Ester-Functionalized Naphthobispyrazine as an Acceptor Building Unit

May 14, 2019 - from pale yellow to purple. Thus, the. reaction must be. carr. ied. out. under. strictly inert atmosphere. All the compounds were chara...
0 downloads 0 Views 5MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Ester-Functionalized Naphthobispyrazine as an Acceptor Building Unit for Semiconducting Polymers: Synthesis, Properties, and Photovoltaic Performance Tsubasa Mikie and Itaru Osaka* Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan

Macromolecules Downloaded from pubs.acs.org by UNIV PARIS-SUD on 05/15/19. For personal use only.

S Supporting Information *

ABSTRACT: Strongly electron-deficient π-conjugated systems are key building units for semiconducting polymers that are used in organic electronic devices, such as organic photovoltaic (OPV) cells. Here, we designed and synthesized a naphthobispyrazine derivative bearing four ester groups (eNPz) as a new electron-deficient building unit and three eNPz-based semiconducting polymers with different donor units, that is, bithiophene (PeNPz2T), terthiophene (PeNPz3T), and quaterthiophene (PeNPz4T). These new polymers have relatively deep lowest unoccupied molecular orbital (LUMO) energy levels of around −3.5 eV, along with narrow optical band gaps of around 1.5 eV. The LUMO energy levels and the optical band gaps are significantly deeper and narrower than those of a polymer based on alkylated naphthobispyrazine. The results indicate that eNPz has a strong electron deficiency. The polymers show reasonably high power conversion efficiency of more than 6% in OPV cells in combination with a fullerene derivative. This study demonstrates that eNPz can be a useful building unit for high-performance semiconducting polymers.

■

INTRODUCTION Semiconducting polymers have attracted considerable attention in the field of organic electronics including field-effect transistors1−4 and solar cells5−10 because of their fascinating optoelectronic properties and solution processability, which have enabled low-cost manufacture of these devices on lightweight and flexible plastic substrates. A promising strategy for creating high-performance semiconducting polymers is to alternately incorporate electron-rich (donor) and electronpoor/deficient (acceptor) π-conjugated building units into the polymer backbone. The resulting donor−acceptor polymers offer two crucial features for improving device performances. One is the possible dipole−dipole interchain interaction, which results in short π−π stacking distances between the polymer chains (backbones), thereby facilitating the charge carrier mobility.11,12 The other is the mixing of the molecular orbitals between the donor and acceptor units, which allows us to finely tune the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels as well as the optical band gap.13,14 In general, deeper HOMO and LUMO energy levels are essential for both p-type and n-type semiconducting polymers: deep HOMO energy levels bring about chemical (oxidative) stability in air and high open-circuit voltages in solar cells (especially for p-type semiconducting polymers),15 and deep LUMO energy levels bring about stable electron transport in air.16 A variety of © XXXX American Chemical Society

acceptor units have been widely explored. Typical acceptor units incorporated into the polymer backbone include imideand amide-based compounds, 17−22 o-quinoidal heterocycles,23−27 and thienoquinoidal compounds.28,29 With respect to o-quinoidal heterocycles, naphthalene-based electrondeficient π-conjugated systems, such as naphtho[1,2-c:5,6c′]bis[1,2,5]oxadiazoles (NOz),30,31 naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazoles (NTz),32−41 naphtho[1,2-c:5,6-c′]bis[1,2,5]selenadiazoles (NSz),30 naphtho[1,2-c:5,6-c′]bis(2-substituted)-[1,2,3]triazole (TZNT),42,43 and naphtho[1,2-b:5,6b′]bispyrazine (NPz),44−47 have been frequently utilized as building units to realize high-performance semiconducting polymers (Figure 1, upper panel). In fact, the performance of semiconducting polymers based on these building units has surpassed that of amorphous silicon in both transistors and organic photovoltaic (OPV) devices.34−38,40,46 Among the naphthalene-based acceptor units, NPz has been the least investigated. Importantly, however, NPz allows the introduction of four substituents at the 2, 3, 8, and 9 positions, in contrast to other naphthalene-based acceptor units that do not do so (NOz, NTz, NSz), or that allow the introduction of only two substituents at the N-position (TZNT). This would Received: March 13, 2019 Revised: April 24, 2019

A

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

route to NPz, in which the methyl group is used as the substituent at these positions (Figure 1, lower panel).47 Although the simple methyl group is beneficial for investigating the synthetic reactions, its electron-donating nature weakens the electron deficiency of NPz, resulting in unfavorable shallow HOMO and LUMO energy levels. Thus, the introduction of functional groups with electron-withdrawing nature is required for developing NPz-based semiconducting polymers with favorable energetics. Herein, we report the synthesis of NPz having one ester group each at the 2, 3, 8, and 9 positions (eNPz) and its copolymers in combination with the oligothiophene donor units. The resulting polymers have more favorable energetics, that is, deep HOMO and LUMO energy levels as well as small optical band gaps compared to the methyl or alkyl analogue. We also studied the film structures and the solar cell performance and discussed the structure−property relationship.

■

Figure 1. Chemical structures of NOz, NTz, NSz, TZNT, NPz, and eNPz.

RESULTS AND DISCUSSION Synthesis. The synthetic routes to the monomers based on eNPz along with the NPz monomer as a reference are illustrated in Scheme 1. Tartaric acid 1 was esterified by 2ethyl-1-hexanol and 2-butyl-1-octanol in the presence of p-

expand the molecular design window and would provide more opportunities to develop high-performance semiconducting polymers. Recently, we have developed an efficient synthetic

Scheme 1. Preparation of Diketone Derivatives and Synthesis of eNPz Derivatives

B

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 2. Synthetic Routes of Polymers

Table 1. Polymerization Results and Polymer Propertiesc

toluenesulfonic acid to give 2a and 2b in 71 and 76% yield, respectively. Compounds 2a and 2b were then oxidized with N-bromosuccinimide to provide dioxosuccinic acid esters 3a and 3b, respectively. The ring-opening reaction of dibrominated NTz (NTz-Br2) was conducted using Zn as the reducing agent, and this was followed by a condensation reaction with 3a and 3b to afford dibrominated eNPz (eNPz-Br2) with 2ethylhexyl (EH) and 2-butyloctyl (BO) groups on the ester moiety, respectively, in reasonably high yields of approximately 60%. The same reaction was carried out using hexacosane13,14-dione to give dibrominated NPz (NPz-Br2) with dodecyl groups in an excellent yield. Then, eNPz-Br2 and NPz-Br2 were cross-coupled with 2-tributylstannylthiophene to afford eNPz2T and NPz2T, and the following dibromination gave eNPz2T-Br2 and NPz2T-Br2, respectively. The synthetic routes to the eNPz-based polymers (PeNPz2T, PeNPz3T, and PeNPz4T) and the NPz-based polymer (PNPz4T) are shown in Scheme 2. All the polymers were synthesized via the Stille coupling reaction. PeNPz2T was synthesized by the copolymerization of eNPz-Br2 (R = EH) and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene using Pd2(dba)3·CHCl3/P(o-tol)3 as the catalyst system. PeNPz3T was synthesized using eNPz2T-Br2 (R = EH) and 2,5bis(trimethylstannyl)thiophene, in which Pd(PPh3)4 was used as the catalyst. Similarly, PeNPz4T was synthesized with eNPz2T-Br2 (R = BO) and bis(trimethylstannyl)-2,2′bithiophene. The BO group was employed as the side chain instead of the EH group to ensure solubility: in fact, PeNPz4T with the EH group had very poor solubility. PNPz4T was also synthesized in the same manner. All the eNPz-based polymers were stable in ambient atmosphere and soluble in chloroform, toluene, chlorobenzene (CB), and o-dichlorobenzene (DCB) even at room temperature. The chemical properties of the three eNPz-based polymers and PNPz4T are summarized in Table 1. The molecular weights of the polymers were evaluated by high-temperature gel-permeation chromatography (GPC) using DCB as the eluent. The number-average molecular weight (Mn) and polydispersity index (PDI) were 12.0 kDa and 2.6 for PeNPz2T, 24.7 kDa and 4.0 for PeNPz3T, 80.2 kDa and 3.0 for PeNPz4T, and 11.2 kDa and 1.9 for PNPz4T, respectively.

polymer

Mn (kDa)a

Mw (kDa)a

PDI

DPnb

Td5 (°C)d

PeNPz2T PeNPz3T PeNPz4T PNPz4T

12.0 24.7 80.2 11.2

31.4 100.0 240.5 21.3

2.6 4.0 3.0 1.9

11.7 22.4 56.9 9.1

326 337 355

a

Determined by GPC using polystyrene standard. DCB was used as the eluent at 145 °C. bOn the basis of the repeating unit. cMelting point determined by DSC measurements at the scan rate of 10 °C/ min. dTemperature for 5% weight loss.

The thermal properties of the eNPz-based polymers were evaluated by differential scanning calorimetry (DSC) (Figure S1) and thermogravimetric analysis (TGA) (Figure S2). All the polymers showed no transition peaks below 300 °C in the DSC curves. The temperatures for 5% weight loss (Td5) were 326, 337, and 355 °C for PeNPz2T, PeNPz3T, and PeNPz4T, respectively. These results indicated that the thermal stability of the eNPz-based polymers is sufficiently high and is slightly enhanced by increasing the number of thiophenes in the repeating unit. Polymer Properties and Ordering Structure. Electrochemical Properties. Then, we evaluated the electronic properties of the polymers by cyclic voltammetry and UV− vis absorption spectroscopy. Figure 2a displays the cyclic voltammograms of the polymers, and the corresponding parameters are summarized in Table 2. The HOMO and LUMO energy levels (EHOMOs and ELUMOs) were determined from the redox onset potentials. Although PeNPz4T had EHOMO of −5.10 eV, which was similar to that of PNPz4T, its ELUMO was −3.47 eV, which was dramatically downshifted by about 0.6 eV from that of PNPz4T. This indicates that the eNPz moiety is strongly electron-deficient compared to alkyl NPz. The EHOMO values of the eNPz-based polymers were downshifted with decreasing size of the donor moiety in the repeating unit: the EHOMO values were −5.24 eV for PeNPz3T and −5.43 eV for PeNPz2T. On the other hand, the ELUMO values of other eNPz-based polymers were also around −3.5 eV: it was −3.47 eV for PeNPz3T and −3.58 eV for PeNPz2T. These trends are in good agreement with the results of DFT calculations at B3LYP 6-31G(d). Figure 3 shows the HOMOs C

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 2. Cyclic voltammograms (a) and UV−vis absorption spectra in the CB solution (5 × 10−6 M) (b) and in the film (c) of the polymers.

Table 2. Electrochemical and Optical Properties of the Polymers λmax(nm)c polymer PeNPz2T PeNPz3T PeNPz4T PNPz4T

EHOMO (eV) −5.43 −5.24 −5.10 −5.13

a,b

ELUMO (eV) −3.58 −3.47 −3.47 −2.87

a,b

ECV g

(eV)

λedge (nm)

d Eopt g (eV)

solution

film

solution

film

solution

film

719 694 677 605

719 708 699 610

787 804 782 673

825 828 835 675

1.58 1.54 1.59 1.84

1.50 1.50 1.49 1.84

1.85 1.77 1.63 2.26

b onset onset Onset potentials for the oxidation (Eonset and ox ) and reduction (Ered ) peaks. Estimated using the following equations: EHOMO = −4.80 − Eox d onset c opt ELUMO = −4.80 − Ered . Absorption maximum. Optical band gap calculated by using the following equation: Eg = 1240/λedge. a

Figure 3. HOMOs and LUMOs of the trimer models for PeNPz2T, PeNPz3T, PeNPz4T, and PNPz4T calculated by the DFT method at B3LYP 6-31G(d). The substituents were replaced by methyl groups to simplify the calculation.

Table 3. Photovoltaic Properties of eNPz-Based Polymer/ PC61BM Cells

and LUMOs of the trimer models for PeNPz2T, PeNPz3T, and PeNPz4T in comparison with that for PNPz4T. The calculation indicated that PeNPz4T has considerably deeper frontier energy levels than PNPz4T. In particular, PeNPz4T gave the calculated ELUMO of −2.96 eV, which was lower by approximately 0.6 eV. The calculated frontier energy levels had trends similar to the experimental data in both EHOMOs and ELUMOs: although EHOMO showed an upfield shift with the decrease of the spacer size, ELUMO did not change. This is most likely ascribed to the fact that HOMOs are delocalized over the backbone including both oligothiophene and eNPz units, whereas LUMOs are mainly localized on the eNPz unit. In contrast, LUMOs are also delocalized over the backbone in the trimer model for PNPz4T. This clearly shows that NPz and eNPz have quite different electronic properties. Optical Properties. The UV−vis absorption spectra of the polymers in the CB solution and in the thin film are shown in Figure 2b,c. The optical properties such as the absorption maximum (λmax) and the optical band gap (Eopt g ) calculated

polymer

Jcalc SC (mA cm−2)

JSC (mA cm−2)

VOC (V)

FF

PCE (%)

PeNPz2T PeNPz3T PeNPz4T

7.6 13.0 11.9

7.1 13.1 11.8

0.83 0.76 0.74

0.46 0.63 0.71

2.7 6.3 6.3

from the absorption edge (λedge) are also summarized in Table 2. In solution, PeNPz4T gave λmax of 677 nm, which was longer by ca. 70 nm compared to that of PNPz4T (λmax = 605 nm). Interestingly, PeNPz3T gave an absorption spectrum having a broad single peak similar to that for PeNPz4T, whereas PeNPz2T gave an absorption spectrum having a sharp peak in the longer wavelength region along with a shoulder in the shorter wavelength region, although the absorption range for both polymers was almost the same as that for PeNPz4T. In the thin film, PeNPz4T gave λmax of 699 nm, whereas D

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. Two-dimensional GIXD images of PeNPz2T (a), PeNPz3T (b), PeNPz4T (c), and PNPz4T (d) thin films, and out-of-plane (e) and inplane (f) profiles cut from the 2D-GIXD images.

Figure 5. Schematic illustrations of plausible 2D polymer structures: (a) PeNPz2T and (b) PeNPz4T.

PNPz4T gave λmax of 610 nm. The larger bathochromic shift between the solution and the film in PeNPz4T than in PNPz4T is attributable to the difference in topology of the alkyl side chain. As PNPz4T has linear alkyl groups, the polymer chains are more aggregated in the solution, relative to PeNPz4T that has bulky branched alkyl groups, which results in the similar absorption spectra between the solution and the film. Accordingly, Eopt for PeNPz4T, which was calculated g using λedge of 835 nm was 1.49 eV, which was much narrower than that for PNPz4T (Eopt g = 1.84 eV, λedge = 675 nm). This is consistent with the computation as well as the electrochemically determined energy gap (ECV g = ELUMO − EHOMO). All the eNPz-based polymers gave similar absorption bands that appeared at around 600−800 nm in the film, regardless of the size of the thiophene spacers. Therefore, all the eNPz-based polymers exhibited similar Eopt g values of approximately 1.5 eV in the film. This contrasts the fact that ECV g widens with the decrease in size of the thiophene spacers, although the reason for this inconsistency is yet unknown: one possible explanation would be the difference in the exciton binding energy.48 Interestingly, the eNPz-based polymers also had an absorption band at approximately 500 nm, which intensified with the increase in size of the thiophene spacers. In particular, PeNPz4T covers almost the whole UV−vis region and, therefore, both the solution and the film are almost black in color. The broad absorption implies that the eNPz-based polymers possess very good light-harvesting ability, which should be beneficial for solar cell applications. Ordering Structures. To investigate the ordering structures of the polymers, two-dimensional grazing incidence X-ray

diffraction (2D-GIXD) studies were carried out using the polymer thin films. Figure 4 shows the 2D-GIXD images of the polymer thin films (Figure 4a−d) and the out-of-plane (along the qz axis) and in-plane (along the qxy axis) profiles (Figure 4e,f). In PeNPz4T, diffractions assignable to the π−π stacking structure (qz ≈ 1.72 Å−1) and the lamellar structure (qxy ≈ 0.26 Å−1) mainly appeared on the qz and qxy axes, respectively. The d-spacings for the π−π stacking structure (dπ) and the lamellar structure (dl) were calculated to be 3.65 and 24.6 Å, respectively. The results indicate that PeNPz4T has the predominant face-on orientation, which is favorable for charge transport in OPV devices. We also investigated the crystallinity of the polymers by calculating the coherence length (LC) of the π−π stacking structures using a simplified Scherrer’s equation (LC = 2π/fwhm, where fwhm is the full width at half-maximum of the diffraction peak).49,50 The LC value of PeNPz4T for π−π stacking was determined to be 23.3 Å (Figure S5). In stark contrast, PNPz4T showed diffraction only in the small-angle region, which likely corresponds to the lamellar structure along the qz axis and is indicative of a low crystallinity. The high crystallinity of PeNPz4T relative to PNPz4T is attributable to the enhanced electron deficiency by the introduction of ester groups, which could promote the dipole−dipole interchain interaction. PeNPz3T showed diffractions similar to PeNPz4T, which were also indicative of the face-on orientation, and dπ, dl, and LC were determined to be 3.63, 22.4, and 22.0 Å, respectively (Figure S4). On the other hand, PeNPz2T showed very weak diffractions, suggesting much lower crystallinity or a less amorphous nature, and dπ and LC were determined to be 3.88 and 11.1 Å, respectively (Figure S3). The difference in the E

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

(J−V) curves of the cells under 1 sun illumination in the AM 1.5 G condition (100 mW cm−2), and Table 3 summarizes the corresponding photovoltaic parameters. The EQE spectra of all the devices covered broad ranges of 300 to over 800 nm, reflecting polymer absorption in the films. Among them, the PeNPz3T cell gave the highest short-circuit current density (JSC) with the maximum EQE value of 54% at λmax around 700 nm. The PeNPz4T cell showed a somewhat lower EQE of 42% at λmax compared to the PeNPz3T cell. In the PeNPz2T cell, EQE was low (33%), which can be correlated with the lower crystallinity of PeNPz2T than PeNPz3T and PeNPz4T. The PeNPz3T cell gave a higher JSC value of 13.1 mA cm−2 than the PeNPz4T cell (11.8 mA cm−2) and the PeNPz2T cell (7.1 mA cm−2), all of which were in good agreement with the calculated current density from the EQE spectrum (Jcalc SC ). The open-circuit voltage (VOC), which is proportional to the gap between the EHOMO of the polymer and the ELUMO of PC61BM, was 0.83, 0.76, and 0.74 V for the cells that used PeNPz2T, PeNPz3T, and PeNPz4T, respectively. Note that, despite the fact that EHOMO of PeNPz4T was shallower than that of PeNPz3T by 0.14 eV, VOC of the PeNPz4T cell was slightly lower than that of the PeNPz3T cell by only 0.02 V. We assume that this is attributed to the weaker polymer−PC61BM interactions in PeNPz4T because of the longer alkyl group.52 The PeNPz3T and PeNPz4T cells showed relatively high fill factors (FFs) of 0.63 and 0.71, respectively, whereas the PeNPz2T cell gave a lower FF of 0.46. Overall, both the PeNPz3T and PeNPz4T cells with more balanced photovoltaic parameters gave higher power conversion efficiencies (PCEs) of 6.3% than PeNPz2T (2.7%). Blend Film Structure and Morphology. To further understand the photovoltaic performance of the cells, we investigated the 2D-GIXD images of the blend films. Figure 7 shows the 2D-GIXD images of the blend films (Figure 7a−c) and the out-of-plane and in-plane profiles (Figure 7d,e). All the films had isotropic diffractions assignable to the aggregation of PC61BM (q ≈ 1.36−1.39 Å−1), where the d-spacing was

crystallinity among these polymers can be explained by the degree of alkyl chain interference (Figure 5). Although we do not know the actual tilt angles of the ester groups with respect to the NPz unit and thus their three-dimensional conformation, increasing the number of thiophene groups in the repeating units is expected to reduce the steric hindrance between the alkyl chains, leading to a more coplanar polymer backbone. Thus, the eNPz-based polymers with a large spacer size (PeNPz3T and PeNPz4T) gave enhanced crystallinity and good ordering in the films. In addition, the relatively low molecular weight of PeNPz2T may have affected its low crystallinity.51 OPV Cells Based on eNPz-Based Polymers. We fabricated bulk heterojunction cells with an inverted structure (indium tin oxide/ZnO/photoactive layer/MoOx/Ag) using eNPz-based polymers and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The optimal polymer to PC61BM ratio was 1:1.5 (w/w). Diiodooctane was used as the solvent additive (2 vol % in CB) for fabricating the eNPz-based

Figure 6. EQE spectra (a) and J−V curves (b) of cells that used the eNPz-based polymers.

polymer/PC61BM lay er. Figure 6a,b shows the external quantum efficiency (EQE) spectra and the current−voltage

Figure 7. 2D-GIXD images of PeNPz2T (a), PeNPz3T (b), and PeNPz4T (c) blend films and (d) out-of-plane and (e) in-plane profiles cut from the 2D-GIXD images of polymer blend films. F

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 8. AFM height images of the blend films on PeNPz2T (a), PeNPz3T (b), and PeNPz4T (c) cells.

■

calculated to be 4.5−4.6 Å. PeNPz3T and PeNPz4T showed diffractions assignable to the π−π stacking structure on the qz axis, whereas PeNPz2T did not show a clear peak; these results were similar to those in neat polymer films. In PeNPz3T, however, dπ in the blend film (3.73 Å) was expanded by approximately 0.1 Å relative to that in the neat film (3.63 Å). Actually, LC in the blend film (20.9 Å) diminished compared to that in the neat film (22.0 Å), suggesting that the crystallinity of PeNPz3T was reduced in the blend film (Figure S6). Meanwhile, PeNPz4T showed almost the same dπ (3.63 Å) and LC (23.3 nm) in the blend film compared to those in the neat film (3.65 Å and 23.3 nm) (Figure S7). The difference in dπ and crystallinity in the blend film can be correlated to the difference in JSC and FF of the cells. We also investigated the morphology of the blend films by atomic force microscopy (AFM) (Figure 8a−c). It seemed that the PeNPz2T blend film had larger domains than the PeNPz3T and PeNPz4T films. Indeed, the PeNPz2T cell had a larger root-mean-square value of 2.04 nm, which represents surface roughness, than the PeNPz3T (1.07 nm) and PeNPz4T cells (1.26 nm). This rougher surface of the PeNPz2T cell implies a less miscible bulk heterojunction network, in agreement with the trend in JSC and FF.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00521.

■

Experimental details; NMR spectra for the compounds; TGA and DSC curves; and fittings of the diffraction profiles (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Itaru Osaka: 0000-0002-9879-2098 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from JST (grant no. JPMJAL 1404) and KAKENHI from JSPS (16H04196). Two-dimensional GIXD experiments were performed at BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2018A1568). The authors thank Dr. T. Koganezawa (JASRI) for support in the 2D-GIXD measurements.

CONCLUSIONS

In conclusion, we synthesized ester-functionalized naphthobispyrazine eNPz as a new electron-deficient building unit and eNPz-based polymers with different donor units: PeNPz2T, PeNPz3T, and, PeNPz4T. The eNPz-based polymers had significantly deeper LUMO energy levels of around −3.5 eV than PNPz4T, which has an alkylated NPz. As a result, the absorption of the eNPz-based polymers reached the nearinfrared region with a small band gap of approximately 1.5 eV. We found that PeNPz4T showed high crystallinity compared to PNPz4T. This could be because of the enhanced intermolecular interaction by the strong electron deficiency of eNPz. We also found that PeNPz3T and PeNPz4T exhibited higher crystallinity than PeNPz2T. The crystallinity increased by increasing the number of thiophene groups in the repeating units, likely because of the decreased steric hindrance between the ester groups. In addition, the eNPz-based polymers gave reasonably high PCEs of up to 6.3% in the polymer/fullerene solar cells. We believe that incorporating the eNPz unit as an electron-deficient building unit into the polymer backbone would allow us to develop semiconducting polymers with far more performance.

■

REFERENCES

(1) Sirringhaus, H.; Tessler, N.; Friend, R. H. Integrated Optoelectronic Devices Based on Conjugated Polymers. Science 1998, 280, 1741−1744. (2) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.; Kastler, M.; Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors. Nature 2009, 457, 679−686. (3) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319−1335. (4) Li, M.; An, C.; Pisula, W.; Müllen, K. CyclopentadithiopheneBenzothiadiazole Donor-Acceptor Polymers as Prototypical Semiconductors for High-Performance Field-Effect Transistors. Acc. Chem. Res. 2018, 51, 1196−1205. (5) Brabec, C.; Scherf, U.; Dyakonov, V. Organic Photovoltaics Materials, Device Physics, and Manufacturing Technologies; Wiley-VCH, 2011. (6) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†. Chem. Mater. 2011, 23, 733−758. (7) Kang, H.; Lee, W.; Oh, J.; Kim, T.; Lee, C.; Kim, B. J. From Fullerene-Polymer to All-Polymer Solar Cells: The Importance of G

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Molecular Packing, Orientation, and Morphology Control. Acc. Chem. Res. 2016, 49, 2424−2434. (8) Hedley, G. J.; Ruseckas, A.; Samuel, I. D. W. Light Harvesting for Organic Photovoltaics. Chem. Rev. 2017, 117, 796−837. (9) Fu, H.; Wang, Z.; Sun, Y. Polymer Donors for High-Performance Non-Fullerene Organic Solar Cells. Angew. Chem., Int. Ed. 2019, 58, 4442−4453. (10) Fan, B.; Zhang, D.; Li, M.; Zhong, W.; Zeng, Z.; Ying, L.; Huang, F.; Cao, Y. Achieving Over 16% Efficiency for Single-Junction Organic Solar Cells. Sci. China: Chem. 2019, 60, 1−7. (11) Tsao, H. N.; Cho, D. M.; Park, I.; Hansen, M. R.; Mavrinskiy, A.; Yoon, D. Y.; Graf, R.; Pisula, W.; Spiess, H. W.; Müllen, K. Ultrahigh Mobility in Polymer Field-Effect Transistors by Design. J. Am. Chem. Soc. 2011, 133, 2605−2612. (12) Yuen, J. D.; Fan, J.; Seifter, J.; Lim, B.; Hufschmid, R.; Heeger, A. J.; Wudl, F. High Performance Weak Donor-Acceptor Polymers in Thin Film Transistors: Effect of the Acceptor on Electronic Properties, Ambipolar Conductivity, Mobility, and Thermal Stability. J. Am. Chem. Soc. 2011, 133, 20799−20807. (13) Ajayaghosh, A. Donor-acceptor type low band gap polymers: polysquaraines and related systems. Chem. Soc. Rev. 2003, 32, 181− 191. (14) Liu, C.; Wang, K.; Gong, X.; Heeger, A. J. Low Bandgap Semiconducting Polymers for Polymeric Photovoltaics. Chem. Soc. Rev. 2016, 45, 4825−4846. (15) Dennler, G.; Scharber, M. C.; Brabec, C. J. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2009, 21, 1323−1338. (16) Usta, H.; Facchetti, A.; Marks, T. J. Air-Stable, SolutionProcessablen-Channel and Ambipolar Semiconductors for Thin-Film Transistors Based on the Indenofluorenebis(dicyanovinylene) Core. J. Am. Chem. Soc. 2008, 130, 8580−8581. (17) Lei, T.; Wang, J.-Y.; Pei, J. Design, Synthesis, and StructureProperty Relationships of Isoindigo-Based Conjugated Polymers. Acc. Chem. Res. 2014, 47, 1117−1126. (18) Guo, X.; Facchetti, A.; Marks, T. J. Imide- and AmideFunctionalized Polymer Semiconductors. Chem. Rev. 2014, 114, 8943−9021. (19) Li, W.; Hendriks, K. H.; Wienk, M. M.; Janssen, R. A. J. Diketopyrrolopyrrole Polymers for Organic Solar Cells. Acc. Chem. Res. 2016, 49, 78−85. (20) Saito, M.; Osaka, I.; Suda, Y.; Yoshida, H.; Takimiya, K. Dithienylthienothiophenebisimide, a Versatile Electron-Deficient Unit for Semiconducting Polymers. Adv. Mater. 2016, 28, 6921−6925. (21) Eckstein, B. J.; Melkonyan, F. S.; Manley, E. F.; Fabiano, S.; Mouat, A. R.; Chen, L. X.; Facchetti, A.; Marks, T. J. Naphthalene Bis(4,8-diamino-1,5-dicarboxyl)amide Building Block for Semiconducting Polymers. J. Am. Chem. Soc. 2017, 139, 14356−14359. (22) Yang, J.; Xiao, B.; Tang, A.; Li, J.; Wang, X.; Zhou, E. AromaticDiimide-Based n-Type Conjugated Polymers for All-Polymer Solar Cell Applications. Adv. Mater. 2018, 0, 1804699. (23) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletête, M.; Durocher, G.; Tao, Y.; Leclerc, M. Toward a Rational Design of Poly(2,7-carbazole) Derivatives for Solar Cells. J. Am. Chem. Soc. 2008, 130, 732−742. (24) Bijleveld, J. C.; Shahid, M.; Gilot, J.; Wienk, M. M.; Janssen, R. A. J. Copolymers of Cyclopentadithiophene and Electron-Deficient Aromatic Units Designed for Photovoltaic Applications. Adv. Funct. Mater. 2009, 19, 3262−3270. (25) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (26) Hu, H.; Chow, P. C. Y.; Zhang, G.; Ma, T.; Liu, J.; Yang, G.; Yan, H. Design of Donor Polymers with Strong TemperatureDependent Aggregation Property for Efficient Organic Photovoltaics. Acc. Chem. Res. 2017, 50, 2519−2528. (27) Osaka, I.; Takimiya, K. Naphthobischalcogenadiazole Conjugated Polymers: Emerging Materials for Organic Electronics. Adv. Mater. 2017, 29, 1605218.

(28) Osaka, I.; Abe, T.; Mori, H.; Saito, M.; Takemura, N.; Koganezawa, T.; Takimiya, K. Small Band Gap Polymers Incorporating a Strong Acceptor, Thieno[3,2-b]thiophene-2,5-dione, with PChannel and Ambipolar Charge Transport Characteristics. J. Mater. Chem. C 2014, 2, 2307−2312. (29) Kawabata, K.; Saito, M.; Osaka, I.; Takimiya, K. Very Small Bandgap π-Conjugated Polymers with Extended Thienoquinoids. J. Am. Chem. Soc. 2016, 138, 7725−7732. (30) Kawashima, K.; Osaka, I.; Takimiya, K. Effect of Chalcogen Atom on the Properties of Naphthobischalcogenadiazole-Based πConjugated Polymers. Chem. Mater. 2015, 27, 6558−6570. (31) Kawashima, K.; Tamai, Y.; Ohkita, H.; Osaka, I.; Takimiya, K. High-Efficiency Polymer Solar Cells with Small Photon Energy Loss. Nat. Commun. 2015, 6, 10085. (32) Wang, M.; Hu, X.; Liu, P.; Li, W.; Gong, X.; Huang, F.; Cao, Y. Donor-Acceptor Conjugated Polymer Based on Naphtho[1,2-c:5,6c]bis[1,2,5]thiadiazole for High-Performance Polymer Solar Cells. J. Am. Chem. Soc. 2011, 133, 9638−9641. (33) Osaka, I.; Shimawaki, M.; Mori, H.; Doi, I.; Miyazaki, E.; Koganezawa, T.; Takimiya, K. Synthesis, Characterization, and Transistor and Solar Cell Applications of a NaphthobisthiadiazoleBased Semiconducting Polymer. J. Am. Chem. Soc. 2012, 134, 3498− 3507. (34) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (35) Vohra, V.; Kawashima, K.; Kakara, T.; Koganezawa, T.; Osaka, I.; Takimiya, K.; Murata, H. Efficient Inverted Polymer Solar Cells Employing Favourable Molecular Orientation. Nat. Photon. 2015, 9, 403−408. (36) Kawashima, K.; Fukuhara, T.; Suda, Y.; Suzuki, Y.; Koganezawa, T.; Yoshida, H.; Ohkita, H.; Osaka, I.; Takimiya, K. Implication of Fluorine Atom on Electronic Properties, Ordering Structures, and Photovoltaic Performance in NaphthobisthiadiazoleBased Semiconducting Polymers. J. Am. Chem. Soc. 2016, 138, 10265−10275. (37) Jin, Y.; Chen, Z.; Dong, S.; Zheng, N.; Ying, L.; Jiang, X. F.; Liu, F.; Huang, F.; Cao, Y. A Novel Naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole)-Based Narrow-Bandgap π-Conjugated Polymer with Power Conversion Efficiency Over 10%. Adv. Mater. 2016, 28, 9811−9818. (38) Jin, Y.; Chen, Z.; Xiao, M.; Peng, J.; Fan, B.; Ying, L.; Zhang, G.; Jiang, X. F.; Yin, Q.; Liang, Z.; Huang, F.; Cao, Y. Thick Film Polymer Solar Cells Based on Naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole Conjugated Polymers with Efficiency Over 11%. Adv. Energy Mater. 2017, 7, 1700944. (39) Li, Z.; Xie, R.; Zhong, W.; Fan, B.; Ali, J.; Ying, L.; Liu, F.; Li, N.; Huang, F.; Cao, Y. High-Performance Green Solvent Processed Ternary Blended All-Polymer Solar Cells Enabled by Complementary Absorption and Improved Morphology. Sol. RRL 2018, 2, 1800196. (40) Chiou, D.-Y.; Su, Y.-C.; Hung, K.-E.; Hsu, J.-Y.; Hsu, T.-G.; Wu, T.-Y.; Cheng, Y.-J. Thiophene-Vinylene-Thiophene-Based Donor-Acceptor Copolymers with Acetylene-Inserted Branched Alkyl Side Chains To Achieve High Field-Effect Mobilities. Chem. Mater. 2018, 30, 7611−7622. (41) Cao, F.-Y.; Lin, F.-Y.; Tseng, C.-C.; Hung, K.-E.; Hsu, J.-Y.; Su, Y.-C.; Cheng, Y.-J. Naphthobisthiadiazole-Based Selenophene-Incorporated Quarterchalcogenophene Copolymers for Field-Effect Transistors and Polymer Solar Cells. ACS Appl. Mater. Interfaces 2019, 11, 11674−11683. (42) Dong, Y.; Hu, X.; Duan, C.; Liu, P.; Liu, S.; Lan, L.; Chen, D.; Ying, L.; Su, S.; Gong, X.; Huang, F.; Cao, Y. A Series of New Medium-Bandgap Conjugated Polymers Based on Naphtho[1,2-c:5,6c]bis(2-octyl-[1,2,3]triazole) for High-Performance Polymer Solar Cells. Adv. Mater. 2013, 25, 3683−3688. (43) Lan, L.; Zhang, G.; Dong, Y.; Ying, L.; Huang, F.; Cao, Y. Novel medium band gap conjugated polymers based on naphtho[1,2H

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules c:5,6-c]bis[1,2,3]triazole for polymer solar cells. Polymer 2015, 67, 40−46. (44) Liu, L.; Zhang, G.; He, B.; Liu, S.; Duan, C.; Huang, F. Novel donor-acceptor type conjugated polymers based on quinoxalino[6,5f]quinoxaline for photovoltaic applications. Mater. Chem. Front. 2017, 1, 499−506. (45) Yu, T.; Xu, X.; Li, Y.; Li, Z.; Peng, Q. Side-Chain Influence of Wide-Bandgap Copolymers Based on Naphtho[1,2-b:5,6-b]bispyrazine and Benzo[1,2-b:4,5-b′]dithiophene for Efficient Photovoltaic Applications. ACS Appl. Mater. Interfaces 2017, 9, 18142− 18150. (46) Yu, T.; Xu, X.; Zhang, G.; Wan, J.; Li, Y.; Peng, Q. Wide Bandgap Copolymers Based on Quinoxalino[6,5-f].quinoxaline for Highly Efficient Nonfullerene Polymer Solar Cells. Adv. Funct. Mater. 2017, 27, 1701491. (47) Mikie, T.; Kawashima, K.; Komeyama, K.; Yoshida, H.; Osaka, I. Naphthobispyrazine as an Electron-deficient Building Unit for πConjugated Polymers: Efficient Synthesis and Polymer Properties. Chem. Lett. 2017, 46, 1193−1196. (48) Bredas, J.-L. Mind the gap! Mater. Horiz. 2013, 1, 17−19. (49) Rogers, J. T.; Schmidt, K.; Toney, M. F.; Kramer, E. J.; Bazan, G. C. Structural Order in Bulk Heterojunction Films Prepared with Solvent Additives. Adv. Mater. 2011, 23, 2284−2288. (50) Rogers, J. T. Methods of X-ray and Neutron Scattering in Polymer Science; Oxford University Press: New York, 2000. (51) Osaka, I.; Saito, M.; Mori, H.; Koganezawa, T.; Takimiya, K. Drastic Change of Molecular Orientation in a Thiazolothiazole Copolymer by Molecular-Weight Control and Blending with PC61BM Leads to High Efficiencies in Solar Cells. Adv. Mater. 2012, 24, 425−430. (52) Osaka, I.; Saito, M.; Koganezawa, T.; Takimiya, K. ThiopheneThiazolothiazole Copolymers: Significant Impact of Side Chain Composition on Backbone Orientation and Solar Cell Performances. Adv. Mater. 2014, 26, 331−338.

I

DOI: 10.1021/acs.macromol.9b00521 Macromolecules XXXX, XXX, XXX−XXX