Does the Electron Donating Polymer Design Criteria Hold True for the

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Does the Electron Donating Polymer Design Criteria Hold True for the Non-Fullerene Bulk Heterojunction Electron Acceptor Boron Subphthalocyanine? Yes. Kathleen L. Sampson, Graham Morse, and Timothy P Bender ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00183 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Energy Materials

Does the Electron Donating Polymer Design Criteria Hold True for the Non-Fullerene Bulk Heterojunction Electron Acceptor Boron Subphthalocyanine? Yes. ǁ Kathleen L. Sampson,† Graham E. Morse , and Timothy P. Bender†,§,‡,*

* to whom correspondences should be addressed. E-mail: [email protected]

† Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5. ǁ Merck Chemicals Ltd., Chilworth Technical Centre, University Parkway, SO16 7QD, Southampton, UK. E-mail: [email protected] § Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4. ‡ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6.

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Abstract The performance of boron subphthalocyanine (BsubPc) and phenyl-C61-butyric acid methyl ester (PC61BM) as electron accepting materials in bulk heterojunction organic photovoltaics (OPVs) with a range of reported literature polymers were simultaneously screened to observe if the design criteria for polymeric electron donating materials, originally designed for pairing with fullerene derivatives, can be applied to non-fullerene electron acceptors. Initially, the morphology and film formation of the BsubPc containing active layer films was improved by using a volatile additive, 1,2-dimethoxybenzene, which is known to solubilize a BsubPc but not a representative high-performing polymer (PBTZT-stat-BDTT-8). Thereafter, with the resulting semi-optimal fabrication parameters, ten polymeric electron donating materials were screened with the BsubPc derivative, PhO-Cl6BsubPc (phenoxy-hexachloro-boronsubphthalocyanine), and PC61BM. Device metrics demonstrate that the BsubPc-based devices perform in-line with their analogs utilizing PC61BM, but can be limited by polymers that have overlapping absorbance with PhO-Cl6BsubPc, low solubility, or large energy level offsets. Overall, the design aspects of electron donating polymeric materials that were considered for more than a decade can be applied to non-fullerene or, specifically, BsubPc-based electron acceptors.

Keywords Boron,

subphthalocyanine,

organic,

photovoltaics,

non-fullerene,

heterojunction.

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acceptor,

bulk,

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Introduction The current solar energy harvesting technology is dominated by inorganic-based devices that require

energy

intensive

manufacturing

processes.1

Photovoltaics

using

organic

semiconducting materials (organic photovoltaics, OPVs) are a highly researched field of study due to their potential as low-cost, large-scale, (and in some cases) solution-printable electronic devices2-4 with relatively low quantities of embodied energy (energy consumed in all processes of chemical, materials, and device production).5-6 Some examples include Kubis et al., who used slot-die coating, a type of roll-coating, solution-printing technique, to fabricate reproducible devices with over 3% efficiency.7 In 2015, semitransparent, large area, flexible solar cells, produced via roll-to-roll fabrication methods, were presented at the German pavilion, Milan EXPO by Merck Chemicals with a power conversion efficiency of 4.5%.8 Typical solution-printed OPVs most commonly consist of a conjugated polymer electron donating and light absorbing material paired with a small molecule electron acceptor and are fabricated by printing both materials simultaneously, producing an intermixed layer, referred to as a bulk heterojunction (BHJ). Research has been largely dominated by considering BHJ OPVs with fullerene derivatives as electron transporters or electron accepting materials (such as phenyl-C61-butyric acid methyl ester, PC61BM, Figure 1).9-12 But, more recent studies have been focusing on fullerene-free devices implementing so-called “non-fullerene” acceptors.13-16 While fullerene derivatives have high electron mobility in three dimensions, form ideal morphology with the polymeric electron donor materials, and have fast electron transfer, they have weak absorption in the visible region of the solar spectrum (where the maximum photon flux from the Sun occurs), are synthesized through energy intensive and low yielding chemical processes, and lack the ability to extensively tune the HOMO/LUMO Page 3 of 38



energy levels (important for charge generation between semiconducting materials).5, 12, 17 A limiting factor in achieving high power conversion efficiency for BHJ OPVs based on nonfullerene acceptors, compared to fullerene counterparts, is the control of the BHJ nano-phase morphology needed to yield well-ordered, phase separated domains of the donor and acceptor materials.14,

18-19

To overcome these issues of inadequate morphology and poor charge

generation, small molecule non-fullerene acceptors with a three-dimensional shape have enabled improvement of morphology and BHJ OPV device performance.18-19

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Figure 1. Structures of electron acceptors (PC61BM and PhO-Cl6BsubPc) and polymer electron donors screened including PTB1, PTB7, PTB7Th, PDTS-TzTz, PBDT-T-TPD, PBTZT-stat-BDTT-8, P3HT, PBDB-T, PCDTBT, and PSBTBT. The thienothiophene and benzodithiophene units are highlighted in blue and green, respectively, in the PTB7-Th structure. Page 5 of 38



Boron subphthalocyanine (BsubPc) derivatives have a unique three-dimensional bowl shape and strong absorption in the visible region of the solar spectrum, which is complementary to the absorption of many polymers and known to contribute considerably to the photocurrent of an OPV, unlike fullerene derivatives.20-21 The majority of studies with BsubPcs focus on utilizing the materials as vacuum deposited electron donor materials in OPVs,22-24 but the energy levels can be fine-tuned through derivatization allowing BsubPcs to function as electron acceptors in both vacuum and solution deposited devices.25-32 We have shown that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are more sensitive to substitutions in the peripheral positions of the BsubPc, than similar modifications to the axial position.33 Peripheral chlorination of a BsubPc, for example, was used to significantly deepen the energy levels to be comparable to fullerene.34 Conversely, the solubility of BsubPc derivatives can be significantly altered with phenoxy substitution at the axial position and other groups on the periphery.20, 35 While there has been a recent focus in literature concerning replacing the fullerene acceptor, there has yet to be a study on whether these non-fullerene acceptors have the same compatibility with the previously designed electron donating polymers. As well, it is difficult to establish this with the variety of other device fabrication conditions and structural variations explored, such as interlayers, deposition techniques, etc., that differ from one lab or study to the other. Thus, in this reported study, we further explored whether the conditions for polymer design with PCBM derivatives holds true for a BsubPc derivative as an acceptor. Inspecting the differences in device performance between polymer:fullerene and polymer:BsubPc BHJ OPVs will help guide future designing of BHJ OPV polymeric materials, discover the limits of new non-fullerene acceptors, improve overall photovoltaic

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performance, and lead research to the optimal, low-cost, solution-printable organic photovoltaic.

Results and Discussion BsubPcs have been incorporated into only two reports of solution-processed BHJ OPVs with high-performing polymer donors, with the most recent report achieving a high power conversion efficiency (PCE) of 4.0% with hexachloro-boron-subphthalocyanine

(Cl-

Cl6BsubPc, with six peripheral chlorines) paired with poly[4,8-bis(5-(2-ethylhexyl)thiophen2-yl)benzo[1,2-b:4,5-b']dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th), which is similar to the performance of some other non-fullerene acceptors.14, 29, 31, 36

Another example is its phenoxy derivative PhO-Cl6BsubPc (phenoxy-hexachloro-boron-

subphthalocyanine, Figure 1), which has deeper HOMO and LUMO energy levels compared to its non-peripherally chlorinated analog phenoxy-boron-subphthalocyanine (PhO-BsubPc, 3.3 and -5.5 eV).29, 33 The solubility of PhO-Cl6BsubPc, which has been estimated to be over 100 mg/mL, enabled its incorporation into a BHJ OPV with a PCE of 3.5 %.29 Due to this enhanced solubility, we chose PhO-Cl6BsubPc as a “prototypical” BsubPc electron acceptor for this study in order to assess the performance of a variety of polymeric materials designed to perform in BHJ OPVs with fullerene derivatives. Polymers containing electron-deficient thienothiophene and electron-rich benzodithiophene monomer units are a family of net-electron donating materials that quickly topped the list of high-performing polymers in OPVs, reaching from 6% to over 10% PCEs.37-42 These polymers are ideal due to their high planarity and charge carrier mobility as well as having synthetic handles to tune the band gap and HOMO/LUMO energy levels with various functional groups and side chains.43-44 Of the wide range of thienothiophene and Page 7 of 38



benzodithiophene

polymers,

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poly[(4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene-2,6-

diyl)(2-((dodecyloxy)carbonyl)thieno[3,4-b]thiophenediyl)]

(PTB1),40

poly[(4,8-bis-(2-

ethylhexyl)oxy)benzo[1,2-b:4,5-b’]dithiophene)-2,6-diyl)(3-fluoro-2-((2ethylhexyl)carbonyl)hieno(3,4-b)thiophenediyl)] (PTB7),45 and PTB7-Th (also called PCE10)38 were chosen for the initial phase of this study (Figure 1). In addition, the PBTZT-statBDTT-8 polymer developed by Merck Chemicals, Ltd., which contains a benzodithiophene monomer and has achieved high performance in both lab-scale and commercially viable OPV prototypes, was also initially included in the study.8 The majority of BHJ OPV device configurations based on this selection of polymers have only utilized a fullerene derivative as the electron acceptor. In the initial phase of our study, the screening of the combination of polymeric PBTZT-statBDTT-8 with PhO-Cl6BsubPc or PC61BM was performed to determine the best solvent, ratio of polymer to electron acceptor (PhO-Cl6BsubPc/PC61BM), total concentration of solids, annealing temperature and time, and type and amount of solvent additive. The parameters were based on those used by Ebenhoch et al for PhO-Cl6BsubPc.29 Dichlorobenzene and oxylene were chosen as the host solvents as PC61BM, PhO-Cl6BsubPc, PBTZT-stat-BDTT-8, and the thienothiophene- and benzodithiophene-based polymers have fairly high solubility in these solvents. While dichlorobenzene has been used previously as a solvent with these polymers as well as with PhO-Cl6BsubPc, o-xylene has been explored as a more environmentally friendly alternative to the typical chlorinated solvents for the related polymer PTB7 yielding a high PCE of 8.7 %.29, 40, 45-47 For a solvent additive, it was found that 5% 1,2-dimethoxybenzene, a solvent with a boiling point higher than that of dichlorobenzene and o-xylene and selective solubility for PhOCl6BsubPc, worked the best. This conclusion coincides with the accepted theory that the main Page 8 of 38


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solvent must dissolve both the polymer and acceptor, while the co-solvent must have a higher boiling point and selective solubility for the electron acceptor material.48-51 Therefore, the main solvent will evaporate first, causing the polymer to aggregate, leaving the BsubPc acceptor remaining in the 1,2-dimethoxybenzene. The slower deposition of the BsubPc between the polymer aggregates may improve the overall morphology and overcome the previously found issues of lower than expected current extraction from the OPV. 1,2Dimethoxybenzene has been used as co-solvent in a BHJ OPV before by Cao et al. with another thienothiophene-based polymer. They found that the low solubility the polymer in 1,2-dimethoxybenzene contributed to narrow polymer fibril widths and higher short-circuit currents (Jsc).52 Rough solubility measurements of the PhO-Cl6BsubPc and PC61BM formulations in dichlorobenzene and o-xylene with the optimal volume percent of 1,2dimethoxybenzene additive are described in the Supporting Information (page S6) and Figure S1. In addition, AFM images of films of PhO-Cl6BsubPc paired with PTB1, PTB7, and PTB7-Th polymers both with and without 1,2-dimethoxybenzene solvent additive are described in the Supporting Information (page S8 – 9) and Figure S2. BHJ OPVs where fabricated using the same conditions established from initial screening with the PBTZT-stat-BDTT-8 polymer to enable direct point-to-point comparisons: a donor to acceptor ratio of 1:1, total solids concentration of 20 mg/mL, and 30 mm/s blade coating speed. An inverted architecture was used for all devices due to preliminary results indicating higher performance, a similar trend to other studies, as well as the ease of solution depositing all layers, except the electrode.53-54 The reason for the improved performance of the inverted architecture for BsubPc electron acceptors is currently being investigated. With these fabrication methods, the PTB1, PTB7, PTB7-Th, and PBTZT-stat-BDTT-8 polymers were screened with both PhO-Cl6BsubPc and PC61BM and by also testing three Page 9 of 38



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other variables: solution depositing from o-xylene and dichlorobenzene solvents, with and without 5% 1,2-dimethoxybenzene solvent additive, and annealing at 120 °C for 5 min. The screening parameters and statistic details are described in Figure S3 of the Supporting Information. The average current density-voltage (J-V) and external quantum efficiency (EQE) characteristics for PhO-Cl6BsubPc and PC61BM devices paired with the thienothiophene-benzodithiophen-based

polymers

are

found

in

Figure

S4

for

dichlorobenzene deposited devices, Figure S5 for o-xylene deposited devices, as well as summarized in Table S1 of the Supporting Information. From the PTB1, PTB7, and PTB7-Th device results, we realized that the polymer performance trend (in terms of PCE) for PhO-Cl6BsubPc-based devices is the same as that for PC61BM, as well as what is reported in the literature: performance increases going from PTB1 to PTB7 to PTB7-Th.29, 31, 42 Moving forward to a second phase to this study, we added six other high-performing polymers known in the field (Figure 1) in order to observe if the trend in performance with PC61BM holds true for BsubPc acceptors when paired with a total of 10 polymers: PTB1,40 PTB7,45 PTB7-Th,38 PBTZT-stat-BDTT-8,8 poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′d]silole)-2,6-diyl-alt-[2,5-bis(3-tetradecylthiophen-2-yl)thiazole[5,4-d]thiazole)-1,8diyl] (PTDS-TzTz or KP115),55-56 poly[4,8-bis(5-(20ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-alt-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione] (PBDT-T-TPD),57-58 poly(3hexylthiophene-2,5-diyl)

(P3HT),59

poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-

benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))]

(PBDB-T),60-61

poly[N-9′-

heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT),62

and

poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-altPage 10 of 38


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(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT).63 The same fabrication methods and three variables described above were tested to determine both the average and the best device performance characteristics for each polymer and acceptor. These 10 polymers cover a wide range of UV-vis absorption spectra and HOMO/LUMO energy levels as observed in Figure 2, Figure 3, and Table 1. Solid state UV-Vis absorption spectra of each electron donating polymer and electron acceptor were analyzed to observe if there is a correlation between total absorption area of the donor and acceptor pairs and overall OPV performance. Optical band gaps were calculated from the onset wavelength of the lowest energy absorption band for each material. To compare the energy levels of the materials more accurately, the onset oxidation/reduction potentials or HOMO/LUMO values were taken from literature and readjusted relative to ferrocene at -5.1 eV with the following equation: .64

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Eq 1


Figure 2. Solid-state absorbance of all polymer donors screened in increasing order of optical band gap (from bottom to top: PSBTBT [brown], PTB7-Th [green], PTB1 [orange], PTB7 [dark blue], PBTZTZ-stat-BDTT-8 [purple], PBDB-T [dark red], PDTS-TzTz [yellow], PBDT-T-TPD [light red], PCDTBT [light blue], P3HT [grey], depicted with electron acceptors PC61BM [black] and PhO-Cl6BsubPc [pink]).

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Figure 3. Energy band diagram of all polymer donors screened with PC61BM and PhOCl6BsubPc acceptors. HOMO and LUMO values were recalculated using reported oxidation and reduction potential onsets from literature and readjusted relative to ferrocene with the following equation:

!"#

$%&! '% () *)

+ , &-.8, 29, 38, 40, 45, 55, 57, 60, 62-63, 65-66

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Table 1. Optical and electronic properties of electron acceptors and polymer electron donors screened. Optical band gaps were calculated from the onset wavelength of the lowest energy absorption band of solid-state films. HOMO and LUMO values were recalculated from literature using reported oxidation and reduction potential onsets and readjusting relative to ferrocene with the following equation:

!"#

$%&! '% () *)

+ , &-.

8, 29, 38, 40, 45,

55, 57, 60, 62-63, 65-66

Polymer Donor

Maximum

Onset

Optical

HOMO

LUMO

Absorbance

Absorbance

Band gap

(eV)

(eV)

(nm)

(nm)

(eV)

600

715

1.73

-5.4

-3.7

PTB1

630

780

1.59

-5.2

-3.5

PTB7

665

760

1.63

-5.5

-3.6

PTB7-Th

700

785

1.58

-5.5

-3.9

PDTS-TzTz

580

685

1.82

-5.4

-3.6

PBDT-T-TPD

615

675

1.83

-5.9

-3.9

P3HT

520

650

1.91

-5.2

-3.3

PBDB-T

630

705

1.76

-5.2

-3.1

PCDTBT

388/550

670

1.85

-5.6

-3.4

PSBTBT

680

835

1.48

-4.9

-3.1

PBTZTZ-statBDTT-8

With a total of eight bulk heterojunction device fabrication conditions tested (1,2dichlorobenzene or o-xylene main solvent, 0 % or 5 % 1,2-dimethoxybenzene solvent additive, and with or without annealing at 120 °C for 5 min), the two acceptors can be Page 14 of 38


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accurately compared across all 10 donor polymers. We compared the average results of all eight conditions tested (Figure 4, 5, 6, and Table S2, herein referred as the average) as well as the best performing device results taking into consideration the best PCE efficiency of the three variables (eight conditions) tested (Variables C – E, Figure S3 with results depicted in Figure 7 and Table 2, herein referred to as the best device). For clarity, because the fabrication conditions and device configurations were set to be the same for point to point comparison, each BHJ OPV was therefore not optimized in terms of active layer deposition, formulation, annealing conditions, device configuration, etc. As a result, performance (i.e. PCE) is likely lower than the top report in the literature associated with each polymer, particularly for the PC61BM results. Overall, our first conclusion is that PhO-Cl6BsubPc is compatible with the polymers originally designed to function with fullerenes, like PC61BM in BHJ OPVs and, therefore, the polymer design criteria, originally for fullerenes, holds true for BsubPcs and likely other non-fullerenes.

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Figure 4. a) Average fill factor (FF), b) average open-circuit voltage (VOC), c) average shortcircuit current (JSC), and d) average power conversion efficiency (PCE) of all eight conditions tested (1,2-dichlorobenzene or o-xylene main solvent, 0 % or 5 % 1,2-dimethoxybenzene solvent, with or without annealing at 120 °C for 5 min) for each electron acceptor (PC61BM in blue, PhO-Cl6BsubPc in pink) in increasing order with respect to PC61BM characteristic (FF, VOC, JSC, and PCE) and the polymer screened. The shading represents the standard deviation over the range of parameters tested.

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Figure 5. Average power conversion efficiency (of all eight conditions tested: solvent type, addition of solvent additive, and annealing) with PC61BM (blue) and PhO-Cl6BsubPc (pink) electron acceptors in increasing order of PC61BM PCE device performance for each polymer with a) average active layer thickness and b) average device active layer absorbance integrated area overlaid. b) Average short-circuit current density (of all eight conditions tested: solvent type, addition of solvent additive, and annealing) with PC61BM (blue) and PhO-Cl6BsubPc (pink) electron acceptors in increasing order of PC61BM JSC device performance for each polymer with average device active layer absorbance integrated area overlaid. The shading represents the standard deviation over the range of devices fabricated. c) Average open-circuit voltage (VOC) (of all eight conditions tested: solvent type, addition of Page 17 of 38



solvent additive, and annealing) compared to the offset between the HOMO of the electron donating polymer and the LUMO of each electron acceptor (PC61BM represented by blue open circles and PhO-Cl6BsubPc represented by pink closed circles) for each polymer electron donor screened. The error associated with the HOMO/LUMO offset is estimated at 0.3 eV, which is the largest difference between the reported HOMO/LUMO value and the recalculated value as detailed in Figure 3. The error associated with the VOC is the standard deviation of all parameters screened (solvent type, solvent additive, and annealing) for all devices fabricated with that electron donor polymer and electron acceptor.

In general, the average PCE for PC61BM devices are higher than those of the PhO-Cl6BsubPc devices due to higher average fill factors (FF) and average short-circuit current densities (JSC) for all polymers screened (Figure 4). However, the average open-circuit voltage (VOC) for PhO-Cl6BsubPc electron acceptor devices is higher for all BHJ OPVs, which is due to the shallower LUMO of PhO-Cl6BsubPc (Figure 4b). Previous studies have shown that the VOC is proportional to the offset in energy levels between the HOMO of the donor material and the LUMO of the acceptor material.25,

67-69

In particular, Cnops et al. found a linear

relationship between VOC and the interfacial energy gap between phthalocyanine-based donor materials and BsubPc acceptors with tuned LUMO levels in planar heterojunction devices.25 However, when observing the correlation between average VOC of the BHJ OPVs and the HOMOdonor - LUMOacceptor, it was found that, in general, higher interfacial energy offsets correlated to higher VOCs, but the relationship is non-linear (Figure 5d). This may be related to other studies that have shown that HOMO/LUMO energy levels of donor polymers can be dependent on the polymer orientation70-71 and is also affected by recombination and charge photogeneration efficiency.72-74 This may then indicate that many of the devices screened Page 18 of 38


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suffer from poor charge generation and/or high rates of charge recombination, perhaps not related to materials only, but the multiple device fabrication conditions (solvent type, solvent additive, and annealing) for which some conditions may produce insufficient morphologies and charge transfer orientations. In addition, the HOMO and LUMO energy levels of the electron donating polymeric materials were measured in different labs using different techniques in different studies and a non-linear correlation could be due to this inconsistency, despite our group recalculating the values using the equation listed above (Eq 1). Regarding the JSC and FF parameters, the poor carrier generation and high recombination could also be attributed to the lower JSC and FF and poor rectification of the PhO-Cl6BsubPc devices compared to the PC61BM-based devices. When ordered in increasing performance of average FF of the PC61BM devices, the average FF varies between 36 ± 8 and 61 ± 6 for PC61BM devices and 28 ± 3 and 54 ± 4 for PhO-Cl6BsubPc devices, but the increase is not in-line with average PCE (Figure 4a and Table S2). The variation in FF between the two acceptors is likely due to different morphologies and recombination mechanisms, which are unique for each donor-acceptor pair. The FF for PhO-Cl6BsubPc is also in line with the FF observed when BsubPcs have been applied as electron acceptors within planar heterojunction OPVs.22, 25-28, 32, 34, 75 When ordered in increasing average JSC of the PC61BM devices, the order of polymer donors is most similar to the order of increasing average PCE, indicating that average JSC is a large contributor to average PCE (Figure 4c and 4d). This correlation is largely due to total integrated absorption area of the device active layer and active layer thickness (Figure 5a – 5c). UV-Vis absorption spectrum and total absorption area of each device film were measured and averaged for each electron donor and electron acceptor (Figure 6, Table S2 of Supporting Information). In general, thicker films had higher integrated absorbance areas of Page 19 of 38



the device active layers, which implies more complimentary absorbing polymer and acceptor materials, more photogeneration and, thus, higher average JSC and PCE. These devices also have the darkest appearance with good film uniformity (Supporting Information, Table S3). The highest three absorption areas are devices with PTB7-Th, PDTS-TzTz, and PBTZT-statBDTT-8 polymers and are also the top performing devices (Figure 6, 7, Table 2). An exception is PDTS-TzTz when paired with PhO-Cl6BsubPc and PC61BM due to its higher band gap, overlapping absorption spectrum with PhO-Cl6BsubPc, and possible poor morphology and charge generation. Other poor performing polymers, such as PCDTBT, PSBTBT, and PBDT-T-TPD, have low integrated areas, and thus average PCE, likely due to their limited solubility.

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Figure 6. Average absorbance of device active layers for all parameters tested (solvent, solvent additive, and annealing) for each polymer screened in increasing order of integrated absorption area for PhO-Cl6BsubPc devices. Solid lines are for PhO-Cl6BsubPc devices and dashed lines are for PC61BM devices (PTB1 [orange], PCDTBT [light blue], PBDT-T-TPD [light red], PDTS-TzTz [yellow], PSBTBT [brown], P3HT [grey], PTB7 [dark blue], PBDBT [dark red], PTB7-Th [green], and PBTZT-stat-BDTT-8 [purple]).

The resulting parameters, J-V and EQE curves, for the best performing PhO-Cl6BsubPc and PC61BM acceptor devices with respect to the three variables and eight tested conditions (main solvent, vol% solvent additive, and thermal annealing) for each polymer screened are tabulated in Table 2 and illustrated in Figure 7. The semi-optimal conditions for the best performing devices are also detailed in Table 2. The best performing devices with PC61BM and PhO-Cl6BsubPc were based on PTB7-Th, PBTZT-stat-BDTT-8, and PTB7 polymers (Figure 7). The PTB7-Th, PBTZT-stat-BDTT-8, and PTB7 polymers have a combination of ideal properties, such as relatively narrow band gaps, good complimentary absorbance to the electron acceptors, favorable energetics, high solubility in dichlorobenzene and o-xylene, and limited solubility in the 1,2-dimethoxybenzene solvent additive. The performance variance between cells on each device is largely due to poor morphology and film formation characteristics. Higher reproducibility with blade-coating can be achieved by optimizing the coating temperature.76 PBDB-T is a high performing polymer with PC61BM, but performs poorly with PhO-Cl6BsubPc. This is likely due to PBDB-T having overlapping absorbance with PhO-Cl6BsubPc and the largest LUMO offset (0.7 eV) between the polymer and PhOCl6BsubPc acceptor. A large LUMO offset is known to decrease overall PCE as most of the exciton energy is lost during charge separation.77 Page 21 of 38



Figure 7. J-V curves and EQE spectrum of screened polymers in increasing order of best device power conversion efficiency of PC61BM devices (PCDTBT light blue, PTB1 orange, PBDT-T-TPD light red, PSBTBT brown, PDTS-TzTz yellow, P3HT grey, PBDB-T dark red, PTB7 dark blue, PTB7-Th green, and PBTZT-stat-BDTT-8 purple) with PhO-Cl6BsubPc (closed squares) and PC61BM (open circles) electron acceptors. The results depicted are the best average devices considering the best of the three variables and eight conditions tested for each polymer and acceptor: main solvent, volume % solvent additive, and thermal annealing.

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Table 2. Summary of the best performing devices considering the optimal of the eight conditions tested (1,2-dichlorobenzene or o-xylene main solvent, 0 or 5 vol% 1,2-dimethoxybenzene solvent additive, and with or without thermal annealing) for all polymers screened with PhOCl6BsubPc and PC61BM acceptors. Two thickness measurements were made for each device and averaged. PhO-Cl6BsubPc Acceptor Donor PTB1 PTB7 PTB7-Th PDTS-TzTz PBDT-T-TPD PBTZTZ-stat-BDTT-8 P3HT PBDB-T PCDTBT PSBTBT PC61BM Acceptor PTB1 PTB7 PTB7-Th PDTS-TzTz PBDT-T-TPD PBTZTZ-stat-BDTT-8 P3HT PBDB-T PCDTBT PSBTBT

PCE (%) 0.7 ± 0.1 2.6 ± 0.2 3.8 ± 0.4 1.8 ± 0.3 0.4 ± 0.1 3.3 ± 0.3 1.5 ± 0.1 1.2 ± 0.1 0.5 ± 0.0 1.0 ± 0.0 1.1 ± 0.1 3.7 ± 0.2 4.8 ± 0.1 2.4 ± 0.2 1.8 ± 0.2 6.1 ± 0.3 2.5 ± 0.4 3.7 ± 0.1 1.0 ± 0.1 1.9 ± 0.1

Device Parameters JSC (mA/cm2) VOC (mV) FF (%) 2.7 ± 0.2 646 ± 5 43 ± 1 7.1 ± 0.4 911 ± 4 40 ± 1 10.0 ± 0.5 905 ± 5 42 ± 2 5.9 ± 0.7 786 ± 24 38 ± 2 0.9 ± 0.1 960 ± 0 46 ± 1 7.7 ± 0.4 926 ± 5 46 ± 2 4.2 ± 0.3 639 ± 4 57 ± 1 3.4 ± 0.1 924 ± 8 39 ± 1 1.5 ± 0.1 960 ± 0 33 ± 0 3.7 ± 0.2 801 ± 6 35 ± 0 5.9 ± 0.3 11.2 ± 0.6 13.6 ± 0.3 8.2 ± 0.2 3.5 ± 0.3 11.0 ± 0.6 7.5 ± 0.3 11.4 ± 0.3 2.9 ± 0.3 7.0 ± 0.2

453 ± 5 758 ± 7 758 ± 8 605 ± 12 853 ± 14 821 ± 4 551 ± 17 756 ± 5 838 ± 8 589 ± 4

42 ± 3 44 ± 1 47 ± 2 47 ± 2 59 ± 1 67 ± 2 60 ± 9 43 ± 0 41 ± 1 47 ± 1

Thickness (nm) 154 213 169 185 22 148 147 236 41 110

Solvent dichlorobenzene dichlorobenzene dichlorobenzene o-xylene o-xylene dichlorobenzene dichlorobenzene dichlorobenzene o-xylene dichlorobenzene

Optimal Conditions Vol% Additive (%) 5 5 5 5 5 5 5 5 0 5

Thermal Annealing none 120 °C 120 °C 120 °C 120 °C 120 °C 120 °C 120 °C 120 °C 120 °C

126.67 175 167 141 57 115 130 211 41 99

dichlorobenzene dichlorobenzene o-xylene dichlorobenzene o-xylene o-xylene dichlorobenzene dichlorobenzene o-xylene dichlorobenzene

5 5 5 0 0 5 5 5 5 5

none none none 120 °C none 120 °C 120 °C none 120 °C none

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The lower light harvesting efficiency and charge carrier generation, and thus lower JSC, is also evident when observing the changes in EQE spectra between PhO-Cl6BsubPc and PC61BM electron acceptor devices for each electron donating polymer screened (Figure 7). While the BsubPc contributes to an increased EQE in the 500 to 650 nm region, there is a lack of light harvesting and charge generation in the 350 to 450 nm region. Further work is needed in optimizing the morphology to limit recombination and finding a BsubPc derivative with more complimentary absorption to the polymers. The majority of best performing PhO-Cl6BsubPc acceptor devices had higher average PCEs with 5 min of annealing at 120 °C after initial testing, except for the devices based on PTB1 (Figure 7, Table 2, and Variable E, Figure S3, Supporting Information). The same is not true for PC61BM acceptor devices, as only four polymers had improved performance with annealing: PDTS-TzTz, PBTZTZ-stat-BDTT-8, P3HT, and PCDTBT. As well, the 5 vol% 1,2-dimethoxybenzene solvent additive (Variable D, Figure S3, Supporting Information) resulted in enhanced PCEs for all polymers paired with PhO-Cl6BsubPc except PCDTBT. This polymer may be more soluble in the 1,2-dimethoxybenzene than the others and, therefore, the additive does not contribute to a more optimal morphology. For the PC61BM devices, the solvent additive also functioned to ameliorate the morphology and performance for the majority of the polymers (except for PDTS-TzTz and PBT-T-TPD). By altering the annealing time and temperature as well as the solvent additive type and volume percent, the performance could be adapted to reflect literature values, but this is not necessary for our comparison study. The majority of devices with the PhO-Cl6BsubPc acceptor performed better with dichlorobenzene, except for PDTS-TzTz, PBT-T-TPD, and PCDTBT, the least soluble Page 24 of 38


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polymers (Variable C, Figure S3, Supporting Information). On the other hand, four of the ten devices paired with PC61BM were better with the o-xylene solvent system (PTB7-Th, PBT-TTPD, PBTZTZ-stat-BDTT-8, and PCDTBT). However, the power conversion efficiencies were often very close between the o-xylene and dichlorobenzene fabricated films for these polymers indicating either solvent is likely a good system for the optimal morphology of PC61BM and the polymer. Finally, we took the best-performing polymer (PTB7-Th) with PhO-Cl6BsubPc and did a further level of semi-optimization by changing the donor:acceptor ratio, total solids concentration, volume percent of solvent additive, blade coating speed, and annealing time. By changing the total solids concentration to 30 mg/mL, ratio of donor to acceptor to 1:2, and producing a thinner film with a slower blade coating speed, the average efficiency improved to 4.42 ± 0.29 % (10.6 ± 0.4 mA/cm2 JSC, 910 ± 0 mV VOC, 46 ± 1 % fill factor), an increase of ~16 %, with a maximum cell efficiency of 4.83 % (Supporting Information, Figure S4 and Table S1). A higher JSC is the largest contributor to increased performance, but the VOC and fill factor (FF) also increased slightly. Further morphology optimization for the PhOCl6BsubPc-containing devices may be achieved with blade coating and annealing temperature. It is known that the film drying conditions, such as temperature and gas flow, determine the morphology once the film is blade-coated. As well, devices coated at lower temperatures, regardless of film thickness, tend to result in lower efficiencies and more variable performance due to non-ideal film formation.76

Conclusion In this study, we solution-processed bulk heterojunctions with PhO-Cl6BsubPc and PC61BM acceptors with 10 high-caliber electron donating polymers under the same processing Page 25 of 38



conditions. The performance of the PhO-Cl6BsubPc acceptor devices in-line with the PC61BM devices confirms that the polymeric electron donors originally designed over the last few decades to function with fullerene acceptors are suitable for application and integration with BsubPc-based electron acceptors. However, the choice of polymer to pair with a BsubPc is important as those with low solubility, high LUMO offsets, and large overlap in absorption the BsubPc acceptor suffered from poor performance characteristics. Overall, the performance of bulk heterojunctions with PhO-Cl6BsubPc as the acceptor was improved with the use of a high-performing benzodithiophene- and thienothiophene-based polymer (PTB7-Th) and with 1,2-dimethoxybenzene as a solvent additive that is BsubPcsoluble and polymer-insoluble. Both the polymer and solvent additive functioned to enhance the active layer film morphology. Even after achieving a PCE of 4.83 %, this study shows there are still limitations to PhOCl6BsubPc as an acceptor in solution-processed bulk heterojunctions as seen from the poor rectification of the J-V curves and the discrepancy between the absorption of the active layer films and the actual extracted current. But, the color tunability of the photoactive films with the uniquely purple colored PhO-Cl6BsubPc, the larger coverage of the absorption spectrum, and the ease of synthesis of PhO-Cl6BsubPc is desirable compared to fullerenes. Each device in this study was not fully optimized for the range of polymers and further exploration of the charge carrier loss mechanisms and device optimization is needed along with the continuing consideration of BsubPc molecular design. However, this study represents a valuable insight into the direction of design criteria for the leading polymers in literature and that the considerations for polymer design hold true for non-fullerene acceptors, like PhO-Cl6BsubPc. Future work should include the selection of one polymer and varying the BsubPc acceptor

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derivative to observe the trend in structure-property relationship (similar to the Cnops et al. study)25 as well as optimization of the device fabrication methods. Supporting Information The Supporting Information is available and contains the Experimental Section with materials, synthesis, and device fabrication and characterization information; solubility measurements; atomic force microscopy (AFM) of thienothiophene- and benzodithiophenebased polymer electron donor materials paired with PhO-Cl6BsubPc electron acceptor; device screening process conditions; J-V and EQE curves and results of thienothiophene- and benzodithiophene-based devices; average device parameters of all eight conditions tested; and device film images. Acknowledgements This work was supported by a Natural Sciences and Engineering Research Council (NSERC) Alexander Graham Bell Canada Graduate Scholarship (Doctoral Level) and NSERC Canadian Graduate Scholarships - Michael Smith Foreign Study Supplements (CGS-MSFSS) to K.L.S. Support was also received through the NSERC via a Discovery Grant to T.P.B.

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TOC Figure 35x15mm (300 x 300 DPI)



Figure 1 r1 522x301mm (300 x 300 DPI)


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Figure 2 r1 132x122mm (300 x 300 DPI)



Figure 3 r1 239x169mm (100 x 100 DPI)


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Figure 4 r1 176x160mm (300 x 300 DPI)



Figure 7 r1 184x154mm (300 x 300 DPI)


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Does the Electron Donating Polymer Design Criteria Hold True for the

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