065 (Figure 1a,c) and 1.03 eV (Figure 1b,d) has been assigned to PA from the trion states in (6,5) and (7,5) tubes, respectively, which can essentially be described as charged exciton transitions.20,33−42 During the first 25 ps (Figure 1a,b), the photoexcitation dynamics are characterized by a strong reduction of the (6,5) and (7,5) PB and of PAE, caused by rapid disappearance of singlet excitons on this time scale, probably mediated by exciton annihilation43 or trapassisted recombination.44 For pump−probe delays of t > 66 ps, (Figure 1c,d), most singlet excitons have decayed, as shown by the lack of redshifted PAE. The remaining PB should therefore be attributed to excited states other than the singlet excitons. According to the phase space filling picture45 the photobleach can be interpreted as nonspecific probe for the presence of excited states. Given the presence of specific optical probes for charged states (PAT and PAC dominating the positive side of the ΔA spectra in Figure 1c,d), we can assign at least part of the PB to charged states. Importantly, in Figure 1c, we find an absolute increase of the PB of the (7,5) chirality at a probe energy of 1.2 eV after long times, showing the delayed creation of excited states on this chirality. Since we do not observe the concomitant (7,5) trion band at 1.03 eV (compare with Figure 1d where this band is clearly displayed), we conclude that this delayed PB(7,5) is not caused by charged states. In order to find out whether further excitations are contributing to the PB bands in Figure 1 c and d, we take advantage of a recent spectroelectrochemical study in a similar set of samples.46 There, it was found that electrochemical doping of (6,5)-rich samples gives rise to specific absorption

Figure 2. Relative contribution of neutral excited states to the total PB signals, calculated according to eq 1, for (6,5) rich (a) and (7,5) rich (b) samples, respectively. The lower row (c and d) shows the dynamics of main PB and PAT for the respective samples, at different pump intensities.

rich and the (7,5) rich samples (panels a and b, respectively) at three different intensities. In both samples, rN(t) is positive over all time scales, evidencing a significant amount of neutral photoexcitations. Two time scales of different time evolution can be distinguished. For t < 10 ps, rN(t) is intensity dependent, and for the highest intensities, rN(t) decreases until a minimum is reached at around 10 ps (in panel b, this behavior is actually found for all intensities). For t > 10 ps, rN(t) becomes intensity independent, and monotonously rises until approximately 400 ps, reaching a stationary value between 60 and 75%. After pumping at high energies, the presence of both singlet excitons and charged states has been shown.18 It is indeed expected that quasi one-dimensional singlet excitons show a net negative TA spectrum, due to the presence of stimulated emission superposing with the bleach;47 therefore we associate the positive rN(t) values on the early time scale with singlet 19780

DOI: 10.1021/acs.jpcc.6b03923 J. Phys. Chem. C 2016, 120, 19778−19784

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proportional to the time-resolved triplet concentration Tn,m on a specific chirality (n,m):

excitons. The reduction of rN(t) at high intensities on the sub10 ps time scale is then explained by rapid bimolecular annihilation of singlet excitons, a commonly observed process in SWNTs.48 On the other hand, the high rN(t) values on the long time scale cannot be explained by singlet excitons. As Figure 2 shows, rN(t) is stable for at least 500 ns, which means that the lifetime of the associated neutral excited state must be at least in the microsecond range. Such a long lifetime strongly argues against a spin allowed relaxation pathway, and therefore excludes singlet excitons. Since it has recently been shown in similar samples that triplet excitons are generated with up to 32% yield,23 we tentatively assign the excess bleach (causing positive rN(t) values) on the time scale t > 50 ps to triplet excitons (see original TA spectra on the nanosecond time scale in the Supporting Information, Figure S2). In order to confirm our notion that the overall bleach after long times contains contributions from two different photoexcited states, we present in parts c and d of Figure 2 decay traces at fixed probe energies corresponding to the main bleach and the trion absorption, both normalized to their value at t = 50 ps. For the (6,5) rich sample, we find that the signal in the trion band is reduced by 40% between 50 and 400 ps, while the signal in the main photobleach is reduced by 70% in the same time interval. It follows that there must be a second contribution to the photobleach which decays much faster than the charged states. In the next section, we show that this decay is caused by triplet transfer to a different chirality. In the (7,5) rich sample, there is still a difference in the normalized trion and bleach kinetics, but not as pronounced as in the (6,5) rich sample. Below, we show that in this sample, no intertube triplet transfer can be observed; the different kinetics therefore reflect different intratube triplet and charge relaxation times. Importantly, these findings are the same for pump intensities ranging over more than an order of magnitude (see different symbols in Figure 2c), from which we conclude that the observed processes are valid also for low-level irradiation under solar conditions. We note that other authors do not find signatures of charged states at low pump intensities10which seems to be in contrast with our findings. As Yuma et al. show,20 maximum singlet concentration and maximum trion absorption are proportional across a range of pump intensities, which agrees with a picture of charge generation by exciton annihilation under certain conditions. Further studies, e.g., showing a deviation of this linear relationship at very low intensities where the exciton annihilation yield is limited by the natural exciton lifetime, are necessary to conclusively resolve this issue. The determination of rN(t) according to eq 1 is the most direct way to show the presence of any negative TA signal that is not compensated by either PAC or PAT and hence is caused by a neutral photoexcitation. However, PAC, PAT, and also PB occur at well-known probe energies that are specific for the respective chirality. Consequently, at pump−probe delays long enough to neglect singlet excitons, the excess bleach becomes a chirality specific optical probe for triplets, giving us the opportunity to trace intertube triplet transfer. To this end we define a chirality specific excess bleach PBnexc, m = a(PBn , m) − [a(PA Cn, m) + a(PA Tn, m)]

nm PBexc (t ) = d × σTn , m × T n , m(t )

(3)

σn,T m

Here, is the absorption cross-section of triplet states on (n,m) tubes, which is caused by phase space filling and related to the triplet exciton size, a quantity which is unknown so far, and d is the film thickness. Since σn,T m does not significantly depend on time, we can use eq 3 to trace triplet exciton dynamics in the time domain by evaluating the excess bleach for different chiralities in order to find evidence for population transfer kinetics. To this end, we need to find the total spectral weights a(X) in eq 2 for each chirality, which due to strong spectral congestion requires a global analysis. To keep the number of free parameters low, we fixed the spectral shapes and relative area ratios of the PAC and PAT bands to the corresponding bands in the electrochemical data46 (for the precise procedure, see Supporting Information). For the (6,5) rich tubes, this gave satisfactory fits, see curves in Figure 3a. For

Figure 3. Global analysis of TA spectra from Figure 1 for the (6,5) rich and (7,5) rich samples (panel a and b, respectively) at t = 167 ps (black curves). The other curves show single contributions to the global fit, as indicated in the legend. The spectral model is adapted from ref 46. The resulting spectral weights are shown in panels c and d.

the (7,5) rich sample, where no analogous electrochemical study is available, we required the spectral offset of PAC against PB to be the same as in (6,5) tubes, and also fixed the ratio 7,5 a(PA7,5 C )/a(PAT ) to the value experimentally obtained for the (6,5) rich sample (for details, see Supporting Information). As shown in Figure 3b, this procedure leads to a good agreement between experiment and simulation. Parts c and d of Figure 3 shows spectral weights a(X)(t) resulting from fits similar to those displayed in parts a and b of Figure 3, for pump−probe delays between 50 and 400 ps, for the (6,5) rich and (7,5) rich samples, respectively. In the (6,5) rich sample in Figure 3c, we find that a(PB6,5)(t) > [a(PA6,5 C )(t) + a(PA6,5 T )(t)] for all times but the difference between the total bleach of the (6,5) tubes and the respective charge-induced PA bands is diminishing after long times, compare black and red symbols, respectively. Consequently, PB6,5 exc → 0 on a 200 ps time scale, see blue symbols, showing that the remaining PB6,5 for t > 400 ps is nearly exclusively due to charged states. The

(2)

∫∞ ω=−∞X(t)

where a(X)(t) = dω is the time-dependent spectral area of a photoinduced band X. After long times where singlet excitons do not significantly contribute to the excess bleach, and applying Lambert−Beer’s law, the excess bleach is 19781

DOI: 10.1021/acs.jpcc.6b03923 J. Phys. Chem. C 2016, 120, 19778−19784

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The Journal of Physical Chemistry C 7,5 decay of PB6,5 exc is corroborated with an absolute increase of PBexc (magenta symbols in Figure 3c) with the same dynamics, from which we conclude that triplet excitons are transferred from (6,5) to (7,5) tubes on a 100 ps time scale. The transfer dynamics can be well described by simple monoexponential functions (see lines in Figure 3c) which might indicate that triplet energy transfer is not diffusion limited. Diffusion control would normally entail dispersive kinetics, which we do not observe. In Table 1, we show that the monoexponental times

exciton transfer, such a dependence on chirality could be easily explained by the lack of spectral overlap between donor and acceptor. This picture is however not applicable to triplet energy transfer, being usually discussed in a Dexter-type transfer model, formally requiring two electron transfer steps in order to conserve the total spin throughout the transfer process.50 As the electrochemical bandgaps (Eb) are diameter dependent,51 our result seems to suggest a higher triplet transfer rate from higher towards lower bandgap tubes like, (6,5)→(7,5), than between tubes of similar diameter (like, (7,5) and (8,4)) and thus similar Eb. However, this notion needs to be confirmed in a broader range of chiralities. From Tables 1 and 2, we get additional insight into the triplet generation mechanism. The approximate triplet yield ηtri tot is clearly reduced at higher pump intensities. As explained in the previous section, the absolute values are error prone because the triplet exciton length is not known. This does however not affect the evolution of the triplet yield with pump intensity, as the triplet size is not expected to depend on pump intensity. We can thus trace the intensity dependence of the triplet yield with a much higher precision than the absolute triplet yield. Thus, the reduction of the triplet yield at higher pump intensity is an indication that triplets are not generated by high-intensity effects such as exciton−exciton annihilation or bimolecular charge recombination; in this case the triplet yield should increase with intensity. The reduction of the triplet yield rather shows that at high pump intensity, the precursor for triplet generation is removed, pointing to relaxed singlet excitons as precursors for triplet generation.

7,5 Table 1. Exponential Fit Result of PB6,5 exc and PBexc and Corresponding Triplet Yield in (6,5) Rich Samplea

pump power (uJ/cm2)

A7,5 PB

τ7,5 PB (ps)

A6,5 exc

τ6,5 exc (ps)

ηtri tot (%, at 50 ps)

33 99 198

−1.65 × 10−4 −3.15 × 10−4 −6.34 × 10−4

110 80 64

5.49 × 10−4 1.16 × 10−3 2.17 × 10−3

82 76 65

4.5 ± 0.5 4.2 ± 0.5 3.8 ± 0.1

y = A × exp(−x/τ) + y0 is used as the fit model, where A is amplitude, x is pump probe delay time, y0 is offset, and τ is lifetime. ηtri tot is the total triplet yield at 50 ps, which was obtained by dividing the spectral weight of excess bleach at 50 ps by the spectral weight of total bleach at 0 ps. a

7,5 for PB6,5 exc decay and PBexc rise match very well, supporting our picture of triplet exciton transfer. In the last columns of Tables 1 and 2, we compare the excess bleach at t = 50 ps with the

Table 2. Exponential Fit Result of PB7,5 exc and Corresponding Triplet Yield in (7,5) Rich Samplea pump power (uJ/cm2)

A8,4 PB

τ8,4 PB (ps)

A7,5 exc

τ7,5 exc(ps)

ηtri tot (%, at 50 ps)

33 99 198

0 0 0

− − −

3.64 × 10−4 6.62 × 10−4 6.48 × 10−4

1.48 × 102 1.40 × 102 2.10 × 102

8.4 ± 0.2 7.1 ± 0.2 5.5 ± 0.2

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CONCLUSIONS We have performed transient absorption spectroscopy on a time scale from femtoseconds to microseconds on chirality sorted single-walled carbon nanotube samples, rich in either (6,5) or (7,5) tubes. By comparing the TA spectra with the results from spectroelectrochemical measurements, we were able to quantify specific contributions of charged states and triplet excitons to the transient photobleach of each chirality. This allowed us to trace, for the first time, the intertube transfer of triplet excitons. We found a complete transfer of triplet excitons from the (6,5) to the (7,5) chirality in 70 ps with first order kinetics, showing that the transfer process is not diffusion limited. In contrast, we observed no transfer of triplets from the (7,5) to the (8,4) tubes. From intensity-dependent measurements, we found relaxed singlet states to be precursors for triplet formation.

y = A × exp(−x/τ) + y0 is used as fit model, where A is amplitude, x is pump probe delay time, y0 is offset, and τ is lifetime. ηtri tot is the total triplet yield at 50 ps which was obtained by dividing the spectral weight of excess bleach at 50 ps by the spectral weight of total bleach at 0 ps. a

total bleach immediately after the pump pulse. Since the latter is expected to sum up over all pump-induced excited states, we can get an order of magnitude guess of the triplet yield. We find values between 4 and 5%. The fact that the values decrease with increasing pump intensity is an indication that exciton annihilation is not the reason for the generation of triplet states. However, in analogy to eq 2 we must highlight that the sizes (longitudinal correlation lengths) of singlets, charges and triplets, all of which are contributing to the total bleach at t = 0, might substantially differ from each other, given the large differences in Coulomb and exchange interactions of these species in a 1D system.49 In the (7,5) rich sample (Figure 3b), we find a high PB7,5 exc, which does not decay to zero on a 400 ps time scale. Moreover, the excess bleach PB8,4 exc of the (8,4) tube remains very low and almost constant over this time scale not showing any sign of triplet exciton transfer into this chirality. This is especially surprising as the relative density of (8,4) as possible acceptor tubes for triplet transfer in the (7,5) rich sample is higher than that of (7,5) acceptor tubes in the (6,5) rich sample (see Supporting Information, Figure S1). In the case of singlet

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03923. Ground state absorption spectra and determination of chirality concentration, rough estimation of the relative excess ground state bleach by spectral integration, nanomicrosecond transient absorption spectra, and global spectral modeling (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(L.L.) E-mail: [email protected]. Telephone: +34 91 299 87 82. 19782

DOI: 10.1021/acs.jpcc.6b03923 J. Phys. Chem. C 2016, 120, 19778−19784

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The Journal of Physical Chemistry C Notes

(16) Qian, H.; Georgi, C.; Anderson, N.; Green, A. A.; Hersam, M. C.; Novotny, L.; Hartschuh, A. Exciton Energy Transfer in Pairs of Single-Walled Carbon Nanotubes. Nano Lett. 2008, 8, 1363−1367. (17) Lüer, L.; Crochet, J.; Hertel, T.; Cerullo, G.; Lanzani, G. Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates. ACS Nano 2010, 4, 4265−4273. (18) Crochet, J. J.; Hoseinkhani, S.; Lüer, L.; Hertel, T.; Doorn, S. K.; Lanzani, G. Free-Carrier Generation in Aggregates of Single-Wall Carbon Nanotubes by Photoexcitation in the Ultraviolet Regime. Phys. Rev. Lett. 2011, 107, 257402. (19) Soavi, G.; Scotognella, F.; Brida, D.; Hefner, T.; Späth, F.; Antognazza, M. R.; Hertel, T.; Lanzani, G.; Cerullo, G. Ultrafast Charge Photogeneration in Semiconducting Carbon Nanotubes. J. Phys. Chem. C 2013, 117, 10849−10855. (20) Yuma, B.; et al. Biexciton, Single Carrier, and Trion Generation Dynamics in Single-Walled Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 205412. (21) Jakubka, F.; Backes, C.; Gannott, F.; Mundloch, U.; Hauke, F.; Hirsch, A.; Zaumseil, J. Mapping Charge Transport by Electroluminescence in Chirality-Selected Carbon Nanotube Networks. ACS Nano 2013, 7, 7428−7435. (22) Stich, D.; Spaeth, F.; Kraus, H.; Sperlich, A.; Dyakonov, V.; Hertel, T. Triplet-Triplet Exciton Dynamics in Single-Walled Carbon Nanotubes. Nat. Photonics 2014, 8, 139−144. (23) Späth, F. L. Preparation and Characterization of Single-Wall Carbon Nanotube-Polyfluorene-Complexes. Doctoral Thesis, Universität Würzburg, Fakultät für Chemie und Pharmazie: 2015. (24) Rao, A.; Wilson, M. W.; Hodgkiss, J. M.; Albert-Seifried, S.; Bassler, H.; Friend, R. H. Exciton Fission and Charge Generation Via Triplet Excitons in Pentacene/C60 Bilayers. J. Am. Chem. Soc. 2010, 132, 12698−12703. (25) Van Stokkum, I. H.; Larsen, D. S.; van Grondelle, R. Global and Target Analysis of Time-Resolved Spectra. Biochim. Biophys. Acta, Bioenerg. 2004, 1657, 82−104. (26) Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J. Highly Selective Dispersion of Single-Walled Carbon Nanotubes Using Aromatic Polymers. Nat. Nanotechnol. 2007, 2, 640−646. (27) Chen, F.; Wang, B.; Chen, Y.; Li, L.-J. Toward the Extraction of Single Species of Single-Walled Carbon Nanotubes Using FluoreneBased Polymers. Nano Lett. 2007, 7, 3013−3017. (28) Ozawa, H.; Ide, N.; Fujigaya, T.; Niidome, Y.; Nakashima, N. One-Pot Separation of Highly Enriched (6, 5)-Single-Walled Carbon Nanotubes Using a Fluorene-Based Copolymer. Chem. Lett. 2011, 40, 239−241. (29) Schöppler, F.; Mann, C.; Hain, T. C.; Neubauer, F. M.; Privitera, G.; Bonaccorso, F.; Chu, D.; Ferrari, A. C.; Hertel, T. Molar Extinction Coefficient of Single-Wall Carbon Nanotubes. J. Phys. Chem. C 2011, 115, 14682−14686. (30) Lüer, L.; Lanzani, G.; Crochet, J.; Hertel, T.; Holt, J.; Vardeny, Z. V. Ultrafast Dynamics in Metallic and Semiconducting Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 205411. (31) Schilling, D.; Mann, C.; Kunkel, P.; Schöppler, F.; Hertel, T. Ultrafast Spectral Exciton Diffusion in Single-Wall Carbon Nanotubes Studied by Time-Resolved Hole Burning. J. Phys. Chem. C 2015, 119, 24116−24123. (32) Sciascia, C.; Crochet, J.; Hertel, T.; Lanzani, G. Long Lived Photo Excitations in (6,5) Carbon Nanotubes. Eur. Phys. J. B 2010, 75, 115−120. (33) Jakubka, F.; Grimm, S. B.; Zakharko, Y.; Gannott, F.; Zaumseil, J. Trion Electroluminescence from Semiconducting Carbon Nanotubes. ACS Nano 2014, 8, 8477−8486. (34) Akizuki, N.; Iwamura, M.; Mouri, S.; Miyauchi, Y.; Kawasaki, T.; Watanabe, H.; Suemoto, T.; Watanabe, K.; Asano, K.; Matsuda, K. Nonlinear Photoluminescence Properties of Trions in Hole-Doped Single-Walled Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 195432. (35) Koyama, T.; Shimizu, S.; Miyata, Y.; Shinohara, H.; Nakamura, A. Ultrafast Formation and Decay Dynamics of Trions in P-Doped

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.L. thanks the EC (Cofund program “amarout”), the Autonomous Community of Madrid (Project “FotoCarbon”), Ministry of economy and competitiveness (“Plan nacional”, Project “MultiCrom”), A.A. and I.N. thank the EC for a Marie Curie fellowship of the FP7 ITN “POCAONTAS”, Project No: 316633, and all authors thank Prof. Thomas Anthopoulos and Ms. Francesca Bottacchi from Imperial College London for providing the raw material.

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REFERENCES

(1) Dürkop, T.; Getty, S.; Cobas, E.; Fuhrer, M. Extraordinary Mobility in Semiconducting Carbon Nanotubes. Nano Lett. 2004, 4, 35−39. (2) Wang, F.; Dukovic, G.; Brus, L. E.; Heinz, T. F. The Optical Resonances in Carbon Nanotubes Arise from Excitons. Science 2005, 308, 838−841. (3) Tune, D. D.; Shapter, J. G. The Potential Sunlight Harvesting Efficiency of Carbon Nanotube Solar Cells. Energy Environ. Sci. 2013, 6, 2572−2577. (4) Kymakis, E.; Amaratunga, G. A. J. Single-Wall Carbon Nanotube/ Conjugated Polymer Photovoltaic Devices. Appl. Phys. Lett. 2002, 80, 112. (5) Ye, Y.; Bindl, D. J.; Jacobberger, R. M.; Wu, M. Y.; Roy, S. S.; Arnold, M. S. Semiconducting Carbon Nanotube Aerogel Bulk Heterojunction Solar Cells. Small 2014, 10, 3299−3306. (6) Mallajosyula, A. T.; Sundar Kumar Iyer, S.; Mazhari, B. Increasing the Efficiency of Charge Extraction Limited Poly-(3-Hexylthiophene): [6, 6] Phenyl C61 Butyric Acid Methyl Ester Solar Cells Using Single Walled Carbon Nanotubes with Metallic Characteristics. J. Appl. Phys. 2011, 109, 124908. (7) Gong, M.; Shastry, T. A.; Xie, Y.; Bernardi, M.; Jasion, D.; Luck, K. A.; Marks, T. J.; Grossman, J. C.; Ren, S.; Hersam, M. C. Polychiral Semiconducting Carbon Nanotube−Fullerene Solar Cells. Nano Lett. 2014, 14, 5308−5314. (8) Gong, M.; Shastry, T. A.; Cui, Q.; Kohlmeyer, R. R.; Luck, K. A.; Rowberg, A.; Marks, T. J.; Durstock, M. F.; Zhao, H.; Hersam, M. C. Understanding Charge Transfer in Carbon Nanotube−Fullerene Bulk Heterojunctions. ACS Appl. Mater. Interfaces 2015, 7, 7428−7435. (9) Shastry, T. A.; Clark, S. C.; Rowberg, A. J.; Luck, K. A.; Chen, K. S.; Marks, T. J.; Hersam, M. C. Enhanced Uniformity and Area Scaling in Carbon Nanotube−Fullerene Bulk-Heterojunction Solar Cells Enabled by Solvent Additives. Adv. Energy Mater. 2016, 6, 1501466. (10) Dowgiallo, A.-M.; Mistry, K. S.; Johnson, J. C.; Blackburn, J. L. Ultrafast Spectroscopic Signature of Charge Transfer between SingleWalled Carbon Nanotubes and C-60. ACS Nano 2014, 8, 8573−8581. (11) Park, J.; Reid, O. G.; Blackburn, J. L.; Rumbles, G. Photoinduced Spontaneous Free-Carrier Generation in Semiconducting Single-Walled Carbon Nanotubes. Nat. Commun. 2015, 6, 8809. (12) Lüer, L.; Hoseinkhani, S.; Polli, D.; Crochet, J.; Hertel, T.; Lanzani, G. Size and Mobility of Excitons in (6, 5) Carbon nanotubes. Nat. Phys. 2009, 5, 54−58. (13) Cognet, L.; Tsyboulski, D. A.; Rocha, J.-D. R.; Doyle, C. D.; Tour, J. M.; Weisman, R. B. Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes by Single-Molecule Reactions. Science 2007, 316, 1465−1468. (14) Hertel, T.; Himmelein, S.; Ackermann, T.; Stich, D.; Crochet, J. Diffusion Limited Photoluminescence Quantum Yields in 1-D Semiconductors: Single-Wall Carbon Nanotubes. ACS Nano 2010, 4, 7161−7168. (15) Harutyunyan, H.; Gokus, T.; Green, A. A.; Hersam, M. C.; Allegrini, M.; Hartschuh, A. Defect-Induced Photoluminescence from Dark Excitonic States in Individual Single-Walled Carbon Nanotubes. Nano Lett. 2009, 9, 2010−2014. 19783

DOI: 10.1021/acs.jpcc.6b03923 J. Phys. Chem. C 2016, 120, 19778−19784

Article

The Journal of Physical Chemistry C Single-Walled Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 165430. (36) Okano, M.; Nishihara, T.; Yamada, Y.; Kanemitsu, Y. Dynamics of Excitons and Trions in Semiconducting Carbon Nanotubes. Proc. SPIE 2013, 8623, 86231F. (37) Nishihara, T.; Yamada, Y.; Okano, M.; Kanemitsu, Y. Trion Formation and Recombination Dynamics in Hole-Doped SingleWalled Carbon Nanotubes. Appl. Phys. Lett. 2013, 103, 023101. (38) Mouri, S.; Miyauchi, Y.; Iwamura, M.; Matsuda, K. Temperature Dependence of Photoluminescence Spectra in Hole-Doped SingleWalled Carbon Nanotubes: Implications of Trion Localization. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 045408. (39) Park, J. S.; Hirana, Y.; Mouri, S.; Miyauchi, Y.; Nakashima, N.; Matsuda, K. Observation of Negative and Positive Trions in the Electrochemically Carrier-Doped Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134, 14461−14466. (40) Watanabe, K.; Asano, K. Trions in Semiconducting SingleWalled Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 035416. (41) Santos, S. M.; Yuma, B.; Berciaud, S.; Shaver, J.; Gallart, M.; Gilliot, P.; Cognet, L.; Lounis, B. All-Optical Trion Generation in Single-Walled Carbon Nanotubes. Phys. Rev. Lett. 2011, 107, 187401. (42) Matsunaga, R.; Matsuda, K.; Kanemitsu, Y. Observation of Charged Excitons in Hole-Doped Carbon Nanotubes Using Photoluminescence and Absorption Spectroscopy. Phys. Rev. Lett. 2011, 106, 037404. (43) Valkunas, L.; Ma, Y.-Z.; Fleming, G. R. Exciton-Exciton Annihilation in Single-Walled Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 115432. (44) Ma, Y.-Z.; Stenger, J.; Zimmermann, J.; Bachilo, S. M.; Smalley, R. E.; Weisman, R. B.; Fleming, G. R. Ultrafast Carrier Dynamics in Single-Walled Carbon Nanotubes Probed by Femtosecond Spectroscopy. J. Chem. Phys. 2004, 120, 3368−3373. (45) Brito Cruz, C. H.; Gordon, J.; Becker, P.; Fork, R.; Shank, C. V. Dynamics of Spectral Hole Burning. IEEE J. Quantum Electron. 1988, 24, 261−269. (46) Hartleb, H.; Späth, F.; Hertel, T. Evidence for Strong Electronic Correlations in the Spectra of Gate-Doped Single-Wall Carbon Nanotubes. ACS Nano 2015, 9, 10461−10470. (47) Van Burgel, M.; Wiersma, D. A.; Duppen, K. The Dynamics of One-Dimensional Excitons in Liquids. J. Chem. Phys. 1995, 102, 20− 33. (48) Valkunas, L.; Ma, Y. Z.; Fleming, G. R. Exciton-Exciton Annihilation in Single-Walled Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 115432. (49) Perebeinos, V.; Tersoff, J.; Avouris, P. Scaling of Excitons in Carbon Nanotubes. Phys. Rev. Lett. 2004, 92, 257402. (50) Monguzzi, A.; Tubino, R.; Meinardi, F. Upconversion-Induced Delayed Fluorescence in Multicomponent Organic Systems: Role of Dexter Energy Transfer. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 155122. (51) Paolucci, D.; Franco, M. M.; Iurlo, M.; Marcaccio, M.; Prato, M.; Zerbetto, F.; Pénicaud, A.; Paolucci, F. Singling out the Electrochemistry of Individual Single-Walled Carbon Nanotubes in Solution. J. Am. Chem. Soc. 2008, 130, 7393−7399.

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DOI: 10.1021/acs.jpcc.6b03923 J. Phys. Chem. C 2016, 120, 19778−19784