A New Low-Bandgap Polymer Containing Benzene-Fused Quinoxaline: Signifi cantly Enhanced Performance Caused by One Additional Benzene Ring
Shuang Li , Aiyuan Li , Jian Yu , Aoshu Zhong , Su’an Chen , Runli Tang , Xianyu Deng , * Jingui Qin , Qianqian Li , Zhen Li *
Two new alkoxy-substituted quinoxaline (Qx)-based copolymers, PBDTQx and PBDTPz, are designed and synthesized. The only difference between these two polymers is that two methyl groups of the Qx are replaced by one additional fused benzene ring. The UV–Vis absorptions, thermal stability, energy levels, fi eld-effect carrier mobility, and photovoltaic characteristics of the two copolymers are systematically evaluated to understand the relationships between the polymer structure at the molecular level and the photovoltaic performances. Photovoltaic cells based on the PBDTPz with a structure of ITO/PEDOT:PSS/Polymer:PC 71 BM/PEO/Ca/Al exhibit a promising effi ciency of 4.40%, while that of PBDTQx is relatively much poorer.
1. Introduction
Polymer solar cells (PSCs), generally based on the bulk heterojunction (BHJ) concept, have attracted tremendous attention due to their unique advantages including low cost, light weight, and potential applications in fl exible and large-area devices. [ 1–7 ] The record high effi ciency has
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S. Li, J. Yu, A. Zhong, S. Chen, R. Tang, Dr. Q. Li, Prof. J. Qin, Prof. Z. LiDepartment of Chemistry, Wuhan University, Wuhan 430072, China E-mail: [email protected]; [email protected] A. Li, X. DengResearch Center for Advanced Functional Materials and Devices, School of Materials Science and Engineering, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, ChinaE-mail: [email protected].
continually increased to its current value of higher than 8%. [ 8 ] This impressive accomplishment is mainly achieved by the molecular engineering of new low-bandgap donor materials, [ 9–11 ] assisted by successes in derivatizing the fullerenes [ 12–17 ] and synergetic advancement in the device optimization. [ 18–22 ] For the further improvement of the per-formance, for chemists, it is still needed to design new con-jugated polymers with low bandgap.
It is known that the bandgap decreases linearly as a function of the increasing quinoid character of a given conjugated polymer. [ 23 ] Thus, combinations of the elec-tron-rich (donor) units and electron defi cient units (acceptor) of heterocycles have become an attractive method to construct low-bandgap polymers for high-effi ciency PSCs since the internal charge transfer (ICT) intrinsic with the D-A polymers effectively promotes con-jugation throughout the polymer backbone by quinoid resonance stabilization, thereby appreciably narrowing the bandgap. [ 24 ] Another approach to increase quinoid character employs fused aromatic units. However, in
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Chart 1. The structure of Quinoxaline (Qx) and Phenazine (Pz).
comparison with the most cases with the efforts to adjust the ICT process in conjugated polymers by choosing dif-ferent donor and acceptor units to lower the bandgap, the control of the bandgap through the second approach, by fusing another aromatic ring, was rarely reported. [ 25 , 26 ]
Quinoxaline unit (Qx) is one of the frequently used acceptor building block in the synthesis of low-bandgap conjugated polymers due to the electron-withdrawing property of two imine nitrogens and relatively stable qui-noid form. [ 27–34 ] As shown in Chart 1 , the Qx unit has a fascinating structure for controlling the electronic struc-ture of the resulting copolymers because it can provide
the versatility of the introduction of substituents easily on its 2 and 3 positions. If, following the above-mentioned second approach, an additional benzene ring was fused to the Qx block to form the Phenzine (Pz) one ( Chart 1 ), how about the bandgap of the resultant conjugated polymers? Perhaps, the performance of the Pz-containing polymers could be further improved. However, so far, there are no reports concerning this.
Prompted by the above points, based on our recent work on the modifi cation of the bandgap of conjugated poly-mers, [ 35 , 36 ] we designed a pair of D-A type low-bandgap poly mers based on quinoxaline and one benzene ring fused quinoxaline, named PBDTQx and PBDTPz (Scheme 2 ). The only difference between these two polymers is that the two methyl group of the Qx is replaced by one additional fused benzene ring. Unexpectedly, the slight difference triggered a signifi cant difference in their optical, elec-trochemical, and photovoltaic properties of the resulting polymers. Excitingly, the PSCs based on PBDTPz as the donor and PC 71 BM as the acceptor exhibits a promising PCE of 4.40% under the illumination of AM 1.5, 100 mW cm − 2 , while that of PBDTQx is relatively much poor. The inter-esting results might provide useful information for the rational design of low-bandgap conjugated polymers for PSCs with even higher performance.
2. Experimental Section
The synthesis and characterization of monomers and polymers are described in Supporting Information. Device fabrication and characterization are also described there.
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Figure 1 . UV–Vis absorption spectra of the polymers in the CHCl 3 solution and in fi lm.
Scheme 2 . Syntheses of Copolymers.
3. Results and Discussion
3.1. Synthesis, Characterizations, and Thermal Stability of Polymers
The synthetic routes to the monomers and polymers were outlined in Scheme 1 and 2 , respectively. Monomer 2,6 ′ -bis(trimethyltin)-4,8-di-2-exylhexyloxybenzo[1,2-b:4,5-b ′ ]di-thiophene M3 was obtained from commercial sources and used without further purifi cation. Compounds 2,3-dihydroxyphenazine 3 [ 37 ] and 4,5-bis(octyloxy)-3,6-dibromo-1,2-phenylendiamine 7 [ 38 , 39 ] were prepared according to the literatures. 3 was reacted with n -octyl bromide in the presence of potassium carbonate in DMF to obtain 4 , which was further brominated to 5 by N-bro-mosuccinimide (NBS). Compound 8 was synthesized via condensation reaction. The Still coupling reaction of the dibromide 5 and 8 with tributyl(thiophen-2-yl)stannane yielded 2,3-bis(octyloxy)-1,4-di(thiophen-2-yl)phenazine 6 and 2,3-dimethyl-6,7-bis(octyloxy)-5,8-di(thiophen-2-yl)quinoxaline 9 , respectively. Bromination of 6 and 9 with NBS in a mixture solvent of CH 3 Cl and DMF afforded the two monomers, 1,4-bis(5-bromothiophen-2-yl)-2,3-bis(octyloxy)phenazine M1 and 5,8-bis(5-bromothiophen-2-yl)-2,3-dimethyl-6,7-bis(octyloxy) quinoxaline M2 .
D-A type two polymers, PBDTPz and PBDTQx , were synthesized by the Stille coupling reaction of M1 and M2 with monomer M3 , respectively. The molecular weights and polydispersity index (PDI) of the polymers were listed in Table S1 (Supporting Information). The polymers exhib-ited good solubility in common organic solvents, such as chloroform, chlorobenzene, and dichlorobenzene, and
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Table 1. Electrochemical and optical properties of the polymers.
Polymer E HOMO a) [eV]
E LUMO a) [eV]
E g (ec) b) [eV]
λ abs [nm] solution [n
PBDTPz –5.17 –3.46 1.71 430 563 4
PBDTQx –5.20 –2.85 2.35 391 497 3
a) HOMO and LUMO levels are estimated from the onset of the oxidation and reduction b) Determined from the cyclic voltammetry; c) Bandgap estimated from optical absorption
possessed a number-average molecular weight ( M n ) of 59.6 and 44.6 kg mol − 1 , with a polydispersity index of 2.27 and 2.30, respectively. Thermogravimetric analysis (TGA) demonstrated a good thermal stability of the two polymers with a 5% weight-loss temperature at 318 and 329 ° C, respectively (as shown in Table S1 and Figure S1, Supporting Information). The results from differ-ential scanning calorimetry (DSC) (heat at 10 ° C min − 1 under nitrogen) showed that the glass transition temperature
of PBDTPz is 82 ° C, while there was no obvious thermal transition between 30 and 200 ° C for PBDTQx (Figure S2, Supporting Information).
3.2. Optical Properties
The normalized UV–vis absorption spectra of PBDTPz and PBDTQx in dilute chloroform solutions with the concentration of 1 × 10 − 5 M and in thin fi lms were shown in Figure 1 , with the maximum absorption wavelengths listed in Table 1 . Although the two polymers had minor differ-ence in chemical structure, they exhibited much different absorption curves to a large degree. The PBDTPz solution
229
λ abs m] fi lm
λ onset [nm] fi lm
E g (opt) c) [eV]
62 641 788 1.57
98 539 638 1.94
peaks of cyclic voltammograms, respectively; band edge of the fi lm.
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displayed two absorption peaks at 430 and 563 nm. The lower energy band (563 nm) could be attributed to the intramolecular charge transfer (ICT), while the higher energy band (430 nm) was originated from the π – π ∗ transition of the conjugated backbone. Actually, PBDTQx also had ICT bands. However, its ICT bands were located at shorter wavelengths and overlapped with the localized π – π ∗ transition band, [ 32 , 34 ] thus, only a very broad band was observed. The optical absorption of PBDTPz in fi lm (641 nm) exhibited red shift of 78 nm compared with that of solution, with the absorption band edge at 788 nm. The obvious red-shifted absorption could be attributed to the strong aggregation of the polymer in the solid state. The optical bandgap ( E g ) of PBDTPz fi lm calculated from the absorption band edge was 1.57 eV. The absorption onset of PBDTPz was dramatically red-shifted (by ≈ 150 nm in the solid state) compared with PBDTQx , refl ecting the enhanced stabilization of the quinoid structure in the former polymers due to the aromatic resonance stabiliza-tion (Figure S3, Supporting Information). The E g of PBDTPz was obviously reduced in comparison with that of 1.94 eV for the copolymer of PBDTQx . Thus, a high J sc for PBDTPz was expected.
3.3. Electrochemical Properties
Cyclic voltammetry (CV) was employed to investigate the redox behavior of the copolymers and estimate their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. All reported potentials were calibrated against the ferrocene/ferrocenium (Fc/Fc + ) couple, which was used as the internal standard. The CV curves were shown in Figure S4 (Sup-porting Information), with the corresponding data sum-marized in Table 1 . The HOMO and LUMO energy levels of the polymers were calculated according to the equation: E HOMO = – e ( E ox + 4.29) eV and E LUMO = – e ( E red + 4.29) eV, respectively, where E ox is the onset oxidation potential of the polymers and E red is the onset reduction potential of the polymers. The HOMO and LUMO energy levels of the two copolymers were − 5.17 and − 3.46 eV for PBDTPz , − 5.20 and − 2.85 eV for PBDTQx , respectively. The two polymers shared almost the same low HOMO energy level, and pos-sessed much different LUMO level, demonstrating that the two methyl groups replaced by one fused benzene ring does not affect the HOMO energy level in a large degree, however, dramatically reduce the LUMO energy lever. The reduced LUMO energy lever should be due to the fused benzene ring of Pz, which could stabilize the electronic quinoid state in the conductive state. The similar HOMO energy level is benefi cial to maintain a relative high V oc and the lower LUMO energy level is conducive to obtain a low-bandgap copolymer.
To get an insight into the fundamentals of the molecular architecture of the polymers, density functional theory (DFT) calculations were performed for PBDTPz and PBDTQx at the B3LYP/6-31G(d) [ 40 , 41 ] level using Gaussian 09, Revi-sion A.02 program and the corresponding Kohn-Sham orbitals were obtained at the same level of theory. [ 42 ] To simplify the calculation, only two repeating units of each polymer were subject to the calculation, with alkyl chains replaced by CH 3 groups. The simulated electron density dis-tributions and the calculated HOMO and LUMO levels along with the optimized geometry were shown in Figure S5 (Supporting Information). The electron density distribu-tions of HOMO energy level for both polymers are nearly identical and both delocalized on the whole polymer skel-eton. However, the electron-density distributions of LUMO levels for PBDTPz primarily localized on Pz units, while for PBDTQx some of the electron density distribution of LUMO level localized on BDT units. This phenomenon indi-cated that the change of acceptor units Qx with Pz had little effect on the HOMO level but signifi cantly affected the LUMO level of the resulting polymer. By changing Qx unit to one benzene fused Pz unit, the calculated LUMO level of PBDTPz was largely decreased in comparison with that of PBDTQx . However, the HOMO level of PBDTPz was identical to that of PBDTQx since both polymer shared the same donor unit. The DFT calculations exhibited the same trends of variation in the HOMO and LUMO energies as experimental results.
3.5. Hole Mobility
In order to get some ideas about the infl uence of the chem-ical structures on the charge-transporting properties of these polymers, the hole mobility of the copolymers was measured by using the space charge limited current (SCLC) method with a device structure of ITO/PEDOT:PSS (40 nm)/polymer:PC 71 BM (1:4, 150 nm)/MoO 3 (5 nm)/Al(100 nm). The SCLC behavior was analyzed using the Mott-Gurney square law: J = (9/8) ε 0 ε r μ (V 2 L − 3 ), where J is the current, μ is the hole mobility, ε 0 is the permittivity of free space, ε r is the dielectric constant of the polymer, L is the thickness of the active layer, and V is the voltage drop across the device. V = V appl - V rs - V bi , where V is the effective voltage, V appl is the applied voltage, V rs is the voltage drop, and V bi is the built-in voltage. The mobility is obtained from the slope of the plot of J 1/2 versus V (Figure S6, Supporting Information). The hole mobility of 4.3 × 10 − 6 cm 2 v − 1 s − 1 was obtained for PBDTPz , which was one order higher than that of PBDTQx (1.7 × 10 − 7 cm 2 v − 1 s − 1 ). Likewise, higher hole mobility was also observed for the blend of PBDTPz with PC 71 BM (3.3 × 10 − 6 cm 2 v − 1 s − 1 ) compared with the blend of PBDTQx with PC 71 BM (1.6 × 10 − 7 cm 2 v − 1 s − 1 ). The enhanced hole mobility
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Figure 3 . External quantum effi ciency (EQE) curves of solar cells with copolymer: PC 71 BM = 1:4 as active layer when using different cathodes.
Figure 2 . J– V curves of photovoltaic cells with the structure of ITO/PEDOT: PSS/polymer: PC 71 BM (1:4, w/w)/PEO(with or without)Ca/Al under illumination of AM 1.5G at 100 mW cm − 2 .
Table 2. Photovoltaic parameters of devices tested under illumination of AM 1.5G at 100 mW cm − 2 .
Polymer PEO V oc [V]
J sc [MA cm − 2 ]
FF [%]
PCE [%]
PBDTPz:PC 71 BM = 1:4 Without 0.65 8.46 42 2.20
PBDTPz:PC 71 BM = 1:4 With 0.75 9.64 60 4.40
PBDTQx:PC 71 BM = 1:4 Without 0.65 1.95 29 0.38
should be ascribed to the enlarged fused planar aromatic ring of Pz, in comparison with Qx. Thus, the π – π interaction of the resultant copolymer PBDTPz was enhanced, and the mobility was improved, which were favorable for the PSCs application.
3.6. Photovoltaic Properties
The PSC devices based on the two copolymers, PBDTPz and PBDTQx , were fabricated and tested with the structure of ITO/PEDOT:PSS (40 nm)/polymers:PC 71 BM (1:4, w/w) (100 nm)/Ca (5 nm)/Al (100 nm). The photovoltaic perform-ance of the PSCs was summarized in Table 2 . Figure 2 dem-onstrated the current density-voltage ( J–V ) curves of the PSCs measured under the illumination of simulated AM 1.5 G conditions (100 W m − 2 ). As shown in the atomic force microscopy (AFM) images (Figure S7, Supporting Informa-tion) of the blend fi lms, the morphology of the blend fi lms was moderately homogeneous and there was no large phase separation, indicating that both of the two polymers possessed good miscibility with PC 71 BM. It was a little sur-prising that the blend of PBDTPz and PC 71 BM exhibited a PCE of 2.20%, ≈ 5.8 times higher than that of PBDTQx (0.38%). The two polymers showed the same V oc because they shared the similar HOMO energy level. The V oc of BHJ
solar cell was mainly governed by the difference between the LUMO level of the acceptor and the HOMO level of the donor. [ 43–45 ] The low J sc should be the main reason for the poor performance of PBDTQx . This should be caused by its relatively narrow absorption spectra. The external quantum effi ciency (EQE) curves of the two devices were shown in Figure 3 . The device based on PBDTPz exhibits broad response covering 330–790 nm, and the EQE exceeds 30% in the spectral region from 400 to 720 nm. However, the device based on PBDTQx showed a narrow and low EQE. The narrow EQE spectrum was due to the large bandgap of PBDTQx . Although the limited EQE may be originated from a low absorption coeffi cient or low internal quantum effi ciency (IQE), that is, a poor conversion of absorbed photons in the collected charges. The absorption spectra of polymers:PC 71 BM = 1:4 thin fi lms were shown in Figure S8 (Supporting Information). The absorption intensity of PBDTQx was stronger than that of PBDTPz from 480 to 570 nm, but the EQE of PBDTQx was still lower than PBDTPz in this range. So we could conclude that the low IQE should be the main reason of the low EQE of PBDTQx .
The performance of the PBDTPz -based device was fur-ther optimized by using a thin layer of poly (ethylene oxide) (PEO) to modify the cathode, [ 20 , 21 ] with the device structure of ITO/PEDOT:PSS (40 nm)/polymers:PC 71 BM
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(1:4, w/w) (100 nm)/PEO (2 nm)/Ca (5 nm)/Al (100 nm). PEO acted as an effective buffer with Ca/Al electrodes, to markedly improve the electrode interface, enhance the V oc , J sc , FF values, and induce double PCE (up to 4.40%) in comparison with those of devices without PEO (Figure 2 and Table 2 ). The EQE of the PSCs is signifi cantly improved, too (Figure 3 ). The device from PBDTPz :PC 71 BM with PEO showed a considerably high incident photon to current effi ciency of 60% at 666 nm. The calculated J sc by inte-grating the spectral response of the cells agreed well with photocurrent obtained by J– V measurements.
4. Conclusions
In summary, two new quinoxaline derivatives, Qx and Pz, were designed and obtained by a facile and effi cient way. On the basis of these two building blocks, two alternating copolymers, PBDTQx and PBDTPz , were synthesized with the bridges of the BDT and thiophene groups, through Stille polycondensation reaction. The only difference between the two polymers was that the two methyl groups of Qx were replaced by one fused benzene ring, but led to totally different properties. Based on the obtained experimental results, including the optical, structure, energy levels, fi eld-effect carrier mobility, and photovoltaic characteristics, it was concluded that to fuse an additional aromatic ring vertically on Qx could obviously improved the photovoltaic properties of the resultant copolymer. This idea should provide some useful clues for the rational design of other types of conjugated polymers with high performance in the research fi eld of PSCs.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements : We are grateful to the National Science Foundation of China (no. 21002075, 21034006) and the Shenzhen Research Foundation Project (JC201005260119A) for fi nancial support.
Received: September 18, 2012 ; Revised: October 23, 2012; Published online: December 11, 2012; DOI: 10.1002/marc.201200623
Keywords: benzene-fused quinoxaline; low-bandgap; polymer solar cells
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