Variation of the Side Chain Branch Position Leads to Vastly ImprovedMolecularWeight and OPV Performance in 4 , 8-dialkoxybenzo [ 1 , 2-b : 4 , 5b ′ ] dithiophene / 2 , 1 , 3-benzothiadiazole Copolymers

Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight (Mw) of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene (BDT)/2,1,3-benzothiadiazole (BT) copolymers. When dodecyl side chains (P1) are employed at the 4and 8-positions of the BDT unit, we obtain a chloroform-soluble copolymer fraction with Mw of 6.3 kg/mol. Surprisingly, by moving to the commonly employed 2-ethylhexyl branch (P2), Mw decreases to 3.4 kg/mol. This is despite numerous reports that this side chain increases solubility and Mw . By moving the ethyl branch in one position relative to the polymer backbone (1-ethylhexyl, P3), Mw is dramatically increased to 68.8 kg/mol. As a result of this Mw increase, the shape of the absorption profile is dramatically altered, with λmax = 637 nm compared with 598 nm for P1 and 579 nm for P2. The hole mobility as determined by thin film transistor (TFT) measurements is improved from ∼ 1× 10−6 cm2/Vs for P1 and P2 to 7× 10−4 cm2/Vs for P3, while solar cell power conversion efficiency in increased to 2.91% for P3 relative to 0.31% and 0.19% for P1 and P2, respectively.


Introduction
Over the past few years considerable attention has been drawn to the field of organic photovoltaics (OPVs) [1][2][3].These devices are lightweight and can be made on flexible substrates via low cost roll-to-roll processing offering a more affordable alternative to traditional PVs based on inorganic semiconductors [4][5][6].However, a major stumbling block has been that OPVs are typically far less efficient than their inorganic counterparts.OPV power conversion efficiencies (PCEs) need to be dramatically improved in order to compete with inorganics and improved further still to achieve grid parity.Recently, researchers have made great strides in improving OPV PCEs from <4% in 2001 to the current record of 8.3% [7].
These devices employ a bulk heterojunction (BHJ) architecture consisting of an active (absorbing) layer, comprised of a phase-separated network of a polymer donor (p-type) phase and a fullerene acceptor (n-type) phase sandwiched between an anode and cathode.Ideally, the domain sizes within the active layer are less than 10 nm, ensuring that excitons generated within the bulk of a phase are able to diffuse to the donor-acceptor interface, where charge separation occurs.It is also necessary that the network is bicontinuous to ensure that the separated charges have unobstructed paths to their respective electrodes.Improving the active layer, both its components (donor and acceptor) and its morphology are the subject of the majority of OPV research and accounts for the recent upsurge in PCEs.Contributing to these improvements are (1) new processing methods that allow for more ideal phase separation and ordering within the absorbing layer [8][9][10][11][12][13][14], (2) new fullerene acceptor materials with elevated lowest unoccupied molecular orbitals (LUMOs) that allow for improved open circuit voltages (V oc ) and in-turn PCEs [15,16] and (3) new polymer materials with highest occupied molecular orbital (HOMO) and LUMO energy levels designed specifically for OPV applications [17].
Hundreds of low bandgap polymers have now been reported for BHJ solar cells [17].However, despite many new materials having attractive HOMO and LUMO levels for OPV applications, only a small fraction has been fabricated into efficient (PCE > 5%) organic solar cells.Frequently, this is a result of low molecular weight and poor solubility.It has been shown that higher molecular weights are generally required for good charge transport and OPV performance, while good solubility is required for the fabrication of uniform thin films and general ease of film deposition.Molecular weight and solubility often go hand-in-hand, since molecular weight growth can be terminated by precipitation of a poorly soluble polymer chain.Fortunately, M n and solubility can often be simultaneously improved by altering the side chains on the polymer.
With this in mind, we were interested in improving the molecular weight of a copolymer (H7) of 4,8didodecoxybenzo[1,2-b:4,5-b ]dithiophene (C 12 -BDT) and 2,1,3-benzothiadiazole (BT) reported by Hou et al. [18].When employed in a bulk heterojunction solar cell, this polymer exhibited a PCE of 0.9%, which seemed particularly low considering that both BDT and BT have been employed in copolymers that have been used to fabricate highly efficient OPVs [11,[19][20][21][22][23][24][25][26].In more recent reports of BDT copolymers, it has been suggested that linear alkoxy chains at the 4 and 8 positions of BDT result in poorly processable low molecular weight (M w ) materials [20,26].We anticipated that by altering the side chains from linear (dodecoxy) to branched, we could enhance M w and solubility, and in turn improve upon the PCE of H7.

Instrumentation.
Flash chromatography was performed on a Biotage Isolera Flash Purification System using Biotage SNAP Flash Purification Cartridges as the stationary phase.Microwave-assisted polymerizations were carried out using a CEM Discover Microwave reactor. 1 H and 13 C NMR spectra were recorded on a Bruker Avance DPX-300 NMR spectrometer. 119Sn NMR spectra were recorded on a Bruker Avance DRX-500 instrument.UV-vis absorption spectra were recorded on an Agilent 8453 diode-array spectrophotometer operating over a range of 190-1100 nm.GC-MS were recorded on an Agilent 6850 Series GC system coupled to an Agilent 5973 mass selective detector run in electron impact mode.Infrared spectra were recorded over the 450-4000 cm −1 region using on a Perkin Elmer Spectrum 100 spectrophotometer with an ATR sampling accessory equipped with a diamond anvil.Gel permeation chromatography (135 • C in 1,2,4-trichlorobenzene) was performed by American Polymer Standards (Ohio).(2).3-Thiophenecarboxylic acid (20.0 g, 0.156 mol) (1) was added to a round bottom flask equipped with a reflux condenser and stopper.Thionyl chloride (100 mL) was then added, and the mixture was allowed to reflux for 4 hours under a blanket of argon.The excess thionyl chloride was distilled over and the solution was dried on a vacuum line to yield colorless crystals (22.7 g, 99%).Characterization was consistent with published data.

N,N-Diethylthiophene-3-carboxamide (3).
3-Thiophenecar-bonyl chloride (13.2 g, 0.09 mol) was dissolved in methylene chloride (45 mL) and added dropwise at 0 • C to a solution of diethylamine (25 mL) in methylene chloride (25 mL).The solution was stirred at 0 • C for 2 hours and then at RT overnight.Diethylamine (10 mL) was added and the mixture was stirred for another 2 hours.The diethylamine hydrochloride precipitate was filtered off and the organic phase was extracted with water.The solution was dried over magnesium sulfate and evaporated to give a brown oil (14.1 g, 85%).Characterization of the product was consistent with the published data.One pot procedure.3-Thiophenecarboxylic acid (7.0 g, 0.055 mol) and thionyl chloride (5.0 mL, 0.068 mol) were added to a round bottom flask containing methylene chloride (200 mL) and cooled to 0 • C. Triethylamine (15 mL) was then added dropwise over 15 minutes resulting in the formation of a white precipitate.
The mixture was allowed to warm to room temperature.After 30 minutes, diethylamine (8 mL) was added dropwise over 10 minutes.The mixture was stirred for an additional 30 minutes at room temperature.The methylene chloride was evaporated, then ether was added, and the triethylamine hydrochloride precipitate was filtered and washed with a small amount of ether.The ether was then evaporated to leave a brown oil (8.6 g, 86%).5), (6), and (7).Compounds 5-7 in dry THF (20 equiv.by wt.) were cooled to −78

General Procedure for Polymer Synthesis of P1-P3.
Under ambient atmosphere in a 5 mL microwave tube equipped with a stir bar was added the bis(trimethyltin) monomer (0.5 mmol) along with the dibromo monomer (0.485 mmol) and 2 mL of chlorobenzene.The mixture was stirred for 5 minutes and tetrakis(triphenylphosphine)palladium(0) (2.5-5 mol%) was added to the tube and the tube was capped and set in the microwave at 200 • C for 10 minutes.The viscous gel was precipitated in methanol and filtered.The solid was then added to a Soxhlet thimble and subjected to extractions with hexanes (6 hrs), THF (24 hrs), and finally chloroform (24 hrs).The chloroform extract was evaporated almost to completion, and methanol was added to precipitate the polymer, which was filtered and dried under vacuum for 24 hours.

Poly[(4,8-Bis(1-dodecyloxy
2.5.Powder X-Ray Diffraction.X-Ray powder diffraction data were collected at room temperature using a BRUKER P4 general-purpose four-circle X-ray diffractometer modified with a GADDS/Hi-Star detector at 40 kv and 30 mA for Cu Kα radiation (λ = 1.5406Å).The GADDS software suite was used to control the goniometer [28].The samples were mounted on a loop and two frames were measured at 2θ = 20 • and 50 • with exposure time of 180 seconds/frame.For the inset powder diffraction spectrum, one frame was measured at 2θ = 25 • with an exposure time of 180 seconds/frame for 10 frames.Area integration techniques were used to reproduce the single powder diffraction pattern for each frame.The powder patterns were each merged and analyzed using the EVA program to produce a single powder diffraction pattern [29].
2.6.Charge Transport Measurements.Bottom-contact thinfilm transistors (TFTs) were fabricated by spin-coating (1000 rpm) polymer solutions (0.75 wt% in 1,2-dichlorobenzene) over the 1.2 cm × 1.2 cm substrate with prefabricated device structures containing a Si gate electrode, 300 nm SiO 2 gate dielectric and 5 nm Ti/45 nm Au source-drain contacts with channel length L = 5 μm and channel width W = 1000 μm.Prior to spin coating, the substrates were cleaned by soaking in hot acetone, followed by isopropyl alcohol.The substrates were then cleaned in UV ozone for five minutes and rinsed with DI water.The surfaces were chemically treated by submerging the substrates for 30 min in a 17 mmol n-octyltrichlorosilane (OTS) solution in hexadecane at room temperature.The substrates were then removed and sonicated in isopropyl alcohol for 10 minutes, sonicated in chloroform for 10 minutes, and rinsed with isopropyl alcohol followed by DI water.The polymer solutions were stirred overnight prior to spin coating to ensure complete dissolution.After spin-coating, the substrates were placed in a vacuum oven overnight prior to measuring the currentvoltage characteristics.The transistors were characterized by measuring the drain current (I D ) against a sweeping gate voltage (V GS ) from −20 V to 60 V in the saturation regime, drain source voltage V DS = −60 V.The measurements were made in ambient conditions using a commercially available probe station and an Agilent 4155C Semiconductor Parameter Analyzer.Five devices were measured for each material.The field-effect mobility (μ) was calculated from the slope of the (I D ) 1/2 versus V GS graph, using the relationship in where C i is the capacitance per unit area for the SiO 2 gate dielectric, W is the channel width, L is the channel length, and V T is the threshold voltage.A value for ∂I 1/2 D /∂V GS was determined from the slope of the linear portion of the (I D ) 1/2 versus V GS plot.

Photovoltaic
Characteristics.ITO-coated glass slides (Delta Technologies) were cleaned by sonication in acetone and isopropanol for 20 minutes each, followed by ozone cleaning for 30 minutes.A solution of PEDOT : PSS (Clevios P-H.C. Starck) at 1 : 1 PEDOT : PSS solution to deionized water was spin-coated onto the slides at 4000 RPM (resulting in a 40 nm layer).The films were then baked in a vacuum oven at 100 • C for 30 minutes.Polymer solutions were then spin-coated onto the PEDOT : PSS-coated substrates.Solutions were prepared by mixing polymer and 1,2dichlorobenzene (8 mg/mL for P2 and P3, 10 mg/mL for P1), then they were heated (80 • C for P3, 100 • C for P1 and P3) and stirred for 24 hours.The solution for P3 was then filtered while hot (0.45 μm PVDF (Millipore)).P1 and P2 were spin-coated from unfiltered, hot solutions, since filtration removed much of the polymer and resulted in poorer device performance and a high frequency of devices shorting.Unfiltered solutions of P3 resulted in devices with ∼10% poorer power conversion efficiency.Optimal spin coating conditions were found to be 600 RPM for 30 s for P1 and P2, and 1500 RPM for 40 s for P3.
The films were set to dry for 30 minutes and then placed under vacuum for thermal deposition of electrodes (2 nm LiF/100 nm aluminum).The active area of the device was 38 mm 2 .All film thicknesses were determined via a JEOL JSPM-5200 operating in AFM tapping mode.
The current-voltage (J-V ) characteristics were measured with a Keithley 236 digital source meter under simulated air mass (AM) 1.5 solar irradiation of 100 mW/cm 2 (Oriel 150 W Xenon Light Source).The light intensity was calibrated with a NREL calibrated reference cell (Newport).The external quantum efficiency (EQE) curve was measured using a 300 W Oriel Xenon light source passed through an Oriel Cornerstone 260 monochromator, a Merlin lock-in amplifier, a calibrated Si UV detector, and an SR 570 low noise preamplifier.

Synthesis and Characterization.
The synthetic routes to the monomers and polymers are shown in Scheme 2. Starting from 3-thiophenecarboxylic acid (1), we developed a new one-pot procedure for accessing the synthetic intermediate N,N-diethylthiophene-3-carboxamide (3).This involves the in situ preparation of thiophene-3-carbonyl chloride (2) via the reaction of 1 with thionyl chloride and excess triethylamine in dichloromethane.The triethylamine functions as a catalyst by generating the more reactive carboxylate and also reacts with the hydrochloric acid (HCl) byproduct generated by the chlorination reaction.After 30 minutes of stirring, a single equivalent of diethylamine was added to the reaction mixture containing 2 to form the desired product.The HCl generated by this reaction was reacted with the remaining triethylamine.The dione 4, the 4,8-dialkoxy-BDTs (5-7) and the bis-stannane monomers (8-10) are prepared by procedures similar to those in the literature [18,21].The purity of monomers 8-10 was confirmed by 119 Sn-NMR spectroscopy (See Supplementary Materials available at doi:10.1155/2011/572329) a very useful technique for determining whether there are any trace monostannane impurities in the monomers.These impurities are detrimental to molecular weight growth.
The polymers P1-P3 were prepared by microwaveassisted Stille cross-coupling polymerization of the bisstannane monomers 8-10 with 4,7-dibromo-2,1,3-benzothiadiazole (BT) in chlorobenzene using tetrakis(triphenylphosphine)palladium(0) ((Ph 3 P) 4 Pd) as the catalyst (see Scheme 1).Microwave heating was used because it is more efficient than conventional heating methods, and it has also been reported to give higher molecular weights [19,22,[30][31][32].After reaction the polymers were precipitated into methanol, collected by filtration, and transferred to a Soxhlet extractor where the material was washed with hexanes and extracted with chloroform.The chloroform-soluble fraction was isolated to ensure the material was sufficiently soluble for the preparation OPV devices.In all cases, material remained in the extraction thimble after chloroform extraction; however, the amount of this material was highly variable.For P1 and P2, yields from the chloroform soluble fractions were often less than 10%, while for P3, yields as high as 55% were obtained.Assuming no crosslinking has occurred, the fact that material remained in the extraction thimble after chloroform extraction means that the chloroform fraction contained material with the highest molecular weight that is soluble in chloroform.The molecular weights of the these fractions were determined by gel permeation chromatography (GPC) at 135 • C using 1,2,4-trichlorobenzene as the solvent, and the results are summarized in Table 1.P1, P2, and P3 were found to have weight average molecular weights (M w ) of 6.3, 3.4, and 68.8 kg/mol, respectively.Our result for P1 differs dramatically from that of Hou et al. and may be due to differences in GPC protocols.Hou et al. ran elutions at RT using THF and found a M w of 31.4 kg/mol, while our GPC protocol was 135 • C using 1,2,4-trichlorobenzene as eluent and found our M w to be 6.3 kg/mol.For P2, we find that despite several reports of the beneficial effect of the 2ethylhexyl side chain on solubility and molecular weight, P2 has a lower M w than P1.Lastly we find that moving the ethyl branch in by one position (P2 compared with P3) results in an unexpectedly large increase in M w .
Thermogravimetric analysis (TGA) was performed on polymers P1-P3 under argon at 5 • C/min as shown in Figure 1.The results indicate that the polymers have similar thermal stabilities with the onsets of degradation ranging from 268 • C and 278 • C.This degradation has been attributed to the loss of the side chains [18], which is supported by the fact that the P1 which has 12 carbon side chain loses a larger weight percentage compared with P2 and P3, which have 8 carbon side chains.

Optical and Electrochemical
Properties.UV-visible abspectra (normalized by area) for polymers P1-P3 in chloroform solution are shown in Figure 2. P2 is considerably blue-shifted of P1, with a λ max of 579 nm compared to 598 nm for P1.The spectrum for P1 is similar in shape and slightly red-shifted of Hou et al.'s H7 (λ max = 591 nm in THF) indicating comparable molecular weight.The shape of the absorption profile for P3 is very different from those of P1 and P2 with a very sharp low-energy feature dominating the spectrum-resulting in a λ max of 637 nm for P3.This sharp low-energy feature has been seen in other low bandgap copolymers and is indicative of high molecular weight and often high solar cell performance [11,19,33,34].UV-Vis spectra of P1 and P3 films (dropcast from chloroform) are shown in the dashed spectra in Figure 2. A spectrum for P2 is not shown as dewetting of the chloroform 300 400 500 700 800 Absorbance (a.u.)

Wavelength (nm)
Figure 2: UV-Vis spectra of P1 (black), P2 (blue), and P3 (red) in chloroform solution (solid) and film (dashed).Spectra are normalized by area.solution prevented preparation of uniform film.The onset of absorption for P1 red shifts 45 nm transitioning from 685 nm in solution to 730 nm in the film.The shift in absorption onset for P3 is not as large, shifting 35 nm from 695 nm in solution to 730 nm in the film.The film spectrum for P1 shows the presence of low-energy shoulder that was not observed in solution, while the spectrum for P3 is broadened but similar in shape to the solution spectrum.The optical bandgaps for both P1 and P3 as determined by absorption onset is 1.69 eV.Cyclic voltammetry measurements were made on polymer films of P1-P3 prepared by dropcasting chloroform solutions of the polymers onto a glassy carbon electrode (see Figure 3).The onsets of the oxidation and reduction potentials for the three polymers were comparable-not surprising given that they share a common backbone.These results are summarized in Table 2.The oxidation onsets, determined as the point where the current differed from the baseline by 2 μA, were found to be 0.15, 0.13, and 0.12 V for P1, P2, and P3 giving rise to HOMO levels of −4.95 ([18] −5.10 eV), −4.93, and −4.92 eV, respectively.Similarly, the reduction onsets were found to be −1.72,−1.62 and −1.59 V for P1, P2, and P3 giving rise to LUMO levels of −3.08 ([18] −3.19 eV), −3.18, and −3.21 eV, respectively.These results give electrochemical bandgaps of 1.87 ([18] 1.91 eV), 1.75, and 1.71 eV for P1, P2, and P3.

Solid State Ordering and Charge Transport.
To give insight into the internal packing structure of the polymers, powder X-ray diffraction (PXRD) was used.Samples were mounted on a loop and two frames were measured at 2θ = 20 and 50 • with exposure time of 180 seconds/frame.The integrated diffraction patterns are shown in Figure 4(a).For the inset powder diffraction pattern, 10 frames were measured at 2θ = 25 • with an exposure time of 180 seconds/frame, then they were added together.The intensities of the diffraction patterns decrease and diffraction peaks broaden from P1 to P2 to P3, indicating that structural order within the solid polymers decreases in the order P1 > P2 > P3.This order correlates with the increasing bulk of the side chain relative to the conjugated backbone.For P3, which has a much higher M w , this lack of order is further exacerbated, since order has been shown to decrease as M w increases [35].
Thin-film transistors were fabricated from the three polymers to assess charge transport.Figure 5 shows the current-voltage characteristics of a representative device for each polymer with channel length L = 5 μm and channel width W = 1000 μm.Here, we plot the log(I D ) versus V GS in solid symbols (right axis) and the (I D ) 1/2 versus V GS in open symbols (left axis) in the saturation regime (V DS = −60 V).The field-effect hole mobility (μ h ) of the devices shown here are μ h = 1.36 × 10 −6 , 9.69 × 10 −7 cm 2 /Vs and 8.17 × 10 −4 cm 2 /Vs for P1, P2, and P3, respectively.These values represent an underestimation of the mobility of these materials, as optical inspection of the devices demonstrated that the film partially dewets the substrates, and the material covers only a fraction of the width of the device.Thus the real width in (3) is lower than the width defined by photolithography.The on/off current ratios for the three devices presented here are 10 2 , 10 2 , and 10 4 , for P1, P2, and P3, respectively.

Photovoltaic Properties.
The photovoltaic characteristics of the polymers were determined using a standard bulk heterojunction device architecture-ITO/PEDOT : PSS/P1-P3 : PC 61 BM/LiF/Al.The devices (0.38 cm 2 ) were fabricated under an inert atmosphere and tested in air.The active layers were deposited from 1,2-dichlorobenzene solutions (8 mg/mL for P2 and P3, 10 mg/mL for P1) and the optimal weight ratios of polymer : PC 61 BM was determined to be 1 : 2.5 for P1 and P3 and 1 : 2.2 for P2.The active layer thickness for the optimal devices as determined by atomic force microscopy (AFM) were 130, 50, and 100 nm for P1, P2, and P3, respectively.Due to the poor solubility of P2 and its low M w , thicker films could not be achieved even at the lowest spin speeds.The current-voltage curves for the  best devices for each polymer are shown in Figure 6, and the results are summarized in Table 3.Consistent with the HOMO levels determined by cyclic voltammetry, the open circuit voltages (V oc ) are similar for the three polymers, near 0.7 V.The short circuit currents (J sc ) for P1 and P2 are similarly low (near 1 mA/cm 2 ), while it is much higher for P3 at 8.93 mA/cm 2 .The fill factors (FFs) increase with increasing M w peaking at 0.46 for P3.Consequently, P3 has a much higher power conversion efficiency (PCE) peaking at 2.91% compared with 0.31% for P1 and 0.19% for P2.
The EQE spectra for the P1 and P2 devices (seen in Figure 7) show poor current generation across the spectrum with peak EQE less than 12%.P3, on the other hand, shows EQE above 30% from 330-670 nm, with a peak at 51% at 370 nm.Between 400 and 600 nm, the EQE dips, indicating that current production could be enhanced by using PC 71 BM  instead of PC 61 BM as PC 71 BM absorbs in this range [26].A comparison between these devices fabricated from P1 with Hou et al.'s H7 [18] reveals similar V oc 's (0.69 V for P1 versus 0.68 V for H7) and FFs (0.42 versus 0.44) though they obtain a higher J sc of 2.97 mA/cm 2 resulting in a higher PCE of 0.90%.The discrepancy in results could be explained by the fact that Hou et al. used a Ca/Al cathode and tested the devices under nitrogen, whereas we used a LiF/Al cathode and tested in air.P3 outperforms H7 across the board, with higher V oc , J sc , FF, and PCE, highlighting the significance of our side chain variation.It is worth noting that the P3 solar cells were prepared from pristine DCB solution without the use of optimization techniques such as annealing or additive processing that could greatly enhance the PCE.3-benzothiadiazole-4,7-diyl](P1).Surprisingly, transitioning from the linear dodecyl side chain (P1) to the branched 2-ethylhexyl side chain (P2) results in a decrease in M w ; however, by moving the ethyl branch in by one position relative to the polymer backbone (P3), we observe a dramatic increase in M w to 68.8 kg/mol compared with 6.3 kg/mol for P1 and 3.4 kg/mol for P2.This results in vastly different optical properties, with the appearance of a sharp low-energy feature in the UV-visible absorption spectrum of P3 that is not present in the spectra of P1 and P2.Despite the increase in the π-π stacking distance due to the 1-ethylhexyl branch in P3 relative to P1 and P2, the increased M w results in a factor of 100 increase in hole mobility, and a factor of 10 increase in the PCE of bulk heterojunction solar cells.We observe a peak PCE of 2.91% for devices based on P3 and PC 61 BM with minimal optimization indicating that this material has the potential to make highly efficient solar cells.The use of the 1-ethylhexyl side chain, and indeed, the general technique of moving side chain branches closer to the polymer backbone is a widely applicable means of increasing M w in conjugated polymers, many of which have not reached their full potential because of insufficient M w .
All measurements were carried out in degassed solutions of acetonitrile with tetrabutylammonium hexafluorophosphate (0.1 M, electrochemical grade) as the supporting electrolyte.The scan rate used was 100 mVs −1 .The electrochemical onsets were determined as the position at which the current differed by 2 μA from the baseline.

Table 1 :
Molecular weight of the polymers.

Table 2 :
Summary of optical and electrochemical properties.

Table 3 :
Photovoltaic characteristics of best devices prepared from P1-P3 and PC 61 BM at a polymer.