Effect of Molecular Shape on the Properties of Non-Fullerene

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Effect of Molecular Shape on the Properties of Non-Fullerene Acceptors: Contrasting Calamitic Versus 3D Design Principles Chase L. Radford, Arthur D. Hendsbee, Maged Abdelsamie, Nicholas M Randell, Yuning Li, Michael F. Toney, and Timothy L Kelly ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01433 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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ACS Applied Energy Materials

Effect of Molecular Shape on the Properties of NonFullerene Acceptors: Contrasting Calamitic Versus 3D Design Principles Chase L. Radford,a Arthur D. Hendsbee,b Maged Abdelsamie,c Nicholas M. Randell,a Yuning Li,b Michael F. Toney, c Timothy L. Kellya* a

Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon,

Saskatchewan S7N 5C9, Canada. b

Department of Chemical Engineering and Waterloo Institute for Nanotechnology (WIN),

University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada. c

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo

Park, California 94025, USA.

KEYWORDS: organic photovoltaics, organic solar cells, non-fullerene acceptors, organic thin film transistors, charge carrier mobility, film morphology

ACS Paragon Plus Environment

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ABSTRACT The recent development of high performance non-fullerene acceptors has relied on two main design principles: the calamitic design, where side-chains are arranged orthogonally to a planar chromophore, and the 3D design, where the chromophore is forced to adopt a non-planar structure. Until now, there has been no direct comparison of these designs. In this work, small molecule non-fullerene acceptors with both the calamitic design (FBRCN) and the 3D design (XFBRCN) were synthesised. These molecules bear the same chromophore unit and have similar solubilities. Our results show that the 3D semiconductor XFBRCN has a higher extinction coefficient, broader absorbance bands, and a higher permittivity than the calamitic FBRCN. This is due to the lower crystallinity and better polarizability of the 3D acceptor. In contrast, the calamitic FBRCN had a higher electron mobility, was more miscible with polymeric OPV donors, and was more crystalline than the 3D XFBRCN. This is attributed to the long alkyl chains of FBRCN encouraging efficient solid-state packing. This work highlights the differences between the two material design principles, and helps to elucidate the role of sidechains in controlling film morphology and the performance of solid-state electronic devices.


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INTRODUCTION Organic solar cells (OSCs) have recently seen a resurgence in interest due to the synthesis of new non-fullerene acceptors (NFAs).1,2 These new acceptors are based around two main design principles – calamitic and 3D – which both produce soluble materials and limit the domain size in bulk heterojunctions, primarily through the partial inhibition of self π-stacking.1–4 In particular, the calamitic4 design has led to many high performance materials1,5–8 that approach 15% power conversion efficiency (PCE).9–13 The calamitic design features a rigid electron-pushpull chromophore system, where side-chains are appended orthogonally to the chromophore core. This imparts steric bulk to the electron-rich molecular core (increasing solubility and limiting over-crystallization), while leaving the electron-deficient termini accessible to facilitate π-overlap and electron transfer.1,2,4,14 A parallel design strategy, driven mainly by work on perylene diimide,15 is to use non-planar 3D semiconductors.16–20 The non-planar design disrupts an otherwise rigid chromophore backbone by imparting a twist or otherwise enforcing a 3D shape.4,14 This limits over-crystallization and creates a more isotropic chromophore arrangement. The reduced anisotropy leads to less orientation-dependence for π-overlap with neighbouring molecules, and facilitates electron-transfer in the solid state. These two design principles are at the forefront of NFA design, although they are rarely compared in the same study,10,21,22 and never with systems having the same electronic structure and solubility. The few comparisons that can be made between electronically similar systems are drawn from different reports, and suggest that the 3D NFAs tend to perform better than the calamitic analogues. One example is a diketopyrrolopyrrole-based system, where the 3D23 and calamitic24 NFAs had PCEs of 5.2% and 3.2%, respectively, when used with the same donor polymer. Similarly, in a thiophene-rhodanine based system the 3D analogue had a higher PCE


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(4.5%)25 than the calamitic derivative (3.1%),26 when used with the same donor. These limited examples suggest that this trend may be more general, and emphasize the importance of further comparing the two design motifs. Alongside the development of new NFA designs, there is tremendous interest in the field of side-chain engineering.27 Many reports on the effect of chain length,28–30 branching,31–34 and position10,35,36 provide insight into the influence of side-chains on intermolecular interactions and solid state morphology. Recent reports show that optimizing chain length can improve crystallinity and charge carrier mobility, implying side-chains play a substantial role in film packing and reorganization.37–40 However, since side-chains are often necessary to solubilize extended π-systems,28 these reports struggle to separate their effect on lamellar packing and donor/acceptor miscibility from simple changes in material solubility. It is also unclear whether side-chains are at all desirable, since their electronically insulating nature and steric bulk have historically limited charge carrier mobility.29,37,41,42 It is therefore important to examine current design principles in systems where structural variables can be isolated (e.g., where side-chains can be removed without affecting the solubility). This study compares the two main NFA design principles: the calamitic and 3D motifs. Two molecules, one with the calamitic design (FBRCN), and one with the 3D design (XFBRCN), were synthesized (Figure 1a); the goal was to determine the effect of molecular shape on the optoelectronic properties of the NFAs and their performance in solid state electronic devices. We show that the 3D semiconductor has a higher extinction coefficient, broader absorbance bands, and a higher permittivity than the calamitic analogue. The calamitic material has a higher electron mobility, is more miscible with polymeric OPV donors, and is more crystalline than the 3D material. Additionally, we found that the alkyl chains of the calamitic NFA played a crucial


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role in controlling the bulk heterojunction morphology in OSCs. Ultimately, the alkyl chains had the biggest impact on OPV efficiency, leaving open the question of whether an appropriatelyalkylated 3D acceptor would perform well in OPV cells. This work compares the two most popular NFA design motifs, and the results will help guide the future design of NFAs.

Figure 1. (a) Chemical structures of the calamitic (FBRCN) and 3D (XFBRCN) NFAs, and frontier orbital isosurfaces for (b) FBRCN and (c) XFBRCN. Calculations were performed at the B3LYP/6-31G(d,p) level of theory and octyl chains were replaced with methyl groups for simplicity. RESULTS AND DISCUSSION Two NFAs (FBRCN and XFBRCN) were synthesized, one with a calamitic (FBRCN) and one with a 3D structure (XFBRCN). The calamitic NFA is based on a chromophore first popularized by Holliday et al. in 2015,8 and has been reported previously,43 while the 3D analogue is new to this work. Identical chromophores were used for both molecules, isolating any observed differences in optoelectronic properties or device performance to the variation in molecular shape and design (calamitic vs. 3D). The two materials were synthesized from their alkyl-fluorene or spirobifluorene precursors via Miyaura borylation, followed by a Suzuki


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coupling with an aldehyde-substituted benzothiadiazole. The final molecules were obtained by either a 2-fold or 4-fold Knovenagel condensation with dicyanorhodanine (Scheme S1). Both molecules had similar solubilities in the same common laboratory solvents (e.g., dichloromethane, chloroform, tetrahydrofuran, toluene, chlorobenzene, dioxane). The solubilities measured in chlorobenzene were roughly the same for both FBRCN (5.7 g/L) and XFBRCN (5.5 g/L), removing solubility as a variable when comparing their properties and performance. The two molecules were then characterized and tested in electronic devices to compare the two different NFA design strategies. Time-Dependent Density Functional Theory Calculations. To evaluate the effects of molecular shape on electronic structure, time-dependant density functional theory (TDDFT) calculations were performed at the B3LYP/6-31G(d,p) level of theory. DFT geometry optimizations show that the structure of the chromophore is the same in both molecules, with the same dihedral angle between the benzothiadiazole and fluorene units (35ᵒ). The orbital isosurfaces are shown in Figure 1; again, no major differences are seen between the calamitic FBRCN and the 3D XFBRCN. The frontier orbitals of XFBRCN closely resemble those of FBRCN, with the exception that the orbitals in XFBRCN are all degenerate (Figure S6). The calculations predict that XFBRCN has a slightly lower energy HOMO (–5.8 eV) than FBRCN (–5.7 eV). This may be due to hyperconjugation of the fluorene π-system with the alkyl chains of FBRCN (which would destabilize the HOMO of FBRCN relative to XFBRCN); alternatively, the more inductively electron withdrawing sp2-hybridized carbons of the spirobifluorene core of XFBRCN may lower the HOMO relative to FBRCN. We also used TDDFT to calculate the electronic transitions of both molecules. Table S1 shows the pertinent TDDFT data for the lowest energy transitions of FBRCN and XFBRCN. The


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oscillator strength of the S1 ← S0 transition of FBRCN (fosc = 1.88) is higher than the same transition in XFBRCN (fosc = 1.43); however, XFBRCN also has a degenerate transition to the S2 state of similar oscillator strength (fosc = 1.40). The net effect is that XFBRCN is expected to have an absorbance ~ 1.5 times higher than FBRCN, even though it contains twice as many chromophores. The transition dipole moments of the S1 ← S0 and S2 ← S0 transitions in XFBRCN are orthogonal to each other (Figure S6, Table S1), and there appears to be minimal coupling of the transitions between the HOMO / HOMO–1 and LUMO / LUMO+1 orbitals. Since the TDDFT calculations predict a 90° angle between the two chromophores in XFBRCN, this lack of electronic coupling is expected; however, any torsion of the spiro-linkage would facilitate exciton coupling of the two chromophores and broaden and blue-shift the absorbance band.44,45 An analysis of the vibrational frequencies in XFBRCN reveals multiple vibrational modes with frequencies in the range of 25 to 200 cm–1 that would twist the spirobifluorene core and potentially allow a modest degree of exciton coupling at room temperature. Optical Properties. To verify the TDDFT calculations and further probe the optoelectronic properties of the two molecules, we analyzed them using UV-Visible spectroscopy. The absorbance and emission spectra, both in chloroform solution and as thin films, are shown in Figure 2. The corresponding data are summarized in Table 1. The absorbance spectra in solution (Figure 2a) are dominated by an intense transition ~ 500 nm, which can be assigned to the S1 ← S0 transition (FBRCN) and the degenerate S2 ← S0 and S1 ← S0 transitions (XFBRCN). The molar extinction coefficient (ε) of XFBRCN (1.06 × 105 M–1·cm–1) is 1.6 times greater than that of FBRCN (6.8 × 104 M–1·cm–1), in good agreement with the ratio of oscillator strengths calculated by TDDFT (1.5 times).


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There are also more subtle differences between the two spectra. The full-width-at-halfmaximum (FWHM) of the lowest-energy absorbance band in XFBRCN (4206 cm–1) is broader than in FBRCN (3968 cm-1). This broadening of the absorption band upon moving from the calamitic to the 3D structure is accompanied by a blue-shift of the absorbance maximum (536 cm–1). This shift is larger than predicted by TDDFT (136 cm–1). The results suggest that in addition to the slightly lower HOMO of XFBRCN (which is already considered in the TDDFT), dynamic torsion of the spirobifluorene core (a result of several low energy vibrational modes) may lead to a small amount of exciton coupling between the two chromophores.44,45 This would explain both the blue-shift and broadening of the absorption band in XFBRCN. A similar blueshift is also observed in the emission maxima, although it is less pronounced (176 cm–1). The smaller blue-shift in the emission maxima is likely due to the hyperconjugation in FBRCN and the inductive effects in XFBRCN discussed previously; these de-stabilize the HOMO of FBRCN and lead to a slightly smaller HOMO-LUMO gap for FBRCN than XFBRCN. The relative magnitudes of the blue-shifts in the absorbance and emission spectra results in a larger Stokes shift for XFBRCN (3418 cm–1) than for FBRCN (3033 cm–1). The data suggests additional structural relaxation pathways in the excited state of XFBRCN, likely due to changes in the torsional angle between the two chromophore units. Figure 2b shows the thin film UV-Vis absorption spectra of the two molecules. Both spectra are red-shifted relative to solution; this red-shift is indicative of aggregation in the solid-state. The magnitude of this red-shift at the onset of absorbance is similar for both FBRCN (1154 cm– 1

) and XFBRCN (1000 cm–1). This contrasts with the solution-to-film red-shift at the absorbance

maxima, which is much larger for FBRCN (735 cm–1) than XFBRCN (209 cm–1). This implies that the maximum degree of aggregation-induced red-shift is similar for both molecules (red-shift


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of the absorption onset), but that the average amount of aggregation is lower for XFBRCN than in FBRCN (red-shift of the absorption maxima). This is likely due to the increased steric bulk of the three-dimensional XFBRCN, which would limit the stacking coherence length of crystallites.21,25,46 This leads to an overall decrease in the ordering of XFBRCN relative to the calamitic FBRCN, which is reinforced by GIWAXS (see below).

Figure 2. (a) UV-Vis absorption (solid line) and emission (dashed line) spectra of FBRCN (blue) and XFBRCN (red) in chloroform solution. Samples were excited at the absorbance maximum of each material, and the emission intensity was corrected for the detector response. (b) UV-Vis absorption spectra of FBRCN (blue) and XFBRCN (red) as thin films spin-coated from chloroform solutions onto glass substrates. Table 1. Optical data of FBRCN and XFBRCN λabs max (nm)

FWHM (cm–1)

ε (M–1·cm–1)

λem max (nm)

Stokes shift (cm–1)

τ1 (ns)

τ2 (ns)

Φ (%)

FBRCN

499 (518)a

3968 (4877)a

6.8 × 104

588

3033

1.37 [0.912]b

2.26 [0.088]b

33

XFBRCN

486 (491)a

4208 (6048)a

1.06 × 105

582

3418

1.20 [0.985]b

2.37 [0.015]b

31

NFA


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Data obtained from chloroform solution except where noted. aMeasured on thin films spincoated from chloroform solutions. bPre-exponential factor of the lifetime component. In order to determine the effect of the molecular design on the excited state, we measured both the emission quantum yield and the excited state lifetime. Time correlated single photon counting (TCSPC) emission spectroscopy was performed on chloroform solutions of both FBRCN and XFBRCN (Figure S7). Both molecules decay by a two-component exponential decay with lifetimes in the nanosecond range. The lifetime of the major decay contribution was longer for FBRCN (τ1 = 1.37 ns) than for XFBRCN (τ1 = 1.20 ns). These data are supported by the emission quantum yield measurements, where FBRCN was found to have a slightly higher quantum yield (Φf = 0.33) than XFBRCN (Φf = 0.31) in chloroform solution. The extra relaxation pathways of XFBRCN are likely due to the additional vibrational modes associated with the spirobifluorene core (see above), but overall the differences in both lifetime and quantum yield are small. The data further support the conclusion that differences in the Stokes shift are due to a more distorted excited state geometry of XFBRCN, which involves torsion at the spirobifluorene core. Electronic properties. To determine the effect of molecule shape on electronic structure, we performed a series of voltammetry experiments on FBRCN and XFBRCN, the results of which are summarized in Table 2. Differential pulse voltammetry (DPV) in dichloromethane solution (Figure 3) revealed multiple low energy reduction events at nearly identical voltages for FBRCN and XFBRCN (–1.19 V vs Fc/Fc+), and a single oxidation event near the solvent window that was at a slightly higher potential for XFBRCN (0.85 V vs Fc/Fc+) than for FBRCN (0.77 V vs Fc/Fc+). These data are consistent with both the orbital energies predicted by TDDFT (Figure 1) and the optical band gaps measured by UV-Vis spectroscopy (Table 2), and clearly show that the alkyl chains of FBRCN reduce the ionization potential of FBRCN relative to XFBRCN.


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Additionally, given the overall similarity between the molecules and their electrochemical responses, we normalized the DPV data to correct for slight differences in analyte concentration. Comparing the molar current differences (Figure 3), we find that the first and second reduction events in XFBRCN involve a transfer of twice as many electrons as in FBRCN. The fact that XFBRCN accepts twice as many electrons as FBRCN without changing the reduction potential suggests that the orthogonal chromophores of XFBRCN are electronically isolated. Table 2. Electrical properties of FBRCN and XFBRCN. E

E

(eV)

(eV)

EHOMO (eV)c

FBRCN

2.07

1.97

–5.84

–3.77

2.7

3 ± 1×10–4

44 ± 2

1 × 10–4

XFBRCN

2.12

2.04

–5.89

–3.77

3.5

8 ± 3×10–5

26 ± 8

4 × 10–5

NFA

g,opt a

g,elec b

ELUMO (eV)d

Relative Permittivitye

µe (OTFT) (cm2·V–1·s–1)f

VTH (V)f

µe (SCLC) (cm2·V–1·s–1)g

a

Measured from the absorption onset of thin films spin-coated from chloroform solution on glass substrates. bMeasured by DPV in dichloromethane solution. cMeasured from the onset of reduction in the cyclic voltammograms of thin films. dEHOMO = ELUMO + Eg,opt. eObtained from the impedance spectra of ITO/NFA/Ag devices. fObtained from measurements on bottom-gate bottom-contact OTFTs. Averages and standard deviations were calculated from measurements on 5 devices. gObtained from SCLC measurements on Al/NFA/Al devices, reported at an electric field strength of 3 × 105 V·cm–1. The cyclic voltammetry of thin films of FBRCN and XFBRCN (Figure S9) shows that the reduction events are mostly irreversible, and occur at the same potential in both molecules (–1.03 V vs Fc/Fc+ in acetonitrile). The small change in reduction potential from solution to thin film is due to both the electronic effects of aggregation and the change in solvent, so direct comparisons can not be made. The thin film voltammetry does, however, give a better approximation of the electron affinity. Assuming that the Fc/Fc+ redox couple lies at –4.80 eV versus the vacuum level,7,10,47,48 the LUMO energy of both FBRCN and XFBRCN is calculated to be –3.77 eV. This was combined with the optical band gap obtained from the thin film UV-Vis data to estimate the energy of the FBRCN (–5.84 eV) and XFBRCN (–5.89 eV) HOMO.


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Figure 3. Differential pulse voltammograms of FBRCN (blue) and XFBRCN (red) in a dichloromethane solution of 0.05 M tetrabutylammonium hexafluorophosphate. Data was acquired with a 5 mV step size and a 0.5 s interval time. The voltammograms were corrected for the concentration of the samples and referenced to the ferrocene/ferrocenium redox couple. In electronic materials, the relative permittivity is a critical parameter that affects the exciton diffusion length,49,50 the charge carrier mobility,51,52 and the charge carrier injection barrier.53,54 The relative permittivities of FBRCN and XFBRCN were determined by impedance spectroscopy (Figure S10) in the low frequency regime, where the permittivity is expected to be frequency independent.55,56 The relative permittivity of XFBRCN (3.5) was found to be higher than that of FBRCN (2.7). This may be due to the two orthogonal π-systems in XFBRCN providing a more isotropic polarizability, resulting in a higher overall permittivity in thin films. Alternatively, the less-efficient packing of XFBRCN (see below) may allow for increased conformational flexibility of the dicyanorhodanine endgroups, increasing dipolar contributions to the permittivity. However, it should be noted that the measured permittivites have a relatively


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high uncertainty, owing to both the difficulty in accurately measuring the permittivity of conjugated materials and the frequency dependence of the permittivity. Next, we measured the electron mobility of the materials by preparing space-charge limited current (SCLC) devices. The cells were fabricated as electron-only devices with an Al/acceptor/Al architecture. Electron mobilities were calculated using the Murgatroyd equation to account for the electric field dependence of the electron mobility.51 This dependence is commonly observed in organic materials, and arises because the built-up charge in the device changes the effective electric field, which in turn changes the electron mobility of the material. Figure 4 shows the SCLC plots as well as the dependence of the electron mobility on electric field strength. The electron mobilities obtained by an analysis of the SCLC data show that both materials have a similar electric field dependence, but that FBRCN has a higher electron mobility (1 × 10–4 cm2·V–1·s–1) than XFBRCN (4 × 10–5 cm2·V–1·s–1) at a representative electric field strength (3 × 105 V·cm–1). Since the electronic structure of both systems is so similar, the mobility data suggests that FBRCN has a more favorable molecular packing, leading to better πoverlap in the solid state. Organic thin-film transistors (OTFTs) can provide additional information about charge carrier mobilities, and the data is highly complementary to that obtained from SCLC devices. We therefore fabricated n-channel OTFTs with a bottom-gate, bottom-contact architecture. Although the currents were relatively small, with small oscillations due to low frequency interference,57–59 an analysis of the transfer curves in the saturation regime (Figure 4c-f, Table S3) revealed the same trend in electron mobility that was observed in the SCLC devices (Table 2). FBRCN again had an electron mobility (3 ± 1 × 10–4 cm2·V–1·s–1) greater than that of XFBRCN (8 ± 3 × 10–5 cm2·V–1·s–1). The bulkier structure of XFBRCN appears to impede efficient packing in the solid


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state, limiting the charge carrier mobility.38,60,61 In both cases, the electron mobilities obtained from OTFT measurements were 2-3 times higher than those obtained from SCLC devices. This may indicate some anisotropy in the electron mobilities of the two materials, although grazing incident wide angle x-ray scattering (GIWAXS) measurements showed both neat films to be relatively isotropic (see below).

Figure 4. SCLC and OTFT data. (a) Current density-voltage curves of electron-only SCLC devices (dots) and the numerical fit (solid lines). (b) SCLC-derived electron mobility as a function of internal electric field strength. The dotted line denotes a representative field strength of 3 × 105 V·cm–1. Representative OTFT output and transfer curves for OTFTs based on: (c,e) FBRCN annealed at 50 ᵒC, and (d,f) XFBRCN annealed at 200 ᵒC. Thermal properties. The thermal properties of FBRCN and XFBRCN were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Both molecules


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had similar thermal stability (Figure S11), with the decomposition temperature of XFBRCN (380 ᵒC) being only slightly higher than that of FBRCN (373 ᵒC). The DSC thermograms (Figure 5) show both FBRCN and XFBRCN as they are taken through a heating, cooling, and reheating cycle. The initial heating of FBRCN shows an exothermic transition at 173 ᵒC due to cold crystallization of the as-prepared sample, followed by a minor endothermic transition at 205 ᵒC and a sharp endothermic melting transition at 229 ᵒC. There follows a small endothermic phase transition at 247 ᵒC that may indicate a mesophase between 229 ᵒC and 247 ᵒC; however, a more in-depth study would be required to definitively assign this feature. No distinct features were observed upon cooling; the lack of an exothermic crystallization peak indicates that the material remains trapped in a highly disordered glass phase. Similar behaviour has been observed previously in a closely related molecule.8 Only a small step at 105 °C was observed on cooling; this feature became more distinct on the second heating cycle (T = 109 °C), and is assigned as the glass transition temperature. Notably, the sharp endothermic melting transition is not observed on the second heating cycle, consistent with the material remaining trapped in the glass phase. In the case of XFBRCN, only a minor endothermic glass transition at 250 ᵒC is seen on the first heating cycle. The glass transition was observed on cooling as a small step at 241 °C, and as a more pronounced glass transition at 250 °C in the second heating cycle. The very different thermal behavior of the two materials can be related back to differences in structure. XFBRCN lacks the long alkyl chains of FBRCN, which appear to be critical to lowering the glass transition temperature and facilitating solid state reorganization. This lack of alkyl chains, combined with the addition of a second large, rigid chromophore, prevents XFBRCN from packing efficiently, trapping it in a disordered state. This likely contributes to the lower electron mobility observed for XFBRCN. Clearly the alkyl chains of FBRCN not only play a role in


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solubilizing the material, but their conformational flexibility helps facilitate reorganization in the solid state.

Figure 5. DSC thermograms of FBRCN (blue) and XFBRCN (red) taken through heating, cooling, and re-heating cycles. The temperature was ramped at 5 ᵒC/min, and samples were measured under nitrogen. Endotherms are shown down. Organic solar cell performance. Since both the calamitic and the 3D designs for NFAs were developed for OSCs, we tested FBRCN and XFBRCN in OSCs to compare the effect of molecular shape on device performance. OSCs were fabricated using the inverted architecture of ITO/ZnO/PTB7-Th:acceptor/MoO3–x/Ag. The donor PTB7-Th (Figure S12) was chosen due to its complimentary light absorbance and its widespread use.62–65 Active layers were spin coated from chlorobenzene solution, and cell parameters were optimized by common pre- and postdeposition methods, as detailed in the Supporting Information (Table S4 and S5). Figure 6a shows the J-V curves of champion devices with no additional processing steps (as cast), after thermal annealing (TA), and with the addition of 1,8-diiodooctane (DIO) prior to spin coating. In all cases, FBRCN produces substantially higher photocurrents than XFBRCN (Jsc = 8.1 and 4.6


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mA/cm2, respectively), leading to higher PCEs (PCE = 3.3% and 1.42%, respectively). The absorbance of both films was very similar (Figure 6b,c), as are the electronics of FBRCN and XFBRCN (Table 2, Figure S12). This suggests that the differences in photocurrent must be caused by changes in the solid state structure of the films.

Figure 6. (a) J-V curves of champion OSCs fabricated using FBRCN (blue) and XFBRCN (red). The devices were prepared using different processing conditions: as cast (solid lines), thermally annealed (TA, dashed lines) and with the addition of DIO (dashed-dotted lines). (b,c) EQEs of champion cells and UV-Vis absorption spectra of thin films processed using the same conditions: (b) PTB7-Th:FBRCN and (c) PTB7-Th:XFBRCN. The data for optimized devices are tabulated in Table 3, and an in-depth summary of the device data (including data for P3HT/PC61BM reference cells) is reported in the Supporting Information (Table S6). FBRCN cells were improved relative to the as cast devices (PCE = 2.1 ± 0.1%) by the addition of DIO (PCE = 3.1 ± 0.1%) and thermal annealing (PCE = 2.5 ± 0.4%), both of which can help organize well-blended mixtures of donor and acceptor into larger and purer crystalline domains.66,67 The larger domain size contributes to a reduction in non-geminate recombination and higher photocurrents. FBRCN is slightly more amenable to the addition of DIO than to thermal annealing, likely due to the improved crystallinity resulting from slow


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solvent removal, as evidenced by GIWAXS (see below). XFBRCN responded differently to both processing treatments. Thermal annealing led to higher photocurrents, along with modest increases in the Voc and FF. DIO treatment, even at very low loadings, led to a reduction in photocurrent; this was coupled with a loss of Voc, FF, and PCE. To help determine the cause of the low photocurrents in XFBRCN, we measured the external quantum efficiency (EQE) of both FBRCN and XFBRCN-based cells. The overall spectral coverage is very similar for both sets of devices (Figure 6b and c), with the only notable difference being the lower overall EQE of XFBRCN as compared to FBRCN. The results suggest that the morphological differences may be responsible for the very different performance of the two NFAs. Table 3. OSC performance of FBRCN and XFBRCN devices with PTB7-Th donor. NFA

Processing Conditions

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω·cm2)

Rsh (Ω·cm2)

µe (SCLC) (cm2·V–1·s–1)a

FBRCN

2% DIO

7.8 ± 0.2 (8.1)

0.998 ± 0.006 (1.001)

39.5 ± 0.7 (40.2)

3.1 ± 0.1 (3.3)

15 ± (14)

3

310 ± 10 (310)

9 × 10–6

XFBRCN

140 ᵒC TA

4.4 ± 0.2 (4.6)

0.910 ± 0.003 (0.910)

33.3 ± 0.4 (33.7)

1.32 ± 0.08 (1.42)

3.0 ± 0.9 (3.2)

333 ± 9 (325)

3 × 10–6

Data for champion devices shown in parentheses. Averages and standard deviations are based on measurements of at least 8 devices. aObtained from SCLC measurements on Al/NFA/Al devices, reported at an electric field strength of 3 × 105 V·cm–1.

Next, we carried out SCLC measurements on the donor:acceptor blends (Figure S15). The same trend of XFBRCN having a lower electron mobility (3 × 10–6 cm2·V–1·s–1) than FBRCN (9 × 10–6 cm2·V–1·s–1) was observed (values reported at an electric field strength of 3 × 105 V·cm– 1

). The electric field dependence of the mobility in the XFBRCN blend was also higher than

FBRCN, exacerbating the effect of the already-low mobility. Both blends had lower electron mobilities than films of the pure NFAs, implying that neither blend formed an ideal morphology with the donor; this is expected, as PTB7-Th is poorly ordered and may limit the overall degree of crystallinity within the blend.68,69


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Figure 7. GIWAXS patterns of (a-d) FBRCN and (e-h) XFBRCN thin films, both pure (neat) and as PTB7-Th:NFA blends (blend). Films were either as cast (AC), thermally annealed for 30 minutes (TA), or cast from a solvent containing DIO (DIO). (i-l) Azimuthally-integrated intensities from the GIWAXS patterns in (a-h). GIWAXS experiments were performed to probe the crystallinity and orientation of the donor:acceptor materials in thin films. Figure 7 shows the scattering patterns of both the pure NFAs and their blends with PTB7-Th; the GIWAXS pattern of pure PTB7-Th70,71 is shown in Figure S16 for comparison. The GIWAXS pattern of pure PTB7-Th is similar to that of many πconjugated polymers; it shows a (100) peak at q ≈ 0.28 - 0.3 Å–1 owing to the lamellar organization of the polymer, and a (010) peak at q ≈ 1.6 Å–1 owing to the π-stacking of polymer chains.70,71 The orientation of the (010) peak along the qz axis is consistent with predominantly face-on PTB7-Th in both neat and blend films (Figure 7 and S16), while the arc-like scattering pattern of the (010) peak reveals that the blend films have a broader distribution of crystallite orientations as compared to neat PTB7-Th (Figure S16). Both the as cast FBRCN and XFBRCN


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films show diffuse, mostly isotropic scattering and are only very weakly ordered (Figure 7a, e, i). In particular, XFBRCN had a very weak lamellar stacking signal (q ~ 0.24 Å–1) with respect to FBRCN (Figure 7i). When processed with DIO, neat FBRCN shows a remarkably high degree of crystallinity (Figure 7c) and strong preferred orientation. In contrast, the XFBRCN films remained only weakly ordered even after thermal annealing (Figure 7g). These results are consistent with the DSC data, where cold crystallization was observed for FBRCN, whereas XFBRCN showed no evidence of crystallization throughout the heat-cool-heat cycle (Figure 5). The GIWAXS patterns of the as cast blends (Figure 7b and f) show the scattering peaks expected for predominantly face-on PTB7-Th, similar to that for the neat PTB7-Th (Figure S16). When the blend films were either processed using DIO or thermally annealed (as optimized in Table 3), there was relatively little change in either of the GIWAXS patterns (Figure 7d and h) compared to those of the as cast films (Figure 7b and f). The PTB7-Th remained weakly ordered, while there was no evidence of distinct scattering peaks arising from either of the NFAs. This contrasts with the behavior of pure FBRCN, which strongly crystallized in thin films cast from solvent containing DIO. In this case, the PTB7-Th appears to disrupt the crystallization of FBRCN, suggesting some degree of mixing of the two materials. Overall, however, the GIWAXS patterns of the two PTB7-Th:NFA blends are quite similar; any small differences are unlikely to result in changes in device performance. We performed atomic force microscopy (AFM) on thin films of the donor:acceptor blends in order to determine whether changes in film morphology could explain the trend in device performance. Figure 8 shows the AFM height images of as cast films of PTB7-Th:FBRCN and PTB7-Th:XFBRCN; analogous images of films processed using the optimized device fabrication conditions are shown in the Supporting Information (Figure S17). The PTB7-


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Th:FBRCN blends appear to form a mixture of the PTB7-Th and FBRCN, with relatively small feature sizes visible in the height image. The root-mean-square roughness (Rq) of the as cast PTB7-Th:FBRCN film is relatively low (Rq = 5.1 nm), and increases slightly when the film is cast using DIO (Rq = 5.4 nm). This fine-tuning of the bulk heterojunction morphology may explain the increase in PCE observed when the FBRCN devices were prepared using DIO as a solvent additive. The XFBRCN blends show an entirely different morphology. Pronounced droplet-shaped features are visible in the AFM image; these features are caused by liquid-liquid phase separation, and are found in many bulk heterojunction films72 where the donor and acceptor are immiscible. These droplets are quite large (d ~ 200 nm), leading to a very high surface roughness (Rq = 33.1 nm). The extensive phase separation observed in the AFM image helps explain the poor performance of XFBRCN in OSCs, as the large domains lead to extensive exciton recombination and correspondingly low photocurrents. The droplets shrink somewhat after thermal annealing (Rq = 31.9 nm), consistent with the improved performance observed for thermally annealed devices (Table S6). When combined with the GIWAXS data, the results suggest that there is extensive phase separation of PTB7-Th and XFBRCN; however, this is not driven by the over-crystallization of either of the two materials, as is commonly observed in other NFAs (e.g., perylene diimide).2 Rather, the lack of alkyl chains on XFBRCN appears to lead to a more endothermic enthalpy of mixing with PTB7-Th, resulting in their phase separation into either amorphous or weakly ordered domains.


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Figure 8. Topographic AFM images of as cast films of: (a) PTB7-Th:FBRCN and (b) PTB7Th:XFBRCN. Images are 2 µm × 2 µm. Based on our results, the alkyl chains (or lack thereof) play an important role in the morphology of FBRCN and XFBRCN thin films. The conformational flexibility of the alkyl chains significantly reduces the glass transition temperature of the materials (Figure 5), facilitating their packing in the solid state. Under certain conditions, this allows for the crystallization of the acceptor (Figure 7c); the higher crystalline order likely explains the higher electron mobility of FBRCN. A lack of alkyl chains on XFBRCN leads to a strongly endothermic enthalpy of mixing with the donor polymer in bulk heterojunctions, causing extensive phase separation. The work shows that alkyl chains are important for ensuring the proper mixing of the donor and acceptor. It helps to elucidate the role that alkyl chains play in dictating solid state film morphology, not just in controlling the solubility.

CONCLUSION In this work, we compared the calamitic and 3D design motifs for NFAs. We synthesized two NFAs with the same chromophore system, one with the calamitic design and one with a 3D structure. We show that the introduction of an orthogonal chromophore at a spirobifluorene center leads to two largely independent chromophores, but that dynamic torsion of the spirobifluorene core leads to a small amount of excitonic coupling. As a result of this second chromophore, the 3D material had promising optical properties: a higher extinction coefficient, broader absorbance bands, and a higher relative permittivity. In contrast, the alkyl chains of the calamitic material facilitate better packing in the solid state, significantly reducing the glass transition temperature and leading to both improved electronic properties (e.g., a higher electron


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mobility) and better performance with polymer donors in OSCs. The alkyl chains ultimately had the biggest impact on OPV performance, and further work will be needed in order to reach more general conclusions regarding the relative merits of the calamitic and 3D designs. We believe that these results will help guide the design of new NFAs. They show that the careful design of molecular shape and structure can be used to tailor the optical and electronic properties, film morphology and crystallinity, as well as the device performance, even for the same chromophore structure. ASSOCIATED CONTENT Supporting Information. Experimental methods, detailed synthetic procedures, cyclic voltammograms, impedance spectroscopy, OSC and OTFT optimization, detailed TDDFT results, AFM images, and TGA thermograms. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Fax: +1-306-966-4730; Tel: +1-306-966-4666 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT


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The Natural Sciences and Engineering Research Council of Canada (NSERC, RGPIN-201703732) and the University of Saskatchewan are acknowledged for financial support. T.L.K. is a Canada Research Chair in Photovoltaics. The research was undertaken, in part, thanks to funding from the Canada Research Chair program. C.L.R. and N.M.R. acknowledge NSERC for scholarship funding. The Saskatchewan Structural Sciences Centre (SSSC) is acknowledged for providing facilities to conduct part of this research. Funding from the Canada Foundation for Innovation (CFI), NSERC and the University of Saskatchewan support research at the SSSC. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Burke Barlow and Dr. Ian Burgess are gratefully acknowledged for their assistance with impedance spectroscopy. REFERENCES (1)

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Effect of Molecular Shape on the Properties of Non-Fullerene

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