Enhanced Light Absorption in Fluorinated Ternary ... - ACS Publications

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Enhanced Light Absorption in Fluorinated Ternary Small Molecule Photovoltaics Nicholas D. Eastham, Alexander S Dudnik, Boris Harutyunyan, Thomas J Aldrich, Matthew J. Leonardi, Eric F. Manley, Melanie R. Butler, Tobias Harschneck, Mark A. Ratner, Lin X. Chen, Michael J. Bedzyk, Ferdinand S. Melkonyan, Antonio Facchetti, Robert P. H. Chang, and Tobin J. Marks ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00486 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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ACS Energy Letters

Enhanced Light Absorption in Fluorinated Ternary Small Molecule Photovoltaics Nicholas D. Eastham,† Alexander S. Dudnik,† Boris Harutyunyan,‡ Thomas J. Aldrich,† Matthew J. Leonardi,† Eric F. Manley,†,# Melanie R. Butler,† Tobias Harschneck,† Mark A. Ratner,*,† Lin X. Chen,*,†,# Michael J. Bedzyk,*,‡, ⊥ Ferdinand S. Melkonyan,*,† Antonio Facchetti,*,† Robert P. H. Chang,*, ⊥ and Tobin J. Marks*,†,⊥ †Department

of Chemistry, the Materials Research Center, and the Argonne-Northwestern Solar

Energy Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡Department

of Physics and Astronomy and the Argonne-Northwestern Solar Energy Research

Center, Northwestern University 2145 Sheridan Road, Evanston, Illinois 60208, United States ⊥

Department of Materials Science and Engineering and the Argonne-Northwestern Solar Energy

Research Center, Northwestern University 2145 Sheridan Road, Evanston, Illinois 60208, United States #Chemical

Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass

Avenue, Lemont, Illinois 60439, United States Corresponding Author *[email protected] *[email protected] *[email protected] *[email protected] *[email protected] *[email protected] *[email protected]

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Abstract: Using small molecule donor (SMD) semiconductors in organic photovoltaics (OPVs) has historically afforded lower power conversion efficiencies (PCEs) than their polymeric counterparts. The PCE difference is attributed to shorter conjugated backbones, resulting in reduced intermolecular interactions. Here, a new pair of SMDs is synthesized based on the diketopyrrolopyrrole–benzodithiophene–diketopyrrolopyrrole (BDT-DPP2) skeleton, but having fluorinated and fluorine-free aromatic side-chain substituents. Ternary OPVs having varied ratios of the two SMDs with PC61BM as the acceptor exhibit tunable open-circuit voltages (Vocs) between 0.833 V and 0.944 V, due to a fluorination-induced shift in energy levels and the electronic “alloy” formed from the miscibility of the two SMDs. A 15% increase in PCE is observed at the optimal ternary SMD ratio, with the short-circuit current density (Jsc) significantly increased to 9.18 mA/cm2. The origin of Jsc enhancement is analyzed via charge generation, transport, and diffuse reflectance measurements, and is attributed to increased optical absorption from a maximum in film crystallinity at this SMD ratio, observed by grazing incidence

wide-angle

X-ray

scattering.

TOC GRAPHIC


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The development of organic photovoltaics (OPVs) has blossomed recently with multiple reports of power conversion efficiencies (PCEs) exceeding 12%.1-3 Advances in polymer or small molecule donor (SMD) design, including band gap modulation, morphology optimization, and selection of complementary absorption profiles between the donor and acceptor semiconductors have led to a continuous series of performance enhancements.4-9 In contrast to polymeric donor semiconductors, solution-deposited SMDs suffer, particularly when blended with acceptors, from limited long-range order, resulting in decreased charge transport capacity and PCEs.10-13 Recently, the use of novel acceptor materials based on indacenodithiophene (IDT) such as IDIC and ITIC with SMDs afforded PCEs over 9%.14-15 Moreover, using the same acceptors with donor-acceptor copolymers such as the benzodithiopene (BDT) based PDBT-T1 and PTFBDT-BZS, yielded PCEs approaching 11%, which were attributed to improved bulkheterojunction (BHJ) domain formation and charge transport, further highlighting the limitations of SMDs.2 Clearly new strategies for enhancing SMD intermolecular interactions to increase OPV performance would be of great interest. Several studies have utilized fluorination as an effective design strategy to increase intermolecular interactions in OPV systems.16-20 This approach has enhanced the charge carrier mobility and overall device performance for several donor semiconductors while simultaneously lowering their highest occupied molecular orbital (HOMO) energies, which enhances Voc and imparts greater environmental stability.21-23 Okamoto and Matsuo showed that perfluorination of an aryl substituent on a diaryl-substituted tetracene SMD yielded smaller d-spacings in the crystalline tetracene lattice via C–H···F bonding, increasing long-range order and photoconductivity.24 Later, You, Neher, and Ade showed that sequential polymer fluorination


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enhances π-π stacking and hole mobility in pristine films due to more favorable intermolecular orbital overlap.25 Introducing a third semiconducting component into binary BHJ blends to produce ternary systems has been recently used to increase polymer, small molecule, and non-fullerene OPV performance.26-30 Ternary acceptor31-32 and donor33-36 additives can enhance solar spectrum absorption, Jsc, and Voc.37 Understanding how to design such ternary components that enable the requisite BHJ phase formation and optimal film morphology should be key objectives.33-34, 38-40 Nevertheless, ternary blends are inherently complicated to model and challenging to process. Thus, co-solvents such as 1,8-diiodooctane (DIO), diphenyl ether (DPE) and post-processing procedures such as thermal annealing and solvent vapor annealing (SVA) make understandingbased adjustment of solar cell performance both challenging and non-obvious. For example, several previously reported systems have immiscible ternary components, evident from pinned Vocs, and limiting potentially beneficial ternary effects.31, 39, 41-42 One of the most extensively studied SMD OPV families comprises a BDT unit flanked by two diketopyrrolopyrrole (DPP) units (Figure 1A), making it an excellent scaffold for modification and exploration in ternary systems, and yielding results applicable to many photovoltaic materials classes.10,

43-44

Previous work from this Laboratory reported effective

BDT-DPP2 modifications, performance effects of extending the BDT core conjugation, and the influence of side chain substituents on film morphology.45-47 From this understanding, we report here two new BDT-DPP2 SMDs, 6,6'-(5,5'-(4,8-bis(4-(2-ethylhexyl)phenyl)benzo[1,2-b:4,5b']dithiophene-2,6-diyl)bis(thiophene-5,2-diyl))bis(2,5-bis(2-ethylhexyl)-3-(thiophen-2yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (PH) and 6,6'-(5,5'-(4,8-bis(4-(2-ethylhexyl)-3,5difluorophenyl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(thiophene-5,2-diyl))bis(2,5-bis(2-


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ethylhexyl)-3-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione)

(PF2),

for

ternary

photovoltaic devices having identical backbones but with varied degrees of conjugated sidechain fluorination. The PH and PF2 donors are fully characterized using a range of techniques including elemental analysis, NMR spectroscopy, optical (UV-vis) absorption

Figure 1. A) Molecular structures of BDT-DPP2 derivatives PH and PF2 used in this study. B) CV-derived HOMO levels and optical absorption-derived LUMO levels of both donor materials. C) Solution and film UV spectra of PH and PF2 exhibiting virtually identical bandgaps due to the minimal backbone effect of fluorination. spectroscopy, cyclic voltammetry (CV), and binary/ternary OPV device evaluation. It is shown that blends of these donors with PC61BM exhibit good photovoltaic performance in binary BHJ OPVs (PCEs ~ 4%), but more notably, a 15% PCE increase is measured here in the ternary architecture -- among the highest performance increase reported to date for ternary OPVs.48 The good miscibility of the two donor materials is evident from the highly tunable Vocs on changing the PH:PF2 ratio as well as from Flory-Huggins interaction parameters, underlying the electronic


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“alloy” formed.37, 49-52 Increases in Jsc while retaining a high Voc accounts for the bulk of the PCE increase. The origin of the increased Jsc is elucidated via light intensity and space-charge limited current measurements (SCLC), grazing incidence wide-angle X-ray scattering (GIWAXS), and diffuse reflectance optical spectroscopy. We show that the maximum Jsc is achieved in an optimal ternary PH:PF2:PC61BM weight ratio with the greatest active layer crystallinity. This study demonstrates the first example of combining fluorinated and hydrogenated donor isomers in ternary OPVs to raise donor crystallinity and light absorption, hence solar cell performance. SMDs PH and PF2 were synthesized according to the procedures described in Scheme S1 in the Supporting Information (SI). Figure 1B shows the PH and PF2 frontier MO energies quantified by combining cyclic voltammetry (CV, Figure S2) and optical absorption (Figs. 1B, C) data. The two SMDs exhibit very similar absorbance profiles in both dilute solutions and as thin films, with nearly identical bandgaps (~1.70 eV). Note that the PF2 EHOMO at -5.47 eV lies slightly lower than that of PH (-5.36 eV), reflecting the electron-withdrawing fluorine substitution. Furthermore, the thermal properties of the new SMDs were investigated by differential scanning calorimetry (DSC, Figure S4). PH and PF2 exhibit phase transitions at 302 and 311 °C, respectively (Table S2), with nearly identical heats of fusion (∆Hf), ~54.5 J/g, consistent with similar crystallinity and lattice cohesion. Conventional architecture5 OPV cells were fabricated using binary and ternary PH and PF2 blends with PC61BM on prefabricated, cleaned ITO substrates. PEDOT:PSS was deposited by spin-coating at 5000 rpm for 30 sec and annealing at 160 °C for 15 min before transferring to a glovebox and annealing for another 10 min at 150 °C. Solutions for the active layer were prepared in chloroform at 10 mg/mL with respect to the donor material in a 1:1 weight ratio with PC61BM. Active layer films were spun-cast onto PEDOT:PSS at 3000 rpm and annealed at 110


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°C for 10 min, yielding films of approximately 100 nm in thickness. The substrates were then transferred to a vacuum deposition chamber and the counter electrode composed of LiF (1 nm) and Al (100 nm) was thermally evaporated on the active layer through a shadow mask. All devices were encapsulated with UV curable resin and cured for 10 min before evaluation. Relevant solar cell parameters are summarized in Table 1. Table 1. OPV Performance Data for PH:PF2:PC61BM OPVs at the Indicated Donor Component Weight % Ratios

a

PH:PF2

Jsc

Voc

FF

PCE

(w:w %)

(mA/cm2)

(V)

(%)

(%)a

100:0

8.36

0.833

59.6

4.15 (4.04 ± 0.18)

75:25

7.51

0.862

62.6

4.05 (4.00 ± 0.11)

50:50

8.06

0.878

61.4

4.35 (4.25 ± 0.19)

25:75

8.19

0.882

62.1

4.49 (4.40 ± 0.12)

15:85

8.74

0.918

59.0

4.73 (4.65 ± 0.14)

10:90

9.18

0.927

57.6

4.90 (4.85 ± 0.11)

5:95

8.71

0.932

55.6

4.55 (4.51 ± 0.12)

0:100

7.81

0.944

57.8

4.26 (4.21 ± 0.10)

Values in parentheses are average PCEs and standard deviations obtained from ≥10 devices

Figure 2A shows the OPV current–voltage (J–V) characteristics of both binary and ternary BHJ blends. Devices containing only a binary active layer blend of PH:PC61BM or PF2:PC61BM exhibit average PCEs of 4.04% and 4.21%, respectively, with a large gain in open-circuit voltage (Voc) from 0.833 V to 0.944 V for fluorinated PF2, attributable to the lower EHOMO relative to PH. The fill factor (FF) and Jsc metrics are similar for each mixture with PH having slightly greater values of 59.6% and 8.364 mA/cm2, respectively compared to PF2 with 57.8% and 7.809mA/cm2 respectively.


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Ternary solar cells were fabricated using the same method as the binary devices while varying the PH:PF2 weight ratio and holding the donor(s):acceptor weight ratio constant at 1:1. The performance shifts considerably as the donor ratio is varied, peaking at a 10:90 ratio of

Figure 2. A) Current density versus voltage, and B) EQE plots of OPVs based on binary and ternary blends of PH and PF2 with PC61BM. C) Trends in PCE, Jsc, and D) Voc with ternary blend composition. PH:PF2 with a PCE of 4.9%, which represents >15% increase in photovoltaic performance relative to the binary systems (Table 1). An increase in Jsc up to 9.18 mA/cm2 accounts for most of this increase along with a relatively high Voc, attributable to the high PF2 fraction in the optimized ternary blend. This enhancement is also evident in the external quantum efficiency (EQE) spectrum (Figure 2B). The ternary mixture exhibits an increased EQE across the entire spectrum and with no significant change in the effective band gap visible from the EQE cutoff. Below we investigate the origins of the tunable Voc and the increased Jsc in the ternary devices.


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Increased Jsc is the primary origin of the ternary device superior performance (Figure 2C) and is accompanied by a near-monotonic Voc shift between the binary endpoints (Table 1). This remarkable tunability of Voc (Figure 2D) reflects the chemical similarity of PH and PF2 but also substantial differences in HOMO energies, evident in pristine film and blend CV measurements (Figure S3). Tunable Voc has been observed in several other ternary systems and has been associated with electronic alloys, solid solutions, and morphologically parallel systems, constituting what is believed to be a favorable molecular morphology for ternary photovoltaics.41,

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To characterize the miscibility of each donor, Flory-Huggins interaction

parameters were evaluated for PH, PF2, and PC61BM using the relationship of χ = (δ − δ ) , where is the molar volume of solvent, is the gas constant, is the temperature, and δ is the surface energy of each component.54-55 Full data and experimental details are given in the SI. The low derived χ of 0.34 for the PH/PF2 pair compared to 0.77 and 0.80 for PH/PC61BM and PF2/PC61BM, respectively, supports an electronic alloy scenario for PH/PF2, favoring a homogenous excitonic environment and a broadly tunable Voc.49, 55-57 Increased OPV Jsc values can originate from one or a combination of sources including light absorption, exciton generation, exciton dissociation, charge transport, and charge recombination. Changes in recombination dynamics are known to contribute to Jsc variations in ternary OPVs. To further probe the cause of the present increase in Jsc, a series of experiments to examine recombination processes in the optimized ternary OPVs was conducted. Previous ternary studies associated reduced recombination with enhanced performance.48 Light intensity (I) studies enable determination of any deviations from ideality in the devices (Figure 3A). Jsc as a function of light intensity can be fit to a power function Jsc α IS to extract an exponent related to the extent of bimolecular recombination in a solar cell. S = 1 indicates weak bimolecular recombination


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while S < 1 implies the presence of bimolecular recombination, space-charge effects, and/or imbalanced mobility in the charge carriers.58 The present ternary devices exhibit S = 0.65, remarkably close to that of PH (0.69) and PF2 (0.67). From these data we conclude that the binary and ternary systems exhibit virtually equal and considerable deviations from ideality, likely reflecting space-charge or bimolecular recombination processes.

Figure 3. A) Jsc versus log(I) outputs and B) J–V measurements used to extract hole-only SCLC mobilities for binary and optimal (10:90) ternary PH + PF2 blends with inset electron/hole mobility ratios. C) Voc versus log(I) outputs and D) Jph versus Veff plots of saturation current for PH, PF2, and the optimal ternary blend. SCLC mobility measurements were next used to probe charge transport perpendicular to the active layer plane and the consequent space-charge properties of each blend. In addition to playing an important role in recombination dynamics, increased carrier mobilities are considered a primary performance-enhancing factor in OPVs containing fluorinated donors. Seminal work


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by You, Neher, and Ade showed that increased donor polymer fluorination enhances charge generation and Jscs in the respective OPVs, primarily due to increases in SCLC hole mobility (µh).25 In the present SMD study, SCLC devices were fabricated in hole-only or electron-only architectures (Table S4), with blend films studied to provide more pertinent OPV information than the pristine films. J–V curves for the hole-only devices are shown in Figure 3B. The highest

µh, 7.45 × 10-6 cm2/V·s, is found in PH:PC61BM films, while the lowest, 4.01 × 10-6 cm2/V·s, is found in PF2:PC61BM films, of similar magnitude to other reported small molecule donors.59-61 The ternary devices exhibit mobilities between these two values, showing minor variations with composition. Note here that the electron-only mobilities (µes) are significantly greater than the

µhs, which is common for blends where the active layer charge transport efficiency of PC61BM exceeds that of the donor.25 As seen with the µh, the PH:PC61BM blend has the highest µe of 1.22 × 10-4 cm2/V·s and PF2:PC61BM the lowest µe of 1.01 × 10-5 cm2/V·s. The ternary devices once again exhibit µe between these two endpoints without a notable trend. However, analysis of the corresponding µe/µh ratios reveals an interesting trend, with PH:PC61BM having the highest ratio of µe/µh = 16.4 and PF2:PC61BM exhibiting a significantly more balanced ratio of µe/µh = 2.52. The optimal ternary blend displays a slightly lower ratio than either pristine material with

µe/µh = 2.02. The more balanced mobilities are in contrast to PH exhibiting a greater Jsc than PF2 in a binary blend and the relatively small fill factor difference between the devices, and therefore suggest bimolecular recombination as a cause of the lower S since space-charge imbalances are more prevalent in PH than in the ternary or PF2-only blends and are not the limiting factor in ultimate device performance. The slope of Voc vs. log(I) plots can be used to assess the dominant recombination mechanism and complements conclusions drawn from the mobility analysis. This value typically


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lies at or above a slope of kBT/q where kB is the Boltzmann constant, T is the temperature, and q is the elementary charge. A slope of kBT/q indicates that bimolecular recombination dominates while a slope greater than kBT/q implicates a combination of mono- and bimolecular processes.58 Note that the Figure 3C data show that the ternary system functions similarly to both binary systems with a slope of 1.250 kBT/q, between the values obtained for PH (1.199) and PF2 (1.336). Clearly bimolecular recombination is a significant recombination factor in all of the present systems (Figure 3A), with more monomolecular recombination occurring in PF2 than in PH (Figure 3C), contributing to differences in Jsc between the two, but not explaining the enhanced Jsc in the ternary devices. Charge generation and exciton dissociation dynamics were next examined to better understand the origin of the increased ternary device current. Photocurrent (Jph) is calculated as Jph = JL-JD where JL is the illuminated current output and JD is the dark current. Note that the ternary blend has a greater saturation current (Jsat) than either binary blend (Figure 3D) as determined by Jph at high bias, consistent with the trend in Jscs. At high effective voltage (Veff) it is assumed that all generated charges are extracted and measured, implying that Jsat is only limited by the exciton generation rate (Gmax). Veff is defined as Veff = V0 – Va where V0 is the voltage at which Jph = 0 and Va is the applied voltage. This relationship simplifies as Jsat = qLGmax, with q as the elementary charge and L the active layer thickness.35 The generation rate of the ternary OPV is found to be 6.95 x 1027 m-3 s-1, greater than that of the PH (5.97 x 1027 m-3 s-1) or PF2 (5.43 x1027 m-3 s-1) OPV. Exciton dissociation probabilities for each blend (P(E,T)) were calculated as the Jph/Jsat ratio. The ternary device has a dissociation probability at Jsc of 85.0%, between those of PH (84.8%) and PF2 (87.8%), indicating that although the ternary OPV outputs greater current, it is not due


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to higher exciton dissociation probability. Combined with the charge transport and recombination analysis, these data implicate increased light absorption as the most reasonable cause of the increased current. Enhanced active layer light absorption is consistent with the greater Jscs of the ternary devices. To verify this, diffuse reflectance UV-vis spectra were acquired for completed devices to eliminate contributions to the absorption spectra from scattering and reflection, using an integrating sphere (Figure 4A), with each measurement corrected for thickness measured by profilometry. Note that the ternary blends exhibit greater absorption than their binary counterparts with PC61BM. The increased absorptivity is not restricted to the small

k Figure 4. A) Thickness corrected diffuse reflectance UV-vis absorption measurements for binary and ternary (10:90) PH:PF2 blends with PC61BM. 3-D Bubble plots depicting the spacing and intensity of crystalline diffractions and the relative size of the crystallite domains as determined by Scherrer analysis of B) PH, C) PF2, and D) the optimal (10:90) ternary blend.


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molecule intramolecular charge transfer absorption spectral region (~650 nm), but is also evident in the π-π* regions (~400 nm) in the optimized ternary blend. Tuning the ratio of PH:PF2 in the active layer influences the morphology of both the donor and acceptor material, affording increased absorption and Jsc. Morphologies leading to enhanced absorption could result from either optimal phase separation in the active layer where both donor and acceptor material exert maximum light absorption or greater active layer material crystallinity, increasing π-system delocalization and light absorption.62-64 This hypothesis is supported by transmission UV-vis spectroscopy of PH:PF2 blend thin films that exhibit similarly enhanced absorptivity when normalized for film thickness by profilometry (Figure S1). Films of PH, PF2, and the optimal ternary ratio (10:90) have thicknesses of 95 nm, 98 nm, and 95 nm, respectively, indicating that increased thickness is not the source of increased absorption. The most plausible origin of the enhanced light absorption in a system composed of identical bandgap materials and device architectures is enhanced active layer crystallinity and creation of a larger optical cross-section.63 Grazing incidence wide-angle X-ray scattering (GIWAXS) was performed on each ternary blend to seek any differences in the active layer morphology. An increase in the microcrystallite phase density at the optimal ternary ratio is observed on analysis of these images, as illustrated in the bubble plots of Figures 4B-D. The bubble size indicates the size of the crystallite domains determined by Scherrer analysis. Upon deconvolution of the broad PC61BM peak at ~4.6 Å in the ternary blends, the presence of new diffraction peaks at 4.13 Å, 4.42 Å, and 5.25 Å in the in-plane line-cut as well as at 4.43 Å and 5.16 Å in the out-of-plane line-cut are observed in the 10:90 ratio films (Figure S8). These reflections are not present in the pristine PH or PF2 films and correspond to ternary blend microcrystalline phases resulting from PH:PF2 in the optimal ratio. Increased crystallinity in the ternary films, peaking at the optimal


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10:90 PH:PF2 ratio, is consistent with the increased optical absorption and increased current in the J–V data. In summary, we have synthesized two new BDT-DPP2-based small molecule OPV donors with a high degree of chemical/structural similarity and miscibility, for implementation in ternary photovoltaics. These materials demonstrate the versatility and impact of core-external donor material fluorination, where it has minimal impact on the bandgap, but can impart significant changes in crystal packing, carrier mobility, optical absorption, and frontier energy levels. Ternary small molecule OPVs composed of fluorinated (PF2) and non-fluorinated (PH) donors and PC61BM as an acceptor achieve a ≥ 15% increase in performance versus their binary counterparts, reflecting in part an increased and extensively tunable Voc resulting from fluorination. A coincident enhancement in light absorption is characterized and attributed to increased crystallinity in the ternary active layer material yielding a high Jsc. We envision that this unprecedented strategy of blending ternary materials with fluorinated and non-fluorinated aromatic side chains could be employed to enhance photovoltaic performance of many other photovoltaic systems. ASSOCIATED CONTENT Supporting Information Synthetic procedures and characterization of donor materials. Device fabrication and characterization details. This material is available free of charge on the ACS Publications website. AUTHOR INFORMATION [email protected] [email protected]


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[email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected]

ACKNOWLEDGMENT This research was supported in part by Argonne-Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001059 (N.D.E., R.P.H.C., and T.J.M. for device fabrication and characterization, B.H., M.J.B. and L.X.C. for Xray characterization) and by AFOSR grant FA9550-08-1-0331 (A.F.). Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under contract No. DE-AC02-06CH11357. This work made use of the EPIC, Keck-II, and/or SPID facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205);


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the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. A.S.D. thanks the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry for a fellowship. T.J.A. and M.R.B. thank the NSF for predoctoral fellowships, and M.J.L thanks the NDSEG for a predoctoral fellowship. F.S.M. was supported by award 70NANB14H012 from U.S. Department of Commerce, National Institute of Standards and Technology as part of the Center for Hierarchical Materials Design (CHiMaD) and E.F.M. by Qatar NPRP grant 7-286-1-046. REFERENCES (1) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (2) Lin, Y.; Zhao, F.; Wu, Y.; Chen, K.; Xia, Y.; Li, G.; Prasad, S. K.; Zhu, J.; Huo, L.; Bin, H., et al. Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells. Adv. Mater. 2017, 29, 1604155. (3) Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade, H.; Hou, J. Energy-Level Modulation of Small-Molecule Electron Acceptors to Achieve over 12% Efficiency in Polymer Solar Cells. Adv. Mater. 2016, 28, 9423-9429. (4) Kippelen, B.; Brédas, J.-L. Organic Photovoltaics. Energy Environ. Sci. 2009, 2, 251-261. (5) Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114, 7006-7043. (6) Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.; Huang, F., et al. Terthiophene-Based D-A Polymer with an Asymmetric Arrangement of Alkyl Chains That Enables Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 14149-14157.


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(15) Bin, H.; Yang, Y.; Zhang, Z.-G.; Ye, L.; Ghasemi, M.; Chen, S.; Zhang, Y.; Zhang, C.; Sun, C.; Xue, L., et al. 9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells with Absorption-Complementary Donor and Acceptor. J. Am. Chem. Soc. 2017, 139, 5085-5094. (16) Wang, M.; Ford, M. J.; Lill, A. T.; Phan, H.; Nguyen, T. Q.; Bazan, G. C. Hole Mobility and Electron Injection Properties of D-A Conjugated Copolymers with Fluorinated Phenylene Acceptor Units. Adv. Mater. 2017, 29, 1603830. (17) Oh, J.; Kranthiraja, K.; Lee, C.; Gunasekar, K.; Kim, S.; Ma, B.; Kim, B. J.; Jin, S.-H. Side-Chain Fluorination: An Effective Approach to Achieving High-Performance All-Polymer Solar Cells with Efficiency Exceeding 7%. Adv. Mater. 2016, 28, 10016-10023. (18) Timalsina, A.; Hartnett, P. E.; Melkonyan, F. S.; Strzalka, J.; Reddy, V. S.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. New Donor Polymer with Tetrafluorinated Blocks for Enhanced Performance in Perylenediimide-Based Solar Cells. J. Mater. Chem. A 2017, 5, 53515361. (19) Kelly, M. A.; Roland, S.; Zhang, Q.; Lee, Y.; Kabius, B.; Wang, Q.; Gomez, E. D.; Neher, D.; You, W. Incorporating Fluorine Substitution into Conjugated Polymers for Solar Cells: Three Different Means, Same Results. J. Phys. Chem. C 2017, 121, 2059-2068. (20) Li, C. Z.; Matsuo, Y.; Niinomi, T.; Sato, Y.; Nakamura, E. Face-to-Face C6F5[60]Fullerene Interaction for Ordering Fullerene Molecules and Application to Thin-Film Organic Photovoltaics. Chem. Commun. 2010, 46, 8582-8584. (21) Kawashima, K.; Fukuhara, T.; Suda, Y.; Suzuki, Y.; Koganezawa, T.; Yoshida, H.; Ohkita, H.; Osaka, I.; Takimiya, K. Implication of Fluorine Atom on Electronic Properties, Ordering Structures, and Photovoltaic Performance in Naphthobisthiadiazole-Based Semiconducting Polymers. J. Am. Chem. Soc. 2016, 138, 10265-10275.


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Processing Additive-Induced Nanofibril in Blend Film for Photovoltaic Applications. Nanoscale 2013, 5, 9536-9540. (61) Shin, W.; Yasuda, T.; Hidaka, Y.; Watanabe, G.; Arai, R.; Nasu, K.; Yamaguchi, T.; Murakami, W.; Makita, K.; Adachi, C. π-Extended Narrow-Bandgap DiketopyrrolopyrroleBased Oligomers for Solution-Processed Inverted Organic Solar Cells. Adv. Energy Mater. 2014, 4, 1400879. (62) Zhokhavets, U.; Erb, T.; Gobsch, G.; Al-Ibrahim, M.; Ambacher, O. Relation between Absorption and Crystallinity of Poly(3-Hexylthiophene)/Fullerene Films for Plastic Solar Cells. Chem. Phys. Lett. 2006, 418, 347-350. (63) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stühn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. J. Correlation between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells. Adv. Funct. Mater. 2005, 15, 1193-1196. (64) Chirvase, D.; Parisi, J.; Hummelen, J. C.; Dyakonov, V. Influence of Nanomorphology on the Photovoltaic Action of Polymer–Fullerene Composites. Nanotechnology 2004, 15, 13171323.


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