Halogenation of a Nonplanar Molecular Semiconductor to Tune

Article pubs.acs.org/cm

Halogenation of a Nonplanar Molecular Semiconductor to Tune Energy Levels and Bandgaps for Electron Transport Anna M. Hiszpanski,† Jonathan D. Saathoff,‡ Leo Shaw,†,⊥ He Wang,†,# Laura Kraya,§,∇ Franziska Lüttich,§,& Michael A. Brady,∥ Michael L. Chabinyc,∥ Antoine Kahn,§ Paulette Clancy,‡ and Yueh-Lin Loo*,† †

Department of Chemical and Biological Engineering, Princeton University, A323 Engineering Quadrangle, Princeton, New Jersey 08544, United States ‡ Department of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14850, United States § Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, United States ∥ Materials Department, University of California, Santa Barbara, California 93106, United States S Supporting Information *

ABSTRACT: Though peripheral halogen substitution is a known strategy to lower the lowest unoccupied (LUMO) and highest occupied (HOMO) molecular orbital energy levels of planar molecular semiconductors, this strategy has not been explored in conformationally contorted systems. We demonstrate that substitution of peripheral hydrogens with fluorine and chlorine can effectively lower the energy levels of contorted hexabenzocoronene (cHBC) despite its nonplanar conformation. The HOMO energy level lowers comparably with fluorine and chlorine substitution. Due to chlorine’s ability to accommodate more electron density than fluorine, chlorination lowers the LUMO energy level more effectively compared to fluorination (31−60 meV/F versus 53−83 meV/Cl), resulting in a narrowing of the optical bandgap. We find the preference for electron transport to increase with increasing halogenation of cHBC. As an example, thin-film transistors fabricated with 8F-8Cl-cHBC demonstrated electron mobilities as high as 10−2 cm2/(V s) and solar cells with 8F-8Cl-cHBC and poly(3-hexylthiophene), P3HT, showed power-conversion efficiencies as high as 1.2%. molecular substructures, such as perylene mers;12−18 this nonplanarity prevents large-scale aggregation and crystallization in bulk-heterojunction active layers.6,10,13,15,16,18 As opposed to linking planar substructures to create conformationally nonplanar compounds, we have chosen to investigate derivatives of an intrinsically nonplanar molecular semiconductor, contorted hexabenzocoronene (cHBC), as potential electron acceptors. The parent compound, cHBC, has been previously shown to be an effective electron donor; solar cells with (6,6)-phenyl C71-butyric acid methyl ester (PC71BM) have yielded power conversion efficiencies (PCEs) as high as 2.41%.19,20 However, cHBC has yet to be considered as an electron acceptor because of the relatively high energy (−2.2 eV) of its lowest unoccupied molecular orbital (LUMO), which precludes efficient charge transfer from the electron donor and to the cathode. Starting with electrically active cHBC, this paper examines the lowering of its LUMO energy level through halogenation.

1. INTRODUCTION Planar aromatic molecules traditionally have been favored as organic semiconductors since nonplanar conformations tend to disrupt conjugation, negatively impacting the intramolecular electron distribution, and to reduce intermolecular π-stacking, thereby hindering charge transport. Fullerenes being molecularly spherical, however, are outstanding counter examples as they have consistently outperformed other molecular electron acceptors in organic solar cells. This exception to the general belief appears to stem from fullerene’s three-dimensionality as it favors partial miscibility with most polymer donors for efficient exciton dissociation and charge separation1−4 and is thought to support more isotropic charge transport in the solid state.5−7 Despite these attractive qualities, fullerenes are costly to produce and have low absorptivity. These drawbacks have spurred interest in developing alternative nonfullerene electron acceptors.8,9 While traditionally planar molecules have been investigated, several groups have recently advocated the development of highly nonplanar electron acceptors that mimic fullerenes’ three-dimensional geometry.6,8,10 Indeed, despite a recent exception,11 the vast majority of the highperforming nonfullerene electron acceptors have nonplanar conformations by linking together two or more planar © 2015 American Chemical Society

Received: January 26, 2015 Revised: February 17, 2015 Published: February 18, 2015 1892

DOI: 10.1021/acs.chemmater.5b00329 Chem. Mater. 2015, 27, 1892−1900

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Chemistry of Materials While fluorination has been a commonly utilized method of lowering the LUMO and highest occupied molecular orbital (HOMO) energy levels of organic semiconductors, functionalization with other halogens has been less explored, in part because fluorine is the most electronegative among the halogens and is thus expected to produce the largest impact on the energy levels per halogen introduced.21,22 However, Tang et al. showed that chlorination lowers the LUMO energy level of planar molecular semiconductors, including acenes, phthalocyanines, and perylene tetracarboxylic diimides, more effectively than fluorination so long as the introduced chlorine atoms do not distort the planarity of the conjugated core.21 Inspired by this work and interested in elucidating the impacts of halogenation on the energy levels of an intrinsically nonplanar compound, we have designed, synthesized, and characterized three halogenated series of cHBC derivatives having increasing fluorines, chlorines, or a mixture of fluorines and chlorines at the outer periphery of the parent compound, the chemical structures of which are shown in Figure 1.

2. EXPERIMENTAL SECTION 2.1. Cyclic Voltammetry (CV). CV was run on ca. 0.25 mM solutions of the cHBC derivatives in tetrahydrofuran (THF) containing 500 mg of tetrabutylammonium hexafluorophorphate. Platinum, glassy carbon, and silver electrodes (CH Instruments, Inc.) were used as the working, counter, and reference electrodes, respectively. Scans were cycled from 1 to −2.5 V three times with a rate of 0.1 V/s to ensure reproducibility. Ferrocene (HOMO: −4.8 eV) was used as an internal standard, and the difference between cHBC derivatives’ reductive half-potential and ferrocene’s oxidative half-potential was used to approximate the cHBC derivatives’ LUMO energy levels. 2.2. Solution-Based Absorption. Solution-based absorption spectra were collected on ca. 19 μM solutions of cHBC derivatives in stabilized tetrahydrofuran using a 10 mm quartz spectrophotometer cell and Agilent 8453 spectrometer. 2.3. Density Functional Theory (DFT). DFT calculations were performed using the Gaussian 09 software package.23 The B3LYP/6-31G(d) basis set and functional were used for singlemolecule geometry optimization, while B3LYP/6-311G(2d, p) was used for energy calculations and population analyses. Rates of electron transfer along k1, k2, and k3 (see the Supporting Information (SI), Figure S1) were calculated using Marcus theory: V2 ⎛ π ⎞ ⎜ ⎟ ℏ ⎝ λkBT ⎠

1/2

k=

⎛ λ ⎞ exp⎜ − ⎟ ⎝ 4kBT ⎠

(1)

where k is the rate of charge transfer, V is the hole or electron transfer integral, λ is the reorganization energy (see the SI, Table S1), and T is the temperature (300 K). Total reorganization energies were estimated by calculating the intramolecular reorganization energies of individual molecules.24 Transfer integrals were calculated between the k1, k2, and k3 pairs of molecules (see the SI, Table S2).25 Relative orientations of cHBC, 8F-cHBC, and 16F-cHBC molecules were determined using structures fit to single crystal diffraction data for the transfer integral calculations.26,27 In the absence of single crystal diffraction data for 8F-8Cl-cHBC, 3 × 3 × 3 unit cells of 8F-8Cl-cHBC with a P21/c space group (similar to that of cHBC)27 were constructed and then optimized using TINKER’s XTALMIN program with the MM3 force field.28,29 Transfer rates along each direction were used to estimate the mobilities (see the SI, Table S3) using the method prescribed by Deng and Goddard.30,31 2.4. Ultraviolet Photoelectron (UPS) and Inverse Photoemission (IPES) Spectroscopy. UPS and IPES measurements were performed on films evaporated in ultrahigh vacuum and transferred in vacuo to an analysis chamber equipped with both techniques. UPS was done using a He discharge lamp delivering He I photons (21.22 eV). The energy resolution in UPS was 0.15 eV. IPES was performed in the isochromat mode, using a setup described elsewhere.32 The energy resolution in IPES was 0.45 eV. 2.5. Grazing-Incidence X-ray Diffraction. Grazingincidence X-ray diffraction was collected at the G1 station (10.53 keV) of the Cornell High Energy Source Synchrotron using a two-dimensional CCD detector, positioned 97.2 mm from the sample. Given the short sample-to-detector distance, samples were made 0.5 cm wide to reduce geometric smearing. The beam energy was selected with synthetic multilayer optics (W/B4C, 23.6 Å d-spacing). 2D-GIXD images were collected

Figure 1. Chemical structures of contorted hexabenzocoronene (cHBC) and its halogenated derivatives studied in this work. The derivatives can be classified into three series with increasing fluorine, chlorine, or mixed fluorine and chlorine substitution. 1893


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Chemistry of Materials

substrates, which were cooled for 30 min prior to analysis on an in-chamber cryostage. A 2 kV O2+ beam, with a current of ∼45 nA, was rastered across a 300 μm × 300 μm lateral area as the material was ablated and larger depths were explored. Of this lateral area, only the central 15% of the area was analyzed for composition by collection of negative secondary ions. 2.9. NEXAFS. NEXAFS experiments were carried out at the NIST/DOW soft X-ray materials characterization facility at the U7A station at the National Synchrotron Light Source at Brookhaven National Laboratories. Partial electron yield (PEY) NEXAFS spectra were acquired at the carbon K-edge at three different X-ray incident angles: 75°, 55°, and 35°. The spectra were normalized by the corresponding incident beam intensity and then subsequently pre- and postedge normalized. The dichroic ratio, a parameter to quantify the molecular orientation, can be extracted from the intensity variation at different X-ray incident angles. The details of the DR extraction were described previously.37

with the beam having a 0.20° incident angle with the ITO substrate to satisfy the grazing condition. 2D-GIXD images have been background subtracted, and polarization and absorption corrections were applied, though these corrections were small.33 2.6. Thin-Film Transistors. cHBC films (80−100 nm thick) were thermally evaporated atop hexamethyldisilazanetreated Si(100) wafers with thermally grown, 300 nm thick oxide layers (Process Specialties, Inc.). Top contacts, Au for hole transport and Al for electron transport, were evaporated through a stencil mask to define channels having 100 μm lengths and 2000 μm widths. For films undergoing postdeposition processing to improve field-effect mobilities (i.e., 8F8Cl-cHBC), thermal annealing was performed under nitrogen and prior to top-contact evaporation. All transistors were tested under nitrogen using an Agilent 4155C semiconductor parameter analyzer, and field-effect mobilities were calculated in the saturation regime. 2.7. Solar Cells. Prepatterned ITO on glass substrates (15 Ω/square, Colorado Concept Coatings) was cleaned by sonication in acetone, isopropyl alcohol, and dionized water and then dried with nitrogen. The cleaned ITO was then oxygen plasma treated for 10 min. A 60 nm layer of PEDOT:PSS (Heraeus Clevios P) was spin-coated and annealed at 150 °C in air for 15 min. To achieve P3HT thicknesses between 18 and 45 nm, the concentration of the P3HT solution and the spin-coating speed were varied. P3HT solutions were stirred overnight in air and spin-coated the following day in air. The best performing devices had a P3HT thickness of 37 nm, which was obtained by spin-coating a filtered 5 mg/mL P3HT (Merck Chemicals Ltd., Batch #98202) solution from chlorobenzene at 500 rpm for 60 s. Devices were then transferred to a glovebox for thermal evaporation of 8F-8Cl-cHBC. After deposition, the films were annealed in the glovebox on a hot plate at 240 °C for 45 min before they were removed from the hot plate and allowed to cool at room temperature. Subsequently, 1 nm LiF and 60 nm Al were evaporated sequentially to define an active area of 0.18 cm2. Current density−voltage (J−V) characteristics were acquired in nitrogen with a Keithley 2400 source measurement unit under AM-1.5G 100 mW/cm2 illumination. For each P3HT and 8F-8Cl-cHBC thickness, 6−18 devices were tested. We note that device characteristics appear to change with illumination on a 1 h time scale, the origin of which is still to be understood. The averages of the temporally best-performing devices are reported. The unusually high thermal annealing temperature, above the melting temperature of P3HT, necessitates consideration of the thermal stability of the other organic material in the device stack. Under nitrogen, P3HT undergoes only minimal degradation, equivalent to 0.3% mass loss, after thermal annealing at 250 °C for 60 min.34 8F-8Cl-cHBC also has excellent thermal stability. Taking the thermal decomposition temperature to be the temperature at which a sample has lost 5% of its mass, 8F-8Cl-cHBC has a thermal decomposition temperature in air of 327 °C (SI, Figure S3). PEDOT:PSS likewise has a high thermal decomposition temperature of approximately 300 °C.35,36 2.8. Secondary Ion Mass Spectrometry (SIMS). Dynamic SIMS was performed on a Physical Electronics 6650 instrument with quadropole detection within the Materials Research Laboratory at UCSB. Depth profiles of the thin films and multilayer were collected on SiO2/Si

3. RESULTS AND DISCUSSION We have previously reported the modular synthesis of a series of cHBC derivatives having 8, 12, 16, or 20 fluorines,26 labeled as the fluorinated series (F-series) in Figure 1. Utilizing this same synthetic pathway with boronic acids having different halogen substitutions, we have synthesized two other series of halogenated cHBCs, functionalized with either chlorines (Clseries) or a combination of fluorines and chlorines (F−Clseries). The LUMO energy levels of the halogenated cHBC derivatives were approximated from their first reduction potential observed using cyclic voltammetry (CV) with ferrocene (HOMO = −4.80 eV)38 as the calibrant (see the SI, Figure S4, for voltammograms). Figure 2a shows these CV-

Figure 2. Energy levels of the lowest unoccupied (LUMO) and highest occupied (HOMO) molecular orbitals of the fluorinated (black circles), fluorinated-chlorinated (red squares), and chlorinated (blue triangles) derivatives of cHBC as determined by (a) cyclic voltammetry and solution absorption spectroscopy, (b) density functional theory (DFT), and (c) inverse photoemission (IPES) and ultraviolet photoelectron (UPS) spectroscopies. Note that in panel c, the energy levels of 8Cl- and 12Cl-cHBC were estimated based on an extrapolation of the linear relationships between the energy levels obtained via DFT and UPS/IPES for the fluorinated and fluorinatedchlorinated compounds.

derived LUMO estimates for the derivatives in the F- (black circles), F−Cl- (red squares), and Cl-series (blue triangles). The linear relationship between their LUMO energy and the number of halogens decorating their periphery is immediately noticeable. Although only a handful of studies have systematically examined how the energy levels of planar molecular semiconductors change with increasing substitution of electron1894


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to their unsubstituted parent compounds, which, based on density functional theory (DFT) calculations, they attributed to chlorine more severely lowering the LUMO energy as compared to the HOMO.45 Comparing various electronwithdrawing functionalities, Kuvychko et al. likewise predicted via DFT calculations that chlorination lowers the LUMO energy level of coroannulenes more effectively than fluorination, though they did not comment on the effect on the HOMO energy levels or optical bandgaps of the derivatives.43 Figure 2b shows the results of our DFT calculations of the molecular orbital energy levels of the F, F−Cl, and Cl series, which, consistent with our characterization, indicate that the LUMO is lowered by 60 meV/F and 83 meV/Cl. While the observation that chlorination has a stronger impact in lowering the LUMO energy levels of molecules and red-shifting their absorption in itself is not new, these prior works have only examined planar systems. Our work demonstrates that this correlation is generalizable to contorted molecular systems as well. In an attempt to address the difference in efficacy fluorine and chlorine substitution have on the energy levels of molecular semiconductors, Tang et al. hypothesized that chlorination introduces empty 3d orbitals, which are able to better accommodate π-electrons.21 Yet another possibility is that strong (2p-2p) π-overlap between fluorine and the aromatic πsystem results in back-donation of electron density from the fluorine to the aromatic π-system via the mesomeric effect, thereby reducing fluorine’s effective electron-withdrawing capability compared to chlorine. In fact, this latter hypothesis was proposed to be responsible for C6Cl5− being a more electron-withdrawing substituent than C6F5− on tri(arly)boranes.46 To follow up, we performed Mulliken population analysis on the DFT-derived HOMO and LUMO energy levels of halogenated cHBCs, from which we can estimate the fraction of total electron density within a given molecular orbital that is located at the edge atoms (i.e., at H, F, and Cl atoms) versus in the core (i.e., all C atoms) of each compound. The results of this analysis are summarized in the SI, Figure S5. Looking specifically at the Mulliken population analysis on 12F- and 12Cl-cHBC’s LUMOs, we find that the fraction of total electron density located at the periphery of 12Cl-cHBC is double that of 12F-cHBC. The trend that the chlorinated cHBC derivatives have greater edge electron density than fluorinated derivatives persists across our study and affirms that, while chlorine may not be as electronegative as fluorine, it is indeed able to accommodate a higher electron density. We also measured the solid-state electronic properties of the halogenated cHBC derivatives via inverse photoemission (IPES) and ultraviolet photoelectron (UPS) spectroscopies of thermally evaporated thin films under vacuum. Figure 2c shows the LUMO and HOMO energy levels of the F- and F−ClcHBC series as determined by IPES and UPS, respectively (see the SI, Figure S6, for spectra). By studying the HOMO and LUMO energy levels via CV, DFT, and UPS/IPES, we have determined linear relationships between the HOMO and LUMO energy levels derived using these various techniques, which has in turn allowed us to predict the UPS/IPES-derived HOMO and LUMO energy levels of the chlorinated derivatives based on their DFT- and CV-derived energy levels. The UPS/ IPES results indicate that cHBC’s LUMO energy levels lower 48 meV/F and 73 meV/Cl. Though the absolute values of the HOMO and LUMO energy levels vary depending on the technique, results from all

withdrawing groups, these reports noted a similar linear relationship between the LUMO energy and the number of electron-withdrawing functional groups.39−43 Our observation, however, is the first indication that halogenation of the peripheral aromatic rings of cHBC can impact the overall electronic properties of the compound despite its nonplanar conformation. The effectiveness of halogen-substitution at the periphery of cHBC in lowering the LUMO energy level is especially surprising as cHBC was previously reported to have two separate π-systems due to its nonplanar nature: a lowenergy planar radialene core, through which charge transport primarily occurs, and high-energy out-of-plane phenyl rings.44 Yet, the appreciable change in the LUMO energy level with halogen substitution on the high-energy out-of-plane phenyl rings suggests intramolecular electronic communication between the core and peripheral aromatic rings. While the LUMO energy level is lowered linearly with the addition of halogen atoms across all three series, chlorine substitution lowers the LUMO energy level more effectively than fluorine substitution, despite Cl being less electronegative than F. Specifically, cHBC’s LUMO lowers by 31 meV/F and 53 meV/Cl, according to our CV measurements. Because of the limited oxidative−reductive window of the solvent (THF), only the reduction potentials of the halogenated cHBCs can be probed via CV. To estimate the HOMO energy levels, the optical bandgaps, as derived from solution absorbance studies, were subtracted from the CV-derived LUMO energy levels. Figure 3 shows the absorption spectra acquired on solutions of

Figure 3. Absorption spectra of the (a) fluorinated, (b) fluorinatedchlorinated, and (c) chlorinated cHBC derivatives in THF. Absorption of the halogenated cHBC derivatives does not significantly change with fluorination but noticeably red-shifts with increasing chlorination.

the halogenated cHBC derivatives in THF. The absorption spectra of the fluorinated cHBCs (Figure 3a) appear comparable to that of the parent compound. Strikingly, the absorption spectra of derivatives comprising chlorines (Figure 3b,c) are red-shifted compared to that of the parent compound. Linearly fitting the optical bandgaps of the Cl series of cHBC derivatives indicates a narrowing of the bandgap of 19 meV/Cl. Since the HOMO energy of the halogenated derivatives appears to primarily depend on the number of halogen substitution and not the type of halogen (i.e., fluorine versus chlorine), the narrowing of the bandgap seen in the compounds in the Cl series, as opposed to the F series, is a direct result of a more rapidly lowering LUMO energy level with chlorination, as opposed to with fluorination. In addition to the previously discussed work of Tang et al.,21 Tan et al. also reported narrower optical bandgaps for a variety of perchlorinated polycyclic aromatic hydrocarbons compared 1895


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We constructed bottom-gate, top-contact TFTs with amorphous films with both Au and Al top contacts to screen for hole and electron transport, respectively. Given that the electrodes and device structures were the same in each case, differences in mobilities must reflect the ability to inject and extract charges into and out of the channel based on energy level differences. Without accounting for contact resistance, which may also influence the extracted field-effect mobilities, we observed only hole-transport in cHBC and 8F-cHBC films with average hole field-effect mobilities of 2.4 × 10−4 cm2/(V s) (from ref 52) and 1.4 × 10−5 cm2/(V s), respectively, and only electron-transport in 16F-cHBC and 8F-8Cl-cHBC with average electron field-effect mobilities of 5.3 × 10−4 cm2/(V s) and 1.7 × 10−3 cm2/(V s), respectively. By thermally annealing 8F-8Cl-cHBC films at 240 °C to induce crystallization, polycrystalline TFTs can exhibit field-effect electron mobilities as high as 1.1 × 10−2 cm2/(V s) (avg. 8.7 ± 2.1 × 10−3 cm2/(V s), see the SI, Figure S8). We separately also estimated the hole and electron mobilities of cHBC and 8F-, 16F-, and 8F-8Cl-cHBC from charge transfer integral calculations performed using crystal structures. Comparing the calculated hole and electron mobilities of each (reported in Table S3 in the SI), we find that our calculations predict hole transport to dominate in cHBC, 8FcHBC, and 16F-cHBC and electron transport to dominate in 8F-8Cl-cHBC. Though not a one-to-one comparison since the experimentally derived mobilities arise from devices with amorphous films whereas the charge transfer integral calculations are performed using crystal structures, it is nonetheless gratifying to see consistency between experiments and calculations. Honing in on 8F-8Cl-cHBC as an electron acceptor for solar cells, we constructed devices by spin-coating P3HT atop PEDOT:PSS-coated prepatterned ITO substrates, thermally evaporating 8F-8Cl-cHBC atop the P3HT, and thermally annealing the material stacks prior to evaporating through shadow masks 1 nm LiF and 60 nm Al layers as top contacts. Optimizing for the thickness of the P3HT and 8F-8Cl-cHBC layers (see the SI, Table S5) as well as the thermal annealing temperature and time, we obtained power PCEs as high as 1.2% (average 0.94 ± 0.06%) with P3HT and 8F-8Cl-cHBC thicknesses of 37 and 25 nm, respectively, and with thermally annealing the active layer at 240 °C for 45 min. For reference, the highest reported PCE of a solar cell with P3HT and a nonfullerene electron acceptor is 4.1%.57 A handful of other nonfullerene electron acceptors have demonstrated device PCEs above 2.0% with P3HT,13,14,58−60 but more typically the PCEs of such devices are between 0.5 to 1.5%.10,61−69 While our best devices suffer from low Jsc (−2.65 ± 0.19 mA/cm2) and low fill factor (0.41 ± 0.01), they demonstrate high Voc (0.86 ± 0.02 V), as can be expected based on the simple difference between P3HT’s HOMO and 8F-8Cl-cHBC’s LUMO energy levels and not accounting for possible vacuum level shifts or the presence of interfacial dipoles when the two constituents are brought into contact.51 Figure 4 contains the external quantum efficiency, EQE, as a function of wavelength of an optimized P3HT/8F-8Cl-cHBC solar cell. Comparing the EQE spectrum with the absorption spectra of 8F-8Cl-cHBC and P3HT films, we see that 8F-8Cl-cHBC efficiently converts photons to carriers from 350 to 450 nm. To better understand the observed solar cell performance and its improvement upon thermally annealing, we performed a suite of structural characterization on optimized active layers

three sets of experiments, CV/UV−vis, DFT, and UPS/IPES, support a consistent conclusion that both fluorination and chlorination can effectively lower the LUMO and HOMO energy levels of cHBC despite its nonplanarity, with chlorination being more effective at lowering the LUMO energy than fluorination, manifesting in a narrowing of the optical bandgap. Variations across the techniques stem from the fact that CV studies are performed in solution and are subject to charge-screening and mass transport effects,47 whereas DFT calculations are performed assuming a single molecule in a vacuum and are highly sensitive to molecular conformation.48 Additionally, UPS/IPES measurements determine the electronic bandgap between the HOMO and LUMO energy levels, which is larger than the optical bandgap derived from absorbance studies because the electronic bandgap includes the exciton binding energy. That we are able to lower the LUMO energy level of cHBC substantially through halogenation led us to believe that the halogenated cHBC derivatives may be suitable as nonfullerene electron acceptors in organic solar cells. Assuming the conventional view of exciton delocalization and charge separation driven by the energy offset between the LUMO levels of the electron donor and acceptor, the LUMO level of a potential electron acceptor must lie at least 0.3 eV below the LUMO of the donor.49,50 However, a LUMO that is too deep is not desirable either, as the open-circuit voltage, VOC, of devices that comprise this material pair scales with the difference between the donor’s HOMO energy level and the acceptor’s LUMO. The LUMO level of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), for example, as measured by IPES, is −3.80 eV, which generally results in a moderate VOC of 0.6 V in solar cells with poly(3-hexylthiophene), P3HT.51 Given these criteria, we chose to screen three compounds, 8F-, 16F-, and 8F-8Cl-cHBC, for electrical activity. Like the parent compound, these halogenated cHBC derivatives form amorphous films upon thermal evaporation (see the SI, Figure S7, for two-dimensional grazing-incidence X-ray diffraction (2D-GIXD) images); postdeposition thermal and solvent-vapor annealing can be applied to induce crystallization.52 These annealed films, however, are polycrystalline and textured to different extents, so the electrical properties derived from thinfilm transistors (TFTs) comprising these structurally heterogeneous films are necessarily complicated by differences in grain boundary density and molecular orientation.53,54 Cases in point are hole mobilities extracted from polycrystalline triethylsilyl anthradithiophene, TES ADT, and cHBC TFTs, which we have shown to vary by 1 to 2 orders of magnitude depending on grain boundary density and molecular orientation, respectively.27,52,55 To assess the electronic properties of the compounds in the absence of structural heterogeneities, we would have liked to construct single-crystal transistors.56 We were, however, unable to grow crystals large enough for this purpose with all the halogenated compounds of interest. Still with the aim to assess and compare intrinsic electrical activity of the compounds in absence of structural heterogeneities, we have instead made TFTs with amorphous films of 8F-, 16F-, and 8F-8Cl-cHBC recognizing that while the extracted mobilities may be low, the amorphous nature of the films allows us to attribute any mobility differences to the electronic nature of the compounds, as opposed to structural heterogeneities that would otherwise be present in polycrystalline films. 1896


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Figure 4. (Left axis) Absorption spectra of unannealed P3HT (red) and 8F-8Cl-cHBC (black) thin films, and an annealed stack of P3HT/ 8F-8Cl-cHBC (blue), as used in solar cells. (Right axis) The external quantum efficiency of a thickness-optimized and thermally annealed P3HT/8F-8Cl-cHBC solar cell.

before and after thermal annealing. Figure 5a shows a twodimensional grazing-incidence X-ray diffraction (2D-GIXD) image of a material stack of P3HT and thermally evaporated 8F-8Cl-cHBC, analogous to that employed in our best solar cells. The intensities along the meridian (qz axis) observed can be assigned to the (h00) reflections of P3HT, indicating that P3HT is semicrystalline and preferentially oriented with its πplane normal to the substrate. The absence of any diffraction attributable to 8F-8Cl-cHBC indicates that the electron acceptor is amorphous as-evaporated, not unlike thermally evaporated thin films of its parent compound. Secondary ion mass spectrometry (SIMS) of the as-evaporated bilayer film is shown in Figure 5b and provides a depth profile. We can identify P3HT by tracking the sulfur signal, and 8F-8Cl-cHBC by tracking the C−Cl and C−F signals. The depth profile of the as-deposited P3HT/8F-8Cl-cHBC stack, measured 2 weeks after film formation, shows a well-defined interface between 8F8Cl-cHBC and P3HT, indicating that diffusion of the small molecule into the polymer is minimal at room temperature or during the course of 8F-8Cl-cHBC thermal evaporation. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) acquired in the partial electron yield (PEY) mode was used to assess the chemical composition and molecular orientation in the top 2 nm70 of the P3HT/8F-8Cl-cHBC films. The NEXAFS spectra of the as-evaporated film, collected with the incident X-rays at three different angles relative to the substrate, are shown in Figure 5c. The orientation-independent magic-angle spectrum37 matches that of pure 8F-8Cl-cHBC (SI, Figure S9); consistent with the results of the SIMS experiments, NEXAFS thus indicates that the top 2 nm of the film is exclusively 8F-8Cl-cHBC. The dichroism, i.e., the intensity inversion exhibited by the C 1s to π* transitions (sharp peaks at ca. 285 eV) and C 1s to σ* transitions associated with the C−C bonds (broad peaks at energies higher than 290 eV) as a function of incident angle, indicates that, although 8F-8ClcHBC is not crystalline (as indicated by 2D-GIXD), 8F-8ClcHBC exhibits a slight preference for a “face-on” orientation with its π-plane parallel to the substrate. The ensemble-average preferred molecular orientation can be quantified with a dichroic ratio (DR),37,71 where a negative DR indicates a preferentially “face-on” orientation; a positive DR indicates a preferentially “edge-on” orientation; and a DR near 0 indicates

Figure 5. (a, e) 2D-GIXD, (b, f) SIMS, and (c, g) carbon-edge NEXAFS spectra of 8F-8Cl-cHBC/P3HT films before (left column) and after (right column) annealing at 240 °C for 45 min. (d, h) Schematics of the morphologies of the active layer before and after thermal annealing.

no preferencial orientation. Analysis of the NEXAFS spectra of the as-evaporated 8F-8Cl-cHBC film yields a DR of −0.25. As a point of reference, highly face-on oriented copper phthalocyanine (CuPc) on graphene yields a dichroic ratio of −0.61.72 Figure 5d summarizes the morphological findings from 2DGIXD, SIMS, and NEXAFS experiments as a schematic depiction of the morphology of the P3HT/8F-8Cl-cHBC film prior to thermal annealing. Figure 5e shows the 2D-GIXD image of the P3HT/8F-8ClcHBC film after thermal annealing at 240 °C for 45 min. We observe reflections associated with crystalline 8F-8Cl-cHBC; the intensities of these reflections are azimuthally isotropic, 1897


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junction active layer can be formed without the need to melt P3HT.

indicating that 8F-8Cl-cHBC is crystalline but not preferentially oriented. The intensities of P3HT’s (h00) reflections have decreased substantially compared to that in the diffraction pattern of the as-cast stack in Figure 5a. Given that, at the grazing incidence angle employed, we are probing through the entire materials stack, this observation indicates that the P3HT is less crystalline after the materials stack has been thermally annealed, an unusual observation as typically the ordering of P3HT increases with thermal annealing of the active layer.73 The SIMS depth profile of the film after annealing is shown in Figure 5f. The sulfur signal associated with P3HT and chlorine and fluorine signals associated with 8F-8Cl-cHBC are now more evenly distributed throughout the depth of the active layer, indicating that a significant amount of mixing occurs during the thermal annealing. We believe that the simultaneous crystallization of 8F-8Cl-cHBC and intermixing of the two constituents during thermal annealing, especially above the melting temperature of P3HT, frustrates P3HT’s subsequent recrystallization upon cooling. While we observe a slightly higher sulfur signal near the bottom of the film, indicating that the active layer-anode interface remains enriched in P3HT, a significant amount of P3HT has also migrated to the top of the film, at the active layer-cathode interface. The NEXAFS spectra of the annealed film are presented in Figure 5g and provide an opportunity to quantify the amount of P3HT present at the surface. Compositional analysis indicates that the top 2 nm of the film is composed of 27 wt % 8F-8Cl-cHBC and 73 wt % P3HT (SI, Figure S9). That P3HT preferentially segregates to the film surface has also been observed in bulk-heterojunction films of P3HT and PC61BM and is not surprising given its low surface energy.51 This observation points out that P3HT is lower in surface energy compared to 8F-8Cl-cHBC. This preferential segregation of P3HT to the surface, however, has previously been shown to not affect electron collection at the cathode in conventional solar cells.74−76 Returning to Figure 5g, the NEXAFS spectra of the annealed film are nearly invariant with incident angle; the lack of any dichroism indicates that the crystallites of 8F-8Cl-cHBC have little preferential orientation after annealing the active layer. Figure 5h summarizes the morphological findings from 2D-GIXD, SIMS, and NEXAFS experiments and depicts how significantly the morphology changes upon thermal annealing. In aggregate, our structural characterization indicates that thermal annealing of the active layer induces 8F-8Cl-cHBC crystallization and substantial intermixing with P3HT, creating in essence a bulk heterojunction and thereby increasing device performance. However, the performance of our devices is handicapped by low fill factors, likely due to contact resistance, and low Jsc, attributable to P3HT’s reduced crystallinity after thermal annealing. As Jsc typically scales with P3HT’s crystallinity,77 this reduction in crystallinity is an undesirable side effect of the thermal annealing process. In an effort to maintain P3HT crystallinity while crystallizing 8F-8Cl-cHBC, we also induced 8F-8Cl-cHBC crystallization via solvent-vapor annealing with hexanes, dichloromethane, carbon disulfide, and tetrahydrofuran. While these solvent vapors do crystallize 8F8Cl-HBC, PCEs of these devices were not markedly improved, presumably because less mixing had occurred between P3HT and 8F-8Cl-cHBC in the absence of P3HT melting. Adding solubilizing groups to 8F-8Cl-cHBC is a potential route to circumvent some of these issues. If 8F-8Cl-cHBC can be made solution-processable alongside P3HT, then a bulk hetero-

4. CONCLUSIONS In conclusion, we have designed, synthesized and characterized the electronic properties of halogenated cHBC derivatives, finding that peripheral halogen substitution effectively decreases the HOMO and LUMO energy levels and increases the likelihood of electron-transporting behavior even in nonplanar molecules. Chlorine and fluorine substitution equally decrease the HOMO energy level of cHBC, but chlorination more effectively decreases the LUMO energy level than fluorination (31−60 meV/F versus 53−83 meV/Cl) due to its ability to accommodate greater electron density. The more rapid decrease of the LUMO energy level with chlorination thereby results in an optical bandgap narrowing of 19 meV/Cl. The ability to tune the energy levels and bandgaps of organic semiconductors has implications for several performance criteria of organic solar cells, including their open-circuit voltage, associated energy barrier for charge transfer to and from electrodes, and coverage of the solar spectrum. We have demonstrated for the first time that a conformationally nonplanar cHBC derivative, having an appropriate energy level by halogen substitution, can be utilized as an electronacceptor in solar cells with P3HT, yielding PCEs as high as 1.2%.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis; NMR spectra; details of DFT and charge transfer integral calculations; TGA; cyclic voltammograms; UPS/IPES; GIXD; device fabrication and testing; NEXAFS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ⊥

Department of Chemical Engineering, Stanford University, Stanford, California # Department of Chemistry, University of California, Berkeley, California ∇ Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey & Department of Physics, Technische Universität Chemnitz, Chemnit, Germany Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M.H., L.S., H.W., and Y.L.L. acknowledge financial support from the NSF MRSEC program through the Princeton Center for Complex Materials (DMR-0819860, DMR-1420531) and the SOLAR Initiative at the NSF (DMR-1035217). J.D.S., P.C., and Y.L.L. acknowledge the financial support of the NSF Nanoelectronics Beyond 2020 (NEB) program, Award No. CHE-1124754, and a gift from the NRI, Award No. (Gift # 2011-NE-2205GB). L.K., F.L., and A.K. acknowledge support by the NSF (DMR-1005892). GIXD experiments were conducted at CHESS, which is supported by NSF and NIH/ NIGMS under Award DMR-0936384. SIMS experiments were conducted at MRL Central Facilities, a member of the NSF1898


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funded Materials Research Facilities, supported by the NSF MRSEC program (DMR-0520415). A.M.H. acknowledges support through the NDSEG Fellowship (Air Force Office of Scientific Research 32 CFR 168a); F.L. was funded by the German Academic Exchange Service; and M.A.B. thanks financial support from an NSF Graduate Research Fellowship. The authors thank Dr. Matthew Bruzek and Prof. John Anthony for providing the fluorinated pentacene quinone precursor, under the auspices of NSF DMR-103527, and Dr. Tom Mates for assistance with the dynamic SIMS instrument.

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Halogenation of a Nonplanar Molecular Semiconductor to Tune

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