WO2021142656A1

WO2021142656A1 - Benzo[1,2-b:4,5-c']dithiophene-4,8-dione-based polymer solar cells

Info

Publication number: WO2021142656A1
Application number: PCT/CN2020/072218
Authority: WO
Inventors: Feng He; Pengjie CHAO; Hui Chen
Original assignee: Southern University Of Science And Technology.
Priority date: 2020-01-15
Filing date: 2020-01-15
Publication date: 2021-07-22

Abstract

Through the strategy of cyclohexane-1, 4-dione embedded into thieno [3, 4-b] thiophene unit, a highly electron-deficient core (TTDO) is synthesized and the corresponding donor polymer (PBTT-F) is also been developed. The nonfullerene photovoltaic device based on this new donor polymer exhibited an outstanding PCE of 16.1%with a very high fill factor of 77.1%, which demonstrate it a very promising donor for high performance solar cells.

Description

Benzo [1, 2-b: 4, 5-c′] dithiophene-4, 8-dione-based Polymer Solar Cells

TECHNICAL FIELD

The invention relates to polymer solar cell field. More particularly, a Benzo [1, 2-b: 4, 5-c'] dithiophene-4, 8-dione-based Polymer Donor, and a preparing method thereof.

BACKGROUND ART

Solution-processed polymer solar cells (PSCs) have become a focus of attention in the academic and industrial fields due to the great advantages of light-weight, potential low cost, flexibility, semitransparency and the large-area fabricated devices. The most common PSCs are fabricated using a bulk heterojunction (BHJ) architecture, where the crucial active layer is comprised of a blend of a donor material and an acceptor material. In the past few decades, fullerene derivatives have been the most commonly used electron acceptor materials. Nevertheless, the development of fullerene-based solar cells is seriously hindered due to the intrinsic disadvantages of fullerenes themselves, such as their weak absorbance, thermal instability and difficulties in structural tunability.

Encouragingly, with the dawn of fused-ring electron acceptors (FREAs) with an acceptor–donor–acceptor (A–D–A) molecular architecture, such as 3, 9-bis (2-methylene- (3- (1, 1-dicyanomethylene) indanone) -5, 5, 11, 11-tetrakis (4-exylphenyl) dithieno [2, 3-: 2′, 3′-d′] -s-indaceno [1, 2-b: 5, 6-b′] -dithiophene (ITIC) and its derivatives, a breakthrough in the performance of OSCs has made it possible to use nonfullerene acceptors (NFAs) as alternatives to the fullerene counterparts. In the past two years, the rapid progress for narrow bandgap NFAs has contributed to improve the performance of OSCs because of their convenient function-tunability and very strong absorbance in the near-infrared spectral region. Very recently, the power conversion efficiency (PCE) of organic photovoltaic (OPV) devices with FREAs Y6 (structure see below) has reached 15.7%for single-junction cells. Such marvelous development of the PCE is mainly attributed to rational structural design and successful synthesis of novel narrow bandgap nonfullerene acceptor materials, and further progress of device engineering.

As one of the two key components for a BHJ blend, the donor-acceptor (D-A) type donor materials also play a vital role in elevating the PCE of nonfullerene polymer solar cells. Accordingly, the large number of donor polymers originally developed for fullerenes provides a rich choice of materials for direct use in nonfullerene polymer solar cells; meanwhile, great efforts and attempts have been devoted to develop new donor polymers matched with NFAs. Although some polymer donors have been successfully used in nonfullerene OSCs, such as typical PTB7-Th, PBDB-T, PBDT-TF (PM6) and J51 materials, the development of donor materials has still lagged far behind that of electron-acceptor materials. Therefore, the further molecular design of D-Atype donor polymers should be one of the most important topics in the field of polymer solar cells.

It is well known that the thieno [3, 4-b] thiophene (TT) is a very important functional unit that has been extensively adopted in the PTB family donor polymers due to its stabilized quinoidal form. Many previous reports have demonstrated that the existence of the TT unit can broaden the absorption band and extend the polymer absorption range to longer wavelength. A representative donor polymer PTB7 containing a TT unit was reported by Yu group in 2010, and a device based on PTB7: PC ₇₁BM exhibited an excellent performance with a record-breaking PCE of 7.4%, which greatly promoted the development of OPVs. Following the success of PTB7, a two-dimensional (2D) conjugated polymer PTB7-Th was developed in the same year; subsequently, the PCE for a PTB7-Th-based device was boosted to 9.35%by Chen and coworkers in 2013. At present, a nonfullerene device with PTB7-Th: IEICO-4F shows an efficiency of 13.2%, and the corresponding flexible nonfullerene device exhibits a high performance of 12.5%, which demonstrates that the TT a very promising donor construction unit. Besides, the π-bridge also plays a quite significant role in modulating planarity of polymer backbone, and thiophene as π-bridge in D-Aconjugated polymer could release the steric torsion of polymer backbone due to extending the distance between D and A cores. Meantime, the polymer containing thiophene π-bridge can obtain more planar polymer backbone due to forming strong quinoidal resonance, leading to enhance the intermolecular interaction.

SUMMARY OF THE INVENTION

It is of great significance to develop efficient donor polymer during the rapid development of acceptor materials for nonfullerene bulk heterojunction (BHJ) polymer solar cells. Herein, a new donor polymer, named PBTT-F, based on a strong electron-deficient core (TTDO) , was developed through the design of cyclohexane-1, 4-dione embedded into a thieno [3, 4-b] thiophene (TT) unit. When blended with the acceptor Y6, the PBTT-F-based photovoltaic device exhibited an outstanding power conversion efficiency (PCE) of 16.1%with a very high fill factor (FF) of 77.1%. This polymer also showed high efficiency for a thick film device, with a PCE of approximately 14.2%realized for an active layer thickness of 190 nm. In addition, the PBTT-F-based polymer solar cells also showed good stability after storage for approximately 700 h in a glove box, with a high PCE of approximately 14.8%, which obviously shows that this kind of polymer is very promising for future commercial applications. This work provides a unique strategy for the molecular synthesis of a donor polymer, and these results demonstrate that PBTT-F is a very promising donor polymer for use in polymer solar cells, and provide an alternative choice for a variety of fullerene-free acceptor materials for the research community.

The present invention provides a PBTT-F donor polymer having the following structure:

wherein Ar is selected from monocyclic, bicyclic and polycyclic arylene, or monocyclic, bicyclic and polycyclic heteroarylene, or may contain one to five such groups, either fused or linked;

wherein

wherein R is independently selected from alkyl groups with up to 18 C atoms, aryls and substituted aryls, and wherein n is an integer greater than 1.

Preferably, Ar is

More preferably, the PBTT-F donor polymer has the following structure:

The present invention further provides a composition, comprising the above PBTT-F donor polymer blended with the acceptor Y6.

The present invention further provides a photovoltaic device based on the above PBTT-F donor polymer or the composition.

The present invention further provides a thick film device based on the above PBTT-F donor polymer or the composition.

The present invention further provides a polymer solar cells based on the above PBTT-F donor polymer or the composition.

The present invention further provides a method for preparing the above PBTT-F donor polymer as following:

wherein compound (3) is prepared as following:

That is, a TO acceptor unit based on a strong electron-deficient TTDO core is prepared through the rational innovation of TT unit, and then a new donor polymer based on TO, called PBTT-F, was developed for building polymer solar cells. PBTT-F has a bandgap of 1.80 eV and the PBTT-F-based inverted devices achieved an outstanding photovoltaic performance of 16.1%in efficiency with a high FF of 77.1%when matched with a commercially available NFA, Y6. Furthermore, as the thickness of the active layer was increased from 80 to 190 nm, the PBTT-F: Y6-based devices maintained an excellent PCE of over 14.2%, which was attributed to a well matched absorption and balanced charge carrier mobilities in the blend active layer. Moreover, after storage in a glove box for over 696 h, the PBTT-F-based devices exhibited very good stability with a PCE of approximately 14.8%for future commercial application. This work proposes a facile strategy for molecular design, and these results demonstrate that PBTT-F is a potential donor polymer for use in polymer solar cells, providing an alternative polymer donor and also additional option to match with diverse nonfullerene acceptors.

DESCRIPTION OF FIGURES

Figure 1 shows (a) TGA and (b) DSC curves of PBTO.

Figure 2 shows molecular structure, optical and electrochemical properties. (a) Chemical structure and calculated UV-Vis absorption spectra for the BDTO and TTDO monomers. Note: EH=2-ethylhexyl. (b) UV-Vis absorption spectra for BDTO and TTDO in diluted solution. (c) Crystal structure of the TTDO and a perpendicular view of the plane. (d) Crystal structure of the BDTO and a perpendicular view of the plane. (e) Packing diagram for TTDO. (f) Packing diagram for BDTO. Note: H atoms and n-butyl chains are omitted for clarity.

Figure 3 shows optimized molecular conformations of TTDO and BDTO using DFT calculations at the B3LYP/6-31g* (d, p) level: the HOMO and LUMO electron distributions.

Figure 4 shows (a) normalized UV-Vis absorption spectra of PBTT-F, PM6 and Y6 in film; (b) energy level diagrams of PBTT-F, PM6 and Y6; (c) optimized molecular backbone conformation of the polymers determined using DFT calculations at the B3LYP/6-31g* (d, p) level: the HOMO and LUMO electron distributions for PBTT-F.

Figure 5 shows (a) normalized UV-Vis absorption spectra of PBTT-F, PM6 and Y6 in chloroform solution.

Figure 6 shows absorbance coefficient spectra of polymer PBTT-F and PM6 film.

Figure 7 shows temperature-dependent absorption spectra of the solutions of PBTT-F in CB solution (10 ^-5 M) as the temperature increased from 20 to 110 ℃.

Figure 8 shows cyclic voltammogram curves of the polymer PBTT-F and PM6 film.

Figure 9 shows optimized molecular backbone conformations of polymers using DFT calculations at the B3LYP/6-31g* (d, p) level: the HOMO and LUMO electron distributions for PM6.

Figure 10 shows (a) The J-V curves of PSCs based on PBTT-F: Y6 (1: 1.1, wt/wt) under the illumination of AM 1.5G at 100 mW cm ^-2. (b) EQE curves based on corresponding PSCs. (c) Short-circuit current density (J _sc) versus light intensity plots of the PBTT-F-based device. (d) PL spectra of the neat Y6 film and blend film. (e) Normalized transient photovoltage (TPV) and transient photocurrent (TPC) of PSCs device. (f) The stability of the PCE determined for unencapsulated devices stored in a glove box.

Figure 11 shows 2D GIWAXS patterns of (a) pristine PBTT-F, (b) neat Y6, (c) PBTT-F: Y6 blend films; (d) GIWAXS line-cut profiles along the out-of-plane (solid line) directions; (e) AFM height image (10 μm × 10 μm) of the optimized PBTT-F: Y6 blend film with thermal annealing at 110 ℃; (f) TEM image of the optimized PBTT-F: Y6 blend film treated by thermal annealing at 110 ℃(scale bar = 1 μm) .

Figure 12 shows ¹H NMR spectrum of compound (1) .

Figure 13 shows ¹³C NMR spectrum of compound (1) .

Figure 14 shows ¹H NMR spectrum of compound (2) .

Figure 15 shows ¹H NMR spectrum of compound (3) .

Figure 16 shows ¹³C NMR spectrum of compound (3) .

Figure 17 shows ¹H NMR spectrum of compound (4) .

Figure 18 shows ¹³C NMR spectrum of compound (4) .

Figure 19 shows ¹H NMR spectrum of compound (5) .

Figure 20 shows ¹³C NMR spectrum of compound (5) .

Figure 21 shows ¹H NMR spectrum of compound (6) .

Figure 22 shows ¹³C NMR spectrum of compound (6) .

Figure 23 shows ¹H NMR spectrum of polymer PBTT-F.

SPECIFIC EMBODIMENTS

Herein, we have embedded cyclohexane-1, 4-dione into the TT unit through the Friedel-Crafts reaction and obtained an fused core 5, 7-dibromo-2, 3-bis (2-ethylhexyl) benzo [1, 2-b: 4, 5-c'] dithiophene-4, 8-dione (TTDO) (Scheme 1a) , which further enhanced the electron deficiency of the TT unit. Although the similar core was used to construct D-Atype polymer, the corresponding donor polymers were much less reported than TT-based polymer donors, and also exhibited relative low performance with efficiency below 6%. In addition, the synthesis of TTDO core was quite facile with high yield according to this strategy, which is more feasible compared to the tedious and low yield of TT cyclization. In comparison with TT unit, TTDO core exhibits stronger electron-withdrawing ability, leading to enhance the intramolecular charge transfer in D-Aconjugated polymer. Thereafter, the acceptor unit based on the TTDO core, named 5, 7-bis (5-bromothiophen-2-yl) -2, 3-bis (2-ethylhexyl) benzo [1, 2-b: 4, 5-c'] dithiophene-4, 8-dione (TO) (Scheme 1 (b) ) , was synthesized through the Stille coupling reaction with thiophene-based π-bridge. Subsequently, a new copolymer, PBTT-F, based on alternating TO and BDT-2F units (structure see below) was obtained as donor material for photovoltaic application, as shown in Scheme 1 (b) . The strong electron-withdrawing property of the central TTDO core is beneficial to broaden absorption and enhance intermolecular interaction. PBTT-F showed good optical absorption and suitable energy level, which is well matched with the highly efficient acceptor material Y6. Finally, the optimized photovoltaic devices achieved an outstanding PCE of 16.1%with a high fill factor (FF) of 77.1%. These remarkable results demonstrated that the PBTT-F copolymer based on the TTDO core is a promising donor material for use in high performance PSCs.

Scheme 1. (a) molecular structure of TT, TTDO and PBTT-F, and (b) synthetic routes to the monomer and the PBTT-F polymer.

The synthetic routes for monomers and the target donor polymer PBTT-F are illustrated in Scheme 1. TTDO can be easily synthesized with high yield compared with TT, as fully exhibited in Scheme 1 and Scheme 2; and the corresponding synthetic procedures and structure characterization are provided in the supporting information. According to the previous route (route a) in Scheme 2, it was extremely difficult to separate compound (3) . Therefore, we designed a new route (route b) , and compound (3) can be easily obtained with a high total yield of 81.5%. All the molecules were characterized by ¹H NMR, ¹³C NMR and MALDI-TOF-MS. PBTT-F was synthesized through the Stille-coupling reaction, which exhibits good solubility in common solvents, such as chloroform and chlorobenzene. Determined by high temperature GPC, the number-average molecular weight (M _n) and polydispersity index (PDI) of PBTT-F was 32.8 kDa and 2.08. PBTT-F exhibited a good thermal stability and corresponding to a decomposition temperature (T _d, 5%weight loss) of 352 ℃, as shown in Figure 1a. The endo-and exothermal peaks cannot be observed from the differential scanning calorimetry (DSC) curve from temperature of 50 ℃ up to 300 ℃ (Figure 1b) , indicating the rigid backbone which limits the motion of polymer chain.

Scheme 2. Synthetic routes of compound (3)

The synthetic routes for monomers and the target donor polymer PBTT-F are illustrated in Scheme 1. TTDO can be easily synthesized with high yield compared with TT, as fully exhibited in Scheme 1 and Scheme S1; and the corresponding synthetic procedures and structure characterization are provided in the supporting information. According to the previous route (route a) in Scheme 2, it was extremely difficult to separate compound (3) . Therefore, we designed a new route (route b) , and compound (3) can be easily obtained with a high total yield of 81.5%. All the molecules were characterized by ¹H NMR, ¹³C NMR and MALDI-TOF-MS. PBTT-F was synthesized through the Stille-coupling reaction, which exhibits good solubility in common solvents, such as chloroform and chlorobenzene. Determined by high temperature GPC, the number-average molecular weight (M _n) and polydispersity index (PDI) of PBTT-F was 32.8 kDa and 2.08. PBTT-F exhibited a good thermal stability and corresponding to a decomposition temperature (T _d, 5%weight loss) of 352 ℃, as shown in Figure 1a. The endo-and exothermal peaks cannot be observed from the differential scanning calorimetry (DSC) curve from temperature of 50 ℃ up to 300 ℃ (Figure 1b) , indicating the rigid backbone which limits the motion of polymer chain.

To elucidate the effects of the TT unit on the properties of the TTDO core, its isomer, BDTO, was also synthesized and investigated (Scheme 3) . As shown in Figure 2a, the main peak of the calculated absorption spectrum of TTDO is redshifted by 45 nm from that of BDTO, and the two isomers exhibited almost the same molar absorption coefficients of 6.87×10 ³ and 6.75×10 ³ M ^-1 cm ^-1 for BDTO and TTDO, respectively. Figure 2b shows the absorption spectra for TTDO and BDTO in diluted chloroform solution, the main TTDO peak displays a distinct redshift by 35 nm compared with that of BDTO, which is consistent with the above result obtained from the simulation calculation. However, TTDO shows a higher molar absorption coefficient (8.47×10 ³ M ^-1 cm ^-1) than BDTO (7.27×10 ³ M ^-1 cm ^-1) . Moreover, as displayed in Figure 3, it should be noted that the molecular energy level of the highest occupied molecular orbital (HOMO) (-6.51 to -6.62 eV) and the lowest unoccupied molecular orbital (LUMO) (-2.29 to -2.83 eV) are downshifted from BDTO to TTDO. In addition, the molecular packing behavior of TTDO and BDTO was investigated through single-crystal X-ray diffraction analysis. As shown by the perpendicular view to plain in Figure 2c and 1d (here –EH was substituted by n-butyl group) , a clear molecular overlap equivalent to approximately half of the benzene ring area is observed for TTDO, but no overlap is observed for BDTO. Furthermore, as exhibited in Figure 2e, the π–π stacking distance for TTDO is approximately

with a slip angle of 47°. However, the adjacent BDTO molecules have a larger distance of

(Figure 2f) and a smaller slip angle of 43° (Figure 2f) compared with that of TTDO. This finding indicates that TTDO possesses an orderly molecular packing structure, and accounts for the redshift of the TTDO absorption.

Scheme 3. Synthetic routes of compound (9) and (11)

Figure 4a and Figure 5 exhibits the normalized absorption spectra for PBTT-F, PM6 and Y6 in diluted chloroform solution and as a neat film. The UV-Vis absorption spectrum of PBTT-F in solution displays two evident absorption bands in the range of 300-750 nm, which is the typical characteristic for D-Atype conjugated polymers. In addition, the maximum absorption peak of PBTT-F is located at about 580 nm in solution with a shoulder peak, implying the presence of molecular aggregation in solution. From solution to film, the maximum absorption is only redshifted to 582 nm with a strong should peak at 616 nm, indicating strong aggregation of polymer backbones and intermolecular π-π interactions in the solid film. In comparison with PBTT-F, as shown in Figure 5 and Figure 4a, PM6 exhibits a narrower band absorption and a distinctly blue-shifted absorption onset with the maximum absorption at 614 and 618 nm in both solution and film states. The absorption onset for PBTT-F and PM6 are 688 and 671 nm, so the corresponding optical bandgap

is 1.80 and 1.85 eV, respectively. As shown in Figure 6, polymer PBTT-F film exhibits a higher absorption coefficient of 4.78×10 ⁴ cm ^-1 in comparison with PM6 (3.97×10 ⁴ cm ^-1) , suggesting PBTT-F based film has stronger light-harvesting ability in the photovoltaic layer. The temperature-dependent absorbance was measured for increasing temperature from 20 to 110 ℃ to study the aggregation behavior of PBTT-F polymer according to our previous procedures. As displayed in Figure 7 the PBTT-F polymer exhibits a strong aggregation in solution, which benefits the formation of nanoscale phase separation and pure phase domains for nonfullerene PSCs. Cyclic voltammetry (CV) was carried out to measure the HOMO level of PBTT-F and PM6. As exhibited in Figure 8, the onset of the oxidation potential (Ф _ox) occurs at about 0.77 V versus Ag/Ag ⁺ according to the equation HOMO= -e (Ф _ox +4.71) , which corresponds to a HOMO level of about -5.48 eV (Figure 4b) for both PBTT-F and PM6, calculated from the optical bandgap and the HOMO level, the LUMO levels were determined to be -3.68 and -3.63 eV as depicted in Figure 4b, implying no almost obvious difference in V _oc for their devices. As shown in Figure 4c and 9, there is an obvious difference in the LUMO levels between PBTT-F and PM6. The LUMO is mainly located on the TTDO unit for PBTT-F polymer, however, the LUMO for PM6 is still partly distributed on the benzo [1, 2-b: 4, 5-b'] dithiophene (BDT) unit in addition to the BDTO unit. And the molecular dipole moment for PBTT-F and PM6 are 1.14 Debye and 0.63 Debye, respectively. Previous research has demonstrated that a larger dipole moment can decrease the Coulomb binding of excitons, contribute to charge separation in a donor: acceptor blend and improve the fill factor (FF) in PSCs. These results all indicate that the TTDO is a stronger electron-deficient group than BDTO, which would be beneficial for enhancing the intramolecular charge transfer (ICT) in a D-Atype polymer.

Herein, the PBTT-F polymer was used as a donor material in a BHJ PSC matched with the narrow bandgap NFAs Y6

due to its complementary absorption and suitable energy level. To investigate the photovoltaic performance of PBTT-F, inverted BHJ PSC devices with the configuration of indium tin oxide (ITO) /ZnO/PBTT-F: Y6/MoO ₃/Ag were fabricated. Chloroform was selected as the processing solvent, the devices were optimized with a blend donor/acceptor (D: A) weight ratio of 1: 1.1 (wt/wt) , 0.5 wt%1-chloronaphthalene (CN) was chosen as the additive, with thermal annealing at 110 ℃ for 10 min. As shown in Figure 10a, the photovoltaic performance based on PBTT-F: Y6 with a varying active thickness was evaluated. The PBTT-F: Y6 (1: 1.1, 0.5 wt%CN) device with a photovoltaic layer thickness of 80 nm achieved an impressive PCE of 15.1%with a V _oc of 0.83 V, FF of 75.8%, and a J _sc of 23.9 mA cm ^-2 after thermal annealing treatment at 110 ℃ for 10 min. Subsequently, the optimized PSCs exhibited an outstanding PCE of 16.1%with a slightly higher V _oc of 0.84 V, higher FF of 77.1%, and higher J _sc of 24.8 mA cm ^-2 when increasing the thickness of the active layer to 100 nm. In addition, a quite low value of E _loss of 0.49 eV was also achieved in such high-performance PSCs. the PBTT-F-based PSCs still showed a very high PCE of 14.6%and a notably enhanced J _sc of 25.0 mA cm ^-2 despite the slightly lower FF observed when raising the thickness to 150 nm. Upon further increasing the thickness of the active layer to 190 nm, a quite decent PCE of 14.2%was still achieved with a distinctly high J _sc of 25.6 mA cm ^-2 and FF of 67.6%. These results further demonstrated that the acceptor TO unit on the base of the TTDO core is a very excellent building unit in D-Adonor polymers, while also confirming that the donor polymer PBTT-F is a promising candidate for use in nonfullerene polymer solar cells. Figure 10b shows the external quantum efficiency (EQE) curves for the corresponding devices above. These PBTT-F: Y6-based devices all exhibited a broad response range, with a maximum EQE value of 70%-80%achieved in the wavelength range of 450 to 850 nm; the maximum EQE of 81%at 630 nm, implies efficient charge transfer with suppressed geminate recombination. The integral J _sc calculated from EQE curves agrees well with the observed J _sc in the J-V measurements, with a mismatch within 4%.

Table 1. Photovoltaic performance parameters of the PSCs based on PBTT-F: Y6 (1: 1.1, wt/wt) , under the illumination of AM 1.5G at 100 mW cm ^-2.

^aThe J _sc calculated from the EQE spectrum. ^bThe average PCE values were obtained from 15 individual cells.

To investigate the bimolecular recombination in the active layer, the dependence of J _sc on light intensity (P _light) was measured. As is well known, the exponent value (α) in the relation J _sc∝P _light ^αtends to be 1 when the bimolecular recombination is negligible. As shown in Figure 10c, the fitted slope for the optimized PBTT-F-based device was 0.99, which is extremely close to 1. This result demonstrates that almost no bimolecular recombination occurred in the PBTT-F: Y6 active layer, which is in line with the high FF and J _sc observed above. Moreover, the photoluminescence (PL) spectra were measured to study the photo-induced charge transfer in the donor: acceptor blend film. Figure 10d displays the PL spectra of the neat and blend films; the fluorescence of PBTT-F or Y6 can be completely quenched when adding them in the blend film, implying very efficient charge transfer between the PBTT-F and Y6.

Meantime, the hole mobility of the PBTT-F neat film was also measured by the SCLC method, and the PBTT-F neat film gave a hole mobility of 1.8×10 ^-4 cm ² V ^-1s ^-1. We also determined the transient photovoltage (TPV) and transient photocurrent (TPC) of PBTT-F-based polymer solar cells to further study the dynamic behavior of the corresponding device. As exhibited in Figure 10e, the value of the carrier lifetime (τ) and the sweeping out time (t _s) was determined to be is 1.22 μs and 0.81 μs, respectively; the suitable value of τ/t _s =1.50 was in good agreement with the observed high FF and J _sc. The ultimate goal of polymer solar cell development is wide application in our daily life, therefore, the stability of the PBTT-F-based device is also a significant evaluation factor. As exhibited in Figure 10f. Distinctly, V _oc shows almost no change, and is highly stable at 0.83 V during the lifetime test. Compared with the stability of V _oc, the J _sc and FF show a slight decline when stored in glove box for over 696 h, which could be attributed to a change in film morphology, degradation of the active layer and interlayer-electrode diffusion. As a result, the PCE remains stable at approximately 14.8%because of the little small changes in J _sc, V _oc and FF. The stability tests indicate that the donor polymer PBTT-F has great potential for commercial applications in the near future. Furthermore, the photo-ability of the PBTT-F: Y6-based device was also measured under light soaking condition.

The photovoltaic performance of the donor polymer tightly depends on the morphology of the blended active layer. To study the structure-morphology relationship, the morphology of pure PBTT-F and its blend film with Y6 was completely investigated through the grazing incidence wide-angle X-ray scattering (GIWAXS) , atomic force microscopy (AFM) , and transmission electron microscopy (TEM) . The 2D-GIWAXS images of the pristine PBTT-F polymer, neat Y6 and blend film are shown in Figure 11a-11c, and the relevant intensity profiles of the out-of-plane (OOP) direction are provided in Figure 11d. As exhibited in Figure 11d, the neat PBTT-F film presents an obvious π-π stacking (010) diffraction peak in the OOP direction at

implying a preferential face-on orientation and good crystallinity property of PBTT-F polymer. After blending the PBTT-F polymer with Y6, a strong π-π stacking peak appears at

in the OOP direction with a face-on orientation. The blend film exhibits a shorter π-π stacking distance compared to that of PBTT-F, indicating very strong intermolecular stacking between PBTT-F donor and Y6 acceptor, the tight π-π stacking can contribute to the observed high fill factor, charge mobility and J _sc. As exhibited in Figure 11e, after being thermal annealed at 110 ℃, the blend film of PBTT-F: Y6 displays a slightly increased roughness with a R _q of 4.94 nm, which is beneficial for exciton diffusion and dissociation in donor-acceptor interface. And the 3D AFM image indicates that the PBTT-F: Y6-based film with the thermal annealing presents a uniform height dimension distribution. The PBTT-F-based blend film was also characterized by TEM, as displayed in Figure 11f, distinctly, the PBTT-F: Y6 blend film exhibits a distinct phase separation with large domain sizes, while fibrillary interpenetrating networks surround the clustered region, demonstrating that the PBTT-F-based film forms a multiscale-length morphology. The TEM result is consistent with that of GIWAXS and AFM. The polymorphous behavior suggests that PBTT-F and Y6 mix and cluster well in solution, before generating a small gathering area in the solid. Meanwhile, the donor material and acceptor material blend well in each cluster, which is favor improved carrier splitting. Moreover, the relatively independent cluster regions also reduce the chance for recombination of electrons and holes, contributing to efficient transportation. These results indicate that this morphology is beneficial to improve the photovoltaic performance.

EXAMPLES

1. Experimental Section

Measurements: ¹H NMR and ¹³C NMR spectra were recorded on Bruker Avance-400/500 spectrometers. Mass spectra (high resolution mass spectrometer (HRMS) ) were determined on an Autoflex III matrix-assisted laser desorption ionization mass spectrometer (MALDI-TOF-MS) . Gel permeation chromatography (GPC) was performed on Agilent Technologies 1260 infinity II high temperature GPC system using 1, 2, 4-trichlorobenzene (TCB) as eluent at 150 ℃. Solution and thin film optical absorption spectra were measured with a UV-Vis spectrophotometer (Shimadzu, UV3600) . The thin films of the polymers were spin-coated from their solutions in chlorobenzene, and then the film absorption spectra were measured. The electrochemical cyclic voltammetry (CV) was carried out on a CHI 660E Electrochemicacl Workstation (Shanghai Chenhua Instrumental Co., Ltd. China) , with glassy carbon disk, Pt wire and Ag/Ag+ electrode as working electrode, counter electrode and reference electrode in an acetonitrile solution of 0.1 mol L ^-1 Tetrabutylammonium phosphorus hexafluoride (n-Bu ₄NPF ₆) at a potential scan rate of 100 mV s ^-1 under a argon atmosphere. Thermogravimetric analysis (TGA) plots were measured with a Discovery series instrument under a nitrogen atmosphere at heating and cooling rates of 10 ℃ min ^-1. Different scanning calorimetry (DSC) measurements were performed on a Discovery series thermal analyzer at a scanning rate of 10 ℃ min ^-1 in N ₂. Tapping mode atom force microscopy (TM-AFM) images were taken on a NanoScope IIIa controller (Veeco Metrology Group/Digital Instruments, Sant a Barbara, CA) , using built-in software (version V6.13R1) to capture images. Transmission electron microscopy (TEM) images were acquired using a HITACHI H-7650 electron microscope operating at an acceleration voltage of 100 kV. The thickness of the blend films was determined by a Dektak 6 M surface profilometer. All J–V curves were captured under an AAA solar simulator (SAN-EI) calibrated by a standard single-crystal Si photovoltaic cell (certificated by National Institute of Metrology) .

2. Device Fabrication and Testing

The inverted device structure was ITO/ZnO/PBTT-F: Y6/MoO ₃/Ag. ITO-coated glass substrates were cleaned with deionized water, acetone and isopropyl alcohol in sequence and dried in the drying oven at 80 ℃ for 12 h before used. The ITO glass was then placed in the UV-ozone for 15 minutes and the sol-gel-derived ZnO films was spin-coated onto the ITO substrate followed by thermal treatment at 200 ℃ for 30 min and cooled to room temperature under vacuum. The mixture of PBTT-F/Y6 (1: 1.1 by wt/wt ratio) was dissolved in chloroform (CF) to obtain 10 mg mL ^-1 of solution. The active layer was spin-coating at 3000 rpm for 60 s to get the blend film. A 10 nm MoO ₃ layer and a 100 nm Ag layer were subsequently evaporated through a shadow mask to define the active area of the devices. The power conversion efficiencies (PCEs) were tested under AM 1.5G irradiation with the intensity of 100 mW cm ^-2 (Enlitech. Inc) which was calibrated by a NREL certified standard silicon cell (4 cm ^-2) . The J-V curves were recorded with the computer-controlled Keithley 2400 sourcemeter in a dry box under an inert atmosphere. The external quantum efficiency (EQE) spectra were measured through the measurement of solar cell spectral response measurement system QE-R3011 (Enli Technology Ltd., Taiwan) .

The hole mobility of the photosensitive layers was measured by the space charge limited current (SCLC) method using hole-only device with the structure of ITO/PEDOT: PSS/PBTT-F: Y6/MoO ₃/Ag, and the electron-only device with the configuration of ITO/ZnO/PBTT-F: Y6/PDINO/Al. The processing conditions used for the active layers were the optimized ones. Charge mobility was extracted by fitting the current density–voltage curves, recorded under dark conditions, with the Mott-Gurney equation. The mobility was determined by fitting the dark current to the model of a single carrier SCLC, which is described by the equation

where J is the current, μ _h is the zero-field mobility, ε ₀ is the permittivity of free space, ε _r is the relative permittivity of the material, d is the thickness of the active layer, and V is the effective voltage. The effective voltage can be obtained by subtracting the built-in voltage (V _bi) and the voltage drop (V _s) from the substrate’s series resistance from the applied voltage (V _appl) , V = V _appl -V _bi -V _s. The photo-ability of device was determined by the long-time stable LED white light soaking test system (Enlitech. Inc) . The light source area is 10 cm *10 cm with favorable uniformity. The light intensity was outputted in 100 mW cm-2 by control system. The operating temperature of setup is about 50 ℃, the humidity is 10%in glove box.

3. Materials

All chemicals and solvents were reagent grades and purchased from Aldrich, Energy, Derthon, Aladdin, Adamas and Sigma-Aldrich. All starting reagents were obtained commercially as analytical grade and used directly without any purification unless stated otherwise.

4. Synthesis and characterization

Synthetic routes of compound (3) are shown in Scheme 2.

2-Bromo-3- (2-ethylhexyl) thiophene (1) .

To a solution of 3- (2-ethylhexyl) thiophene (20 g, 101.9 mmol) in 250 mL of AcOH/CHCl ₃ (1: 1) was added N-bromosuccinimide (NBS) (18.13 g, 101.9 mmol) in one portion under nitrogen protection in ice bath, and the reaction mixture was kept stirring overnight. Then water was added into the mixture, the mixture was extracted with hexane, and the organic layer was washed with brine and dried over anhydrous sodium sulfate. Removal of the solvent and column purification on silica gel using hexane as eluent, the residue was further purified through reduced pressure distillation to obtain a colorless oil (25.33 g, 90.34%) . ¹H NMR (400 MHz, Chloroform-d) δ 7.18 (d, J = 5.6 Hz, 1H) , 6.76 (d, J = 5.6 Hz, 1H) , 2.50 (d, J = 7.2 Hz, 2H) , 1.65 –1.56 (m, 1H) , 1.36 –1.21 (m, 8H) , 0.88 (t, J = 7.4 Hz, 6H) . ¹³C NMR (101 MHz, Chloroform-d) δ 141.16, 128.79, 124.93, 109.41, 39.97, 33.60, 32.47, 28.78, 25.65, 23.03, 14.11, 10.81.

(5-bromo-4- (2-ethylhexyl) thiophen-2-yl) triisopropylsilane (2) .

2-Bromo-3- (2-ethylhexyl) thiophene (20.00 g, 72.66 mmol) was added dropwise to a solution of lithium diisopropylamide (39.96 ml, 79.93 mmol) at -78 ℃. After stirring the mixture for 2 h, triisopropylsilyl chloride (15.41 g, 79.93 mmol) was added to the mixture. After the addition, the mixture was naturally warmed up to room temperature and stirred overnight. The reaction mixture was poured into the water, extracted with ethyl acetate. The organic layer was dried over magnesium sulfate and the solvent was removed via rotary evaporation, and further purification was carried out by column chromatography using petroleum ether as eluent to obtain a crude colorless liquid 30.23 g. ¹H NMR (400 MHz, Chloroform-d) δ 6.88 (s, 1H) , 2.51 (dd, J = 7.3, 4.9 Hz, 2H) , 1.61 (m, 1H) , 1.27 (m, 11H) , 1.09 (m, 18H) , 0.88 (m, 6H) .

2, 3-bis (2-ethylhexyl) thiophene (3)

To a solution of (5-bromo-4- (2-ethylhexyl) thiophen-2-yl) triisopropylsilane (30.00 g, 69.51 mmol) in THF at -78 ℃ was added dropwise n-BuLi (57.93 ml, 139.02 mmol, 2.4 M in hexane) under nitrogen. After stirring the mixture for 2 h, 2-ethylhexyl bromide (24.40 mL, 137.20 mmol) was added to the mixture at -78 ℃ and stirred overnight at room temperature. The reaction mixture was poured into the water, extracted with petroleum ether. The organic layer was dried over magnesium sulfate and the solvent was removed under reduced pressure. And then the tetrabutylammonium fluoride (TBAF) (100 ml, 1M) was added to the residue and stirred 2h. The reaction mixture was poured into the water, extracted with petroleum ether, dried over magnesium sulphate, removed the solvent, and further purified through vacuum distillation to give a colorless liquid (18.27 g, total yield of 2 and 3= 81.49%) . ¹H NMR (400 MHz, Chloroform-d) δ 7.02 (d, J = 5.1 Hz, 1H) , 6.77 (d, J = 5.2 Hz, 1H) , 2.63 (d, J = 7.1 Hz, 2H) , 2.42 (d, J = 7.2 Hz, 2H) , 1.55 (m, 2H) , 1.40 –1.20 (m, 16H) , 0.94 –0.80 (m, 12H) . ¹³C NMR (101 MHz, Chloroform-d) δ 138.44, 137.45, 129.13, 120.86, 41.67, 40.58, 32.64, 32.63, 32.54, 32.16, 28.93, 28.89, 25.75, 25.70, 23.10, 23.06, 18.15, 14.14, 10.91, 10.88.

5, 7-dibromo-2, 3-bis (2-ethylhexyl) benzo [1, 2-b: 4, 5-c'] dithiophene-4, 8-dione (4)

Oxalyl chloride (2.1 mL) was slowly added to 2, 5-dibromothiophene-3, 4-dicarboxylic acid (1 g, 3.03 mmol) and DMF (1 drop) in dry dichloromethane (DCM) (20 mL) . The mixture was stirred for 12 h at room temperature. The solvent was removed under vacuum to obtain crude 2, 5-dibromothiophene-3, 4-dicarbonyl dichloride, which was used for next step without further purification. To a stirred solution of the dicarbonyl dichloride (1.23 g, 3.35 mmol) and 2, 3-bis (2-ethylhexyl) thiophene (1.03 g, 3.35 mmol) in dry 1, 2-dichloroethane, AlCl ₃ (1.79 g, 13.40 mmol) was added in small portions at 0 ℃. The mixture was allowed to stir at 0 ℃ for 30 min and then at room temperature for 6 h. The mixture was poured into ice with 1 mol/L hydrochloric acid and then extracted with DCM . The organic layer was collected and the volatile solvent was removed under vacuum. The crude product was puri fi ed through a silica gel column with petroleum ether/dichloromethane (5: 1 by volume) to give a pale yellow sticky oil (1.07 g, 53.23%) . ¹H NMR (400 MHz, Chloroform-d) δ 2.90 (d, J = 7.3 Hz, 2H) , 2.75 (d, J = 7.2 Hz, 2H) , 1.76 –1.66 (m, 1H) , 1.63 –1.55 (m, 1H) , 1.52 –1.15 (m, 16H) , 0.90 (m, 12H) . ¹³C NMR (101 MHz, Chloroform-d) δ 174.77, 172.17, 152.49, 145.27, 141.34, 141.04, 133.93, 132.78, 120.26, 119.64, 41.63, 40.10, 33.21, 32.56, 32.40, 31.75, 28.86, 28.69, 25.82, 25.66, 23.17, 22.98, 14.17, 14.12, 11.04, 10.88. HRMS (MALDI+) : calculated for C ₂₆H ₃₄Br ₂O ₂S ₂ [M+] , 602.0347; found: 601.8983.

2, 3-bis (2-ethylhexyl) -5, 7-di (thiophen-2-yl) benzo [1, 2-b: 4, 5-c'] dithiophene-4, 8-dione (5)

Pd (PPh ₃) ₄ (100 mg) was added to a solution of compound 4 (2.14 g, 3.55 mmolg) and thimethyl (thiophen-2-yl) stannane (3.98 g, 10.66 mmol) in 100 mL of dry toluene. The mixture was refluxed in an argon atmosphere for 24 h. After the removal of the solvent at a reduced pressure, the residue was purified by column chromatography on a silica gel column with petroleum ether/dichloromethane (5: 1 by volume) to give an red sticky oil (2.11 g, 97.69%) . ¹H NMR (400 MHz, Chloroform-d) δ 7.83 (ddd, J = 21.5, 3.8, 1.2 Hz, 2H) , 7.52 (d, J = 4.7 Hz, 2H) , 7.14 (dd, J = 5.1, 3.8 Hz, 2H) , 2.91 (m, 2H) , 2.75 (d, J = 7.2 Hz, 2H) , 1.71 –1.64 (m, 1H) , 1.60 (m, 1H) , 1.40 –1.21 (m, 16H) , 1.00 –0.81 (m, 12H) . ¹³C NMR (101 MHz, Chloroform-d) δ 177.02, 174.38, 151.15, 145.76, 143.17, 142.84, 141.05, 140.86, 133.17, 132.89, 131.10, 131.07, 130.85, 129.81, 129.71, 129.37, 127.31, 127.10, 41.57, 40.32, 33.14, 32.75, 32.62, 31.74, 28.94, 28.92, 28.89, 25.87, 23.28, 23.07, 14.31, 14.21, 11.20, 10.94. HRMS (MALDI+) : calculated for C ₃₄H ₄₀O ₂S ₄ [M+] , 608.9320; found: 610.0260.

5, 7-bis (5-bromothiophen-2-yl) -2, 3-bis (2-ethylhexyl) benzo [1, 2-b: 4, 5-c'] dithioph-ene-4, 8-dione (6)

Compound 5 (2.1 g, 3.45 mmol) was added into THF (30 mL) . After the solid dissolved completely, N-bromosuccinimide (NBS) (1.35 g, 7.59 mmol) was added in one portion. The reaction mixture was stirred at room temperature for 4 h, water was added into the mixture, the mixture was extracted with ethyl acetate, and the organic layer was washed with brine and dried over anhydrous magnesium sulfate. The solvent was removed at a reduced pressure, the residue was purified by column chromatography on silica gel with petroleum ether to give a red sticky oil (2.26 g, 85.61%) . ¹H NMR (400 MHz, Chloroform-d) δ 7.52 (dd, J = 11.2, 4.1 Hz, 2H) , 7.10 (dd, J = 4.0, 2.7 Hz, 2H) , 2.92 (m, 2H) , 2.76 (m, 2H) , 1.69 (m, 1H) , 1.57 (m, 1H) , 1.28 (m, 16H) , 0.90 (m, 12H) . ¹³C NMR (101 MHz, Chloroform-d) δ 176.75, 174.05, 151.91, 145.24, 142.15, 141.60, 140.99, 140.70, 134.34, 134.09, 130.69, 130.54, 130.28, 129.77, 129.63, 128.61, 119.06, 118.97, 41.43, 40.36, 33.19, 32.93, 32.64, 31.74, 29.12, 28.91, 28.87, 25.85, 23.34, 23.06, 14.42, 14.19, 11.21, 10.92. HRMS (MALDI+) : calculated for C ₃₄H ₃₈Br ₂O ₂S ₄ [M+] , 766.7240; found: 768.2027.

Polymerization of PBTT-F. To a 25 mL flask, compound 6 (150 mg, 0.19 mmol) , compound BDT-2F (184 mg, 0.19 mmol) and Pd (PPh ₃) ₄ (9.04 mg, 0.0078 mmol) were added under argon, then the reaction container was purged with argon for 20 min to remove O ₂. After the addition of toluene (8 mL) , the reactant mixture was heated to reflux for 18 h. After cooling to room temperature, the mixture was poured into methanol (200 ml) , then filtered through a Soxhlet thimble, which was then subjected to Soxhlet extraction with methanol, acetone, hexane, dichloromethame and chloroform. The chloroform fraction was concentrated and added dropwise into methanol. Subsequently, the precipitates were collected and dried under vacuum overnight to get polymer as blue black solid. Yield: 125.30 mg (76.84%) . GPC: M _w=68.54 kDa; M _n=32.82 kDa; PDI=2.08. The polymer was thermally stable up to 352 ℃ (5%weight loss by TGA) . ¹H NMR (400 MHz, CDCl ₂CDCl ₂, 100 ℃) δ (ppm) 7.73-6.07 (br, ArH) , 2.79 (br, Ar-CH ₂) , 1.33-1.20 (br, CH and CH ₂ ) , 0.88 (br, CH ₃) .

The synthetic procedures of compound (9) and (11) are shown in Scheme 3 and are similar to the above methods for compound TTDO, and their characterization data are as follow:

(5-bromo-4-butylthiophen-2-yl) triisopropylsilane: colorless liquid 16.70 g, 97.49%. ¹H NMR (400 MHz, Chloroform-d) δ 6.91 (s, 1H) , 2.62 –2.54 (m, 2H) , 1.62 –1.53 (m, 2H) , 1.44 –1.21 (m, 5H) , 1.09 (d, J = 7.4 Hz, 18H) , 0.93 (t, J = 7.3 Hz, 3H) . ¹³C NMR (101 MHz, Chloroform-d) δ142.83, 136.75, 134.60, 113.38, 31.98, 28.99, 22.35, 18.52, 13.92, 11.62.

2, 3-dibutylthiophene: colorless liquid 8.35 g, 99.04%. ¹H NMR (400 MHz, Chloroform-d) δ 7.02 (d, J = 5.1 Hz, 1H) , 6.81 (d, J = 5.1 Hz, 1H) , 2.76 –2.65 (m, 2H) , 2.61 –2.43 (m, 2H) , 1.68 –1.55 (m, 2H) , 1.54 (m, 2H) , 1.37 (m, 4H) , 0.93 (m, 6H) . ¹³C NMR (101 MHz, Chloroform-d) δ 138.77, 137.66, 128.69, 120.88, 34.10, 33.04, 27.91, 27.48, 22.57, 22.45, 13.99, 13.90.

5, 7-dibromo-2, 3-dibutylbenzo [1, 2-b: 4, 5-c'] dithiophene-4, 8-dione (7) : yellow crystal 1.14 g, 55.61%. ¹H NMR (400 MHz, Chloroform-d) δ 3.09 –2.87 (m, 2H) , 2.82 (m, 2H) , 1.80 –1.62 (m, 2H) , 1.56 –1.35 (m, 6H) , 1.09 –0.80 (m, 6H) . ¹³C NMR (101 MHz, Chloroform-d) δ 175.04, 172.26, 152.55, 145.02, 141.39, 140.73, 133.90, 132.81, 120.31, 119.76, 33.34, 32.35, 28.24, 27.09, 22.86, 22.45, 14.03, 13.81. HRMS (MALDI+) : calculated for C ₁₈H ₁₈Br ₂O ₂S ₂ [M+] , 490.2680; found: 490.7938.

2, 5-dibutylthiophene: colorless liquid 6.37 g, 54.58%. ¹H NMR (400 MHz, Chloroform-d) δ 6.48 (s, 1H) , 2.67 (t, J = 7.6 Hz, 4H) , 1.56 (m, 4H) , 1.31 (m, 4H) , 0.86 (t, J = 7.3 Hz, 6H) . ¹³C NMR (101 MHz, Chloroform-d) δ 143.25, 123.29, 33.85, 29.86, 22.24, 13.86.

1, 3-dibromo-5, 7-dibutyl-4H, 8H-benzo [1, 2-c: 4, 5-c'] dithiophene-4, 8-dione (8) : light yellow solid 0.84 g, 46.63%. ¹H NMR (400 MHz, Chloroform-d) δ 3.41 –3.26 (m, 4H) , 1.73 (m, 4H) , 1.47 (m, 4H) , 0.97 (t, J = 7.3 Hz, 6H) . ¹³C NMR (101 MHz, Chloroform-d) δ 175.43, 155.47, 134.71, 132.09, 119.51, 32.69, 29.76, 22.55, 13.91. HRMS (MALDI+) : calculated for C ₁₈H ₁₈Br ₂O ₂S ₂ [M+] , 490.2680; found: 490.8632.

Claims

A PBTT-F donor polymer having the following structure:

wherein Ar is selected from monocyclic, bicyclic and polycyclic arylene, or monocyclic, bicyclic and polycyclic heteroarylene, or may contain one to five such groups, either fused or linked;

wherein
wherein R is independently selected from alkyl groups with up to 18 C atoms, aryls and substituted aryls, and

wherein n is an integer greater than 1.
The PBTT-F donor polymer according to claim 1, wherein Ar is
The PBTT-F donor polymer according to claim 1, wherein the PBTT-F donor polymer has the following structure:
A composition, comprising the PBTT-F donor polymer according to claim 1 or 2 blended with the acceptor Y6.
A photovoltaic device based on the PBTT-F donor polymer according to claim 1 or 2 or the composition according to claim 3.
A thick film device based on the PBTT-F donor polymer according to claim 1 or 2 or the composition according to claim 3.
A polymer solar cells based on the PBTT-F donor polymer according to claim 1 or 2 or the composition according to claim 3.
A method for preparing the PBTT-F donor polymer according to claim 2 as following:

wherein compound (3) is prepared as following: