Poly(naphthalene diimide-alt-bithiophene) Prepared by Direct (Hetero

Article Cite This: Chem. Mater. 2018, 30, 5353−5361

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Poly(naphthalene diimide-alt-bithiophene) Prepared by Direct (Hetero)arylation Polymerization for Efficient All-Polymer Solar Cells Ameĺ ie Robitaille,†,‡ Samson A. Jenekhe,*,‡ and Mario Leclerc*,† †

Département de Chimie, Université Laval, Québec City, Québec G1V 0A6, Canada Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1750, United States

‡

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S Supporting Information *

ABSTRACT: A high molecular weight alternating naphthalene diimide−bithiophene copolymer (poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt5,5′-(2,2′-bithiophene)} (PNDIOD-T2)) has been prepared by direct (hetero)arylation polymerization (DHAP). Its structure and properties were investigated in comparison with the Stille-prepared commercial analog, N2200. It was found that the new DHAP-derived PNDIOD-T2 and known N2200 have similar optical absorption spectra and HOMO/ LUMO energy levels but slightly different 1H NMR spectra. Inverted all-polymer solar cells using PNDIOD-T2 as the acceptor and PCE12 or PCE10 as the donor polymer showed slightly superior performances compared to similar N2200 devices. All-polymer solar cells with 7.3% efficiency could be achieved with PNDIOD-T2 blends without the use of a processing additive. These results demonstrate that low-cost and eco-friendly DHAP-based n-type semiconducting polymers are promising for developing high performance all-polymer solar cells.

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INTRODUCTION Over the past few years, extensive research has been carried out on the development of new π-conjugated polymers. Organic conductors and semiconductors have advantages over their inorganic counterparts such as easily modulated optoelectronic properties and solubility in common solvents as well as the mechanical properties of plastics. This synergy enables one possibility to develop efficient, flexible, and lightweight devices using low-cost, simple, and large-area roll-to-roll printing techniques for the fabrication of many organic electronics devices such as solar cells,1−5 field-effect transistors,6−8 and Liion batteries,9,10 just to name a few. However, some drawbacks remain in most organic electronic devices, mainly due to the synthetic procedures used to obtain such polymers.11,12 Stateof-the-art methods for synthesis of π-conjugated alternating copolymers (e.g., Migita-Stille and Suzuki-Miyaura couplings) are far from being optimal. Indeed, the synthetic pathways for the preparation of the monomers require numerous synthetic steps, and they also call for expensive organometallics reagents, which are, most of the time, unstable, hard to purify, and even toxic in the case of organostannanes involved in the Stille coupling.13−15 Since organic electronic devices should be low-cost and ecofriendly, the choice of the synthetic pathway should be in the same vein. Direct (hetero)arylation polymerization (DHAP) allows the formation of C−C bonds between (hetero)arenes and (hetero)aryl halides without organometallics intermediates © 2018 American Chemical Society

reducing the number of synthetic steps. DHAP also enables the synthesis of high molecular weight and high-performing organic materials.16−18 DHAP has recently lead to low-defect polymers even with the use of thiophene derivative and 2,2′-bithiophene.19−21 One of them is one of the most popular n-type π-conjugated polymer, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDIOD-T2; analog of N2200 active ink which has achieved excellent devices’ performances in OFET,22,23 Li-ion batteries,24 and all-polymer solar cells25) realized by Sommers and co-workers.26 They have shown relatively good molecular weight (31 kDa) that offers electron mobilities of 2.9 cm2/(V s) in organic field-effect transistors. This was slightly lower than the 3.2 cm2/(V s) from the Stille control (32 kDa). Although this performance is excellent, it is important to know that the commercial N2200 normally has a much higher molecular weight, which could probably explain the slightly better performance and its use to fabricate efficient all-PSCs. All-PSCs have attracted attention since they can overcome the problems of fullerene acceptors such as their weak molar absorption coefficient, their low stability under light and stress, and their poor stability over time due to the formation of Received: May 22, 2018 Revised: July 9, 2018 Published: July 9, 2018 5353

DOI: 10.1021/acs.chemmater.8b02160 Chem. Mater. 2018, 30, 5353−5361

Article

Chemistry of Materials crystalline domains in the active layer.27−30 Indeed, using an active layer of two polymers, a donor and an acceptor, provides a better optimization of the energy levels, band gap, and absorption spectra as well as a higher molar absorption coefficient.31−33 All-PSC have already demonstrated a better thermal and mechanical stability and suitability for the roll-toroll large area with a conversion efficiency (PCE) >5%.34 The effect, in all-PSCs, of the molecular weight of both donor and acceptor polymers has been widely studied35,36 as well as the effect of crystallinity and regioregularity.37−41 Additionally, the optimization of the morphology by the utilization of additives, the effect of solvents and different posttreatment, and the utilization of ternary blend have allowed an increase of PCE from 2% to 9%.42−48 In this paper, we present the synthesis of high-molecular weight PNDIOD-T2 (76 kDa) via DHAP. It has been compared to a commercially available high-molecular weight N2200. All-polymer solar cells have been realized with PNDIOD-T2 and N2200. PNDIOD-T2 has demonstrated slightly better performance than the Stille coupled analog, the commercial N2200. Air aging process of the all-polymer solar cells with PBDB-T:PNDIOD-T2 (DHAP) allows a high-PCE of 7.3%.

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measurements. The optical bandgap was calculated from the onset of the absorption band. Cyclic voltammetry (CV) was performed with a Solartron 1287 potentiostat using platinum electrodes at a scan rate of 10 mV/s and a Ag/Ag+ (0.01 M of AgNO3 in acetonitrile) reference electrode in an anhydrous and argon-saturated solution of 0.1 M of tetrabutylammonium tetrafluoborate (Bu4NBF4) in acetonitrile. In these conditions, the oxidation potential (Eox) of ferrocene was 0.314 V versus Ag/Ag+, whereas the Eox of ferrocene was 0.446 V versus standard calomel electrode (SCE). The HOMO and LUMO energy levels were determined from the oxidation and reduction onset from the voltammogram assuming the SCE is −4.7 eV in vacuum, as reported in the literature. Electrochemical onsets were determined at the position where the current starts to differ from the baseline.49 Thermogravimetric analyses of the polymers were performed with a TGA/SDTA 851e from Metler-Toledo. The acquisitions were recorded under argon with a 10 °C/min scan rate from 50 to 800 °C. The reported degradation temperatures (DT) correspond to a 5% mass lost. Fabrication of All-Polymer Solar Cells. Inverted BHJ solar cells were prepared on a commercial ITO coated glass substrate. ITO glass substrates were cleaned sequentially in ultrasonic baths with acetone and isopropyl alcohol for 20 min, dried using nitrogen gas, and stored in a glovebox. The ITO glass substrates were UV−ozone treated for 2 min right before coating the ZnO/PEI layer. The ZnO precursor solution (250 mg of zinc acetate, 0.069 mL of ethanolamine, and 2.91 mL of 2-methoxyethanol) was spin-coated onto the ITO glass at 5000 rpm for 40 s and annealed at 250 °C on a hot plate in air for 1 h to make the 20−30 nm thick ZnO layer. A 0.05 wt % polyethylenimine (PEI, Mw ∼ 25 000, Aldrich 408727) in 2-methoxyethanol solution was spin-coated onto the ZnO layer and dried at 110 °C on a hot plate in air for 10 min. Then, the glass/ITO/ZnO/PEI substrate was transferred into an argon filled glovebox. The active layer was deposited over the ZnO layer by spin coating the polymer/polymer solution which is then dried at 130 °C for 90 s. The blend solutions were prepared by dissolving the polymers in chlorobenzene and stirring the mixtures on a hot plate at 60 °C for 2 h and then stirred at 40 °C overnight. The different blends used were PTB7-Th:PNDIODT2 with a 1:1 ratio with a concentration of 15 mg/mL and PBDBT:PNDIOD-T2 with a 1:1 ratio with a concentration of 10 mg/mL. Finally, MoO3 (7.5 nm) and Ag anode (100 nm) were deposited onto the active layer through a shadow mask on top of the active layer by thermal evaporation under high vacuum (1 × 10−7 Torr) onto the active layer. Five pixels, each with an active area of 4 mm2, were fabricated per ITO substrate. Characterization of All-Polymer Solar Cells. The photovoltaic cells were tested under AM 1.5 G solar illumination at 100 mW/cm2 in ambient condition using a Solar Simulator (model 16S, Solar Light Co., Philadelphia, PA) with a 200 W xenon Lamp Power Supply (Model XPS 200, Solar Light Co., Philadelphia, PA) calibrated by NREL certified Si photodiode (Model 1787−04, Hamamatsu Photonics K.K., Japan) and a HP4155A semiconductor parameter analyzer (Yokogawa Hewlett-Packard, Japan). After the J−V measurement, the external quantum efficiency (EQE) was measured by using a solar cell quantum efficiency measurement system (Model QEX10, PV Measurements, Inc., Boulder, CO) with a 2 mm2 (2 mm × 1 mm) size masked incident light source and TF Mini Super measurement apparatus for multiple devices in a single substrate. The EQE system was calibrated with a Si photodiode before each measurement.

EXPERIMENTAL SECTION

Materials. Naphthalene-1,4,5,8-tetracarboxylic dianhydride was obtained from TCI America and used without any further purification. 2,2′-Bithiophene was obtained from Sigma-Aldrich and purified by low-pressure distillation. Tris(dibenzylideneacetone)dipalladium (0), pivalic acid, and cesium carbonate as well as other precursors for the preparation of the 2-octyl-1-dodecylamine were obtained from SigmaAldrich and used without further purifications. Extra dry chlorobenzene was obtained from Accros Organic and degassed with argon for 15 min prior to utilization. Other starting materials were purchased from Sigma-Aldrich, TCI America, or Alfa Aesar. The donor polymer PTB7-Th (PCE10) and the commercial N2200 were obtained from 1-Material (Montréal, Québec, Canada), and PBDT-T (PCE12) was obtained from Brilliant Matters (Québec, Québec, Canada); all were used without any further purification. Measurements. 1H spectra were recorded using a Varian AS400 in deuterated chloroform at 298 K. Chemicals shifts were reported as δ values (ppm) relative to the chloroform value of 7.26 ppm. 1H spectra of the polymers were recorded on a Varian Agilent DD2 500 MHz apparatus in 1,1,2,2-tetrachloroethane-d2 at 90 °C. Number-average (Mn) and weight-average (Mw) molecular weights were determined by size exclusion chromatography (SEC) using a high temperature Varian Polymer Laboratories GPC220 equipped with an RI detector and a PL BV400 HT Bridge Viscometer. The column set consists of 2 PL gel Mixed C (300 × 7.5 mm) columns and a PL gel Mixed C guard column. The flow rate was fixed at 1 mL/ min using 1,2,4-trichlorobenzene (TCB) (with 0.0125% BHT w/v) as the eluent. The temperature of the system was set to 110 °C. The samples (2 mg) were dissolved in 2 mL of TCB in a 5 mL chromatography vial and then stirred and heated to 110 °C for 1 h for a complete dissolution. Then, a filtration through a 0.45 μm cellulose fiber film in a 5 mL chromatography vial lead to a homogeneous polymeric solution. Dissolution of the polymers was performed through a loop of 200 μL with a Varian Polymer Laboratories PL-SP 260VC sample preparation system. The calibration method used to generate the reported data was the classical polystyrene method using polystyrene narrow standards Easi-Vials PS-M from Varian Polymer Laboratories which were dissolved in TCB. UV−vis−NIR absorption spectra were recorded using a Varian Cary 500 UV−vis−NIR spectrophotometer using 1 cm path length quartz cells. A polymeric solution (10 mg/mL in chloroform) was spin coated on untreated glass substrate to perform the solid-state

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RESULTS AND DISCUSSION The synthesis of 2,6-dibromonaphthalene-1,4,5,8-diimide is a two-step procedure inspired by reported literature; a detailed protocol is available in the Supporting Information.50,51 As shown in Scheme 1, to afford a PNDIOD-T2, 2,6dibromonaphthalene-1,4,5,8-diimide has been polymerized with 2,2′-bithiophene with modified conditions from the literature.26 The concentration, the polymerization time, and the concentration over the polymerization reaction have been 5354


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Chemistry of Materials Scheme 1. Synthesis of PNDIOD-T2

Table 1. Molecular Weight, Electronic Structure, and Optical Bandgap of PNDIOD-T2 and N2200 polymer PNDIODT2 N2200 PTB7-The PBDB-Tf

Mn (kDa)

PDI

HOMOa,c (eV)

LUMOb,c (eV)

Egel (eV)

Egoptc,d (eV)

76

3.2

−5.8

−3.9

1.9

1.5

76 63 66

2.5 3.0 1.9

−5.8 −5.2 −5.3

−3.9 −3.7 −3.5

1.9 1.6 1.8

1.5 1.6 1.8

From the onset of the oxidation (CV thin film). bFrom the onset of the reduction (CV thin film). cSpin-cast film from CHCl3 solution. d From the edge of the absorption spectra of thin films. eData provided by 1-Material. fData provided by Brilliant Matters. a

modified. Details for the synthesis of poly{[N,N′-bis(2octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDIOD-T2) are presented in the Supporting Information. To fully study the structural and electronic properties, both polymers (PNDIOD-T2 and commercial N2200) have been characterized under the same conditions. 1H NMR spectra of both polymers, PNDIOD-T2 (red) and N2200 (blue), are presented in Figures 1, S2, S3, and S4. The spectra demonstrated that both polymers possess welldefined structures. In the aromatic region, PNDIOD-T2 showed a 2:4 proton ratio, which corresponds to the desired structure. In the case of N2200, a 2:3.95 proton ratio was observed and the deviation from 2:4 could be associated with homocoupling. Indeed, it is known that stannylated monomers in Stille coupling can undergo homocoupling reactions.52,53 Results of the characterizations are presented in Table 1 and are detailed in the Supporting Information. The numberaverage molecular weights (Mn) and polydispersity index (PDI) were measured by gel permeation chromatography

(GPC) in 1,2,4-trichlorobenzene at 110 °C by using polystyrene standards. The HOMO and LUMO levels of the polymers were calculated from the oxidation and reduction potentials of the cyclic voltammetry (CV) measurements. Thermal analysis (TGA and DSC) and OFET have been carried on both polymers, and they exhibited essentially similar thermal properties as well as electron mobilities. They are presented in the Supporting Information. The optical absorption spectra of PNDIOD-T2 and N2200 were recorded in dilute chloroform solutions and as thin films are shown in Figure 2. Both polymers displayed similar UV− visible absorption spectra; each has characteristic two-bands with a high energy band peak at 391 nm and lower energy band centered at 699 nm due to intramolecular charge transfer (ICT) between the NDI and bithiophene moieties in the

Figure 1. 1H NMR (a) of aromatic region of PNDIOD-T2 (DHAP), (b) of aromatic region of N2200 (Stille coupled), (c) comparison of PNDIOD-T2 (red) and N2200 (blue), and (d) comparison of aromatic region of PNDIOD-T2 (red) and N2200 (blue). 5355


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Chemistry of Materials

PBDT-T. These donor polymers have optical absorption spectra that are complementary to that of PNDIOD-T2 (Figure 3). We note that the HOMO/LUMO energy level offsets of the donor polymers with those of PNDIOD-T2 are 0.2−0.6 eV, which are sufficient to drive efficient photoinduced charge transfer and charge photogeneration in the photovoltaic blends. All devices were prepared in a glovebox using an inverted configuration: indium tin oxide (ITO)/ZnO/polyethylenimine (PEI)/active layer/MoO3/Ag (Figure 3d). The optimized donor/acceptor polymer blend ratios were 1:1 (w/ w), and the active layers were spin coated from chlorobenzene (CB) solutions with or without a processing additive. The active area of each cell was 4 mm2, and the current density− voltage (J−V) measurements were carried out in ambient condition under simulated AM1.5 solar illumination calibrated at 100 mW cm−2. The corresponding all-PSC parameters are summarized in (Table 2), and the current density−voltage (J− V) curves EQE spectra are depicted in (Figure 4). The reference solar cells based on the commercial N2200 and donor PTB7-Th gave a PCE of 4.61% with Jsc of 10.63 mA/cm2, Voc of 0.82 V, FF of 0.53, and a maximum EQE of 51%. In contrast, the PBDB-T:N2200 blend system gave a PCE of 5.57% with Jsc of 9.43 mA/cm2, Voc of 0.89 V, FF of 0.67, and maximum EQE of 50%. Compared to the N2200 reference devices, PNDIOD-T2-based all-PSCs gave rise to enhanced photovoltaic performance in both all-PSC blend systems: PCE = 4.83%, Jsc = 10.90 mA/cm2, Voc = 0.82 V, FF = 0.54, and EQEmax = 56% for PTB7-Th:PNDIOD-T2; PCE = 6.06%, Jsc = 11.87 mA/cm2, Voc = 0.89 V, FF = 0.58, and EQEmax = 53% for PBDB-T:PNDIOD-T2. Following literature precedence,55,56 we attempted to use different amounts of processing additives such as 1,8-diiodooctane (DIO), diphenyl ether (DPE), or 1-chloronaphthalene (CN) in the fabrication

Figure 2. UV−visible absorption spectra of PNDIOD-T2 (red) and N2200 (blue): (a) solution and (b) thin film.

backbone.54 The optical absorption edge bandgap (Egopt) was found to be 1.5 eV for both PNDIOD-T2 and N2200. To further compare the commercial N2200 synthesized by Stille coupling and PNDIOD-T2 synthesized via DHAP, both polymers were investigated in bulk-heterojunction all-polymer solar cells (all-PSCs) with donor polymers PTB7-Th and

Figure 3. (a) Molecular structures of acceptor polymers, PNDIOD-T2 and N2200, and donor polymers, PTB7-Th and PBDB-T. (b) Normalized optical absorption spectra of PNDIOD-T2, N2200, PTB7-Th, and PBDB-T. (c) HOMO/LUMO energy levels of the donor and acceptor polymers. (d) Schematic of inverted solar cell configuration used in this work. 5356


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Chemistry of Materials Table 2. Photovoltaic Parameters of All-PSCsa blend PTB7-Th:PNDIOD-T2 PTB7-Th:N2200 PTB7-Th:PNDIOD-T2 PTB7-Th:N2200 PBDB-T:PNDIOD-T2 PBDB-T:N2200 PBDB-T:PNDIOD-T2 PBDB-T:N2200

film thickness (nm) 93 99 101 106 66 63 65 72

± ± ± ± ± ± ± ±

6 7 9 9 7 9 4 2

additive

1%DIO 1%DIO

1%DIO 1%DPE

Voc (V) 0.81 0.82 0.82 0.82 0.88 0.87 0.88 0.87

± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01

Jsc (mA cm−2) 11.73 10.17 10.20 8.80 10.58 9.84 8.39 9.27

± ± ± ± ± ± ± ±

0.35 0.64 0.28 0.26 0.71 0.47 0.39 0.45

FF 0.50 0.47 0.48 0.48 0.59 0.58 0.67 0.62

± ± ± ± ± ± ± ±

PCEave (PCEmax) (%) 0.01 0.03 0.02 0.03 0.05 0.05 0.07 0.02

4.71 4.02 3.99 3.45 5.42 4.97 4.68 4.92

(4.83) (4.61) (4.24) (3.79) (6.06) (5.57) (5.78) (5.66)

a

Average over 20 cells.

Figure 4. J−V characteristics of (a) PTB7-Th-based and (c) PBDB-T-based all-PSCs. The corresponding EQE spectra of the all-PSCs (b, d).

of the all-PSCs . However, all-PSCs fabricated from PNDIODT2 and N2200 with each of the different processing additives resulted in a lower performance. The absence of enhancement from the use of a processing additive means that it is not necessary in these blends systems and it could actually be an advantage in large-scale production. Our comparative investigation of PNDIOD-T2- and N2200based all-PSCs found that PNDIOD-T2-based devices always showed a better performance than the commercial N2200based devices. The possible influence of molecular weight as a factor to explain the superiority of PNDIOD-T2 is eliminated since both polymers have an identical molecular weight (Mn = 76 kDa). We suggest that the difference in photovoltaic performance of PNDIOD-T2 versus N2200 could be a result of the demonstrated higher structural defects in N2200, which in turn could influence the self-organization and morphology of the two blend systems. Clearly, blend morphology plays a key in achieving efficient exciton dissociation, charge transport, and charge collection in BHJ solar cells.57−60 Atomic force microscopy (AFM) topography images of the PBDB-T:PNDIOD-T2 and PBDB-T:N2200 blend films, as shown in Figure 5, reveal a subtle difference in surface morphology. A relatively rougher surface is observed on the

Figure 5. Topographic AFM images (1 × 1 μm2) of all-PSC blend films: (a) PBDB-T:N2200 height; (b) PBDB-T:N2200 phase; (c) PBDB-T:PNDIOD-T2 height; (d) PBDB-T:PNDIOD-T2 phase.

PBDB-T:N2200 blend film (root-mean-square (RMS) roughness value of 1.71 nm for PNDIOD-T2 blend film and 2.70 nm for N2200 blend film). Also, a larger phase separation domain is observed in the N2200 blend films. These morphological observations imply that the slightly lower performance of N2200 blend devices is probably caused by the different end 5357


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Chemistry of Materials Table 3. Variation of the Photovoltaic Parameters of PBDB-T:PNDIOD-T2 (1:1 w/w) All-PSCs with Aging Time blend a

PBDB-T:PNDIOD-T2 PBDB-T:PNDIOD-T2b PBDB-T:PNDIOD-T2b PBDB-T:PNDIOD-T2b PBDB-T:PNDIOD-T2c

film thickness (nm)

air-aging (days)

± ± ± ± ±

0 1 2 3 4

90 90 93 91 93

10 5 4 2 2

Voc (V) 0.88 0.88 0.89 0.88 0.88

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

Jsc (mA cm−2) 11.34 12.08 12.36 11.99 11.60

± ± ± ± ±

0.76 0.94 0.59 0.78 0.12

FF 0.57 0.58 0.60 0.58 0.57

± ± ± ± ±

PCEave (PCEmax) (%) 0.04 0.04 0.03 0.04 0.02

5.60 6.11 6.58 6.12 5.87

(6.32) (6.85) (7.27) (6.80) (6.26)

a

Average over 40 cells. bAverage over 20 cells. cAverage over 5 cells.

Figure 6. J−V characteristics (a), EQE spectra (b), and photovoltaic parameters Voc (c), FF (d), Jsc (e), and PCE (f) of PBDB-T:PNDIOD-T2 allPSCs aged for various times.

groups and bithiophene homocoupling observed in the 1H NMR spectra.61−63 To further control the self-organization of the polymer/ polymer blends, the aging process, previously reported in recent publications,64,65 was applied to the PBDB-T:PNDIODT2 1:1 (w/w) blends. As used here, the aging process involved brief annealing of the spin coated blend active layer at 130 °C for 90 s in the glovebox and was followed by aging of the active layer in a closed box at room temperature (25 °C) in air for various periods (0−4 days). This aging process prior to the deposition of the MoO3 interlayer and silver electrode

presumably facilitated the self-organization of the blend active layers to form the final morphology existing in the all-PSC devices. The resulting photovoltaic parameters of the aged allPSC devices, including Jsc, Voc, fill factor (FF), and the PCE, are summarized in Table 3. The current density−voltage curves, EQE spectra, and aging time dependence of the photovoltaic parameters of the PBDB-T:PNDIOD-T2 devices are presented in Figure 6. The reference devices without any aging gave a PCE of 6.32% (with Jsc = 12.05 mA/cm2, Voc = 0.89 V, FF = 0.59) and maximum EQE of 59%. However, we found the photovoltaic is enhanced with aging in air up to 2 5358


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Figure 7. Topographic AFM images (1 μm2) of PBDB-T:PNDIOD-T2 blend active layer films annealed at 130 °C for 1.5 min and followed by various air-aging times (height (top) and phase (bottom)).

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days, reaching a maximum PCE of 7.27% (with Jsc = 12.55 mA/cm2, Voc = 0.90 V, FF = 0.65) and EQE of 62%. These results further highlight the critical role of controlling the phase-separated morphology toward enhancing the efficiency of all-PSCs. To investigate the effect of aging on the morphology of the all-PSCs, we used AFM topographic imaging to observe blend films prepared and aged under identical conditions as the devices. As observed in Figure 7, the AFM topography images (height and phase) of the PBDB-T:PNDIOD-T2 blend films (1:1 w/w) clearly show a different topography over aging time. This is supported by the surface root-mean-square (RMS) roughness value of 1.71 nm for the fresh film, 1.81 nm for 1 day aging, 2.03 nm for 2 days aging, 2.99 nm for 3 days aging, and 3.10 nm for 4 days aging. These results imply that selforganization of the photovoltaic blend reaches optimal phaseseparated morphology at the 2-day aging mark in accord with the observed performance of the PBDB-T:PNDIOD-T2 allPSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02160.

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Detailed experimental procedures and additional characterization data (UV−vis, 1H NMR, TGA, GPC, CV, μe of thin-film OFET and their preparation) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mario Leclerc: 0000-0003-2458-9633 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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CONCLUSIONS

ACKNOWLEDGMENTS The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for their support. We would also like to acknowledge the financial support of the FRQNT international training program via the Centre québecois sur le matériaux fonctionnels (CQMF). Work at the University of Washington was supported by the US NSF (DMR-1708450) and the Office of Naval Research (N0001417-1-2203). We acknowledge Arthur D. Hendsbee and Prof. Yuning Li from the University of Waterloo for the OFET preliminary tests. We also acknowledge Xiaomei Ding and Nagesh Kolhe for instructions on instruments at UW and for helpful discussions.

In summary, we have synthesized a high molecular weight PNDIOD-T2 via DHAP and investigated its structural and photovoltaic properties in comparison with the Stille-coupled, commercial, N2200. The commercial N2200 was found to have some bithiophene homocoupling defects, which were not found in the DHAP polymer. The slightly lower performance of N2200-based all-polymer solar cells was partly attributed to such structural defects. PBDB-T:PNDIOD-T2 blend film aging was found to facilitate the achievement of optimal phaseseparated morphology, which led to highly efficient all-PSCs with a maximum PCE of 7.3%, Jsc of 12.55 mA/cm2, Voc of 0.90 V, and FF of 0.65. The results of the present comparative study highlight the promise of DHAP toward the synthesis of improved n-type semiconducting polymers for all-PSCs.

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