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

The tetrazine monomer 2OD-TTz (3,6-bis(5-bromo-4-(2-octyldodecyl)thiophen-2-yl)-1,2,4,5-tetrazine) was synthesized in analogy to previously published protocols [27‐29]. Details for the synthetic procedures of 2OD-TTz can be found in the supporting information.

For the copolymerization, 2OD-TTz (100.16 mg, 0.1037 mmol, 1 eq), 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (51.05 mg, 0.1038 mmol, 1 eq), Pd₂(dba)₃ (2.28 mg, 2.49 µmol, 0.024 eq) and P(o-tol)₃ (3.15 mg, 1.04 µmol, 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. ¹H-NMR (δ, 20 °C, CDCl₃, 500 MHz): 8.06 (br, 2H), 7.23 (br, 4H), 2.82 (br, 4H), 2.11 (br, 2H), 1.93–0.55 (m, 76H), ¹³C-NMR (δ, 20 °C, CDCl₃, 125 MHz): 160.90, 138.66, 138.48, 134.64, 134.23, 127.98, 124.44, 124.11, 38.80, 37.16, 34.27, 33.52, 33.38, 32.09, 31.59, 30.07, 29.71, 29.39, 26.57, 22.82, 22.72, 14.14, FT-IR (cm⁻¹): 3062, 2956, 2924, 2854, 1548, 1504, 1455, 1349, 1067.

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 KMnO₄ (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 (λ = 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 1,2,4-trichlorobenzene as eluent with a PLgel MIXED-B LS 300 × 7.5 mm column and a refractive index detector (1.00 mL min⁻¹, 150 °C, 200 µL 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 × 15 mm, 15 Ω/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 (Bu₄NPF₆, 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 viawhere 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 µm² pixel size and using Cu K_α 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 × 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™ 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™ 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-µm-radius stylus tip in order to determine the layer thickness of the thin-film samples. Line scans were recorded over a length of 1000 µm, with a stylus force of 3 mg, and a resolution of 0.33 µm pt⁻¹. Layer thickness values were derived from two-dimensional surface profiles using Vision 64 software (Bruker).

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-μm 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. PTz4T-2OD was dissolved in chlorobenzene at 90 °C, blended with ITIC-F in a donor–acceptor ratio of 1:1 by weight (16 mg mL⁻¹ total concentration), and spin-coated onto preheated substrates to obtain a layer thicknesses of about 150 nm. A molybdenum(VI) oxide anode interfacial layer (10 nm, deposition rate: ca. 0.1–0.4 Å s⁻¹) and a silver anode (100 nm, 0.2–1.0 Å s⁻¹) were deposited by thermal evaporation under reduced pressure (ca. 10⁻⁵ mbar) through a shadow mask, defining the active area (9 mm²). For bulk-heterojunction solar cells in normal device configuration, poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS, Heraeus Clevios P VP AI 4083) was filtrated through a 0.45-µm polyvinylidene difluoride (PVDF) syringe filter and spin-coated onto the pre-cleaned ITO substrates under ambient conditions, followed by annealing (120 °C, 20 min) to obtain layer thicknesses of ca. 40 nm. The absorber layer was processed using the same procedure as described above. An aluminium cathode (100 nm, 0.3–8.0 Å s⁻¹) was deposited by thermal evaporation under reduced pressure (ca. 10⁻⁵ mbar) through a shadow mask, defining the active area (9 mm²).

Current density–voltage (J–V) curves were recorded under illuminated and dark conditions in an inert atmosphere using a Keithley 2400 source meter and a Dedolight DLH400D metal halide lamp (1000 W m⁻²), which was calibrated with a standard reference silicon solar cell (Fraunhofer ISE). Photovoltaic characteristic parameters were determined from the J–V curves under illumination and averaged over five devices, unless otherwise stated. Series (R_S) and shunt (R_SH) resistance values were extracted from the J–V curves under illumination. External quantum efficiency (EQE) measurements were performed under ambient conditions using an Amko MuLTImode 4-AT monochromator equipped with a 75-W xenon lamp (LPS 210-U, Amko), a lock-in amplifier (Stanford Research Systems, Model SR830) and a Keithley 2400 source meter. The monochromatic light was chopped at a frequency of 30 Hz. The EQE spectra were measured in the wavelength range of 350–1000 nm (increment: 10 nm). The measurement set-up was spectrally calibrated with a reference photodiode (Newport Corporation, 818-UV/DB).

3,6-Bis(5-bromo-4-(2-octyldodecyl)thiophen-2-yl)-1,2,4,5-tetrazine (2OD-TTz) was synthesized from 5-bromo-4-(2-octyldodecyl)thiophene-2-carbonitrile 5. In order to avoid any traces of monosubstituted tetrazine monomer causing the formation of shorter polymer chains, the tetrazine formation step was performed last in the synthetic sequence. In order to obtain 5, 3-bromothiophene was subjected to a Kumada coupling reaction with 2-octyldodecyl magnesium bromide leading to 3-(2-octyldodecyl)thiophene [27], followed by bromination with N-bromosuccinimide (NBS) resulting in 2-bromo-3-(2-octyldodecyl)thiophene. Subsequently, this compound was deprotonated followed by quenching with DMF to introduce an aldehyde functionality which was subsequently converted to the corresponding nitrile 5 [28, 29]. The synthetic procedures for 5 including the intermediates 1–4 are described in detail in the supporting information. From 5, the monomer was prepared using Pinner conditions with sulphur as catalyst. Due to the large hydrophobic 2-octyldodecyl moiety, the standard solvent (ethanol) had to be replaced with isopropanol. The intermediately formed dihydrotetrazine was isolated as a crude product and directly converted to the corresponding tetrazine using sodium nitrite and acetic acid, as shown in Scheme 1.

PTz4T-2OD (poly[(1,2,4,5-tetrazin-3,6-diyl)-alt-(3,3′′′-di(2-octyldodecyl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)]) was synthesized via a Stille cross-coupling polymerization of 2OD-TTz and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene using Pd₂(dba)₃ as catalyst, as shown in Scheme 2 [34].

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 Đ_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 N₂). 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).

Figure 1b and 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 α at the absorption maximum is approx. 1.9 × 10⁵ 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.

Moreover, two-dimensional grazing incidence wide-angle 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 π–π 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.

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)/MoO₃ (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_OCs) 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 values remained low. For example, FFs of approx. 43% were obtained in the inverted device architecture and in the normal configuration only FFs of 37% could be realized. The slightly higher R_S and the significantly lower R_SH in the normal device architecture might originate from the absence of a hole-blocking layer between the absorber layer and the Al-electrode. Overall, solar cells with PCEs of up to 2.6% could be realized in the inverted architecture and maximum PCEs of 2.35% were reached with solar cells prepared in normal architecture.

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 × 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].