Molecular Rectification in Conjugated Block Copolymer Photovoltaics

Mar 21, 2016 - It is necessary to characterize block copolymer photovoltaic systems on a batch-by-batch basis because the synthesis of fully conjugate...
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Molecular Rectification in Conjugated Block Copolymer Photovoltaics Christopher Grieco,† Melissa P. Aplan,‡ Adam Rimshaw,† Youngmin Lee,‡ Thinh P. Le,‡ Wenlin Zhang,‡ Qing Wang,§ Scott T. Milner,‡ Enrique D. Gomez,*,‡,∥ and John B. Asbury*,† †

Department of Chemistry, ‡Department of Chemical Engineering, §Department of Materials Science and Engineering, and Materials Research Institute, The Pennsylvania State University, State College, Pennsylvania 16801, United States

∥

S Supporting Information *

ABSTRACT: We investigate the influence that covalent linkage of electron donating and accepting blocks in high performance fully conjugated block copolymer photovoltaics has on charge generation and recombination using ultrafast mid-infrared transient absorption spectroscopy. We show that block copolymer architectures containing a conjugated bridge between the donor and acceptor groups can be used to form ordered mesoscale morphologies that lead to improved photovoltaic performance without enhancing charge recombination. Judicious placement of an electron-rich moiety in the electron accepting block of the block copolymer creates a donor−bridge−acceptor architecture that slows intramolecular charge transfer across the covalent linkage. Charge recombination in such donor−bridge−acceptor block copolymer films proceeds at the same rate as it does in their corresponding homopolymer blends for which the donor and acceptor blocks are not covalently linked, indicating that recombination is dominated by intermolecular charge transfer in both systems. The electrical and morphological properties of functional block copolymer photovoltaics are correlated with their underlying charge generation and recombination kinetics, permitting us to identify key design rules for further improvements in the power conversion efficiency of fully conjugated block copolymer solar cells.

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INTRODUCTION Conjugated block copolymers provide a means of systematically tuning electron donor and acceptor phase segregation with nanoscale precision.1−6 For example, tuning the total molecular weight, volume fractions, and interaction energies of the blocks in diblock copolymers permits lamellar or bicontinuous mesoscale phases to be obtained, 7,8 which can create continuous charge transport paths for efficient charge extraction. Prior work on fully conjugated donor−acceptor diblock copolymers for photovoltaic applications led to promising phase separation behavior on length scales wellmatched to the ∼10 nm exciton diffusion length of conjugated molecular systems under quasi-equilibrium conditions.1−6 Despite successes in controlling mesoscale morphology, these systems have struggled to achieve photovoltaic power conversion efficiencies greater than 1%.6,9 It was hypothesized that fully conjugated block copolymers may undergo fast charge recombination because the covalent bond linking the donor and acceptor blocks enables strong electronic coupling due to wave function overlap between the donor and acceptor.10−17 Recent advances in fully conjugated block copolymer photovoltaics have demonstrated 3% power conversion efficiency using poly(3-hexylthiophene-2,5-diyl)-block-poly((9,9-bis(2-octyl)fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)2,1,3-benzothiadiazole)-5′,5″-diyl) (P3HT-b-PFTBT, Figure 1a).5 This block copolymer system exhibits approximately 3fold greater power conversion efficiency in comparison to the © 2016 American Chemical Society

optimized blend of P3HT/PFTBT homopolymers. Significantly, the enhancement of photovoltaic performance of the block copolymer system was associated with formation of a mesoscale morphology with ∼20 nm periodicity that resulted from lamellar ordering of the donor and acceptor blocks. These results suggested that the kinetics of charge extraction were competitive with charge recombination processes, although direct spectroscopic evidence for this favorable recombination behavior has not been explored. Here, we examine charge carrier dynamics in P3HT-bPFTBT block copolymer films in comparison to the corresponding P3HT/PFTBT homopolymer blends that serve as control samples to assess the influence that the covalent linkage between the P3HT and PFTBT blocks in P3HT-b-PFTBT has on charge recombination. By directly observing charge generation and recombination in the midinfrared on time scales ranging from 100 fs to 100 μs and over 3 decades in signal amplitude, we demonstrate that the covalent linkage has no measurable influence on charge recombination in P3HT-b-PFTBT films. Charge recombination occurs at the same rate in both the block copolymer and the P3HT/PFTBT blend for which the electron donor and acceptor blocks are not covalently linked. Furthermore, we show that conducting Received: January 5, 2016 Revised: March 18, 2016 Published: March 21, 2016 6978

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the same conditions. The molecular weights and polydispersity of P3HT and PFTBT used to form the P3HT/PFTBT blends were matched to the blocks used to form the P3HT-b-PFTBT block copolymer. Similar to our initial report, the open-circuit voltage, shortcircuit current, and fill factor of the block copolymer devices are markedly improved by thermal annealing at 165 °C, leading to an average power conversion efficiency of these devices of 2.1% (see Supporting Information, Table S1). Furthermore, the electrical properties of the block copolymer films are superior to P3HT/PFTBT blends for all annealing conditions, consistent with our earlier observations.5 The methods to fabricate and characterize the materials and devices reported here are summarized in the Methods section and described in detail in the Supporting Information. We investigated whether the improvement in block copolymer photovoltaic performance is associated with formation of mesoscale order following annealing at 165 °C. Figure 1c depicts Kratky plots in which resonant soft X-ray scattering (RSOXS) intensities I(q) are plotted as I(q)q2 versus scattering vector q. These measurements were performed on P3HT-b-PFTBT and the P3HT/PFTBT blend films corresponding to those used in the active layer of the devices. Because polymeric systems frequently exhibit q−2 dependence due to scattering from polymer chains, presenting scattering intensities as I(q)q2 is commonly used to ensure that scattering features are apparent.18−22 The Kratky plots demonstrate that the block copolymer film annealed at 165 °C exhibits a peak at a scattering vector around q = 0.03 Å−1 and a shoulder around 3q = 0.09 Å−1 corresponding to an ∼ 20 nm lamellar domain size. This domain size corresponds to the approximate end-toend distance of the polymer chains as observed previously.5 In contrast, the P3HT/PFTBT blend film annealed at 165 °C exhibits a weak shoulder around at 0.04 Å−1, indicating the formation of an ∼15 nm structure that may be associated with P3HT fibril formation. Examining the mesoscale morphology of the P3HT-bPFTBT and P3HT/PFTBT blend films annealed at 100 °C provides insight about the structural evolution that gives rise to the improved device performance of the block copolymer following annealing at 165 °C. Scattering from the blend annealed at 165 or 100 °C and from the block copolymer at 100 °C share a similar weak scattering feature near q = 0.04− 0.05 Å−1. We attribute this feature to the presence of crystalline P3HT fibrils because the features are present in the blends as well as the block copolymer film annealed at 100 °C. Nevertheless, there are clear differences between the scattering from the block copolymer and blend films. The scattering data from the blend at low q has a negative slope, suggesting the presence of large-scale features (>50 nm) that are consistent with macrophase separation. In contrast, scattering from the block copolymer is either independent of q or decreases with q at low q. After annealing at 165 °C, the scattering at low q increases slightly for the blends, suggesting further coarsening of the morphology. But, for the block copolymer, a significant change in the scattering profile occurs with annealing at 165 °C, where a peak near 0.03 Å−1 becomes apparent and a second feature near 0.09 Å−1 is also visible. Similar to our earlier report,5 this evolution in the active layer morphology is associated with marked improvement of the block copolymer photovoltaic device performance. We therefore take the P3HT-b-PFTBT block copolymer batch examined in this work as nearly equivalent to the batch

Figure 1. (a) Structure of the block copolymer examined in this investigation is compared to the homopolymers P3HT and PFTBT that are examined as controls. (b) Current−voltage characterization reveals that annealing of the block copolymer active layer results in improved photovoltaic device performance in comparison to homopolymer controls. (c) RSOXS measurements demonstrate that the formation of a block copolymer mesophase with ∼20 nm periodicity is associated with significant improvement in power conversion efficiency of photovoltaic devices. In contrast, annealing the homopolymer blend degrades the photovoltaic performance due to coarsening of the P3HT and PFTBT phase-separated domains.

transient absorbance measurements in the mid-infrared where charge-separated polarons uniquely absorb is critical to accurately measuring charge generation and recombination in such systems. The combination of ultrafast spectroscopy of functional materials with morphology and device characterization permits us to suggest design rules to guide future development of fully conjugated block copolymers for high performance organic photovoltaic applications.

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RESULTS AND DISCUSSION In our initial report of P3HT-b-PFTBT photovoltaic devices, we demonstrated significant enhancement in electrical properties of the material after annealing at 165 °C and consequent formation of a lamellar mesophase.5 In the present study, in which we seek to characterize the charge recombination kinetics that underpin this record device performance, we must examine a different block copolymer batch than the one reported initially. It is necessary to characterize block copolymer photovoltaic systems on a batch-by-batch basis because the synthesis of fully conjugated block copolymers remains a challenge.4,6,11 Therefore, we repeated the morphological and device characterization on the present batch of P3HT-b-PFTBT to motivate spectroscopic measurements and to demonstrate comparable results to our initial report. The most important results of the device and morphological characterization are represented in Figures 1b and 1c. Figure 1b displays current−voltage curves measured in P3HT-b-PFTBT block copolymer devices annealed at 100 and 165 °C in comparison to P3HT/PFTBT blend controls annealed under 6979

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regarding the transient absorption spectra are provided in the Methods section. The nomenclature used in the literature to describe the electronic species that form in conjugated polymers following photoexcitation is varied. Therefore, to be clear, we adopt the following naming conventions when discussing the electronic species that give rise to the spectroscopic features appearing in the transient spectra represented in Figure 2. Photogenerated charge carriers (electrons and holes) that form as a result of electron transfer from the donor to the acceptor blocks of block copolymers or polymer blends are termed charge separated polarons in accord with Durrant and co-workers.24 Such charge-separated polarons are capable of contributing to photocurrent measured in the corresponding photovoltaic devices if they do not first recombine. Electronic species that form in neat P3HT homopolymer films (or domains in the polymer blends) following initial relaxation of the photogenerated singlet excitons are termed polaron pairs in accord with usage by other groups in the field.23,26,27 While these electronic species exhibit some spectroscopic features that resemble those of charge-separated polarons, they contribute little to photocurrent measured in devices.28 Finally, PFTBT exhibits distinct electronic states that have intramolecular charge transfer character because it is a push−pull copolymer.29,30 In this work, we refer to such electronic species as pseudo-charge-transfer states in accord with Chen and coworkers.31 These electronic species have also been termed intramolecular polaron pairs.29,30 We chose not to use this term for clarity because the block copolymer includes P3HT in which polaron pairs are known to form. Referring to the spectra in Figure 2, the data reveal significant transient absorptions in the visible and near-infrared regions in all samples. Furthermore, the near-infrared transient absorption signals measured in the neat PFTBT homopolymer control are equal in amplitude to the transient signals measured in the block copolymer and polymer blend samples on all time scales. Such near-infrared absorptions in PFTBT can include contributions from pseudo-charge-transfer states, polarons pairs, and triplet excitons.30 Observing transient absorptions of this magnitude in neat PFTBT films in the near-infrared is particularly problematic because charge collection at electrodes of devices occurs on this time scale. It is challenging to determine whether the visible and near-infrared transient absorptions measured in the block copolymer and polymer blend films arise from charge-separated polarons or from pseudo-charge-transfer states, polaron pairs, or triplets. Figure S1 provides further evidence of the interference from other transient electronic species such as triplet states that absorb in the visible and near-infrared and that complicate measurements of charge carrier dynamics in this spectral region. In contrast to the visible and near-infrared regions, the transient absorptions measured in the mid-infrared26,32 provide distinct spectral signatures that can be uniquely identified with the formation of charge separated polarons arising from charge transfer from the electron donor to the acceptor. The midinfrared region of the spectra displayed in Figure 2 (highlighted by the gray box) reveals the presence of a broad mid-infrared polaron absorption in the 0.1−0.5 eV range that is present on the nanosecond time scale and persists into the microsecond time scale only when both the P3HT donor and the PFTBT acceptor are present in the filmeither in the form of the block copolymer or the polymer blend. In contrast, films containing only PFTBT or P3HT exhibit modest transient absorptions on

reported previously on the basis of the superior photovoltaic performance in comparison to the P3HT/PFTBT blend control and as a result of the formation of mesoscale order. Thus, measurements of the charge generation and recombination kinetics measured in the block copolymer films annealed under various conditions are needed to elucidate the mechanism for charge photogeneration and recombination. We used ultrafast and nanosecond transient absorption spectroscopy to measure the charge recombination kinetics in the P3HT-b-PFTBT films that were processed in the same manner and from the same batch as those examined by current−voltage characterization and RSOXS in Figure 1. Nanosecond transient absorption spectra were measured spanning the visible to the mid-infrared region in an effort to identify the spectral region that provides the most direct probe of the photogenerated charge carrier dynamics with minimal interference from other transient electronic species.23−25 The spectra appearing in Figure 2 represent averages of time delays measured between 25 and 50 ns (upper panel) and between 1 and 2 μs (lower panel) following excitation of the films at 532 nm with absorbed excitation densities below 50 μJ/cm2. Note the order of magnitude difference in signal level between the upper and lower panels. Additional experimental details

Figure 2. Transient absorption spectra of P3HT-b-PFTBT films, homopolymer blends, and their neat homopolymer controls are depicted in the visible, near-IR, and mid-IR regions. The spectra have been averaged between 25 and 50 ns time delays (upper panel) and between 1 and 2 μs (lower panel) following 532 nm excitation and have been scaled by the number of absorbed pump photons in each sample. Transient species present in the near-infrared spectrum of PFTBT persist into the microsecond time scale, which are not easily distinguished from charge-separated polarons that are relevant to charge extraction. In contrast, mid-IR polaron absorptions around 0.3 eV are observed on the microsecond time scale on which charges are collected in electrodes of devices only in films containing both the donor and acceptor blocks, indicating that the mid-IR spectral region provides a unique probe of such long-lived polarons with the least interference from other transient electronic species. 6980

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Supporting Information. The kinetics traces collected from the various samples have been scaled by the density of 532 nm pump photons that were absorbed in the films (∼15 μJ/cm2) and are plotted on logarithmic amplitude and time axes. For reference, the gray vertical lines appearing in Figure 3 indicate the time delays at which the transient absorption spectra appearing in Figure 2 were measured. Importantly, the charge recombination kinetics traces of both P3HT-b-PFTBT films displayed in Figure 3 are nearly indistinguishable from the charge recombination kinetics of the P3HT/PFTBT blend control annealed at 100 °C. For comparison, the P3HT/PFTBT blend film annealed at 165 °C exhibits a faster decay on the 100 ps−100 ns time scale that is intermediate between the P3HT/PFTBT blend annealed at 100 °C and the neat P3HT and neat PFTBT control samples. Annealing the homopolymer blend film at 165 °C leads to growth of the phase-separated domains of the P3HT and PFTBT. As a consequence, some excitons formed in these larger domains cannot reach donor−acceptor interfaces consistent with the lower short-circuit currents that are observed in the P3HT/PFTBT blend photovoltaic device that was also annealed at 165 °C (Figure 1b). The observation of charge recombination kinetics that are very similar in P3HT-b-PFTBT and the P3HT/PFTBT blend annealed at 100 °C suggests that the covalent linkage between the P3HT and PFTBT blocks of the block copolymer does not cause rapid charge recombination. The through-bond backelectron-transfer process that is unique to the block copolymer is not the dominant charge recombination pathway. The observation is in contrast to conventional wisdom in the field, which suggests that covalently linking electron donor and acceptor moieties should enhance their electronic overlap leading to fast charge separation but also to fast charge recombination.10,12−15 In fact, previous work demonstrated that covalently linking electron donor and acceptor blocks containing an electron-deficient moiety such as benzothiadiazole,10 fullerene,12−14 or perylenediimide15 units (that serve as electron accepting groups) leads to the formation of strongly bound charge transfer states or fast charge recombination. The P3HT-b-PFTBT is the first fully conjugated block copolymer system that does not exhibit this deleterious property. Intrigued by this unusual behavior, we examined the charge carrier dynamics in the P3HT-b-PFTBT and the P3HT/ PFTBT blend films more closely by fitting the kinetic decays in an effort to understand how they depend on the underlying molecular structure and mesoscale morphology. Figure 4 reproduces the mid-infrared kinetic decay traces from Figure 3 in individual panels so they can be more easily compared to their overlapping fit functions (represented as solid curves). Power law functions were selected to fit the kinetic decay curves because this functional form is the solution to a secondorder rate equation describing bimolecular decay processes. It is customary to modify the power law function with an empirical exponent α of the form n(t) = n(0)/(1 + t/τ)α. This model has been shown to describe trap assisted bimolecular polaron recombination in organic semiconducting polymers with exponents varying between unity and near zero.25 On a logarithmic plot, the exponent determines the “slope” of the kinetic decay curve (see Figure S4) with smaller exponents being associated with slower decays caused by deeper traps and wider trap distributions.33−35

the nanosecond time scale that then decay to negligible values in the mid-infrared on the 1−2 μs time scale. As a consequence, we assign the microsecond time-scale mid-infrared transient absorption feature to the absorption of charge-separated polarons that are photogenerated by charge transfer between the P3HT and PFTBT moieties in both the block copolymer and the homopolymer blend films. We compared the charge recombination kinetics measured in the mid-infrared in P3HT-b-PFTBT and P3HT/PFTBT blend films as a means to assess the influence that the covalent linkage between the P3HT and PFTBT moieties in the block copolymer has on the charge recombination kinetics. We hypothesized that if the covalent linkage significantly increased the charge recombination rate due to wave function overlap between the PFTBT anion and the oxidized P3HT block,10,16 then this would appear as faster loss of the mid-infrared polaron absorption in the block copolymer films in comparison to the P3HT/PFTBT blend controls. Figure 3 depicts transient absorption kinetics traces measured in the mid-infrared in a series of P3HT-b-PFTBT

Figure 3. Transient mid-infrared absorption kinetics traces measured in P3HT-b-PFTBT and P3HT/PFTBT blend films are compared to neat homopolymer controls following 532 nm excitation. The kinetics traces have been scaled by the number of pump photons absorbed in each sample. The kinetics are nearly identical in the block copolymer films annealed under different conditions in comparison to the P3HT/ PFTBT blend annealed at 100 °C. The comparison reveals that the covalent linkage between P3HT and PFTBT blocks has negligible influence on recombination. The curves through the data (symbols) are empirical power law fit functions described in the text. The vertical gray lines indicate the time delays at which the spectra appearing in Figure 2 were measured.

and P3HT/PFTBT blend films. The films were deposited under identical conditions used to fabricate photovoltaic devices from which the current−voltage curves appearing in Figure 1b were obtained. The kinetics traces spanning more than 8 decades in time were constructed by combining ultrafast mid-infrared transient absorption kinetics measured from zero time delay to 2 ns with mid-infrared transient absorption data collected on a separate instrument under identical excitation conditions from 20 ns to 100 μs. The method of combining these measurements to form a single kinetics trace for each sample that can be quantitatively analyzed is described in the 6981

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decays are well described by single power law functions indicating that the transient electronic species undergo bimolecular decays with exponents of α = 0.53 (α = 0.76) for the P3HT (PFTBT) homopolymer films. Polaron pair formation has been observed on the picosecond time scale in conjugated polymers.36,37 Therefore, we assign the mid-infrared kinetic decays to the absorptions of polaron pairs or pseudocharge-transfer states formed in P3HT23,26,27 and PFTBT29 films. However, the polaron pairs and pseudo-charge-transfer states do not lead to efficient generation of charges that can be collected at electrodes in photovoltaic devices because these are homopolymer films that do not contain mixtures of electron donating and accepting blocks. Importantly, their mid-infrared absorptions decay more rapidly in comparison to the block copolymer films. As a consequence, polaron pairs or pseudocharge-transfer states contribute little to the microsecond time scale mid-infrared transient spectra of P3HT or PFTBT appearing in Figure 2, although their contributions in the visible and near-infrared are substantial even at 1 μs. The mid-infrared kinetic decay curves of the polymer blend films represented in Figures 4e and 4f reveal more complex photophysics. The kinetic decay measured in the P3HT/ PFTBT blend annealed at 100 °C exhibits modest deviations from the linear appearance of a single power law function at longer time delays. These deviations are more pronounced in the P3HT/PFTBT blend annealed at 165 °C. We note that other models such as multiexponential or Kohlrausch functions are not suitable to describe the kinetics because large numbers of exponential terms are required to adequately fit the data within the single-to-noise. Therefore, we fit the kinetic decay curves with a single power law function over the amplitude and time range that is well described by a single power law in order to quantitatively compare the kinetics measured in the blends with those observed in the block copolymer films. The parameters and uncertainty limits associated with this fitting procedure are represented in Table S2. The best fit functions overlaid on the mid-infrared kinetic decays represented in Figures 4e and 4f reveal that ∼95% of the kinetic decays measured in the films are characterized a single power law decay indicative of trap-mediated bimolecular charge recombination with exponents of α = 0.35 (α = 0.44) for the P3HT/PFTBT blend films annealed at 100 °C (165 °C). Both exponents are intermediate between the exponents measured in the block copolymer films versus the homopolymer films, suggesting that two processes contribute to the decay curves. The polymer blends are capable of coarser morphologies in comparison to the block copolymer films because the P3HT and PFTBT polymer chains are not covalently linked. As a consequence, islands of P3HT (PFTBT) form that are too large for all photogenerated excitons to reach acceptor (donor) interfaces to undergo efficient electron transfer to form chargeseparated polarons. A fraction of the initial excitations formed by the pump pulse therefore form polaron pairs or pseudocharge-transfer states that absorb in the mid-infrared and that decay on the same time scale as they do in the neat homopolymer films. This process causes a portion of the kinetics measured in the polymer blend film to resemble the faster decay observed in the neat homopolymers. However, charge-separated polarons can form in polymer blend films as well, though with lower overall yields. These charge-separated polarons undergo charge recombination that is characterized by the slower decay and smaller exponents observed in the block copolymer films. The overlap of these parallel photophysical

Figure 4. Transient mid-infrared absorption kinetics appearing in Figure 3 are plotted in individual panels for quantitative comparison with fit functions used to quantify the data. Single power law functions describe the kinetic decays in the block copolymer (a, b) and homopolymer (c, d) films. The kinetics measured in the polymer blend films (e, f) exhibit more complex behavior arising from the increase of phase-separated morphology found in the blends.

Comparison of the fit functions with the mid-infrared kinetic decays measured in P3HT-b-PFTBT films annealed at both 100 and 165 °C (Figures 4a and 4b) reveals that the charge carrier dynamics in both films are well described by single power law functions spanning the full time range from 1 ps to 100 μs. The best fit functions reveal exponents of α = 0.33 (α = 0.33) for the P3HT-b-PFTBT films annealed at 100 °C (165 °C). We note that the time constants in the power law fits describe the curvature of the functions on the picosecond time scale rather than the time dependence of the subsequent decays (see Figure S4). In the fitting procedure the time constants of the fits were allowed to vary. None the less, the time constants have similar values because the kinetics traces have similar curvature on the picosecond time scale. We therefore focus the following discussion on the exponents of the power law functions, α. The detailed fitting procedures, quantitative results, and uncertainty limits associated with this analysis are described in the Supporting Information and tabulated in Table S2. The observation of a single power law function describing the charge recombination kinetics over 8 decades in time and 2 decades in amplitude in both block copolymer films indicates that their photophysics are comparatively simple. Chargeseparated polarons are efficiently formed in the block copolymer films on ultrafast time scales and then undergo trap-mediated bimolecular charge recombination on time scales slow enough to permit their collection at electrodes of the corresponding photovoltaic devices (Figure 1b). Figures 4c and 4d depict mid-infrared kinetic decay curves measured in neat P3HT and neat PFTBT homopolymer films, respectively. Similar to the P3HT-b-PFTBT films, the kinetic 6982

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P3HT block of the same block copolymer chain such that back electron transfer across the their mutual covalent linkage is suppressed. Instead, charge recombination between donor and acceptor blocks of different polymer chains dominates the recombination mechanism in both P3HT-b-PFTBT and the P3HT/PFTBT blend. The fluorene unit serves as a bridge between P3HT and the PFTBT block and slows the intramolecular recombination pathway sufficiently to render it noncompetitive with intermolecular charge recombination that occurs in both block copolymer and polymer blend films. We examined visible absorption and photoluminescence spectra of P3HT-b-PFT6BT to determine whether the spectra are consistent with weak coupling between P3HT and PFT6BT blocks in the block copolymer. Figure 5a compares the visible

processes leads to the intermediate exponents observed in the polymer blend films in comparison to the block copolymer and homopolymer films. Because fewer initially excited excitons are able to form charge-separated polarons in the polymer blend film annealed at higher temperature, the decay kinetics measured in this film have greater contribution from polaron pairs and pseudocharge-transfer states. As a consequence, the exponent describing the kinetic decay measured in the polymer blend film annealed at 165 °C (α = 0.44) is larger than the film annealed at 100 °C (α = 0.35) and more closely resembles the decays measured in neat homopolymer films. The increased phase separation in the polymer blend annealed at higher temperature also leads to lower photocurrents measured in the corresponding devices (Figure 1b) because of the lower yield of charge-separated polarons. However, the photocurrent may also be affected by changes in charge transport that accompany annealing at higher temperature. Despite the increased complexity of the photophysics occurring in the polymer blend films, the similarity of the exponent of the polymer blend annealed at 100 °C to those obtained from fitting the kinetic decay curves measured P3HTb-PFTBT films confirms the qualitative conclusion drawn by inspection of the kinetics traces in Figure 3. The covalent linkage between the P3HT and PFTBT blocks of the block copolymer does not cause rapid charge recombination. Instead, the fitting results indicate that the power law decay of the P3HT-b-PFTBT films may actually be slightly slower in comparison to the polymer blend film annealed at 100 °C, although the difference is near the experimental precision of the measurements and fitting results. Like the polymer blend annealed at 100 °C, a low amplitude decay component with much longer decay time is observed in the polymer blend film annealed at 165 °C. We believe this slower component arises from photogenerated carriers that become trapped in islands of P3HT or PFTBT that are present in the blend annealed at 165 °C and to a lesser extent in the blend annealed at 100 °C. Such islands are not likely in the block copolymer films because the donor and acceptor blocks are covalently linked and cannot undergo macroscopic phase separation. Consequently, the kinetic decay curves of the block copolymer films are well described by single power law functions within the measured time range. It is interesting to consider the molecular structures of the P3HT-b-PFTBT block copolymer in an effort to identify why the covalent linkage does not influence the charge recombination time scale. A recent study of the photophysics of P3HT end-capped with a fluorene and dithianylbenzodiathiazole (TBT) unit in solution reported the formation of a strongly emissive CT state when the strong acceptor TBT was covalently bonded directly to P3HT.10 However, when the fluorene unit bridged the P3HT and TBT moieties, as in the case of the P3HT-b-PFTBT block copolymer examined here, the formation of a strongly bound CT state was suppressed. Similar findings were reported in a study of block copolymer films in which the TBT or the fluorene unit were bonded directly to P3HT.9 Covalent bonding of the TBT unit directly to P3HT decreased photovoltaic device performance likely through formation of a strongly bound CT state. The data in Figure 3 indicate that the fluorene unit has a more profound effect than simply modulating the CT state characteristics. The fluorene unit actually prevents the anionic PFTBT block from being strongly coupled to the oxidized

Figure 5. (a, b) Absorption and photoluminescence spectra of isolated block copolymer chains in solution (red) cannot be distinguished from the sum of the absorption and photoluminescence spectra of the homopolymers in solution (black), indicating that they have relatively weak electronic overlap. (c) Frontier molecular orbitals of sexithiophene as a model of P3HT and one repeat unit as model of PFTBT from DFT calculations. The frontier orbitals of the ground state of both blocks are delocalized over the entire conjugated framework, indicating they have significant electronic overlap in the block copolymer. However, the frontier orbital contributing to the charge-separated state on the PFTBT monomer is largely localized on the TBT moiety, suggesting little electronic overlap with orbitals of P3HT.

absorption spectrum of a dilute solution of P3HT-b-PFT6BT in CHCl3 (1.0 μg/mL) to absorption spectra of solutions of neat P3HT and neat PFT6BT also in CHCl3 of the same total solids concentrations, respectively. A spectrum constructed from a mass-weighted linear combination of the neat homopolymer solution spectra is overlaid onto the experimental spectrum of P3HT-b-PFT6BT. The comparison indicates that the covalent linkage between the P3HT and PFT6BT in the block copolymer has negligible influence on the electronic structure of the polymer blocks. Note that a hexyl group was added to the block copolymer structure to suppress aggregation in 6983

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acceptor blocks does not limit device performance for this block copolymer system. Our findings demonstrate that block copolymers, if engineered correctly at the molecular level, can provide the mesoscale structural control needed to improve device efficiency, while preventing deleterious intramolecular charge recombination.

solution for these studies (see Supporting Information for polymer structures). Furthermore, the photoluminescence spectrum of the 1.0 μg/mL solution of P3HT-b-PFT6BT in CHCl3 is represented in Figure 5b in comparison to photoluminescence spectra of the neat homopolymer solutions. A spectrum constructed from a linear combination of the neat homopolymer solution photoluminescence spectra is overlaid on that of the block copolymer. The spectrum constructed from the neat homopolymer spectra cannot be distinguished from the experimental block copolymer photoluminescence spectrum, again indicating that the electronic structure of the P3HT and PFT6BT blocks are negligibly affected by their covalent linkage. Figure S5 provides further evidence that the covalent linkage does not lead to CT states that influence the absorption and photoluminescence spectra of the P3HT and PFTBT blocks of P3HT-b-PFTBT even in the solid state. These findings demonstrate that the absorption and photoluminescence spectra are consistent with predictions from the charge recombination kinetics that the P3HT donor and PFTBT acceptor blocks are only weakly coupled through their covalent linkage. It should be noted that the model spectrum used to fit the measured photoluminescence spectrum in Figure 5b contains a smaller contribution from P3HT than might be expected from the absorption of the polymer blocks. The lower quantum yield for emission from P3HT is partially responsible for this difference, though it is necessary to include energy transfer from P3HT to PFT6BT to quantitatively account for the relative weighting of the P3HT and PFT6BT emission in the block copolymer solution. This energy transfer pathway will be a topic of a forthcoming publication. Consideration of the frontier orbitals contributing to the states of P3HT and PFTBT that are involved in charge transfer provides an explanation for the weak coupling that exists between the donor and acceptor blocks of P3HT-b-PFTBT. We modeled the P3HT block with α-sexithiophene (6T) and the PFTBT block with 4-(thiophene-2-yl)-7-(5-(fluorene-2-yl)thiophene-2-yl)-2,1,3-benzothiadiazole (FTBT) in order to calculate the relevant frontier orbitals of the blocks (Figure 5c) using density functional theory (DFT).38 The calculations reveal that the HOMO of FTBT is delocalized over both the fluorene and dithienylbenzothiadiazole moieties. However, the LUMO of FTBT is largely localized on the dithienylbenzothiadiazole moiety.9,39 Extending these results to P3HT-b-PFTBT, we suggest that the HOMO of the PFTBT block can be strongly coupled to the frontier orbitals of P3HT, but the fluorene unit serves as a bridge that selectively reduces the electronic coupling of the LUMO of PFTBT to the frontier orbitals of P3HT. Importantly, the LUMO figures prominently in the anionic state of the PFTBT block. Therefore, intramolecular charge recombination, which involves the transfer of an electron from PFTBT back to P3HT of the same polymer chain, is expected to be slow both because it occurs in the Marcus inverted region, as previously reported using DFT calculations of the P3HT-bPFTBT system,38 and because the electronic coupling of these states is low. Charge recombination can occur between neighboring block copolymer chains because the electronic overlap of P3HT and PFTBT depends on their relative alignment and orientation, leading to similar recombination kinetics in block copolymer and P3HT/PFTBT blend films. As a consequence, the covalent linkage between donor and

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CONCLUSION AND OUTLOOK In summary, we examined charge recombination kinetics measured in fully conjugated block copolymer photovoltaic materials composed of P3HT-b-PFTBT and annealed under different conditions using ultrafast and nanosecond transient absorption spectroscopy. The charge recombination kinetics were compared to the corresponding polymer blends including the same P3HT and PFTBT blocks but without their mutual covalent linkage. Transient absorption spectra covering a wide range spanning the visible to mid-infrared spectral regions were used to identify the region best suited to unambiguously measure charge recombination kinetics without interference from other electronic species. By inclusion of correct homopolymer controls and comparison to transient absorption spectra of block copolymer and polymer blend samples, we identified the mid-infrared region as uniquely suited to measure charge recombination kinetics that are relevant to charges photogenerated in photovoltaic devices. Using this spectral region and measuring charge recombination kinetics over 8 decades in time and 3 decades in signal amplitude leads to the surprising conclusion that the covalent linkage between donor and acceptor blocks in P3HT-b-PFTBT films does not lead to faster charge recombination in comparison to the polymer blends. The results of this investigation permit us to suggest a design rule for further development of fully conjugated block copolymer photovoltaicsnamely that high performance block copolymers may require the inclusion of a push−pull copolymer as the acceptor or the donor block. If the acceptor block is a push−pull copolymer as in the case of P3HT-bPFTBT, then the LUMO should be localized to the strong acceptor moiety. The weak donor moiety of the push−pull copolymer should be placed between the donor and acceptor blocks to inhibit intramolecular charge recombination as observed here. Alternatively, it may be advantageous to disrupt the electronic coupling of the donor and acceptor blocks by inclusion of a short nonconjugated bridge such as a few methylene groups.

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EXPERIMENTAL METHODS Synthesis of Block Copolymers and Homopolymers. Synthesis for Poly(3-hexylthiophene-2,5-diyl) (P3HT). Regioregular P3HT was synthesized according to the Grignard metathesis method. 2,5-Dibromo-3-hexylthiophene (2.55 g, 7.82 mmol) was placed in a reactor and purified under vacuum for several minutes. After refilling with argon, freshly dried THF was injected as solvent. Isopropylmagnesium chloride with LiCl in THF (7.43 mmol) was added dropwise at 0 °C. After 3 h at room temperature, the reaction was diluted with additional THF. Polymerization began with addition of 1.2 mol % of [1,3bis(diphenylphosphino)propane]dichloronickel(II) (Ni(dppp)Cl2) (50.9 mg, 0.094 mmol) as catalyst. After 20 min at room temperature, the reaction was terminated by the addition of 5 M HCl (3 mL). The reaction mixture was precipitated in chilled methanol and subsequently purified by 6984

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Characterization of the Polymers. Molar mass and dispersity of the polymers were determined by gel permeation chromatography (GPC) with polystyrene standards. RI detector was applied and mobile phase was THF. Photovoltaic Device Fabrication and Characterization. A photovoltaic device was prepared with conventional architecture of ITO/PEDOT:PSS (70 nm)/active layer (80− 100 nm)/Al electrode (75 nm). The film thicknesses were determined on a TENCOR P-10 surface profiler. PEDOT:PSS (Clevios P, Heraeus) was covered onto ITO-coated glass substrates (20 ohm/sq, Xin Yan Technology, Hong Kong) by spin coating at 4000 rpm for 2 min, followed by drying at 165 °C for 10 min. The substrate was transferred into a nitrogenfilled glovebox for the further process. Solutions of P3HT/ PFTBT mixture (5.0 mg/mL, 1:1 in weight) and P3HT-bPFTBT (5.0 mg/mL) were prepared in anhydrous chloroform (≥99%, amylenes as stabilizer, Sigma-Aldrich) and stirred at 90 °C for about 20−22 h in a tightly sealed container in a nitrogen-filled glovebox. The active layers were covered by spin coating at 1000 rpm for 60 s. The devices were completed by vacuum thermal evaporation of aluminum at 10−6 Torr on top of the active layer through a shadow mask (device area = 16.2 mm2). Integrated solar cells were further annealed at 100 °C for 20 min or at 165 °C for 10 min. Photovoltaic measurements were performed in a nitrogen atmosphere under simulated AM 1.5G illumination (97 mW/ cm2) from a xenon lamp solar simulator (Newport Model SP92250A-1000). The illumination intensity was calibrated using an optical power meter and NREL certified Si reference photocell (Newport). A Keithley 2636A sourcemeter was used to measure the current−voltage characteristics of solar cells. RSOXS Sample Preparation and Measurement. Thin films of either P3HT-b-PFTBT or P3HT and PFTBT blends were spin cast in a N2 glovebox from 7 mg/mL solution of chloroform onto poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), PEDOT:PSS (Clevios P, H.C. Starck), films deposited on silicon wafers. Prior to spin coating, silicon wafers were sonicated in acetone and isopropanol followed by ultraviolet light ozonation. As-cast films were subsequently floated off in deionized water and picked up with 5 mm × 5 mm silicon frames supporting a 1 mm × 1 mm, 100 nm thick Si3N4 window. Samples were then dried for 24 h under vacuum at room temperature and then annealed at different temperatures inside a N2-filled glovebox. RSOXS measurements were carried out in transmission geometry in vacuum at beamline 11.0.1.2 at the Advanced Light Source, Lawrence Berkeley National Laboratory. Scattering patterns obtained using polarized X-ray at an energy of 285.4 eV were radially integrated as a function of scattering vector q, and dark currents were taken into account. Optical Spectroscopies. Sample Preparation for Solution Measurements. All polymers were dissolved in anhydrous chloroform (Sigma-Aldrich) at 1 mg/mL in a nitrogen-filled glovebox and stirred overnight. Before measurement, solutions were diluted to 1 μg/mL, loaded into quartz cuvettes (1 cm path length), and sealed. Once removed from the glovebox, measurements were taken within 30 min. Sample Preparation for Film Measurements. Solutions of P3HT/PFTBT mixture (5.0 mg/mL, 1:1 in weight) and P3HT-b-PFTBT (5.0 mg/mL) were prepared in anhydrous chloroform (≥99%, amylenes as stabilizer, Sigma-Aldrich) and stirred at 90 °C for about 20−22 h in a tightly sealed container in a nitrogen-filled glovebox. Films were formed onto CaF2

sequential Soxhlet extraction with methanol, acetone, hexane, and chloroform. The chloroform part was collected as product. The yield was 28.0% (364 mg, Mn 10.4 kg/mol, Mw 11.8 kg/ mol, Đ 1.13). Synthesis for Poly((9,9-bis(2-octyl)fluorene-2,7-diyl)-alt(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl) (PFTBT). PFTBT was prepared through a Suzuki−Miyaura polycondensation reaction. 9,9-Dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester (F) (300 mg, 0.54 mmol) and 4,7-bis(5-bromothiopen-2-yl)-2,1,3-benzothiadiazole (TBT) (222 mg, 0.49 mmol) were dissolved in 36 mL of anhydrous toluene. Tetraethylammonium hydroxide solution (20% in water, 12 mL), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (22 mg, 0.019 mmol), and a few drop of Aliquat 336 were added. The reaction mixture was degassed by three “freeze−pump−thaw” cycles and then backfilled with argon. The polymerization proceeded at at 90 °C for 20 h. After termination by bromobenzene (6 mL), the aqueous phase was eliminated by a separation funnel, and then the organic phase was precipitated in chilled methanol. The raw product was dissolved in toluene, precipitated again in chilled methanol, and washed with methanol, acetone, and hexane using a Soxhlet extractor. The yield was 83.8% (297 mg, Mn 7.8 kg/mol, Mw 13.7 kg/mol, Đ 1.75). Synthesis for Poly((9,9-bis(2-octyl)fluorene-2,7-diyl)-alt(4,7-di(4-hexylthiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″diyl) (PFT6BT). PFT6BT was prepared in the same manner as PFTBT except using 4,7-bis(5-bromo-4-hexylthiophen-2-yl)2,1,3-benzothiadiazole (T6BT) instead of TBT. F (112 mg, 0.20 mmol) and T6BT (105 mg, 0.17 mmol) were dissolved in 15 mL of anhydrous toluene. Pd(PPh3)4 (26 mg, 0.023 mmol), tetraethylammonium hydroxide (20% in water, 4.5 mL), and a few drops of Aliquat 336 were added. The reaction mixture was degassed by three freeze−pump−thaw cycles, backfilled with argon, and stirred at 90 °C for 20 h. The reaction was terminated with 4 mL of bromobenzene. The aqueous phase was discarded and polymer precipitated into chilled methanol. The raw product was dissolved in toluene, precipitated again in chilled methanol, and washed with methanol and acetone using a Soxhlet extractor. The yield was 67% (104 mg, Mn 27 kg/mol, Mw 55 kg/mol, Đ 2.0). Synthesis for Poly(3-hexylthiophene-2,5-diyl)-block-poly((9,9-bis(2-octyl)fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2yl)-2,1,3-benzothiadiazole)-5′,5″-diyl) (P3HT-b-PFTBT). P3HT-b-PFTBT block copolymer was prepared in the same manner as PFTBT except addition of P3HT macroreagent as a donor block. P3HT macroreagent (Mn 10.4 kg/mol, Mw 11.8 kg/mol, Đ 1.13, 150 mg, 0.015 mmol), F (168 mg, 0.30 mmol), and TBT (124 mg, 0.27 mmol) were dissolved in 20 mL of anhydrous toluene. Tetraethylammonium hydroxide solution (20% in water, 6 mL) was injected. Pd(PPh3)4 (13 mg, 0.011 mmol) as catalyst and a few drops of Aliquat 336 were added. The reaction mixture was degassed by three freeze−pump− thaw cycles and then refilled with argon. The polymerization went at 90 °C for 20 h. Bromobenzene (6 mL) was injected as terminator. The aqueous phase was eliminated by a separation funnel, and then the organic phase was precipitated in chilled methanol. The raw product was dissolved in toluene, precipitated again in chilled methanol, and subsequently purified by Soxhlet extraction with methanol, acetone, and hexane. The yield of the block copolymer was 80.3% (279 mg, Mn 16.3 kg/mol, Mw 23.5 kg/mol, Đ 1.44). 6985

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oscilloscope (Pico Technology). Pump fluences used were the same as reported for the nanosecond mid-IR spectroscopy. Steady-State Absorbance and Photoluminescence Spectroscopy. For polymer solutions, absorbance spectra were measured using a UV−vis spectrometer (Agilent Technologies, Cary60). Steady-state photoluminescence spectra were collected using a spectrofluorometer (Photon Technology International) with an excitation wavelength of 470 nm and a spectral resolution was 2 nm. For polymer films, absorbance spectra were collected using a UV−vis spectrometer (Beckman, DU 520). Quantum Chemical Calculations. Density functional theory (DFT) calculations were done with the Gaussian 09 package at the B3LYP 6-31G(d,p) level. The shapes of the HOMO and LUMO for optimized structures of α-sexithiophene and 7-(5-thienyl)-2,1,3-benzothiadiazole-4-(2-fluorene5-thienyl) were rendered using the VMD package.

substrates by spin coating at 1000 rpm, followed by thermal annealed at 100 °C for 20 min or at 165 °C for 10 min. Ultrafast Mid-IR Transient Absorption Spectroscopy of Films. Ultrafast visible-pump/mid-IR-probe spectroscopy was performed using a home-built system as described elsewhere.40 Briefly, a Ti:sapphire regenerative laser amplifier (Integra, Quantronix) seeded by a Ti:sapphire laser (Coherent) was used as the source for the pump and the probe. For the vis pump, the laser drove an OPA (TOPAZ, Light Conversion Ltd.), which was tuned to 532 nm. For the mid-IR probe, a second OPA was tuned to ∼4000 nm. The pump was mechanically delayed relative to the probe using a motorized delay stage (Newport). After passing through the sample, the probe beams were dispersed in a monochromator (Horiba) before being detected to an MCT array detector (Infrared Associates/ Infrared Systems Development). The fluence was kept below 30 μJ/cm2 in order to avoid a nonlinear response from the sample (see Supporting Information). Nanosecond Mid-IR Transient Absorption Spectroscopy. Nanosecond mid-IR transient absorption measurements were performed using a home-built mid-IR flash photolysis instrument.41 A silicon nitride emitter (Spectral Products) was used as the probe source. A 30 Hz frequency-doubled Nd:YAG laser (Continuum) was used as the pump, which had a pulse duration of ∼8 ns and a wavelength of 532 nm. Spectral resolution was obtained by dispersing the probe beam using a monochromator (Spectral Products) with slits set to obtain a 40 nm effective bandwidth. For kinetic measurements the probe was filtered using a 2.5 μm long-pass filter and a 10 μm short-pass filter (CaF2 lenses) and detected and amplified using a 100 MHz MCT Photodiode (Kolmar Technologies, KV104). For spectral measurements the probe was dispersed using a monochromator (Spectral Products, DK240) and detected and amplified using a 16 MHz MCT photodiode (Kolmar Technologies, KMPV11-1-J1). The signal was then digitized using a 100 MHz PC oscilloscope (Pico Technology). The pump laser fluence was kept as close as to 30 μJ/cm2 as possible for all kinetic measurements in order to match the fluence used in ultrafast measurements. Spectra were collected using a slightly higher fluence (∼50−60 μJ/cm2) in order to achieve better S/N to minimize data acquisition time. Nanosecond Vis/Near-IR Transient Absorption Spectroscopy. Nanosecond vis/near-IR transient absorption measurements were performed using a home-built vis/near-IR flash photolysis instrument as described previously.42 In particular, a tungsten halogen light source (Spectral Products) filtered using an assortment of long-pass dielectric filters (depending on the spectral region measured) was used as the probe. The excitation source was the same laser used for nanosecond mid-IR transient absorbance spectroscopy. Spectral resolution was obtained by dispersing the probe beam using a monochromator (Spectral Products) with slits set to obtain a 10 nm effective bandwidth. The dispersed beam was measured using an assortment of photodiodes with similar rise times depending of the spectral region being scanned. For the visible region (∼575−800 nm), a silicon photodiode was used (Thorlabs, DET10A). For the near-IR region (∼800−1300 nm), an InGaAs photodiode was used (DET10N, Thorlabs). For the far-near-IR region (∼1300−2300 nm), an InGaAs transimpedance amplified photodiode (PDA10D, Thorlabs) was used. For all but the latter detector, the signal was further amplified using a 200 MHz preamplifier (Femto). The amplified signal was then digitized using a 200 MHz PC

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00103. Device characterization, motivation for selection of midIR spectral region for polaron absorption kinetics, method to combine ultrafast and mid-IR kinetics, fitting routine and power-law functions, steady-state absorption and emission spectra, excitation energy density dependence of ultrafast transient absorption kinetics (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; (814) 863-6309 (J.B.A.). *E-mail [email protected]; (814) 867-3428 (E.D.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Office of Naval Research under Grant N000141410532 is gratefully acknowledged. The Advanced Light Source is an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Lawrence Berkeley National Laboratory and is supported by the U.S. Department of Energy under Contract DE-AC0205CH11231.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b00103 J. Phys. Chem. C 2016, 120, 6978−6988

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The Journal of Physical Chemistry C (42) Rimshaw, A.; Grieco, C.; Asbury, J. B. Note: Using Fast Digitizer Acquisition and Flexible Resolution to Enhance Noise Cancellation for High Performance Nanosecond Transient Absorbance Spectroscopy. Rev. Sci. Instrum. 2015, 86, 066107(3).

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DOI: 10.1021/acs.jpcc.6b00103 J. Phys. Chem. C 2016, 120, 6978−6988