Morphological and Device Evaluation of an Amphiphilic Block

Article pubs.acs.org/Macromolecules

Morphological and Device Evaluation of an Amphiphilic Block Copolymer for Organic Photovoltaic Applications Valerie D. Mitchell,† Eliot Gann,‡,⊥ Sven Huettner,§ Chetan R. Singh,∥ Jegadesan Subbiah,† Lars Thomsen,⊥ Christopher R. McNeill,‡ Mukundan Thelakkat,∥ and David J. Jones*,† †

School of Chemistry, University of Melbourne, Bio21 Institute, 30 Flemington Road, Parkville, Victoria 3010, Australia Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia § Organic and Hybrid Electronics, Macromolecular Chemistry I, and ∥Applied Functional Polymers, Macromolecular Chemistry I, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany ⊥ Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia ‡

S Supporting Information *

ABSTRACT: We report the morphological and photovoltaic evaluation of a novel fully conjugated donor/acceptor block copolymer system based on the P3HT-b-PFTBT scaffold. The incorporation of hydrophilic tetraethylene glycol side-chains into the PFTBT acceptor block generates an amphiphilic species whose properties provide demonstrable benefits over traditional systems. This design strategy facilitates isolation of the block copolymer from homopolymer impurities present in the reaction mixture, and we show that this purification leads to betterdefined morphologies. The chemical disparity introduced between donor and acceptor blocks causes spontaneous microphase separation into well-defined domains, which we demonstrate with a combination of spectroscopy, microscopy, and X-ray scattering. The morphological advantages of this system are significant; however, preliminary device characterization indicates a loss of electron mobility in the hydrophilic acceptor block.

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INTRODUCTION Block copolymers (BCPs) are a compelling and highly investigated class of compounds. These materials, in which two or more dissimilar polymers are covalently linked, are capable of self-assembly into ordered morphologies with length scales determined by the degree of polymerization.1 The ability to control the spatial arrangement of functional moieties at the nanoscale has facilitated the bottom-up design of materials for applications such as nanotemplating and nanolithography.2 The objective of this research is to exploit the control provided by block copolymer self-assembly to provide morphological stability in a system in which thin film structure is crucial: organic photovoltaics (OPV). In OPV, free charges arise from the separation of a photogenerated exciton at the interface between a donor and acceptor material, with electrons moving into the acceptor and positively charged holes moving into the donor. For this process to be efficient the interface between the two materials must be maximized, while donor/ acceptor domain sizes exceeding the Coulombic capture radius are required to prevent bimolecular recombination.3 Additionally, excitons are short-lived species with diffusion lengths of ∼10 nm.4 Thus, ideal morphologies form a contiguous, interpenetrating network of pure donor and acceptor domains with widths of 10−20 nm. In the majority of ongoing OPV research, these morphologies are obtained by dissolving the donor and acceptor materials together in a suitable solvent and depositing the blend © XXXX American Chemical Society

on the chosen electrode. This blend is then encouraged to partially phase separate, often through thermal treatment or solvent vapor annealing, to obtain what is called the bulk heterojunction active layer (Figure 1).5−8 If properly optimized and the appropriate morphology is obtained, the bulk heterojunction is often still only metastable; that is, the

Figure 1. Comparison of bulk heterojunction morphology (a) with vertically aligned block copolymer morphology (b). Received: February 20, 2017 Revised: June 18, 2017

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DOI: 10.1021/acs.macromol.7b00377 Macromolecules XXXX, XXX, XXX−XXX

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predominantly involved polymeric mixtures for this reason.27,31 The previously reported material P3HT-b-PFTBT, for example, was 52 wt % BCP with the remaining mass composed of homopolymer and triblock copolymer. The composition of these mixtures is difficult to determine and control, leading to decreased synthetic reproducibility. Additionally, because the presence of polymeric impurities can have a large effect on phase behavior,32,33 thorough purification of the block copolymer is crucial for morphological reproducibility. In this work we demonstrate the morphological benefit afforded by this strategy and evaluate P3HT-b-PFTEGT6BT (Figure 2) as

donor and acceptor phases are kinetically trapped above their thermodynamic minimum.9 Over the lifetime of the device, these films tend to phase separate further, coarsening and increasing domain size to suboptimal length scales.10,11 Topdown approaches to controlling morphology have shown improved performance as the domain sizes approach those required; however, physical limits to nanoimprinting processes, especially for the high aspect ratios required, ultimately restrict the applicability of these processes.12 Incorporating donor and acceptor into a single block copolymer physically prohibits macrophase separation, instead favoring self-assembly into domains with shape and dimension determined by polymer chain length and flexibility.1 The promise of morphological stability and microstructure formation has led researchers to design donor-block-acceptor copolymers for application in photovoltaic systems.13−16 The first generation of such block copolymers primarily employed n-type blocks composed of a flexible aliphatic backbone functionalized with pendant acceptor groups.17,18 While these systems achieved morphological control, the nonconjugated backbone is electrically insulating and so may inhibit charge transport.19 Thus, investigation of fully conjugated block copolymers is expected to produce materials with improved charge mobility. Additionally, the rigidity of fully conjugated block copolymers has the effect that lamellar structures dominate a larger space of the morphological phase diagram.20 A lamellar morphology, if oriented perpendicular to the electrodes, could maximize donor and acceptor interface while provide direct percolation paths for charge collection as shown in Figure 1b. These potential advantages have led several groups to explore fully conjugated block copolymer systems.21−25 The most successful application of this strategy employed poly(3hexylthiophene) (P3HT) as the electron donor linked directly to a poly(2,7-(9,9′-dioctylfluorene)-alt-5,5-(4′,7′-dithienyl2′,1′,3′-benzothiadiazole) (PFTBT) acceptor. The resultant block copolymer achieved a power conversion efficiency (PCE) of 3.1%, higher than optimized blends of the same materials.26 This result was quite promising; however, it remains unmatched even in a series of 30 closely related P3HT-bPFTBT derivatives with varying solubilizing side-chains.27 The authors attributed the lower PCEs to poor domain resolution, a function of increased miscibility between the blocks conferred by the addition of side-chains. This confirms separate work on P3HT-b-PFT6BT, which incorporated hexyl side-chains at the thienyl units, in which no indication of phase separation was seen.28 We have recently reported the design and synthesis of amphiphilic derivatives of the P3HT-b-PFTBT system in which tetraethylene glycol side-chains are affixed to the fluorenyl unit.29 This modification was intended to increase the Flory− Huggins parameter30 and thus amplify phase separation in the system. Additionally, the chemical disparity introduced between the isolated blocks and the target BCP was exploited to enable isolation of the product from the polymeric reaction mixture. A lack of controlled polymerization techniques for the production of n-type polymers means that often polycondensations must be employed. This results in reaction mixtures of multiple polymeric species, including the isolated homopolymers and triblock copolymers in addition to the diblock copolymer product. Because the donor, acceptor, and block copolymer often have similar solubilities, it is difficult to isolate the block copolymer product. Previous work on block copolymers has

Figure 2. Structure and summary of P3HT-b-PFTEGT6BT properties. Number-average molecular weights (Mn) were determined from gel permeation chromatography (GPC) with PS calibration and THF as eluent or integration of 1H NMR backbone signals.a HOMO and LUMO were determined from cyclic voltammetry.

OPV active layer material. While the thin-film behavior was promising, device performance was low as a result of significantly decreased electron mobility in the hydrophilic block.

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EXPERIMENTAL SECTION

Materials and Methods. All commercial reagents were used without further purification unless otherwise noted. Synthesis and purification of the monomers, block copolymer P3HT-b-PFTEGT6BT, the homopolymer PFTEGT6BT, and the macroinitiator P3HT were reported previously.29 Charge mobility measurements and device construction are detailed in the Supporting Information. Absorption measurements were recorded of block copolymer films spin-cast on quartz slides using a Varian Cary 50 UV/vis spectrophotometer. Sample concentrations were 1 wt % in the various solvents described. The quartz substrates were cleaned by sonication in acetone followed by isopropanol for 15 min each and then UV/ozone treated. Atomic force microscopy measurements were conducted on an Asylum MFP-3D using Olympus model AC240TS-R3 probes with a spring constant of 2 N/m and a frequency of 70 kHz. Samples for AFM Analysis were prepared by spin coating 2 wt % solutions of the block copolymer, homopolymer blend, or block copolymer/homopolymer blend in o-xylene with 2 wt % 4-bromoanisole content. The substrates were silicon wafers that had been sonicated in acetone and isopropanol for 30 min each followed by 15 min of UV/ozone treatment. X-ray Scattering Measurements and Analysis. Samples for wideand small-angle X-ray scattering transmission measurements were prepared by loading the powdered polymeric materials into perforated aluminum disks and sealed with Kapton tape or left free-standing. The samples were analyzed at the SAXS/WAXS beamline at the Australian synchrotron34 with an X-ray energy of 10 keV. A Pilatus 200K detector was used for wide-angle measurements, and a Pilatus 1M detector was used for small-angle measurements. Block copolymer samples for GIWAXS were prepared by spin-coating 2 wt % solutions in o-xylene with 2 wt % 4-bromoanisole content. The substrates were silicon wafers that had been sonicated in acetone and isopropanol for 30 min each followed by 15 min of UV/ozone treatment. Samples were analyzed at the SAXS/WAXS beamline at the Australian synchrotron34 B


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Macromolecules with an X-ray energy of 11 keV and a range of incident angles from Ω = 0.02−0.35 in 0.005 increments to allow signal optimization near the critical angle of the polymer film but below the critical angle of the substrate. Data from GIWAXS experiments were analyzed using a customized version of NIKA 2D based in IgorPro.35 Hermans orientation parameters were calculated for incident angles ranging between 0.105 and 0.15 in steps of 0.05 and averaged over this range for each sample. An estimation of error was calculated from the standard deviation of these results and given in parentheses in Table 1.

correlate with a decreased free exciton bandwidth and increased conjugation length, leading to higher hole mobilities in thin film.43 Thus, UV/vis absorption spectroscopy can be employed to identify solvent systems expected to produce the highest performance in BCP devices. The effect of solvent deposition on UV/vis absorption of P3HT-b-PFTEGT6BT is summarized in Figure 3. The spectra were normalized to absorbance at 390

Table 1. Summary of GIWAXS Scattering Dataa sample

S

CCL (nm)

d[100] (nm)

d[020] (nm)

as-spun 15 min 2h

0.35(1) 0.39(2) 0.31(1)

10 22 22

1.67 1.70 1.70

0.38 0.38 0.38

a

S is the Herman’s orientation parameter with the uncertainty of the final digit given in parentheses, CCL is the crystallite correlation length in nm, d[100] is the alkyl spacing taken from the out-of-plane scattering, and d[020] is the π-spacing taken from in-plane scattering.

Samples for resonant soft X-ray scattering (RSOXS) were prepared in identical solvent concentration and composition as for GIWAXS analysis. The substrates were glass slides that had been sonicated in acetone and isopropanol for 30 min each followed by 15 min of UV/ ozone treatment. Following deposition of a sacrificial NaPSS layer, the block copolymer films were deposited and analyzed as is or after annealing under nitrogen for 1 h 45 min at 200 °C followed by 15 min at 220 °C. NEXAFS spectra of samples were first collected at the SXR beamline of the Australian Synchrotron36 to allow identification of the spectral features of different samples and predict where contrast between domains might be maximized. Samples for NEXAFS were prepared by spin-coating the polymer solutions on n-doped silicon wafers. NEXAFS spectra were analyzed and contrast functions calculated using QANT.37 RSoXS was collected at beamline 11.0.1.2 of the Advanced Light Source in Lawrence Berkeley National Laboratory.38 From the glass slides, films were floated off onto 100 nm thick silicon nitride windows. The films were aligned normal to the X-ray beam, and scattering patterns were collected at energies across the carbon K absorption edge (260−320 eV). Analysis was completed using a customized version of NIKA 2D based in IgorPro.35

Figure 3. UV/vis absorption spectra of P3HT-b-PFTEGT6BT in films deposited from solvents including chloroform (CHCl3), chlorobenzene (CB), 4-bromoanisole (BrAni), and o-xylene. Features corresponding to the 0−1 and 0−0 transitions are indicated.

nm, where contribution from P3HT is minimal and PFTBT materials exhibit a characteristic feature which has been attributed to fluorene absorbance.44 4-Bromoanisole is a poor solvent for P3HT, leading to aggregation in solution and increased domain purity in films.28,45 Additionally, 4-bromoanisole (BrAni) is categorized as nonhazardous and so is a potential candidate for industrial processes. For these reasons, a small amount of 4-bromoanisole was added to a selection of solvents with varying boiling points and solute compatibility. Chloroform with its low boiling point showed the smallest degree of P3HT crystallinity as determined from the vibronic structure, with no significant benefit from the addition of 2-bromoanisole. This is expected from previous work showing that deposition from chloroform freezes in poorly crystalline structures in P3HT. Chlorobenzene has a higher boiling point, allowing better organization during film formation; however, the 0−0 and 0−1 transitions are only poorly defined. The absorbance of films spun from chlorobenzene with added 2-bromoanisole, however, shows increased intensity at 610 and 550 nm, indicative of interchain interactions in P3HT. While the chlorobenzene/4-bromoanisole solvent combination was promising, a move away from halogenated solvents is crucial for commercial application. Xylene has been classified as problematic but nonhazardous in a ranking of industrially relevant solvents and has been widely investigated as a nonchlorinated alternative to existing OPV solvents.46−48 Xylene has also been shown to be a marginal solvent for P3HT, causing aggregation in similar fashion to 4bromoanisole. In the system at hand, films spun from xylene as solvent did indeed show a slight increase in vibronic absorption over those of chlorobenzene, and another slight enhancement was achieved with the addition of 4-bromoanisole. From this analysis films spun from 2% 4-bromoanisole in o-xylene (BrAni/Xyl) were predicted to show the largest degree of microphase separation and hole mobility within P3HT domains.

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RESULTS AND DISCUSSION Spectroscopic Evaluation of Solvent Effects. P3HT is a semicrystalline polymer known to adopt an anisotropic lamellar structure, a characteristic giving rise to distinct spectral signatures.39 Although the relationship between crystallinity and microphase separation is often competitive and varies widely between block copolymer systems, it is certainly true that if no phase separation is obtained then crystallization will be inhibited.28,40,41 The amphiphilic block copolymer at hand was designed to be a strongly segregated system, and so crystallization is expected to be confined to the domains defined by microphase separation.42 Accordingly, the ability of P3HT to form well-ordered crystallites may be indicative of block segregation. To begin the confirmation of microphase separation, absorption spectroscopy was performed on block copolymer films deposited from a range of solvents. The spectroscopic properties of P3HT have been correlated with the conjugation length, crystallinity, and charge mobility of the material. Specifically, crystalline P3HT has absorption features at 515, 550, and 610 nm which correlate to the 0−2, 0−1, and 0−0 transitions of the material, respectively.39 The intensity of the 0−0 transition can be related to the crystallinity of the material. An increase in absorption attributed to the 0−0 transition relative to the 0−1 feature has been shown to C


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conjugated block copolymers in the literature have employed blends of the block copolymer and homopolymer. To examine the importance and benefit of the amphiphilic block copolymer strategy in both purification and phase separation, the AFM of the blended homopolymers and a ternary blend of the block copolymer with P3HT and PFTEGT6BT were collected. The P3HT/PFTEG T6BT blend was prepared to mimic the proportions present in the block copolymer, with 47% P3HT and 53% PFTEGT6BT by weight. AFM of the homopolymer blend is given in Figure 4a. As-spun films are mostly featureless, with some areas showing a concentration of rod-like structures which we identify as the well-known P3HT nanofibrils.52 After annealing, the film becomes dominated by nodules with the elimination of any fibrillar features, possibly indicative of preferential segregation of the amorphous PF TEGT6BT homopolymer to the film surface. In Figure 4b, the AFM of a block copolymer/homopolymer blend is presented. The composition of the BCP/HP blend was 50% P3HT-b-PFTEGT6BT and 50% of the homopolymer blend by weight. While this represents a significant amount of homopolymer contamination, these ratios are representative of blends previously reported.27,31 In as-spun films, the BCP/HP mixture shows some structure reminiscent of swollen lamella in a matrix of featureless film. This is consistent with analyses done on nonconjugated BCP/HP blends in which the films phase separate into domains rich in HP and domains rich in BCP. It is easily surmised that this morphology would not be optimal for charge generation in OPV. With annealing, any linear structure in the BCP/HP blend film is eliminated. The film becomes largely featureless but fails to develop the nodules seen in the HP blend material. The AFM of films prepared from the isolated block copolymer are given in Figure 4c. In contrast to the blended films, the BCP samples show well-defined structure in both asspun and annealed films. The linear domains present in as-spun materials are on average 20 nm in width, and after annealing the morphology appears to sharpen, with the phase contrast between domains increasing slightly. While flexible polymers adopt coiled configurations leading to smaller domain sizes which scale with N2/3,53 rigidity in fully conjugated systems leads to chain extension and domain sizes that can approach the polymeric contour length.54 P3HT-b-PFTEGT6BT is composed of semiflexible polymers,55 and so domain sizes are expected to exceed coil-type dimensions but may not reach those of fully extended rods. The contour length of P3HT-b-PFTEGT6BT was calculated from the molecular weight. In rod-like polymers GPC overestimates the molecular weight by a factor of 1.5− 1.7,54,56 and after adjusting for this inflation the predicted contour length of the entire block copolymer is between 44 and 50 nm. From this calculation domain sizes in the 22−25 nm range would be expected for a fully extended polymer, which is consistent with the 20 nm domains observed in AFM. This comparison between the morphologies of the HP blend, HP/ BCP blend, and the BCP films illustrates the control attainable through the covalent linkage of donor and acceptor in OPV, while highlighting the importance of BCP purification and the large impact that homopolymer impurities can have on domain structure. Analysis of Crystallinity, Domain Size, and Orientation. While AFM can give some indication of film morphology, as a surface technique it is best considered in conjunction with measurements capable of probing internal structure. To further describe the morphology of the P3HT-b-PFTEGT6BT system,

In order to further elucidate the effects of the chosen deposition solvent, UV/vis of the block copolymer and homopolymers in solution was examined. It has been recently demonstrated that aggregation in solution prior to deposition can greatly enhance block copolymer ordering in films and may even be necessary for successful self-assembly.49 The absorption spectra of P3HT-b-PFTEGT6BT, P3HT, and PFTEGT6BT are given in Figure 4 for the materials dissolved in chloroform (an

Figure 4. Solution UV/vis absorption spectra of P3HT-b-PFTEGT6BT, P3HT, and PFTEGT6BT in chloroform and 2% 4-bromoanisole in oxylene.

excellent solvent) and BrAni/Xyl. The block copolymer shows an enhanced absorption at longer wavelengths in the xylene solution, indicating that the chosen solvent does have an effect on block copolymer behavior in solution. To elucidate the nature of this shift, we compare the absorption of the isolated donor and acceptor in the two solutions. P3HT in BrAni/Xyl shows a small red-shift, but no absorption at 610 nm which has previously been observed in aggregated solutions of P3HT.45 This implies that there is some rotational restriction of P3HT in BrAni/Xyl, but no significant interchain interaction. PFTEGT6BT shows a slight enhancement of the absorption band typically assigned to the thiophene−benzothiadiazole charge transfer state relative to the higher energy absorption band at 370 nm.50 This relative enhancement of the chargetransfer absorption has been observed in temperature-dependent absorption measurements, where it was attributed to a reduced flexibility between the thiophene and benzothiadiazole moieties upon cooling.51 Thus, we conclude that both PFTEGT6BT and P3HT experience restricted rotation in BrAni/Xyl, indicative of weak aggregation in solution. This behavior of the individual donor and acceptor is mirrored in the block copolymer, leading to the observed shift in absorption and the enhanced ordering in films. AFM Interrogation of Block Copolymer and Blend Films. To further elucidate the thin film behavior of these materials, atomic force microscopy (AFM) in tapping mode was employed. The phase images collected from films spun from BrAni/Xyl are shown in Figure 4. As previously reported,29 the P3HT-b-PFTEGT6BT material under investigation was carefully isolated from the complex reaction mixture resulting from the synthetic techniques employed. This purification methodology stands in contrast to techniques such as Soxhlet extraction, which are incapable of separating the desired block copolymer from the constituent homopolymers. It has been shown that homopolymer contamination can have significant effects on the morphology obtained, but due to the difficulties associated with purification, most reports of fully D


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the increased molecular width of the PFTEGT6BT repeat units disrupting the close packing of the P3HT crystallites. Crystallite correlation lengths (CCLs) can be estimated through application of the Scherrer equation in which CCL = 2πΔq−1, where Δq is the full width at half-maximum of the scattering intensity fitted with a Lorentz function.62 Applying this analysis to the (100) peak, the bulk block copolymer asprecipitated has estimated CCLs of 7.72 nm, and following annealing the BCP CCL increases to 13.5 nm. For comparison, the annealed P3HT in isolation showed CCLs of 22.1 nm. Thus, the block copolymer system as-precipitated shows significant phase separation and P3HT crystallization, which almost doubles with thermal annealing. However, the crystallite size is still significantly diminished compared to pristine P3HT. Small-angle X-ray scattering can also give information about the morphology adopted by block copolymers. If lamellar domains with spacing of 44 nm (as expected from the block copolymer contour length) are achieved, then a series of scattering peaks at Nq are expected with q = 0.014 Å−1.63 No well-defined peaks are observed in the low q range of the block copolymers; however, the lowest value of q reached in these measurements was q = 0.017 Å−1. Thus, it is possible that the lamellar scattering peak is out of the experimental range, and given the lack of long-range order present in the AFM strong second-order reflections are not expected. A detailed analysis of the microphase separation is attempted using resonant soft Xray scattering (RSoXS), which is discussed below. In order to investigate P3HT crystallization behavior in thin block copolymer film, grazing incidence wide-angle X-ray scattering (GIWAXS) was employed. Because charge transport in P3HT occurs most readily through the stacked aromatic π orbitals, for optimum OPV performance these charge percolation pathways should be oriented perpendicular to the electrode. GIWAXS analysis of BCP thin films is given in Figure 6. The π−π scattering peak is clearly visible along the inplane axis at 1.6 Å−1 while the scattering attributed to the alkyl lamella are oriented out of plane. This indicates that the P3HT crystallites align preferentially in an edge-on fashion, even in asspun films. Previously it has been shown that a series of block copolymers based on the P3HT-b-PFTBT scaffold adopted a face-on orientation when spun from chloroform, transitioning to edge-on when heated above the melting point.27 The different behavior demonstrated in films of P3HT-bPFTEGT6BT is most likely a result of the solvent choice, with the high-boiling o-xylene allowing sufficient reorganization time for the polymer to adopt the thermodynamically favored edgeon configuration. This illustrates a difficulty in optimization of these systems, as deposition from solvents with a low boiling point will enhance the desired face-on orientation of P3HT, but at the expense of crystallinity and phase separation which require slower film formation. Crystallite correlation lengths as estimated from Scherrer analysis as well as Herman’s orientation parameters and domain spacing are given in Table 1. Herman’s orientation parameters are determined from the intensity of the (100) reflection as a function of the azimuthal angle and averaged over a range of incident angles between 0.1° and 0.15° for each sample. In Herman’s orientation analysis a value of −0.5 indicates complete alignment of the crystal plane parallel to the substrate (faceon), a value of 1 indicates perfect alignment normal to the substrate (edge-on), and random orientation is indicated by a value of 0.64 The values in Table 1 confirm quantitatively what

wide- and small-angle X-ray (WAXS/SAXS) scattering profiles were collected on bulk samples of the BCP and as well as PFTEGT6BT and P3HT (Figure 5). Previous analysis of P3HT-

Figure 5. Tapping mode AFM of films of (a) PFTEGT6BT/P3HT blend, (b) PFTEGT6BT/P3HT/P3HT-b-PFTEGT6BT blend, and (c) P3HT-b-PFTEGT6BT deposited from 2% BrAni in xylene before and after annealing.

b-PFTEGT6BT crystallinity using differential scanning calorimetry (DSC) revealed a melting point at 209 °C attributable to the P3HT block, while no transitions attributable to the PFTEGT6BT were observed. The enthalpy of melting (ΔHm) of the P3HT was 2.4 J/g, indicating a 25% degree of crystallinity for the P3HT block relative to the isolated macroinitiator.29 This analysis confirms that microphase separation in the BCP material is sufficient to allow P3HT crystallization and also provides a guideline for high-temperature annealing. The polymer materials were analyzed as precipitated in methanol and after prolonged annealing above the melting point (220 °C) followed by a slow cooling to room temperature. As expected, the PFTEGT6BT homopolymer was amorphous and showed only the halos associated with disordered polymers.57−59 In the as-precipitated P3HT sample we identify the (100) and (200) reflections at q = 0.38 and q = 0.76 Å−1, respectively, with the (002) and (020) reflections overlapping at q = 1.65 and 1.71 Å−1.60 With annealing the distinct (002) reflection is lost and the lamellar (100) is slightly shifted to lower q (0.375 Å−1), indicating a small reorganization of the side-chains to fully form the polymorph I unit cell.61 P3HT crystallites in the block copolymer showed marginally different unit cell dimensions from the isolated donor: the lamellar spacing was fairly well maintained in the block copolymer, but the (020) reflection indicated a larger π-stacking distance of 3.77 Å compared to 3.67 Å in P3HT. This could be a result of E


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scattering contrast of traditional hard X-ray scattering techniques is dependent on electron density differences which are very similar in most polymeric materials, and so domains in organic thin films only scatter weakly. However, adjusting the X-ray photon energy so that it lies near the absorption edge of one constituent of the organic film increases the sensitivity and can greatly enhance the scattering contrast. This is the principle behind RSoXS which allows the determination of nanoscale morphologies in heterogeneous organic materials. The radially averaged RSoXS of P3HT-b-PFTEGT6BT is shown in Figure 7.

Figure 6. SAXS/WAXS scattering profiles of P3HT-b-PFTEGT6BT (BCP), PFTEGT6BT (HP), and P3HT before and after annealing at 220 °C for 17, 5, and 4 h, respectively.

is observable in Figure 6, which is that the polymer orients weakly edge-on. The edge-on alignment is slightly enhanced with annealing for 15 min, but with prolonged hightemperature annealing the isotropy of the film increases. This development could be a result of an increase in face-on crystallites formed during extended annealing. Conjugated polyelectrolytes containing triethylene glycol-substituted fluorene have been shown to adopt and maintain a face-on orientation even with annealing, inducing face-on order in subsequently deposited active layers.65 It is therefore reasonable to hypothesize that PF TEG T6BT would favor face-on orientation even more so than its alkylated analogues. Possibly, the deposition solvent employed stabilizes the edge-on orientation while prolonged annealing allows the growth of crystallites dominated by the orientation of the hydrophilic block. In as-spun films, the crystallite correlation length was 9.5 nm, which more than doubled to 22 nm upon annealing for 15 min. Prolonged 2 h annealing led to only a marginal increase in CCL over the 15 min annealing treatment. These correlation lengths are significantly larger than those attained in bulk measurements and illustrate the benefit of solvent induced preaggregation to P3HT crystallization. As in the bulk measurements, annealing led to a shift in the (100) peak to slightly lower q values as the lamellar spacing increased. The (010) spacing was nearly identical between the thin film and bulk measurements, and the comparability between the two measurements indicates minimal effects of the thin film confinement on the crystalline P3HT polymorph. To conclusively determine the average domain spacing in films of P3HT-b-PFTEGT6BT, resonant soft X-ray scattering (RSoXS) was employed. RSoXS has become an invaluable tool for microstructural evaluation in organic thin films.66 The

Figure 7. GIWAXS diffraction patterns of P3HT-b-PFTEGT6BT films as-spun, annealed at 220 °C for 15 min or annealed at 200 °C for 1 h 45 min followed by 220 °C for 15 min and line cuts along the Qxy and Qz axes.

A clear scattering peak corresponding to domain spacing of 35 nm is seen in as-spun films. By examining the spectral behavior of the scattering feature, it is determined that this peak is

Figure 8. RSoXS scattering profiles of P3HT-b-PFTEGT6BT as-spun and after annealing at 200 °C for 1 h 45 min followed by 220 °C for 15 min. Solid lines were collected at 287 eV where contrast between P3HT and PFTEGT6BT domains is maximized. Dashed lines were collected at 260 eV where the contrast between domains is minimized, showing the background structure and roughness not correlated with contrast between chemical species. F


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In order to investigate the efficiency of this charge separation, the photoluminescence (PL) of the block copolymer in films deposited from BrAni/Xyl was measured and compared with that of the PFTEGT6BT homopolymer before and after annealing. Charge transfer between donor and acceptor provides an alternative route for exciton decay and if efficient will lead to an observable quenching of photoluminescent intensity. The fluorescence of the block copolymer and PFTEGT6BT homopolymer is presented in Figure 10. An excitation wavelength of 370 nm was used to isolate the PFTEGT6BT component selectively and facilitate direct comparison between the block copolymer and homopolymer, and the resulting PL was adjusted for differences in absorption at 370 nm between the films. Blend films were prepared to mimic the ratio of donor and acceptor for comparison. As-spun films of the block copolymer were quenched by 85% relative to as-spun PFTEGT6BT, while the as-spun blend showed a smaller degree of quenching at 69%. After annealing, the block copolymer quenching decreases to 74%, consistent with domain purification and increased microphase separation. The PL quenching of the blend decreases by a marginal amount with annealing, down to 66% that of the homopolymer. The larger amount of quenching observed in the block copolymer relative to the blend is most likely related to morphological control and inhibition of macrophase separation. These results also give no indication that charge transfer is detrimentally affected by the covalent linkage between donor and acceptor, a potential hazard in these systems. To further evaluate the P3HT-b-PFTEGT6BT system, the electron mobility of the isolated acceptor block was determined using space charge limited current analysis (SCLC) of electrononly diodes with the device architecture described in the Experimental Section. The average electron mobility for three devices of thickness ranging from 180 to 350 nm was determined to be 1.3 × 10−8 cm2/(V s). This surprisingly low value seems to indicate that the incorporation of tetraethylene glycol side-chains has significantly diminished the electron transport capability of the material. Previously, ambipolar behavior and good electron transport were measured in organic field effect transistors (OFETs) composed of PFTBT derivatives.57,69 In order to confirm the lack of electron mobility in the PFTEGT6BT acceptor material, OFETs were prepared in bottom contact, bottom gate configuration with a

attributed to spacing between material domains and not any other source. The larger size scale (low q) scattering background is observed at nonresonant energies; thus, we can attribute it to structure between both materials and vacuum either void or more likely the overall film roughness. The peak, on the other hand, has exactly the spectral behavior of the contrast between P3HT and PFTEGT6BT measured by NEXAFS, firmly identifying it as a material domain feature. This peak increases in intensity, decreases in width, and the corresponding domain spacing increases slightly with annealing to 40 nm, corresponding to average domain sizes of 20 nm. This behavior nicely confirms that the local surface morphology obtained from AFM applies to the film as a whole, validating the morphological benefits of the amphiphilic block copolymer design strategy. Photovoltaic Evaluation. To begin the photovoltaic evaluation of P3HT-b-PFTEGT6BT as OPV active layer materials, the potential for charge transfer between the donor and acceptor materials was evaluated. The energy levels of the two homopolymers was determined using a combination of cyclic voltammetry and UV/vis spectroscopy and are given in Figure 9. These values are consistent with those previously

Figure 9. Schematic of energy levels of P3HT and PFTEGT6BT as determined by cyclic voltammetry and UV/vis spectroscopy and the energy of the interlayers and electrodes employed.

reported for P3HT and other PFTBT analogues. The energetic offset between the LUMO of P3HT and PFTEGT6BT is 0.4 eV, which is expected to be sufficient for separation of the Frenkel exciton into free charges.

Figure 10. (a) UV/vis absorption spectra of films P3HT-b-PFTEGT6BT, PFTEGT6BT, and P3HT/PFTEGT6BT blend normalized to absorption at 370 nm. (b) Fluorescence spectra of the same films, excited at 370 nm and adjusted for differences in absorption at that wavelength. G


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Macromolecules

processing is not required to induce domain formation. This represents an improvement over previously reported P3HT-bPFTBT systems in which phase separation was difficult to obtain. Despite these benefits, the performance of devices prepared from this material was unexpectedly low. The measured energy levels and photoluminsecent quenching experiments indicate a potential for efficient charge transfer, and so we attribute this negative result to the greatly decreased electron mobility in the tetraethylene glycol materials and a concomitant decrease in short circuit current. This low mobility may be a function of disrupted electron transport through steric-induced twisting of the acceptor backbone.

P3HT-b-PFTEGT6BT active layer as described in the Experimental Section. In OFETs, no electron transport was observed. However, hole transport was maintained, with as-spun films of BCP displaying hole mobilities 3 orders of magnitude lower than that of a pristine P3HT film. With annealing, hole transport increased 2 orders of magnitude, indicating the effective development of charge percolation pathways. The effect of annealing on hole mobility in the block copolymer OFET is summarized in Table 2. Table 2. OFET Mobilities of P3HT-b-PFTEGT6BT at Different Annealing Temperaturesa

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μh (cm2/(V s)) 100 °C

150 °C

220 °C

4.22 × 10−5

9.89 × 10−4

1.13 × 10−3

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00377. Details of device fabrication and charge mobility measurements (PDF)

μh are hole mobilities as measured after annealing for 15 min under nitrogen at the specified temperature. a

To complete the evaluation, devices were prepared with the standard inverted configuration and depicted schematically in Figure 9. As expected from the poor electron mobility measurements, the short circuit current (JSC) was low with and without annealing (0.11 and 0.13 mA/cm2, respectively), presumably due to the charge carrier mobility imbalance. This contributed to the low power conversion efficiencies of 0.023 and 0.025% (see the Supporting Information for full device characterization in). To place this performance in context, we turn to other block copolymer systems based on the P3HT-bPFTBT scaffold for comparison. The previously reported highperforming block copolymer system26 produced a JSC of 5.0 mA/cm2, and a second ternary blend reported in 201170 achieved a JSC of 3.4 mA/cm2. However, in a recent library of P3HT-b-PFTBT materials with an assortment of side-chains found that incorporation of alkyl chains on the thienyl unit led to a dramatic decrease in both the JSC (ranging between 0.006 and 0.67 mA/cm2) and VOC.27 When considered against this data set, the performance of the block copolymer system at hand does not seem so anomalous. Rather, it may indicate that the current and historical issue with this class of block copolymer is not purely one of phase separation but also of electron mobility through the amorphous acceptor material. Notably, the P3HT-b-PFTBT derivatives with the highest JSC were the polymers with no side-chains on the TBT unit, indicating that the linearity of this monomer may contribute significantly to efficient charge transport.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (D.J.J.). ORCID

Valerie D. Mitchell: 0000-0001-9097-4500 Jegadesan Subbiah: 0000-0002-5852-9121 Christopher R. McNeill: 0000-0001-5221-878X Mukundan Thelakkat: 0000-0001-8675-1398 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was made possible by support from the Australian Renewable Energy Agency which funds the project grants within the Australian Centre for Advanced Photovoltaics. Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. This work was supported in part by funding from the German Academic Exchange Service (DAAD). AFM measurements were made possible through ARC LIEF Grant LE110100161. The authors thank Mr. Jianing Lu for his expert assistance with AFM measurements and Anna Gräser for her invaluable support and assistance during beamtime. This research was undertaken in part on the SAXS/WAXS and Soft X-ray beamlines at the Australian Synchrotron, Victoria, Australia, and at beamline 11.0.1.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. C.R.M. and E.G. acknowledge travel funding provided by the International Synchrotron Access Program (ISAP) managed by the Australian Synchrotron and funded by the Australian Government. S.H. acknowledges the Bavarian framework program “Soltech” for funding.

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CONCLUSION The design and synthesis of the amphiphilic block copolymer P3HT-b-PFTEGT6BT was undertaken to address issues facing the adoption of fully conjugated block copolymers as active layer material for organic photovoltaic applications. Incorporating hydrophilic moieties into the acceptor block of the BCP system facilitated isolation of the block copolymer from the polymeric mixture resulting from the polycondensation synthetic route. We demonstrate that this purification contributes to morphological control, as homopolymer impurities disrupt the BCP domain structure. The amphiphilic nature led to spontaneous phase separation into well-defined, linear domains as determined by AFM and X-ray scattering techniques. The ready self-organization of this block copolymer system could prove industrially beneficial, as postdeposition

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Morphological and Device Evaluation of an Amphiphilic Block

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