Long-Lived Charge Separation at Heterojunctions betweenSemiconducting Single-Walled Carbon Nanotubes and PeryleneDiimide Electron AcceptorsHyun Suk Kang,† Thomas J. Sisto,‡,# Samuel Peurifoy,‡,# Dylan H. Arias,† Boyuan Zhang,‡

Colin Nuckolls,‡ and Jeffrey L. Blackburn*,†

†National Renewable Energy Laboratory, Golden, Colorado 80401, United States‡Department of Chemistry, Columbia University, New York, New York 10027, United States

*S Supporting Information

ABSTRACT: Nonfullerene electron acceptors have facilitated a recent surge inthe efficiencies of organic solar cells, although fundamental studies of the natureof exciton dissociation at interfaces with nonfullerene electron acceptors are stillrelatively sparse. Semiconducting single-walled carbon nanotubes (s-SWCNTs),unique one-dimensional electron donors with molecule-like absorption andhighly mobile charges, provide a model system for studying interfacial excitondissociation. Here, we investigate excited-state photodynamics at the hetero-junction between (6,5) s-SWCNTs and two perylene diimide (PDI)-basedelectron acceptors. Each of the PDI-based acceptors, hPDI2-pyr-hPDI2 andTrip-hPDI2, is deposited onto (6,5) s-SWCNT films to form a heterojunctionbilayer. Transient absorption measurements demonstrate that photoinducedhole/electron transfer occurs at the photoexcited bilayer interfaces, producinglong-lived separated charges with lifetimes exceeding 1.0 μs. Both excitondissociation and charge recombination occur more slowly for the hPDI2-pyr-hPDI2 bilayer than for the Trip-hPDI2 bilayer. Toexplain such differences, we discuss the potential roles of the thermodynamic charge transfer driving force available at eachinterface and the different molecular structure and intermolecular interactions of PDI-based acceptors. Detailed photophysicalanalysis of these model systems can develop the fundamental understanding of exciton dissociation between organic electrondonors and nonfullerene acceptors, which has not been systematically studied.

■ INTRODUCTION

Semiconducting single-walled carbon nanotubes (s-SWCNTs)have been intensely studied as the components of variousphotovoltaic cells, both as active layer materials (mostly aselectron donor) in organic photovoltaic (OPV) cells and ashole transport layers in organic, inorganic, and hybrid (e.g.,perovskite) solar cells.1−8 Semiconducting SWCNTs havesignificant mechanical advantages for PV devices, such as highflexibility, chemical robustness, giant aspect ratio, and hydro-phobicity, and are also lightweight.9 Moreover, their excellentbandgap tunability allows s-SWCNTs to selectively overlapwith specific regions of the solar spectrum,10 potentiallyenabling novel applications such as building-integrated photo-voltaics (BIPV).6,11 Tuning the bandgap can be easily realizedby the selective polymer extraction technique to obtain specifics-SWCNT diameters and electronic structures.12−14 Polymer-extracted s-SWCNTs with one (or only a few) species shownarrow optical transitions that assist the fundamental photo-dynamic studies of s-SWCNT systems, since their spectro-scopic signatures can be easily distinguished from those of theother components within the systems.15−17

Fullerene derivative electron acceptors, such as phenyl-C61-butyric acid methyl ester (PC61BM) and phenyl-C71-butyric

acid methyl ester (PC71BM), have been prevalent in OPVs formany years. Importantly, s-SWCNTs with appropriateelectronic band gaps form a Type-II heterojunction withfullerene derivative electron acceptors, with sufficient excitondissociation driving force to generate photocurrent.1−4 As such,there have been many fundamental studies of s-SWCNT/fullerene heterojunctions, aimed at understanding the kineticsand thermodynamics of interfacial exciton dissociation andcharge recombination in this model system.5,7,15,17 However,the expensive synthetic costs, limited spectral absorption, andlimited synthetic tunability of fullerenes have raised the interestin development of nonfullerene electron acceptors.18,19 Despitethis pivot within the OPV community, there have been rarefundamental or device studies on heterojunctions betweenSWCNTs and nonfullerene acceptors.Perylene diimides (PDIs) have seen mounting interest as a

prominent class of nonfullerene acceptor due to several

Special Issue: Prashant V. Kamat Festschrift

Received: February 9, 2018Revised: March 24, 2018

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advantages.20−22 Their electron affinity values are in the samerange as those of fullerenes, their electron mobility is high, andthey tend to be more stable than fullerenes, particularly withrespect to photodegradation.19,23 Spectral absorption for PDIderivatives tends to be broader and more synthetically tunablethan that of fullerenes, and their synthetic cost is typicallysignificantly cheaper than that of fullerene derivatives (as anexample, PDIs are used in many common pigments in theautomotive, and other, industries). As with fullerenes, PDIs alsoaggregate within films such that π−π stacking betweenmolecules can facilitate charge transport. However, they tendto aggregate into large crystallites that reduce donor−acceptorinterfacial area for exciton dissociation in bulk heterojunctions(BHJs).24 Hence, avoiding excess aggregation without losingappreciable long-range charge transport has been a key strategyin the development of PDI-based acceptors, and a primarymolecular design rule invokes the disruption of π−π stackingbetween acceptors by adding substituents to PDIs or covalentlyconnecting multiple PDIs.25−28

In a recent study, a new class of PDI-based electronacceptors, referred to as PDI nanoribbons, was synthesized.29

These acceptors were constructed from the alternation of anelectron-rich donor (alkoxy pyrene) and electron-poor accept-ors (PDI oligomers, hPDIx, x = the number of PDIs), and aredenoted as hPDI2-pyr-hPDI2 and hPDI3-pyr-hPDI3. Thesestructural motifs lower the bandgap of the PDI acceptors toshift optical absorption deeper into the visible region, enablingbetter solar energy harvesting. Furthermore, the energy-minimized nanoribbon geometries, predicted by densityfunctional theory (DFT) calculations, are twistacene helicalstructures that utilize steric hindrance to inhibit the excessiveaggregation. In thin films, hPDI3-pyr-hPDI3 forms anorganized lamella structure on highly ordered pyrolyticgraphite, indicating that efficient π−π stacking is stillmaintained. This study explored BHJs with PTB7-Th, adonor polymer showing exceptional promise for high-efficiencyOPV devices, and the power conversion efficiencies (PCEs) ofthe BHJs were recorded as 6.9 and 7.6% for hPDI2-pyr-hPDI2and hPDI3-pyr-hPDI3, respectively. The PCE value of 7.6%obtained from the hPDI3-pyr-hPDI3 device is comparablewith the values of the best PTB7-Th-based nonfullerene solarcells, which typically show a PCE < 7.5% without solventadditives. However, fundamental studies of these PDI-basedacceptors have not been performed to resolve the chargeseparation and recombination processes from PDI-basedacceptors.In this study, the detailed photodynamics of heterojunctions

between (6,5) s-SWCNTs and two PDI-based electronacceptors were investigated by transient absorption spectros-copy. The two PDI-based acceptors were chosen to havedistinct molecular geometries and frontier orbital energies thatestablish sufficient thermodynamic driving force for excitondissociation at the heterojunction with (6,5) s-SWCNTs. Assuch, we hope for this study to serve as the starting point for anongoing effort aimed at disentangling the effects of interfacialthermodynamics (i.e., exciton dissociation driving force) andmolecular structure considerations (i.e., steric effects, electroniccoupling, reorganization energy) on photoinduced chargetransfer at model interfaces between s-SWCNTs and a broadarray of acceptors. In the current study, we focus primarily onphotoinduced hole transfer (PHT) from the PDI molecules to(6,5) s-SWCNTs. Since the PHT event utilizes the broadvisible absorption (with high extinction coefficient) of the PDI

molecules that captures significantly more visible photons thanfullerenes, this PHT event comprises a strategy for significantlyimproving energy harvesting for these PDI heterojunctionsrelative to the more broadly studied SWCNT/fullereneheterojunctions. We find that both PDI-based acceptorsfacilitate interfacial exciton dissociation by photoinduced holetransfer from the PDI acceptors to (6,5) s-SWCNTs, enablingvery long-lived charge-separated states. The two PDI-basedbilayers show measurable differences in the rates of photo-induced charge transfer and recombination, with both PHT andrecombination being appreciably slower in the hPDI2-pyr-hPDI2 bilayer. These differences are discussed with regards tothe potential effects of exciton dissociation driving force,interfacial coupling, and PDI molecular structure.

■ EXPERIMENTAL METHODSSyntheses of PDI-Based Electron Acceptors. Synthesis

of the hPDI2-pyr-hPDI2 nanoribbon PDI-based electronacceptor was described previously.29 The synthesis of Trip-hPDI2 will be described in detail in a forthcoming work.

Film Preparation. SWCNTs for (6,5) samples werepurchased from CHASM (CoMoCAT SG65i), and poly-[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,60-[2,20-bipyridine])](PFO-bpy) was purchased from American Dye Source. Toextract pure (6,5) s-SWCNTs from SG65i, SWCNTs (1 mg/mL) were dispersed in PFO-bpy in toluene (∼4 mg/mL) by tipsonication for 15 min at 40% intensity (Cole-Palmer CPX 750,1/2” tip) in a bath of cool (∼18 °C) flowing water. The tip-sonicated SG65i/PFO-bpy mixture in toluene was immediatelytransferred to a centrifuge tube and centrifuged at 20 °C and13 200 rpm for 5 min (Beckman Coulter L-100 XPultracentrifuge, SW-32 Ti rotor). The supernatant, containingPFO-bpy-wrapped (6,5) SWCNTs and excess PFO-bpy, wasseparated from the pellet by pipet, and it was centrifuged at 20°C and 24 100 rpm for 20 h to remove excess polymer. Thepellet, containing PFO-bpy-wrapped (6,5) SWCNTS, wasseparated from the supernatant, which contained excess PFO-bpy. This long centrifuge process was repeated for theredispersed pellet in toluene to arrive at an ∼1:1 mass ratioof (6,5) SWCNTs and PFO-bpy. The pellet was thenredispersed in toluene in a heated ultrasonic bath for morethan an hour to yield homogeneous (6,5)/PFO-bpy ink.The (6,5)/PFO-bpy ink was spray-coated onto clean glass

substrates, which were rinsed by acetone and isopropyl alcohol(IPA) and treated by O3 plasma for 10 min prior to SWCNTdeposition. The cleaned glass substrates were placed on aheated stage (T = 130 °C) to evaporate solvent as the ink wasapplied onto substrates, and an ultrasonic sprayer (SonoTek)operated at 0.8 W was used to spray (6,5)/PFO-bpy ink at asolution flow rate of 0.3 mL/min and an N2 flow rate of 7.0standard liters per minute. The (6,5)/PFO-bpy films weresoaked in a hot toluene bath (78 °C) for 10 min to removeexcess polymer. This step helps to couple SWCNTs betterwithin the film. The PDI-based electron acceptors were thendeposited onto both clean glass substrates for neat films and(6,5)/PFO-bpy films for bilayers. Each acceptor solution inchloroform (2.5 mg/mL) was spin-coated at 500−2000 rpm for1 min by dispensing 100 μL of solution while the substrate wasspinning.

Static and Transient Absorption. The ground-stateabsorption spectra of neat and bilayer films were measuredon a Varian Cary 5000 spectrophotometer in the wavelengthrange of 200−1500 nm with baseline correction. The (6,5) s-

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SWCNT film thickness was estimated using the (6,5) S11 peakoptical density at 1000 nm4 and is estimated to be 10 nm forthe films studied here. Transient absorption spectra anddynamics of neat and bilayer films were measured by twodifferent configurations of an ultrafast pump−probe time-resolved spectroscopic measurement system (Ultrafast Sys-tems). Dynamics covering the temporal regime from femto-seconds to 5 ns (ns) were measured by the “Helios”configuration, which employs pump and probe pulses withwidths of ∼200 fs. Dynamics covering the range from ∼1 ns to∼100 μs were measured by the “EOS” configuration, whichemploys the same pump pulse but utilizes subnanosecondprobe pulses. The femtosecond probe setup was used for allfilms, and the subnanosecond setup was used for bilayers toprobe the long-lived species in these films. Both experimentswere performed on 1 kHz regeneratively amplified Ti:sapphirelaser system that produces 4 mJ laser pulses at 800 nm. TheTi:sapphire laser then pumps an optical parametric amplifier(OPA) to generate 400 nm (mostly exciting PDI-basedelectron acceptors) pump light. The excitation pulse energyemployed for bilayer films was ca. 1.5−2.5 nJ at 400 nm(absorbed photon flux of ca. 1.5 × 1012 photons·pulse−1·cm−2)

and was 48 nJ at 400 nm (absorbed photon flux of ca. 2.4 ×1013 photons·pulse−1·cm−2) for the PDI acceptor neat films andfor the bilayer dynamics measured over long pump−probedelays on the EOS experiment (Vide Infra). The pump andprobe beams are spatially overlapped at the glass slide. For thefemtosecond probe setup, a portion of the amplified 800 nmlight was passed through a sapphire plate to generate either avisible (Vis, 400 nm < λprobe < 800 nm) or near-infrared (NIR,800 nm < λprobe < 1700 nm) continuum probe pulse. The probepulses were delayed in time with respect to the pump pulseusing a motorized translation stage mounted with aretroreflecting mirror. The average time for data collectionwas 2−5 s at each pump−probe delay. The measurements wereoperated from 0.5 ps before time zero to 5.5 ns after time zero.For the subnanosecond probe setup, a supercontinuumgenerated in a photonic crystal fiber via an Nd:YAG is usedwith controlled timing to within ∼100 ps to achieve a longerpump−probe delay electronically.

Time-Resolved Photoluminescence. Time-resolved pho-toluminescence (TRPL) was measured using a supercontinuumfiber laser (Fianium, SC-450-PP) operating at 2 MHz as theexcitation source. The excitation wavelength used was 500 nm,

Figure 1. Molecular structures (blue: donor, red: acceptor) and DFT-optimized molecular geometries of (a) hPDI2-pyr-hPDI2 and (b) Trip-hPDI2.

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and the pulse energy was approximately 1 nJ. A streak camerafor visible detection (Hamamatsu C10910−04) was used todetect time-resolved spectra. The instrument response function(IRF) lifetime was measured to be 220 ps by scattering theexcitation beam from a Ludox solution, and the time windowsize was 20 ns. All time-resolved photoluminescence spectrawere, then, integrated with respect to time to yield steady-statephotoluminescence.Electrochemistry. Cyclic voltammograms (CVs) were

recorded on a CH166 electrochemical workstation using anAg/AgCl electrode as the reference electrode. 0.1 Mtetrabutylammonium hexafluorophosphate (TBAPF6) in tetra-hydrofuran was used as the supporting electrolyte. All cyclicvoltammetry data were referenced to the known ferrocene/ferrocenium couple. The first reduction peaks of both hPDI2-pyr-hPDI2 and Trip-hPDI2 rest at −1.07 V. The electro-chemical profile of Trip-hPDI2 will be reported in aforthcoming publication, and the profile of hPDI2-Pyr-hPDI2 has been previously reported.29

Computational Calculations. All quantum mechanicalcalculations were performed using Jaguar, version 8.2,Schrodinger, Inc., New York, NY, 2013.30 All geometrieswere optimized using either the B3LYP or M06-2X functionaland the 6-31G** basis set.

■ RESULTS AND DISCUSSION

Two PDI-based acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, were examined for this study, and their molecularstructures and DFT-calculated molecular geometries aredisplayed in Figure 1. Unlike hPDI2-pyr-hPDI2, which waspreviously introduced,29 Trip-hPDI2 is a newly synthesizedPDI-based electron acceptor that is introduced for the first timein this work. A detailed report on the synthesis andcharacterization of this acceptor is forthcoming. This acceptoris, like hPDI2-pyr-hPDI2, constructed by alternating theelectron-rich donor (triptycene) and electron-poor acceptors(PDI dimers). The three-dimensional structure for Trip-hPDI2may help to avoid excess aggregation by steric hindrance. It isalso possible that these structural differences affect the degreeof π−π interactions at the interfaces formed between the (6,5)s-SWCNTs and each acceptor. These considerations will bediscussed in more detail below.The ground-state absorption spectra of PDI-based acceptors

in both solution and as neat films are displayed in Figure 2. Theabsorption spectra of PDI-based acceptors in both chloroformand neat film span from 350 to 700 nm. The absorptionspectrum of both PDI neat films show absorption bands atsimilar wavelengths to those in solution, but with broadenedpeaks. The primary absorption characteristics of Trip-hPDI2 inchloroform (Figure 2b) are two main vibronic progressionsstarting from 398 and 557 nm. The Trip-hPDI2 neat filmshows a typical vibronic progression with the lowest energy S1transition being the most intense peak. This observationsuggests that the molecular stacking of Trip-hPDI2 is likelysignificantly disrupted in the film due to steric hindrancebetween molecules. The dominant PL from the Trip-hPDI2 inCHCl3 occurs at 578 nm, a Stokes shift of ca. 81 meV from theS1 absorption at 557 nm. This 578 nm PL peak is stillprominent in the Trip-hPDI2 film, again consistent withdisrupted molecular stacking and the retention of the emissioncharacteristics of Trip-hPDI2 monomers. A broad emissionpeak occurs at 633 nm in the Trip-hPDI2 film. This peak may

be excimer-like, but the assignment of this peak is outside thescope of the current study.The hPDI2-pyr-hPDI2 absorption and emission spectra

(Figure 2a) are markedly different than that of Trip-hPDI2.First, the absorption spectra of hPDI2-pyr-hPDI2 in bothsolution and film do not show a clean vibronic progressionfrom an intense S1 peak but instead show relatively weak lowestenergy transitions at 606 nm. We also note that the relativeintensity of this 606 nm peak can vary, both within solutionsand films, suggesting that it is not a characteristic transition ofhPDI2-pyr-hPDI2 monomers. Furthermore, the lowest energypeak in the PL spectrum of the hPDI2-pyr-hPDI2 solutionoccurs at 586 nm, ca. 70 meV higher in energy than the lowestenergy peak in the absorbance spectrum at 606 nm. Theseobservations suggest that the relatively weak shoulder in thehPDI2-pyr-hPDI2 absorbance spectrum at 555 nm corre-sponds to monomer absorption (i.e., similar S1 absorption andemission as Trip-hPDI2 monomers), whereas the red-shifted606 nm peak originates from hPDI2-pyr-hPDI2 aggregates.Emission from the hPDI2-pyr-hPDI2 film undergoes adramatic bathochromic shift of ca. 230 meV relative to thesolution-phase emission. This large bathochromic shift isconsistent with efficient energy transfer from hPDI2-pyr-hPDI2 monomers to aggregates within the film, such that thePL spectrum is dominated by the emission from aggregates. Insum, Figure 2 suggests a higher degree of π−π stacking for thehPDI2-pyr-hPDI2 molecules and a significant contribution ofhPDI2-pyr-hPDI2 aggregates to the photophysics within thesefilms.Exciton dissociation across donor−acceptor interfaces

depends sensitively upon the interfacial thermodynamics, andthe role of exciton dissociation driving force (ΔGPET or ΔGPHTfor photoinduced electron or hole transfer, respectively) hasbeen explored for a number of heterojunctions, includingSWCNT/fullerene, polymer/fullerene, and other interfa-ces.7,31−33 ΔGPET/PHT can be calculated as

Figure 2. (a) Ground-state absorption and PL spectra of neat hPDI2-pyr-hPDI2 solution and film. (b) Ground-state absorption and PLspectra of neat Trip-hPDI2 solution and film.

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Δ = − −G E(IP EA )PET/PHT D A opt (1)

where IPD is the ionization potential of electron donor ((6,5) s-SWCNT in this study), EAA is the electron affinity of electronacceptor (PDI-based acceptors in this study), and Eopt is theoptical bandgap of the donor or acceptor during either PET orPHT, respectively.31 The estimated interfacial energetics for theexplored heterojunctions are displayed in Figure 3 and

tabulated in Table 1. The methodology used to estimatethese values is detailed in the SI. Figure 3 schematicallydemonstrates that each PDI-based acceptor forms a Type-IIheterojunction with the (6,5) s-SWCNTs. Both systems havenegative values for ΔGPET and ΔGPHT, implying that photo-induced electron and hole transfer should both be spontaneousfor each bilayer. Importantly, the optical band gap utilized inthese estimates for hPDI2-pyr-hPDI2 is the band gapassociated with the lower energy aggregate optical transition(λopt = 602 nm, Eopt = 2.06 eV), since the PL measurementssuggest that the photophysics within hPDI2-pyr-hPDI2 filmsare dominated by the aggregates. However, the driving force foraggregates should be reduced relative to that for monomers,

since the S1 transition of aggregates is appreciably lower inenergy than that of monomers. As such, ΔGPHT frommonomers of both PDI acceptors may be quite similar. Suchsubtleties may play a role in hole transfer kinetics, as discussedin more detail below.Ground-state absorption spectra of each of the (6,5) s-

SWCNT/PDI bilayers are displayed in Figure 4. The thickness

of each PDI-based acceptor layer is kept relatively thin(estimated to be <10 nm) to minimize the need for excitondiffusion to the exciton dissociation interface. Each staticabsorption spectrum of the (6,5) s-SWCNT/PDI-basedacceptor bilayer is essentially the sum of each component.The absorption spectra of (6,5) s-SWCNT/PDI-based acceptorbilayers with different thicknesses of PDI-based acceptors aredisplayed in Figure S1; no differences in TA dynamics wereobserved for bilayers with different PDI thicknesses. The (6,5)S11 absorption band in the bilayers is at 1000−1002 nm, red-shifted from that in (6,5) s-SWCNT neat film at 998 nm. Themagnitude of this bathochromic shift is 2.5−5 meV, and therewas no apparent relationship between PDI-based acceptor filmthickness and (6,5) S11 band shift (Figure S1).Figure 5 shows the transient absorption (TA) spectra of the

neat films of (6,5) s-SWCNT and PDI-based acceptors. The(6,5) s-SWCNT neat film is excited at the position of the S11exciton transition (1000 nm), and the PDI-based acceptor neatfilms are excited at 400 nm where the absorption from PDI-based acceptors is dominant (optical density of 0.025 for (6,5)s-SWCNT layer, 0.14 for hPDI2-pyr-hPDI2, and 0.26 forTrip-hPDI2). The TA spectra of the (6,5) s-SWCNT neat filmshowed similar spectroscopic signatures to those observed inpast studies.15,17 In the NIR region, the S11 ground-state bleach(GSB) appears at 1011 nm, along with a broad excited-stateabsorption band, centered around at 1122 nm, which haspreviously been assigned to biexciton formation.36 In the visible

Figure 3. Schematic of the ionization potential and electron affinityvalues estimated for (6,5) s-SWCNTs and the two PDI-basedacceptors. The data and assumptions underlying the estimation aredetailed in the SI for SWCNT levels and in Table 1 footnotes for PDIacceptors.

Table 1. IP, EA, and Eopt Values for (6,5) s-SWCNT, hPDI2-pyr-hPDI2, and Trip-hPDI2 and Calculated ExcitonDissociation Driving Force Values for the Bilayers

IP (eV) EA (eV) Eopt (eV)

(6,5) s-SWCNT 5.40 3.86 1.24hPDI2-pyr-hPDI2 6.63a 4.27b 2.06Trip-hPDI2 6.81a 4.27b 2.24

ΔGPHT (eV) ΔGPET (eV)

(6,5) s-SWCNT− hPDI2-pyr-hPDI2 −0.93 −0.11(6,5) s-SWCNT− Trip-hPDI2 −1.11 −0.11

aThe ionization potential values of PDI-based acceptors weredetermined by adding Eopt and 0.3 eV (average exciton bindingenergy for organic semiconductors)34 to EA. Their IP values should beaccurately determined by several experimental techniques such asultraviolet photoelectron spectroscopy (UPS) in future studies. bTheelectron affinity values of PDI-based acceptors were determined bycomparing their first reduction potential (Ered) values to that of C60.

35

Figure 4. Ground-state absorption spectra of (6,5) s-SWCNT bilayerfilms (black: PDI-based acceptor neat film; dotted: (6,5) SWCNT neatfilm; red and blue: (6,5) SWCNT−PDI-based acceptor bilayer) of (a)hPDI2-pyr-hPDI2 and (b) Trip-hPDI2.

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region, the (6,5) S22 GSB occurs at 575 nm. These bleachescompletely decay in less than ∼1 ns with 80% decay within 20ps (inset, Figure 5a). The (6,5) S11 and S22 GSB in the neat filmoccur primarily due to Pauli blocking of the S11 transition byexcitons generated by the pump pulse.The GSB features for PDI-based acceptor neat films (Figure

5b,c) span the range of 560−610 nm, and broad (but weak)excited-state absorption appears above 610 nm, extending intothe NIR. The insets of Figure 5b,c show the TA dynamicsprobed at the peak of the lowest energy GSB for each PDIacceptor, and the kinetics of the GSB decay for the neat PDIfilms are tabulated in the SI (Table S1). The TA spectra of theTrip-hPDI2 film are dominated by the GSB of the monomer S1transition at 560 nm, and the rise time of this GSB isinstrument response limited (τIRF ≈ 120 fs). Interestingly, theTA spectra for the hPDI2-pyr-hPDI2 film show GSB signalsfor both monomers (530 and 560 nm) and aggregates (610nm) at very early times (tdelay < 1 ps), whereas the spectra atlater delays are dominated by the GSB of aggregates at 610 nm.The rise times for monomer GSB features are instrument

response limited, whereas the GSB for aggregates at 610 nmrises more slowly (τrise ≈ 540 ± 230 fs, see SI, Figure S2). Thisslow rise time matches the GSB decay for the monomerfeatures and is thus consistent with rapid downhill energytransfer from monomers to aggregates within the film,consistent with the mechanism inferred from the largebathochromic emission shift observed in Figure 2 for the neathPDI2-pyr-hPDI2 film. Importantly, this 540 fs energy transfertime represents the time frame against which PHT fromhPDI2-pyr-hPDI2 monomers must compete to be efficient.For both hPDI2-pyr-hPDI2 and Trip-hPDI2 films, the lowestenergy GSB decays in less than ∼1 ns with 80% decay within 30ps (Figure 5b,c, insets). The initial GSB decay of the Trip-hPDI2 film is slightly faster than that of the hPDI2-pyr-hPDI2film, potentially reflecting differences between recombinationwithin monomers and aggregates.Figure 6 displays TA spectra of the (6,5) s-SWCNT/PDI-

based acceptor bilayers, excited at the strong PDI-basedtransitions at 400 nm to mainly probe photoinduced holetransfer from PDI-based acceptors to (6,5) s-SWCNTs. TheTA spectra of the bilayers, excited at 400 nm and probed in theNIR, showed two distinct spectroscopic signatures relative toneat (6,5) s-SWCNTs. The first one is that the (6,5) s-SWCNTGSB decay became significantly slower at the heterojunctionbetween (6,5) s-SWCNT and PDI-based acceptors. At themeasured pump−probe delay time of 1.06 ns, about 70 and40% of the (6,5) S11 GSB at 1005 nm remains for the hPDI2-pyr-hPDI2 and Trip-hPDI2 bilayers, respectively, whereas theGSB from (6,5) s-SWCNT neat film has fully recovered overthe same time period (Figure 6, right column). The secondimportant observation is the appearance of the (6,5) s-SWCNTtrion absorption band (X+) at 1174 nm,37,38 which conclusivelydemonstrates that charge carriers are present in the (6,5) s-SWCNTs at appreciable concentrations.7,15−17,36 Since thePDI-based acceptor layer dominantly absorbs the incident 400nm photons, the observation of the (6,5) trion absorptionclearly indicates that excitons created in the PDI acceptor layersdissociate at the heterojunction interface as a result of holetransfer to the (6,5) s-SWCNT donor layer. In turn, thepresence of a long-lived trion absorption implies that theconcomitant observation of the long-lived GSB similarly reflectsthe charge carrier population that results from excitondissociation, in this case arising from a reduction in oscillatorstrength for the S11 exciton transition due to the phase-spacefilling effect.15−17 We note that we also observe the samespectroscopic signature for charged (6,5) s-SWCNTs followingexcitation of the s-SWCNT layer at 1000 nm (Figure S3),indicating that exciton dissociation also occurs via photo-induced electron transfer. For brevity, we focus on hole transferin this manuscript, but electron transfer will be treated in moredetail in a future manuscript.Figure 7 shows the TA kinetics of the bilayers, probed at the

(6,5) s-SWCNT S11 GSB and trion when the bilayers areexcited at 400 nm. The TA kinetic profiles of (6,5) s-SWCNTS11 GSB and trion absorption in both bilayers showed distinctfeatures relative to the dynamics of neat (6,5) s-SWCNT films.The first feature of note is that both the S11 GSB and trionabsorption TA signals in the bilayers (Figure 7a,b, respectively)are significantly longer-lived than those probed in the neat(6,5) s-SWCNT films (the trion absorption is absent in theneat film, but the dynamics are probed at the samewavelength). This slow decay for each signal indicates thatinterfacial charge separation, driven by PHT from PDI to the

Figure 5. Transient absorption spectra of the neat films of (a) (6,5) s-SWCNT, (b) hPDI2-pyr-hPDI2, and (c) Trip-hPDI2.

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(6,5) s-SWCNT layer, dramatically lengthens the electron−hole recombination time, since the two charge carriers are nowspatially separated into different phases. Interestingly, bothsignals decay more slowly for the hPDI2-pyr-hPDI2 bilayer,relative to the Trip-hPDI2 bilayer, indicating slower electron−hole recombination for the hPDI2-pyr-hPDI2 bilayer. The TAkinetics of each peak (GSB and X+) were fit with multi-exponential decays, and the extracted time constants aretabulated in the SI (Table S2). The long lifetimes observedhere are comparable to those observed for (6,5) s-SWCNT-C60

heterojunctions,15,17 which suggests that PDI-based acceptorsare good candidates as nonfullerene electron acceptors inSWCNT-based solar energy harvesting systems.Figure 7c,d focuses on the early time dynamics of the GSB

and trion absorption for the hPDI2-pyr-hPDI2 and Trip-hPDI2 bilayers, respectively. For the Trip-hPDI2 bilayer(Figure 7d), the GSB and X+ kinetics are essentially identical,but with opposite sign. This correspondence is expected if bothfeatures arise predominantly from the same species. Since thetrion absorption only arises from charge carriers, thisequivalence implies that the GSB for the Trip-hPDI2 bilayermust also correspond solely to the holes that reside in the (6,5)s-SWCNT layer as a result of PHT from Trip-hPDI2. As such,the rise time of each of these signals (τ = 1.4 ± 0.2/1.8 ± 0.07ps, < τ > ≈ 1.6 ± 0.15 ps,) corresponds to the hole injectiontime.

The situation for the hPDI2-pyr-hPDI2 bilayer is morecomplex (Figure 7c). In this case, the rise time of the trionabsorption is biphasic, with a fast component (τ ≈ 1.2 ± 1.11ps, amplitude = 39%) and a slower component (τ ≈ 17 ± 8.7ps, amplitude = 61%) that rises significantly more slowly thanthat of the Trip-hPDI2 bilayer. For reasons discussed below,we assign the faster rise time to hole injection from hPDI2-pyr-hPDI2 monomers and the slower rise time to hole injectionfrom hPDI2-pyr-hPDI2 aggregates. We do not believe that therelatively fast hole injection times (1.2−1.8 ps) observed foreach bilayer contain appreciable kinetic contributions fromexciton diffusion to the interface for two reasons. First, wecompared (6,5) s-SWCNT/PDI bilayers over a small range ofvarying PDI acceptor layer thicknesses (Figure S1) andobserved no change in the TA dynamics. Second, these holeinjection times are relatively similar to those observed in priorstudies exploring poly-disperse SWCNTs interfaced withdifferent PDI molecules in solution.20−22 In these studies,hole transfer occurs on time scales in the range of 2−4 ps forinterfaces between SWCNTs and individual PDI moleculeswhere diffusion within a PDI film cannot occur. In contrast,exciton diffusion within hPDI2-pyr-hPDI2 aggregates couldcontribute to the slower PHT time, as discussed in more detailbelow.Interestingly, the slow rising component (τ ≈ 17 ± 8.7 ps,

amplitude = 61%) observed in the trion dynamics for thehPDI2-pyr-hPDI2 bilayer is not observed in the GSB

Figure 6. Transient absorption spectra (left) of the bilayers of (a) (6,5) s-SWCNT−hPDI2-pyr-hPDI2 and (b) (6,5) s-SWCNT−Trip-hPDI2,excited at 400 nm, and their comparison with (6,5) SWCNT neat film transient absorption spectrum at 1062 ps (right).

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dynamics. In this case, the relatively fast GSB rise (τ ≈ 0.6 ±0.07 ps) is followed by a rapid GSB decay that is not observedin the X+ signal. To explore this discrepancy in more detail, weplot the normalized dynamics of the GSB and trion absorptionfor the hPDI2-pyr-hPDI2 bilayer in Figure 8. Here, the sign ofthe GSB is reversed, and the two kinetic traces are normalizedat long pump−probe delays. This normalization proceduredemonstrates that the long-lived signals for both species haveidentical kinetics and thus originate from the same species.Since the trion signal solely reflects the dynamics of chargecarriers, this long-lived species must be the holes in the (6,5) s-SWCNTs. Since the GSB can arise from either holes (phase-space filling) or excitons (Pauli blocking), it follows that theadditional dynamics observed only in the GSB correspond toan exciton population.The origin of this exciton population is not entirely clear at

the moment, although we can speculate on two possiblesources. First, it is possible that it originates from ultrafastexcitation energy transfer (ET) from some population ofphotoexcited hPDI2-pyr-hPDI2 molecules, where the energytransfer time is faster than that of the instrument response (τIRF≈ 120 fs). Second, it may simply arise from some non-negligible exciton population created directly in the s-SWCNTfilm by excitation at 400 nm. As shown in Figure 2, theabsorption of the hPDI2-pyr-hPDI2 layer (OD400 = 0.14) isroughly half that of the Trip-hPDI2 layer (OD400 = 0.26).

Since the TA measurements of the two bilayers are performedat equivalent absorbed photon f luences, the incident photonfluence is roughly doubled for the hPDI2-pyr-hPDI2 bilayer.As such, more photons will ultimately reach the s-SWCNTlayer and may generate a small, but non-negligible, excitonpopulation in the s-SWCNT layer that can be observed in theS11 GSB.The lifetime of the charge-separated state dictates the time

scale available to do useful work with photogenerated freecharge carriers, e.g., by extracting the charges as current in asolar cell or by driving a chemical reaction. It is clear that thecharge-separated state in the (6,5) s-SWCNT/hPDI2-pyr-hPDI2 bilayer is quite long-lived. To fully resolve the kineticsof this slow recombination, we utilized the electronic delay ofour TA system to probe the GSB of this bilayer into themicrosecond time scale. Although the pulse energy in thisexperiment was increased by an order of magnitude to achievesufficient signal-to-noise, the GSB is very long-lived, with ca.30% of the signal still remaining after 1.0 μs (Figure 9, inset).The main panel of Figure 9 demonstrates that the GSB doesnot fully decay until ca. 100 μs, demonstrating that the charge-separated state in this bilayer is exceptionally long-lived. Amultiexponential fit (solid fit line) to the transient revealed fourtime constants of 11 ± 3.7 ns (40%), 114 ± 40 ns (27%), 1.9 ±0.6 μs (19%), and 74.6 ± 22.6 μs (14%). These exceptionallylong lifetimes for the charge-separated state in our s-SWCNT/

Figure 7. Transient absorption kinetic profiles of (6,5) s-SWCNT−PDI-based acceptor bilayers excited at 400 nm, probed at (a) (6,5) s-SWCNTS11 ground-state bleaching and (b) (6,5) s-SWCNT trion absorption and early time transients of the (c) (6,5) s-SWCNT−hPDI2-pyr-hPDI2 bilayerand the (d) (6,5) s-SWCNT−Trip-hPDI2 bilayer.

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PDI bilayers (ns to μs) range from 2 to 6 orders of magnitudelonger than those observed for SWCNTs interfaced withindividual PDI molecules in solution (i.e., 10s of ps).20−22

The time-resolved spectroscopic studies in the previoussections reveal that PHT across the (6,5) s-SWCNT-hPDI2-pyr-hPDI2 bilayer (1) proceeds more slowly and (2) yields alonger lifetime for the charge-separated state than that of (6,5)s-SWCNT−Trip-hPDI2 bilayer. The biphasic PHT kinetics of

the hPDI2-pyr-hPDI2 bilayer suggest the possibility of twopopulations of hPDI2-pyr-hPDI2 molecules that undergoPHT to the (6,5) s-SWCNT layer. From the above discussions,these two populations are likely hPDI2-pyr-hPDI2 monomersand hPDI2-pyr-hPDI2 aggregates. The smaller amplitude forthe relatively fast PHT component (τPHT ≈ 1.2 ± 1.11 ps, 39%)is consistent with this component reflecting the PHT dynamicsfrom monomers, where the PHT event must compete withdownhill energy transfer to hPDI2-pyr-hPDI2 aggregates (τET≈ 0.54 ± 0.23 ps). This assignment is also consistent with thesimilar time scale for this PHT component and the PHTdynamics observed for the Trip-hPDI2 bilayer, where thephotophysics are dominated by monomers. In turn, the slowerPHT component (τPHT ≈ 17 ± 8.7 ps, 60%) reflects holetransfer from hPDI2-pyr-hPDI2 aggregates, where the PHTevent must compete with the multiexponential decay of theaggregate excited state. Figure 10 summarizes the kineticscheme derived from these TA measurements.It is useful to try and understand these results in light of the

interfacial energetics and interfacial structure present for eachbilayer. As discussed above, exciton dissociation at the donor−acceptor heterojunction is intimately connected to the excitondissociation driving force (ΔGPHT) and charge transferreorganization energy.7 As shown in Table 1, the estimatedΔGPHT value for the hPDI2-pyr-hPDI2 bilayer is approx-imately 0.18 eV smaller in magnitude than the ΔGPHTestimated for the Trip-hPDI2 bilayer, when assuming holetransfer proceeds from hPDI2-pyr-hPDI2 aggregates. How-ever, the ΔGPHT values are likely roughly equivalent if holetransfer proceeds from hPDI2-pyr-hPDI2 monomers. Thus, itis feasible that the nearly equivalent fast PHT dynamics (τPHT≈ 1.2−1.6 ps) observed for both types of PDI monomers reflectthe similar PHT driving force values and in turn that the slowerPHT dynamics observed for hPDI2-pyr-hPDI2 aggregates(τPHT ≈ 17 ± 8.7 ps) reflect the reduction of ΔGPHT that occurswhen excitons are localized onto the hPDI2-pyr-hPDI2aggregates. While this assessment makes sense qualitatively,there is still significant uncertainty around the extent to whichdriving force affects exciton dissociation in (6,5) s-SWCNT−PDI-based acceptor bilayers, since these two acceptors providea limited range of driving force. Additionally, since themolecular geometries of these PDI-based acceptors are quitedifferent from fullerenes, the reorganization energy associatedwith PHT at the s-SWCNT/PDI interfaces may differappreciably from that of s-SWCNT/fullerene interfaces. Thus,at this point, it is unclear whether these donor/acceptor pairsoccupy the normal or inverted regime, as described by theMarcus formulation of electron transfer.6,31

The unique molecular geometries of the PDI acceptors mayalso influence the PHT and recombination kinetics bymodifying the electronic coupling and/or interfacial molecularstructure present at the s-SWCNT/PDI interface. A compar-ison to previously studied s-SWCNT/C60 interfaces is usefulhere. Since fullerenes are spherical molecules, and C60 hasparticularly high rotational symmetry, there should besignificant overlap between the π-electron clouds at the s-SWCNT/fullerene interface. The three-dimensional structureof fullerene films has also been suggested to decrease geminaterecombination, since electrons can rapidly diffuse away fromthe interface following exciton dissociation.18,39,40 In contrast,as illustrated by Figure 1, achieving good cofacial π−πinteractions at the s-SWCNT/PDI interface will dependsensitively on the orientation adopted by each PDI molecule

Figure 8. Normalized transient absorbance kinetics for the S11 GSBand the trion absorption for the (6,5) s-SWCNT/hPDI2-pyr-hPDI2bilayer. The traces are normalized at long pump−probe delays. (a)Dynamics over a full ∼5 ns TA window to highlight the matchingkinetics at long time scales and (b) over a 200 ps window to highlightthe deviation of the two transients at early pump−probe delays, withthe GSB displaying a fast rise and decay that is not observed in thetrion dynamics.

Figure 9. Microsecond TA dynamics of the (6,5) s-SWCNT/hPDI2-pyr-hPDI2 bilayer following photoexcitation at 400 nm. Inset: TAdynamics over the first 1.0 μs.

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or aggregate in the immediate vicinity of a (6,5) s-SWCNTbundle. Additionally, the absorption spectra in Figure 2 suggestthat intermolecular interactions occur to a larger degree withinthe hPDI2-pyr-hPDI2 layer than in the Trip-hPDI2 layer.These factors may contribute to biphasic PHT kineticsobserved for the hPDI2-pyr-hPDI2 bilayer. For example, it isfeasible that the electronic coupling between hPDI2-pyr-hPDI2 aggregates and the (6,5) s-SWCNT layer is reducedrelative to monomer/(6,5) coupling, which could help toexplain the slower PHT kinetics for aggregates. It is alsoimportant to note that a random-walk-based diffusion ofFrenkel excitons within hPDI2-pyr-hPDI2 aggregates couldcontribute to the slower PHT time, since only excitons at theimmediate interface with the s-SWCNT layer will likelyundergo efficient PHT.Finally, we note that the prevalence of longer-range π−π

stacking in the hPDI2-pyr-hPDI2 layer is likely beneficial forprolonging the charge-separated state in that bilayer, since theelectron can move away from the interface within an aggregateto reduce geminate recombination. In contrast, the lowerdegree of π−π interaction in the Trip-hPDI2 layer mayprohibit the electron from successfully diffusing away from theinterface following interfacial exciton dissociation, enhancingthe probability of geminate recombination and shortening thelifetime of the charge-separated state.

■ CONCLUSIONSIn this study, we performed transient absorption studies on theheterojunction between (6,5) s-SWCNT and two PDI-basedacceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2. Probing therepresentative spectroscopic signatures from the monochiral s-SWCNT layer, namely, the S11 ground-state bleach and trionabsorption, we were able to track the photoinduced holetransfer process in real time. For both bilayers, PHT occurs ona picosecond time scale, generating long-lived charge-separatedstates. Although PHT occurs more rapidly for the Trip-hPDI2bilayer, recombination is also faster. Steady-state absorption,PL, and TA measurements suggest that rapid energy transferfrom monomers to aggregates occurs within hPDI2-pyr-hPDI2layers and that the slower PHT and recombination dynamicsfor these layers must be analyzed with both monomer andaggregate populations in mind. We discuss the potential effectsof aggregate formation within hPDI2-pyr-hPDI2 layers byconsidering the concomitant effects on exciton dissociationdriving force, donor−acceptor electronic coupling, andinterfacial molecular structure. In our current study, we cannotconclusively determine the degree to which each of these

factors influence the PHT and recombination kinetics.However, future studies (both experimental and computa-tional) can shed light on how the different moleculargeometries, dimensionality, and frontier orbital energies ofPDI-based acceptors influence important factors for excitondissociation at s-SWCNT/PDI heterojunctions, such asreorganization energy, electronic coupling, and Coulombscreening of separated electrons and holes. Importantly, thePDI acceptors utilized here harvest significantly more light inthe visible region of the solar spectrum than typical fullerene-based acceptors. Since exciton dissociation at these interfacesproduce charge-separated species that live substantially longerthan 1.0 μs, these results demonstrate that this system ispromising for efficient solar energy harvesting in the visibleregion. PDI-based acceptors have been developed to the levelthat they can substitute fullerene derivative acceptors in organicphotovoltaics, so the systems introduced here provide a routetoward better fundamental understanding of exciton dissocia-tion at model interfaces between organic semiconductors andnonfullerene acceptors.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.8b01400.

DFT-optimized XYZ coordinates of PDI-based accept-ors; absorption spectra of the bilayers of (6,5) s-SWCNT−PDI-based acceptors with different layerthicknesses of the PDI-based acceptors; calculation ofionization potential and electron affinity for (6,5) s-SWCNT; transient absorption kinetic features of PDI-based acceptor neat films; tabulated subnanosecondtransient absorption kinetics of (6,5) s-SWCNT ground-state bleaching of the bilayers of (6,5) s-SWCNT−PDI-based acceptors; transient absorption spectra of thebilayers of (6,5) s-SWCNT−PDI-based acceptors fromphotoinduced electron transfer (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jeffrey.blackburn@nrel.gov.

ORCIDSamuel Peurifoy: 0000-0003-3875-3035Dylan H. Arias: 0000-0003-2358-6967Colin Nuckolls: 0000-0002-0384-5493

Figure 10. Kinetic scheme deduced from TA measurements in this study. The τx values represent characteristic time constants extracted fromexponential fits of TA dynamics, where τR, τET, and τPHT are the characteristic time constants for recombination, energy transfer, and photoinducedhole transfer, respectively. These time constants are also tabulated in the SI, Tables S1−S3. Note that time constants with relatively large uncertaintyvalues and/or low amplitudes are not included in this kinetic scheme.

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Jeffrey L. Blackburn: 0000-0002-9237-5891Author Contributions#T.J.S. and S.P. contributed equally to this work

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was funded by the Solar PhotochemistryProgram, Division of Chemical Sciences, Geosciences, andBiosciences, Office of Basic Energy Sciences, U.S. Departmentof Energy (DOE). NREL is supported by the DOE underContract No. DE-AC36-08GO28308. The U.S. Governmentretains (and the publisher, by accepting the article forpublication, acknowledges that the U.S. Government retains)a nonexclusive, paid up, irrevocable, worldwide license topublish or reproduce the published form of this work, or allowothers to do so, for U.S. Government purposes. The synthesesof PDI acceptors from C.N. were supported by the U.S.Department of Energy (DOE) under Award No. DE-SC0014563 and the Office of Naval Research (ONR) underAward No. N00014-17-1-2205. The authors thank AndrewFerguson for helpful discussion.

■ REFERENCES(1) Shea, M. J.; Arnold, M. S. 1% Solar Cells Derived from UltrathinCarbon Nanotube Photoabsorbing Films. Appl. Phys. Lett. 2013, 102,243101.(2) 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. PolychiralSemiconducting Carbon Nanotube−Fullerene Solar Cells. Nano Lett.2014, 14, 5308−5314.(3) Ye, Y.; Bindl, D. J.; Jacobberger, R. M.; Wu, M.-Y.; Roy, S. S.;Arnold, M. S. Semiconducting Carbon Nanotube Aerogel BulkHeterojunction Solar Cells. Small 2014, 10, 3299−3306.(4) Guillot, S. L.; Mistry, K. S.; Avery, A. D.; Richard, J.; Dowgiallo,A.-M.; Ndione, P. F.; van de Lagemaat, J.; Reese, M. O.; Blackburn, J.L. Precision Printing and Optical Modeling of Ultrathin Swcnt/C60Heterojunction Solar Cells. Nanoscale 2015, 7, 6556−6566.(5) Bindl, D. J.; Ferguson, A. J.; Wu, M.-Y.; Kopidakis, N.; Blackburn,J. L.; Arnold, M. S. Free Carrier Generation and Recombination inPolymer-Wrapped Semiconducting Carbon Nanotube Films andHeterojunctions. J. Phys. Chem. Lett. 2013, 4, 3550−3559.(6) Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.;Nicholas, R. J.; Snaith, H. J. Carbon Nanotube/Polymer Compositesas a Highly Stable Hole Collection Layer in Perovskite Solar Cells.Nano Lett. 2014, 14, 5561−5568.(7) Ihly, R.; Mistry, K. S.; Ferguson, A. J.; Clikeman, T. T.; Larson, B.W.; Reid, O.; Boltalina, O. V.; Strauss, S. H.; Rumbles, G.; Blackburn,J. L. Tuning the Driving Force for Exciton Dissociation in Single-Walled Carbon Nanotube Heterojunctions. Nat. Chem. 2016, 8, 603.(8) Schulz, P.; Dowgiallo, A.-M.; Yang, M.; Zhu, K.; Blackburn, J. L.;Berry, J. J. Charge Transfer Dynamics between Carbon Nanotubes andHybrid Organic Metal Halide Perovskite Films. J. Phys. Chem. Lett.2016, 7, 418−425.(9) Blackburn, J. L. Semiconducting Single-Walled Carbon Nano-tubes in Solar Energy Harvesting. ACS Energy Letters 2017, 2, 1598−1613.(10) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley,R. E.; Weisman, R. B. Structure-Assigned Optical Spectra of Single-Walled Carbon Nanotubes. Science 2002, 298, 2361−2366.(11) Wheeler, L. M.; Moore, D. T.; Ihly, R.; Stanton, N. J.; Miller, E.M.; Tenent, R. C.; Blackburn, J. L.; Neale, N. R. SwitchablePhotovoltaic Windows Enabled by Reversible Photothermal ComplexDissociation from Methylammonium Lead Iodide. Nat. Commun.2017, 8, 1722.

(12) Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J. Highly SelectiveDispersion of Single-Walled Carbon Nanotubes Using AromaticPolymers. Nat. Nanotechnol. 2007, 2, 640.(13) Mistry, K. S.; Larsen, B. A.; Blackburn, J. L. High-YieldDispersions of Large-Diameter Semiconducting Single-Walled CarbonNanotubes with Tunable Narrow Chirality Distributions. ACS Nano2013, 7, 2231−2239.(14) Pochorovski, I.; Wang, H.; Feldblyum, J. I.; Zhang, X.; Antaris,A. L.; Bao, Z. H-Bonded Supramolecular Polymer for the SelectiveDispersion and Subsequent Release of Large-Diameter Semiconduct-ing Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2015, 137,4328−4331.(15) Dowgiallo, A.-M.; Mistry, K. S.; Johnson, J. C.; Blackburn, J. L.Ultrafast Spectroscopic Signature of Charge Transfer between Single-Walled Carbon Nanotubes and C60. ACS Nano 2014, 8, 8573−8581.(16) Ihly, R.; Dowgiallo, A.-M.; Yang, M.; Schulz, P.; Stanton, N. J.;Reid, O. G.; Ferguson, A. J.; Zhu, K.; Berry, J. J.; Blackburn, J. L.Efficient Charge Extraction and Slow Recombination in Organic-Inorganic Perovskites Capped with Semiconducting Single-WalledCarbon Nanotubes. Energy Environ. Sci. 2016, 9, 1439−1449.(17) Dowgiallo, A.-M.; Mistry, K. S.; Johnson, J. C.; Reid, O. G.;Blackburn, J. L. Probing Exciton Diffusion and Dissociation in Single-Walled Carbon Nanotube−C60 Heterojunctions. J. Phys. Chem. Lett.2016, 7, 1794−1799.(18) Liu, T.; Troisi, A. What Makes Fullerene Acceptors Special asElectron Acceptors in Organic Solar Cells and How to Replace Them.Adv. Mater. 2013, 25, 1038−1041.(19) Nielsen, C. B.; Holliday, S.; Chen, H.-Y.; Cryer, S. J.;McCulloch, I. Non-Fullerene Electron Acceptors for Use in OrganicSolar Cells. Acc. Chem. Res. 2015, 48, 2803−2812.(20) Hahn, U.; Engmann, S.; Oelsner, C.; Ehli, C.; Guldi, D. M.;Torres, T. Immobilizing Water-Soluble Dendritic Electron Donors andElectron AcceptorsPhthalocyanines and PerylenediimidesontoSingle Wall Carbon Nanotubes. J. Am. Chem. Soc. 2010, 132, 6392−6401.(21) Oelsner, C.; Schmidt, C.; Hauke, F.; Prato, M.; Hirsch, A.;Guldi, D. M. Interfacing Strong Electron Acceptors with Single WallCarbon Nanotubes. J. Am. Chem. Soc. 2011, 133, 4580−4586.(22) Tsarfati, Y.; Strauss, V.; Kuhri, S.; Krieg, E.; Weissman, H.;Shimoni, E.; Baram, J.; Guldi, D. M.; Rybtchinski, B. DispersingPerylene Diimide/Swcnt Hybrids: Structural Insights at the MolecularLevel and Fabricating Advanced Materials. J. Am. Chem. Soc. 2015,137, 7429−7440.(23) Li, C.; Wonneberger, H. Perylene Imides for OrganicPhotovoltaics: Yesterday, Today, and Tomorrow. Adv. Mater. 2012,24, 613−636.(24) Howard, I. A.; Laquai, F.; Keivanidis, P. E.; Friend, R. H.;Greenham, N. C. Perylene Tetracarboxydiimide as an ElectronAcceptor in Organic Solar Cells: A Study of Charge Generation andRecombination. J. Phys. Chem. C 2009, 113, 21225−21232.(25) Shivanna, R.; Shoaee, S.; Dimitrov, S.; Kandappa, S. K.; Rajaram,S.; Durrant, J. R.; Narayan, K. S. Charge Generation and Transport inEfficient Organic Bulk Heterojunction Solar Cells with a PeryleneAcceptor. Energy Environ. Sci. 2014, 7, 435−441.(26) Sun, D.; Meng, D.; Cai, Y.; Fan, B.; Li, Y.; Jiang, W.; Huo, L.;Sun, Y.; Wang, Z. Non-Fullerene-Acceptor-Based Bulk-HeterojunctionOrganic Solar Cells with Efficiency over 7%. J. Am. Chem. Soc. 2015,137, 11156−11162.(27) Zhong, Y.; Trinh, M. T.; Chen, R.; Wang, W.; Khlyabich, P. P.;Kumar, B.; Xu, Q.; Nam, C.-Y.; Sfeir, M. Y.; Black, C.; et al. EfficientOrganic Solar Cells with Helical Perylene Diimide Electron Acceptors.J. Am. Chem. Soc. 2014, 136, 15215−15221.(28) Liu, Y.; Lai, J. Y. L.; Chen, S.; Li, Y.; Jiang, K.; Zhao, J.; Li, Z.;Hu, H.; Ma, T.; Lin, H.; et al. Efficient Non-Fullerene Polymer SolarCells Enabled by Tetrahedron-Shaped Core Based 3d-Structure Small-Molecular Electron Acceptors. J. Mater. Chem. A 2015, 3, 13632−13636.(29) Sisto, T. J.; Zhong, Y.; Zhang, B.; Trinh, M. T.; Miyata, K.;Zhong, X.; Zhu, X. Y.; Steigerwald, M. L.; Ng, F.; Nuckolls, C. Long,

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b01400J. Phys. Chem. C XXXX, XXX, XXX−XXX

K

Atomically Precise Donor−Acceptor Cove-Edge Nanoribbons asElectron Acceptors. J. Am. Chem. Soc. 2017, 139, 5648−5651.(30) Bochevarov, A. D.; Harder, E.; Hughes, T. F.; Greenwood, J. R.;Braden, D. A.; Philipp, D. M.; Rinaldo, D.; Halls, M. D.; Zhang, J.;Friesner, R. A. Jaguar: A High-Performance Quantum ChemistrySoftware Program with Strengths in Life and Materials Sciences. Int. J.Quantum Chem. 2013, 113, 2110−2142.(31) Coffey, D. C.; Larson, B. W.; Hains, A. W.; Whitaker, J. B.;Kopidakis, N.; Boltalina, O. V.; Strauss, S. H.; Rumbles, G. An OptimalDriving Force for Converting Excitons into Free Carriers in ExcitonicSolar Cells. J. Phys. Chem. C 2012, 116, 8916−8923.(32) Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V. PhotoinducedElectron Transfer from Semiconductor Quantum Dots to Metal OxideNanoparticles. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 29−34.(33) Zhu, H.; Yang, Y.; Hyeon-Deuk, K.; Califano, M.; Song, N.;Wang, Y.; Zhang, W.; Prezhdo, O. V.; Lian, T. Auger-Assisted ElectronTransfer from Photoexcited Semiconductor Quantum Dots. Nano Lett.2014, 14, 1263−1269.(34) Friend, R. H.; Phillips, M.; Rao, A.; Wilson, M. W. B.; Li, Z.;McNeill, C. R. Excitons and Charges at Organic SemiconductorHeterojunctions. Faraday Discuss. 2012, 155, 339−348.(35) Larson, B. W.; Whitaker, J. B.; Wang, X.-B.; Popov, A. A.;Rumbles, G.; Kopidakis, N.; Strauss, S. H.; Boltalina, O. V. ElectronAffinity of Phenyl−C61−Butyric Acid Methyl Ester (Pcbm). J. Phys.Chem. C 2013, 117, 14958−14964.(36) Yuma, B.; Berciaud, S.; Besbas, J.; Shaver, J.; Santos, S.; Ghosh,S.; Weisman, R. B.; Cognet, L.; Gallart, M.; Ziegler, M.; et al.Biexciton, Single Carrier, and Trion Generation Dynamics in Single-Walled Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys.2013, 87, 205412.(37) Matsunaga, R.; Matsuda, K.; Kanemitsu, Y. Observation ofCharged Excitons in Hole-Doped Carbon Nanotubes Using Photo-luminescence and Absorption Spectroscopy. Phys. Rev. Lett. 2011, 106,037404.(38) Park, J. S.; Hirana, Y.; Mouri, S.; Miyauchi, Y.; Nakashima, N.;Matsuda, K. Observation of Negative and Positive Trions in theElectrochemically Carrier-Doped Single-Walled Carbon Nanotubes. J.Am. Chem. Soc. 2012, 134, 14461−14466.(39) Hammond, M. R.; Kline, R. J.; Herzing, A. A.; Richter, L. J.;Germack, D. S.; Ro, H.-W.; Soles, C. L.; Fischer, D. A.; Xu, T.; Yu, L.;et al. Molecular Order in High-Efficiency Polymer/Fullerene BulkHeterojunction Solar Cells. ACS Nano 2011, 5, 8248−8257.(40) Chen, W.; Xu, T.; He, F.; Wang, W.; Wang, C.; Strzalka, J.; Liu,Y.; Wen, J.; Miller, D. J.; Chen, J.; et al. Hierarchical Nano-morphologies Promote Exciton Dissociation in Polymer/FullereneBulk Heterojunction Solar Cells. Nano Lett. 2011, 11, 3707−3713.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b01400J. Phys. Chem. C XXXX, XXX, XXX−XXX

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