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Effect of Ester Side Chains on Photovoltaic Performance in Thiophene-Thiazolothiazole Copolymers

2021, Vol.94, No.8

2019-2027

Effect of Ester Side Chains on Photovoltaic Performance in Thiophene-Thiazolothiazole Copolymers

Kodai Yamanaka ,Masahiko Saito ,Tsubasa Mikie , and Itaru Osaka ^*

Graduate School of Advanced Science and Engineering, Hiroshima University, 1-4-1 Kagamiyama Higashi-Hiroshima, Hiroshima 739-8527, Japan

https://doi.org/10.1246/bcsj.20210172

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Abstract

Section:

Thiazolothiazole-based π-conjugated polymers are promising semiconducting materials in organic photovoltaics (OPVs). In this study, we report on a series of thiophene-thiazolothiazole based polymers having ester side chains, PTzBTE and PTzBTEE. We first show a new synthetic methodology for their common monomer having ester group, in which the total yield was significantly improved by a factor of ten compared to the previous methodology. This resulted in polymer samples with high molecular weights. We then show that OPV cells using PTzBTE, in combination with PC₆₁BM, gave quite high efficiencies of as high as 8.9% despite its limited absorption range. This value is significantly higher than that for PTzBT having only alkyl groups as the side chain and that obtained for a low-molecular weight PTzBTEE sample reported previously. On the other hand, although PTzBTEE had a well-ordered structure similar to PTzBTE, it showed lower OPV performances. Through investigations of electronic properties and structural order and morphology in thin films, structure-property-device performance relationships are carefully discussed. In addition, the OPV cells fabricated with a non-halogenated solvent showed efficiencies comparable to the cells fabricated by a conventional halogenated solvent.

We report the synthesis of a series of thiophene-thiazolothiazole copolymers having alkyl and/or ester groups as the side chain. Polymers having ester side chains showed higher crystalline order in the thin films. These polymers showed quite high photovoltaic performances, although the polymer with all-ester side chain showed lower performances, in the solar cells fabricated with a non-halogenated solvent as well as a conventional halogenated solvent.

1. Introduction

Section:

Organic photovoltaics (OPVs) that use π-conjugated polymers (semiconducting polymers) as the photoactive layer have been attracting considerable attention because of their light weight and flexibility as well as low-cost and low-environmental-impact solution processability.¹^–³ Typically, a semiconducting polymer is used as the p-type (donor) material, which is combined with a fullerene derivative⁴^,⁵ or a non-fullerene material as the n-type (acceptor) material.⁶^–⁸ A number of new semiconducting materials have been developed in order to improve the power conversion efficiency (PCE) of OPVs in the last decade.⁸^–¹⁶ In general, control of the ordering structures in thin film, such as crystallinity and backbone orientation, and tuning of the frontier molecular orbital (FMO) energy levels of semiconducting polymers are important approaches for improving the PCE. In order to confer higher ordering structures, π-extended fused rings as well as donor–acceptor motifs are typically introduced in the backbone, which would increase the coplanarity of the backbone and the intermolecular interaction.¹⁰^,¹⁶^–²² Side chain engineering, i.e., choice of the substitution position, length, and topology, is also important for the control of ordering structures.²³^–²⁶ With respect to the FMO energy levels, as the open-circuit voltage (V_OC) depends on the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, the donor is required to have a deeper HOMO energy level. In addition, as the offset energy of the LUMOs or HOMOs between the donor and acceptor affect the photon energy loss, also called voltage loss, and thus the V_OC, the energy levels must be carefully fine-tuned.²⁷^,²⁸

The introduction of functional groups that can induce non-covalent interactions within the polymer backbone is known to enhance the polymer order.²⁹ For example, a fluorine atom, when introduced in a heteroaromatic ring, can non-covalently interact with the sulfur atom in the neighboring thiophene, suppressing the bond rotation in between the rings, which results in enhanced coplanarity.¹¹^,¹²^,¹⁶^,³⁰^–³² Further, as an oxygen atom can also interact with the thiophene sulfur, alkoxy and ester groups having a flexible alkyl moiety are used as the side chain instead of simple alkyl groups.³³^–³⁵ In particular, electron withdrawing ester groups are promising side chains that can realize a high crystallinity and a deep HOMO energy level at the same time.

Previously, we reported a series of semiconducting polymers based on thiophene and thiazolothiazole, which have been shown to have high crystallinity and high charge carrier transport property.²³^,³⁶^–⁴¹ In particular, PTzBT (Figure 1a) showed reasonably high OPV performances when combined with [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) as the acceptor, with PCEs of more than 7% despite the limited absorption range covering up to 670 nm.²³ Further, we also synthesized PTzBTE, where two of the four alkyl groups in the repeat unit of PTzBT were replaced with ester groups (Figure 1b).⁴⁰ On the one hand, PTzBTE gave a higher V_OC than PTzBT in the PC₆₁BM-based OPV cells, due to the deeper HOMO energy level, but on the other hand, it showed a lower PCE than PTzBT due to the lower short-circuit current density (J_SC) and the fill factor (FF). We assume that the low PCE was ascribed to the relatively low molecular weight of PTzBTE i.e., number-average molecular weight (M_n) of 25 kDa.

Figure 1. Chemical structures of thiophene-thiazolothiazole copolymers: (a) PTzBT with only alkyl groups as the side chain, (b) PTzBTE with both alkyl and ester groups, and (c) PTzBTEE with only ester groups, where R¹ and R² are 2-butyloctyl and 2-hexyldecyl groups, respectively. Dotted lines represent possible non-covalent interactions between the ester oxygen and the thiophene sulfur.

In this study, we first show an improved synthesis of one of the two monomers for PTzBTE, i.e., one with ester substituents, in which the total synthetic yield was 10-times higher than that of the previous synthesis. By using the monomer synthesized by the new methodology, we successfully had PTzBTE samples with significantly improved molecular weight (M_n > 50 kDa). We also synthesized a new thiophene-thiazolothiazole polymer having ester groups for all the side chains (PTzBTEE) (Figure 1c). We then discuss the properties, ordering structures in the thin film, and OPV properties of the polymers in comparison with PTzBT.

2. Results and Discussion

Section:

2.1 Synthesis.

Previously we have reported the synthesis of PTzBTE, in which the monomer having ester groups (TzBTE-Br2) was synthesized according to the synthetic route shown in Scheme 1a (upper, denoted as Previous route). In this route, 3-thiophenecarboxylic acid (1) was first brominated to give 5-bromothiophene-3-carboxylic acid (2), which was formylated at the 2-position to yield 5-bromo-2-formyl-3-thiophene-carboxylic acid (3). 3 was then esterified to yield 4, which was then reacted with dithiooxamide to undergo cyclization to form a TzTz ring, thus affording the ester-substituted monomer TzBTE-Br2. However, the final cyclization reaction resulted in a very low yield of approximately 11%, consequently led to the total reaction yield of only 4%. We hypothesized that the low yield in the cyclization could originate in the reactive bromo group, which could cause some side reactions. We thus devised a new route to TzBTE-Br2, where the bromo groups were introduced in the last step (Scheme 1a, lower, denoted as New route). In the new route, 1 was first formylated at the 2-position through lithiation with lithium diisopropylamide (LDA) and the following treatment with 1-formylpiperidine. The resulting 2-formyl-3-thiophene-carboxylic acid (5) was subjected to an esterification reaction using 2-hexyl-1-bromodecane in the presence of potassium carbonate to afford 6, which was reacted with dithiooxamide to form a TzTz ring, thus yielding TzBTE. Notably, the yield of the cyclization was as high as 82%, which was markedly higher than that in the previous route. TzBTE was easily dibrominated to give TzBTE-Br2 in a fairly high yield of 86%. As a result, the total yield of the new route was approximately 40%, that is, ten-fold higher than the previous route.

Scheme 1. (a) Synthesis of the monomer. (b) Synthesis of PTzBTE and PTzBTEE.

PTzBTE was synthesized via the Migita-Kosugi-Stille coupling reaction using a stannylated monomer with the 2-butyloctyl groups on the thiophene rings (TzBT-Sn2) and TzBTE-Br2 (Scheme 1b, upper). On the other hand, PTzBTEE was synthesized via the Migita-Kosugi-Stille coupling reaction using TzBTE-Br2 and hexamethylditin (Scheme 1b, lower). The molecular weights were evaluated by high-temperature gel-permeation chromatography (GPC). Interestingly, PTzBTE had M_n of 102 kDa and weight-average molecular weight (M_w) of 983 kDa, which was significantly higher than those reported previously (M_n = 25 kDa, M_w = 115 kDa). This could be due to the higher purity of the monomer TzBTE-Br2: In the cyclization reaction in the new route, side reactions were suppressed and thereby the monomer was able to be much more highly purified. PTzBTEE was also found to have reasonably high M_n of 47 kDa. These molecular weights were comparable to that for PTzBT (M_n = 64 kDa, M_w = 164 kDa) that was also synthesized here. Note that PTzBT possess 2-butyloctyl and 2-hexyldecyl groups as the side chain (Figure 1). The molecular weights for these polymers were quite reproducible.

Thermal properties of the polymers were studied by the DSC measurements (Figure S7). The DSC curves showed that whereas PTzBT had melting peak at 308 °C, PTzBTE and PTzBTEE had no phase-transition peaks below 350 °C.

2.2 Polymer Properties.

The HOMO and LUMO energy levels (E_HOMO and E_LUMO) of the three polymers were investigated by cyclic voltammetry using the polymer thin films (Figure 2a). The cyclic voltammograms for all the polymers gave oxidation and reduction waves, in which the onset potentials were used for estimating the energy levels. Whereas E_HOMO and E_LUMO of PTzBT were −5.46 eV and −3.46 eV, those of PTzBTE were −5.59 eV and −3.48 eV and of PTzBTEE were −5.66 eV and −3.59 eV, respectively (Table 1). Thus, as expected, PTzBTEE had the deepest energy levels among the three polymers due to the fact that all the side chains were replaced with the ester group. Note that, however, these values possibly include some experimental errors, since the downward shift of the E_LUMO is expected to be larger than that of the E_HOMO according to the DFT calculation (Figure S8).

Figure 2. (a) UV-vis absorption spectra of polymers in thin films. (b) Cyclic voltammogram of polymers in thin films. (c–e) Temperature-dependence UV-vis absorption spectra of polymers in chlorobenzene.

Table 1. Electronic properties of polymers.

Polymer	E_HOMO (eV)	E_LUMO (eV)	λ_max (nm)^a	λ_edge (nm)^b	E_g^opt (eV)^c
PTzBT	−5.46	−3.46	620	658	1.88
PTzBTE	−5.59	−3.48	631	671	1.85
PTzBTEE	−5.66	−3.59	629	667	1.86

a) Absorption maximum, b) absorption onset, c) optical bandgap determined from λ_edge.

Figure 2b shows the UV-vis absorption spectra of polymers in thin film. PTzBTE and PTzBTEE showed almost the same absorption spectrum with absorption maxima (λ_max) of 631 and 629 nm and absorption onsets (λ_edge) of 671 and 667 nm, respectively (Table 1). These absorption spectra were slightly red-shifted compared to that of PTzBT (λ_max = 620 nm, λ_edge = 658 nm) (Table 1), which is consistent with the variation of the energy levels calculated by the DFT method (Figure S8). We also studied the temperature dependence of UV-vis absorption spectra for these polymers in solution (Figures 2c–e). Interestingly, in PTzBTE and PTzBTEE, the low-energy absorption band (600–650 nm) had only one peak, whereas in PTzBT the low-energy absorption band split into two peaks, although the reason is yet unclear. The slightly sharp spectra for PTzBTEE compared to that for PTzBTE could be attributed to the enhanced interlocking by the increased ester substitution. In PTzBT, with increasing the temperature from 20 to 100 °C, the two peaks in the low-energy absorption band merged into the higher energy peak, while the low-energy absorption band slightly blue-shifted and weakened. In PTzBTE and PTzBTEE, the low-energy absorption band slightly blue-shifted and weakened as is the case in PTzBT. These results suggest that although the ester substitution would suppress the torsion of the backbone by the non-covalent interlocking, it does not significantly enhance the aggregation ability and reduce the solubility of the polymers.

2.3 OPV Properties.

Photovoltaic properties of the polymers were investigated using inverted solar cells (ITO/ZnO/polymer:PC₆₁BM/MoO_x/Ag). The active layer was first fabricated by spin-coating the blend solution in chlorobenzene (CB), in which the polymer to PC₆₁BM weight ratio was 1:2. Figures 3a and 3b depict the current density–voltage (J–V) curves and the external quantum efficiency (EQE) spectra of the best cells, respectively, and Table 2 summarizes the photovoltaic parameters. Owing to the deeper E_HOMO’s, the cells that used PTzBTE and PTzBTEE exhibited higher V_OC’s of 0.92 V and 0.97 V, respectively, compared to the cell that used PTzBT (0.87 V). PTzBTE showed a J_SC of 13.0 mA cm⁻², which was slightly higher than PTzBT (12.4 mA cm⁻²) and a FF of 0.74, which was significantly higher than PTzBT (0.66). The difference in the J_SC consistent with the difference in the EQE in particular at the polymer absorption region. As a result, PTzBTE showed a PCE of 8.88%, which was significantly higher than that for PTzBT (7.10%) and is quite high considering that the polymer has relatively wide E_g^opt of 1.85 eV. We also note that the PCE of PTzBTE observed here was markedly higher than that reported previously (4.2%), indicating the importance of molecular weight.⁴⁰ On the other hand, PTzBTEE showed a lower PCE of 3.74%, because of the low J_SC (6.79 mA cm⁻²) and FF (0.57).

Figure 3. Photovoltaics characteristics of the cells based on PC₆₁BM fabricated by chlorobenzene. (a) J-V curves. (b) EQE spectra. Fabricated by o-xylene. (c) J-V curves. (d) EQE spectra.

Table 2. Photovoltaics properties of the polymers.

Active layer	Solvent	Additive	Thickness (nm)^b	J_SC (mA cm⁻²)	V_OC (V)	FF	PCE (%)
PTzBT/PC₆₁BM	CB	—	320	12.4	0.86	0.66	7.04
PTzBT/PC₆₁BM	o-xylene	DPE^a	240	12.5	0.83	0.68	7.06
PTzBTE/PC₆₁BM	CB	—	280	13.0	0.92	0.74	8.88
PTzBTE/PC₆₁BM	o-xylene	DPE^a	370	14.3	0.90	0.66	8.52
PTzBTEE/PC₆₁BM	CB	—	230	6.8	0.97	0.57	3.74
PTzBTEE/PC₆₁BM	o-xylene	DPE^a	200	10.1	0.93	0.71	6.60

(a) Cyclic voltammograms of the polymers in the thin film. (b) UV-vis absorption spectra of the polymers in the thin film.

We also fabricated the active layer using a non-halogenated solvent, o-xylene, which is more environmentally friendly than CB (Figures 3c and 3d). In this case, 0.5 vol% of diphenylether (DPE) was also added as the solvent additive, which enhanced the photovoltaic performance. PTzBT and PTzBTE showed PCEs of 7.03% and 8.53% that are comparable to PCEs for the cells fabricated by CB, although the parameters somewhat differed. This indicates that these polymers are of practical use. Interestingly, PTzBTEE showed a PCE of 6.60% that is significantly higher than the value obtained for the cell fabricated by CB, which was due to the improved J_SC and FF.

2.4 Structural Order and Morphology.

The ordering structures of the polymers in thin films were investigated by grazing incidence X-ray diffraction (GIXD) measurements. Figures 4a–c depict two-dimensional (2D) GIXD patterns of the polymer neat films. In all cases, a diffraction assignable to the stacking structure appeared along the q_z axis (out of plane) at around q_z of 1.7 Å⁻¹, indicative of the face-on orientation, which would facilitate the charge transport in vertical direction against the substrate. Notably, PTzBTE and PTzBTEE showed a more intense diffraction corresponding the lamellar structure along the q_xy axis (q_xy ≈ 0.25 Å⁻¹) relative to that along q_z axis (also see Figure S10), indicating that the ester substitution enhanced the fraction of face-on orientation. The distance of π–π stacking (d_π) was slightly decreased by increasing the ester-substitution: d_π’s for PTzBT, PTzBTE, and PTzBTEE were 3.55, 3.52, and 3.48 Å, respectively. This is most likely due to the suppressed torsion of the backbone and thereby higher coplanarity originating in the increased number of non-covalent interactions. The crystallite coherence length (L_C) calculated by the Scherrer’s equation using the π–π stacking diffraction tended to slightly increase as the ester substitution increased, consistently with the d_π: L_C’s for PTzBT, PTzBTE, and PTzBTEE were 22, 25, and 36 Å, respectively.

Figure 4. 2D-GIXD images of neat films and PC₆₁BM blend films. (a–c) polymer neat films fabricated by chlorobenzene. (d–f) polymer/PC₆₁BM blend films fabricated by chlorobenzene. (g–i) polymer/PC₆₁BM blend films fabricated by o-xylene.

We also conducted the GIXD measurements for the polymer/PC₆₁BM blend films fabricated by CB (Figures 4d–f) and o-xylene (Figures 4g–i), in which all the polymers showed 2D GIXD patterns corresponding to the face-on orientation as is the case in the neat film. In the blend film fabricated by CB, PTzBTE had significantly higher fraction of face-on orientation compared to PTzBTE (also see Figure S10), and a narrower d_π of 3.56 Å and a larger L_C of 34 Å, though not significantly, than those for PTzBT (d_π = 3.57 Å, L_C = 32 Å). This agrees well with the fact that PTzBTE showed higher J_SC and FF in the OPV cell than PTzBT. In the blend film fabricated by o-xylene, each polymer gave a similar 2D GIXD pattern to that in the blend film fabricated by CB. However, the L_C’s of 27 Å for PTzBT and 31 Å for PTzBTE were slightly lower than those in the CB-fabricated film, which agrees with the lower FF. We note that although PTzBTEE showed lower photovoltaic performances, 2D GIXD patterns for the blend films fabricated by both CB and o-xylene were quite similar and the d_π and L_C were even narrower and larger than those for PTzBT and PTzBTE.

To gain further insight into the difference in the photovoltaic performances, the morphology of the blend films was investigated by the transmission electron microscope (TEM) measurements (Figure 5). PTzBT and PTzBTE similarly provided fibrillar structures, forming fine networks, in both the blend films fabricated by CB and o-xylene as often seen in the blend films showing high photovoltaic performance. On the other hand, PTzBTEE showed large fibrils with more needle-like structures, suggesting larger phase separation. Such clear difference in the morphology should account for the difference in the photovoltaic performance.

Figure 5. TEM images of polymer/PC₆₁BM blend films fabricated by (a–c) CB and (d–f) o-xylene. (a, d) PTzBT, (b, e) PTzBTE, (c, f) PTzBTEE.

3. Conclusion

Section:

A series of thiophene-thiazolothiazole with ester side chains were studied. We successfully developed a more efficient synthetic route to the ester-substituted monomer for thiophene-thiazolothiazole polymers. The total synthetic yield of the monomer in the new route was 40% in 5 steps from a commercially available compound, which was about 10 times as high as that in the previous route. By using the monomer synthesized via the new route, we were able to synthesize PTzBTE samples with significantly higher molecular weights than that reported previously. In addition, we further synthesized PTzBTEE, in which all the side chains were ester groups. With increasing the number of ester groups, the polymers consistently had deeper FMO energy levels than PTzBT having alkyl groups for all the side chains. Interestingly, although ester-substitution slightly enhanced the backbone order, it did not significantly reduce the solubility. Due to the improved molecular weight, PTzBTE showed markedly improved PCEs in PC₆₁BM-based solar cells than the previously reported value in a low molecular weight sample. Further, PTzBTE showed a higher PCE than PTzBT, which was consistent with the increased crystallinity as well as the fraction of favorable face-on backbone orientation. On the other hand, PTzBTEE showed lower PCE, though it showed the highest V_OC. In addition, when the solar cells were fabricated in a non-halogenated solvent (o-xylene), these polymers showed comparable or even higher photovoltaic performances relative to the cells fabricated in a conventional chlorinated solvent (CB), which is important for practical use. Further studies on these polymer systems are underway in our group.

4. Experimental

Section:

4.1 Materials.

All chemicals were used as purchased. Super dehydrated solvents such as tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were purchased from Wako Pure Chemical Industries, Ltd. TzBT-Sn2 and PTzBT were synthesized according to the reported procedures.²³ Nuclear magnetic resonance (NMR) measurements were conducted with a Varian-400 spectrometer or a Varian-500 spectrometer. High-resolution mass spectrum (HRMS) was performed using LTQ Orbitrap XL (Thermo Fisher Scientific, Inc.). Microwave reactor, Initiator (Biotage Initiator), was used for polymerizations. Molecular weights of the polymers were determined by high-temperature gel permeation chromatography (GPC), using TOSOH HLC-8321GPC/HT, at 140 °C with o-dichlorobenzene (DCB) as the solvent, which was calibrated with polystyrene standards.

4.2 Synthesis.

2-Formyl-3-thiophene-carboxylic Acid (5):

Lithium diisopropyl amide (LDA), which was prepared from diisopropylamine (11.0 mL, 78.0 mmol) and 1.57 M n-butyllithium (49.7 mL, 78.0 mmol) in THF (120 mL), was added dropwise to a THF (120 mL) solution of 3-thiophenecarboxylic acid (4.00 g, 31.2 mmol) at −30 °C. After the mixture was stirred for 1 h, 1-formylpiperidine (8.65 mL, 78.0 mmol) was added in one portion at −30 °C. After stirring for another 1 h, the mixture was warmed to room temperature. Dilute hydrochloric acid was added to the mixture, which was extracted with ether three times. The organic layer was dried over magnesium sulfate, and the solvent was evaporated under a reduced pressure, giving 5 as yellow solids (4.70 g). 5 was subjected to the next reaction without further purification.

2-Hexyldecyl 2-Formylthiophene-3-carboxylate (6):

5 (4.70 g), 2-hexyl-1-bromodecane (11.0 g, 36.1 mmol), and sodium carbonate (31.9 g, 301 mmol) in DMF (200 mL) were heated at 125 °C for overnight. The reaction mixture was cooled to room temperature, and then was filtered. The filtrate was poured into water and extracted with hexane three times. The crude product was purified by column chromatography on silica gel, using hexane:dichloromethane (4:1) to give 6 as yellow oil (6.38 g, 54% in two steps). ¹H NMR (400 MHz, CDCl₃) δ 10.63 (d, J = 1.2 Hz, 1H), 7.64 (dd, J = 5.1, 1.2 Hz, 1H), 7.57 (d, J = 5.1 Hz, 1H), 4.27 (d, J = 5.6 Hz, 2H), 1.77 (p, J = 5.9 Hz, 1H), 1.42–1.24 (m, 24H), 0.87 (m, 6H). ¹³C NMR (126 MHz, CDCl₃) δ 184.74, 162.15, 147.36, 136.87, 132.68, 130.92, 68.42, 37.34, 31.86, 31.77, 31.36, 29.88, 29.55, 29.51, 29.27, 26.69, 26.67, 22.65, 22.62, 14.09, 14.06. HRMS Calcd for C₂₃H₃₉O₃S [M + CH₃]+: 395.26199. Found: 395.26093.

Bis(2-hexyldecyl) 2,2′-(Thiazolo[5,4-d]thiazole-2,5-diyl) Bis(thiophene-3-carboxylate) (TzBTE):

A mixture of 6 (0.60 g, 1.6 mmol) and dithiooxamide (0.18 g, 1.5 mmol) was heated at 140 °C for 24 h. After cooling to room temperature, the reaction mixture was poured into water and extracted with hexane three times. The combined organic layer was dried over magnesium sulfate and concentrated under a reduced pressure. The crude product was purified by column chromatography on silica gel, using hexane:ethyl acetate (10:1) and recrystallized from hexane: ethanol (1:2) to give TzBTE as orange solids (0.54 g, 82%). ¹H NMR (400 MHz, CDCl₃) δ 7.52 (d, J = 5.4 Hz, 2H), 7.39 (d, J = 5.4 Hz, 2H), 4.26 (d, J = 5.7 Hz, 4H), 1.77 (p, J = 5.9 Hz, 2H), 1.41–1.24 (m, 48H), 0.87 (m, 12H). ¹³C NMR (126 MHz, CDCl₃) δ 163.23, 160.41, 153.29, 144.28, 130.45, 128.95, 127.50, 68.15, 37.35, 31.89, 31.80, 31.33, 29.95, 29.61, 29.55, 29.30, 26.71, 26.69, 22.67, 22.64, 14.12, 14.09. HRMS Calcd for C₄₆H₇₀N₂O₄S₄ [M + H]+: 843.42967. Found: 843.42914.

Bis(2-hexyldecyl) 2,2′-(Thiazolo[5,4-d]thiazole-2,5-diyl) Bis(5-bromothiophene-3-carboxylate) (TzBTE-Br2):

To a solution of TzBTE (4.34 g, 5.14 mmol) in 150 mL of chloroform and 10 mL of acetic acid, N-bromosuccinimide (NBS) (5.91 g, 33.2 mmol) was added in one portion. The reaction solution was refluxed for 2 days. Then, it was cooled to room temperature and washed with water. The organic layer was dried over magnesium sulfate and concentrated by evaporation. The resulting orange solid was recrystallized from hexane:ethanol (1:2) to give TzBTE-Br2 as orange solids (4.40 g, 86%). ¹H NMR (500 MHz, CDCl₃) δ 7.45 (s, 2H), 4.25 (d, J = 5.7 Hz, 4H), 1.83–1.71 (m, 2H), 1.41–1.20 (m, 48H), 0.87 (m, 6H). ¹³C NMR (126 MHz, CDCl₃) δ 162.14, 159.46, 153.62, 145.92, 132.57, 128.75, 115.76, 68.52, 37.37, 31.87, 31.78, 31.35, 29.89, 29.56, 29.52, 29.27, 26.70, 26.69, 22.64, 22.61, 14.05. HRMS Calcd for C₄₆H₆₈Br₂N₂O₄S₄ [M + H]+: 998.24287. Found: 999.25037.

PTzBTE:

TzBT-Sn2 (48 mg, 50 µmol), TzBTE-Br2 (50 mg, 50 µmol), Pd₂(dba)₃·CHCl₃ (1.0 mg, 2 mol%), and P(o-tolyl)₃ (2.4 mg, 16 mol%), and chlorobenzene (2 mL) were added to a reaction tube, which was heated at 250 °C for 2 hours in a microwave reactor. After cooling to room temperature, the reaction solution was poured into 50 mL of methanol. The precipitate was washed sequentially with methanol, hexane, and dichloromethane using a Soxhlet apparatus to remove impurities and low-molecular weight fractions. The residue was finally extracted with chloroform, and reprecipitated in 50 mL of methanol. The precipitate was dried in vacuo to yield the polymer sample (134 mg, 93%). Anal. Calced for C₈₂H₁₂₀N₄O₄S₈: C, 66.71, H, 8.40, N, 3.70. Found: C, 66.41, H, 7.97, N, 3.62. M_n = 102 kDa, M_w = 983 kDa, PDI = 9.7.

PTzBTEE:

TzBTE-Br2 (102 mg, 0.10 mmol), hexamethylditin (33 mg, 0.10 mmol), Pd(PPh₃)₄ (2.3 mg, 2 mol%), and toluene (4 mL) were added to a reaction tube, which was heated at 200 °C for 2 hours in a microwave reactor. After cooling to room temperature, the reaction solution was poured into 50 mL of methanol and stirred for 1 hour. Then the precipitate was washed sequentially with methanol, hexane, and dichloromethane using a Soxhlet apparatus to remove impurities and low-molecular weight fractions. The residue was finally extracted with chloroform, and reprecipitated in 50 mL of methanol. The precipitate was dried in vacuo to yield the polymer sample (79 mg, 94%). Anal. Calced for C₄₈H₇₄N₂O₄S₄: C, 66.16, H, 8.56, N, 3.21. Found: C, 66.08, H, 8.50, N, 3.21. M_n = 47 kDa, M_w = 276 kDa, PDI = 5.8.

4.3 Instrumentation.

Differential scanning calorimetry (DSC) analysis was carried out with DSC7020 (HITACHI) at a cooling and heating rate of 10 °C/min. Cyclic voltammetry (CV) was carried out, using an ALS Electrochemical Analyzer Model 610E, in acetonitrile containing tetrabutylammonium hexafluorophosphate (Bu₄NPF₆, 0.1 M) as the supporting electrolyte. The counter and working electrodes were made of Pt, and the reference electrode was Ag/Ag+. Polymer thin films were fabricated on the working electrode by dipping the electrode in the polymer solution. All potentials were calibrated with the standard ferrocene/ferrocenium redox couple (Fc/Fc+: E_1/2 = +0.07 V measured under the identical conditions). HOMO energy levels (E_HOMO) and LUMO energy levels (E_LUMO) were calculated with the following equations:

\begin{equation*} E_{\text{HOMO}}\ (\text{eV}) = - 4.80 + 0.131 - E_{\text{ox}} \end{equation*}

\begin{equation*} E_{\text{LUMO}}\ (\text{eV}) = - 4.80 + 0.131 + E_{\text{red}} \end{equation*}

where E_ox and E_red are the onset oxidation and reduction potentials of cyclic voltammograms, respectively, and −4.80 eV is the E_HOMO of ferrocene against the vacuum level. UV-vis absorption spectra were measured using a Shimadzu UV-3600 spectrometer. Grazing incidence X-ray diffraction (GIXD) measurements were conducted at the SPring-8 on the beamline BL46XU. The sample was irradiated at a fixed incident angle on the order of 0.12° through a Huber diffractometer with an X-ray energy of 12.39 keV (λ = 1 Å). Two dimensional (2D) GIXD patterns were recorded with a 2D image detector (Pilatus 300K). Atomic force microscopy (AFM) study was carried out with a SPM-9700HT scanning probe microscope (Shimadzu Corp.). Transmission electron microscopy (TEM) was conducted on JEM-2021 (JEOL).

4.4 Fabrication of OPV Cells.

Indium tin oxide (ITO) substrates were first pre-cleaned sequentially by sonicating in a detergent bath, deionized water, acetone and 2-propanol at room temperature, and in boiled 2-propanol for 10 min each, and then baked at 120 °C for 10 min in air. The substrates were then subjected to UV/ozone treatment at room temperature for 20 min. The ZnO layer was prepared by spin-coating (at 1200 rpm, 10 s) a dispersion of ZnO nanoparticles in chloroform. The photoactive layer was deposited in a glove box (KOREA KIYON, KK-011AS-EXTRA), by spin coating a hot chloroform solution containing a polymer and PC₆₁BM (total concentration: 18 g L⁻¹, p:n = 1:2 w/w) at 600–800 rpm for 30 s at 100 °C. The thin films were transferred into a vacuum evaporator (ALS Technology, E-100J) connected to the glove box, and the MoO_x (7.5 nm) and Ag (100 nm) layers were sequentially deposited through a shadow mask. The active area of the cells was 0.1256 cm². J–V characteristics of the cells were measured using a Keithley 2400 source–measure unit in the glove box under the 1 sun (AM1.5G) condition using a solar simulator (SAN-EI Electric, XES-40S1). The light intensity was calibrated with a reference PV cell (KONICA MINOLTA AK-100 certified at National Institute of Advanced Industrial Science and Technology, Japan). EQE spectra were measured with a Spectral Response Measuring System (Soma Optics, Ltd., S-9241). The thickness of the active layer was measured with a microfigure measuring instrument ET200 (Kosaka Laboratory, Ltd.).

This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from JST (grant no. JPMJAL 1404) and MIRAI Program (grant no. JPMJMI20E2) from JST. This work was partly supported by Next Generation Photovoltaics at Hiroshima University (the Program for Promoting the Enhancement of Research Universities from the Ministry of Education, Culture, Sports, Science and Technology, Japan). 2D GIXD experiments were performed at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2020A1742 and 2021A1558). The authors thank Dr. T. Koganezawa (JASRI) for the support on 2D GIXD measurements and Dr. Maeda for the TEM measurements.

Supporting Information

NMR spectra, DSC thermogram, FMO geometries, 2D GIXD patterns, and AFM images. This material is available on https://doi.org/10.1246/bcsj.20210172.

Itaru Osaka

Professor Itaru Osaka received his doctoral degree from University of Tsukuba in 2002. After a 4-year research stint at Fujifilm, he worked as a postdoctoral researcher at Carnegie Mellon University in 2006–2009. He then started his professional carrier at Hiroshima University as an Assistant Professor in 2009, and moved to RIKEN as a Senior Research Scientist in 2013. He was appointed as a Professor at Hiroshima University in 2016. His research interests include design and synthesis of π-conjugated materials, in particular, polymers for organic electronics such as field-effect transistors and solar cells.

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2021, Vol.94, No.8

Effect of Ester Side Chains on Photovoltaic Performance in Thiophene-Thiazolothiazole Copolymers

Abstract

2.1 Synthesis.

2.2 Polymer Properties.

2.3 OPV Properties.

2.4 Structural Order and Morphology.

4.1 Materials.

4.2 Synthesis.

4.3 Instrumentation.

4.4 Fabrication of OPV Cells.

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