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Design and Computational Characterization of Non-Fullerene Acceptors for Use in Solution-Processable Solar Cells Lesley R. Rutledge, Seth M. McAfee, and Gregory C. Welch* Department of Chemistry, Dalhousie University, 6274 Coburg Road, P.O. Box 15000, Halifax, Nova Scotia, Canada B3H 4R2 S Supporting Information *

ABSTRACT: In an effort to seek high-performance small molecule electron acceptor materials for use in heterojunction solar cells, computational chemistry was used to examine a variety of terminal acceptor−conjugated bridge−core acceptor− conjugated bridge−terminal acceptor small molecules. In particular, we have systematically predicted the geometric, electronic, and optical properties of 16 potential smallmolecule acceptors based upon a series of electron deficient π-conjugated building blocks that have been incorporated into materials exhibiting good electron transport properties. Results show that the band gap, HOMO/LUMO energy levels, orbital spatial distribution, and intrinsic dipole moments can be systematically altered by varying the electron properties of the terminal or core acceptor units. In addition, the identity of the conjugated bridge can help fine-tune the electronic properties of the molecule, where this study showed that the strongest electron affinity of the conjugated π-bridge increased the stability in the HOMO and LUMO energies and increased the band gap of these small-molecule acceptors. As a result, this work points toward an isoindigo (C5) core combined with C2-thienopyrrole dione (A5) terminal units as the most promising small molecule acceptor material that can be fine-tuned with the choice of conjugated bridge and may be considered as reasonable candidates for synthesis and incorporation into organic solar cells.

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BHJ solar cells is an extremely attractive alternative.15−18 Molecular systems can offer many advantages due to their lowcost and facile synthesis, purification, and stability and welldefined structures with readily tunable electronic energy levels, as well as higher ordered molecular packing and hole mobility than their polymer counterparts. Small molecule based,19−22 solution-processed OSCs have seen PCE’s rise from 4 to 8% over the past few years,19,23−32 although the performance still lags behind that of their polymeric based counterparts. The design of new donor and acceptor materials to improve device PCE is the current concentration and goal of many research efforts, yet significant advances still need to be made, especially with respect to their use as fullerene (acceptor) replacements.12,33−35 Small-molecule acceptor materials are often designed to have electronic properties that compliment the polymeric donor material poly(3-hexylthiophene) (P3HT),36−42 with PCEs reaching 3%. However, in theory, OSCs designed with both small-molecule donors and acceptors can be tailored to have complementary electronic and optical properties that aim to maximize both solar spectrum overlap and frontier molecular orbital offsets, which do not closely resemble those of their

INTRODUCTION Organic photovoltaics are considered to be one of the most promising green technologies to address the increasing energy problems worldwide.1,2 These plastic or organic solar cells (OSCs) offer enormous potential due to their ability of being processed from solution onto flexible, lightweight substrates, which would allow them to be easily transported, stored, and installed with low costs.3−6 Over the past few years, power conversion efficiencies (PCEs) of 9% for single-layer7−9 and 10.6% for tandem polymer10 solar cells have been achieved for the most promising bulk-heterojunction (BHJ) architectures. While these results are encouraging and commercialization is becoming viable in the near future, these conjugated polymeric electron donors and fullerene electron acceptors within the active layer are not without their drawbacks. Difficulties with obtaining defined and uniform polymeric donors can result in large batch-to-batch variations limiting reproducibly,11 while the high cost, poor photo/air stability, and low absorption coefficients of fullerenes are not ideal for large-scale OSC production.12 While some recent research has focused on polymer−polymer BHJ solar cells,13 owing to the great filmforming abilities of π-conjugated polymers, all small molecule, nonfullerene based BHJ cells are a desirable composition.14 Recent developments have indicated that use of soluble organic small molecules in place of the polymeric donor and/or the fullerene acceptor components within the active layer of © XXXX American Chemical Society

Received: June 12, 2014 Revised: August 10, 2014

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dx.doi.org/10.1021/jp505867y | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. Representation of the (a) molecular framework used for all small-molecule acceptors examined in this study. Molecular models used to represent the (b) terminal acceptors (A1−A6), (c) conjugated bridges (π1−π6), and (d) core acceptors (C1−C6).

each structure as this property can affect charge transport properties 18,49 and influence self-assembly properties,50 although we recognize that π−π interactions and side-chains typically direct molecular self-assembly.51,52 Finally we hope to provide an in-depth insight into how structure can define and fine-tune these acceptor molecules for use in organic solar cells.

polymer−fullerene counterparts (i.e., narrow band gap polymer with weakly absorbing fullerene). On the other hand, we believe that we first need to understand how these electronic and optical properties of small molecule donors, and especially acceptors, can be fine-tuned before significant advances can be made. With this in mind, we theoretically designed a series of molecules within the terminal acceptor−conjugated bridge−core acceptor−conjugated bridge−terminal acceptor framework (Figure 1a) that has shown promise through recent research efforts.43,44 This molecular scaffold allows us to examine a combination of small-molecule acceptors that contains strong central core acceptors and electron-deficient terminal fragments that have been utilized in the construction of high performance organic materials, which are connected through a variety of conjugated bridge systems (Figure 1). Due to the promise shown in use of computational chemistry to study materials for OSCs,45−48 we have systematically predicted the geometric, electronic, and optical properties of these proposed small-molecule acceptors. In particular, we have concentrated on examining the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, viewing the level of HOMO/LUMO orbital delocalization, and predicting the optical absorption spectra for these acceptors. Criteria for uncovering promising n-type materials using computational methods include: (1) low lying HOMO and LUMO energies comparable to the well-known [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and perylene diimide (PDI) acceptors that will allow for efficient charge separation when paired with high performance donor materials, (2) planar structures with HOMO/LUMO orbital delocalization to promote intermolecular charge transport, and (3) low energy excitations with high oscillator strengths as this will correlate to visible and near-IR optical absorption important for maximizing photon harvesting. In addition, we have described the intrinsic dipole moment of

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METHODS Building Block Selection. A terminal acceptor−conjugated bridge−core acceptor−conjugated bridge−terminal acceptor molecular scaffold was selected for the following reasons: (1) Three electron deficient organic segments to ensure relatively low lying HOMO/LUMO energy levels important for charge separation and to enable electron transport, (2) heterocyclic conjugated bridges to extend conjugation and assist in backbone planarization, important for broadening optical abortion and promoting π−π intermolecular interactions, and (3) symmetrical, three building block framework to ensure facile synthesis of the predicted materials which is important for building molecular libraries and potential industrial scaleup.53−55 The architecture is opposite to that of the molecular donor, D1−A−D2−A−D1 design (where D1 and D2 are electron rich heterocyclic units and A is an electron deficient heterocyclic unit) successfully employed by Bazan et al. to achieve unprecedented performance in small-molecule-fullerene based solar cells.51,56 Core building blocks phthalimide (C1),57−61 pyromellitic diimide (C2),62−65 angular and symmetric naphthalene diimides (C3,C5),66−70 and isoindigo (C4)71−74 were selected based on their known ability to act as electron transport materials, incorporation of pendant alkyl chains perpendicular to the molecular backbone to ensure solution processability, and related imide/amide functionalities known for high thermal stability and propensity to readily self-assemble in the solid B



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Figure 2. Molecular representation of the small-molecule acceptors examined in this study. See Figure 1 for chemical structures.

While these values are outside of the range of typical experimental values (ca. −3.8 to −4.2 eV)67,97 and caution must be used when predicting electron affinities,98 the values do provide a relative reference to compare predicted energy levels of our proposed electron deficient small molecules. Due to the large conformational flexibility of the compounds within this molecular design framework (Figure 1a), smaller building blocks consisting of a fewer number of conjugated πsystems were examined prior to the desired small-molecule acceptor. Specifically, the terminal acceptors (A, Figure 1b) were bonded to either one thiophene or one thiazole, and the preferred dihedral angle that dictates the relative orientations of these two π-systems was determined through full geometry optimizations. Additionally, the core acceptors (C, Figure 1d) were sandwiched between either one thiophene or one thiazole, and the preferred dihedral angles between these three πsystems were determined with B3LYP/6-31G(d,p). Subsequently, the preferred dihedral angles determined for these building blocks were used to generate the input geometries for each small-molecule acceptor examined in this study. When building block geometries resulted in multiple minima that were within 5 kJ mol−1 of one another, all of the corresponding dihedral angle combinations were examined in the larger smallmolecule acceptor structure to examine a variety of local minima structures. It should be noted here that all smallmolecule acceptors examined in this study are symmetric, and structures that involve π2 or π3 are built to have the thiophene or thiazole ring bonded to the core acceptor, respectively. In addition, to help determine the global minimum structure for these compounds, conformational searches were performed on all small-molecule acceptors using Monte Carlo with MMFF as implemented in the Spartan software package.99 The 10 lowest energy structures determined through these Monte Carlo scans were further optimized with B3LYP/6-31G(d,p) and compared to the original optimized geometries. Using the predicted global minimum ground-state structure optimized from the above procedure, time-dependent (TD) DFT calculations were completed using TD-B3LYP/6-31G(d,p). Recent reports have determined that B3LYP can have drawbacks regarding the description of charge-transfer

state. Fluorene (substituted in the 2,7 positions, C6), a commonly used donor building block, was used to give perspective to the electron withdrawing ability of all the organic units.75−77 Terminal end acceptors dicyano vinyl (A1),78−81 indan-1,3dione (A2), 38,82 N-phthalimide (A3), C 4 -phthalimide (A4),44,83,84 and C2-thienopyrrole dione (A5)85,86 were selected based on their electron deficient conjugated structure and synthetic accessibility. A1, A2, A4, and A5 compare varying electron accepting ability, with A4 and A5 having the added functionality of incorporating alkyl groups parallel to the conjugated backbone that can assist with uniform film formation from solution and direct self-assembly.20,87,88 A simple 2-methylthiophene (A6) terminal end group was used as a control building block. Conjugated π-bridges thiophene (π6), bithiophene (π1), and thieno[3,2-b]thiophene (π5) were used to explore the effect of conjugation length and rigidity on calculated properties. Thiazole based conjugated bridges (π2−π4), which have recently been incorporated into high performance organic solar cell materials,89 were investigated to evaluate the effect of incorporation of electron withdrawing N atoms on electronic properties. Computational Details. To examine the geometry and electronic transitions of the proposed small-molecule acceptors, ground state (S0) geometry optimizations were completed at the B3LYP/6-31G(d,p) level of theory. B3LYP is arguably the most used functional within the density functional theory (DFT) framework, and this level of theory and basis set combination has been shown to provide reliable electronic trends (when compared to available experimental data) for organic solar cell molecules.46−49,90−93 In addition, all geometries examined in this work were subsequently examined through harmonic vibrational frequency calculations to verify their position on the B3LYP/6-31G(d,p) potential energy surface. This methodology was also used to examine the widely used acceptor molecules, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)94 and perylene diimide (PDI).95,96 Using our theoretical methods, the calculated LUMO energies of PC61BM and PDI were found to be −3.00 and −3.58 eV, respectively. C



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Figure 3. Energies (eV) of HOMO (bottom of rectangular box) and LUMO (top of rectangular box) and corresponding band gaps (center of rectangular box) of the phase I C1−π1 small-molecule acceptors with terminal acceptors A1−A6. The HOMO/LUMO energies of PC61BM and PDI are included for reference.

excitations;100−104 therefore, initial tests were conducted on a fluorene/indan(1,3)dione based small-molecule acceptor, that has been utilized in high performance organic solar cells.37 Using a truncated crystal structure geometry for this acceptor (see Supporting Information for full details), TD-DFT absorption spectra with PBE0, M06, M06-2X, CAM-B3LYP, and B3LYP with 6-31G(d,p) were examined. Results (Supporting Information Figure SI-1) indicated that B3LYP revealed reliable results when compared to experimental UV spectra and was therefore used throughout this study. Absorption spectra were simulated through convolution of the vertical transition energies and oscillator strengths with Gaussian functions characterized by a full-width at halfmaximum (fwhm) of 3000 cm−1. All DFT and TD-DFT calculations were completed in Gaussian09.105

2) with the conjugated bridge (π1, Figure 1c) to examine all possible core acceptors (C1−C6, Figure 1d). Finally, the results from phase II were used to choose a core acceptor, which was then used in phase III (Figure 2) with the terminal acceptor identified in phase I to examine the conjugated bridge (π1−π6, Figure 1c). The following sections will provide information about the specific HOMO/LUMO energies and illustrations, followed by the predicted optical absorption data for each small-molecule acceptor examined within each phase in this study and the dipole moments of the global minimum smallmolecule acceptors. The figures showing plots of optimized structures and dipole moment vectors can be found in the Supporting Information. Phase I: Alter Terminal Acceptor (A). To study the influence of terminal acceptor, we utilized a simple structure with phthalimide as a core acceptor and bithiophene as the conjugated bridge. Phthalimide is relatively weakly electron deficient and was thought to not strongly influence the electronic properties of the system,57 allowing for emphasis on the comparison of the terminal acceptors, while bithiophene is ubiquitous as a conjugated building block in organic electronics.106 The results of phase I theoretical experiments provided insight for the selection of the most appropriate terminal acceptor based on the aforementioned criteria. The optimized geometries of each molecule are shown in Supporting Information Figure SI-2. Each molecule adopts a near linear π-conjugated backbone with only slight deviations from planarity. The HOMO and LUMO energies for the small-

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RESULTS AND DISCUSSION Due to the large number (216) of possible small-molecule acceptors that could be examined within the chosen molecular framework (Figure 1), only select compounds were examined in three different phases within this study (Figure 2). Specifically, to identify a promising terminal acceptor, phase I examined only one core acceptor (C1, Figure 1d) and one conjugated bridge (π1, Figure 1c) with all possible terminal acceptors (A1−A6, Figure 1b). The results from phase I were used in conjunction with available experimental data in the literature to identify a promising candidate for the terminal acceptor, which was then subsequently used in phase II (Figure D



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Information. These molecules have dipole moments between 1.600 (A5) and 4.824 D (A1), indicating strong intermolecular dipole−dipole forces would be present and may assist in directing self-assembly for these acceptor molecules. Summarizing these results, the first structure C1−π1−A1 with dicyano vinyl terminal (A1) units is conjugated throughout the backbone structure and was found to possess a fully delocalized HOMO and LUMO system, and an appropriate simulated absorption spectrum, pronounced in the visible region of the solar spectrum that absorbs out past 650 nm, approaching the region of maximum solar flux. Due to the strong electron withdrawing character of the cyano functional groups the HOMO/LUMO energy levels were found to be considerably lower than PC61BM, and so this material was determined to be excessively electron deficient, which might lead to low open circuit voltages when paired with common electron donors. Indan-1,3-dione (A2) returned appropriate HOMO/LUMO energy levels that most closely matched that of PC61BM while also displaying a red-shifted simulated absorption spectrum in comparison to A1; however, the lack of sites for incorporating solubilizing side chains raises concerns about solution processability. The N-phthalimide (A3) moiety is bound via the amide nitrogen and would not be an effective terminal acceptor due to the orbital depiction of C1−π1−A3, showing a highly localized LUMO π-system. Furthermore, the energy levels show the highest values of all the terminal acceptors investigated in this study, while the simulated absorption spectrum shows an undesirable blue shift in relation to the other alternatives. C4−phthalimide (A4), bound through its phenyl ring, is a much more acceptable terminal acceptor than A3, and it offers lower HOMO/LUMO energy levels, fully delocalized HOMO and LUMO systems, and an acceptable simulated absorption spectrum. The nature of a C4-bound phthalimide also offers the ability to functionalize the amide nitrogen with flexible side chains, a feature that has been shown to have a significant influence on the solubility and self-assembly of the material.88 C2-thienopyrrole dione (A5) is similar to A4, and the orbital depictions of the target small molecule again show fully delocalized HOMO and LUMO systems that offer lower HOMO/LUMO energy levels consistent with the substitution of a phenyl for thienyl group. The structure still offers amide functionalization with flexible side chains but in addition demonstrates a red shift in the simulated absorption spectrum in relation to A4. Considering the analysis of our results, C2-thienopyrrole, A5, is most properly aligned with our design criteria for a πconjugated terminal acceptor. It possesses extensively delocalized HOMO and LUMO π-systems, appropriate energy levels in reference to PC61BM and PDI, and a pronounced simulated absorption in the visible region of the solar spectrum that extends past 600 nm toward the region of maximum solar flux. Phase II: Alter Core Acceptor (C). The results from phase I identified A5 as a promising terminal acceptor due to its aforementioned favorable electronic and optical properties. Therefore, A5 was then subsequently used in phase II (Figure 2) with the conjugated bridge (π1, Figure 1c) to examine all possible core acceptors (C1−C6, Figure 1d). The HOMO and LUMO energies for the small-molecule acceptors with C1−C6 core acceptors are shown in Figure 6. Unlike in phase I, the HOMO energies are very close together (range from −5.50 to −5.26 eV), yet the LUMO energies have a larger variance in energy (−3.57 to −2.67 eV). Due to the variance in core acceptor properties, the HOMO stabilities

molecule acceptors with C1−π1−[A1−A6] are shown in Figure 3. The HOMO and LUMO energies range between −5.96 and −4.96 eV and −3.40 and −2.28 eV, respectively, where their stabilities decrease according to the same trend within the terminal acceptors of A1 > A2 > A5 > A4 > A3 > A6. Comparatively, structures incorporating terminal acceptors A1, A2, and A5 have HOMO and LUMO energies comparable to the calculated energies of the standard electron transport materials PC61BM and PDI. Illustrations of the HOMO and LUMO frontier molecular orbitals (FMO, Figure 4) generally show orbital delocalization over the entire π-system. However, the structures with the least stable HOMO and LUMO energies (C1−π1−A3 and C1−π1−A6) were found to have highly localized LUMO orbitals centered on either the terminal acceptor or the core acceptor, respectively. Here, the A3 moieties are bound via the amide nitrogen atom and therefore are not fully through conjugated, while the A6 moiety is a relatively electron rich thiophene ring and does not significantly contribute to the LUMO. With the purpose of estimating the electronic transition energies of the small-molecule acceptors examined in phase I of this study, the simulated absorption spectra and the relevant singlet excited-state transitions are presented in Figure 5 and Table 1. As seen in Figure 5, all of the investigated compounds show absorption within the range of the solar spectrum.107 Generally, the spectrum of each compound can be characterized by one main low-energy transition with large oscillator strength and a series of highenergy transitions with relatively small oscillator strengths. As expected from the HOMO and LUMO illustrations, the maximum absorption peaks arise from the HOMO → LUMO transition, except for the cases when the LUMO orbital was highly localized (C1−π1−A3 and C1−π1−A6, Figure 4). The dipole moment magnitudes and vectors for the global minimum structures for the small-molecule acceptors examined in phase I are shown in Figure SI-2 in the Supporting

Figure 4. HOMO (left) and LUMO (right) orbitals (isovalue of 0.02) for small-molecule acceptors examined in phase I of this study, where the core acceptor C1 and conjugated bridge π1 were used when altering the terminal acceptors A1−A6. Generally delocalized orbitals are observed (with terminal acceptors A1, A2, A4, and A5), while highly localized LUMO orbitals are found for terminal acceptor A3 and A6. E



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Figure 5. Simulated absorption spectra of phase I small-molecule acceptors with structures corresponding to C1−π1, where terminal acceptors A1− A3 (left) and A4−A6 (right) are altered. See Figure 1 for chemical structures.

Table 1. Calculated Vertical Transition Energy (E, eV), Wavelengths (λ, nm), Oscillator Strengths (f), and Dominant Electronic Configuration for the S0 → S1 Excitationa Phase I

C1−π1−A1 C1−π1−A2 C1−π1−A3 C1−π1−A4 C1−π1−A5 C1−π1−A6

Phase II

C1−π1−A5 C2−π1−A5 C3−π1−A5 C4−π1−A5 C5−π1−A5

Phase III

a

C6−π1−A5 C4−π1−A5 C4−π2−A5 C4−π3−A5 C4−π4−A5 C4−π5−A5 C4−π6−A5

state

E (eV)

λ (nm)

f

S1 S1 S1 S6 S1 S1 S1 S2 S1 S1 S3 S1 S3 S1 S1 S3 S1 S1 S1 S1 S1 S1 S1

2.31 2.25 2.06 2.58 2.45 2.33 2.31 2.42 2.33 1.84 2.44 1.79 2.35 1.90 1.69 2.58 2.30 1.90 1.94 1.95 1.98 1.97 2.02

536 551 603 480 505 533 536 513 533 673 508 692 527 653 734 480 539 653 638 637 625 631 613

2.044 2.537 0.002 2.126 2.207 2.065 0.055 2.098 2.065 0.008 1.855 0.068 2.364 2.205 0.959 1.134 1.864 2.205 2.158 2.113 1.974 1.926 1.519

electronic configuration HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO

→ → → → → → → → → → → → → → → → → → → → → → →

LUMO (99%) LUMO (98%) LUMO (84%) LUMO+3 (97%) LUMO (97%) LUMO (95%) LUMO (98%) LUMO+1 (98%) LUMO (93%) LUMO (98%) LUMO+1 (90%) LUMO (99%) LUMO+1 (93%) LUMO (97%) LUMO (99%) LUMO+2 (80%) LUMO (93%) LUMO (97%) LUMO (97%) LUMO (96%) LUMO (96%) LUMO (97%) LUMO (97%)

In cases where the S0 → S1 transition does not correspond to the largest oscillator strength, this transition is also included.

decrease as C2 ≈ C5 > C3 ≈ C1 > C4 ≈ C6, and the LUMO stabilities decrease as C5 > C3 ≈ C2 > C4 > C1 > C6. Interestingly, illustrations of the HOMO and LUMO orbitals (Figure 7) show delocalization of the HOMO over the entire πsystem, yet C2 and C3 have highly localized LUMO orbitals that are centralized at their core structures. Not surprisingly, the maximum absorption peaks (Figure 8 and Table 1) do not arise from the expected HOMO → LUMO transition for the C2 and C3 small-molecule acceptors. However, another exception was observed for the C5 acceptor, where both the HOMO → LUMO and the HOMO → LUMO+2 electronic transitions

have large oscillator strengths (0.959 and 1.134 (Table 1), respectively). The global minimum structure involving the core acceptor C2 resulted in a small overall dipole moment of 0.001 D (Supporting Information Figure SI-3). However, the remaining core acceptors examined in phase II had resulting dipole moment vectors with magnitudes ranging from 1.276 (C4) to 2.086 D (C6). The results of phase II theoretical experiments facilitated the selection of the most appropriate core acceptor in conjunction with the model structure containing terminal C2-thienopyyrole, A5, acceptor units. The small-molecule structure incorporating F



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Figure 6. Energies (eV) of HOMO (bottom of rectangular box) and LUMO (top of rectangular box), and corresponding band gaps (center of rectangular box), of the phase II π1−A5 small-molecule acceptors with core acceptors C1−C6. The HOMO/LUMO energies of (c) PC61BM and PDI are included for reference.

The pyromellitic diimide based molecule (C2−π−A5) has a lower-lying LUMO level than C1, but this characteristic is also true in comparison to PC61BM, relating to a significantly isolated and electron deficient core acceptor. This is revealed further in the molecular orbital diagrams where significant deviation from planarity along the π-conjugated backbone (Supporting Information Figure SI-3) results in the LUMO being highly localized at the pyromellitic diimide core. Furthermore, the predicted absorption spectrum shows a maximum absorption at 508 nm resulting from a HOMO → LUMO+1 transition and only a low probability HOMO → LUMO transition at 673 nm. The nonplanar structure and isolated LUMO energy level may not be ideal for electron transport.44 The structure incorporating angular naphthalene diimide (C3) is similar to C2, with a low-lying, highly localized LUMO (on the acceptor core) and low probability of a strong HOMO−LUMO absorption transition. In this case, there is minimal deviation from planarity across the π-conjugated backbone, indicating that the localized LUMO is largely an electronic effect. Incorporating the isoindigo (C4) unit offers a narrow band gap molecule with a LUMO level that most closely matches that of PC61BM. The molecular orbital representations show an extensively delocalized HOMO πsystem, and while the LUMO π-system is primarily localized at the isoindigo core it also adequately extends toward the bridging units and terminal end-cap. The predicted optical absorption spectrum of this molecule highlights its potential, with a favorable simulated absorption band near 650 nm. Symmetric naphthalene diimide (C5) also displays an

Figure 7. HOMO (left) and LUMO (right) orbitals (isovalue of 0.02) for small-molecule acceptors examined in phase II of this study, where the conjugated bridge π1 and terminal acceptor A5 were used when altering the core acceptor C1−C6.

phthalimide (C1−π−A5) displays extensively delocalized HOMO and LUMO π-systems; however, it has a relatively high lying LUMO level and relatively large band gap in comparison to the other core acceptors in this series and would be best suited for pairing with a small-band gap donor material. G



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Figure 8. Simulated absorption spectra of phase II small-molecule acceptors with structures corresponding to π1−A5, where core acceptors C1−C3 (left) and C4−C6 (right) are altered. See Figure 1 for chemical structures.

Figure 9. Energies (eV) of HOMO (bottom of rectangular box) and LUMO (top of rectangular box) and corresponding band gaps (center of rectangular box) of the phase III C4 and A5 small-molecule acceptors with conjugated bridges π1−π6. The HOMO/LUMO energies of PC61BM and PDI are included for reference.

Reflecting on the results of our core acceptor investigation, isoindigo, C4, has been chosen as the most attractive in this series. C4−π−A5 is a narrow band gap molecule with LUMO levels that most closely match that of PC61BM. Moreover, the conjugated π-system is not entirely localized at the core, and it has an excellent predicted optical absorption spectrum that

impressive predicted optical absorption spectrum with a strong absorption band past 700 nm. While this material shows the smallest band gap of those in the series it also returns the lowest LUMO level and a localized LUMO π-system at the naphthalene diimide core. H



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Furthermore, the simulated absorption spectra for all π1−π6 complexes are very similar in shape (Figure 11), where the maximum absorption peaks (Table 1) all arise from the expected HOMO → LUMO transition. We have previously discussed the significant influence changes to the core and terminal acceptors have on the electronic and optical properties of π-conjugated materials; alternatively, modification to the π-bridge has a more subtle effect. The results of phase III theoretical experiments demonstrate that, in general, all of the bridging units show extensive HOMO/LUMO orbital delocalization; however, analysis of the energy levels and simulated absorption spectra highlights how minor changes to the π-conjugated bridge can fine-tune the electronics of a given material. Each of the bridging units investigated incorporates two aryl components, which effectively extends the π-conjugation leading to a narrowing of the band gap. This is illustrated through the comparison of thiophene (π6), bithiophene (π1), and thieno[3,2-b]thiophene (π5) where thiophene has the largest band gap and an absorption maximum closer to 600 nm. On the other hand, bithiophene has the most narrow band gap and absorption maximum closer to 700 nm. Thieno[3,2-b]thiophene also has a slightly smaller band gap than thiophene and a red-shifted absorption spectrum; however, the fused nature of the bridge induces rigidity leading to a more planar structure. The band gap can be further tailored by introducing electron-withdrawing nitrogen atoms to the thiophene rings. These thiazole bridging units stabilize the HOMO and LUMO levels and show acceptable simulated absorption spectra extending beyond 700 nm. The three thiazole derivatives investigated (π2−π4) demonstrate the influence of substitution location of a single thiazole unit and also the impact two thiazole units have on the π-conjugated material. When the thiazole, π3, is located nearest to the more electronwithdrawing component, the acceptor core, it leads to a slight increase in energy level stabilization when compared to π2, where the thiazole is located nearest to the terminal acceptor. Furthermore, the inclusion of two thiazole units displays the deepest HOMO and LUMO levels of the series due to the aforementioned electron-withdrawing nature of the nitrogen

Figure 10. HOMO (left) and LUMO (right) orbitals (isovalue of 0.02) for small-molecule acceptors examined in phase III of this study, where the core acceptor C4 and terminal acceptor A5 were used when altering the conjugated bridge π1−π6.

extends appreciably past 700 nm, ideal for maximizing photon harvesting. In addition, isoindigo benefits from ease of synthetic functionalization and formation from sustainable building blocks.108 Phase III: Alter Conjugated Bridge (π). The results from phase II indicated that the isoindigo core (C4) was a promising core acceptor for the final phase of this study (Figure 2) in conjunction with the terminal acceptor identified in phase I (A5) to examine the conjugated bridge (π1−π6, Figure 1c). As expected, the HOMO and LUMO energies (Figure 9) are not highly dependent on the identity of the conjugated bridge (HOMO energies differ by less than 0.34 eV and LUMO energies differ by less than 0.23 eV). Additionally, as the electron affinity of the conjugated π-bridge increases from the thiophene to the thiazole, the HOMO and LUMO stabilities increase, and the band gap increases. Illustrations of both the HOMO and the LUMO orbitals for all complexes in phase III (Figure 10) show delocalization over the entire complex.

Figure 11. Simulated absorption spectra of phase III small-molecule acceptors with structures corresponding to C4 and A5, where conjugated bridge π1−π3 (left) and π4−π6 (right) are altered. See Figure 1 for chemical structures. I



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heteroatom. Thus, we have demonstrated that appropriate alterations to the bridging units of a material offers a means to tailor its electronic properties through the stabilization of energy levels, narrowing of the band gap, and lowering the energy of the strongest electronic transitions. On the basis of our acquired knowledge of these various π-conjugated bridging units, we can suggest that molecular design of this component for π-conjugated materials should be primarily influenced by the desired impact on the band gap.

CONCLUSIONS In this study, we theoretically designed a series of molecules within the terminal acceptor−conjugated bridge−core acceptor− conjugated bridge−terminal acceptor framework in order to examine a combination of small-molecule acceptors that contain strong central core acceptors and electron-deficient terminal fragments that are connected through a variety of conjugated bridge systems. Through the use of computational chemistry, this systematic approach allowed for the prediction of the geometric, electronic, and optical properties of a large number of potential solar cell acceptors. Results suggest that the HOMO and LUMO energies are largely dictated by the properties of both the core and terminal acceptors and that the identity of the conjugated bridge can help fine-tune the electronic properties of the molecule. These results provide an insight into how structure can define and adjust these acceptor molecules for use in organic solar cells. Finally, this study indicates that isoindigo (C5), combined with C2-thienopyrrole dione (A5), can yield attractive small molecule acceptor systems that can be fine-tuned with the choice of conjugated bridge. It is worth mentioning that, during the course of this study, Frechet and co-workers reported on a series of small-molecule acceptors utilizing thienopyrrole dione as a terminal acceptor and achieved respectable organic solar cell device performance, thus reinforcing the potential of such systems to act as fullerene alternatives.109 Thus, it is expected that the molecular backbones presented in this study can be further developed experimentally to help improve the performance of small organic molecule solar cells and is the subject of ongoing investigations in our laboratory. ASSOCIATED CONTENT

S Supporting Information *

Full computational details and results from TD-DFT method comparisons, illustrations of small-molecule acceptor dipole moment magnitudes and vectors, and full references. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 1 (902) 494 4245. Fax: 1 (902) 494 1310. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dalhousie University, the Canada Research Chairs Program, and the Natural Science and Engineering Council of Canada are acknowledged for financial support. Computational time was provided by the Atlantic Computational Excellence Network (ACEnet) and Compute Calcul Canada. J



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