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Proceeding Paper

Weak Donor, Strong Acceptor Thienopyrazine-Based Polymers for Fine Tuning of LUMO Levels—Suitable Materials for Energy and Storage Solutions †

by
Benedetta Maria Squeo
1,*,
Elisa Lassi
1,
Chiara Botta
1,
Silvia Luzzati
1,
Barbara Vercelli
2,
Stefania Zappia
1 and
Mariacecilia Pasini
1,*
1
Istituto di Scienza e Tecnologie Chimiche (SCITEC), Consiglio Nazionale Delle Ricerche, Via Corti 12, 20133 Milano, Italy
2
Istituto di Chimica Della Materia Condensata e di Tecnologie per l’Energia (ICMATE), Consiglio Nazionale Delle Ricerche, Via Roberto Cozzi 53, 20125 Milano, Italy
*
Authors to whom correspondence should be addressed.
†
Presented at the 25th International Electronic Conference on Synthetic Organic Chemistry, 15–30 November 2021; Available online: https://ecsoc-25.sciforum.net/.
Chem. Proc. 2022, 8(1), 71; https://doi.org/10.3390/ecsoc-25-11742
Published: 14 November 2021
(This article belongs to the Proceedings of The 25th International Electronic Conference on Synthetic Organic Chemistry)
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Versions Notes

Abstract

:
One approach creating and to tuning the energy levels of small band gap polymers or molecules is alternating donor-like (electron donating) and acceptor-like (electron withdrawing) monomers. Polymer for solar cells need to maintain a low highest occupied molecular orbital (HOMO) energy level in order to maximize the open circuit voltage (Voc) and have a small band gap in order to maximize the short-circuit current (Jsh). Alternating a “weak donor” and a “strong acceptor” will maintain a low HOMO energy level and reduce the band gap via intramolecular charge transfer (ICT). Here we present the synthesis and characterization of a series of polymers based on thienopyrazine (acceptor) and carbazole (donor). We compared the optoelectronic properties of the classic polymer poly-carbazole-dithiophene-benzothiadiazole (PCDTBT) with one where benzothiadiazole was substituted with thienopyrazine and one where we randomly alternated thienopyrazine and benzothiadiazole as the acceptor unit while maintaining the same donor unit. The approach of alternating weak donor and strong acceptor moieties allowed us to obtain fine LUMO level modulation of the materials, opening the way to the design of new donor materials.

1. Introduction

The increasing global energy demand and the necessity to shift from fossil fuels to renewable energy sources demand changes in energy production and storage solutions.
Conjugated semiconducting materials in between conjugated polymers may be the protagonists of this epochal change of paradigm because they can be low-cost, lightweight, flexible, derived from abundant raw materials, low in toxicity, low in environmental impact and able to meet to circular chemistry demands [1,2,3,4,5,6,7].
Additionally, a wide range of electronic properties can be accessed through well-developed synthetic organic chemistry. This has allowed conjugated polymers to provide optical and electronic properties for many applications, such as light-emitting diodes (OLEDs), organic photovoltaics (OPVs), field-effect transistors (OFETs), biosensors and energy storage devices [8,9,10,11,12,13,14,15].
As far as p-type materials are concerned, satisfactory results have already been obtained, but there is still much to be done with n-type materials [16,17,18]. The development of new n-type materials is fundamental for the creation of organic solar cells with efficiencies exceeding 10%, but it is also particularly significant for pseudocapacitors based on to conjugated polymers [19]. In fact, most of these conjugated polymers are only capable of storing charge in a “positive” potential range, which results in low operating voltages and limited energy and power densities due to the lack of stable n-type redox-active materials [20].
The D–A approach with the repeating units consisting of an electron-donating (donor) moiety and an electron-withdrawing (acceptor) moiety has been successfully proposed for developing this class of materials [21,22,23,24]; specifically, donor–acceptor copolymers are combinations of an electron donor and an acceptor unit copolymerized via traditional organometallic coupling [25]. As shown in Figure 1, hybridization of frontier molecular orbitals (HOMO and LUMO) of donor and acceptor units yields a low bandgap material with absorption extended into the near-IR region, which has made these systems popular in OPVs’ active layers. Moreover, polymers for solar cells need to maintain a low HOMO energy level in order to maximize the open circuit voltage (Voc), and a small band gap in order to maximize the short-circuit current (Jsh) [26,27,28]: alternating a “weak” donor and a “strong” acceptor will maintain a low HOMO energy level thanks to the weak donor, and reduce the band gap via Intramolecular charge transfer (ICT) thanks to the “strong” acceptor [29,30].
Very recently [31], the use of the proquinoidal acceptors strategy has been proposed for the development of n-type copolymers, as it is able to promote strong π-electron correlations, extensive delocalization and a very narrow bandgap. An aromatic moiety can in fact be described as “proquinoidal” if it has an important resonance contribution from the quinoidal form, and the proquinoidal character can be improved by fusing additional aromatic rings [32]. Notable examples of proquinoidal–aromatic copolymers with proquinoidal building blocks, including thieno[3,4-b]pyrazine (TP) [33], have been developed as active materials for organic field-effect transistors, photovoltaics, organic light-emitting diodes and many other applications [34].
The TP structure is an isomer of the well-known 2,1,3-benzothiadiazole (BT) [35], which has been successfully employed as an OPV active material, achieving PCEs over 18% [36,37]. In addition, the TP unit, unlike BT (Figure 1), has an important resonance contribution from the quinoidal form in the resonance structures, forming a six membered aromatic sextet, and thus additionally lowering the band gap of conjugated polymers with respect to BT [38]. Another potential advantage of TP in conjugated polymers is that the TP moiety provides a more planar backbone between repeating units owing to decreased steric hindrance of the thiophene ring compared to the benzene ring in BT-based copolymers
TP-based materials have been developed for OPV applications, and moreover, they possess many attributes that are advantageous to pseudo-capacitors, such as the ability to reversibly store negative charge [19,39].
By miming the well-known poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), we have synthetized the analogue poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2-thieno[3,4-e]pyrazines)] and a random copolymer 9-(9-Heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester containing 4,7-Bis(2-bromo-5-thienyl)-2,1,3-benzothiadiazole and 4,7-Bis(2-bromo-5-thienyl)-2 Thieno[3,4-e]pyrazines)]. In our polymers, there is significant internal charge transfer character between electron-rich (donor) and electron-deficient (acceptor) components; furthermore, they maintain the ability to reversibly store negative charge, and the random copolymer approaches [40] seem to be effective for the fine modulation of HOMO and LUMO levels. Finally, the polymers are highly soluble in organic solvents and can be processed by solution deposition approaches, rendering them versatile and scalable.

2. Materials and Methods

General Information for synthesis. Unless specifically mentioned, all reagents were purchased from a commercial source and used without further purification. Toluene was freshly distilled prior to use according to a procedure in the literature. All reactions were carried out in inert atmosphere. Compound M1 was prepared according to the procedures reported in the literature [41,42].
PCDTBT was purchased from 1-Materials. PC70BM (99.5% purity) was purchased from Solenne BV.
Electrochemistry. Electrochemistry was performed at room temperature in acetonitrile under nitrogen in three electrode cells. The counter electrode was platinum; reference electrode was Ag/Ag+ (0.1 m AgNO3 in acetonitrile, 0.34 V vs. SCE, −4.73 V vs. vacuum); supporting electrolyte was 0.1 m tetrabutylammonium perchlorate (TBAP). The voltammetric apparatus was Metrohm Autolab 128N potentiostat/galvanostat. The working electrode for cyclic voltammetry (CV) was a glassy-carbon (CG) minidisc electrode (0.2 cm2). HOMO and LUMO levels were estimated according to the equation EHOMO = −(Eox + 4.39 + 0.34) [43]. PCDTBT, PCDTTB and PC(DTTP)5(DTBT)0.5 films were casted on a preheated electrode surface at around 60 °C. By means of a micro-syringe, 2 μL of 2 mg mL−1 chloroform solutions of the polymers were dropped on the electrode surface.
Size Exclusion Chromatography (SEC). The molecular weight distribution (MWD) of every sample was obtained by an integrated high-temperature (HT) size exclusion chromatography (SEC) system. The chromatographic system consisted of a GPCV2000 system from Waters (USA) equipped on-line with a differential refractometer (DRI) as concentration detector.
Experimental Conditions: column set constituted a 3 Shodex HT from Showa Denko (Japan); mobile phase: o-dichlorobenzene (o-DCB); flow rate: 0.8 mL/min; temperature: 135 °C.
Nuclear Magnetic Resonance (NMR). 1H NMR and 13C NMR spectra were recorded with a Bruker DRX 600 MHz spectrometer.
Synthesis of compound M2. Compound M1 (1 40 mg, 0.47 mmol, 1 eq.) was dissolved in dry chloroform (CHCl3, 25 mL) and cooled down to 0 °C. Then, N-bromosuccinimide (NBS; 174 mg, 0.98 mmol, 2.1 eq.) and glacial acetic acid (CH3COOH, 25 mL) were added. The reaction was allowed to reach room temperature in the dark by shielding the flask with aluminum foil. After 1 h at room temperature, the crude product was diluted with CHCl3 and washed 3 times with a saturated sodium bicarbonate (NaHCO3) solution. The organic solvent was removed under reduced pressure, and the product was purified by silica gel chromatography and recrystallized from hexane/dicloromethane (143 mg, 66% yield). 1H NMR (CDCl3) δ (ppm): 8.51 (s, 2H), 7.31 (d, 2H), 7.1(d, 2H).
Synthesis of polymer PCDTTP. A mixture of monomer M2(65 mg, 0.142 mmol, 1 eq.), 9-(9-heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester) (93 mg, 0.142 mmol, 1 eq.), Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (3.3 mg, 2% mmol) and tetrabutylammonium bromide (TEBAB) was added to a predegassed schlenk, followed by three vacuum/nitrogen cycles. Then, dry toluene (4 mL) and degassed potassium carbonate aqueous (2 mL) solution were added. The mixture was stirred at 110 °C. After 5 h, there was precipitation of the polymer, and the reaction was quenched. The crude was precipitated in methanol. The polymer was washed on Soxhlet apparatus with methanol, hexane and chloroform. The chloroform fraction was evaporated under reduced pressure, and the polymer was precipitated in methanol, filtered and dried under high vacuum, providing a dark solid (25 mg, 35% yield of chloroform fraction). 1H NMR (CDCl3) δ (ppm): 8.6 (broad, 2H), 8.1–8.0 (broad, 2H), 7.9–7.7 (broad, 4H), 7.58 (broad, 2H), 7.4 (broad, 2H, 4.1 (broad, 1H) 1.2–1.1 (broad, 26H), 0.8 (broad, 8H); MW = 3.7 kDa, MN = 8.1 kDa, PD = 2.22.
Synthesis of polymer PC(DTTP)0.5(DTBT)0.5. A mixture of monomer M2 (58 mg, 0.126 mmol, 0.5 eq.), 4,7-Bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (58 mg, 0.126 mmol, 0.5 eq.), 9-(9-heptadecanyl)-9H-carbazole-2,7-diboronic acid bis(pinacol) ester) (166 mg, 0.253 mmol, 1 eq.), Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (6 mg, 2% mol) and tetrabutylammonium bromide (TEBAB) was added to a predegassed schlenk, followed by three vacuum/nitrogen cycles. Then, dry toluene (6 mL) and degassed potassium carbonate aqueous solution (3 mL) were added. The mixture was stirred at 110 °C. After 6 h, the polymer started to precipitate, and the reaction was quenched. The crude was precipitated in methanol. The polymer was washed on Soxhlet apparatus with methanol, hexane and chloroform. The chloroform fraction was evaporated under reduced pressure, and the polymer was precipitated in methanol, filtered and dried under high vacuum, providing a dark solid (95 mg, 46% yield of chloroform fraction). 1H NMR (CDCl3) δ (ppm): 8.54 (broad, 2H), 8.2–8.1 (broad, 4H), 7.9–7.7 (broad, 6H), 7.6–7.4 (broad, 4H), 7.2–7.0 (broad, 2H), 4.8–4.7 (broad, 1H) 1.2–1.1 (broad, 26 H), 0.8 (broad, 8H). (MW = 3.2 kDa, MN = 6.1 kDa, PD = 1.81).

Device Fabrication and Photovoltaic Characterization

Direct Geometry devices’ fabrication. Solar cells were assembled with the conventional structure: glass/ITO/PEDOT:PSS/PCDTTP:PC70BM/Ca/Al. Glass ITO (Kintec) 15 Ω/sq substrates were mechanically cleaned with peeling tape and paper with acetone and then were washed in a sonic bath at 50 °C for 10 min sequentially with water, acetone and isopropanol. After drying with compressed nitrogen flow, 10 min plasma treatment in the air was used to enhance ITO wettability for the next deposition. PEDOT:PSS (Al VP 8030 from Heraus) was filtered on a 0.45 μm nylon filter and spin-coated in the air at 2500 rpm for 50 s. The device assembly was then performed in a glovebox. The active layer was composed by a blend dissolved at 1:4 wt/wt of PCDTTP:PC70BM solution in a 1,2-dichlorobenzene–chlorobenzene 3:1 v:v mixture at a total concentration of 30 mg/mL. The solution was stirred for 12 h on a hotplate in glovebox at 60 °C. The active layer was spin-coated from the warm solution at 1000 rpm for 60 s, which resulted in a thickness of 95 nm; at 1200 rpm for 60 s, which resulted in a thickness of 85; and at 1500 rpm for 60 s, which resulted in a thickness of 70 nm. Finally, the substrates were stored in a glovebox and annealed at 70 °C for 15 min. Finally, 10 nm of calcium and a 100 nm thick aluminum electrode were evaporated on the top of the device through a shadow mask under a pressure of 2 × 10−6 mbar. The deposition rate was 0.5 nm/s. There were six devices on a single substrate, each with an active area of 6.1 mm2.
Device characterization and measurements. The devices were characterized through current density–voltage and external quantum efficiency characterization. Current density–voltage measurements were performed directly in the glovebox where the solar cells were assembled, with a Keithley 2602 source meter, under an AM 1.5G solar simulator (ABET 2000). The incident power, measured with a calibrated photodiode (Si cell + KG5 filter), was 100 mW/cm2. The EQE spectral responses were recorded by dispersing an Xe lamp through a monochromator, using an Si solar cell with a calibrated spectral response to measure the incident light power intensity at each wavelength. The devices were taken outside the glovebox for the EQE measurements, after mounting them on a sealed cell to avoid moisture and oxygen exposure.

3. Results and Discussion

3.1. Chemical Characterization

Polymers PCDTTP and PC(DTTP)0.5(DTBT)0.5 were synthesized through Suzuki cross-coupling of a dibromide of the acceptor units and the bis-boronic ester of carbazole, with Pd(PPh3)4 as a catalyst and K2CO3 as a base in degassed toluene as solvent (Scheme 1). Both polymers showed low molecular weight due to the poor solubility of the growing chain during polymerization.
The yields of the polymers were 35% for PCDTTP and 46% PC(DTTP)0.5(DTBT)0.5 and were calculated for only the chloroform soluble fraction. As reported in the literature, randomly distributed conjugated terpolymers may have higher solubility and a lower tendency to aggregate in comparison to their parent polymers with more regularly repeating units [44]. Additionally, in this case the different yields was most probably due to the lower solubility of PCDTTP with respect to PC(DTTP)0.5(DTBT)0.5. After the extraction in chloroform, there was still some PCDTTP that was not soluble even in dichlorobenzene.
We decided to use a non-substituted TP derivative in order to have a direct comparison with the well-known PCDTBT copolymer. Moreover, the strategy of donor–acceptor conjugated polymers with minimally substituted acceptor moieties [45] has been revealed to be particularly useful in order to understand the electronic role of the acceptor, and the impact of the steric hindrance of the acceptor can be clarified in future studies by inserting linear or branched lateral alkyl chains into TP, or thienyl moieties. For this reason, it was not possible to use palladium-catalyzed direct C–H arylation coupling due to the formation of cross-linked byproducts in the unsubstituted positions [46].
The 1H NMR spectra of the two synthesized polymers confirmed the presence of the polymers. The peaks are large and broad, in line with the polydispersity and poor solubility of the materials in room temperature CDCl3. The FT-IR spectra showed the characteristic aliphatic and aromatic C−H bending between 3000 and 2700 cm−1, and the C=C and C=N stretching that is characteristic of aromatic compounds.

3.2. Optoelectronic Characterization

The absorption spectra in thin films and CHCl3 solution of the synthesized polymers PCDTTP and PC(DTTP)0.5(DTBT)0.5 and the purchased PCDTB were recorded. The polymers showed two major absorption peaks, a common feature for alternating D–A copolymers, and the two new polymers showed red-shifted absorption compared to PCDTBT. In particular, the random copolymer PC(DTTP)0.5(DTBT)0.5 showed a panchromatic absorption spectrum from 450 to 700 nm (Figure 2), and the low energy peak, ascribed to intramolecular charge transfer (ICT), showed the two contributions of the precursor copolymers.
The optical energy gap (Table 1) was calculated from the onset of the solid-state absorption spectra: the two new polymers have roughly the same energy gap (1.60 eV), smaller than that of the PCDBT polymer (1.89 eV).
Cyclic voltammetry (CV) determinations (Figure 3b) showed that PCDTBT and PCDTTB presented one irreversible oxidation process each, at 0.589 and 0.627 V, respectively, which probably involves their electron-donating (donor) moieties. In PC(DTTP)0.5(DTBT)0.5, the oxidation process split into two reversible ones separated by ca. 0.24 V (see Table 1). The latter could be a solid-state effect which may be accounted for by the formation of mixed-valence states analogous to those found in TTF polymers [47]. All samples presented a single reversible reduction process, at comparable potentials, which is probably localized on the electron-withdrawing (acceptor) moieties of the molecules.
It is important to underline a lowering of the electrochemical Eg from PCDTBT to PCDTTP, and then to PC(DTTP)0.5(DTBT)0.5. This behavior can be attributed to the greater contribution of the quinoid form in the polymer containing the TP unit additionally lowering the band gap. Furthermore, the presence of the TP moiety resulted in increased backbone planarity thanks to the decreased steric hindrance of the five membered thiophenic ring compared to the six membered benzenic ring. The further decrease in electrochemical Eg in the random copolymer can be attributed to the increase in the mean length of conjugation in the D–A segments.
We have investigated the influences of the solvent polarity on the optical properties of the polymers PCDTTP and PC(DTTP)0.5(DTBT)0.5, as shown in Figure 4. We have analyzed absorption and emission spectra in six different solvents, and we observed that absorption spectra were poorly influenced by the polarity of the solvent. In the emission spectra, we observed solvatochromism. The emission spectra of the polymers were redshifted with increasing solvent polarity. To confirm, we plotted the redshift vs. solvent polarity: in Figure 4e, a clear linear correlation can be observed, which is ascribed to the stabilization of the highly polar, excited charge-transfer states in more polar solvents, typical of donor–acceptor emitters [48,49].

3.3. Photovoltaic Characterization

The photovoltaic characteristics were investigated upon blending PCDTTP with PC71BM in bulk heterojunction devices. The current density–voltage curves (J–V) of devices with different active layer thicknesses under AM1.5-simulated solar light illumination (100 mW/cm2) are shown in Figure 5a, and the corresponding photovoltaic parameters are depicted in Table 2. The power conversion efficiency (η) obtained with PCDTTP reached 1.24%, which is in line to the values reported in the literature for other TP-based OPV devices [39]. The PCDTTP devices exhibited an open circuit voltage (Voc) of 0.58 V, which is about 0.2–0.3 V lower than the benchmark PCDTBT solar cells according to the literature [50]. Such a reduction in the Voc could result from the upward shift of the HOMO level of PCDTTP, as displayed in Figure 3c [26]. In addition, the short-circuit current density Jsc reached 5.5 mA/cm2 in PCDTTP devices, which is about a half of the typical values reported for benchmark PCDTBT cells [49]. One of the reasons that may eventually contribute to the relatively lower Jsc and EQE values is the lower LUMO-energy-level offset between the polymer and the fullerene component, which may limit the charge photogeneration yield [51]. The relatively low FF of 0.39 suggests non-optimal charge separation or transport to the electrodes. Interestingly, the EQE spectrum displayed in Figure 5b exhibits an onset at 780 nm (1.59 eV), which is red-shifted when compared to PCDTBT devices [52]. This is consistent with the reduced optical energy gap of PCDTTP displayed in Table 1.

4. Conclusions

In summary, we synthesized an alternating donor–acceptor copolymer via Suzuki coupling based on 9′-heptadecanyl-2,7-carbazole as the donor and 2-thienyl-2 thieno[3,4-e]pyrazines as the acceptor; the random terpolymer from the same donor unit; and two different acceptors unit—namely, 2-thienyl-2 thieno[3,4-e]pyrazine and its isomer, 2-thienyl-2′,1′,3′-benzothiadiazole. The polymers were chemically characterized; and their optical, electrochemical and photovoltaic properties were discussed also in comparison to the well-known PCDTBT.
The TP-based copolymer showed red-shifted absorption and emission spectra both in solution and as a thin film with respect to the isomeric-BT-based copolymer, likely due to the greater contribution of the quinoid form in the presence of TP. The random terpolymer showed extended absorption and lower electrochemical Eg compared to the parent polymers, and this behavior could be ascribed to the increased delocalization of the system due to the presence of two acceptors with different strengths, as previously observed [40].
This work will help guide the synthetic design of n-type materials that can be applied as acceptors in OPV devices, but also will contribute to the long-standing challenge of finding a stable n-type material for supercapacitors for energy storage. Specifically, we showed that it is possible to lower the LUMO levels by using random tercopolymer approaches with two different acceptor units while maintaining the electrochemical reversibility of the reduction.

Author Contributions

B.M.S. performed the synthesis of materials; E.L. contributed to OPV preparation and characterization; C.B. was responsible for PL spectra; S.L. contributed to the discussion of PV results; B.V. was responsible for CV; S.Z. was responsible for SEC and FT-IR characterization; M.P. and B.M.S. designed the study, discussed the data and prepared the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This project has received funding from the ATTRACT-POSEIDON project funded by the EC under Grant Agreement 777222.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Chemproc 08 00071 g001 550
Figure 1. Aromatic and quinoide resonance structures of thienopyrazine and benzothiadiazole units (left). Hybridization of frontier molecular orbitals (HOMO and LUMO) of donor and acceptor units (right).
Figure 1. Aromatic and quinoide resonance structures of thienopyrazine and benzothiadiazole units (left). Hybridization of frontier molecular orbitals (HOMO and LUMO) of donor and acceptor units (right).
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Scheme 1. Synthesis of compound M2, polymer PCDTTB and polymer PC(DTTP)0.5(DTBT)0.5.
Scheme 1. Synthesis of compound M2, polymer PCDTTB and polymer PC(DTTP)0.5(DTBT)0.5.
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Figure 2. FT-IR spectra of polymers PCDTTP and PC(DTTP)0.5(DTBT)0.5.
Figure 2. FT-IR spectra of polymers PCDTTP and PC(DTTP)0.5(DTBT)0.5.
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Figure 3. (a) Absorption spectra in an CHCl3 solution and in a solid state for polymers PCDTBT, PCDTTP, PC(DTTP)0.5(DTBT)0.5. (b) Voltammograms of polymers PCDTBT, PCDTTP, PC(DTTP)0.5(DTBT)0.5. (c) Energy levels diagram.
Figure 3. (a) Absorption spectra in an CHCl3 solution and in a solid state for polymers PCDTBT, PCDTTP, PC(DTTP)0.5(DTBT)0.5. (b) Voltammograms of polymers PCDTBT, PCDTTP, PC(DTTP)0.5(DTBT)0.5. (c) Energy levels diagram.
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Figure 4. Absorption and emission spectra of polymers PCDTTP (a,b) and PC(DTTP)0.5(DTBT)0.5 (c,d) in different solvents (hexane, toluene, chloroform, tetrahydrofuran, N,N-dimethylformamide, ethyl acetate). (e) Stokes shifts (solid points) against normalized solvent polarity parameter ETN with the best linear fit (solid lines).
Figure 4. Absorption and emission spectra of polymers PCDTTP (a,b) and PC(DTTP)0.5(DTBT)0.5 (c,d) in different solvents (hexane, toluene, chloroform, tetrahydrofuran, N,N-dimethylformamide, ethyl acetate). (e) Stokes shifts (solid points) against normalized solvent polarity parameter ETN with the best linear fit (solid lines).
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Figure 5. (a) Current density–voltage curves vs. active layer thicknesses of PCDTTP:PC71BM devices. Continuous lines: under 1 sun of AM1.5G solar simulation (100 mW/cm2); dotted lines: dark curves. (b) EQE spectra vs. active layer thicknesses of PCDTTP:PC71BM devices.
Figure 5. (a) Current density–voltage curves vs. active layer thicknesses of PCDTTP:PC71BM devices. Continuous lines: under 1 sun of AM1.5G solar simulation (100 mW/cm2); dotted lines: dark curves. (b) EQE spectra vs. active layer thicknesses of PCDTTP:PC71BM devices.
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Table
Table 1. Optoelectronic properties of polymers PCDTBT, PCDTTP, PC(DTTP)0.5(DTBT)0.5.
Table 1. Optoelectronic properties of polymers PCDTBT, PCDTTP, PC(DTTP)0.5(DTBT)0.5.
PolymerSolution [nm] aFilm [nm] bEgopt
[eV] c		Eox
[eV]		Ered
[eV]		HOMO
[eV] d		LUMO
[eV]		Egcv
[eV] e
λmaxλmaxλonset
PCDTBT5605816561.890.53−1.70−5.27−3.032.24
PCDTTP6356557751.610.35−1.56−5.08−3.171.91
PC(DTTP)0.5(DTBT)0.5518; 611550; 6407661.600.20−1.51−4.93−3.231.7
a Diluted polymer solution in CHCl3; b thin film on glass; c estimated from the absorption band edge in the film, Egopt = 1240/λonset (eV); d EHOMO = −4.39 − (Eox + 0.34) eV; ELUMO = −4.39 − (Ered + 0.34) eV; e Egcv = ELUMO − EHOMO.
Table
Table 2. Summary of the photovoltaic parameters.
Table 2. Summary of the photovoltaic parameters.
ThicknessVoc [V]FF [-]Jsc [mAcm−2]η [-]JscEQE [mAcm−2] a
701.890.53−5.27−3.032.24
751.610.35−5.08−3.171.91
951.600.20−4.93−3.231.7
a Calculated from the AM1.5G and EQE spectra.
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Squeo, B.M.; Lassi, E.; Botta, C.; Luzzati, S.; Vercelli, B.; Zappia, S.; Pasini, M. Weak Donor, Strong Acceptor Thienopyrazine-Based Polymers for Fine Tuning of LUMO Levels—Suitable Materials for Energy and Storage Solutions. Chem. Proc. 2022, 8, 71. https://doi.org/10.3390/ecsoc-25-11742

AMA Style

Squeo BM, Lassi E, Botta C, Luzzati S, Vercelli B, Zappia S, Pasini M. Weak Donor, Strong Acceptor Thienopyrazine-Based Polymers for Fine Tuning of LUMO Levels—Suitable Materials for Energy and Storage Solutions. Chemistry Proceedings. 2022; 8(1):71. https://doi.org/10.3390/ecsoc-25-11742

Chicago/Turabian Style

Squeo, Benedetta Maria, Elisa Lassi, Chiara Botta, Silvia Luzzati, Barbara Vercelli, Stefania Zappia, and Mariacecilia Pasini. 2022. "Weak Donor, Strong Acceptor Thienopyrazine-Based Polymers for Fine Tuning of LUMO Levels—Suitable Materials for Energy and Storage Solutions" Chemistry Proceedings 8, no. 1: 71. https://doi.org/10.3390/ecsoc-25-11742

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