Donor or Acceptor? How Selection of Rylene Imide Endcap Impacts

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Donor or Acceptor? How Selection of Rylene Imide Endcap Impacts Polarity of #-Conjugated Molecules for Organic Electronics Abby-Jo Payne, Nicole Rice, Seth M McAfee, Shi Li, Pierre Josse, Clément Cabanetos, Chad Risko, Benoît H. Lessard, and Gregory C Welch ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00929 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Energy Materials

Donor or Acceptor? How Selection of Rylene Imide Endcap Impacts Polarity of π-Conjugated Molecules for Organic Electronics Abby-Jo Paynea, Nicole A. Riceb, Seth M. McAfeea, Shi Lic, Pierre Jossed, Clément Cabanetosd*, Chad Riskoc*, Benoît H. Lessardb*, Gregory C. Welcha*

a

Department of Chemistry University of Calgary, 2500 University Drive NW Calgary, Alberta T2N 1N4, Canada

b

University of Ottawa, Department of Chemical and Biological Engineering, 161 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada

c

Department of Chemistry & Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky, 40506, USA d

CNRS UMR 6200, MOLTECH-Anjou, University of Angers, 2 Bd Lavoisier, Angers, France, 49045

KEYWORDS: Organic Electronics, Organic Field-Effect Transistors, Organic Solar Cells, Rylene Imides, Perylene Diimides, Diketopyrrolopyrrole, Direct (Hetero)Arylation

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Abstract Three molecular semiconductors are compared and evaluated in organic field-effect transistors and organic solar cells. The molecules are constructed from the dyes diketopyrrolopyrrole (DPP), perylene diimide (PDI), and N-(alkyl)benzothioxanthene- 3,4-dicarboximide (BTXI). The compound PDI-DPP-PDI (1) has previously been reported and used as a non-fullerene acceptor. The compounds PDI-DPP-BTXI (2) and BTXI-DPP-BTXI (3) were synthesized using direct (hetero)arylation methods and fully identified using NMR spectroscopy and mass spectrometry. All three compounds were characterized using UV-visible spectroscopy, cyclic voltammetry, and density functional theory calculations. Increasing the BTXI content results in a progressive destabilization of the electronic energy levels. For all compounds, no significant changes in the optical absorption spectra are observed when compared to a combination of the constituent optical absorption spectra. Compound 1 exhibits electron transport characteristics and functions as an electron acceptor in solar cells that produce a power conversion efficiency of 5%. Compound 2 exhibits unbalanced (electron transporting dominate) ambipolar charge transport characteristics and performs better as a non-fullerene acceptor in solar cells. Compound 3 exhibits balanced ambipolar charge transport characteristics and performs best as a donor in solar cell devices. The ability to tune the optical and charge-carrier transport characteristics of these panchromatic dyes through direct (hetero)arylation synthesis offers a distinctive way to create organic semiconductors that span a range of device performance metrics.


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Introduction Aryl imides are an important class of materials and have found extensive use in organic electronic applications.1–3 These building blocks are routinely incorporated into both molecular4–8 and polymeric9–13 designs typically for their electron deficient character and self-assembly properties. Among them, the rylene-based imides, perylene and naphthalene diimide (PDI, NDI, respectively), have found great success as electron transporting materials in organic field-effect transistors (OFETs)14–18 and organic solar cells (OSCs).19–25 Until recently, N-(alkyl)benzothioxanthene3,4-dicarboximide (BTXI), a sulfur containing rylene-imide dye used in bio imaging26,27 and for its anti-tumor activity28, had not been considered as a building block for use in organic electronics. Cabanetos et al. demonstrated an efficient selective bromination of the BTXI dye that shows excellent compatibility among commonly used palladium catalyzed C-C bond forming reactions including Stille, Suzuki, Sonagashira, as well as the atom economical direct (hetero)arylation (DHA).29 In addition to the importance of simply developing/incorporating new building blocks in organic semiconductors, it is important to understand how these building blocks modify key molecular and material characteristics in order to enhance design strategies. Electron deficient and electron rich building blocks are often used interchangeably in both electron and hole transporting materials, but the roles that these building blocks play in molecular design is often unclear. In this study, we set out to investigate the role of BTXI upon its incorporation into various molecular architectures towards π-conjugated materials for application in organic electronics. With interest in how the building block compares to its much more studied high performance relative, PDI, we chose to modulate a PDI-based molecule known to be an excellent non-fullerene acceptor in OSCs.20 T20he molecule PDI-DPP-PDI (compound 1, Figure 1) has an A-A’-A type frame-


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work where A’ = bis-thienyldiketopyrrolopyrrole (DPP) and A = N-annulated PDI. The hexyl and 1-ethyl propyl aliphatic chains on the PDI render the molecule soluble in organic solvents, while the octyl chain on DPP helps to drive self-assembly.30,31 Like PDI, DPP is another dye-based building block which has been extensively explored and has found success in a variety of organic electronic applications.32–41 A key feature of 1 is that it can be readily synthesized via DHA between the DPP monomer and the brominated N-annulated PDI unit in high yields, rendering the preparation straightforward. Using this modular framework and optimized DHA reaction conditions, we were able access the full BTXI derivative of 1 using BTXI instead of PDI as the terminal units (3). For further comparison, the asymmetric derivative was also synthesized through reacting PDI-DPP with BTXI (2). To elucidate the role of BTXI in this system, a series of optoelectronic properties were evaluated using cyclic voltammetry and UV-Vis spectroscopy, supplemented by density functional theory (DFT) calculations, in addition to the evaluation of each compound in organic thin-film transistors (OTFTs) and OSCs.

Figure 1. Compounds synthesized and characterized in this study.


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Synthesis Compound 1 with PDI terminal units was synthesized as previously reported via DHA methods using the silica supported catalyst SiliaCat® DPP-Pd in 70% yield.20,42 For the synthesis of compound 3, the same DHA methods were applied, using the BTXI building block instead of PDI. The reaction and purification proceeded smoothly to give compound 3 as a dark solid in 70% isolated yield (Scheme 1). Using the same DHA methods, a 1:1 reaction between PDI and DPP yielded the monosubstituted product PDI-DPP in 45% yield (Scheme 1).42 Further reaction of PDI-DPP with the BTXI building block under the same DHA coupling conditions gave the target compound 2 in 30% yield (Scheme 1). The reduced product yield in the synthesis of 2 compared to that of compounds 1 and 3 was due to loss in purification (via silica-gel column chromatography) where an unidentified minor impurity required rigorous separation. The final product was isolated in 70% yield with ~90% purity before column purification, thus a significant amount of product was lost during the purification step (Figure S1). Perhaps lower temperatures and/or reduced reaction times may prove beneficial in avoiding any unwanted by-products; however, for consistency, 80 ºC and 24 hours was chosen to match the conditions used for the synthesis of symmetric compounds 1 and 3. Aromatic proton assignments of 1-3 are displayed in Figure 2. An upfield shift in the DPP-thiophene C-H resonances are observed upon replacing PDI with BTXI, most notably for the C-H closest to the terminal unit (PDI or BTXI) where an upfield shift of 0.17 ppm (9.38 ppm to 9.21 ppm) is observed. Indeed, the aromatic resonances for BTXI are shifted upfield compared to PDI, suggesting the PDI units are comparatively more electron deficient. See supporting information (SI) for full synthetic details and complete material identification.


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Scheme 1. Synthetic routes towards compounds 2 and 3 via direct (hetero)arylation coupling reactions.


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Figure 2. Aromatic region of the 1H NMR spectra with proton assignment for compounds 1-3. Resonances for DPP (blue), PDI (red), and BTXI (orange) are highlighted.

Optoelectronic Characterization The electrochemical and optical properties of compounds 1-3 were determined by cyclic voltammetry and UV-Vis spectroscopy respectively. Cyclic voltammograms are presented in Figure 3 with tabulated data in Table 1. Compounds 1-3 each display two reversible oxidation waves


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attributed to the DPP core. All compounds exhibit reversible reduction waves with the number of reduction waves varying based on the terminal units. Compound 1 flanked with PDI units has three reduction waves with the first two being attributed to PDI and the third to DPP. Asymmetric compound 2 displays a total of four reversible reduction waves with the first two reductions attributed to PDI followed by a single reduction wave each for DPP and BTXI, respectively. Lastly, compound 3 with BTXI end groups exhibits one reduction wave for the DPP core followed by another reduction wave for the BTXI endcaps. The assignments of the oxidation and reduction waves are based on the overlaid cyclic voltammograms of each of the final compounds 1-3 with their individual components (Figure S9). The ionization potentials (IP) and electron affinities (EA) were estimated by correlating the onsets of oxidation and reduction, respectively, to the normal hydrogen electrode (NHE), assuming the IP of Fc/Fc+ to be 4.80 eV. Overall, the IPs are dictated by the DPP core and are between 5.2-5.3 eV. For compounds 1 and 2, the EAs (ca. 3.7 eV) are dictated by the PDI units. For compound 3 containing only BTXI terminal groups with no PDI, the EA is dictated by DPP with some influence from BTXI, which results in a large decrease in the EA from 3.7 to 3.3 eV suggesting that BTXI is significantly more electron rich than PDI.


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Figure 3. A) Normalized cyclic voltammograms of compounds 1-3 obtained in solution. B) Corresponding electrochemically determined energy levels (HOMO and LUMO).

Table 1. Tabulated electrochemical data. Compound

Oxonset

IP

Ox E1/2

Redonset

EA

Eg Red E1/2 (V)

(V)

(eV)

(V)

(V)

(eV)

1

0.5

5.3

0.55, 0.83

-1.1

3.7

-1.22, -1.48, -1.85

1.6

2

0.5

5.3

0.57, 0.80

-1.1

3.7

-1.19, -1.41, -1.65, -1.77

1.6

3

0.4

5.2

0.49, 0.70

-1.5

3.3

-1.59, -1.84

1.9

(eV)

The assignment of the IP and EA of compounds 1 and 2 being a function of the DPP and PDI units, respectively, is confirmed by evaluation of the frontier molecular orbitals as determined by DFT calculations at the OT-ωB97X-D/6-31g(d,p) [OT = optimally tuned] level of theory43–45 (see SI for further computational details, where the gap-tuned ω values46–49 are also provided). The highest-occupied molecular orbitals (HOMO: 1, –6.45 eV, and 2, –6.33 eV) for 1 and 2 reside on the DPP moiety, while the lowest-unoccupied molecular orbitals (LUMO; 1, –2.06 eV,

and

2,

–2.03 eV) are localized on the PDI units (Figure 4). For 3, the assignment of IP and EA being a


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function of DPP with some influence from BTXI is also confirmed through evaluation of the frontier molecular orbitals. Here the HOMO (–6.19 eV) is mainly localized on the DPP core, though it does extend onto the BTXI sulfur atoms, while the LUMO (–1.50 eV) is delocalized across the DPP and BTXI π-conjugated framework (Figure 4); note that though there is more π electron delocalization of the LUMO of 3 when compared to 1 and 2, the stronger reducing power of PDI results in a more energetically stabilized LUMO when it is part of the full molecular construct. Overall, the frontier molecular orbital energies follow the redox potential trends. Further, adiabatic IP and EA (AIP and AEA, respectively) determined at the OT-ωB97X-D/631g(d,p) level of theory correspond well with experiment. The subsequent replacement of PDI with BTXI results in an energetic destabilization of both the AIP (6.18 eV for 1, 6.05 eV for 2, and 5.94 eV for 3) and AEA (–2.21 eV for 1, –2.19 eV for 2, and –1.67 eV for 3). Finally, we also note that the intramolecular reorganization energies for hole (0.43 eV for 1, 0.48 eV for 2, and 0.49 eV for 3) and electron (0.38 eV for 1, 0.39 eV for 2, and 0.31 eV for 3) transport are rather considerable in these systems; for the acenes, for instance, the reorganization energies are typically of the order of 0.10 eV.50 These large reorganization energies, which constitute the sum of the geometric relaxation processes that molecules undergo during the oxidation and reduction events that occur as charges move through the molecular materials, stem in part from considerable changes in the dihedral angles between the DPP and PDI or BTXI when the molecules are oxidized or reduced.


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Figure 4. Pictorial representations of select frontier molecular orbitals of 1, 2, and 3 as determined at the OT-ωB97X-D/6-31g(d,p) level of theory.

Solution and thin film optical absorption spectra for 1-3 can be seen in Figure 5 with tabulated data in Table 2. For compound 1 the optical absorption spectrum is dominated by PDI absorption with a strong peak at ~530 nm and slightly weaker higher energy peak at ~500 nm; a low energy tail extends to ~700 nm. Compound 2 has a similar profile but with minor differences. First, the PDI absorption band has lower intensity and a lower energy peak at ~600 nm has emerged while a small peak at ~400 nm is also evident. For compound 3, in the absence of PDI, the low energy peak centered at ~600 nm is more defined and higher in intensity, while a second band of equal intensity is seen at ~480 nm. Additionally, the peak at ~400 nm is more intense. Upon transitioning from solution to thin-film, an overall broadening of the absorption profile for each compound is observed, accompanied by significant changes in band shape or position. Thin film UV-Vis spectra of the individual components (PDI, DPP, BTXI) of compounds 1-3 can be viewed in Figure S8 and assist in the assignment of the optical transitions. For compounds 1 and 2 the optical absorption spectra has a major contribution from the PDI chromophore, but with 2 the DPP core has some contribution. With compound 3, the optical profile is almost a simple combination of the DPP and BTXI building blocks. Overall, these features imply


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that there is minimal donor-acceptor charge transfer character within these compounds. For the PDI-DPP moiety the PDI is the acceptor and DPP is the donor, but steric strain prevents any significant charge transfer and thus no strong low-energy bands are observed. This is confirmed with a previous report where we installed acetylene units in between the PDI and DPP chromophores to relieve the steric strain and observed the emergence of a strong low-energy donoracceptor charge transfer band.51 For the BTXI-DPP moiety, there is less steric strain between the two units rendering the molecule more co-planar. However, the lack of any significant low energy donor-acceptor band implies BTXI is neither a stronger acceptor nor donor relative to DPP.

Figure 5. A) Solution absorption of 1-3 (CHCl3). B) Thin film absorption of 1-3 (Films spun from 100 µL of 1 % w/v CHCl3 solution at 1500 RPM, 10,000 RPM/s, 30 sec. onto 2 x 2 cm glass substrates).


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Table 2. Tabulated optical absorption data. Solution Data

Thin Film Data

λmax

λon

Eg(opt)

ε(λmax)

λmax

λon

Eg(opt)

(nm)

(nm)

(eV)

(M-1cm-1)

(nm)

(nm)

(eV)

1

534

660

1.9

121,352

534

775

1.6

2

534

660

1.9

94,136

538

756

1.6

3

414, 487, 584

650

1.9

62,883

427, 487, 600

731

1.7

Compound

To assist in the understanding of the optical characteristics, the molecular electronic transitions were examined via time-dependent DFT (TD-DFT) calculations at the OT-ωB97X-D/631g(d,p) level of theory. Simulated absorption spectra derived from the TD-DFT calculations agree reasonably well with experiment (Figure S12); it should be noted that while the TD-DFT calculations reproduce the experimental trends in terms of transition energies, the relative intensities of the DPP-based and PDI-based transitions are inconsistent. For 1 and 2, the S0S1 transitions (both at 2.45 eV; 506 nm) are centralized on the DPP core, which is a function of the highly twisted nature of the PDI moieties with respect to the DPP; in each case, the transitions mainly involve DPP-localized orbitals (HOMOLUMO+2 for 1, and HOMOLUMO+1 for 2). These DPP-centric transitions for 1 and 2 are confirmed by natural transition orbital (NTO) analyses, where the hole and electron wavefunctions are located on the DPP moiety (Figure 6). The next transitions with significant oscillator strength – S0S4 for 1 (2.75 eV; 450 nm) and S0S3 for 2 (2.75 eV; 450 nm) – are PDI localized, as again demonstrated by the NTO (see SI). For 3, though it is generally more planar and the LUMO extends partly into the BTXI, the hole and electron NTO for the S0S1 transition (HOMOLUMO) are DPP centered. Indeed, even for the higherlying excited states (see NTO in the SI) of 3, there is more DPP character associated with each


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transition. This result confirms the idea that BTXI is neither a strong donor or acceptor with respect to DPP.

Figure 6. Pictorial representations of natural transition orbitals (NTO) of the S0S1 of compound 1, 2 and 3 as determined at the TD-OT-ωB97X-D/6-31g(d,p) level of theory. λ is the fraction of the hole-electron contribution to the excitation.

Organic Thin-Film Transistors We further investigated the impact of BTXI for PDI substitution on materials electronic properties by determining the charge-carrier mobility of compounds 1-3 using OTFTs (Figure 7, Table 3). Bottom-gate bottom-contact (BGBC) OTFTs were prepared in air using pre-fabricated Fraunhofer wafers. Each chip contained four different channel lengths (2.5, 5, 10 and 20 µm), all of which have a channel width of 2000 µm, allowing for four different W/L ratios to be investigated. Semiconducting layers of 1-3 were prepared by solution drop-casting 0.5 µL into the device channel. Two different solvents, CHCl3 and o-dichlorobenzene (oDCB), as well as two dif-


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ferent concentrations (0.5 and 1 mg/mL) were investigated. The effect of surface treatment with octyltrichlorosilane (OTS), annealing the devices at 150 °C, and environmental conditions during testing (vacuum or air) were also investigated. Full optimization details can be found in the supporting information. Compound 1 resulted in electron transporting devices when characterized under vacuum (pressure less than 0.1 Pa), which was to be expected given the presence of two PDI units. Output curves displayed typical linear-saturation behavior (Figure 7A and D), and OTFTs prepared using this compound consistently had higher mobilities and Ion/off ratios compared to devices prepared from compounds 2 and 3. Device performance increased after annealing, with a maximum mobility of 3.23 x 10-4 cm2/Vs achieved (for a channel length of 10 µm). When the devices were exposed to air and characterized, transistor performance began to drop off rapidly, with the devices barely working after a few hours. Replacing one PDI unit with BTXI had a drastic impact on OTFT device performance. Devices prepared from compound 2 performed poorly compared to the other two compounds (Figure 7B and E); the mobilities were one-to-two orders of magnitude lower than those for devices prepared from 1 and 3, with the maximum mobility only reaching 8.63 x 10-6 cm2/Vs (at a channel length of 2.5 µm). When characterized under vacuum, compound 2 resulted in electron transporting devices, but when exposed to air, the electron transporting behavior was completely suppressed, and weak hole transporting performance was observed. However, these holetransporting devices performed worse compared to the electron-transporting devices characterized under vacuum. Only a minor increase in mobility was observed after annealing at 150 °C, but interestingly weak ambipolar behavior was observed in the 2.5 and 5 µm channel devices


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when characterized under vacuum. The electron transporting capability was still dominant compared to hole transport. Substituting both PDI units with BTXI resulted in a shift to predominantly hole transporting OTFTs (Figure 7C and F). Compound 3 produced hole transport devices in both air and vacuum, with the transistors performing slightly better when characterized in air. After annealing at 150 °C, the devices became ambipolar, with both electron and hole transport behavior observed at all channel lengths when characterized under vacuum. The electron transport behavior degraded rapidly when characterized in air; however, only moderate changes were observed for the hole transporting devices.

Figure 7. Example OTFT output and transfer curves for compound 1 (A and D), 2 (B and E) and 3 (C and F). All data for annealed samples (channel length = 5 µm), characterized under vacuum, prepared from 1 mg/mL solution in oDCB.


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Table 3. OTFT device data for devices prepared from 1 mg/mL oDCB (characterized under vacuum) at a channel length of 5 µm, before and after annealing at 150 °C. Compounds 2 and 3 were ambipolar after annealing. Active Compound

Layer*

Charge

†

†

Maximum µ

Average µ

Average VT

†

Ion/off Transport

(cm /Vs) (x10 )

(cm /Vs) (x10 )

(V)

2

-5

2

-5

1

as-cast

electron

2.40 ± 0.27

2.82

21.9 ± 1.3

102-103

2

as-cast

electron

0.158 ± 0.0072

0.172

21.5 ± 0.99

102

3

as-cast

hole

0.332 ± 0.031

0.439

-17.7 ± 1.3

10-102

1

annealed

electron

13.3 ± 1.3

28.2

18.6 ± 0.68

103-104

electron

0.172 ± 0.0051

0.194

17.3 ± 2.8

10-102

hole

0.0973 ± 0.016

0.160

-35.9 ± 1.3

10

electron

1.14 ± 0.079

1.44

29.6 ± 0.49

102

hole

1.12 ± 0.087

1.40

-20.8 ± 0.59

10-102

2

3

annealed

annealed

*“annealed” = OTFT active layers measured after being thermally annealed at 150 °C for 1 hour. †

µ = charge mobility (either electron or hole transporting), VT = threshold voltage

Organic Solar Cells Next, the structure-property relationships of the three materials in this series were correlated with their performance in OSCs. Considering the results of the mobility measurements, each material was investigated for its performance both as an acceptor with the polymer donor PTB7-Th and as a donor with the fullerene acceptor PC60BM. Devices were fabricated in a straightforward approach following an air-processed and air-tested protocol utilizing the inverted device architecture: ITO/ZnO/BHJ/MoOx/Ag (Figure 8) following previously optimized conditions, solutions of the active layer materials were prepared at 1.0 % wt/v in CHCl3.21,31


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Figure 8. Device architecture and energy levels of active layer materials. Energy levels estimated from solution cyclic voltammetry measurements (black) or from the onset of absorption (white) by subtracting from the ionization potential determined by cyclic voltammetry.

Focusing first on devices utilizing 1-3 as acceptors (Figure 9 and Table 4), two donor:acceptor blend ratios were screened, 1:1.5 and 1.5:1. All three materials saw high opencircuit voltages (VOC) realized (> 1 V) with 3 being the highest at 1.1 V, which correlates with its lower EA value (3.3 eV versus 3.7 eV). With high VOC values and similar fill factors (FF), it was the short-circuit current (JSC) that had the most influence on device performance. In accordance with the mobility results, JSC values decreased with increasing BTXI content in the molecular structure. Compound 1, with flanking PDI units responds with a JSC of over 4 mA·cm-2, which is reduced by half for the asymmetric compound 2 and further reduced to less than 0.2 mA·cm-2 for compound 3 with no PDI content. By switching from an acceptor heavy blend (1:1.5) to a donor heavy one (1.5:1) JSC values did improve for 3, now not needing to carry as much of the charge transport burden; however, for 1 and 2, JSC values decreased highlighting the efficiency of the PDI unit for charge transport (Figure S23, Table 4).


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Voltage (V) 0.4

Voltage (V)

0.6

0.8

1.0

1.2

-0.4

-0.2

-2

-6

-10

0.0

0.2

0.4

-10

1.2

0.3 0.2 0.1 0 700

Wavelength (nm)

800

0.4

0.6

900

1.0

1.2

-2

-6

-10

PTB7-Th:3 As-cast PC₆₀ BM:3 As-cast

0.6

PTB7-Th:2 As-cast PTB7-Th:2 SVA

0.5

0.8

PTB7-Th:3 SVA

PTB7-Th:3 As-cast PTB7-Th:3 SVA PC₆₀ ₆₀BM:3 As-cast ₆₀

0.5

PC₆₀ ₆₀BM:2 As-cast ₆₀ 0.4 0.3 0.2

0.4 0.3 0.2 0.1

0 600

0.2

-14

0.1

500

0.0

PC₆₀ BM:2 As-cast

0.6

Film Absorbance

PC₆₀ ₆₀BM:1 As-cast ₆₀ 0.4

400

-0.2

PTB7-Th:2 As-cast

-14

PTB7-Th:1 As-cast PTB7-Th:1 SVA

300

-0.4

PTB7-Th:2 SVA

PC₆₀ BM:1 As-cast

0.5

1.0

-6

PTB7-Th:1 SVA

0.6

0.8

-2

PTB7-Th:1 As-cast

-14

Voltage (V)

0.6

Current (mA cm-2)

0.2

Film Absorbance

0.0

Current (mA cm-2)

-0.2

Current (mA cm-2)

-0.4

Film Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 300

400

500

600

700

800

900

Wavelength (nm)

300

400

500

600

700

800

900

Wavelength (nm)

Figure 9. Current-voltage curves and UV-Vis absorption profiles for OSC devices utilizing 1-3 as acceptors with PTB7-Th in a 1:1.5 ratio (solid) or as a donor with PC60BM in a 1:3 ratio (dashed). Compound 1 (left, green), compound 2 (middle, blue), compound 3 (right, red).

As recently reported, thin-films of compound 1 change upon being exposed to solvent vapor.31 This post-film deposition solvent vapor annealing (SVA) induces structural rearrangement forming a more ordered nanostructure that was found to greatly enhance solar cell performance.52,53 This change was easily monitored through UV-Vis absorption spectroscopy with the emergence of a new, well defined peak at 586 nm.31 Thus we explored the impact of SVA on thin-films of each compound to see could induce a morphology change that would impact OSC performance. We have recently identified that tetrahydrofuran (THF) is an excellent annealing solvent to favorably alter the thin-film structure of 1, and again such changes were detected by optical absorption spectroscopy54, so the same protocol was followed in this study. Thin-films of com-


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pounds 1-3 were exposed to THF vapour for varying times, the optical absorption spectra are shown in Figure S10. SVA treatment of thin-films of 1 results in a sharpening of the absorption onset and the appearance of a new electronic transition at 586 nm. These changes occur within 10 minutes of solvent exposure. For thin-films of 2, a significant increase in the low energy absorption band at 586 nm is observed. This change reached a maximum after 60 minutes of solvent treatment. Upon SVA treatment of thin-films of 3, a red shift of ~50 nm in the absorption onset is observed. Appearance of fine structure across the absorption profile is also notable, especially the low energy peak centered at 600 nm, which increases in intensity and broadens into three more well-defined transitions. Such changes are typical for DPP containing molecules.55–57 Ultimately, the replacement of PDI with BTXI does not inhibit the response observed by UV-Vis spectroscopy upon SVA treatment, further suggesting that the DPP core plays a significant role in the molecular reorganization responsible for the changes observed in the absorption profile upon SVA. To investigate the relationship between possible changes in molecular structure and optical characteristics with SVA, further TD-DFT calculations (at the OT-ωB97X-D/6-31g(d,p) level of theory) were carried out as a function of the degree of twist within the molecular structure (among the DPP and rylene-based substituents) and the potential to form tightly packed dimers with co-planar structures (see SI for further details).42 For 1, a more co-planar conformation, though higher in energy, leads to a significant red-shift of the S0S1 transition (by ~0.4 eV). For 2 and 3, however, the already more co-planar conformation between the DPP and BTXI moieties in the optimized structure limits changes in S0S1 transition energies if fully planar configurations are considered. Instead, the potential to form closer contacts among the π systems of neighboring, co-planar molecules can lead to the appearance of lower-energy transitions (by ~0.2 eV)


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in 2 and 3. In each case, the low-lying transitions with appreciable oscillator strength reveal some charge-transfer-like character from the DPP to the respective rylene-based substituents. All as-cast devices were subject to SVA based on the observed re-organization in the absorption profiles of the neat materials. When blended with PTB7-Th, changes in the absorption profiles of both symmetric materials (1 and 3) were observed; however, no significant difference in the as-cast blend was observed with 2 even after up to 60 minutes of THF SVA exposure (Figure 9, Figure S22). Despite this lack of distinct changes in the optical absorption spectrum for 2, the as-cast device performance of all three materials was improved upon SVA treatment. The post-SVA improvement in performance was most influential on the JSC, which naturally led to the most significant increases observed for 1 with a near 3-fold increase in current. Both 2 and 3 were met with respectable increases in JSC, approximately double that of their as-cast performance. These increases can be visualized in the current-voltage curves of the devices and the external quantum efficiencies (EQE) where significant photocurrent generation increases are observed upon SVA (Figure 9, Figure S23). For the same reasons as discussed for as-cast devices, increases in JSC and PCE were further enhanced for 3 post-SVA when switching to donor heavy blends (1.5:1), while the improvement was diminished for 1 and 2 when compared to acceptor heavy blends (1:1.5) (Figure S23, Table 4). With similar as-cast and post-SVA roughness (Figure S26 and S27) measured by atomic force microscopy (AFM) we suggest that charge transport within the individual acceptors rather than morphology is what distinguishes the higher performance for 1 over 2 and 3.


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Table 4. OSC device results for 1-3 used as an acceptor with PTB7-Th or as a donor with PC60BM. Active layers cast from 1.0 w/v % solutions in CHCl3 and annealed with THF solvent vapour where indicated. Donor

Acceptor

Ratio Processing

VOC

JSC (mAcm-

FF

(V)

2)

(%)

PCE (%)

PTB7-Th

1

1:1.5

As-cast

1.03

4.41

27.5

1.3

PTB7-Th

1

1:1.5

SVA – 10 min

0.98

12.48

43.1

5.3

PTB7-Th

1

1.5:1

As-cast

0.99

4.43

28.5

1.3

PTB7-Th

1

1.5:1

SVA – 10 min

0.98

11.23

46.4

5.1

1

PC60BM

1:3

As-cast

0.28

0.81

45.3

0.1

PTB7-Th

2

1:1.5

As-cast

1.01

2.08

26.8

0.6

PTB7-Th

2

1:1.5

SVA – 10 min

0.95

3.78

31.3

1.1

PTB7-Th

2

1.5:1

As-cast

1.01

2.20

26.4

0.6

PTB7-Th

2

1.5:1

SVA – 10 min

0.94

2.93

30.6

0.8

2

PC60BM

1:3

As-cast

0.36

1.78

43.3

0.3

PTB7-Th

3

1:1.5

As-cast

1.12

0.17

26.7

0.1

PTB7-Th

3

1:1.5

SVA – 5 min

1.09

0.43

25.0

0.1

PTB7-Th

3

1.5:1

As-cast

1.10

0.31

27.2

0.1

PTB7-Th

3

1.5:1

SVA – 5 min

1.08

0.70

23.9

0.2

3

PC60BM

1:3

As-cast

0.40

2.92

44.0

0.5

Considering these results, and the propensity for the compounds containing BTXI moieties to move holes over electrons, we screened all three compounds as donors with PC60BM to see if the trend would reverse. Utilizing active layer processing conditions from previous BTXI


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work,29 1.0 % w/v solutions in CHCl3 in a 1:3 donor:acceptor ratio, devices were fabricated in the same inverted architecture used to investigate this series of materials as acceptors (Figure 9, Figure S24, Table 4). As expected, as-cast devices reversed the trend in JSC, where 3, with the highest hole mobility, led to the highest JSC values and 1, with negligible hole mobility measured the lowest JSC values in the series. In comparing the use of 1-3 as donors with PC60BM with their results as acceptors with PTB7-Th (Figure 10) served to complement the electron and hole mobilities measured in OTFTs and identified key structure-property relationships within this molecular framework, and the BTXI building block. In this design, the alternative rylene diimide BTXI serves more effectively as an electron donating component rather than a PDI alternative that can impart strong electron withdrawing properties.

Figure 10. Comparison of PCEs of compounds 1-3 when used as acceptors with PTB7-Th (blue) and as donors with PC60BM (red).

Conclusion In this contribution we have directly compared the impact of a rylene imide dye on the synthesis and materials properties of DPP molecular semiconductors. Three molecular materials with a


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dye-DPP-dye structure were studied where the dyes used are perylene diimide and N(alkyl)benzothioxanthene- 3,4-dicarboximide. All materials were synthesized via direct (hetero)arylation methods. While PDI is a well know electron deficient organic dye, the BTXI dye has yet to be fully explored as a building block in organic electronics and thus its impact on materials physical and electronic properties is not yet fully understood. Through a systematic evaluation of optical, electrochemical, and electronic properties, in addition to theoretical analysis, we have discovered that the use of BTXI not only renders the entire molecule less electron deficient, it also imparts hole transporting character. Indeed, the molecular materials using PDI units functioned best as electron transporting materials in field-effect transistors and electron acceptors in solar cell devices while molecular materials using BTXI functioned best as hole transporting materials in field-effect transistors and electron donors in solar cell devices. A mixed molecule gave intermediate characteristics. This work clearly demonstrates how subtle alteration of dye-based building blocks can have a major impact on materials properties and role in organic electronic devices. Finally, the BTXI-based dye is new to organic electronics, possess unique properties, and should prove a useful building block for the construction of next generation electronic materials where its electron-donating character is desired.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Methods/materials, experimental, characterization, computations, and device data (PDF) Corresponding Authors * [email protected], [email protected], [email protected],


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[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. FUNDING SOURCES GCW acknowledges NSERC DG (435715-2013), CFI JELF (34102) and the Canadian Research Chairs Program. BL acknowledges NSERC DG (03987-2015). This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund (CFREF). For work completed at the University of Kentucky, CR acknowledges the Department of the Navy, Office of Naval Research (ONR Award No. N00014-16-1-2985) ACKNOWLEDGMENT AJP acknowledges Alberta Innovates and the University of Calgary. SMM acknowledges NSERC, Killam Laureates, and the University of Calgary. NAR acknowledges NSERC PDF. Supercomputing resources on the Lipscomb High Performance Computing Cluster were provided by the UK Information Technology Department and Center for Computational Sciences (CCS). ABBREVIATIONS PDI, perylene diimide; NDI, naphthalene diimide; OFETs, organic field-effect transistors; OSCs, organic solar cells; BTXI, N(alkyl)benzothioxanthene 3,4-dicarboximide; DHA, direct heteroarylation; DPP, diketopyrrolopyrrole; DFT, density functional theory; OTFTs, organic thin-film transistors; IP, ionization potential; EA, electron affinity; NHE; normal hydrogen electrode; TDDFT, time-dependent density functional theory; NTO, natural transition orbital;; BGBC, bottom-gate bottom-contact; oDCB, o-dichlorobenzene; OTS, octyltrichlorosilane; ITO, indium tin


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Annealing Optimization, and BDT-Based Polymer Screening. Sol. RRL 2018, DOI: 10.1002/solr.201800143. (55) Ryan, J. W.; Matsuo, Y. Increased Efficiency in Small Molecule Organic Solar Cells Through the Use of a 56-π Electron Acceptor – Methano Indene Fullerene. Sci. Rep. 2015, 5, 8319. (56) Viterisi, A.; Gispert-Guirado, F.; Ryan, J. W.; Palomares, E. Formation of Highly Crystalline and Texturized Donor Domains in DPP(TBFu)2:PC71BM SM-BHJ Devices via Solvent Vapour Annealing: Implications for Device Function. J. Mater. Chem. 2012, 22, 15175–15182. (57) Wang, J.-L.; Wu, Z.; Miao, J.-S.; Liu, K.-K.; Chang, Z.-F.; Zhang, R.-B.; Wu, H.-B.; Cao, Y. Solution-Processed Diketopyrrolopyrrole-Containing Small-Molecule Organic Solar Cells with 7.0% Efficiency: In-Depth Investigation on the Effects of Structure Modification and Solvent Vapor Annealing. Chem. Mater. 2015, 27, 4338–4348.

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