Ethynylene-Linked Donor–Acceptor Alternating Copolymers

Article pubs.acs.org/Macromolecules

Ethynylene-Linked Donor−Acceptor Alternating Copolymers Wade A. Braunecker,*,† Stefan D. Oosterhout,† Zbyslaw R. Owczarczyk,† Ross E. Larsen,† Bryon W. Larson,‡ David S. Ginley,† Olga V. Boltalina,‡ Steven H. Strauss,‡ Nikos Kopidakis,† and Dana C. Olson† †

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States

‡

S Supporting Information *

ABSTRACT: Controlling steric interactions between neighboring repeat units in donor−acceptor (D−A) alternating copolymers can positively impact morphologies and intermolecular electronic interactions necessary to obtain high performances in organic photovoltaic (OPV) devices. Herein, we design and synthesize 12 new conjugated D−A copolymers, employing ethynylene linkages for this control. We explore D−A combinations of fluorene, benzodithiophene, and diketopyrrolopyrrole with analogues of pyromellitic diimide, thienoisoindoledione, isothianaphthene, thienopyrazine, and thienopyrroledione. Computational modeling suggests the ethynylene-containing polymers can adopt virtually planar conformations, while many of the analogous polyarylenes lacking the ethynylene linkage are predicted to have quite twisted backbones (>35°). The introduction of ethynylene linkages into these D−A systems universally results in a significant blue-shift in the absorbance spectra (by as much as 100 nm) and a deeper HOMO value (∼0.1 eV) as compared to the polyarylene analogues. The contactless time-resolved microwave conductivity technique is used to measure the photoconductance of polymer/fullerene blends and is further discussed as a tool for screening potential active layer materials for OPV devices. Finally, we demonstrate that an ethynylene-linked alternating copolymer of diketopyrrolopyrrole and thienopyrroledione, with a rather deep LUMO estimated at −4.2 eV, shows increased photoconductance when blended with a perfluoroalkyl fullerene C60(CF3)2 as compared to the standard PC61BM. We attribute the change in increased free carrier generation to the higher electron affinity of C60(CF3)2 that is more appropriately matched with the deeper LUMO of the polymer.

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INTRODUCTION Conjugated polymers have been used extensively in recent years for a number of organic electronic applications, including light-emitting diodes,1 field effect transistors,2 and organic photovoltaic (OPV) cells.3−5 Through the design and modification of the individual electron donating (D) and accepting (A) components in alternating conjugated D−A copolymers,6,7 intramolecular charge transfer within these systems can be readily manipulated with endless permutations to fine-tune the electronic and optoelectronic properties of the polymers for specific applications. In the field of OPV, this strategy has enabled the development of new materials with highly tailored band gaps having both improved open circuit voltages and improved capacity for harvesting photons from the solar spectrum.8 OPV technology has begun to find commercial application,9 but further systematic research will be necessary to drive device efficiencies higher before adoption of the technology is realized on a large industrial scale. The successful design of a D−A copolymer for OPV applications must take into account certain steric and conformational constraints. Repulsive steric interactions between neighboring aromatic repeat units can impart twisting © 2013 American Chemical Society

throughout a polymer backbone. Such twisting is generally considered undesirable, as planarity can effectively increase the polymer conjugation length in D−A systems and can also facilitate π−π stacking and/or crystalline packing of the polymer chains, all of which can enhance intermolecular electronic interactions critical to the function of an OPV device.10 When steric interactions between conjugated aryl components do impart twisting through a polymer, an alkyne linkage could in principle relieve that strain. With the increasing availability of efficient procedures developed for palladium-catalyzed alkynylation reactions over the past few decades,11−13 poly(arylene ethynylenes), also known as poly(arylacetylenes), are finding broad application as a promising class of organic semiconductors.14,15 The use of ethynylene-based materials in OPV has recently been reviewed.16 Ethynylene-containing materials have found application in a number of small molecule,17,18 polymer,19−22 and metallopolyyne23−25 photovoltaic devices. While device Received: February 1, 2013 Revised: April 10, 2013 Published: April 29, 2013 3367

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efficiencies generally have not exceeded 4%,26 the ethynylenebased OPV materials sometimes have advantages over their fully cyclic analogues. For example, the open circuit voltage of polythiophene was improved by nearly 0.4 V upon the introduction of alkyne linkages, which was attributed to the electron-withdrawing nature of the ethynylene group that significantly lowered the highest occupied molecular orbital (HOMO) of the polymer.21 Alkyne linkages were also demonstrated to systematically improve (red-shift) the absorption spectrum of certain sterically constrained polyarylenes more than 100 nm, consistent with a more highly conjugated and planar backbone.27 Thus, when used appropriately, the introduction of an alkyne linkage could be an important tool for fine-tuning the electronic and optoelectronic properties of certain OPV polymers. However, only a handful of ethynylene-containing D−A copolymers have been reported in the literature.22,28−32 Even fewer systematic studies have probed how the introduction of an ethynylene group into a polycyclic D−A system will affect intramolecular charge transfer relative to its fully heterocyclic analogue.30 This work details the design, synthesis, and characterization of 12 new D−A copolymers containing alkyne linkages for a number of different systematic studies. Several D−A copolymers are synthesized with and without alkyne linkages; their optical and electronic properties are characterized, and the effect of the alkyne linkages on the photoconductance of the materials is examined, as determined by time-resolved microwave conductivity (TRMC) experiments. Other D−A copolymers are predicted by computational modeling to be so twisted without alkyne linkages that only the triple-bond-based analogues were synthesized and characterized. Furthermore, after designing and synthesizing some new “electron deficient” low band gap materials by linking traditional electronwithdrawing comonomers together with electron-withdrawing ethynylene, we illustrate how the photoconductance of materials with rather deep LUMO levels can be improved over systems with PC61BM by appropriately matching the polymers with the higher electron affinity fullerene C60(CF3)2, which has a deeper LUMO.

Figure 1. Benzodithiophene (BDT)-based polymers with thienoisoindoledione (TID) and diketopyrrolopyrrole (DPP) cores.

were considerably more planar and were shown to form partially ordered domains. In an effort to improve the geometry of P-TID-BDT, and in turn the morphology and photoconductance of the material, we designed an analogous D−A copolymer with alkyne linkages (P-TID-≡-BDT) as a follow-up to our original effort. Figure 2

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BACKGROUND AND DESIGN OF NEW MATERIALS In a recent study, our group synthesized a D−A copolymer of benzodithiophene (BDT) and thienoisoindoledione (TID) (PTID-BDT in Figure 1).33 Intramolecular charge transfer between the alternating units of electron-donating and electron-accepting moieties in this copolymer effectively promoted conjugation throughout its backbone by stabilizing the polymer in its quinoidal state. Additionally, the fused aromatic ring in TID provides further stabilization of the quinoidal state of the polymer, as the dearomatization of the thiophene ring to assume a quinoid structure is accompanied by a gain in aromatic resonance energy in the fused benzene ring. The resulting band gap of P-TID-BDT was thus appreciably lower (by ∼0.4 eV) than in an analogous copolymer of thienopyrroledione (TPD) and BDT that was not stabilized by such aromatic resonance. However, despite the optimized band gap, P-TID-BDT displayed lower photoconductance, as determined by TRMC, and decreased device efficiency (2.1% vs 4.8%) as compared with the TPD analogue.33 These results were partially attributed to morphology, as computational modeling suggested the TID copolymers would have a twisted backbone, and X-ray diffraction data indicated the polymer films did not form ordered domains, whereas TPD copolymers

Figure 2. Illustration of the twist in P-TID-BDT (top) and planarity of P-TID-≡-BDT (middle and bottom).

illustrates the geometric structures for oligomers of P-TIDBDT and P-TID-≡-BDT, as calculated by density functional theory (DFT) and optimized in vacuum as described in the Experimental Section. As can be seen in the top of Figure 2 for P-TID-BDT, the hydrogen atom on the isoindole unit of TID creates sufficient steric hindrance with the neighboring BDT unit that a dihedral angle of ∼34° is induced between these 3368

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reveals that the FLR-based systems have greater torsion angles. For example, the copolymer P-TID-FLR is predicted to have a dihedral angle of 41° (Figure S1 in the Supporting Information) compared to 34° for P-TID-BDT. This can be attributed to the greater intrinsic steric constraints imposed on the TID comonomer by the six-member ring in FLR compared to the five-member ring in BDT. In the case of pyromellitic diimide (PMDI) and FLR, the dihedral angle is as high as 57° without ethynylene linkages (Figure S2). Given the extreme twisting in these polymers, only analogues containing ethynylene were synthesized (Figure 3). Such polymers are predicted by DFT to be virtually planar (Figures S1 and S2). The optical/electronic properties and photoconductance of the FLR-based ethynylene-containing copolymers with TID, PMDI, and isothianaphthene (ITN) are compared with the DPP-containing copolymer P-DPP-≡-FLR (Figure 3), which has shown OPV device efficiencies of ∼2% in the literature.22 Finally, the incorporation of ethynylene linking units between electron-withdrawing comonomers opens a synthetic pathway for us to design rather electron-withdrawing alternating copolymers with narrow band gaps and relatively deep HOMO values. For example, the bromine-functionalized monomer TID cannot be copolymerized in alternating fashion with bromine-functionalized DPP to give P-TID-DPP. However, following the introduction of ethynylene onto the DPP core, the copolymer P-TID-≡-DPP can now be synthesized through a Sonogashira cross-coupling reaction. This relatively electron-withdrawing copolymer should have a deep HOMO level, and the aromatic resonance stabilization afforded by the TID group should keep the band gap relatively narrow. Six new copolymers of DPP with comonomers of various electron withdrawing strength (TPD, PMDI, TID, ITN, DPP, and thienopyrazine (TP), Figure 4) were synthesized in an effort to provide materials with a range of energy levels for systematic studies in OPV.

comonomers, introducing significant twist throughout the polymer backbone. However, the middle and bottom of Figure 2 illustrate the relative planarity of P-TID-≡-BDT, with the dihedral angle being less than 4°. The planarity of P-TID-≡-BDT will in principle enhance π−π stacking interactions. However, it is not straightforward to predict the impact of the ethynylene unit on the optoelectronic properties. In a recent study of a twisted quinoxaline-based homopolymer, ethynylene linkages were introduced to allow the polymer to adopt a more planar structure.27 The absorption of the resultant polymer was red-shifted more than 100 nm as compared with the original material, consistent with a more highly conjugated and planar backbone. However, in another study concerning a benzothiadiazole-based D−A copolymer that was relatively planar to begin with, the introduction of an ethynylene linkage between the D−A components blue-shif ted the absorption ∼100 nm relative to its fully cyclic analogue.30 The “push-pull” effect between donor and acceptor moieties was apparently disrupted in the latter system by the ethynylene linkage. Thus, it is not immediately clear the effect that ethynylene will have in P-TID-≡-BDT, as conjugation and π−π stacking interactions could be enhanced through planarization of the twisted polymer backbone, yet intramolecular charge transfer between TID and BDT could be negatively impacted by ethynylene. We further investigate the effect of ethynylene linkages on D−A copolymers of diketopyrrolopyrrole (DPP) and BDT (P-DPP-BDT and P-DPP-≡-BDT in Figure 1). The optimized DFT structure for P-DPP-BDT shows a dihedral angle of 8°−19° (for anti and syn conformations, respectively) between comonomers. P-DPP-≡-BDT is significantly more planar (2°−6°). Next, a number of fluorene (FLR)-based D−A copolymers were designed that incorporate ethynylene linkages, shown in Figure 3. A comparison of the optimized DFT structures of FLR-based copolymers with analogous BDT-based systems

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EXPERIMENTAL SECTION

General. All reagents employed in this study were obtained from commercial sources at the highest available purity and used without further purification, unless otherwise noted. All reactions were performed under dry N2. Methylene chloride, toluene, and THF were purified by passing through alumina in an MBraun solvent purification system. Column chromatography was performed with Fluka Silica Gel 60 (220−440 mesh). All small molecules were characterized by 1H NMR (400 MHz) and 13C NMR (100 MHz) on a Varian Unity Inova. Monomers were >99% pure as determined by 1H NMR. PCBM was purchased from Nano-C, Inc., was >99% pure and was used without further purification. C60(CF3)2 was synthesized according to the literature method34 and purified to >99% as determined by HPLC, 19F NMR, and APCI-MS, described in detail in the Supporting Information. UV−vis absorption measurements were performed using a Hewlett-Packard 8453 UV−vis spectrophotometer. Polymer Molecular Weight Determination. Polymer samples were dissolved in HPLC grade chloroform (∼1 mg/mL), stirred and heated at 50 °C for 2 h, stirred overnight at rt, and then filtered through a 0.45 μm PVDF filter. Size exclusion chromatography was then performed on a PL-Gel 300 × 7.5 mm (5 μm) mixed D column using an Agilent 1200 series autosampler, inline degasser, and diode array detector. The column and detector temperatures were 35 °C. HPLC grade chloroform was used as eluent (1 mL/min). Linear polystyrene standards were used for calibration. Cyclic Voltammetry. All voltammograms were recorded at 25 °C with a CH Instruments Model 600D potentiostat. Unless otherwise specified, measurements were carried out under nitrogen at a scanning rate of 0.1 V s−1 using a platinum wire as the working electrode and a

Figure 3. Fluorene (FLR)-based polymers with thienoisoindoledione (TID), isothianaphthene (ITN), pyromellitic diimide (PMDI), and diketopyrrolopyrrole (DPP) cores. 3369

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Figure 4. Diketopyrrolopyrrole (DPP)-based polymers with thienopyrrolodione (TPD), pyromellitic diimide (PMDI), thienoisoindoledione (TID), isothianaphthene (ITN), DPP, and thienopyrazine (TP) cores.

Table 1. Number-Average Molecular Weight (Mn), Polydispersity Index (PDI), and Optical and Electrochemical Properties of Polymersa λmax, λ0.1max (nm) polymer

Mn (kDa)

PDI

solutiona

P-TID-BDT P-TID-≡-BDT P-DPP-BDT P-DPP-≡-BDT P-TID-≡-FLR P-ITN-≡-FLR P-PMDI-≡-FLR P-DPP-≡-FLR P-TPD-≡-DPP P-PMDI-≡-DPP P-TID-≡-DPP P-ITN-≡-DPP P-DPP-≡-DPP P-TP-≡-DPP

37 8.2 22 40 72 42 60 65 14 30 30 20 8.5 38

2.7 1.5 3.5 2.5 3.1 3.0 5.0 3.2 1.5 2.9 3.4 3.2 3.5 3.7

651, 564, 742, 729, 540, 523, 476, 632, 655, 743, 712, 697, 690, 740,

793 706 836 788 574 563 502 667 829 811 843 835 823 870

film 662, 583, 742, 729, 546, 532, 483, 643, 672, 750, 758, 814, 738, 768,

921 767 852 808 617 612 540 746 902 862 908 945 903 944

Egopt b (eV)

EHOMOc (eV)

1.35 1.62 1.46 1.53 2.01 2.03 2.30 1.66 1.37 1.44 1.37 1.31 1.37 1.31

−5.4 −5.5 −5.3 −5.4 −5.6 −5.7 −6.0 −5.5 −5.6 −5.6 −5.4 −5.4 −5.5 −5.4

Measured in chloroform solution; λ0.1max = wavelength at which absorption is 0.1 its maximum value. bCalculated from film λ0.1max. cEHOMO estimated from onset potential measured at 0.1 V/s vs Ag/Ag+ and calibrated against Fc/Fc+ (measured as 0.1 V vs Ag/Ag+); Fc/Fc+ energy level used in HOMO calculations was −4.8 eV.41 a

platinum wire as the counter electrode. Potentials were measured vs Ag/Ag+ (and calibrated vs Fc/Fc+) using 0.01 M AgClO4 and a 0.1 M Bu4NBF4 salt bridge to minimize contamination of the analyte with Ag+ ions. Polymer films were drop cast onto a platinum wire working electrode from a 1 mg/mL chloroform solution and dried under a stream of nitrogen prior to measurement in a 0.1 M Bu4NBF4− acetonitrile solution. Theory. Density functional theory (DFT) was used to predict the structural properties of the polymers reported in this work for hydrogen-terminated oligomers with n = 1−4. All calculations were performed with the default settings in the Gaussian 09 electronic structure package, revision B.01.35 The geometric structure of each oligomer was optimized in vacuum using the Becke-style threeparameter density functional with the Lee−Yang−Parr correlation function (B3LYP) with the 6-31G(d) basis set. Time-Resolved Microwave Conductivity. TRMC is a pump− probe technique that can be used to measure the photoconductance of a film without the need for charge collection at electrical contacts.36,37 The details of the experimental methodology have been presented elsewhere.37,38 In brief, the sample is placed in a microwave cavity at the end of an X-band waveguide operating at ca. 9 GHz and is photoexcited through a grid with a 5 ns laser pulse from an OPO

pumped by the third harmonic of an Nd:YAG laser. The relative change of the microwave power, P, in the cavity, due to abrorption of the microwaves by the photoinduced free electrons and holes, is related to the transient photoconductance, ΔG, by ΔP/P = −KΔG, where the calibration factor K is experimentally determined individually for each sample. Taking into account that the electrons and holes are generated in pairs, the peak photoconductance during the laser pulse can be expressed as37 ΔG = βqeFAI0(ϕ∑ μ)

(1)

where qe is the elementary charge, β = 2.2 is the geometric factor for the X-band waveguide used, I0 is the incident photon flux, FA the fraction of light absorbed at the excitation wavelength, ϕ is the quantum efficiency of free carrier generation per photon absorbed, and ∑μ is the sum of the mobilities of electrons and holes.37 Equation 1 is used to evaluate the quantum efficiency or free carrier generation per photon absorbed and the local mobility of free carriers. These quantities can then be correlated to the molecular structure to provide insight into the mechanisms for free carrier generation and transport in polymer−fullerene composites as a function of the microstructure. The photoconductance decay after the end of the laser pulse is also a 3370

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useful tool for the characterization of free carrier decay mechanisms by recombination and trapping.

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RESULTS AND DISCUSSION Material Synthesis. Synthetic procedures for all monomers and their precursors are described in the Supporting Information, as are full synthetic proceedures for the polymerizations. The two non-ethynylene-containing copolymers in this study, P-TID-BDT and P-DPP-BDT, were synthesized by a palladium-catalyzed Stille coupling at 110 °C over 36 h in chlorobenzene. After this time, the polymers were end-capped with thiophene reagents, as it has been demonstrated that such end-capping can improve the performance of photovoltaic devices.39 A palladium scavenger40 was then stirred with the polymer to complex the catalyst before the polymer was precipitated into MeOH, after which the polymer was purified via Soxhlet extraction for 12 h with MeOH and 2 h with acetone. The 12 ethynylene-containing polymers were synthesized by a palladium-catalyzed Sonogashira crosscoupling reaction at 60 °C over 6 h. The polymers were endcapped with ethynyltrimethylsilane, after which the polymer was purified in an analogous manner as described above. As can be seen in Table 1, most of the Sonogashira polymerizations proceeded smoothly to high molecular weights; however, the lower molecular weights (Mn < 30 kDa) of P-TPD-≡-DPP, P-ITN-≡-DPP, and P-DPP-≡-DPP could generally be attributed to solubility issues, as the polymers began to precipitate from solution during the reaction. However, P-TID-≡-BDT (with Mn ∼ 8.2 kDa) remained soluble during the polymerization reaction, but molecular weights did not increase after 6 h. Furthermore, if left to react for greater than 24 h, the brilliant purple color of the polymer began fading to a gray-purple, indicating significant decomposition of P-TID-≡-BDT under the longer reaction times; thus, all subsequent measurements on this polymer were conducted on a sample obtained after 6 h. Optical Characterization of Polymers. The absorption spectra of the polymers were measured both in solution (chloroform) and in the solid state as thin films. Several important observations are worth noting. First, the introduction of ethynylene into these alternating copolymers universally resulted in a blue-shift in the absorbance spectrum in both solution and the solid state. These results were not necessarily anticipated, especially for P-TID-≡-BDT, given the recent literature results that demonstrated the introduction of ethynylene into a twisted quinoxaline homopolymer red-shifted absorbance more than 100 nm in the resultant polymer, consistent with a more highly conjugated and planar structure.27 Despite the more planar structure predicted for P-TID-≡-BDT (see Figure 2), λmax of the thin flim of P-TID≡-BDT blue-shifts 79 nm relative to P-TID-BDT (Figure 5). The presence of the electron-withdrawing ethynylene group thus seemingly interupts the “push−pull” effect between the BDT and TID units that dictates the optical band gap in the fully heterocyclic polymer analogue. For P-DPP-≡-BDT, the blue-shift in λmax of the film is not quite as dramatic with just a 13 nm difference between P-DPP-≡-BDT and P-DPP-BDT (Figure 5). Similarly, λmax of the P-DPP-≡-FLR film blue-shifts between 9 and 21 nm as compared to several literature analogues of P-DPP-FLR with different alkyl chains.42 For PDPP-≡-DPP, the effect of ethynylene is substantial, as λ0.1max blue-shifts ∼100 nm relative to the literature polymer P-DPPDPP.42 The latter polymer was recently used as the active layer

Figure 5. Effect of ethynylene linkages on polymer film absorbance (normalized).

in an OPV device, but only 0.3% device efficiency was obtained. This was attributed to the small LUMO−LUMO offset with PC61BM that was an insufficient driving force for charge separation in the solar cell device. The absorbance spectra of P-TID-≡-FLR and P-DPP-≡-FLR are blue-shifted relative to their BDT containing analogues. The thin film optical band gap of these FLR-containing polymers are 390 and 130 meV wider, respectively, than P-TID-≡-BDT and P-DPP-≡-BDT. This can likely be attributed to the weaker electron-donating power of the FLR unit relative to BDT. Similar trends have been seen in the literature for nonethynylene-containing D−A copolymers. For example, the optical band gap we measured for P-DPP-BDT is 340 meV smaller than that reported in the literature for P-DPP-FLR.42 The polymer with the largest band gap of the 14 polymers investigated in this study was P-PMDI-≡-FLR. This band gap of 2.30 eV results from the combination of the strong electronwithdrawing PMDI unit with the weak electron-donating FLR unit. Also, the three polymers in this study with the smallest band gaps, P-TID-≡-DPP, P-ITN-≡-DPP, and P-TP-≡-DPP, all employ electron-withdrawing comonomers that contain fused aromatic rings. As discussed earlier, it has been well documented that in such systems the dearomatization of the thiophene ring to assume a quinoid structure is stabilized both by the push−pull effect of the D−A groups and by a gain in aromatic resonance energy in the fused benzene ring.33 Absorption of the acceptor-rich DPP-based polymers is also quite broad compared to their FLR-based analogues (compare the solid state spectra of P-TID-≡-DPP and P-ITN-≡-DPP with P-TID-≡-FLR and P-ITN-≡-FLR in Figure 6), allowing the DPP-based polymers to potentially harvest photons from a broader range of the solar spectrum. All solution spectra are presented in the Supporting Information. Electrochemical Characterization of Polymers. Cyclic voltammetry was utilized to evaluate the HOMO energy levels of all the polymers. The data are summarized in Table 1. Given that the redox processes were generally irreversible (illustrated in the Supporting Information), EHOMO was estimated from the onset potential. Potentials were measured vs Ag/Ag+ and calibrated against the ferrocene/ferrocenium couple (Fc/Fc+ measured as 0.1 V vs Ag/Ag+). The Fc/Fc+ energy level used in HOMO calculations was assumed to be −4.8 eV.41 Thus, EHOMO = −(ϕox + 4.7) eV, where ϕox is the oxidation onset potential of the sample vs Ag/Ag+. Polymer films were dropcast onto a platinum electrode from solutions with uniform concentrations (1 mg/mL). The data are summarized in Figure 3371

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Figure 6. Normalized absorbance spectra of polymer films. Fluorene-based materials (top panel) and DPP-based materials (bottom panel).

Figure 7. Optical band gaps (determined from λ0.1max) and HOMO levels (determined from CV) for polymer films.

7, where the LUMO, as we define it in this paper, was determined from the addition of the optical band gap to the HOMO. A couple of important observations can be made about the energy level diagram in Figure 7. First, the introduction of the electron-withdrawing ethynylene into a polymer lowers the HOMO level with respect to the fully heterocyclic analogue. The HOMO of P-TID-≡-BDT was measured as 0.1 eV deeper than P-TID-BDT, as was the HOMO of P-DPP-≡-BDT vs P-

DPP-BDT (Figure 7). This is generally consistent with literature observations for the introduction of ethynylene into poly(3-hexylthiophene)21 and a D−A copolymer of dioxythiophene and benzothiadiazole.30 The HOMO of both of those literature polymers was lowered by ∼0.3 eV upon introduction of ethynylene. The HOMO of P-PMDI-≡-FLR is quite deep, approximately −6.0 V, with the LUMO estimated at −3.7 eV. In a recent study of a series D−A ethynylene-containing copolymers based 3372

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on PMDI, the LUMO values of five different PMDI-based polymers were all approximately −3.6 eV.32 The PMDI unit dictated the LUMO, while the HOMO varied 0.8 eV depending on the strength of the donating unit. Weak donating units had a HOMO around −5.9 eV. Thus, our estimation of the HOMO and LUMO for P-PMDI-≡-FLR, with the weakly donating FLR group, is fully consistent with literature observations for similar polymers. The other FLR-containing polymers also have relatively deep HOMO values. While this would translate to large desirable open circuit voltages in OPV devices, the relatively wide band gaps (>2 V) of these polymers make them less than optimal for OPV applications. These wide band gaps are present despite the aromatic resonance stabilization afforded by the ITN and TID groups. Of the four FLRcontaining polymers investigated in this work, only P-DPP≡-FLR is promising for OPV applications given its band gap closer to 1.7 eV. The band gaps of the new acceptor-rich DPP-containing polymers are in a much more optimal range for OPV than the FLR polymers. However, the estimated energy levels indicate that the LUMO values for these polymers (between −4.0 and −4.2 eV) are likely too low for use in OPV devices with traditional fullerene acceptors (i.e., PC61BM, with a LUMO around −4.2 eV4). Previous results already suggested that the LUMO of P-TID-BDT (−4.0 V) was too low to allow efficient charge separation at the polymer−PC61BM interface.33,43 The LUMOs of these six polymers are all estimated to lie deeper than that of P-TID-BDT. Such polymers would need to be used in conjunction with deeper LUMO fullerene acceptors if they were to be successfully employed in OPV devices. Time-Resolved Microwave Conductivity (TRMC). Contactless photoconductivity has previously been used by us33,37,44 and others36,45 to study the photophysics of free carrier generation and decay in bulk heterojunctions of polymer donor materials with PC61BM. Clear correlations are emerging between the magnitude of the photoconductivity measured by TRMC and the performance of complete OPV devices.33,45,46 In this paper, TRMC was used to evaluate the effect of the alkyne linkage on the photocarrier generation and decay dynamics. For this purpose, P-DPP-BDT and P-DPP-≡-BDT were compared in pure films and in bulk heterojunctions with 5% and 50% PC61BM loading by weight. The low, 5% PC61BM loading is used to evaluate the efficiency of free carrier generation at the polymer−fullerene interface: assuming fullerenes are dispersed into the polymer matrix in this “dilute” blend, small or negligible contribution to the sum of the free carrier mobilities, ∑μ, from the electron mobility is expected allowing us to attribute the increase in ϕ∑μ from the pure polymer to an increase in the quantum yield for free carrier generation per photon absorbed, ϕ, due to the presence of the PC61BM. The ϕ∑μ product for the P-DPP-BDT and P-DPP-≡-BDT systems is shown in Figure 8. While ϕ∑μ was measured over an intensity range spanning ca. 5 orders of magnitude, in Figure 8 we compare the magnitude of ϕ∑μ at an absorbed photon flux of 1013 photons/(cm2 pulse) for simplicity. However, the same trend was observed throughout the excitation intensity range (Figure S40). Structure simulations (discussed above) showed a twist of ca. 20° in the backbone of P-DPP-BDT and a significanly more planar structure for P-DPP-≡-BDT. The results of Figure 8 show that planarizing the polymer backbone has no effect on the magnitude of ϕ∑μ for the pure polymer or

Figure 8. Product of the yield for free carrier generation ϕ and the sum of mobilities ∑μ of electrons and holes obtained from the peak photoconductance at an absorbed photon flux, FAI0, of 1013 photons/ (cm2 pulse) for thin films of P-DPP-BDT and P-DPP-≡-BDT, photoexcited at 680 nm. Blue bar: pure polymers; red bar: blend with 5 wt % PCBM; black bar: blend with 50 wt % PC61BM.

the blends. We note that adding 5% PC61BM to P-DPP-BDT causes a small increase of ϕ∑μ, contrary to what is observed with polymers such as P3HT, where addition of 5% PC61BM by weight increases ϕ∑μ by a factor of 40.37 Furthermore, the photoconductance decays of both P-DPP-BDT and P-DPP≡-BDT are relatively fast (Figure S21), indicating significant carrier loss during the 5 ns laser pulse that limits the density of free carriers observed with TRMC. This indicates the presence of an inherent limitation to the creation of free carriers in these systems, which is not overcome by planarizing the polymer via the addition of the alkyne linkage. TRMC was then used to test the films of seven additional polymers: P-TID-BDT (excitation at 660 nm), P-PMDI-≡-FLR (480 nm), P-ITN-≡-FLR (530 nm), P-TID-≡-FLR (540 nm), P-DPP-≡-FLR (640 nm), P-TPD-≡-DPP (680 nm), and PTID-≡-DPP (680 nm), blended with PC61BM (1:1 weight ratios). Universally lower magnitudes of ϕ∑μ (by a factor 10− 100) were observed as compared to high performance polymers reported previously.33,37 The results are illustrated in Figure S20. A recent study of ours correlated the photoconductance of several polymers determined by TRMC with their performance in OPV devices.33 Among those polymers was P-TID-BDT, for which ϕ∑μ of P-TID-BDT:PC61BM was found to be lower by a factor of 7 than that of an analogous polymer P-TPD-BDT. Device efficiencies of P-TID-BDT have been reported between 2.1% and 3.0%,33,43 while efficiencies for P-TPD-BDT have been reported between 4.8% and 6.8%,33,47,48 in good agreement with the relative difference in ϕ∑μ for these two polymers.33 The lower photoconductance and OPV performance of P-TID-BDT was partially attributed to morphological issues, as P-TID-BDT has a quite twisted backbone and could not order very well;33 it was also partially attributed to the lowlying LUMO of P-TID-BDT with respect to the LUMO of PC61BM.43 The introduction of ethynylene in theory gave the resultant polymer P-TID-≡-BDT a more planar structure; it also widened the band gap so that the LUMO was not so deep (see Figure 7). However, as discussed earlier in the Material Synthesis section, this polymer was not stable at long reaction times, and its potential partial decomposition during the 3373

dx.doi.org/10.1021/ma400238t | Macromolecules 2013, 46, 3367−3375

Macromolecules

Article

by ∼0.1 eV,52 was blended with P-TPD-≡-DPP to probe the extent to which ϕ is limited by poor free carrier generation in such systems. In Figure 9, the ϕ∑μ product for P-TPD-≡-DPP with 5 wt % ratio of PC61BM and the same molar equivalent of C60(CF3)2 is shown. For these measurements, a higher loading of fullerene was not pursued since C60(CF3)2 forms coarse clusters at high loading and phase-separates from the polymer phase. Figure 9 shows that the ϕ∑μ product for P-TPD-≡-DPP increases by a factor of 4 upon going to C60(CF3)2. This evidence suggests that ϕ may be limited by poor LUMO−LUMO offset in the PTPD-≡-DPP system with PC61BM and can be improved with an appropriate fullerene.

polymerization deters us from drawing any hard conclusions about its photoconductance data. It should be noted that of all the polymers in this study, PTID-BDT gave the highest ϕ∑μ signal. Thus, the remaining polymers are not anticipated to produce OPV device efficiencies higher than 3.0% reported for this polymer in the literature. Literature values for optimized device efficiencies of P-DPP-BDT49 and P-DPP-≡-FLR22 are 2.8% and 2.3%, respectively, in good agreement with our TRMC data that suggest they should be under 3%. As discussed above, the presence of ethynylene in P-DPP-≡-BDT has virtually no effect on the TRMC signal compared to the fully heterocyclic analogue. Device efficiency is thus anticipated to be in the range of 2−3% for this polymer as well. As generally quite low TRMC signals were obtained for the remainder of the polymers, no OPV devices were made from these new materials. It is possible that while ethynylene in principle allows these polymers to adopt a planar structure that could improve packing over their cyclic analogues, the huge bulky side chains needed to solubilize the polymers actually prevent them from doing so. A lack of ordering has previously been correlated with slower transport of photogenerated carriers,50 which in turn implies higher recombination losses. A detailed study of the effect of side-chain size on long-lived carrier generation in push−pull polymers will be presented in an upcoming publication. In addition to morphological effects limiting free carrier generation, for the acceptor-rich polymers investigated by TRMC (namely, P-TPD-≡-DPP and P-TID-≡-DPP), the poor LUMO−LUMO offset with PC61BM likely contributes to a low yield of free carrier generation. As mentioned earlier, the LUMO of P-TID-≡-DPP (−4.0 eV) is similar to the LUMO of P-TID-BDT, which is already believed to be quite deep for application with PC61BM. The LUMO of P-TPD-≡-DPP (−4.2 eV) is deeper yet. The other four DPP based polymers in Figure 7 with band gaps