Synthesis of a tetrazine–quaterthiophene copolymer and its optical, structural and photovoltaic properties

Herein, we report the synthesis of a novel, tetrazine-based conjugated polymer. Tetrazines have the benefit of being strong electron acceptors, while little steric hindrance is imposed on the flanking thiophene rings. Conversion of a suitably substituted nitrile precursor led to 3,6-bis(5-bromo-4-(2-octyldodecyl)thiophen-2-yl)-1,2,4,5-tetrazine (2OD-TTz). Palladium-catalyzed copolymerization of 2OD-TTz with a bithiophene monomer yielded an alternating tetrazine–quaterthiophene copolymer (PTz4T-2OD). The polymer PTz4T-2OD showed an optical band gap of 1.8 eV, a deep HOMO energy level of − 5.58 eV and good solubility. In combination with the non-fullerene acceptor ITIC-F, solar cells with power conversion efficiencies of up to 2.6% were obtained. Electronic supplementary material The online version of this article (10.1007/s10853-019-03551-3) contains supplementary material, which is available to authorized users.


Introduction
Organic photovoltaics are gathering a tremendous amount of attention in the scientific community. In particular, fullerene-free organic solar cells have sparked interest amongst researchers since novel donor and acceptor structures have extended the range of possible combinations to tune the device performance to an optimum. By combining the right conjugated polymer donor and non-fullerene acceptor (NFA) partners, power conversion efficiencies (PCEs) have reached values of up to 15.6% in singlejunction solar cells [1][2][3] and 17% in tandem devices [4], while the quest for suitable materials on both sides is still ongoing. Thus, NFA-based solar cells have already outperformed PCBM ( [6,6]-phenyl-C 71butyric acid methyl ester)-based devices. It is not only possible to prepare NFAs in large quantities and good purity, but even more importantly, they can be Also for bis(thienyl)tetrazine-based polymers, placing the alkyl chains in 4-position instead of the 3-position on the flanking thiophenes leads to increased coplanarity due to less steric hindrance [19,20]. In combination with a cyclopenta[2,1-b:3,4b 0 ]dithiophene (CPDT) monomer, this type of tetrazine-based monomer led to the so far highest observed PCE of around 5.5% using PCBM as acceptor [21,22], while 5.1% were reached for benzo [1,2-b:4,5-b']dithiophene (BDT)-type tetrazine copolymers [23,24]. Alternative polymer structures led to less efficient solar cells; copolymers with dithieno[3,2-b:2 0 ,3 0 -d]silole (DTS) reached PCE values of up to 4.2% [25], whereas the best-performing fluorene [20] and carbazole [20] copolymers resulted in devices with efficiencies of up to 0.8% and around 2%, respectively.

Materials and methods
All reagents and solvents were purchased from commercial sources (Sigma-Aldrich or TCI) with reagent-grade quality and were used as received. All solvents were dried using a column-based solvent purification system except CH 2 Cl 2 , which was dried by distillation over CaH 2 as drying agent. ITIC-F was synthesized according to Refs. [14,26].

Polymer synthesis
For the copolymerization, 2OD-TTz (100.16 mg, 0.1037 mmol, 1 eq), 5,5 0 -bis(trimethylstannyl)-2,2 0bithiophene (51.05 mg, 0.1038 mmol, 1 eq), Pd 2 (dba) 3 (2.28 mg, 2.49 lmol, 0.024 eq) and P(o-tol) 3 (3.15 mg, 1.04 lmol, 0.1 eq) were placed in a 10-mL glass tube and dissolved in chlorobenzene (4 mL). The mixture was degassed with nitrogen for 30 min. Afterwards, the tube was sealed and placed into a conventionally heated synthesis reactor (Monowave 50 from Anton Paar GmbH, Graz, Austria) and subjected to the following temperature program: ramp to 180°C (for 10 min), 180°C (30 min holding time). The dark blue mixture was added dropwise to cold methanol to precipitate a blue polymer. The crude product was purified by Soxhlet extractions in acetone, cyclohexane and chloroform. The majority of the product was dissolved in the chloroform fraction. This solution was concentrated to a volume of about 5 mL and precipitated into cold methanol to obtain the purified polymer. Yield: 82.2 mg, blue-violet powder. 1

Characterization techniques
Nuclear magnetic resonance (NMR) spectroscopy was performed on Bruker Avance 300 MHz and Varian Inova 500 MHz spectrometers. Deuterated solvents were obtained from Cambridge Isotope Laboratories Inc. Spectra were referenced against the residual proton signals of the solvent according to the literature [30]. Peak shapes are specified as follows: s (singlet), bs (broad singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quadruplet) and m (multiplet). FT-IR spectroscopy measurements were acquired on a Bruker Alpha FT-IR spectrometer in transmission using undoped Si-wafers as substrates or in ATR-mode using an ALPHA Platinum ATR single reflection diamond ATR module. Silica gel 60 F254 and aluminium oxide 60 F254 (both from Merck) on aluminium sheets were used for thin-layer chromatography. Visualization was done under UV light or by dipping into an aqueous solution of KMnO 4 (0.1 wt%). MALDI-TOF mass spectrometry was performed on a Micromass TofSpec 2E time-of-flight mass spectrometer. The instrument was equipped with a nitrogen laser (k = 337 nm, operated at a frequency of 5 Hz) and a time lag focusing unit. Ions were generated just above the threshold laser power. Positive ion spectra were recorded in reflection mode with an accelerating voltage of 20 kV. The spectra were externally calibrated with a polyethylene glycol standard. Analysis of data was done with MassLynx-Software V3.5 (Micromass/Waters, Manchester, UK). High-temperature gel permeation chromatography (GPC) measurements were performed on an Agilent Technologies PL-GPC220 instrument with 1,2,4trichlorobenzene as eluent with a PLgel MIXED-B LS 300 9 7.5 mm column and a refractive index detector (1.00 mL min -1 , 150°C, 200 lL injection volume). Thermogravimetric analysis measurements were performed on a Netzsch STA 449 C thermogravimetric analyser using aluminium oxide crucibles in the temperature range between 20 and 550°C with helium as purge gas (flow rate: 50 mL min -1 ) and a heating rate of 10 K min -1 . Absorption spectra of the polymer thin films were recorded on a Shimadzu UV-1800 UV-Vis spectrophotometer in the range of 300-1000 nm. Absorption coefficients were determined from thin films deposited by spin coating from chlorobenzene solutions. Cyclic voltammetry measurements were carried out in acetonitrile using a three-electrode set-up consisting of a platinum (Pt) mesh (counter electrode), an Ag/Ag ? reference electrode [31] and an indium tin oxide (ITO)-coated glass substrate (15 9 15 mm, 15 X/sq, Kintec) coated with a thin film of PTz4T-2OD as working electrode. The reference electrode was calibrated against a ferrocene-ferrocenium solution (Fc/Fc ? ) in deoxygenated and anhydrous acetonitrile using tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 , 0.1 M) as supporting electrolyte and a scan rate of 50 mV s -1 . The ionization potential (IP) and the electron affinity (EA) were calculated from the onset of the oxidation and the reduction potential (E ox , E red ) of the polymer considering the energy level of Fc/Fc ? to be -4.8 eV below the vacuum level via where E ox(Fc) is the oxidation potential of ferrocene [32]. 2D-GIWAXS measurements of polymer thin films spin-coated on silicon substrates were performed on an Anton Paar SAXSpoint 2.0 system equipped with a Dectris 2D EIGER R 1 M hybrid photon counting detector with 75 lm 2 pixel size and using Cu K a radiation at 50 kV and 1 mA, which was point-collimated using automated scatterless slits. The incidence angle was set to 0.12°, and the exposure time was 10 9 120 s. A spin-coated silver behenate film was used for the angular calibration. Atomic force microscopy (AFM) measurements were performed on an Anton Paar Tosca TM 400 atomic force microscope in tapping mode using Al-coated cantilevers (ARROW-NCR, NanoWorld AG) with a resonance frequency of 285 kHz and a force constant of 42 N m -1 . All measurements were acquired at room temperature under ambient conditions. All calculations and image processing were done with Tosca TM analysis software (V7.4.8341, Anton Paar). Surface profilometry measurements were performed on a Bruker DektakXT stylus surface profiling system equipped with a 12.5-lm-radius stylus tip in order to determine the layer thickness of the thin-film samples. Line scans were recorded over a length of 1000 lm, with a stylus force of 3 mg, and a resolution of 0.33 lm pt -1 . Layer thickness values were derived from two-dimensional surface profiles using Vision 64 software (Bruker).

Solar cell fabrication
Pre-patterned ITO-coated glass substrates were cleaned by sonication in 2-propanol (40-50°C, 60 min) and oxygen plasma treatment (FEMTO, Diener Electronic, 3 min). For inverted bulk-heterojunction solar cells, ZnO thin films were derived from a sol-gel reaction of a zinc oxide precursor solution consisting of zinc acetate dihydrate (0.5 g, 2.3 mmol) in 2-methoxyethanol (5 mL) using ethanolamine (150 mL, 2.5 mmol) as the stabilizer [33]. The zinc oxide precursor solution was vigorously stirred overnight under ambient conditions for the hydrolysis reaction, followed by filtration through a 0.45-lm polytetrafluoroethylene (PTFE) syringe filter before spin coating (4000 rpm, 30 s). The ZnO films were annealed under ambient conditions (150°C, 15 min) to achieve layer thicknesses in the range of 30-40 nm.
One drawback of microwave-assisted polymerization is the risk of local overheating (''hot spots'') which can be observed in monomode microwave reactors (due to a higher local field density) and, in particular, at later stages of the polymerization procedure where higher viscosity of the reaction solution can potentially cause the magnetic stirring to fail. Since tetrazines are sensitive towards thermal decomposition, a conventionally heated synthesis reactor, which allows monitoring of the internal reaction temperature and permits reaction pressures of up to 20 bar, was employed. The resulting conjugated D-A polymer PTz4T-2OD consists of a 1,2,4,5-tetrazine acceptor moiety and four thiophene units partially substituted with branched aliphatic side chains as donor building block. The conjugated polymer exhibits good solubility in common organic solvents such as chloroform, chlorobenzene and ortho-dichlorobenzene. In the FT-IR spectrum of the polymer, characteristic bands for both the tetrazine (1504 cm -1 ) and the quaterthiophene (1067 cm -1 ) units are observed ( Figure S5) [35]. High-temperature gel permeation chromatography (GPC) measurements gave a number-average molecular weight (M n ) of 24.1 kDa, a weight-average molecular weight of 59.3 kDa (M w ) and a dispersity Ð M of 2.46 (Table 1, Figure S6). Thermogravimetric analysis (TGA) under helium atmosphere demonstrates good thermal stability of PTz4T-2OD up to 288°C. At this temperature, the tetrazine rings in the backbone are known to decompose to the corresponding quaterthiophene dinitriles [19,22] and nitrogen is released which accounts for the observed mass loss of around 3% (considering a molar weight of 1000 g mol -1 for the repeating unit and 28 g mol -1 for N 2 ). Previously, a yield of more than 90% has been reported for this reaction [19]. The remaining organic moieties further decompose at a decomposition temperature (T d ) of approx. 460°C (Fig. 1a). Table 2 show the absorption spectrum of a PTz4T-2OD thin film and the corresponding optical properties, respectively. PTz4T-2OD exhibits a broad absorption peak with a maximum at 583 nm together with a small peak at ca. 377 nm. The optical absorption coefficient a at the absorption maximum is approx. 1.9 9 10 5 cm -1 . The optical band gap (E g opt ) was determined from the onset of the thin-film absorption spectrum to be 1.80 eV. Cyclic voltammetry was performed to determine the highest occupied molecular orbital (HOMO) energy level of PTz4T-2OD using ferrocene-ferrocenium (Fc/Fc ? ) as external standard. From the cyclic voltammetry data (Figure S7), an ionization potential of -5.58 eV and an electron affinity of -3.10 eV were determined, giving an electrochemical energy gap of 2.48 eV. The optical band gap (1.80 eV) is significantly lower. Setting the value of the ionization potential equal to the HOMO energy level and using the optical band gap, a LUMO energy value of -3.78 eV was calculated. The corresponding data are summarized in Table 2. The difference between the optical and electrochemical band gap most likely resides in the very different conditions in which these two determinations were carried out. First, it is known that the solvents, in which chemical species are dissolved, may influence strongly the position of the HOMO and LUMO levels. Although the PTz4T-  2OD was prepared as a film on ITO, it is very likely that some swelling of the polymer in acetonitrile occurs. For optical band gap determination there are no solvents in which the polymer film is immersed. Second, as this polymer is a donor it is expected to undergo a much faster oxidation reaction than a reduction reaction. This is indeed clear from the cyclic voltammogram in Fig. S7; the kinetics of reduction are probably much slower than the kinetics of oxidation. This will introduce an overpotential, and thus the reduction potential used to estimate the electron affinity will be shifted to lower values. Third, although we use a supporting electrolyte we cannot be sure that all ohmic drops are negligible. While they will certainly not be very high due to the reasonable conductivity of the ITO and the thin polymer film used, they might add up with the other two effects and contribute to the increased value of the electrochemically determined energy gap.

Molecular packing and film morphology
Moreover, two-dimensional grazing incidence wideangle X-ray scattering (2D-GIWAXS) measurements of thin films of PTz4T-2OD were performed to gain information about the crystallization behaviour and molecular packing (Fig. 2). The GIWAXS image reveals a weak ring-like feature at q = 15.5 nm -1 , which is more pronounced in the out-of-plane direction. This suggests a preferred face-on packing orientation of the polymer chains in the film, and the d-spacing of approx. 0.40 nm is indicative of p-p stacking. In addition to the lamellar diffraction peak at q * 2.3 nm -1 (interlamellar distance of approx. 2.7 nm) in the in-plane direction, which is expected for a preferential face-on orientation with respect to the substrate, a further intense area is found in the out-of-plane direction, indicating a certain isotropy in the molecular packing. The crystalline coherence length (CCL), determined from the lamellar diffraction peak in the in-plane direction, is 12 nm.
The AFM topography image (Fig. 3a) of the polymer film spin-coated from a chlorobenzene solution reveals a very smooth surface morphology, and a very low surface roughness with an S q value of 1.5 nm is observed. For the application as absorber layer in organic solar cells, the polymer is blended with ITIC-F. The morphology of the blend film appears similar to the pristine polymer film, and the surface roughness is essentially the same (S q = 1.5 nm). Furthermore, the phase contrast in the blend film is very low due to the rather similar chemical composition of PTz4T-2OD and ITIC-F in the blend (Fig. 3c, d), which makes it difficult to draw any distinct conclusions about phase separation based on the AFM phase images.

Photovoltaic performance
PTz4T-2OD was investigated in bulk-heterojunction non-fullerene organic solar cells in inverted (indium tin oxide (ITO, ca. 140 nm)/ZnO (ca. 30-40 nm)/ PTz4T-2OD-ITIC-F (ca. 150 nm)/MoO 3 (10 nm)/Ag (100 nm)) and normal device configurations (ITO (ca. 140 nm)/PEDOT-PSS (ca. 40 nm)/PTz4T-2OD-ITIC-F (ca. 120 nm)/Al (100 nm)). A schematic representation of the device architectures and the corresponding energy level diagrams are given in Figure S8A and B. In addition to the well matching energy levels of PTz4T-2OD and ITIC-F, these compounds have complementary absorption properties, as can be seen in Figure S9. For the preparation of the solar cells, a PTz4T-2OD-ITIC-F weight ratio of 1:1 was used, as this ratio gave the best results in preliminary experiments. Table 3 and Fig. 4a show the photovoltaic performance parameters and the J-V curves measured under illuminated and dark conditions. In both device architectures, comparably high open-circuit voltages (V OC s) of around 0.9 V and short-circuit current density (J SC ) values between 6 and 7 mA cm -2 were obtained. In particular, the FF As already mentioned above, the best solar cell performance was obtained with an absorber layer thickness of about 150 nm. By using thinner absorber layers, the FF could be slightly improved, however, only at the expense of a reduced J SC (Table S1). Additional annealing of the absorber layers did not lead to improved solar cell characteristics. The  corresponding results are shown in Figure S10. Moreover, initial shelf life tests of the solar cells revealed promising results ( Figure S11). After 83 days of storage in inert atmosphere, the solar cells retained 92%, and after 184 days about 76% of their initial PCE.
The EQE spectra (Fig. 4b) show a broad photoresponse from 450 to 800 nm with a maximum at ca. 700 nm. The shapes of the EQE spectra are in good agreement with the optical absorption properties of the polymer and the acceptor. The region between 450 and 600 nm is mainly governed by the absorption of the conjugated polymer, while the photocurrent generated in the wavelength region between 650 and 800 nm originates from the contribution of the NFA ( Figure S5). The cumulated J SC values of 5.30 mA cm -2 (ZnO, inverted) and 5.21 mA cm -2 (PEDOT-PSS, normal) correlate well with the measured J SC (5.46 mA cm -2 for ZnO, 4.92 mA cm -2 for PEDOT-PSS) using a shadow mask (2.65 9 2.65 mm).
Furthermore, there is potential for improving the photovoltaic performance of the polymer since the good solubility of PTz4T-2OD would allow preparing batches with a higher molecular weight, while maintaining good processability. Increased molecular weight has been identified as a key to improving the efficiency of polymer-based solar cells [32,36]. To further optimize the material, diligent purification steps, including preparative GPC to selectively isolate high molecular weight fractions with a low polydispersity, are required and are expected to further push the performance towards higher PCEs [37].

Conclusions
The synthesized conjugated tetrazine-quaterthiophene copolymer PTz4T-2OD features a band gap of 1.8 eV, an absorption coefficient of up to 1.9 9 10 5 cm -1 and a good solubility with a molecular weight (M n ) of 24.1 kDa. In thin films, the polymer reveals a preferred face-on orientation with respect to the substrate and a smooth surface morphology. Moreover, a good thermal stability was observed up to 288°C, the temperature at which the tetrazine rings in the backbone start to decompose.
The photovoltaic performance of PTz4T-2OD was investigated in organic solar cells in combination with the non-fullerene acceptor ITIC-F in inverted and normal device configurations. PTz4T-2OD and  ITIC-F have complementary absorption properties, and the EQE spectra confirm a contribution of both materials to the photocurrent generation in a wavelength range between 400 and 800 nm. The solar cells showed high open-circuit voltages of approx. 900 mV; however, the photocurrents (6-7 mA cm -2 ) and FF values (40-44%) remained limited. The highest power conversion efficiencies of 2.6% were obtained in the inverted solar cell architecture.