Thiazole-Induced Quinoid Polymers for Efficient Solar Cells: Influence

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Thiazole-Induced Quinoid Polymers for Efficient Solar Cells: Influence of Molecular Skeleton, Regioselectivity and Regioregularity Dangqiang Zhu, Qian Wang, Yingying Wang, Xichang Bao, Meng Qiu, Bilal Shahid, Yonghai Li, and Renqiang Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01250 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Chemistry of Materials

Thiazole-Induced Quinoid Polymers for Efficient Solar Cells: Influence of Molecular Skeleton, Regioselectivity and Regioregularity Dangqiang Zhu†, Qian Wang†, Yingying Wang, Xichang Bao, Meng Qiu, Bilal Shahid, Yonghai Li, and Renqiang Yang* CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China ABSTRACT: Fluorination strategy has been regarded as a promising approach to improve the photovoltaic performance in polymer solar cells. However, the synthesis is relatively tedious and costly for most fluorinated momomers. In this work, differently from that fluorine atoms usually incorporated on the side chain, we successfully developed a thiazoleinduced strategy to construct efficient photovoltaic materials via inserting one thiazole unit into the backbone of nonfluorinated quinoid polymer, which would enhance the intermolecular interactions and decrease the ionization potential (IP) of the resulted polymers, benefiting from the desirable molecular skeleton. And then, considering the asymmetry nature of acceptor segments, the influence of molecular regioselectivity with different orientations of thiazole unit on optoelectronic properties was systematically investigated. Encouragingly, a superior power conversion efficiency (PCE) of 9.36% for PBTzT-4-based photovoltaic device was obtained, higher than that of the isomer polymer PBTzT-6 (PCE=8.52%) and a signficant increase of 50% compared to the widely reported analog polymer PBDTTT-E-T (PCE=6.21%) just without thiazole unit, which can be ascribed to more planar molecular conformation, stronger crystallinity and excellent phase separation. More interestingly, compared with random polymers, the regioregular copolymers exhibit enhanced red-shifted absorption, better crystallinity and compatiblity with PC71BM, leading to more desirable efficicency for PBTzT-4R-based devices (PCE=9.63%) with higher JSC of 17.56 mA/cm2, which can be comparable to the typical polymer PTB7-Th. This work not only provides a new strategy to improve the intermolecular interaction through backbone design, but also reveals that the orientations of asymmetric unit (that is regioselectivity and regioregularity) along the polymer backbone play a crucial role and should be taken into account in future molecule design.

o

Introduction Conjugated polymers have attracted considerable attentions in organic electronics due to their distinct characteristics of the electronic properties, structure diversity, light weight, and large-area fabrication of flexible devices via ease of solution processing. Nowadays, power conversion efficiency (PCE) of over 11% in polymer/fullerene solar cells (PSCs) has been realized with continuous efforts in design of new building blocks, backbone modification, side-chain engineering 1, 2 and device optimization. Hereinto, molecular skeleton, such as configuration, conformation, and planarity, has been put forward to regulate the energy levels, intermolecular interactions and carrier mobility, as well as the photovoltaic 3-9 performance. Andersson tailored the polymer backbone conformation from zigzagged to linear type by the change of π-conjugated spacer from thiophene to thieno[3,2b]thiophene, which exhibited stronger structural rigidity and 10 planarity. Consequently, the corresponding polymer presented highly ordered intermolecular stacking, leading to superior hole mobility and photovoltaic performance. Similarly, the widely reported polymer PBDTTT system based thieno[3,4-b]thiophene (TT) and benzo[1,2-b:4,5b′]dithiophene (BDT) also presents a zigzagged backbone o with the big included angle (36 ), likely due to the linear BDT and one five-membered thiophene (a twisted angle of

150 between chemical bond 2 and 5). Hou introduced two thiophene units as π bridges to obtain the polymers with a 4,11 more straight conformation. The aforementioned examples indicate that it could be an efficient strategy to improve molecular packing and photovoltaic performance through rational backbone modulation. However, the electrondonating thiophene or thieno[3,2-b]thiophene unit could make the ionization potential (IP) upshift, which is unfavorable to the open circuit voltage (VOC). By contrast, thiazole can significantly decrease the frontier orbital energy levels and reduce the steric hindrance with the neighboring units, 12-14 in favor of enhancing backbone planarity. Moreover, con sidering the asymmetry nature of TT segment, molecular regioselectivity and regioregularity should also be the critical 15 factors for the molecular design. For example, Bazan found that regioselectivity significantly influenced the intermolecular contacts, charge transport, and the photovoltaic performance via the investigation of isomeric polymers based another asymmetric mono-fluorination 2,1,3-benzothiadiazole 16 (BT). Meanwhile, some works also reveal that the regioregular copolymers usually exhibit enhanced red-shifted absorption, better crystallinity and higher

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Figure 1. Molecular design diagram and chemical structures of four polymers synthesized in this work. photovoltaic performance in comparison with random poly17-20 mers. Here, we propose a new strategy to reduce the twisted backbone skeleton through the introduction of five-membered heterocycle thiazole into the backbone of PBDTTT system, and four polymers, i.e., PBTzT-4, PBTzT-6, PBTzT-4R, and PBTzT-6R (R represents regioregularity) were designed and synthesized, to systemically investigate the influence of molecular skeleton, orientation of TT and regioregularity (Figure 1). Encouragingly, just as expected, the introduction of thiazole significantly enhances the intermolecular interactions with smaller π–π stacking distance (3.65 Å), as well as decreases IP value, leading to an obvious increase of VOC (~0.1 V). As a result, the polymer PBTzT-4 exhibits more planar molecular conformation, stronger crystallinity and higher hole mobility in comparison with those of PBTzT-6. Combined with excellent phase separation with PC71BM, a superior photovoltaic performance (PCE=9.36%) was obtained in the corresponding devices with a short-circuit 2 current density (JSC) of 16.92 mA/cm , higher than that of PBTzT-6 (PCE=8.52%) and a signficant increase of 50% compared to PBDTTT-E-T (PCE=6.21%) just without thiazole unit. In addition, compared with the random polymers, the regioregular copolymers exhibit red-shifted absorption, better crystallinity and hole mobility. Finally, PBTzT-4R-based devices gave more desirable efficiency of 9.63% with a 2 remarkable JSC of 17.56 mA/cm , which can be comparable to the classical polymer PTB7-Th, and among the highest values for the TT-based solar cell. In general, this work not only provides a method to improve the crystallinity and π–π stacking through rational molecular design, but also reveals the crucial role of the orientations of asymmetric unit along the polymer backbone. More importantly, different from most high-performance photovoltaic materials by introducing fluorine atoms on the side chain, this new strategy employing thiazole to construct highly efficient light-harvesting polymers would be more suitable for commercialization in terms of cost in future. Results and Discussion The synthetic routes of monomers are shown in Scheme 1. The compounds 1 and 3 were synthesized according to the 21 previous literatures. To construct 4-position linked polymer, the key monomer M1 was obtained in two steps of Stille

coupling reaction and bromination by N-bromosuccinimide 1 (NBS) starting from compound 1. For the HNMR spectra of the isomer monomers, the chemical shift of =CH (3-position of TT) for M1 (4-position thiazole) was significantly shifted to 7.97 ppm from 7.60 ppm for M2 (6-position thiazole), which could be due to the inductive effect of N=C in thiazole. Meanwhile, from the ultraviolet-visible (UV-vis) spectra (Figure S1, Supporting Information), M1 shows a significant red-shift of 25 nm for the maximum absorption peak (λmax) in comparison with that of M2, which would be ascribed to the longer conjugated length for M1 (5 conjugated double bonds including the C=O unit). Furthermore, to realize the complete regioregularity, we first synthesized the compound 5 with mono-iodo and mono-bromo substituents, and then selectively coupled with BDT to give a chemical regioregular M3 due to the higher activity of I atom in coupling reaction. The M2 and M4 were obtained through the similar synthetic routes. For the polymerizations, Stille coupling reaction and Soxhlet extraction purification were adopted to yield the target polymers. These polymers exhibited similar numberaverage molecular weights (Mn) ranging from 50.0 to 60.5 kDa with polydispersity index (PDI) < 2.6 (Figure S2, Supporting Information), implying the polymerization shows negligible influece on the molecular weight. Moreover, the resulted polymers can be dissolved very slowly in chlorobenzene (CB) and o-dichlorobenzene (o-DCB) at room temperao ture, however, once heated to 50-60 C, they can well dissolve in the CB solution in several minutes, which is suitable for the solution-processed device fabrication. In addition, there is almost no obvious difference in the solubility for these four polymers even at the concentration of 10 mg/mL. To explore the influence of the introduction of thiazole unit on molecular skeleton, grazing-incidence wide angle X-ray scattering (GIWAXS) was employed to probe the molecular packing characteristics of PBTzT-4 and PBTzT-6 in neat 22 film. The polymer PBDTTT-E-T only exhibited a weak re-1 flection at 0.274 Å , corresponding to a lamellar d-spacing of 23 22.9 Å. By contrast, the both new polymer films showed clear (100) reflection in the in-plane direction and (010) diffraction signal in out-of-plane direction, which indicated a favorable face-on orientation (Figure 2). Since the d100spacing was mainly determined by the long alkyl side chains, -1 the difference of (100) peaks (0.263 Å ) for both polymer films was inconspicuous. Additionally, the strong (010) π-π


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Scheme 1. Synthetic routes of the monomers and polymers. Reagents and conditions: (i) 2-(tributylstannyl)thiazole, Pd(PPh3)4, o o PhMe, 110 C, 12 h; (ii) NBS, CHCl3, rt, 30 min; (iii) NIS, CHCl3, rt, 1 h; (iv) 2D-BDT-Sn, Pd(PPh3)4, PhMe, 100 C, 24h; (v) o Pd(PPh3)4, PhMe/DMF, 110 C, 15h. fied the feasibility of the strategy that the introducing thiazole as π bridge can decrease the energy level. Interestingly, some great differences were observed in optical absorption, crystalline, and hole mobility of the two polymers. From the ultraviolet-visible (UV-vis) spectra in Figure 3, Table 1, and Figure S5, both polymers PBTzT-4 and PBTzT-6 cover the broad absorption ranging from 300 to 750 nm and show similar λonset at ~750 nm as films, slightly red-shifted by 10-15 nm in comparison with those in solution. Notably, the maximum absorption peak of PBTzT-4 has a tiny bathochromic-shift of ~10 nm compared to that of PBTzT-6, and the two polymers exhibit significantly different absorption profiles in solution and as film. In details, PBTzT-4 shows a defined vibronic structure with absorption peak at ~680 nm even in dilute -5 solution (10 M), which is absent in the case of PBTzT-6, implying there should have strong inter-molecular aggregation in solution. Meanwhile, the

Figure 2. GIWAXS profiles of PBTzT-4 and PBTzT-6 in neat films. -1

stacking peaks at about 1.72 Å were observed, corresponding to a π−π stacking distance of 3.65 Å, obviously smaller than 24 that of the PBDTTT system, such as PTB7-Th (3.85 Å) , 5 19 PBDTTT-S-T (3.88 Å) , PBDTTT-C-T (3.93 Å) . The aforementioned results suggest the formation of more ordered structures and more compact π-π stacking when introducing additional five-membered heterocycle into the backbone, which confirms our initial expectation to modulate molecular skeleton by this strategy. The influence of regioselectivity on physiochemical properties was further investigated. Owing to the polymer backbone composed of the same building blocks, both polymers show excellent thermal stability with the similar decomposition temperatures (Td, 5% weight loss) at 332 °C and 323 °C for PBTzT-4, and PBTzT-6, respectively (Figure S3), as well as the almost same redox peaks according to the cyclic voltammetry (CV) measurement (Table 1 and Figure S4). The IP values for PBTzT-4 and PBTzT-6 from the CV curves are estimated to be -5.35 eV and -5.31 eV, respectively, much lower than that of PBDTTT-E-T (-5.22 eV), which veri-

Figure 3. UV-vis absorption spectra of four polymers in solution (a), intensity ratio values of λ0-0 and λ0-1 of the polymers in solution at different temperature (b).



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Table 1. The optical and electrochemical properties of the polymers Solution

εmax

λmax (nm)

(M cm )

PBTzT-4

334, 622, 672

5.4×10 @672 nm

PBTzT-6

321, 627, 662

4.8×10 @627 nm

PBTzT-4R

336, 625, 674

5.6×10 @674 nm

PBTzT-6R

316, 631, 673

5.1×10 @631 nm

Polymers

a

opt

Film

Film

Eg

λmax (nm)

λonset (nm)

(eV)

4

340, 624, 681

751

4

330, 628, 671

4

4

-1

IP

µh

(eV)

(cm V S )

1.65

-5.35

1.09×10

747

1.66

-5.31

8.42×10

341, 630, 689

759

1.63

-5.35

1.99×10

328, 633, 683

757

1.64

-5.32

1.26×10

-1

a

2 -1 -1

-3

-4

-3

-3

opt

Estimated from the onset wavelength of the optical absorption: Eg = 1240/λonset.

photoluminescence (PL) spectra of the polymer/PC71BM blend films were carried out to investigate the charge transfer. It can be observed from Figure S5 (b) that the high PL intensity for the neat films can be completely quenched when blended with PC71BM, implying the efficient charge transfer between polymer donor and electron acceptor. To further verify this spectra of the polymers in dilute solutions are recorded to estimate the aggregation degree as 25 shown in Figure S6. One can observe that the defined vibronic structures are gradually weakened, accompaning with a blue-shift of several nanometers for the main absorption at ~625 nm, and the intensity ratio of I0-0/I0-1 between λ0-0 and λ0-1 almost linearly decreased as the temperature increased (Table S1), implying the formation of isolated polymer chain when continuously heated (Figure 3b). More importantly, the I0-0/I0-1 values for PBTzT-4 are greatly higher than those of PBTzT-6 at the same temperature, indicating it shows stronger crystalline. Combined stronger and sharper diffraction signal observed in the GIWAXS image of PBTzT-4 film (Figure 2), it can be concluded that PBTzT-4 possesses more ordered structure and higher crystalline compared to PBTzT-6. Accordingly, PBTzT-4 neat film exhibits higher hole mobility than that of PBTzT-6 film (Table 1). This phenomenon may be explained by the backbone planarity for the two polymers. As we know, the steric hindrance caused by C-H bond (3 position of TT) will yield a bigger dihedral angle at the connection 4 position of TT and 26， 27 BDT moiety, leading to a twisted configuration. Considering the thiazole unit can efficiently decrease the dihedral angle, it could make the polymer PBTzT-4 present a more planar struture. The “planar” polymer backbone will yield strong driving force to facilitate intermolecular stacking, which would be the main reason for different behaviors in UV-vis absorption spectra and GIWAXS images. Furthermore,compared to the random polymers, the corresponding regioregular polymers show red-shifted absorption peak of 8-10 nm and slightly enhanced molar extinction coefficient, which could be in favor of sunlight utilization in PSCs. On the other hand, the obviously enhanced I0-0/I0-1 values (Figure 3b), as well as the stronger diffraction signal presented in the GIWAXS images (Figure S7), also confirms that the regioregu lar polymers possess stronger inter-molecular interaction and higher ordered structures than those of the

random polymer. As a result, higher hole mobilities of the regioregular polymers are obtained (Table 1, Figure S8), which can be in favor of charge transport in PSCs.

Figure 4. The J–V curves of devices based on polymer: PC71BM (a) and EQE curves at the optimal conditions (b). The bulk heterojunction PSCs were fabricated with the structure of ITO/PEDOT:PSS/polymer:PC71BM/PFN-Br/Al. The photovoltaic performance was optimized by various conditions (different D/A weight ratios and additive amount) as shown in Figure 4, Table 2, and Table S2. The current density-voltage (J–V) characteristics and photovoltaic parameters under the optimized conditions are shown in Figure 4a and Table 2, and the average efficiencies are obtained from over 10 separated devices. All the devices exhibit desirable VOC of ~0.78 V, obviously higher than that


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Chemistry of Materials Table 2. Photovoltaic parameters of PSCs based on polymer/PC71BM VOC

Polymer

2

PCE(%)

a

µh 2 -1 -1

(V)

(mA/cm )

(%)

max.

ave.

(cm V S )

0.68

14.59

62.6

6.21

---

---

PTB7-Th

0.78

16.76

64.5

8.43

---

---

PBTzT-4

0.78(0.77±0.01)

16.92(16.48±0.53)

70.9(69.7±1.6)

9.36

9.19

2.08×10

PBTzT-6

0.77(0.77±0.01)

15.72(15.45±0.47)

70.4(69.1±1.7)

8.52

8.28

1.16×10

PBTzT-4R

0.77(0.77±0.01)

17.56(17.31±0.40)

71.1(70.1±1.5)

9.63

9.48

4.25×10

PBTzT-6R

0.77(0.77±0.01)

16.84(16.38±0.56)

70.3(69.2±1.8)

9.12

9.01

2.44×10

PBDTTT-E-T 12

a

FF

JSC

30

-4

-4 -4 -4

Data are obtained from 10 separate devices.

of PBDTTT-E-T (0.68 V), which can be comparable to that of the fluorinated TT-based polymer PTB7-Th. The results also confirm that the introduction of thiazole as π bridge is in favor of higher VOC, which is as efficient as the fluorine strategy. Without any additive, the solar cells exhibit very poor performance due to the unfavorable phase separation which will be discussed later. When DIO added, the JSC and FF are simultaneously increased, which result in the PCEs significantly enhanced. Noticeably, the photovoltaic devices exhibit high FF of >70%, which are superior to most analogue polymers (Table S3). In details, the best device based on PBTzT-4 exhibits a maximum PCE of 9.36%, with a JSC of 2 16.92 mA/cm , an increase of 50% in comparison with the widely reported polymer PBDTTT-E-T (PCE=6.21%). In contrast, the isomeric polymer PBTzT-6-based solar cell exhibits a slightly lower PCE of 8.52% and a decrease of JSC (15.72 2 mA/cm ), which may be ascribed to the slightly blue-shifted absorption and a little low hole mobility. The significant improvements verify that the thiazole-induced strategy in the main chain is effective. However, the regioregularity does not have great influence on its molecular energy levels, leading to almost same VOC of ~0.77 V. On the contrary, the JSC values are obviously increased due to the red-shifted absorption, enhanced crystallinity and higher hole mobility. Consequently, a superior efficiency of 9.63% is obtained for PBTzT2 4R with a desirable JSC value of 17.56 mA/cm , which is one of the highest values among the TT-based solar cells, and obviously higher than that of non-fluorinated polymers (generall PCE4 nm without any additives from AFM images. Meanwhile, some PC71BM show strong aggregations from TEM images, especially for PBTzT-6/PC71BM film with a domain size of over 200 nm, which is very unfavorable for exciton separation. Surprisingly, when solvent additive DIO is added, the morphologies of all blend films demonstrate a dramatic change to relatively desirable nano-phase separation, which is in favor of charge transfer, leading to the



Figure 6. AFM and TEM images of polymer:PC71BM blend films. (a, b) for PBTzT-4, (c, d) for PBTzT-6, (e, f) for PBTzT-4R, (g, h) for PBTzT-6R. a-1 to h-1 for AFM height image, a-2 to h-2 for AFM phase image, a-3 to h-3 for TEM image. (a, c, e, g) without DIO, (b, d, f, h) with DIO. The size is 4×4 µm for AFM images. significant increase of JSC and FF in the photovoltaic devices (Table S2). Meanwhile, the PBTzT-4/PC71BM film also exhibits more uniform bicontinuous interpenetrating network compared to PBTzT-6/PC71BM film (which shows a little nutty structures from AFM and TEM amplified images shown in Figure S11 and Figure S12). Finally, compared to random polymers, the regioregular polymers exhibit more desirable fibril-like nano-phase separation, especially for PBTzT4R/PC71BM film, which would be one important reason why the PBTzT-4R polymer enables high photovoltaic performance. The significant difference in morphology is closely related to the backbone planarity and regioregularity. Conclusion In this work, thiazole unit was introduced into the backbone of quinoid polymer as π bridge to modulate the molecular skeleton. As a result, the intermolecular interactions of the resulted polymers were enhanced and their ionization poten tials were decreased. It can be seen that the thiazole unit in the main chain functions exactly as the fluorine atoms, which usually is incorporated on the side chain. Interestingly, the orientation of thiazole unit and regioregularity of the polymers show significant influence on the physiochemical and photovoltaic performance. The device based on PBTzT-4R gave a superior PCE of 9.63% with a remarkable JSC of 17.56 2 mA/cm , which can be ascribed to more planar molecular conformation, red-shifted absorption, stronger crystallinity and excellent phase separation compared to the random analog polymer PBTzT-4 and its isomer PBTzT-6R. This work not only provides a promising and simple strategy to regulate the energy levels, the crystallinity and π–π stacking through employing thiazole-induced effect, which functions as effi ciently as the widely recognized F strategy, but also reveals

that the orientations of asymmetric unit along the polymer backbone play a crucial role and should be taken into account in future molecular design. ASSOCIATED CONTENT Supporting Information. The synthesis procedure and characterization of materials, fabrication of PSC devices, TGA curves, temperaturedependent UV-vis absorption spectra, electrochemical properties, PL spectra, hole mobility of the polymers, the photovoltaic performance of the polymer under different fabrication conditions, AFM and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (R. Y.)

Author Contributions All authors have given approval to the final version of the manuscript. †D. Zhu and Q. Wang contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are deeply grateful to the National Natural Science Foundation of China (21604092, 51573205 and 51773220),


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Chemistry of Materials the Ministry of Science and Technology of China (2014CB643501), China Postdoctoral Science Foundation (2017M610453), and the Youth Innovation Promotion Association CAS (2016194). The authors thank beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing beam time.

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Thiazole-Induced Quinoid Polymers for Efficient Solar Cells: Influence

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