Non-fullerene Acceptors for Harvesting Excitons fromSemiconducting Carbon NanotubesJialiang Wang,† Samuel R. Peurifoy,‡ Michael T. Bender,§ Fay Ng,‡ Kyoung-Shin Choi,§

Colin Nuckolls,‡ and Michael S. Arnold*,†

†Department of Materials Science and Engineering, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States‡Department of Chemistry, Columbia University, New York, New York 10027, United States§Department of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States

*S Supporting Information

ABSTRACT: Semiconducting single-walled carbon nanotubes(s-SWCNTs) are promising materials for solar energyconversion and photodetectors. Fullerenes and their deriva-tives, being widely employed as electron acceptors in s-SWCNT photovoltaic devices, effectively dissociate s-SWCNTexcitons by forming heterojunctions with favorable energeticoffsets. However, their limited tunability and poor co-processability with s-SWCNTs in blends have been majorobstacles for further improving the device performance. Here,we investigate the exciton dissociating capability of a series ofnon-fullerene acceptors (NFAs) based on indacenodithiophene and perylene diimides, by measuring internal quantumefficiency (QE) for exciton dissociation and electron transfer in bilayer s-SWCNT/NFA devices. A maximum internal QE of∼50% is achieved with a (6,5) s-SWCNT/indacenodithiophene-based acceptor heterojunction. Our results indicate that non-fullerene acceptors with deeper electron affinities could potentially replace traditional fullerene acceptors in s-SWCNTphotovoltaic devices.

■ INTRODUCTIONSemiconducting single-walled carbon nanotubes (s-SWCNTs)have attracted significant attention as light absorbing materialsin photovoltaic (PV) devices and photodetectors1−11 becauseof their strong optical absorptivity,12,13 solution processability,and tunable bandgaps spanning from visible to infraredwavelengths. s-SWCNTs are excitonic with binding energiesof 0.2−0.3 eV.14,15 Therefore, in a typical s-SWCNT PV cell,photocurrent is mostly generated at a type-II heterojunctionbetween the s-SWCNTs and a layer of a complementarysemiconductor that provides a sufficient energetic band offsetgreater than the exciton binding energy. This offset dissociatesthe exciton resulting in electron transfer from nanotube to theacceptor, and dissociation and electron-transfer efficiencies ofover 85% have been obtained for favorable small-diameter s-SWCNTs and acceptor pairs.1

Over the past years, great progress has been made onimproving the exciton harvesting efficiency of s-SWCNT PVdevices.3,8,11,16−18 The highest reported external quantumefficiency (QE) [the fraction of collected electron−hole pairsper incident photon] to-date at the nanotube S11 opticalbandgap is 49%, obtained by employing relatively pristine,shear-mixed, nearly monochiral (6,5) s-SWCNTs with an S11resonance at 1000 nm in a bilayer planar heterojunction withthe fullerene electron-acceptor C60.

2 However, the optimized s-SWCNT layer thickness in these bilayer heterojunctions is

only 5−10 nm due to poor out-of-plane exciton diffusion,limiting the external QE away from the S11 transition to ≪49%due to weaker absorption. More soluble fullerene derivatives,such as phenyl-C61-butyric acid methyl ester (PC61BM) andphenyl-C71-butyric acid methyl ester (PC71BM), have beenblended with s-SWCNTs in attempts to create bulkheterojunction devices that increase the heterointerfacial areabetween the two components so that excitons can be morereadily dissociated, even in thicker films with stronger, morebroadband absorption. To-date, the highest reported externalQE at the nanotube S11 transition in blended heterojunctions is19%, less than in bilayers.4 One explanation for the relativelypoor performance may be that PCBM/s-SWCNT blends sufferfrom excessive phase separation and morphological inhomo-geneity, resulting in inefficient exciton harvesting and/ordiscontinuous pathways for charge carrier collection.19

Fullerenes have been widely used as electron acceptors in s-SWCNT PV devices because of their suitable lowestunoccupied molecular orbital (LUMO) energy levels andrelatively high electron mobility.18,20 Moreover, s-SWCNT/fullerene heterojunctions show excellent exciton dissociationcapabilities with low energy losses due to a small reorganiza-

Received: July 4, 2019Revised: August 3, 2019Published: August 9, 2019

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C 2019, 123, 21395−21402

© 2019 American Chemical Society 21395 DOI: 10.1021/acs.jpcc.9b06381J. Phys. Chem. C 2019, 123, 21395−21402

Dow

nloa

ded

via

CO

LU

MB

IA U

NIV

on

July

23,

202

0 at

21:

50:2

0 (U

TC

).Se

e ht

tps:

//pub

s.ac

s.or

g/sh

arin

ggui

delin

es f

or o

ptio

ns o

n ho

w to

legi

timat

ely

shar

e pu

blis

hed

artic

les.

pubs.acs.org/JPCChttp://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.jpcc.9b06381http://dx.doi.org/10.1021/acs.jpcc.9b06381

tional energy associated with photoinduced electron transferand the absence of charge-transfer (CT) states.10,21 Theunderlying dynamics relating charge carrier generation, trans-port, and recombination has also been extensively investigatedin this energy conversion system.22,23 However, there aredrawbacks to using fullerene acceptors in s-SWCNT PVdevices, such as their weak optical absorptivity24 andphotochemical instability.25−27 In addition, the poor co-processability of s-SWCNTs with fullerenes in blends hasmade it difficult to move beyond bilayers to generally moreefficient blended heterojunction device architectures.In recent years, non-fullerene acceptors have been developed

as alternatives that have outperformed traditional fullereneacceptors in small molecule and polymer solar cells.28−32

These non-fullerene acceptors offer several advantages: (1)tunable energy levels that can better match the levels of a givendonor material;33 (2) stronger and broader spectral absorptionenabling more complementary donor/acceptor photon cap-ture;34,35 (3) improved stability;36 and/or (4) more econom-ical synthesis.37 The potential of non-fullerene acceptors hasrecently motivated researchers to explore the photophysics ofs-SWCNT/non-fullerene acceptor systems. In an initial study,Kang et al. reported on the existence of long-lived, separatedphotogenerated charges in (6,5) s-SWCNT/perylene diimide(PDI)-based acceptor bilayers and investigated photoinducedcharge-transfer dynamics.38 This work points to the promise ofusing non-fullerene acceptors in s-SWCNT photovoltaicdevices.Here, we investigate non-fullerene acceptors recently

developed for small molecule and polymer photovoltaic cells,with the goal of determining if these acceptors are capable ofefficiently driving s-SWCNT exciton dissociation and photo-excited electron transfer from s-SWCNT to the acceptor. Twoclasses of non-fullerene acceptors, indacenodithiophenes andperylene diimides, along with C60, PC61BM, and PC71BM arepaired with six different (n,m) chiralities of s-SWCNTs in thinbilayer heterojunctions relatively free of exciton and charge-transport limitations. The internal quantum efficiency (QE)[the fraction of collected electron−hole pairs per absorbedphoton] is quantified for 43 of the resulting s-SWCNT/acceptor combinations to determine the efficiency of excitondissociation and photoexcited electron transfer for each pair.The LUMO energy levels of the acceptors are quantified viasolution-phase cyclovoltammetry, and the relationship betweeninternal QE and the energetic driving force for electron transferis compared against the Marcus electron-transfer theory. Theultimate goal of this direction of research is to identify newacceptors that are as efficient of electron acceptors as fullerenessuch as C60 but that offer enhanced, more complementaryabsorption, improved stability, and/or improved co-process-ability in blends.

■ EXPERIMENTAL METHODSPreparation of s-SWCNTs. s-SWCNTs are selectively

separated using methods adopted from Nish et al. and Graf etal.39,40 To prepare (6,5)-enriched s-SWCNTs, poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-cobipyridine] (PFO-BPy) (Amer-ican Dye Source, ADS153UV, Lot #18H004B1) is firstdissolved in toluene at a concentration of 1 mg mL−1 bymagnetic stirring and heating. CoMoCAT SG65i (Sigma, Lot#MKBN5945V) (0.5 mg mL−1) is added to the above polymersolution, and the mixture is shear force mixed (Silverson L5M-A) for 24 h with a total volume of 600 mL. The resulting

dispersion is then centrifuged at 300 000g for 10 min toremove the undispersed material. The top supernatant iscollected and filtered through a 5 μm PTFE membrane toremove residual impurities. The bottom pellet is mixed withthe fresh PFO-BPy/toluene solution, and the above shear forcemixing and centrifugation steps are repeated. The combinedsupernatant is distilled under reduced pressure with a rotaryevaporator (IKA RV 10), and the obtained powder isredispersed in hot tetrahydrofuran (THF). The dispersion isthen centrifuged at 50 000g at 21 °C for 24 h to precipitate thes-SWCNTs and leave the free polymer in the supernatant. Theresulting pellets of s-SWCNTs are redispersed into fresh THFand centrifuged again. The above steps are repeated for fivetimes to remove the excess polymer, and the final ratio ofpolymer to s-SWCNTs is about 1:1 by weight. Finally, the s-SWCNT pellets are dispersed in o-dichlorobenzene (ODCB)with mild tip sonication.To prepare multichiral small-bandgap s-SWCNTs, poly(9,9-

dioctylfluorene-2,7-diyl) (PFO) (American Dye Source,ADS329BE, Lot #15L008A1) and HiPCO-grown SWCNTs(NanoIntegris, Batch #H24-118R) are mixed together andsonicated at 35% amplitude for 30 min (Fisher model 500horn sonicator, 400 W). The remainder of the processing stepsis the same as above.

Electron Acceptors. C60 is purchased from Sigma (Lot#MKBZ0154V). PC61BM and PC71BM are purchased fromNano-C. ITIC-4F (3,9-bis(2-methylene-((3-(1,1-dicyano-methylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hex-ylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene) is purchased from 1-Material Inc. All chemicalsare used as-received. Perylene diimide compounds aresynthesized according to previously reported methods,31,41,42

and chemical structures are listed in the SupportingInformation.

Photovoltaic Device Fabrication. Indium tin oxide(ITO)-coated glass substrates (15 Ω sq−1, Prazisions Glas &Optik) are first cleaned with acetone and isopropyl alcohol andexposed to ultraviolet-ozone for 15 min. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PE-DOT:PSS, Clevios Al 4083) is spin-coated onto ITOsubstrates at 3000 rpm for 2 min and annealed at 120 °Cfor 30 min in ambient. The s-SWCNT layer is deposited viablade-casting and further soaked in boiling toluene for 30 minto remove excess polymers and other impurities. The thicknessof the s-SWCNT layer is restricted to

with wavelength cutoffs of 420, 680, and 1090 nm are used toprevent excitation of the sample by higher-energy multiplepassthrough from the monochromator. The beam is modulatedwith a mechanical chopper, and the intensity is measured withNewport 818-UV and 818-IR photodetectors, which arecalibrated over the range of 300−1088 and 780−1450 nm,respectively. The device photocurrent is measured with a lock-in amplifier (Stanford Research Systems, Model SR830) anddivided by the photon flux measured by the photodetectors tocalculate external QE. Optical reflectance spectra, R, aremeasured and fit to determine the fraction of incident photonsabsorbed at the S11 bandgap transition of various chiralities ofs-SWCNTs, ηA. The internal QE is calculated as external QE/ηA. The internal QE is measured for five different devices foreach s-SWCNT/acceptor pair.

■ RESULTS AND DISCUSSIONFive different non-fullerene acceptors are evaluated, and theirchemical structures are presented in Figure S1. The first is anindacenodithiophene (3,9-bis(2-methylene-((3-(1,1-dicyano-methylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hex-ylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene, ITIC-4F), which has been used as an electronacceptor in conjunction with PBDB-T-SF (a fluorinatedderivative of a co-polymer consisting of alternating 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c′]-dithiophene-4,8-dione and (4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5- b′]dithiophene-2,6-diyl) units) as donorsin photovoltaic devices with external QE as high as 83%.35 Theremainder are perylene diimide (PDI)-based acceptors,specifically hPDI4 (four PDI units fused through their baypositions to two-carbon bridges), hPDI3-Pyr-hPDI3 (twohPDI3 ribbons fused through their bay positions to a pyrenebridge), Trip-hPDI2 (three hPDI2 ribbons fused through theirbay positions to a triptycene hub), and Trip-hPDI3 (threehPDI3 ribbons fused through their bay positions to atriptycene hub). These PDIs have been employed inphotovoltaic devices in conjunction with PTB7 and PTB7-Th (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]-dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]-thieno[3,4-b]thiophenediyl]] and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-2-carboxylate-2-6-diyl)], respectively), reaching external QE ashigh as 70%.29,31 C60, PC61BM and PC71BM are tested, forcomparison. The absorption spectra of C60 and tworepresentative non-fullerene acceptors, ITIC-4F and hPDI4,are shown in Figure 1. The absorbance spectra of the

remainder of these acceptors and their chemical structure arepresented in Figure S2 and discussed in the text of theSupporting Information. These electron acceptors all haverelatively deep LUMOs with the possibility of forming a type-IIheterojunction with s-SWCNTs that drive exciton dissociationand electron transfer from photoexcited s-SWCNT to theacceptor. Additionally, these acceptors strongly absorb overvisible wavelengths but not in the near-infrared where s-SWCNTs absorb, facilitating the quantification of photo-current originating exclusively from excitons generated in the s-SWCNTs.The acceptors are paired in bilayer planar heterojunctions

with six different chiralities of s-SWCNTs with different opticalbandgaps and LUMO to systematically investigate the effect ofthermodynamic driving force on the efficiency of excitondissociation and charge transfer at the donor/acceptorinterface. Nearly, monochiral (6,5) s-SWCNTs are extractedfrom as-produced CoMoCAT-grown material using theconjugated polymer PFO-BPy, whereas (7,5), (7,6), (8,6),(8,7), (9,7) chirality-mixed s-SWCNTs are extracted from as-produced HiPCO-grown material using PFO. Each of these s-SWCNTs has well-resolved absorbance features in the near-infrared in the range of 1000−1400 nm, with S11 opticalbandgaps spanning from 0.92 to 1.24 eV, as seen in Figure 1.The architecture of the bilayer device stack used to measure

exciton dissociation and charge-transfer efficiency is shown inFigure 2A. Upon illumination of the s-SWCNTs at the S11transition, a photocurrent is generated at zero-bias by thedissociation and separation of photoexcited electron−holepairs at the heterointerface (Figure 2B). The external QE ateach wavelength is determined from the ratio of the measuredphotocurrent to the incident photon flux, whereas the internalQE is determined by normalizing the external QE by ηA. Inturn, ηA is determined by measuring the reflectance from thedevice stack at each wavelength.The internal QE in the near-infrared quantifies the efficiency

by which a heterojunction converts photons absorbed by s-SWCNTs into collected charge carriers. The internal QE isgenerally related to three processes in a planar heterojunction:(1) the efficiency of exciton diffusion to the donor/acceptorheterointerface; (2) the efficiency of exciton dissociation andelectron transfer at the heterointerface; and, (3) the efficiencyby which separated electrons and holes are collected at theelectrodes. For thin s-SWCNT layers (

To determine ηA from reflectance spectra, losses due toabsorption by the ITO, PEDOT:PSS, and Ag and scatteringmust be subtracted. 1 − R quantifies the fraction of photonsthat are not reflected but rather absorbed by the s-SWCNTs orlost to these processes. The 1 − R spectra are fit followingmethods established by Bindl et al.,1,43 to a linear combinationof: Gaussian−Lorentzian sum (GLS) functions for each of the(7,5), (7,6), (8,6), (8,7), and (9,7) s-SWCNTs; a backgroundreflectance spectrum measured from a control device stackfabricated without s-SWCNTs; and a constant backgroundoffset. A small spectral shift (

versus film (e.g., due to solvation effects or the formal potentialof the Fc+/Fc redox couple that is used), these differencesshould be systematic, enabling quantitative comparison ofΔGPET between different s-SWCNT/acceptor pairs.The ΔGPET of the pairs tested is plotted along the horizontal

axis of Figure 3 and specified in Table 1. The ΔGPET vary bymore than 350 meV, from approximately −50 meV for (6,5)/C60 bilayers to 300 meV for (9,7)/Trip-hPDI2 bilayers, wheremore negative ΔGPET specifies that electron transfer isincreasingly exothermic.Figure 3 shows that internal QE generally decreases with

increasing (less exothermic) ΔGPET. The data for all of thedifferent s-SWCNT/acceptor pairs roughly fall onto a singlecurve, in which the internal QE reaches approximately 90% for−50 meV ≤ ΔGPET ≤ 0 meV but then falls off as the drivingforce decreases for ΔGPET > 0 meV, for example, declining to20% for ΔGPET = 100 meV.Ihly et al.21 have shown that the kinetics of electron transfer

from photoexcited s-SWCNTs to fullerene acceptors followthe Marcus electron-transfer theory with a reorganizationenergy, λ. To compare this theory to our experimental data inFigure 3, we must relate electron-transfer rate predicted by thetheory to an electron-transfer yield. In the Marcus theory,47−49

the electron-transfer rate (kPET) is described byÄ

Ç

ÅÅÅÅÅÅÅÅÅÅ

É

Ö

ÑÑÑÑÑÑÑÑÑÑk A

Gk T

exp( )

4PET PETPET

2

B

λλ

= −+ Δ

(1)

where APET is a prefactor that depends on the electroniccoupling strength between the donor and acceptor, λ is thereorganization energy, kB is the Boltzmann constant, and T isthe absolute temperature. Following Ihly et al., the electron-transfer rate can be correlated to the electron-transfer yield,ηPET, as

k k Gk k k G

( )( )PET

CG PET PET

CG loss PET PETη =

+ Δ+ + Δ (2)

where ηPET is equal to the internal QE measured in Figure 3(assuming lossless exciton diffusion and charge collectionbecause of the thin layers), kCG and kloss are the chargegeneration and exciton decay rates in neat carbon nanotubefilms, respectively, and kPET is from eq 1. The sum of kCG and

kloss represents the overall exciton decay rate in neat carbonnanotube layers, which is the inverse of the exciton lifetime. Inthis case, the charge carrier yield in neat carbon nanotube films(ηneat) can be written as

kk kneat

CG

CG lossη =

+ (3)

Expressing the combination of the above equations, in terms ofthe peak yield for carrier generation at the donor/acceptorinterface (ηPET

max ), achieved when GPET = −λ, we arrive atÄÇÅÅÅÅÅÅÅ

ÉÖÑÑÑÑÑÑÑÄ

ÇÅÅÅÅÅÅÅ

ÉÖÑÑÑÑÑÑÑ

G( )exp

1 exp

Gk T

Gk T

PET PET

neat( )

1( )

4

( )

1( )

4

PETmax

neat

PETmax

PET2

B

PETmax

neat

PETmax

PET2

B

ηη

Δ =+ −

+ −

η ηη

λλ

η ηη

λλ

−−

+ Δ

−−

+ Δ

(4)

The experimental data in the aggregate (combining all of the s-SWCNT/acceptor pairs) roughly follow a dependencepredicted by eq 4 with a ηPET

max of 90%, a ηneat of 0.1%, and aλ of 130 meV (dashed gray line, Figure 3), although it is notpossible to constrain the model in the inverted region due tothe lack of acceptors with deeper LUMO. The λ of 130 meV issimilar to the λ extracted from the previous analysis of the s-SWCNT/fullerene derivative system, whereby the intrinsicallysmall reorganization energy of the s-SWCNTs limits theoverall λ of the pair so that it is dominated by the acceptor.21

Each acceptor pair should have a unique λ and ηPETmax .

However, we do not have enough data to separately extractthese parameters for each pair. For example, it is possible to fitthe ηPET versus ΔGPET data for the hPDI4 acceptor to twocases (see Figure S7): one case in which ηPET

max is large (90%),similar to fullerene acceptors, with a λ of 250 meV and anothercase in which ηPET

max is smaller (75%) with a smaller λ of 130meV. In all cases, ηneat is small, less than a few percent, asexpected from the large EB ≫ kBT at room temperature,consistent with the low ηneat measured via time-resolvedmicrowave conductivity measurements.23,50 The developmentof a more thorough understanding of the relationship betweenηPET versus ΔGPET in the future will necessitate the synthesis ofnon-fullerene acceptors with deeper LUMO levels or theisolation of s-SWCNTs with larger bandgaps, thereby

Table 1. Summary of Internal QE (IQE) and Exciton Dissociation Driving Force (ΔGPET)

acceptorsLUMO(eV)

ΔG(6,5) (eV)IQE(6,5)

ΔG(7,5) (eV)IQE(7,5)

ΔG(7,6) (eV)IQE(7,6)

ΔG(8,6) (eV)IQE(8,6)

ΔG(8,7) (eV)IQE(8,7)

ΔG(9,7)(eV)IQE(9,7)

C60 4.07 −0.06 0 0.02 0.06 0.07 0.100.90 ± 0.05 0.85 ± 0.13 0.63 ± 0.09 0.55 ± 0.09 0.26 ± 0.13 0.19 ± 0.13

PC61BM 3.98 0.03 0.09 0.11 0.15 0.16 0.200.73 ± 0.03 0.26 ± 0.03 0.11 ± 0.03 0.09 ± 0.03 0.02 ± 0.01 0.02 ± 0.01

PC71BM 4.00 0.020.75 ± 0.05

ITIC-4F 3.99 0.03 0.09 0.10 0.14 0.15 0.190.50 ± 0.03 0.24 ± 0.08 0.21 ± 0.05 0.17 ± 0.04 0.03 ± 0.01 0.01 ± 0.01

hPDI4 4.00 0.02 0.08 0.09 0.13 0.14 0.180.33 ± 0.08 0.09 ± 0.02 0.07 ± 0.01 0.05 ± 0.01 0.04 ± 0.01 0.05 ± 0.02

hPDI3-Pyr-hPDI3 3.92 0.10 0.16 0.17 0.22 0.22 0.260.08 ± 0.01 0.05 ± 0.01 0.03 ± 0.01 0.02 ± 0.01 0.02 ± 0.01 0.02 ± 0.01

Trip-hPDI2 3.90 0.13 0.19 0.20 0.24 0.25 0.290.17 ± 0.03 0.03 ± 0.01 0.03 ± 0.01 0.02 ± 0.01 0.04 ± 0.01 0.06 ± 0.01

Trip-hPDI3 3.93 0.09 0.14 0.16 0.20 0.21 0.250.26 ± 0.08 0.04 ± 0.01 0.04 ± 0.01 0.03 ± 0.01 0.03 ± 0.01 0.03 ± 0.01

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.9b06381J. Phys. Chem. C 2019, 123, 21395−21402

21399

http://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.9b06381/suppl_file/jp9b06381_si_001.pdfhttp://dx.doi.org/10.1021/acs.jpcc.9b06381

producing a broader range of driving forces (including negativeΔGPET).

■ CONCLUSIONSIn conclusion, non-fullerene acceptors for harvesting s-SWCNT excitons have been systematically investigated usingbilayer heterojunction devices. Among the tested s-SWCNT/non-fullerene acceptor combinations, the highest internal QEvalues have been achieved using (6,5) s-SWCNT/ITIC-4F(50%) and (6,5) s-SWCNT/hPDI4 (30%) pairs. However,these internal QEs are still lower than in correspondingPC61BM and PC71BM devices with similar ΔGPET. Thisdiscrepancy may indicate that the CV method underestimatesthe LUMO energies of the non-fullerene acceptors relative tothose of PC61BM and PC71BM. Alternatively, if the LUMOenergies of the PCBM and these two non-fullerene acceptorsare indeed similar, then this discrepancy may point to otherfactors besides driving force that may suppress internal QE,such as (i) slow electron-transfer kinetics and/or poorelectronic orbital overlap at the heterointerface arising fromdifferences in acceptor shape and size (both of which couldsuppress ηPET

max ); (ii) a larger λ; or (iii) the existence ofinterfacial charge-transfer (CT) states that promote recombi-nation.51−53 Recent experiments have shown an absence of CTstates at s-SWCNT/fullerene heterojunctions;10 however, thepossibility of CT states at s-SWCNT/non-fullerene acceptorheterointerfaces is still not clear.Regardless, the data in Figure 3 show that the internal QE

for s-SWCNT/non-fullerene acceptor pairs should continue toincrease with decreasing ΔGPET. The ΔGPET of (6,5)/non-fullerene acceptor pairs is still 80 meV smaller in magnitudethan the ΔGPET of the most efficient (6,5)/C60 pair. Thus, ourwork indicates that the non-fullerene acceptors tested here arelimited by their relatively shallow LUMO and that non-fullerene acceptors with deeper LUMO will be needed toincrease the efficiency. Overall, this work is important becauseit provides guidance on the need to design and synthesize newelectron acceptors with more favorable, deeper LUMO energylevels54,55 that can potentially replace conventional fullerene-based electron acceptors in s-SWCNT photovoltaic devices.Moreover, it will be important to exploit the structuraltunability of non-fullerene acceptors to realize acceptors thatcan be more favorably co-cast with s-SWCNTs into blendswith more optimized microstructures so that more efficientblended heterojunction devices can be realized.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.9b06381.

Chemical structures, absorption spectra, CV curves, andLUMO level determination of electron acceptors anddetails on internal QE calculation (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: michael.arnold@wisc.edu.ORCIDJialiang Wang: 0000-0002-2946-5480Samuel R. Peurifoy: 0000-0003-3875-3035Kyoung-Shin Choi: 0000-0003-1945-8794

Colin Nuckolls: 0000-0002-0384-5493NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSs-SWCNT preparation and photovoltaic device fabrication andcharacterization were supported by the Air Force Office ofScientific Research (FA9550-19-1-0093). The synthesis ofperylene diimide-based electron acceptors was supported bythe Office of Naval Research (N00014-17-1-2205). The cyclicvoltammetry characterization was supported by the Division ofChemical Sciences, Geosciences, and Biosciences, Office ofBasic Energy Sciences of the U.S. Department of Energy (DE-SC0008707).

■ REFERENCES(1) Bindl, D. J.; Wu, M. Y.; Prehn, F. C.; Arnold, M. S. EfficientlyHarvesting Excitons from Electronic Type-Controlled Semiconduct-ing Carbon Nanotube Films. Nano Lett. 2011, 11, 455−460.(2) Shea, M. J.; Wang, J. L.; Flach, J. T.; Zanni, M. T.; Arnold, M. S.Less Severe Processing Improves Carbon Nanotube PhotovoltaicPerformance. APL Mater. 2018, 6, No. 056104.(3) Pfohl, M.; Glaser, K.; Ludwig, J.; Tune, D. D.; Dehm, S.; Kayser,C.; Colsmann, A.; Krupke, R.; Flavel, B. S. Performance Enhancementof Polymer-Free Carbon Nanotube Solar Cells via Transfer MatrixModeling. Adv. Energy Mater. 2016, 6, No. 1501345.(4) Ye, Y. M.; 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.(5) Shea, M. J.; Arnold, M. S. 1% Solar Cells Derived from UltrathinCarbon Nanotube Photoabsorbing Films. Appl. Phys. Lett. 2013, 102,No. 243101.(6) Jain, R. M.; Howden, R.; Tvrdy, K.; Shimizu, S.; Hilmer, A. J.;McNicholas, T. P.; Gleason, K. K.; Strano, M. S. Polymer-Free Near-Infrared Photovoltaics with Single Chirality (6,5) SemiconductingCarbon Nanotube Active Layers. Adv. Mater. 2012, 24, 4436−4439.(7) Xie, Y.; Gong, M. G.; Shastry, T. A.; Lohrman, J.; Hersam, M.C.; Ren, S. Q. Broad-Spectral-Response Nanocarbon Bulk-Hetero-junction Excitonic Photodetectors. Adv. Mater. 2013, 25, 3433−3437.(8) Koleilat, G. I.; Vosgueritchian, M.; Lei, T.; Zhou, Y.; Lin, D. W.;Lissel, F.; Lin, P.; To, J. W. F.; Xie, T.; England, K.; et al. Surpassingthe Exciton Diffusion Limit in Single-Walled Carbon NanotubeSensitized Solar Cells. ACS Nano 2016, 10, 11258−11265.(9) Gomulya, W.; Gao, J.; Loi, M. A. Conjugated Polymer-WrappedCarbon Nanotubes: Physical Properties and Device Applications. Eur.Phys. J. B 2013, 86, No. 404.(10) Classen, A.; Einsiedler, L.; Heumuelier, T.; Graf, A.; Brohmann,M.; Berger, F.; Kahmann, S.; Richter, M.; Matt, G. J.; Forberich, K.;et al. Absence of Charge Transfer State Enables Very Low Voc Lossesin Swcnt:Fullerene Solar Cells. Adv Energy Mater. 2019, 9,No. 1801913.(11) Blackburn, J. L. Semiconducting Single-Walled CarbonNanotubes in Solar Energy Harvesting. ACS Energy Lett. 2017, 2,1598−1613.(12) Streit, J. K.; Bachilo, S. M.; Ghosh, S.; Lin, C. W.; Weisman, R.B. Directly Measured Optical Absorption Cross Sections forStructure-Selected Single-Walled Carbon Nanotubes. Nano Lett.2014, 14, 1530−1536.(13) Sanchez, S. R.; Bachilo, S. M.; Kadria-Vili, Y.; Lin, C. W.;Weisman, R. B. n,m)-Specific Absorption Cross Sections of Single-Walled Carbon Nanotubes Measured by Variance Spectroscopy. NanoLett. 2016, 16, 6903−6909.(14) Wang, F.; Dukovic, G.; Brus, L. E.; Heinz, T. F. The OpticalResonances in Carbon Nanotubes Arise from Excitons. Science 2005,308, 838−841.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.9b06381J. Phys. Chem. C 2019, 123, 21395−21402

21400

http://pubs.acs.orghttp://pubs.acs.org/doi/abs/10.1021/acs.jpcc.9b06381http://pubs.acs.org/doi/suppl/10.1021/acs.jpcc.9b06381/suppl_file/jp9b06381_si_001.pdfmailto:michael.arnold@wisc.eduhttp://orcid.org/0000-0002-2946-5480http://orcid.org/0000-0003-3875-3035http://orcid.org/0000-0003-1945-8794http://orcid.org/0000-0002-0384-5493http://dx.doi.org/10.1021/acs.jpcc.9b06381

(15) Ma, Y. Z.; Valkunas, L.; Bachilo, S. M.; Fleming, G. R. ExcitonBinding Energy in Semiconducting Single-Walled Carbon Nanotubes.J. Phys. Chem. B 2005, 109, 15671−15674.(16) Mallajosyula, A. T.; Nie, W. Y.; Gupta, G.; Blackburn, J. L.;Doorn, S. K.; Mohite, A. D. Critical Role of the Sorting Polymer inCarbon Nanotube-Based Minority Carrier Devices. ACS Nano 2016,10, 10808−10815.(17) Pfohl, M.; Glaser, K.; Graf, A.; Mertens, A.; Tune, D. D.;Puerckhauer, T.; Alam, A.; Wei, L.; Chen, Y.; Zaumseil, J.; et al.Probing the Diameter Limit of Single Walled Carbon Nanotubes inSWCNT: Fullerene Solar Cells. Adv. Energy Mater. 2016, 6,No. 1600890.(18) Gong, M. G.; Shastry, T. A.; Xie, Y.; Bernardi, M.; Jasion, D.;Luck, K. A.; Marks, T. J.; Grossman, J. C.; Ren, S. Q.; Hersam, M. C.Polychiral Semiconducting Carbon Nanotube-Fullerene Solar Cells.Nano Lett. 2014, 14, 5308−5314.(19) Shastry, T. A.; Clark, S. C.; Rowberg, A. J. E.; Luck, K. A.;Chen, K. S.; Marks, T. J.; Hersam, M. C. Enhanced Uniformity andArea Scaling in Carbon Nanotube-Fullerene Bulk-HeterojunctionSolar Cells Enabled by Solvent Additives. Adv. Energy Mater. 2016, 6,No. 1501466.(20) Bindl, D. J.; Brewer, A. S.; Arnold, M. S. SemiconductingCarbon Nanotube/Fullerene Blended Heterojunctions for Photo-voltaic Near-Infrared Photon Harvesting. Nano Res. 2011, 4, 1174−1179.(21) 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 inSingle-Walled Carbon Nanotube Heterojunctions. Nat. Chem. 2016,8, 603−609.(22) Ferguson, A. J.; Dowgiallo, A. M.; Bindl, D. J.; Mistry, K. S.;Reid, O. G.; Kopidakis, N.; Arnold, M. S.; Blackburn, J. L. Trap-Limited Carrier Recombination in Single-Walled Carbon NanotubeHeterojunctions with Fullerene Acceptor Layers. Phys. Rev. B 2015,91, No. 245311.(23) Bindl, D. J.; Ferguson, A. J.; Wu, M. Y.; Kopidakis, N.;Blackburn, J. L.; Arnold, M. S. Free Carrier Generation andRecombination in Polymer-Wrapped Semiconducting Carbon Nano-tube Films and Heterojunctions. J. Phys. Chem. Lett. 2013, 4, 3550−3559.(24) He, Y. J.; Li, Y. F. Fullerene Derivative Acceptors for HighPerformance Polymer Solar Cells. Phys. Chem. Chem. Phys. 2011, 13,1970−1983.(25) Burlingame, Q.; Tong, X. R.; Hankett, J.; Slootsky, M.; Chen,Z.; Forrest, S. R. Photochemical Origins of Burn-in Degradation inSmall Molecular Weight Organic Photovoltaic Cells. Energy Environ.Sci. 2015, 8, 1005−1010.(26) Rao, A. M.; Zhou, P.; Wang, K. A.; Hager, G. T.; Holden, J. M.;Wang, Y.; Lee, W. T.; Bi, X. X.; Eklund, P. C.; Cornett, D. S.; et al.Photoinduced Polymerization of Solid C60 Films. Science 1993, 259,955−957.(27) Li, Z.; Wong, H. C.; Huang, Z. G.; Zhong, H. L.; Tan, C. H.;Tsoi, W. C.; Kim, J. S.; Durrant, J. R.; Cabral, J. T. PerformanceEnhancement of Fullerene-Based Solar Cells by Light Processing. Nat.Commun. 2013, 4, No. 2227.(28) Li, T. Y.; Meyer, T.; Ma, Z. F.; Benduhn, J.; Korner, C.; Zeika,O.; Vandewal, K.; Leo, K. Small Molecule near-Infrared BoronDipyrromethene Donors for Organic Tandem Solar Cells. J. Am.Chem. Soc. 2017, 139, 13636−13639.(29) Zhong, Y.; Trinh, M. T.; Chen, R. S.; Purdum, G. E.;Khlyabich, P. P.; Sezen, M.; Oh, S.; Zhu, H. M.; Fowler, B.; Zhang, B.Y.; et al. Molecular Helices as Electron Acceptors in High-Performance Bulk Heterojunction Solar Cells. Nat. Commun. 2015,6, No. 8242.(30) Cheng, P.; Li, G.; Zhan, X. W.; Yang, Y. Next-GenerationOrganic Photovoltaics Based on Non-Fullerene Acceptors. Nat.Photonics 2018, 12, 131−142.(31) Sisto, T. J.; Zhong, Y.; Zhang, B. Y.; Trinh, M. T.; Miyata, K.;Zhong, X. J.; Zhu, X. Y.; Steigerwald, M. L.; Ng, F.; Nuckolls, C.

Long, Atomically Precise Donor-Acceptor Cove-Edge as ElectronAcceptors. J. Am. Chem. Soc. 2017, 139, 5648−5651.(32) Zhang, J. Q.; Tan, H. S.; Guo, X. G.; Facchetti, A.; Yan, H.Material Insights and Challenges for Non-Fullerene Organic SolarCells Based on Small Molecular Acceptors. Nat. Energy 2018, 3, 720−731.(33) Hou, J. H.; Inganas, O.; Friend, R. H.; Gao, F. Organic SolarCells Based on Non-Fullerene Acceptors. Nat. Mater. 2018, 17, 119−128.(34) Lin, Y. Z.; Wang, J. Y.; Zhang, Z. G.; Bai, H. T.; Li, Y. F.; Zhu,D. B.; Zhan, X. W. An Electron Acceptor Challenging Fullerenes forEfficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170−1174.(35) Zhao, W. C.; Li, S. S.; Yao, H. F.; Zhang, S. Q.; Zhang, Y.;Yang, B.; Hou, J. H. Molecular Optimization Enables over 13%Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148−7151.(36) Zhao, W. C.; Qian, D. P.; Zhang, S. Q.; Li, S. S.; Inganas, O.;Gao, F.; Hou, J. H. Fullerene-free Polymer Solar Cells with over 11%Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28,4734−4739.(37) Baran, D.; Ashraf, R. S.; Hanifi, D. A.; Abdelsamie, M.;Gasparini, N.; Rohr, J. A.; Holliday, S.; Wadsworth, A.; Lockett, S.;Neophytou, M.; et al. Reducing the Efficiency-Stability-Cost Gap ofOrganic Photovoltaics with Highly Efficient and Stable SmallMolecule Acceptor Ternary Solar Cells. Nat. Mater. 2017, 16, 363−369.(38) Kang, H. S.; Sisto, T. J.; Peurifoy, S.; Arias, D. H.; Zhang, B. Y.;Nuckolls, C.; Blackburn, J. L. Long-Lived Charge Separation atHeterojunctions between Semiconducting Single-Walled CarbonNanotubes and Perylene Diimide Electron Acceptors. J. Phys. Chem.C 2018, 122, 14150−14161.(39) Nish, A.; Hwang, J. Y.; Doig, J.; Nicholas, R. J. Highly SelectiveDispersion of Singlewalled Carbon Nanotubes Using AromaticPolymers. Nat. Nanotechnol. 2007, 2, 640−646.(40) Graf, A.; Zakharko, Y.; Schiessl, S. P.; Backes, C.; Pfohl, M.;Flavel, B. S.; Zaumseil, J. Large Scale, Selective Dispersion of LongSingle-Walled Carbon Nanotubes with High PhotoluminescenceQuantum Yield by Shear Force Mixing. Carbon 2016, 105, 593−599.(41) Peurifoy, S. R.; Castro, E.; Liu, F.; Zhu, X. Y.; Ng, F.; Jockusch,S.; Steigerwald, M. L.; Echegoyen, L.; Nuckolls, C.; Sisto, T. J. Three-Dimensional Graphene Nanostructures. J. Am. Chem. Soc. 2018, 140,9341−9345.(42) Zhong, Y.; Kumar, B.; Oh, S.; Trinh, M. T.; Wu, Y.; Elbert, K.;Li, P. P.; Zhu, X. Y.; Xiao, S. X.; Ng, F.; et al. Helical Ribbons forMolecular Electronics. J. Am. Chem. Soc. 2014, 136, 8122−8130.(43) Bindl, D. J.; Arnold, M. S. Efficient Exciton Relaxation andCharge Generation in Nearly Monochiral (7,5) Carbon Nanotube/C60 Thin-Film Photovoltaics. J. Phys. Chem. C 2013, 117, 2390−2395.(44) Barone, V.; Peralta, J. E.; Uddin, J.; Scuseria, G. E. ScreenedExchange Hybrid Density-Functional Study of the Work Function ofPristine and Doped Single-Walled Carbon Nanotubes. J. Chem. Phys.2006, 124, No. 024709.(45) Capaz, R. B.; Spataru, C. D.; Ismail-Beigi, S.; Louie, S. G.Excitons in Carbon Nanotubes: Diameter and Chirality Trends. Phys.Status Solidi B 2007, 244, 4016−4020.(46) Tanaka, Y.; Hirana, Y.; Niidome, Y.; Kato, K.; Saito, S.;Nakashima, N. Experimentally Determined Redox Potentials ofIndividual (n,m) Single-Walled Carbon Nanotubes. Angew. Chem.,Int. Ed. 2009, 48, 7655−7659.(47) Marcus, R. A. On the Theory of Oxidation-ReductionReactions Involving Electron Transfer. J. Chem. Phys. 1956, 24,966−978.(48) Coffey, D. C.; Larson, B. W.; Hains, A. W.; Whitaker, J. B.;Kopidakis, N.; Boltalina, O. V.; Strauss, S. H.; Rumbles, G. AnOptimal Driving Force for Converting Excitons into Free Carriers inExcitonic Solar Cells. J. Phys. Chem. C 2012, 116, 8916−8923.(49) Hilmer, A. J.; Tvrdy, K.; Zhang, J. Q.; Strano, M. S. ChargeTransfer Structure-Reactivity Dependence of Fullerene-Single-Walled

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.9b06381J. Phys. Chem. C 2019, 123, 21395−21402

21401

http://dx.doi.org/10.1021/acs.jpcc.9b06381

Carbon Nanotube Heterojunctions. J. Am. Chem. Soc. 2013, 135,11901−11910.(50) Park, J.; Reid, O. G.; Blackburn, J. L.; Rumbles, G.Photoinduced Spontaneous Free-Carrier Generation in Semiconduct-ing Single-Walled Carbon Nanotubes. Nat. Commun. 2015, 6,No. 8809.(51) Deibel, C.; Strobel, T.; Dyakonov, V. Role of the ChargeTransfer State in Organic Donor-Acceptor Solar Cells. Adv. Mater.2010, 22, 4097−4111.(52) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. The Energy ofCharge-Transfer States in Electron Donor-Acceptor Blends: Insightinto the Energy Losses in Organic Solar Cells. Adv. Funct. Mater.2009, 19, 1939−1948.(53) Liu, X.; Ding, K.; Panda, A.; Forrest, S. R. Charge TransferStates in Dilute Donor-Acceptor Blend Organic Heterojunctions. ACSNano 2016, 10, 7619−7626.(54) Zhang, A. D.; Xiao, C. Y.; Meng, D.; Wang, Q.; Zhang, X. T.;Hu, W. P.; Zhan, X. W.; Wang, Z. H.; Janssen, R. A. J.; Li, W. W.Conjugated Polymers with Deep LUMO Levels for Field-EffectTransistors and Polymer-Polymer Solar Cells. J. Mater. Chem. C 2015,3, 8255−8261.(55) An, C. B.; Puniredd, S. R.; Guo, X.; Stelzig, T.; Zhao, Y. F.;Pisula, W.; Baumgarten, M. Benzodithiophene-Thiadiazoloquinoxa-line as an Acceptor for Ambipolar Copolymers with Deep LUMOLevel and Distinct Linkage Pattern. Macromolecules 2014, 47, 979−986.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.9b06381J. Phys. Chem. C 2019, 123, 21395−21402

21402

http://dx.doi.org/10.1021/acs.jpcc.9b06381