Nature of the Binding Interactions between Conjugated Polymer

Article pubs.acs.org/cm

Nature of the Binding Interactions between Conjugated Polymer Chains and Fullerenes in Bulk Heterojunction Organic Solar Cells Mahesh Kumar Ravva,‡ Tonghui Wang,‡ and Jean-Luc Brédas* Laboratory for Computational and Theoretical Chemistry of Advanced Materials, KAUST Solar Center, Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: Blends of π-conjugated polymers and fullerene derivatives are ubiquitous as the active layers of organic solar cells. However, a detailed understanding of the weak noncovalent interactions at the molecular level between the polymer chains and fullerenes is still lacking and could help in the design of more efficient photoactive layers. Here, using a combination of long-range corrected density functional theory calculations and molecular dynamic simulations, we report a thorough characterization of the nature of binding between fullerenes (C60 and PC61BM) and poly(benzo[1,2-b:4,5-b′]dithiophene− thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) chains. We illustrate the variations in binding strength when the fullerenes dock on the electron-rich vs electron-poor units of the polymer as well as the importance of the role played by the polymer and fullerene side chains and the orientations of the PC61BM molecules with respect to the polymer backbones.

I. INTRODUCTION Polymer−fullerene based bulk heterojunction (BHJ) solar cells are considered as promising candidates for renewable energy production, owing to their potential to be printed on flexible substrates at low cost and their light weight.1 Over the past decade, various strategies founded on the development of lowoptical-gap conjugated polymers, optimization of the energy levels between electron-donor and electron-acceptor materials, and tuning of the morphology with the help, e.g., of solvent additives, have led to substantial improvement in the power conversion efficiencies (PCEs) that now reach on the order of 11.7%.2−7 In spite of these advances, higher power conversion efficiencies are still desirable for most major commercial applications. One of the key parameters to realize high PCEs in organic solar cells is an optimal morphology of the polymer−fullerene blend in the active layer.3 Indeed, the morphology of the active layer, the corresponding intermolecular arrangements between electron-donor and electron-acceptor materials, and the resulting energy landscapes at the interfaces strongly influence device performance.3,8−11 The intermolecular interactions are expected to impact the electron-transfer rates12 and exciton binding energies,13,14 as well as the charge-separation15,16 and charge-recombination12,17 processes, with a number of reports having addressed the role of interfacial geometry and longrange electrostatic interactions.18−20 Furthermore, the energetic disorder in the blends mostly depends on the variations in positions, orientations, and conformations of the materials21 and also impacts device efficiency.22,23 Hence, a detailed molecular-level understanding of the noncovalent binding interactions at the organic−organic interfaces between the conjugated polymer chains and the fullerene molecules will © 2016 American Chemical Society

provide useful information, which could help in the development of more efficient materials. A recent study by Graham et al. has shown that, in the case of the poly(benzo[1,2-b:4,5-b′]dithiophene−thieno[3,4-c]pyrrole4,6-dione) (PBDTTPD) family of polymers (see Figure 1)

Figure 1. Molecular structures of poly(benzo[1,2-b:4,5-b′]dithiophene−thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) chains considered in this work.

even a priori minor changes in the nature, length, and positions of the side chains can affect the efficiency of solar cells.4 For instance, when the thieno[3,4-c]pyrrole-4,6-dione (TPD, electron-poor) moiety carries a sterically accessible linear alkyl side chain and the benzodithiophene (BDT, electronrich) moiety carries more sterically hindered branched side chains, the fullerene molecules are preferred to dock on the electron-poor part of the polymer chain; the resulting Received: July 18, 2016 Revised: October 17, 2016 Published: October 24, 2016 8181

DOI: 10.1021/acs.chemmater.6b02930 Chem. Mater. 2016, 28, 8181−8189

Article

Chemistry of Materials

II. COMPUTATIONAL METHODOLOGY

arrangement has given power conversion efficiencies up to 8% and open-circuit voltages up to 1 V.24−27 On the other hand, for the polymer backbone, much lower power conversion efficiencies are observed if TPD units carry a bulky side chain and BDT units carry two linear side chains. More recently, Laquai and co-workers have reported that a PBDTTPD polymer with a pattern of branched alkyl side chains on the electron-rich moieties and linear alkyl side chain on the electron-poor moieties shows lower geminate and nongeminate charge-carrier recombination losses compared to the corresponding PBDTTPD polymer with linear alkyl groups on both BDT and TPD units.28 We have shown recently, using molecular dynamics simulations, the impact of the alkyl side chains on polymer chain aggregation in the pure phase and the affinity of PC61BM molecules for BDT or TPD units of PBDTTPD polymer in the mixed region.29 However, our goal here is to determine the nature of the binding interactions between the polymer chain and the f ullerene at the molecular scale. There have been a number of experimental investigations devoted to describe the energy landscape of polymers and fullerenes in polymer:fullerene bulk heterojunctions with pure and mixed phases.30−36 For instance, using a combination of cyclic voltammetry and ultraviolet photoelectron spectroscopy, McGehee and co-workers have highlighted the occurrence of energy-level shifts in the polymer phases due to polymer disorder and intermolecular interactions between the polymer chains and fullerenes as well as the importance of these energylevel modifications on solar-cell efficiency.36 Also, it has been recently shown that no significant ground-state charge transfer occurs between a thiophene-based conjugated polymer and fullerene molecules; hence, any energy-level shift caused by mixing polymer and fullerene is expected to come mainly from van der Waals interactions (especially dispersion forces).32 Also, the efficiency of photoinduced charge-transfer between polymer and fullerene depends on whether the polymer can accommodate the fullerene on top of the acceptor units of the polymer.30,31 Since it remains challenging to probe experimentally at the molecular scale the nature of the binding interactions and intermolecular arrangements between the polymer chains and the fullerenes in the BHJ blends, the insight brought by computational studies can prove to be extremely useful in this context. A systematic comparison of polymer−PC61BM interactions in the presence and absence of PC61BM− PC61BM interactions can provide a useful insight in order to optimize the morphology of bulk heterojunctions and ultimately the solar cell efficiency. Hence, in this study, we describe the intermolecular interactions between the polymer chains and fullerenes via long-range corrected density functional theory (DFT) calculations, complemented by classical molecular dynamics (MD) simulations. Given the extensive experimental data available on solar cells based on PBDTTPD−fullerene bulk heterojunctions,4,24−28 we have chosenas in our earlier work29three PBDTTPD polymers (Figure 1) with different alkyl side-chain patterns as electrondonor materials. First, with the aid of long-range corrected DFT calculations, we elucidate the nature and magnitude of the binding interactions between the polymer chains and fullerenes (C60 and PC61BM) and shed light on the importance of the various intermolecular forces operating at the organic−organic interfaces. Then, molecular dynamics simulations are used to demonstrate the role of the side chains on the preferred locations of the fullerenes on top/around the polymer chains.

II.a. Density Functional Theory Calculations. To obtain a reliable description of polymer−fullerene complexes, geometry optimizations have been carried out at the ωB97XD/6-31G(d,p) DFT level of theory. The ωB97XD functional allows us to take properly into account both the extent of the wave function delocalization along the π-conjugated backbone and the intermolecular interactions between the backbone and fullerene.37 We have used a PBDTTPD oligomer model consisting of four BTD and three TPD units, given that such a chain length provides convergence of the interaction energies between oligomer and fullerene (see the Supporting Information and Figure S1 for a more detailed discussion). As a first step, we were interested in gaining insight into the polymer−fullerene interactions in the absence of side chains. Thus, we considered an electron-donor/electron-acceptor complex formed by a coplanar PBDTTPD oligomer (containing, as mentioned above, four BTD and three TPD units) with the alkyl groups replaced by hydrogens, interacting with a C60 molecule. As a second step, we added complexity by turning to the phenyl-C61-butyric acid methyl ester (PC61BM) molecule, thus taking explicit account of the fullerene functional group. We chose a number of starting configurations, which led to different local minima: (i) three face-on configurations, for which the fullerene appears on top of either the central BDT unit (referred to as the “face-on D” configuration), an inner TPD unit (face-on A configuration), or the bond between the central BDT unit and an adjacent TPD unit (face-on DA configuration) (ii) an edge-on configuration, for which the fullerene lies on the side of the backbone The counterpoise-corrected total interaction energies were calculated on the basis of the optimized geometries. In addition, to gain a deeper insight on the nature the intermolecular interactions, we performed energy-component analyses using symmetry-adapted perturbation theory (SAPT) at the SAPT0/cc-pVDZ level of theory.38 In this way, the total interaction energy can be decomposed into its various, physically meaningful contributions, i.e., the electrostatic, exchange-repulsion, induction (permanent dipole−induced dipole), and dispersion (induced dipole−induced dipole) energies. In this study, we have considered spin-component-scaled dispersion energies.39 All DFT and SAPT calculations were performed with the Gaussian 0940 and PSI-441 packages, respectively. II.b. Molecular Dynamics Simulations. All molecular dynamics (MD) simulations were carried out using the LAMMPS package.42 The force field was derived from the Optimized Potentials for Liquid Simulations−All Atom (OPLS-AA) force field.43,44 The torsion potential between adjacent BDT and TPD moieties was parametrized according to results at the ωB97XD/6-31G(d,p) level of theory.29 For PC61BM, the force-field parameters were taken from Troisi et al.45 Atomic partial charges were determined by the restricted electrostatic potential (RESP)46fitting scheme at the ωB97XD/cc-PVTZ level; the RESP calculations were carried out with the Red software.47 In order to describe the effects of polymer side-chain variations on the binding between PBDTTPD and PC61BM, we have considered the same three polymers as in our other work:29 PBDTTPD(C14-C8), PBDTTPD(C14-EH), and PBDTTPD(EH-C8) (see Figure 1); here, C14-C8, C14-EH, and EH-C8 denote PBDTTPD with two linear C14 alkyl groups on BDT and a linear C8H17 (C8) alkyl groups on TPD; two linear C14H29 (C14) alkyl chains on BDT and one bulky 2-ethylhexyl (EH) group on TPD; and two bulky EH groups on BDT and one linear C8 alkyl group on TPD, respectively. The complete methodological details are given in the Supporting Information.

III. RESULTS AND DISCUSSION III.a. Interaction between C60 and the PBDTTPD Backbone. We start our discussion by evaluating the interaction energies between the bare π-conjugated polymer backbone and C60. Of particular interest here is the question of 8182


Article


difference in going from the configuration for which C60 sits on top of the BDT (D) unit with respect to the one where it sits on top of the TPD (A) unit, −12.3 kcal/mol vs −11.7 kcal/mol, remains within thermal energy at room temperature (∼0.6 kcal/mol). (ii) Edge-on configurations are much less favored as their interaction energies are only about half those of the faceon configurations. Preference for face-on configurations has also been seen, for instance, in the case of pentacene−C60 interactions.48 (As a consequence, in the remainder of this work, we focus on face-on configurations). (iii) In all instances, the dispersion (induced dipole−induced dipole) forces are the main contributor to the total interaction energy. We also note that the wave function delocalization along the polymer backbone is the key contributor to ensuring that faceon configurations have similar interaction energies whatever the location of the C60 molecule. Indeed, calculations involving C60 with either a single BDT unit or a single TPD unit give a substantially stronger interaction energy in the former case (−10.3 kcal/mol) than in the latter case (−6.3 kcal/mol), which is merely a reflection of the larger size of the BDT unit (see Figure S1). III.b. Interaction between PC61BM and the PBDTTPD Oligomer Backbone. We now add to the complexity of the system by considering the phenyl butyric acid methyl ester functional group on C60, i.e., PC61BM. A first important aspect (which has often been overlooked although it has been pointed out before49,50) is that PC61BM has a substantial dipole moment of 5.2 D (ωB97XD/6-31G(d,p)) in its optimal geometry (see Figure 3); this dipole is related to an ∼0.1 |e|

whether intrinsically, i.e., in the absence of any side chain, the fullerene has a higher affinity for the electron-poor TPD unit of the backbone (hereafter referred to, for the sake of simplicity, as the “acceptor”, A, copolymer moiety) or rather for the electronrich BDT moiety (“donor”, D). The optimized geometries of the PBDTTPD oligomer−C60 complexes are illustrated in Figure 2 along with the total interaction energies. A comparison

Figure 2. Illustration of the optimized geometries for the three face-on and the edge-on configurations of the oligomer−fullerene complex using the ωB97XD/6-31G(d,p) method. The calculated counterpoisecorrected interaction energies are also given.

Figure 3. Influence of the phenyl butyric acid methyl ester functional group on the PC61BM dipole moment. Left: Optimized geometry of PC61BM at the ωB97XD/6-31G(d,p) level. Right: Charge distribution in PC61BM with the charges (in |e|) evaluated using a natural population analysis.

of the interaction energies for the face-on D, face-on A, face-on DA, and edge-on configurations then reflects the binding preference of the fullerene interacting with the PBDTTPD oligomer. The counterpoise-corrected interaction energies obtained from DFT (ωB97XD/6-31G(d,p) level of theory) and the components of the interaction energies calculated from SAPT0/cc-pVDZ are listed in Table S1. We emphasize that the interaction energies obtained from DFT and SAPT0 are fully consistent with each other. The conclusions that are to be drawn from Figure 2 are that, in the absence of alkyl groups on polymer backbone and f unctional group on f ullerene: (i) The fullerene adopts face-on configurations but has hardly any preference as to where it lies on the aromatic surface of the polymer backbone. The interaction energy

transfer from the functional group to the fullerene cage. This can potentially add a significant electrostatic component to its interactions with the polymer backbone, in addition to the larger dispersion forces due to the presence of the functional group. Note that the dipole moment of the isolated PC61BM molecule can fluctuate between 5.2 and ca. 1.5 D, depending on the exact orientation of the ester group (see Supporting Information for a detailed discussion). When PC61BM interacts with the polymer backbone (where the long and the short axes of the chain are taken to be the xand y-axes, respectively), it adopts three main configurations;29 8183


Article

Chemistry of Materials see Figure 4 for schematic representations: A first configuration, denoted as conf-1, has the long axis of the PC61BM molecule

The optimized geometries of PC61BM on top of the BTD moiety and on top of the TPD moiety are described in more detail in the Supporting Information Figures S4 and S5. (We note that the introduction of the functional group on the fullerene cage makes it more difficult to make a clear distinction between face-on DA and face-on D or face-on A configurations; hence, all geometries have been simply classified as face-on D or face-on A, depending on whether the center of mass of the C60 cage is closer to BDT or TPD). Not surprisingly, the interaction energies strongly depend upon the orientation of the long axis of the PC61BM molecule (Figure 4), leading to a range of interaction energies from −11.3 to −21.6 kcal/mol; see Table 1. For the conf-1 and conf-2 instances, where the PC61BM functional group interacts with the backbone, the interaction energies increase significantly with respect to the C60 case: by nearly 50% up to −20.1 kcal/mol [−18.7 kcal/mol] for conf-1 [conf-2] and PC61BM on top of the polymer BDT moiety and up to −17.0 kcal/mol [−16.2 kcal/mol] for conf-1 [conf-2] when PC61BM is on top the TPD moiety. In the instance of conf-3, the fact that the functional group of PC61BM is away from the polymer backbone leads to interaction energies very similar to the C60 cases. Overall, these results point out the importance of the PC61BM orientations with respect to the polymer backbone. They also indicate that, without alkyl side chains on the polymer but with the phenyl butyric acid methyl ester functional group on the fullerene, there occurs some preference for PC61BM to sit on top of the polymer BDT electron-rich moiety. Thus, the presence of side chains attached to the polymer backbones is clearly expected to play a major role. III.c. Interaction between PC61BM and the PBDTTPD Oligomer with Side Chains. We go another step into the complexity of the systems and now consider the alkyl side chains on the PBDTTPD polymers. At this stage, in order to most easily describe the results, it is best to turn our discussion to the molecular dynamics simulations. We note that, in the presence of side chains on both backbone and fullerene, in addition to the intermolecular interactions seen in the previous sections, there will occur dispersion and induction binding interactions involving the polymer side chains and fullerene cages as well as the polymer side chains and the fullerene side chains. We have investigated the three polymers: PBDTTPD(C14EH), PBDTTPD(C14-C8), and PBDTTPD(EH-C8), illustrated in Figure 1. We performed MD simulations for two situations: In the first case, we took just one PC61BM molecule and one PBDTTPD chain (dissolved in chloroform solvent), in order to make an easy comparison to the results presented above. In the second case, we considered a single polymer chain interacting with a large number of PC61BM molecules, which mimics the photoactive layer at very low polymer concentration.4 (We recall that in all instances the MD force-field parameters have been optimized on the basis of the ωB97XD/ 6-31G(d,p) results). We first discuss the results of the MD simulations with one polymer chain interacting with one PC61BM molecule in a CHCl3 solution. Since the interactions between the PBDTTPD chain and the PC61BM molecule are expected to be modulated as a function of the exact location of the fullerene, its orientation with respect to the polymer backbone, and the dynamics of the polymer−fullerene complex (at 300 K), in order to gain an appreciation of the variations in interaction

Figure 4. Schematic representation of the optimal configurations calculated for the PBDTTPD oligomer−PC61BM complex using the ωB97XD/6-31G(d,p) method. The top panel gives the reference frame for the PBDTTPD oligomer chain. The arrow illustrates the PC61BM long axis.

oriented along the x-axis of the oligomer backbone; conf-2 has the long axis of the PC61BM molecule oriented along the y-axis of the oligomer backbone; and conf-3 has the functional group of PC61BM oriented normal to the oligomer backbone (along the z-axis of the polymer backbone). 8184


Article


Table 1. Energy Component Analysis (SAPT0/cc-pVDZ) and DFT Total Interaction Energies (ωB97XD/6-31G(d,p)) of the Optimal PBDTTPD−PC61BM Complex Configurations for PC61BM on Top of a BDT or TPD Moietya face-on D conf-1 conf-2 conf-3 face-on A conf-1 conf-2 conf-3 a

Es

Ex

Ind

Disp

SAPT0 interaction energy

DFT interaction energy

−20.84 −17.18 −10.01

+37.38 +36.01 +21.78

−5.50 −4.13 −2.73

−31.09 −33.35 −22.17

−20.06 −18.66 −13.13

−24.01 −22.31 −12.36

−16.46 −14.10 −8.00

+36.17 +30.83 +18.03

−4.17 −3.44 −1.88

−32.56 −29.51 −20.03

−17.03 −16.22 −11.88

−21.69 −20.37 −11.38

All values in kcal/mol. The SAPT components are Es = electrostatic; Ex = exchange-repulsion; Ind = induction; and Disp = dispersion.

Table 2. Average (avg) and Standard Deviation (sd) Values of the Interaction Energies between the PBDTTPD Tetramer and the PC61BM Molecule as a Function of Interfacial Geometry Obtained from Molecular Dynamics Snapshotsa BDT avg ± sd conf-1 conf-2 conf-3 for all configurations

−15.9 −13.5 −10.1 −13.2

± ± ± ±

3.0 2.6 1.4 3.4

TPD avg ± sd

max −21.7 −16.5 −13.9 −21.7

(−24.01) (−22.31) (−12.36)

−16.1 −14.7 −10.9 −13.8

± ± ± ±

2.2 2.8 1.6 3.0

max −20.5 −20.9 −14.3 −20.9

(−21.69) (−20.37) (−11.38)

Interaction energies obtained from ωB97XD/6-31G(d,p) method (all values in kcal/mol). Values given in parentheses are interaction energies obtained on DFT optimized geometries.

a

polymers were collected after every 5 ps for data analyses. We evaluated the probabilities of finding the PC61BM molecule on top of either BDT or TPD in the following way: the probability of finding the PC61BM molecule on top of the BDT moiety, for instance, is calculated as the ratio between the number of configurations where PC61BM is on top of BDT over the total number of configurations where PC61BM is on top of the backbone. The results are illustrated in Figure 5 and Table S2; the probabilities are further decomposed into the three preferred orientations of PC61BM (the conf-1, conf-2, and conf-3 configurations shown in Figure 4). The main conclusion to be drawn from Figure 5 is that, for the PBDTTPD (C14-EH) polymer where the branched side chains are appended to the TPD unit, there is only a 26.1% probability of finding PC61BM on top of the TPD moiety. The probability increases to 37.8% when the branched side chains on TPD are replaced by linear C8 alkyl group (PBDTTPD(C14-C8) polymer) and to 43.7% in the PBDTTPD (EH-C8) polymer, where the bulky side chains are located on the BDT unit. Overall, however, it is important to keep in mind that PC61BM tends to be located on top of the BDT unit in a majority of instances. It is also interesting to note that (i) The variations in the nature and size of the polymer side chains influence the relative orientations of PC61BM on top of the polymer backbone. We have recently shown that the longer and more flexible C14 side chains either interact with their own backbone or orient nearly normal to the backbone, while the shorter and less flexible EH and C8 side chains make the backbone more open to PC61BM molecules; although the EH side chains can also orient normal to the backbone, they are too short to interact significantly with the PC61BM functional group.29 Hence, we found substantial presence of the conf-3 configurations in the cases where the PBDTTPD backbone carries C14 alkyl side chains (C14-EH and C14-

energies, we collected 20 snapshots from the MD simulations for each of the three configurations (i.e., conf-1, conf-2, or conf-3) of PC61BM in proximity to either a BDT or a TPD moiety; thus, in total, 120 configurations of PBDDTPD− PC61BM complexes were considered for interaction energy calculations using the ωB97XD/6-31G(d,p) method (we note that these calculations involve a PBDTTPD tetramer with the alkyl side chains replaced with hydrogen atoms and no further geometry optimizations were performed). The results are collected in the Table 2. The average interaction energies obtained for each configuration provide the same trends, although they are somewhat smaller than the interaction energies obtained from the DFT-optimized geometries; the latter finding is likely related to the flexibility and conformational modulations of the copolymer backbone in the course of the MD simulations.29 In addition, we have generated the potential energy surface corresponding to the displacement of the PC61BM molecule on top of the conjugated copolymer backbone, using the coordinates extracted from the MD simulations. We have chosen a period of 300 ps, in the course of which the PC61BM molecule can translate fully from a BDT moiety to an adjacent TPD moiety along the backbone. We extracted the geometries of the corresponding PBDTTPD-PC61BM complexes at every 5-ps time step. The ωB97XD/6-31G(d,p) interaction energies of all these complexes (based on a PBDTTPD tetramer) were evaluated. The results are given in Figure S6 of the Supporting Information and illustrate the fluctuations in interaction energy as a function the modulations in PC61BM positions and orientations (we note that since the BDT moiety is larger in size than the TPD unit, in the course of its translation, the PC61BM molecule spends more time over BDT compared to TPD). We now turn to the discussion of the impact of the presence of the alkyl side chains on the orientation of the PC61BM molecule and its preference for the BDT or TPD moiety. Here, snapshots from the MD simulations on the three PBDTTPD 8185


Article


Figure 5. Representative configurations obtained from the MD simulations for one PBDTTPD chain and one PC61BM molecule in chloroform. (a), (b), and (c) correspond to the PBDTTPD(C14-EH), PBDTTPD(C14-C8), and PBDTTPD(EH-C8) cases, respectively. The face-on D and face-on A percentages represent the total probability of finding the PC61BM molecule on top of a BDT or TPD moiety, respectively; in each face-on D and face-on A case, we also give the decomposition of these probabilities in terms of the orientations of PC61BM (conf-1, conf-2, and conf-3).

and 5.9% for the PBDTTPD (C14-EH, C14-C8, and, EH-C8) systems, respectively. On the other hand, the PC61BM− PC61BM interactions have a more important role in the determination of the orientations of the PC61BM functional groups with respect to the polymer backbone; see Table S3. It is important to note that these results provide the same trends as those found in the case where the polymer−fullerene mixed phase is examined.29 Hence, it can be concluded that both PC61BM−PC61BM and PBDTTPD−PBDTTPD interactions have little impact on the probability of finding PC61BM close to either a BDT or TPD unit but play a determining role on the orientations of the PC61BM molecules. Experimentally, the power conversion efficiencies reported for solar cells in which the bulk heterojunctions contains 1 wt % in polymer evolve from 0.07% to 0.22% in going from PBDTTPD (C14-EH) to (EH-C8). For the optimal 1:1.5 ratio by weight of polymer vs PC61BM in the photoactive layers, the

C8), whereas conf-3 configurations are less probable in the PBDTTPD(EH-C8) case. (ii) The nature and length of alkyl side chain have subtle effect on the distances between PC61BM and the PBDTTPD backbone, which lie in the range of 3.43− 3.49 Å (Table S2). Finally, we investigate the impact of the presence of many PC61BM molecules by simulating a thin film with 1 wt % concentration in polymer. The major difference with the previous cases is that there now occur interactions among PC61BM molecules. Interestingly, these PC61BM−PC61BM interactions are found to have only limited impact. In all three polymer cases, they simply tend to increase slightly the probability of finding PC61BM on top of a TPD moiety (see Table 3), without altering the trend in going from PBDTTPD(C14-EH) to (C14-C8) and (EH-C8), except some edge-on configurations do appear but remain a minority: 11.8%, 6.3%, 8186


Article


roughly 25% to nearly 50% in going from PBDTTPD (C14-EH) to (EH-C8). Thus, shifting the more sterically bulky branched side chains from the TPD to the BDT units significantly enhances the number of close contacts between the PC61BM molecules and the TPD moieties. This underlines the important role played by the polymer side chains in the locations (as well as orientations) of the PC61BM molecules. Experimentally, such a trend has been suggested to correlate with higher power conversion efficiencies.4 (iv) The probability of finding PC61BM in proximity to an electron-rich (BDT) or electron-poor (TPD) unit has now been examined in a series of systems such as, here, a single polymer chain interacting with a single PC61BM molecule or a myriad PC61BM molecules (and, in other work,29 in the case of many, interacting polymer chains). Similar trends are found in all instances. Thus, it consistently appears that both PC61BM−PC61BM and PBDTTPD−PBDTTPD interactions have little impact on the probability of finding PC61BM close to BDT or TPD units but play a major role in determining the orientation of PC61BM with respect to the polymer backbone. We believe that the present work represents a stepping stone for and provides useful information in the investigation of the intermolecular packing (“local” morphology) in the polymer− fullerene mixed phases that are present in bulk-heterojunction active layers. Also, the variety of polymer−fullerene configurations described here can be used to determine the nature and energetic distribution of the charge-transfer electronic states at the polymer−fullerene interfaces. Such investigations are currently in progress in our laboratory.

Table 3. Probability of Finding a PC61BM Molecule Next to an Electron-Rich BDT or Electron-Poor TPD Moiety and Average Distance between PC61BM and the Polymer Moiety in the Case of MD Simulations of a Thin Film with One PBDTTPD Chain and Many PC61BM Molecules at a PBDTTPD Concentration of 1 wt %a PBDTTPD C14-EH C14-C8 EH-C8

BDT

TPD

65.1% (73.9%) 58.3% (62.2%) 53.5% (56.3%)

34.9% (26.1%) 41.7% (37.8%) 46.5% (43.7%)

dBDT (Å)

dTPD (Å)

3.35 ± 0.26

3.38 ± 0.22

3.46 ± 0.35

3.65 ± 0.70

3.41 ± 0.26

3.37 ± 0.23

a

Note that the distance between PC61BM and BDT (TPD) is calculated by subtracting the radius of the C60 cage from the distance between the mass centers of the C60 cage and the nearest aromatic ring in BDT (TPD). [For the sake of easy comparison, we have also given in parentheses the probability of finding PC61BM next to BDT or TPD in the case where one PBDTTPD chain interacts with one PC61BM molecule in chloroform].

corresponding efficiencies evolve from ∼1.7% to ∼6.0%.4 Thus, whether at low or optimal polymer concentration in the bulk heterojunction, the presence of side chains that increase the probability of finding the PC61BM molecules in close contact with the electron-poor TPD units appears to correlate with an increase in device performance.

IV. CONCLUSIONS In this work, we used a combination of long-range corrected DFT calculations and molecular dynamics simulations to elucidate the nature of the noncovalent interactions between the conjugated polymer chains and the fullerenes that are present in the photoactive layers of organic solar cells. We chose three representative poly(benzo[1,2-b:4,5-b′]dithiophene−thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers, in order to gain the insights on the impact that alkyl side chains play in dictating the locations and orientations of PC61BM molecules on top of and around the polymer chains. The main conclusions we can draw are (i) In the absence of any side chains on the polymer and fullerene, the C60 molecules favor face-on configurations on the aromatic surface of the polymer backbone but have hardly any preference for the electron-rich (benzodithiophene, BDT) or electron-poor (thieno[3,4c]pyrrole-4,6-dione, TPD) moieties of the polymer backbone. The largest interaction energies are on the order of −12 kcal/mol. (ii) When the fullerene functional groups (PC61BM) are taken into account, there exist three main orientations of these functional groups over the backbone; in the instances where both the fullerene cages and functional groups interact with the polymer backbone, the interaction energies can increase up to ca. −20 kcal/ mol. The binding interactions between the polymer backbone and PC61BM are found to be dominated by dispersion (London) forces. (iii) In the presence of side chains on the polymer backbones, whether the polymers interact with a single or a myriad PC61BM molecules, the probability of finding PC61BM on top of the electron-poor TPD unit increases from

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02930. Detailed computational methodologies; computed energy differences (at ωB97XD/6-31G(d,p) and SAPT0/ cc-pVDZ levels) due to oligomer chain length; influence of the functional-group orientation on the PC61BM dipole moment; potential energy surfaces; and additional information on classical MD simulations are provided (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(J.-L.B.) E-mail: [email protected]. Author Contributions ‡

M.K.R. and T.W. have contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to Professors Mike McGehee, Pierre Beaujuge, Aram Amassian, and Chad Risko for many stimulating discussions. This work has been supported by King Abdullah University of Science and Technology (KAUST), the KAUST Competitive Research Grant program, and the Office of Naval Research Global (Award N62909-15-18187


Article


(17) D’Avino, G.; Muccioli, L.; Olivier, Y.; Beljonne, D. Charge Separation and Recombination at Polymer−Fullerene Heterojunctions: Delocalization and Hybridization Effects. J. Phys. Chem. Lett. 2016, 7 (3), 536−540. (18) Castet, F.; D’Avino, G.; Muccioli, L.; Cornil, J.; Beljonne, D. Charge Separation Energetics at Organic Heterojunctions: On the Role of Structural and Electrostatic Disorder. Phys. Chem. Chem. Phys. 2014, 16 (38), 20279−20290. (19) Poelking, C.; Andrienko, D. Design Rules for Organic Donor− Acceptor Heterojunctions: Pathway for Charge Splitting and Detrapping. J. Am. Chem. Soc. 2015, 137 (19), 6320−6326. (20) Poelking, C.; Tietze, M.; Elschner, C.; Olthof, S.; Hertel, D.; Baumeier, B.; Würthner, F.; Meerholz, K.; Leo, K.; Andrienko, D. Impact of Mesoscale Order on Open-Circuit Voltage in Organic Solar Cells. Nat. Mater. 2015, 14 (4), 434−439. (21) Tummala, N. R.; Zheng, Z.; Aziz, S. G.; Coropceanu, V.; Brédas, J.-L. Static and Dynamic Energetic Disorders in the C60, PC61BM, C70, and PC71BM Fullerenes. J. Phys. Chem. Lett. 2015, 6 (18), 3657−3662. (22) Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson, M. E.; Forrest, S. R. Independent Control of Bulk and Interfacial Morphologies of Small Molecular Weight Organic Heterojunction Solar Cells. Nano Lett. 2012, 12 (8), 4366−4371. (23) Fu, Y.-T.; da Silva Filho, D. A.; Sini, G.; Asiri, A. M.; Aziz, S. G.; Risko, C.; Brédas, J.-L. Structure and Disorder in Squaraine−C60 Organic Solar Cells: A Theoretical Description of Molecular Packing and Electronic Coupling at the Donor−Acceptor Interface. Adv. Funct. Mater. 2014, 24 (24), 3790−3798. (24) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fréchet, J. M. J. Synthetic Control of Structural Order in N-Alkylthieno[3,4-C]pyrrole-4,6-Dione-Based Polymers for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132 (22), 7595−7597. (25) Zou, Y.; Najari, A.; Berrouard, P.; Beaupré, S.; Réda Aïch, B.; Tao, Y.; Leclerc, M. A Thieno[3,4-C]pyrrole-4,6-Dione-Based Copolymer for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132 (15), 5330−5331. (26) Zhang, Y.; Hau, S. K.; Yip, H.-L.; Sun, Y.; Acton, O.; Jen, A. K.Y. Efficient Polymer Solar Cells Based on the Copolymers of Benzodithiophene and Thienopyrroledione. Chem. Mater. 2010, 22 (9), 2696−2698. (27) Cabanetos, C.; El Labban, A.; Bartelt, J. A.; Douglas, J. D.; Mateker, W. R.; Frechet, J. M. J.; McGehee, M. D.; Beaujuge, P. M. Linear Side Chains in Benzo[1,2-b:4,5-B′]dithiophene-Thieno[3,4C]pyrrole-4,6-Dione Polymers Direct Self-Assembly and Solar Cell Performance. J. Am. Chem. Soc. 2013, 135 (12), 4656−4659. (28) Dyer-Smith, C.; Howard, I. A.; Cabanetos, C.; El Labban, A.; Beaujuge, P. M.; Laquai, F. Interplay Between Side Chain Pattern, Polymer Aggregation, and Charge Carrier Dynamics in PBDTTPD:PCBM Bulk-Heterojunction Solar Cells. Adv. Energy Mater. 2015, 5 (9), 1401778−1401778. (29) Wang, T.; Ravva, M. K.; Brédas, J.-L. Impact of the Nature of the Side-Chains on the Polymer-Fullerene Packing in the Mixed Regions of Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2016, 26, 5913−5921. (30) Grand, C.; Baek, S.; Lai, T.-H.; Deb, N.; Zajaczkowski, W.; Stalder, R.; Müllen, K.; Pisula, W.; Bucknall, D. G.; So, F.; Reynolds, J. R. Structure−Property Relationships Directing Transport and Charge Separation in Isoindigo Polymers. Macromolecules 2016, 49, 4008− 4022. (31) Lai, T.-H.; Constantinou, I.; Grand, C. M.; Klump, E. D.; Baek, S.; Hsu, H.-Y.; Tsang, S.-W.; Schanze, K. S.; Reynolds, J. R.; So, F. Evidence of Molecular Structure Dependent Charge Transfer between Isoindigo-Based Polymers and Fullerene. Chem. Mater. 2016, 28 (7), 2433−2440. (32) Sweetnam, S.; Vandewal, K.; Cho, E.; Risko, C.; Coropceanu, V.; Salleo, A.; Bredas, J.-L.; McGehee, M. D. Characterizing the Polymer:Fullerene Intermolecular Interactions. Chem. Mater. 2016, 28, 1446−1452.

2003). We acknowledge the IT Research Computing Team and Supercomputing Laboratory at KAUST for providing outstanding assistance and computational and storage resources.

■

REFERENCES

(1) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258 (5087), 1474−1476. (2) Brédas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular Understanding of Organic Solar Cells: The Challenges. Acc. Chem. Res. 2009, 42 (11), 1691−1699. (3) Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114 (14), 7006−7043. (4) Graham, K. R.; Cabanetos, C.; Jahnke, J. P.; Idso, M. N.; El Labban, A.; Ngongang Ndjawa, G. O.; Heumueller, T.; Vandewal, K.; Salleo, A.; Chmelka, B. F.; Amassian, A.; Beaujuge, P. M.; McGehee, M. D. Importance of the Donor:Fullerene Intermolecular Arrangement for High-Efficiency Organic Photovoltaics. J. Am. Chem. Soc. 2014, 136 (27), 9608−9618. (5) Mazzio, K. A.; Luscombe, C. K. The Future of Organic Photovoltaics. Chem. Soc. Rev. 2015, 44 (1), 78−90. (6) Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.; Huang, F.; Qu, Y.; Ma, W.; Yan, H. Terthiophene-Based D−A Polymer with an Asymmetric Arrangement of Alkyl Chains That Enables Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137 (44), 14149−14157. (7) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (8) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. Molecular and Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices. J. Am. Chem. Soc. 2009, 131 (26), 9281−9286. (9) Tumbleston, J. R.; Collins, B. A.; Yang, L.; Stuart, A. C.; Gann, E.; Ma, W.; You, W.; Ade, H. The Influence of Molecular Orientation on Organic Bulk Heterojunction Solar Cells. Nat. Photonics 2014, 8 (5), 385−391. (10) Cha, H.; Baek, J. Y.; An, T. K.; Kim, S.-O.; Kwon, S.-K.; Kim, Y.H.; Park, C. E. Development of Bulk Heterojunction Morphology by the Difference of Intermolecular Interaction Behaviors. Org. Electron. 2014, 15 (12), 3558−3567. (11) Rand, B. P.; Cheyns, D.; Vasseur, K.; Giebink, N. C.; Mothy, S.; Yi, Y.; Coropceanu, V.; Beljonne, D.; Cornil, J.; Brédas, J.-L.; Genoe, J. The Impact of Molecular Orientation on the Photovoltaic Properties of a Phthalocyanine/Fullerene Heterojunction. Adv. Funct. Mater. 2012, 22 (14), 2987−2995. (12) Yi, Y.; Coropceanu, V.; Brédas, J.-L. Exciton-Dissociation and Charge-Recombination Processes in Pentacene/C60 Solar Cells: Theoretical Insight into the Impact of Interface Geometry. J. Am. Chem. Soc. 2009, 131 (43), 15777−15783. (13) Linares, M.; Beljonne, D.; Cornil, J.; Lancaster, K.; Brédas, J.-L.; Verlaak, S.; Mityashin, A.; Heremans, P.; Fuchs, A.; Lennartz, C.; Idé, J.; Méreau, R.; Aurel, P.; Ducasse, L.; Castet, F. On the Interface Dipole at the Pentacene−Fullerene Heterojunction: A Theoretical Study. J. Phys. Chem. C 2010, 114 (7), 3215−3224. (14) Verlaak, S.; Beljonne, D.; Cheyns, D.; Rolin, C.; Linares, M.; Castet, F.; Cornil, J.; Heremans, P. Electronic Structure and Geminate Pair Energetics at Organic−Organic Interfaces: The Case of Pentacene/C60 Heterojunctions. Adv. Funct. Mater. 2009, 19 (23), 3809−3814. (15) Idé, J.; Mothy, S.; Savoyant, A.; Fritsch, A.; Aurel, P.; Méreau, R.; Ducasse, L.; Cornil, J.; Beljonne, D.; Castet, F. Interfacial Dipole and Band Bending in Model pentacene/C60 Heterojunctions. Int. J. Quantum Chem. 2013, 113 (4), 580−584. (16) Chen, W.; Qi, D.-C.; Huang, H.; Gao, X.; Wee, A. T. S. Organic−Organic Heterojunction Interfaces: Effect of Molecular Orientation. Adv. Funct. Mater. 2011, 21 (3), 410−424. 8188


Article

Chemistry of Materials (33) Westacott, P.; Tumbleston, J. R.; Shoaee, S.; Fearn, S.; Bannock, J. H.; Gilchrist, J. B.; Heutz, S.; deMello, J.; Heeney, M.; Ade, H.; Durrant, J.; McPhail, D. S.; Stingelin, N. On the Role of Intermixed Phases in Organic Photovoltaic Blends. Energy Environ. Sci. 2013, 6 (9), 2756−2764. (34) Hedley, G. J.; Ward, A. J.; Alekseev, A.; Howells, C. T.; Martins, E. R.; Serrano, L. A.; Cooke, G.; Ruseckas, A.; Samuel, I. D. W. Determining the Optimum Morphology in High-Performance Polymer-Fullerene Organic Photovoltaic Cells. Nat. Commun. 2013, 4, 2867. (35) Burke, T. M.; McGehee, M. D. How High Local Charge Carrier Mobility and an Energy Cascade in a Three-Phase Bulk Heterojunction Enable > 90% Quantum Efficiency. Adv. Mater. 2014, 26 (12), 1923−1928. (36) Sweetnam, S.; Graham, K. R.; Ngongang Ndjawa, G. O.; Heumüller, T.; Bartelt, J. A.; Burke, T. M.; Li, W.; You, W.; Amassian, A.; McGehee, M. D. Characterization of the Polymer Energy Landscape in Polymer:Fullerene Bulk Heterojunctions with Pure and Mixed Phases. J. Am. Chem. Soc. 2014, 136 (40), 14078−14088. (37) Chai, J.-D.; Head-Gordon, M. Long-Range Corrected Hybrid Density Functionals with Damped Atom-Atom Dispersion Corrections. Phys. Chem. Chem. Phys. 2008, 10 (44), 6615−6620. (38) Jeziorski, B.; Moszynski, R.; Szalewicz, K. Perturbation Theory Approach to Intermolecular Potential Energy Surfaces of van Der Waals Complexes. Chem. Rev. 1994, 94 (7), 1887−1930. (39) Parker, T. M.; Burns, L. A.; Parrish, R. M.; Ryno, A. G.; Sherrill, C. D. Levels of Symmetry Adapted Perturbation Theory (SAPT). I. Efficiency and Performance for Interaction Energies. J. Chem. Phys. 2014, 140 (9), 094106. (40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wallingford, CT, U.S.A., 2009. (41) Turney, J. M.; Simmonett, A. C.; Parrish, R. M.; Hohenstein, E. G.; Evangelista, F. A.; Fermann, J. T.; Mintz, B. J.; Burns, L. A.; Wilke, J. J.; Abrams, M. L.; Russ, N. J.; Leininger, M. L.; Janssen, C. L.; Seidl, E. T.; Allen, W. D.; Schaefer, H. F.; King, R. A.; Valeev, E. F.; Sherrill, C. D.; Crawford, T. D. Psi4: An Open-Source Ab Initio Electronic Structure Program. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2 (4), 556−565. (42) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117 (1), 1−19. (43) Dahlgren, M. K.; Schyman, P.; Tirado-Rives, J.; Jorgensen, W. L. Characterization of Biaryl Torsional Energetics and Its Treatment in OPLS All-Atom Force Fields. J. Chem. Inf. Model. 2013, 53 (5), 1191− 1199. (44) Di Pierro, M.; Elber, R. Automated Optimization of Potential Parameters. J. Chem. Theory Comput. 2013, 9 (8), 3311−3320. (45) Cheung, D. L.; Troisi, A. Theoretical Study of the Organic Photovoltaic Electron Acceptor PCBM: Morphology, Electronic Structure, and Charge Localization. J. Phys. Chem. C 2010, 114 (48), 20479−20488. (46) Bayly, C. I.; Cieplak, P.; Cornell, W.; Kollman, P. A. A WellBehaved Electrostatic Potential Based Method Using Charge Restraints for Deriving Atomic Charges: The RESP Model. J. Phys. Chem. 1993, 97 (40), 10269−10280.

(47) Dupradeau, F.-Y.; Pigache, A.; Zaffran, T.; Savineau, C.; Lelong, R.; Grivel, N.; Lelong, D.; Rosanski, W.; Cieplak, P. The R.E.D. Tools: Advances in RESP and ESP Charge Derivation and Force Field Library Building. Phys. Chem. Chem. Phys. 2010, 12 (28), 7821−7839. (48) Zhang, C.-R.; Sears, J. S.; Yang, B.; Aziz, S. G.; Coropceanu, V.; Brédas, J.-L. Theoretical Study of the Local and Charge-Transfer Excitations in Model Complexes of Pentacene-C60 Using Tuned Range-Separated Hybrid Functionals. J. Chem. Theory Comput. 2014, 10 (6), 2379−2388. (49) Marchiori, C. F. N.; Koehler, M. Density Functional Theory Study of the Dipole across the P3HT:PCBM Complex: The Role of Polarization and Charge Transfer. J. Phys. D: Appl. Phys. 2014, 47 (21), 215104. (50) D’Avino, G.; Olivier, Y.; Muccioli, L.; Beljonne, D. Do Charges Delocalize over Multiple Molecules in Fullerene Derivatives? J. Mater. Chem. C 2016, 4 (17), 3747−3756.

8189


Nature of the Binding Interactions between Conjugated Polymer

Recommend Documents