π-Conjugation Effects of Oligo(thienylenevinylene) Side Chains in

Apr 18, 2017 - Using the copolymers that had π-conjugated side chains as the PSC electron ... Jiyoung Kim , Jong Baek Park , Woo-Hyung Lee , Jiwon Mo...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/Macromolecules

π‑Conjugation Effects of Oligo(thienylenevinylene) Side Chains in Semiconducting Polymers on Photovoltaic Performance Jianming Huang,† Kyohei Nakano,† Kaori Suzuki,† Yujiao Chen,† Fanji Wang,†,§ Tomoyuki Koganezawa,∥ and Keisuke Tajima*,†,‡ †

RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako 351-198, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Department of Applied Chemistry Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ∥ Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan ‡

S Supporting Information *

ABSTRACT: Semiconducting copolymers based on benzo[1,2-b:4,5-b′]dithiophene and thieno[3,4-c]pyrole-4,6-dione containing oligo(thienylenevinylene) side chains with different lengths were synthesized to examine the effect of the πconjugated side chains on the performance of polymer solar cells (PSCs). Using the copolymers that had π-conjugated side chains as the PSC electron donor resulted in a higher short circuit current and fill factor compared with the reference copolymers, which had no side chain or an analogous side chain with no π-conjugation, resulting in an increase of power conversion efficiency of 10−22%. Measurements of hole mobility by space-charge-limited current and internal quantum efficiency indicated that introducing the π-conjugated side-chain units can facilitate both charge transport and charge separation in the polymer:PC71BM blend films.

1. INTRODUCTION Polymer solar cells (PSCs) are lightweight, flexible, and suitable for inexpensive large-area production by roll-to-roll solution processes, and they have attracted intense interest for use in printed electronics.1−6 Bulk heterojunction (BHJ) PSCs based on interpenetrating networks of conjugated polymers as electron donors and fullerene derivatives as electron acceptors have achieved power conversion efficiencies (PCEs) higher than 11% under solar light with single-cell structures.7−9 Much effort has been made to improve the performance of PSC devices by designing new semiconducting polymers. In particular, the design principle of the donor−acceptor (D−A) copolymer, in which the electron donating and withdrawing monomer units alternate in the polymer main chains, are effective for tuning the optical and electronic properties to form efficient PSCs in combination with fullerene derivatives.10−12 Major strategies for designing polymers to enhance the PCEs in PSCs include the development of new D and A units and combinations,13,14 the optimization of the alkyl side chains to balance crystallinity and solubility,15,16 and the introduction of π-spacers in D−π−A polymers and substituents such as fluorine.17−20 In addition to the D−A interactions in the main chains, the side chains can be also used to tune the electronic properties of © XXXX American Chemical Society

the semiconducting polymers. These 2D-conjugated polymers have been studied to extend π-conjugation into higher dimensions.21−23 In an early study, Hou et al. developed a series of poly(3-alkylthiophene) copolymers containing a conjugated side chain of bis(thienylenevinylene) (bis-TTV). The PSC based on the mixed BHJ of the copolymers and PC61BM showed the highest PCE of 3.2%, which was higher than that of the P3HT:PC61BM device because of the broader absorption, higher hole mobility, and lower-lying HOMO level of the copolymer.24 Zhang et al. synthesized a series of D−A conjugated random terpolymers based on three units of a donor, an acceptor, and a thiophene-containing isoindigo-based π-conjugated side chain. Compared with the corresponding alternating copolymer, the PCE of the PSCs based on the terpolymer was increased from 4.37% to 5.62%.25 Most recently, Zhou et al. developed a series of D−A copolymers with tris(thienylenevinylene) (tris-TTV) as the π-conjugated side chain.26,27 The introduction of the tris-TTV side chain to the monomer units in a ratio of 20% greatly improved PCE by 25−43% compared with the original polymers owing to the Received: February 27, 2017 Revised: April 12, 2017

A

DOI: 10.1021/acs.macromol.7b00439 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Chemical structures of the copolymers investigated in this study.

Table 1. Summary of the Polymer Properties λmaxc (nm) polymers

Mna (kg mol−1)

PDI

Tdb (°C)

P1 P2 P2NC P3

20.0 29.5 20.9 30.4

2.27 2.03 5.70 2.16

312 312 305

solution 552, 555, 549, 551,

627 599 596 599

film 548, 556, 551, 552,

599 599 595 598

Egopt d (eV)

EHOMOe (eV)

ELUMOf (eV)

1.79 1.81 1.79 1.80

−5.42 −5.32 −5.31 −5.30

−3.63 −3.51 −3.52 −3.50

Determined by GPC using chloroform and TEA as the eluent. bDetermined by TGA at 5% weight loss with a heating rate of 10 °C min−1 under N2. cAbsorption maxima in chloroform and film. dEnergy bandgaps determined from the absorption onset in films. eDetermined by photoelectron spectroscopy in air. fEstimated by ELUMO = Egopt + EHOMO. a

increase of JSC and FF regardless of the main chain structures. These studies demonstrated that introducing π-conjugated side chains to D−A copolymers can be a simple, effective approach for high PSC performance. However, although it was hypothesized that polymers with conjugated side chains can affect the charge separation efficiency at donor/acceptor interfaces, the origin of the enhancement effect is still unclear because of the lack of systematic studies. In this work, we synthesized D−A copolymers based on the combination of benzo[1,2-b:4,5-b′]dithiophene (BDT) and thieno[3,4-c]pyrole-4,6-dione (TPD) as the donor and acceptor units, respectively (Figure 1). Base copolymer P1 has been reported as a D−A copolymer with high performance.28−31 We introduced bis-TTV and tris-TTV side chains to the copolymers to obtain P2 and P3, respectively, to examine the effects of the length of the conjugated side chain on the PSC performance. Furthermore, copolymer P2NC was synthesized to control P2. P2NC has an analogous structure to P2, but the π-conjugation of the side chains is removed by hydrogenation. This would allow us to discuss the effects of π-conjugation in

the side chain in a more comparable way. The copolymers were used to fabricate PSCs in combination with PC71BM as the acceptor. The origin of the improvements in the PSC performance was also investigated by analyzing the film properties and charge mobility in detail.

2. RESULTS AND DISCUSSION Synthesis of Polymers. The scheme and the detailed procedures for the synthesis of the copolymers are described in the Supporting Information. 2,5-Dibromothiophene monomers with bis-TTV (SC1) and tris-TTV (SC2) attached at the 3position were synthesized according to a reported procedure.24 The nonconjugated side-chain unit was synthesized by hydrogenation of SC1 under H 2 in the presence of tris(triphenylphosphine)rhodium(I) chloride as a catalyst.32 Copolymers P1, P2, P2NC, and P3 were synthesized by Stille coupling polymerization of the mixed donor monomer of BDT (0.8 equiv), the acceptor monomer of thieno[3,4-c]pyrrole-4,6dione (TPD) (1.0 equiv), and either SC1, nonconjugated SC1, or SC2 (0.2 equiv) in the presence of Pd2(dba)3·CHCl3 and B

DOI: 10.1021/acs.macromol.7b00439 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules P(o-tolyl)3 in toluene at 150 °C for 1 h by microwave irradiation (Schemes S1 and S2). The introduction rates of the side chains were confirmed by 1H NMR to be close to the feed ratios (20−24%). The target copolymers were purified by sequential Soxhlet extraction to remove low-molecular-weight fractions. The number-averaged molecular weights (Mn) and polydispersity index (PDI) determined by analytical gel permeation chromatography (GPC) are summarized in Table 1. All the copolymers exhibited good solubilities in chlorinated solvents such as CHCl3, chlorobenzene, and o-dichlorobenzene. The thermal properties of the polymers were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under N2, and the degradation temperatures are summarized in Table 1. TGA data showed that all the polymers possess high thermal stabilities with a 5% weight loss at temperature (Td) of over 300 °C. The DSC analysis showed no thermal transitions up to 300 °C (Figure S1). Photophysical Properties of Polymers. UV−vis absorption spectra of the copolymers in CHCl3 solutions and films are shown in Figures 2a and 2b, respectively, and the absorption

absorption at 350−500 nm and a strong absorption at 500−680 nm. This is a typical feature of the D−A configuration, and the absorption with the lowest energy can be assigned to the intramolecular charge transfer (ICT) absorption. The absorption maxima of P2 and P3 in solution are blue-shifted compared with those of P1, which indicates that the presence of a conjugated side chain helps to decrease intermolecular interactions and suppresses aggregation in solution. P2 and P3 showed larger absorptions than P1 and P2NC in the range of 400−500 nm, which was attributed to the independent absorption of TTV side chains.27 In the P1 films, the ICT absorption bands were slightly blue-shifted compared with those obtained in solution, whereas the absorption spectra of P2 and P3 films showed no major differences from their solution spectra. These results suggest that the introduction of the TTV side chain hinders the aggregation of the BDT-TPD main chains. The optical energy gaps (Egopt) of the copolymers determined by the onset of absorption in films were similar (1.79−1.81 eV), indicating that the lowest transition energy was not affected much by the introduction of the side chains. The ionization potentials (IPs) in the solid state were evaluated by photoelectron spectroscopy in air (Figure S2) and are summarized in Table 1. Based on EHOMO and ELUMO for the polymers, which are estimated by −IP and Egopt − IP, respectively, the energy level diagrams of the copolymers, the electrodes, and the acceptor (PC71BM) for PSCs are presented in Figure 2c. Introducing the side chains shifts EHOMO and ELUMO of P2, P2NC, and P3 upward relative to P1, suggesting that the TTV side chains affect the energy level of polymers owing to their electron donating ability. This is reasonable because the acceptor unit (TPD) was replaced with the donor unit (thiophene with the side chains) in P2, P3, and P2NC (see also Theoretical Calculations section). The orbital energy levels of P2NC are almost the same as those of P2. The ELUMO values of the copolymers are higher than that of PC71BM by about 0.3 eV, indicating effective charge separation in the BHJ PSCs system.33 This ELUMO value of the polymers determined by Egopt − IP includes the exciton binding energy in the copolymers. PSC Performance. The BHJ PSC devices were fabricated with a typical structure of glass/indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) PEDOT:PSS/active layers/Ca/Al. The fabrication conditions, such as the blend ratios of the copolymer and PC71BM, processing solvent, and additive, were optimized based on reported conditions.31 The best PSC performance was achieved with a D:A ratio of 1:1.5 by weight and with 5% 1chloronaphthalene (CN) added to chlorobenzene in the

Figure 2. Normalized UV−vis absorption spectra of P1, P2, P2NC, and P3 in (a) chloroform solution and (b) in films (solid line). (c) Energy diagram for PSCs.

maxima and the optical band gaps are summarized in Table 1. All the polymers showed two absorption regions: a weak

Figure 3. (a) J−V curves under the illumination of AM 1.5G, 100 mW cm−2, (b) EQE and IQE spectra, and (c) reflective UV−vis spectra of the P1:PC71BM, P2:PC71BM, P2NC:PC71BM, and P3:PC71BM PSC devices. C

DOI: 10.1021/acs.macromol.7b00439 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Table 2. Performance of PSCs Based on the Blend Films of the Polymers and PC71BM with D/A Ratio of 1:1.5 (w/w) under the Illumination of AM 1.5G, 100 mW cm−2

a

blend films

thickness (nm)

P1:PC71BM P2:PC71BM P2NC:PC71BM P3:PC71BM

85 92 90 85

VOC (V) 0.90 0.84 0.85 0.80

± ± ± ±

0.00 0.01 0.01 0.02

JSC (mA cm−2) 10.22 11.19 10.23 10.60

± ± ± ±

0.03 0.07 0.11 0.07

FF 0.56 0.67 0.53 0.67

± ± ± ±

PCE (%) 0.01 0.01 0.02 0.01

5.11 6.21 4.61 5.64

± ± ± ±

0.09 0.10 0.17 0.0.14

μha (10−3 cm2 V−1 s−1) 3.02 8.36 3.96 6.83

± ± ± ±

1.73 1.36 1.23 1.46

Determined by SCLC measurement.

Figure 4. AFM height images of (a) P1:PC71BM, (b) P2:PC71BM, (c) P2NC:PC71BM, and (d) P3:PC71BM (image size: 2 × 2 μm) with Ra. GIWAXS 2D patterns of (e) P1:PC71BM, (f) P2:PC71BM, (g) P2NC:PC71BM, and (h) P3:PC71BM.

spectra were calculated by IQE = EQE/(1 − R − T), where R and T are the measured reflectance and transmittance spectra of the devices, respectively (Figure 3b). Compared with P1-based devices, the IQE spectra of P2-, P2NC-, and P3-based devices are greatly enhanced over a broad range from 350 to 650 nm. In particular, the P2-based device showed a peak value of approximately 80% in the IQE spectrum. This result reveals that the EQE enhancement in the copolymers with TTV side chains may arise from the improved charge separation in the blend films. The surface morphology of the blend films was investigated by atomic force microscopy (AFM). All the blend films exhibited nanoscale aggregated domains with average roughness (Ra) of 2.51 nm for P1:PC71BM, 0.94 nm for P2:PC71BM, 1.09 nm for P2NC:PC71BM, and 0.60 nm for P3:PC71BM (Figures 4a−d). This suggests that the mixed BHJs had an optimized morphology and that the PSCs had good charge separation. The difference in the roughness may suggest that the P1:PC71BM blend film formed a larger domain than the P2:PC71BM, P2NC:PC71BM, and P3:PC71BM blend films, which may be due to the higher crystallinity of P1. The morphologies of P2:PC71BM, P2NC:PC71BM, and P3:PC71BM may help efficient charge separation and the formation of holetransporting pathways, contributing to the higher FF and JSC compared with the solar cells based on P1 with no conjugated side chain. The molecular packing and orientation of the copolymers were investigated by X-ray diffraction. Two-dimensional grazing incidence wide-angle X-ray scattering (GIWAXS) images of blend films on the ITO/PEDOT:PSS substrate are shown in Figures 4e−h. In all the films, a characteristic broad ring

solutions. The current density−voltage (J−V) curves and external quantum efficiency (EQE) spectra for these optimized devices are shown in Figures 3a and 3b, respectively, and the photovoltaic performance is summarized in Table 2. The observed VOC values are in qualitative agreement with the EHOMO of the polymers shown in the energy diagram in Figure 2c; the PSC device based on P1:PC71BM exhibited a highest VOC of 0.90 V that was mainly attributed to the lowest-lying EHOMO of P1. In contrast, P2-, P2NC-, and P3-based devices showed lower VOC values of 0.84, 0.85, and 0.80 V, respectively, than P1-based devices, which reflects the order of the EHOMO values. In contrast, the PSCs based on P2 and P3 with the conjugated side chain showed higher JSC and FF than those of P1-based devices. P2-based devices showed the best PCE among the copolymers owing to the large increase of JSC and FF. In contrast, P2NC-based devices showed FF and JSC comparable to P1-based devices, suggesting that the gains in FF and JSC observed for P2-based devices were related to the πconjugation of the TTV side chains. JSC calculated for AM1.5 irradiation by integrating the EQE spectra agreed well with the JSC data from the J−V measurements (within ±5%). UV−vis absorption spectra of the blend films showed that the absorbance intensity of the P1:PC71BM blend film is larger than those of P2:PC71BM, P2NC:PC71BM, and P3:PC71BM (Figure 3c). In contrast, the EQE values between 450 and 650 nm of the P1:PC71BM device are lower than those of P2:PC71BM, P2NC:PC71BM, and P3:PC71BM. The lower EQE value of the P1 device may be attributed to lower charge generation efficiency and to the difference in the charge mobility, as discussed below. To explain the enhanced EQE, the internal quantum efficiency (IQE) D

DOI: 10.1021/acs.macromol.7b00439 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 5. Dependence of JSC and FF on the film thickness of the PSCs with P1:PC71BM, P2:PC71BM, P2NC:PC71BM, and P3:PC71BM. The error bars are the standard deviations calculated from at least eight devices.

originating from PC71BM was observed at q of 1.3 Å−1. In the P1:PC71BM blend film, the scattering pattern showed lamellar (qz = 0.23 Å−1) and π−π stacking structures (qxy = 1.75 Å−1), corresponding to spacings of 19.5 and 3.6 Å, indicating that the blend film contained mainly face-on crystallites of the polymer. The predominant face-on orientation allows efficient charge transport in BHJ PSCs.34−36 P2:PC71BM, P2NC:PC71BM, and P3:PC71BM blend films showed weaker diffraction from PC71BM and the out-of-plane π−π stacking, although weaker lamellar peaks were still observed in the in-plane direction. To discuss the diffraction intensity quantitatively, in-plane and outof-plane 1D profiles were measured by using a goniometer with a θ/2θ scan (Figure S3). P1:PC71BM showed larger lamellar peaks in both the profiles than other blend films. These results suggest that the copolymers with the side chains (P2, P2NC, and P3) had less crystalline structures in the blend films with PC71BM compared with P1. These results contradict the general relationship of the crystallinity and the chain orientation of the polymers with FF in PSCs; the higher crystallinity and the more face-on orientation lead to the higher FF.37,38 However, P2- and P3-based devices showed higher FF and JSC than P1, which may be due to the effect of the πconjugated side chains. To elucidate the reasons for the differences in PSC performance further, the relationship between the film thickness and FF and JSC of the devices was investigated. The devices were fabricated under the same conditions, and the film thickness was controlled by changing the solution concentrations. The dependences of JSC and FF on the film thickness are shown in Figure 5. As the film thickness increased, JSC showed a similar trend for all the polymers and there was an optimum thickness of around 90 nm. In contrast, FF monotonically decreased as the film became thicker, but the rate of the decrease depended on the system; P2:PC71BM and P3:PC71BM devices showed much more gradual decreases of FF than P1:PC71BM or P2NC:PC71BM. These results suggest that the π-conjugated side-chain unit in a D−A copolymer system could improve the charge collection process in PSCs even though these units decrease the crystallinity of the films. Charge Transport Properties. Space-charge-limited current (SCLC) was measured to investigate the carrier mobility in the blend films.39 Hole-only devices with various film thicknesses were fabricated with a structure of ITO/ PEDOT:PSS/blend film/Au. Hole mobility (μh) was calculated from μh = 8d3[J/(V − Vbi)2]/(9εε0), where d is the blend film

thickness, ε is the dielectric constant of the polymers, ε0 is the permittivity of free space, and Vbi is the built-in voltage. The J− V characteristics of the devices are shown in Figure S4, and the hole mobilities are summarized in Table 2. Three devices with different film thickness were fabricated for each blend film to confirm the measurements in the SCLC condition. The μh values increased in the order of P1:PC71BM < P2NC:PC71BM < P3:PC71BM < P2:PC71BM, which is in good agreement with the increase in FF in P2- and P3-based devices. Notably, P2NC:PC71BM showed substantially lower μh than P2:PC71BM despite the similar film structure observed by AFM and X-ray analysis. These results suggest that the TTV side chains increase the hole mobility in the mixed BHJ films. This could be the main reason for the improved FF in PSCs for P2. The πconjugated side chains between the copolymer chains may increase the electronic coupling in the interchain charge transport process. This effect may be greater in mixed films where the crystallinity and the mobility values are lower and the interchain hopping process could be more important than in the pristine polymer films. The mobility enhancement partly explains the increase of JSC, but there could be other mechanisms related to the charge generation process. Further study of the relationship between the structures of the conjugated side chains and the PSC performance are necessary to elucidate the whole picture. Theoretical Calculations. The long-range corrected Coulomb attenuated method of the B3LYP level (CAMB3LYP) has been used to calculate the geometry and orbital energy levels of conjugated oligomers or polymers as a better model stating charge transfer.40−43 Therefore, to understand the electronic properties of P1 and P2 better, the frontier molecular orbitals of their model pentamers (P15 and P25, respectively) were calculated by density functional theory (DFT) at the CAM-B3LYP/6-31G(d,p) level. To simplify the calculation, the alkyl groups were replaced by methyl groups. The dihedral angles between BDT and TPD units in the optimized structure for P15 were 174.8°−178.3°, suggesting a stable planar conformation (Figure S5). However, for the optimized P25 structure, the dihedral angles between the BDT unit and the thiophene with the conjugated side chain were 162.6° and 145.0°, respectively, indicating a large torsion. The electron densities of the LUMO and LUMO+1s in P15 and P25 were mainly distributed over the main chains with some localization on the TPD units because of their electron withdrawing nature. In contrast to the unoccupied molecular E

DOI: 10.1021/acs.macromol.7b00439 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

3. CONCLUSION A systematic study of the effect of D−A copolymers with and without π-conjugated side chains on organic photovoltaic performance revealed that introducing conjugated side chains improved the photovoltaic performance, even though they decreased the crystallinity of the polymers. The origin of these effects could be partly attributed to the enhanced hole mobility in the mixed BHJ films. The conjugated side chains may improve the interchain charge hopping by forming a better 3D network of copolymers. However, the charge generation process may also be affected by the conjugated side chains at the D/A interface. This effect seems small in this system but could be increased by manipulating the energy levels of the side chain to improve the coupling with the LUMO levels of the copolymers.

orbitals, the occupied molecular orbitals of P15 and P25 showed large differences. The electron densities of the HOMO and HOMO−1 were localized on the conjugated backbone in P15. However, the electron density of the HOMO was mainly localized on the bis-TTV side-chain unit in P25 (Figure 6a), and

4. EXPERIMENTAL SECTION Fabrication of PSC and Evaluation. The BHJ PSCs were fabricated with a typical structure of glass/ITO/PEDOT:PSS/active layers/Ca/Al. The active layers were composed of the polymer/ PC71BM blend films. The PSC devices were fabricated as follows. Patterned ITO substrates were sequentially cleaned by ultrasonication in toluene, acetone, deionized water, and 2-propanol for 10 min. Next, the ITO substrates were treated with UV-ozone for 20 min. The PEDOT/PSS layer was spin-coated to a thickness of 30−40 nm on top of the ITO substrate at 3000 rpm for 30 s and heated at 150 °C for 15 min in air. Then, the active layers were spin-coated onto ITO/ PEDOT:PSS from chlorobenzene solution with 5% CN at 60 °C. Lastly, the device fabrication was completed by thermal evaporation of 20 nm of Ca and 80 nm of Al as the cathode under vacuum below a pressure of 2 × 10−4 Pa. The area of the devices was 0.12 cm2. The current density−voltage (J−V) characteristics of the PSC devices were measured by a source meter (2400SMU, Keithley) under air mass 1.5 global (AM 1.5G) simulated solar illumination with an irradiation intensity of 100 mW cm−2, and the electrical photovoltaic parameters of PCE, VOC, JSC, and FF were obtained. The intensity of the solar simulator was calibrated carefully with a standard silicon photodiode. The EQE spectra were obtained with a Hypermonolight system (SM250F, Bunkou Keiki). The thickness of the active layers was determined by a surface profiler (Dektak 6M, ULVAC). Characterization. UV−vis spectra and IQE measurements were performed on double-beam spectrophotometers (V670 and V650, JASCO, respectively). X-ray diffraction (XRD) measurements were performed on an X-ray diffractometer (Smartlab, Rigaku) by using monochromatized Cu Kα radiation (λ = 0.154 nm) generated at 45 kV and 200 mA. AFM measurements were performed with a scanning probe microscope (550, Agilent Technologies) in tapping mode. The IP measurements were carried out by using a photoelectron spectrometer (Keiki AC-2, Riken). TGA measurements were performed on a thermogravimetric analyzer (Thermoplus TG8120, Rigaku). DSC measurements were performed on a differential scanning calorimeter (DSC8230, Rigaku). GIWAXS experiments were conducted at an incident angle of 0.12° by using synchrotron radiation at beamline BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute.

Figure 6. (a) HOMO of the neutral ground state and (b) SOMO of singly oxidized doublet state of the model pentamer P25 obtained by DFT calculations at the CAM-B3LYP/6-31G(d,p) level.

the electron density of HOMO−1 also extended to the conjugated backbone from the main chain. Because the interchain hole hopping through the alkyl side chain can be slow and is the rate-determining step for the hole conduction in the films, the extension of the HOMO to the next chain in P2 could improve this hopping process. To clarify the charge transport process, the structure optimization was performed on oxidized (+1) doublet state of P25, and the frontier orbitals are compared (Figure 6b). In contrast to the HOMO of the neutral singlet ground state in P25, the electron density of the singly occupied molecular orbital (SOMO) in the oxidized doublet state is localized on the conjugated backbone. This suggests that the intramolecular charge transport would be through the main chain, and the TTV would not act as the charge trapping site. Moreover, the intermolecular hole hopping would involve the electron transfer from the TTV side chain of the neutral molecule to the main chain of the oxidized molecule. This process could be more efficient than the interchain hopping process through the insulating long alkyl chains, which could be reflected in the larger mobility observed for P2 and P3 than P1 or P2NC. These results imply that the conjugated side chain may play an important role in helping interchain hole hopping and contribute to the higher hole mobilities, the higher FF, and the tolerance of the FF to the film thickness observed in the PSCs.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00439. Details of synthetic procedures, NMR spectra, DSC and TGA, IP, XRD, hole mobility, and DFT calculations (PDF) F

DOI: 10.1021/acs.macromol.7b00439 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

Polymer Solar Cells with Low-Bandgap Polymer (PTB7-Th) for High Performance. Adv. Mater. 2013, 25, 4766−4771. (15) Osaka, I.; Saito, M.; Koganezawa, T.; Takimiya, K. Thiophene− Thiazolothiazole Copolymers: Significant Impact of Side Chain Composition on Backbone Orientation and Solar Cell Performances. Adv. Mater. 2014, 26, 331−338. (16) Warnan, J.; Cabanetos, C.; Labban, A. E.; Hansen, M. R.; Tassone, C.; Toney, M. F.; Beaujuge, P. M. Ordering Effects in Benzo[1,2-b:4,5-b′]difuran-thieno[3,4-c]pyrrole-4,6-dione Polymers with > 7% Solar Cell Efficiency. Adv. Mater. 2014, 26, 4357−4362. (17) Min, J.; Zhang, Z.-G.; Zhang, S.; Zhang, M.; Zhang, J.; Li, Y. Synthesis and Photovoltaic Properties of D-A Copolymers Based on Dithienosilole and Benzotriazole. Macromolecules 2011, 44, 7632− 7638. (18) Huang, J.; Ie, Y.; Karakawa, M.; Aso, Y. Low band-gap donor− acceptor copolymers based on dioxocyclopenta[c]thiophene derivatives as acceptor units: synthesis, properties, and photovoltaic performances. J. Mater. Chem. A 2013, 1, 15000−15009. (19) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. Highly Efficient Solar Cell Polymers Developed via Fine-Tuning of Structural and Electronic Properties. J. Am. Chem. Soc. 2009, 131, 7792−7799. (20) Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D. Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2012, 134, 14932−14944. (21) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45, 723−733. (22) Ye, L.; Zhang, S.; Huo, L.; Zhang, M.; Hou, J. Molecular Design toward Highly Efficient Photovoltaic Polymers Based on TwoDimensional Conjugated Benzodithiophene. Acc. Chem. Res. 2014, 47, 1595−1603. (23) Zhang, Z.-G; Li, Y. Side-chain engineering of high-efficiency conjugated polymer photovoltaic materials. Sci. China: Chem. 2015, 58, 192−209. (24) Hou, J.; Tan, Z.; Yan, Y.; He, Y.; Yang, C.; Li, Y. Synthesis and photovoltaic properties of two-dimensional conjugated polythiophenes with bi(thienylenevinylene) side chain. J. Am. Chem. Soc. 2006, 128, 4911−4916. (25) Zhang, M.; Wu, F.; Cao, Z.; Shen, T.; Chen, H.; Li, X.; Tan, S. Improved photovoltaic properties of terpolymers containing diketopyrrolopyrrole and an isoindigo side chain. Polym. Chem. 2014, 5, 4054−4060. (26) Zhou, E.; Cong, J.; Hashimoto, K.; Tajima, K. Control of miscibility and aggregration via the material design and coating process for high-performance polymer blend solar cells. Adv. Mater. 2013, 25, 6991−6996. (27) Zhou, E.; Cong, J.; Hashimoto, K.; Tajima, K. Introduction of a conjugated side chain as an effective approach to improving donoracceptor photovoltaic polymers. Energy Environ. Sci. 2012, 5, 9756− 9759. (28) Zou, Y.; Najari, A.; Berrouard, P.; Beaupré, S.; Aich, B. R.; Tao, Y.; Leclerc, M. A Thieno[3,4-c]pyrrole-4,6-dione-Based Copolymer for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132, 5330−5331. (29) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fréchet, J. M. J. Synthetic Control of Structural Order in N-Alkylthieno[3,4-c]pyrrole-4,6-dione-Based Polymers for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132, 7595−7597. (30) Zhang, Y.; Hau, S. K.; Yip, H.-L; Sun, Y.; Acton, O.; Jen, A. K.-Y. Efficient Polymer Solar Cells Based on the Copolymers of Benzodithiophene and Thienopyrroledione. Chem. Mater. 2010, 22, 2696−2698. (31) Cabanetos, C.; El Labban, A.; Bartelt, J. A.; Douglas, J. D.; Mateker, W. R.; Frechet, J. M. J.; McGehee, M. D.; Beaujuge, P. M. Linear side chains in benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c]pyrrole-4,6-dione polymers direct self-assembly and solar cell performance. J. Am. Chem. Soc. 2013, 135, 4656−4659.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.T.). ORCID

Kyohei Nakano: 0000-0003-2493-2817 Keisuke Tajima: 0000-0003-1590-2640 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS 2D GIWAXS experiments were performed at beamline BL46XU and BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals 2016B1875 and 2013B1719). HRMS was carried out at the Materials Characterization Support Unit, Advanced Technology Support Division, RIKEN. DFT calculations were performed by using HOKUSAI-Great Wave system of Advanced Center for Computing and Communication, RIKEN. This research was supported in part by the Japan Science and Technology Agency (JST), PRESTO.

■

REFERENCES

(1) Dennler, G.; Scharber, M. C.; Brabec, C. J. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2009, 21, 1323−1328. (2) Arias, A. C.; MacKenzie, J. D.; McCulloch, L.; Rivnay, J.; Salleo, A. Materials and Applications for Large Area Electronics: SolutionBased Approaches. Chem. Rev. 2010, 110, 3−24. (3) Beaujuge, P. M.; Fréchet, J. M. J. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009−20029. (4) Lipomi, D. J.; Tee, B. C. K.; Vosgueritchian, M.; Bao, Z. Stretchable Organic Solar cells. Adv. Mater. 2011, 23, 1771−1775. (5) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733−758. (6) Krebs, F. C.; Espinosa, N.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M. 25th Anniversary Article: Rise to Power − OPV-Based Solar Parks. Adv. Mater. 2014, 26, 29−39. (7) 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, 15027−15034. (8) Yang, Y.; Zhang, Z.-G.; Bin, H.; Chen, S.; Gao, L.; Xue, L.; Yang, C.; Li, Y. Side-Chain Isomerization on an n-type Organic Semiconductor ITIC Acceptor Makes 11.77% High Efficiency Polymer Solar Cells. J. Am. Chem. Soc. 2016, 138, 15011−15018. (9) 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. (10) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficienies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789− 1791. (11) Zhang, Z.-G.; Wang, J. Structures and properties of conjugated Donor−Acceptor copolymers for solar cell applications. J. Mater. Chem. 2012, 22, 4178−4187. (12) Zhou, H.; Yang, L.; You, W. Rational Design of High Performance Conjugated Polymers for Organic Solar Cells. Macromolecules 2012, 45, 607−632. (13) Dou, L.; Chen, C.-C.; Yoshimura, K.; Ohya, K.; Chang, W.-H.; Gao, J.; Liu, Y.; Richard, E.; Yang, Y. Synthesis of 5H-Dithieno[3,2b:2′,3′-d]pyran as an Electron-Rich Building Block for DonorAcceptor Type Low-Bandgap Polymers. Macromolecules 2013, 46, 3384−3390. (14) Liao, S.-H.; Jhuo, H.-J.; Cheng, Y.-S.; Chen, S.-A. Fullerene Derivative-Doped Zinc Oxide Nanofilm as the Cathode of Inverted G

DOI: 10.1021/acs.macromol.7b00439 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (32) Liang, Y.; Chen, Z.; Jing, Y.; Rong, Y.; Facchetti, A.; Yao, Y. Heavily n-Dopable π-Conjugated Redox Polymers with Ultrafast Energy Storage Capability. J. Am. Chem. Soc. 2015, 137, 4956−4959. (33) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar Cells- Towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789−794. (34) He, F.; Wang, W.; Chen, W.; Xu, T.; Darling, S.; Strzalka, J.; Liu, Y.; Yu, L. Tetrathienoanthracene-Based Copolymers for Efficient Solar Cells. J. Am. Chem. Soc. 2011, 133, 3284−3287. (35) Rivnay, J.; Steyrleuthner, R.; Jimison, L. H.; Casadei, A.; Chen, Z.; Toney, M. F.; Facchetti, A.; Neher, D.; Salleo, A. Drastic Control of Texture in a High Performance n-Type Polymeric Semiconductor and Implications for Charge Transport. Macromolecules 2011, 44, 5246− 5255. (36) Vohra, V.; Kawashima, K.; Kakara, T.; Koganezawa, T.; Osaka, I.; Takimiya, K.; Murata, H. Efficient inverted polymer solar cells employing favourable molecular orientation. Nat. Photonics 2015, 9, 403−408. (37) Guo, X.; Zhou, N.; Lou, S. J.; Smith, J.; Tice, D. B.; Hennek, J. W.; Ortiz, R. P.; Navarrete, J. T. L.; Li, S.; Strzalka, J.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Polymer solar cells with enhanced fill factors. Nat. Photonics 2013, 7, 825−833. (38) Proctor, C. M.; Love, J. A.; Nguyen, T.-Q. Mobility Guidelines for High Fill Factor Solution-Processed Small Molecule Solar Cells. Adv. Mater. 2014, 26, 5957−5961. (39) Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Fréchet, J. M. J. Molecular-weight-dependent mobilities in regioregular poly(3-hexyl-thiophene) diodes. Appl. Phys. Lett. 2005, 86, 122110. (40) Yanai, T.; Tew, D. P.; Handy, N. C. A new hybrid exchangecorrelation functional using the Coulomb-attenuating method (CAMB3LYP). Chem. Phys. Lett. 2004, 393, 51−57. (41) Salzner, U.; Aydin, A. Improved Prdiction of Properties of πConjugated Oligomers with Range-Separated Hybrid Density Functionals. J. Chem. Theory Comput. 2011, 7, 2568−2583. (42) Peach, M. J. G.; Tellgren, E. I.; Salek, P.; Helgaker, T.; Tozer, D. Structural and Electronic Properties of Polyacetylene and Polyyne from Hybrid and Coulomb-Attenuated Density Functionals. J. Phys. Chem. A 2007, 111, 11930−11935. (43) Pandey, L.; Doiron, C.; Sears, J. S.; Brédas, J.-L. Lowest excited states and optical absorption spectra of donor−acceptor copolymers for organic photovoltaics: a new picture emerging from tuned longrange corrected density functionals. Phys. Chem. Chem. Phys. 2012, 14, 14243−14248.

H

DOI: 10.1021/acs.macromol.7b00439 Macromolecules XXXX, XXX, XXX−XXX