Ultra Low Band Gap α,β-unsubstituted BODIPY-based Copolymer Synthesized by Palladium Catalyzed Cross-Coupling Polymerization for Near Infrared Organic Photovoltaics

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Introduction
Low band gap (LBG) organic materials that absorb into the near-infrared (NIR) are of great interest in the recent years for a number of potential applications. 1 For example, the use of NIR-absorbing or NIR photovoltaic organic materials (small molecules or polymers) could extend the material's absorption into the NIR spectral region and even beyond 1000 nm wavelength, which in principle could enhance the current power conversion efficiency (PCE) of organic photovoltaics (OPVs). 2 Even though semiconducting polymers with ultra LBGs have been synthesized before, [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] the challenge in designing and synthesizing materials that have a good photoresponse beyond 900 nm and an appreciable PCE in polymer:fullerene solar cells lies (among others) in the precise energy level control that is required.Two major methodologies are employed to fine tune the band gap and energy level alignment of conjugated polymers.One methodology relies on the donor-acceptor (D-A) approach and the other on the stabilization of the quinoid structure. 20D-A approach is the most common tool to synthesize NIR conjugated polymers due to the plethora of functional electron rich and electron deficient building blocks.In order to develop ultra LBG D-A polymers, usually strong electron rich units and strong electron deficient building blocks are used.Common monomer units function as strong electron donors include pyrrole, thiophene, ethylenedioxythiophene (EDOT), bridged bithiophenes derivatives etc. 21 Moreover, the most wellknown strong electron deficient units used to construct NIR D-A polymers are diketopyrrolopyrrole dyes, 4c,13a,14c benzobisthiadiazole, 3a,3g pyrazinoquinoxaline derivatives, 9b,10a thiadiazoloquinoxaline derivatives, 10a,11b-c annelated benzotriazole, 10b thienoisoindigo, 4b,13b emreladicene, 3f cyclopentadithiophenone, 14a tetraazabenzodifluoranthene diimides, 8a-c etc. 3g, 22 One of the less explored electron deficient monomers for the synthesis of NIR conjugated polymers is the so-called, 4,4difluoro-4-bora-3a,4a-diaza-s-indacene, commonly known as BODIPY (Scheme 1). 23BODIPY dyes were first discovered in 1968 by Treibs and Kreuzer 24 and exhibit unique properties such as large absorption coefficients, high fluorescence quantum yields, and remarkable photostability. 23In addition, straightforward chemical synthesis and structural robustness have enabled fine tuning of optical properties of BODIPY dyes via systematic structural variations. 23Owing to large extinction coefficients, intense absorption spectra that extend into the red region of the visible spectrum, and decent hole mobility, small-molecule BODIPY derivatives have been recently employed as p-type or donor materials in conjunction with PCBM in bulk heterojunction (BHJ) solar cells. 25espite the fact that BODIPY-based conjugated polymers have also been used in different applications, [26][27][28][29][30][31][32][33][34][35] such as near-IR emitters, nonlinear optics, light harvesting, electrochromics and OPVs, limited examples have been presented up to now with ultra LBG conjugated polymers (Eg < 1.4-1.5 eV) consisting of the BODIPY core.As a matter of fact, Algi and Cihaner 32 designed and synthesized a BODIPY-based polymer for electrochromic applications.In their study, a 1,3,5,7tetramethyl substituted BODIPY derivative was used as an acceptor unit, featuring a non-planar repeating unit and distorted conjugation between EDOT (strong electron rich) and BODIPY (electron deficient) units due to the steric effect of the methyl groups.To avoid the steric effect of the methyl groups, Vobecka et al. 33 and Samuel et al. 34 have synthesized a BODIPY core with halogens at 3-and 5-positions (α-positions; Scheme 1) and effectively synthesized an EDOT-BODIPY-EDOT conjugated polymer both with electropolymerization and Stille cross coupling polymerization, respectively.However, the resulting copolymers are kinked because the linearity of the polymer was lost (Scheme 1).Last year, Stoddart et al. 36 succeeded to synthesize the first ultra LBG DAD type α,βunsubstituted BODIPY-based copolymer using EDOT as the electron rich (D) unit by electropolymerization which displays electrochromic behavior.In this work, it is presented for the first time the successful synthesis and optoelectronic characterization of an ultra LBG (Eg = 1.15 eV) linear BODIPY-based copolymer by conventional cross coupling polymerization procedure (Stille cross coupling).The new synthesized NIR BODIPY copolymer exhibits a broad (panchromatic) absorption spectrum ranging from 300 nm to 1100 nm, while initial photovoltaic characterization as electron donor in BHJ OPVs reveals very interesting and promising photovoltaic characteristics such as good fill factor (FF) and open circuit voltage (Voc).

Results and discussion
The preferred method of choice for the synthesis of α,βunsubstituted BODIPY dyes is the combination of aldehydes and pyrrole under neat conditions. 23,37The aldehyde can be dissolved in excess pyrrole at room temperature, and the dipyrromethane intermediate (the reduced form of the dipyrromethene) is formed and isolated.The BODIPY core can then be obtained after oxidation and complexation with boron.In this work, 5-octylthiophene-2-carbaldehyde has been chosen as the aldehyde, where upon condensation with pyrrole, catalyzed by few drops of trifluoroacetic acid, the resulting 2,2'-((5-octylthiophen-2-yl)methylene)bis(1H-pyrrole) (1) is obtained (Scheme 2).The design concept of choosing an alkylthiophene-carbaldehyde is the formation of a functional BODIPY building block (3) with a 2D extension for the development of a series of BODIPY based copolymers soluble in common organic solvents.Treatment of monomer 1 with the strong oxidant 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ), then diisopropylethylamine (Hünig's base) and finally with trifluoroborane dietherate [BF3O(Et)2] provided the corresponding borondipyrromethene monomer 2. Recently, new synthetic protocols to introduce bromine atoms at the 2-and 6-positions on the unsubstituted BODIPY core have been presented in the literature. 3838c Scheme 2. Reaction procedures towards the synthesis of dibromo borondipyrromethene monomer 3 For the synthesis of TBDPTV, Stille cross-coupling polymerization using 1:1 monomer feed ratios was used (Scheme 3).A solution of the commercially available (E)-1,2bis(3-dodecyl-5-(trimethylstannyl)thiophen-2-yl)ethane and the dibromo borondipyrromethene monomer 3 were combined in dry deoxygenated toluene in the presence of tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] (5 mol%) Please do not adjust margins Please do not adjust margins and tri-o-tolylphosphine [P(o-Tol)3] (40 mol%) and the mixture was heated to ca. 120 o C for 48 h to provide the desired crude polymer.Purification was achieved by Soxhlet extraction with methanol (200 mL, 1 d), hexane (200 mL, 1 d) and chloroform (200 mL, 1 d).The chloroform fraction was then concentrated under reduced pressure, precipitated in methanol, filtered and dried in vacuum.The resulting TBDPTV is readily soluble in chloroform, chlorobenzene and o-dichlorobenzene (o-DCB).The polymer was characterized with 1 H NMR spectroscopy and gel permeation chromatography (GPC) [see Supporting Information (Fig. S1-S2)].The typical molecular weight characteristics of TBDPTV were estimated by GPC and presented in Table 1.

Table 1
The normalized UV-vis absorption spectra of TBDPTV in chloroform solution and in the solid state are presented in Figure 1, and the corresponding optical properties are summarized in Table 1.Two major absorption bands can be observed in solution, a feature, which is commonly observed for alternating D-A copolymers.The low-wavelength peak observed at 444 nm for can be attributed to a π-π* transition, while the highwavelength transition with maximum absorption at 817 nm is believed to be related to an intramolecular charge transfer (ICT) between the electron donor and the electron deficient of the repeating unit.
Passing from solution to the solid state, the absorption spectrum of TBDPTV becomes broader covering all the range from 300 nm to 1100 nm.All the absorption bands are redshifted when passing from solution to the solid state, the peak at 444 nm in solution is situated at 451 nm in the solid state whereas the peak at 817 nm in solution has been shifted to 848 nm in the solid state, indicating the presence of strong intermolecular π-π interactions.The optical band gap (Eg opt ) deduced from the absorption onset in the solid state is estimated at 1.15 eV.
The energy levels of TBDPTV have been determined utilizing both cyclic voltammetry (CV) and atmospheric pressure photoelectron spectroscopy (AAPPS, AC-2).TBDPTV exhibits reduction and oxidation peak potentials versus saturated calomel electrode (SCE) at -0.68 V and 0.46 V, respectively as obtained by cyclic voltammetry (Figure 2).The resulting HOMO (EHOMO) and LUMO (ELUMO) energy levels as derived from the equations EHOMO = -(4.7 + Eonset ox ) eV and ELUMO = -(4.7 + Eonset red ) eV are -5.16eV and -4.02 eV vs. vacuum, respectively.The electrochemical bandgap (Eg CV ) of 1.14 eV is in excellent agreement with the Eg opt .The HOMO level as calculated by AAPPS (Figure S3 in Supporting Information) is situated at -5.29 eV, slightly different from the one obtained by cyclic voltammetry.An attempt to probe the photovoltaic properties of the synthesized polymer was carried out.Organic BHJ solar cells were fabricated in inverted device structure consisting of ITO/ZnO/TBDPTV:PC71BM/MoOx/Ag, using the three different D:A composition ratio (1:2, 1:3 and 1:4).The active layer solution was spin coated under inert atmosphere condition using 97 to 3 vol % of chlorobenzene (CB) and 1,8 diiodooctane (DIO).Figure 3a and 3b show the J-V curves of the optimized BHJ solar cells under simulated AM 1.5G solar irradiance (100mW cm -2 ) and in the dark, respectively.A PCE of 1.1% was Please do not adjust margins Please do not adjust margins obtained for the 1:3 ratio, with a short circuit current density (Jsc) of 3.39 mA cm -2 , Voc of 0.59 V and FF of 0.56.Using the same fabrication conditions, solar cells containing 1:2 and 1:4 D:A ratio achieved a maximum efficiency of 1.03 and 1.01, respectively.A strong correlation between the charge carrier mobility and FFs is also obtained for these devices.In fact, as shown in Table 2, solar cells based on 1:3 and 1:4 ratio, display the highest FF of 0.56 and 0.57, respectively.On the other hand, the 1:2 based devices depicted a lower FF of 0.52, costing with the lower charge carrier mobility obtained.It is worth to mention that for all the devices, Jsc values are similar and this can be possible attributed to the not perfect energy alignment with PC71BM for achieving efficient exciton splitting rather than to morphological issues as can be seen on Figure 5. Furthermore, external quantum efficiency (EQE) measurements have been carried out (Figure S4).An EQE signal around 950-1000 nm, even though very weak, is observed, suggesting an existing path towards charge generation from the polymer.As reported by Janssen et al., 39 a fast recombination path becomes essential when the LUMO of the acceptor become too shallow, leading to low Jsc values.Noticeably, the rectification behavior of the device slightly improved with increasing the D:A ratio (Figure 3b).The Jsc calculated from the EQE is 3.08 mA/cm -2 , which is less than 10% smaller compared to the Jsc obtained from the solar simulator.

Table 2.
In order to understand the transport properties of TBDPTVbased solar cells, the charge carrier mobility μ of the aforementioned devices were determined by employing the technique of photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV). 40Charges are photogenerated by a strongly absorbed laser pulse and extracted after an adjustable delay time.Figure 4 shows the photo-CELIV transients of the TBDPTV:PC71BM system in the three different composition ratio (1:2, 1:3 and 1:4), which were recorded by applying a 2V/20 μs linearly increasing reverse bias pulse and a delay time (td) of 1 μs.From the measured photocurrent transients, the charge carrier mobility (μ) is calculated using the following equation: where d is the active layer thickness, A is the voltage rise speed  = U/dt, U is the applied voltage, tmax is the time corresponding to the maximum of the extraction peak, and j(0) is the displacement current. 41The photocurrent transients in Figure 4 reveal that tmax occurs significantly earlier for the 1:4 weight ratio.As a result, the calculated charge carrier mobility (μ) is higher for the 1:4 ratio (1.05±0.2)x10 - cm 2 V -1 s -1 , then for the 1:3 ratio (9.18±0.2)x10 - cm 2 V -1 s -1 , and finally for the 1:2 ratio (7.50±0.1)x10 - cm 2 V -1 s -1 , respectively.Investigations on the surface microstructure were also carried out through intermitted contact mode atomic force microscopy (AFM, Figure 5).A good intermixing between the donor and the acceptor materials is obtained, consequentially the topography and the phase images do not reveal obvious differences between the three systems studied, supporting the hypothesis that the main limitations on TBDPTV-based solar cells are attributed to the low LUMO offset rather than to morphological issues.

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Please do not adjust margins A dibromo α,β-unsubstituted BODIPY building block suitable for conventional catalyzed cross coupling polymerizations has been effectively synthesized.In order to confirm the proof of concept, the dibromo BODIPY monomer 3 has been successfully polymerized through Stille cross coupling with (E)-1,2-bis(3-dodecyl-5-(trimethylstannyl)thiophen-2-yl)ethane as the comonomer providing the new ultra LBG (Eg opt = 1.15 eV) TBDPTV copolymer.To the best of our knowledge TBDPTV is the first α,β-unsubstituted BODIPY-based NIR copolymer synthesized by conventional catalyzed cross coupling polymerization methods.TBDPTV exhibits a panchromatic absorption spectrum ranging from 300 nm to 1100 nm, and a promising PCE of 1.1% in NIR BHJ OPVs using PC71BM as the electron acceptor with very interesting photovoltaic characteristics, such as good fill factor (FF) and open circuit voltage (Voc).

Experimental
All reactions were treated as air and light sensitive and performed under argon and in the dark.All glassware used were washed using teepol surfactant, rinsing with excess water, acetone and methylene dichloride and dried in an oven at 120 °C.All solvents and reagents were sourced commercially from Aldrich, except (E)-1,2-bis(3-dodecyl-5-(trimethylstannyl)thiophen-2-yl)ethane which was obtained from Solarmer Materials Inc.

Instrumentation and Materials Characterization
Nuclear Magnetic Resonance (NMR): 1 H and 13 C NMR spectra were recorded on a Bruker AV-400 (400 MHz for 1 H and 100 MHz for 13 C), using the residual solvent resonance of CDCl3 as an internal reference.Gel Permeation Chromatography (GPC): Mn and Mw of the polymer have been determined by GPC on a PSS/Agilent SECurity GPC system, equipped with two PSS SDV analytical linear M columns, ALSG1329A DAD detector, and RID G1362A RI detector.The measurement was performed with chloroform as eluent, with a sample concentration of 0.8 g/L.Absorption spectroscopy: UV/vis spectra were measured on a Jasco V-670 spectrophotometer.
Cyclic Voltammetry: Cyclic voltammetry was executed in chloroform with 0.1 M (n-Bu)4NClO4 against standard calomel electrode.HOMO and LUMO levels were calculated using the formulae HOMO=-(Eox+4.7)eV and LUMO=-(Ered+4.7) eV, respectively.The potensiostat was a PAR VersaSTAT4 and the working electrode used is platinum.Atmospheric pressure photoelectron spectroscopy: AAPPS has been performed on a Riken Keiki AC-2 spectrometer in thin film at room temperature.
Fabrication of Photovoltaic devices: All devices were fabricated using doctor-blading under ambient conditions.Prestructured indium tin oxide (ITO) substrates were cleaned with acetone and isopropyl alcohol in an ultrasonic bath for 10 minutes each.After drying, the substrates were successively coated with 40 nm of zinc oxide (ZnO), 10 nm of Ba(OH)2 and finally a 80-90 nm thick active layer based on TBDPTV:PC70BM (20 g L -1 ).To complete the fabrication of the devices 10 nm of MoOx and 100 nm of Ag were thermally evaporated through a mask (with a 10.4 mm² active area opening) under a vacuum of ~5x10 -6 mbar.J-V measurements: The J-V characteristics was measured using a source measurement unit from BoTest.Illumination was provided by a solar simulator (Oriel Sol 1A, from Newport) with AM1.5G spectrum at 100 mW/cm 2 .EQEs were measured using an integrated system from Enlitech, Taiwan.In order to study the light intensity dependence of current density, we used a series of neutral color density filters.The intensity of light transmitted through the filter was independently measured via a power meter.All the devices were tested in ambient air.Photo-CELIV: In photo-CELIV measurements, the devices were illuminated with a 405 nm laser-diode.Current transients were recorded across an internal 50 Ω resistor of an oscilloscope (Agilent Technologies DSO-X 2024A).We used a fast electrical switch to isolate the cell and prevent charge extraction or sweep out during the laser pulse and the delay time.After a variable delay time, a linear extraction ramp is applied via a function generator.The ramp, which was 20 μs long and 2 V in amplitude, was set to start with an offset matching the Voc of the cell for each delay time.
Atomic Force Microscopy: AFM measurements were performed on a solver nano from NT-MDT using 300 kHz single crystal silicon cantilevers (Nt-MDT, NSG30).

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Scheme 1 .
Scheme 1.Chemical structure of BODIPY dye and assignment of the different positions and forms

Figure 1 .
Figure 1.Absorption spectra of TBDPTV in chloroform solution and as thin film.
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Figure 4 .
Figure 4. Time-dependent photo-CELIV traces of TBDPTV-based solar cells at a fixed delay time of 1 μs.

Table 1 .
Molecular weight characteristics, optical and electrochemical properties of TBDPTV

Table 2 .
Photovoltaic parameters of TBDPTV:PC71BM system in different composition ratio