Double Acceptor Blocks-Containing Copolymers with Deep HOMO

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Organic Electronic Devices

Double Acceptor Blocks-Containing Copolymers with Deep HOMO Levels for Organic Solar Cells: Adjusting Carboxylate Substituent Position for Planarity Hui Guo, Bin Huang, Lifu Zhang, Lie Chen, Qian Xie, Zhihui Liao, Shaorong Huang, and Yiwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02212 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Materials & Interfaces

Double Acceptor Blocks-Containing Copolymers with Deep HOMO Levels for Organic Solar Cells: Adjusting Carboxylate Substituent Position for Planarity Hui Guoa, Bin Huanga, Lifu Zhanga, Lie Chen*a, Qian Xiea, Zhihui Liaoa, Shaorong Huanga, and Yiwang Chena a

College of Chemistry/Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, Nanchang 330031, (P. R. China).

ABSTRACT: A deep highest occupied molecular orbital (HOMO) level is a prerequisite for polymer donor material to boost the organic solar cells (OSCs) performance by achieving high open circuit voltage (Voc). Abandoning the traditional concept of donor-acceptor (D-A) structure, two copolymers PBTZ-4TC and PBTZ-C4T based on acceptor1-π-acceptor2 (A1-π-A2) architecture where thiophene as the bridge, the difluorinated benzotriazole (BTZ) as A1 unit alternating copolymerized with

4,4’-dicarboxylate-substituted

difluorotetrathiophene

(4TC)

and

3,3’-dicarboxylate-substituted difluorotetrathiophene (C4T) as A2, respectively, are developed. Due to the double acceptor blocks with high electron affinity, both A1-π-A2 type copolymers possess the lower HOMO levels of 5.52-5.56 eV, which are lower than most D-A type donors.

Polymer PBTZ-4TC and PBTZ-C4T have the

same backbone but only differ with the position of carboxylate substituent on the A2 unit. Intriguingly, subtle optimizing the position of the carboxylate-substitute causes a significantly difference on the properties of the A1-π-A2 type copolymers. PBTZ-C4T with more planar geometry is demonstrated with better light absorption, higher crystallinity, more pronounced temperature-dependent aggregation effect and favorable bulk heterojunction morphology, but with slightly higher HOMO level and more emission energy loss relative to the PBTZ-4TC. The PBTZ-C4T device exhibits the higher power conversion efficiency (PCE) of 9.34% than the PBTZ-4TC-based one (8.75%). These results reveal that concept of A1-π-A2 type copolymers not only can afford more flexibility in tuning the energy levels to achieve the deep HOMO 1

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levels, but also can provide a facial strategy to greatly enrich the types of polymer donors for high performance OSCs. KEYWORDS: organic solar cells, acceptor1-π-acceptor2 (A1-π-A2) copolymers, more planar geometry, deep energy level, high performance.

INTRODUCTION Organic solar cells (OSCs) have been attracted the increasing attention during the past decades, because of their low-cost, light weight, flexibility, and roll-to-roll printing. Generally, the bulk-heterojunction OSCs consist of a p-type organic semiconductor as an electron donor and an n-type organic semiconductor as an acceptor.1-9 Traditional OSCs are based on fullerene derivatives as acceptors, and the PCEs of fullerene OSCs have exceeded 11%.10, 11 But the performance of fullerene derivatives are limited by some drawbacks, such as difficulty to modify, high cost, poor thermal stability and limited absorption. Compared to fullerene acceptors, the non-fullerene acceptors (NFAs) exhibit wider absorption to harvest sunlight and greater tunability of the molecular energy levels. Currently, the non-fullerene based OSCs with single junction achieved the PCE over 15% recently.12-14 In order to achieve complementary absorption with non-fullerene acceptors, the development of wide bandgap (WBG) polymer donor materials is also a critical factor to boosting the OSCs efficiency. Most of the reported polymer donors are based on the donor–acceptor (D-A) architecture, where electron-deficient units alternate with electron-rich unit. The push-pull character of the D-A alternating copolymers can effectively improve the light harvesting and has good complimentary absorption with low-bandgap NFAs. A variety of acceptor blocks have been development, such as 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c’]dithiophene-4,8-dione (BDD),15,16

ﬂuorine

benzotriazole,17-22

substituted

bithiazole,23-25

dicarboxylate-substituted thiophene, etc. In contrast, the number of donor blocks lags large behind that of the acceptor blocks. There are few donor blocks to construct efficient D-A alternating copolymers, except benzodithiophene (BDT) donor block. 2


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The limited donor building blocks restrict the diversification in the development of the donor conductive materials. Apart from the light harvesting properties, the deep HOMO level is also a prerequisite for a donor material to achieve high Voc of the resulting devices. However, the HOMO levels of most BDT-based D-A copolymers are normally 5.2-5.4 eV, which is not deep enough to maximize the Voc, especially when matched with some efficient non-fullerene acceptors with deep lowest unoccupied molecular orbital (LUMO) levels. Incorporation of electron-withdrawing groups, such as fluorine or chlorine atoms into the BDT unit to lower the energy level. Nevertheless, this strategy normally requires tedious synthesis, low yield and high cost in preparation of the monomers, incompatible to the commercialization of OSCs. Therefore, synthetic simplified of new donor polymers warrants significant attention. How to lower the HOMO level of the donor materials by simple synthesis should be taken into careful consideration. In this work, we designed and synthesized two polymers based on the acceptor1-π-acceptor2 (A1-π-A2) architecture to replace the traditional D-A counterpart. Reasonable combination of different acceptor blocks is a facial strategy to greatly enrich the types of polymer donor materials for OSCs. More importantly, compared with the traditional D-A polymers, A1-π-A2 type architecture can obtain the deep HOMO levels by tuning the energy levels, for the improvement in the Voc and the device performance. The two A1-π-A2 donor polymers, namely PBTZ-4TC and PBTZ-C4T, are displayed in Scheme 1, in which thiophene ring as the bridge, difluorinated benzotriazole (BTZ) as A1 unit alternating copolymerized with 4, 4’-dicarboxylate-substituted

difluorotetrathiophene

(4TC)

and

3,

3’-dicarboxylate-substituted difluorotetrathiophene (C4T) as A2, respectively. Note that the 4TC and C4T are first developed as acceptor units. Combination of difluorothiophene with dicarboxylthiophene is expected to effectively pull down the energy levels. Furthermore, intermolecular S∙∙∙F, F∙∙∙H, and F∙∙∙π noncovalent interactions can also favor a compact molecular packing in the copolymers to 3



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optimize bulk-heterojunction morphology and facilitate charge dissociation and transport. A1-π-A2 polymer PBTZ-4TC and PBTZ-C4T have the same backbone but only differ with the position of carboxylate substituent on the A2 unit. We systematically

investigated

the

light

physical

and

chemical

properties,

bulk-heterojunction morphology and the resulting OSCs performance of the two A1-π-A2 copolymers. And the impact of the position of carboxylate substituent on the properties of copolymers has also been given a special attention.

RESULTS AND DISCUSSION The final synthetic routes of the two A1-π-A2 copolymers PBTZ-4TC and PBTZ-C4T are displayed in the Scheme 1. The detailed synthesis procedures are described in the supporting information Scheme S1. The PBTZ-4TC and PBTZ-C4T, were easily synthesized by dithiophenyl flanked BTZ monomer Stille coupling with 4, 4’-dicarboxylate-substituted 3’-dicarboxylate-substituted

difluorotetrathiophene difluorotetrathiophene

(4TC) (C4T),

and

respectively.

3, The

molecular weights of the polymers were measured by gel permeation chromatography (GPC). The number molecular weights ( M n ) of PBTZ-C4T and PBTZ-4TC are 48.9 and 14.8 kDa, and the polydispersity index (PDI) are 1.75 and 2.05. Thermogravimetric analysis (TGA) was used to characterize the thermal stability of the polymer (seen Supporting Information Figure S1). The decomposition temperatures (Td) at 5% weight loss of the copolymers of PBTZ-4TC and PBTZ-C4T are 375 and 380 °C (seen Table 1), respectively, indicating the two polymers are thermal stable for OSCs application. Since the two A1-π-A2 type copolymers only differ with the position of carboxylate substituent, we are interested in how the bulk carboxylate substituent may cause some possible influence on the conformation of the polymer backbone. To study backbone geometry of two copolymers, theoretical calculations were performed by density functional theory (DFT) at the B3LYP/6-31G (d, p) level using Gaussian. In Figure 1, 4


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the calculations show that the dihedral angle between dithiophenyl flanked BTZ and 4TC unit of PBTZ-4TC is 24°. After moving carboxylate group from 4, 4’ position to 3, 3’ position of the flanked thiophene rings, the dihedral angle between dithiophenyl flanked BTZ and C4T distinctly reduced to 11° in PBTZ-C4T. From the side-view we can see that the PBTZ-C4T reveals a better planar conformation than PBTZ-4TC. The better planar geometry of PBTZ-C4T helps to charge transport. The reduced hindrance between A1 and A2 unit in PBTZ-C4T may the reason to induce a higher reactivity and lead to the higher molecular weight. In view of the position of carboxylate substituent caused an obvious difference on the backbone conformation, the molecular packing of the polymers would also be influenced. X-Ray diffraction (XRD) analysis was thereby used to measure the crystalline behavior of the polymer neat films. In Figure 2a, the PBTZ-4TC only shows a halo-diffraction peak, indicating the amorphous arrangement of the polymer. In contrast, a sharp diffraction peaks at 4.1° is observed for PBTZ-C4T, indicative of an ordering crystal structure. Such ordered stacking of PBTZ-C4T should originate from the planar conformation of backbone, which can facilitate the formation of the pure domain for efficient charge transport. The temperature-dependent aggregation property of the polymer donors have been demonstrated as an effective protocol for the well-defined film-state packing in high performance organic photovoltaics. The temperature-dependent absorbance (Td-Abs) was measured in 0.2 mg/ml chlorobenzene (CB) solution to detect the self-aggregation property of the copolymers. In Figure 2b, when the temperature is 95 °C, the polymer PBTZ-4TC shows a peak in the wavelength 496 nm. After cooled to 25 °C, the PBTZ-4TC has a significantly red-shifted absorption from 496 to 530 nm, implying a typical temperature-dependent aggregations existing in the chlorobenzene solution. For PBTZ-C4T (Figure 2c), a broad peak also can be observed in the UV-spectra at 95 °C, but this peak is much red-shifted relative to the peak of PBTZ-4TC at 95 °C (540 nm vs 496 nm). During the cooling process from the 95 °C to 25 °C, a typical temperature-dependent aggregation characteristic is also 5



detected for PBTZ-C4T. Temperature-dependent property of the PBTZ-C4T is quite different relative to that of PBTZ-4TC. For PBTZ-4TC, there is only one peak during the cooling process. However, moving the position of carboxylate substituents on the thiophene rings allows for a significantly stronger aggregation of PBTZ-C4T at low temperatures, as revealed by the distinct shoulder peaks appearing at 606 nm. The carboxylate position enhanced temperature-dependent aggregation is ascribed to the more planar conformation of the PBTZ-C4T backbone. The solution aggregation can favor strong intermolecular interactions, which would contribute to the improvement in charge transport. The normalized UV-vis absorption spectra of the PBTZ-4TC, PBTZ-C4T and ITIC-Th1 in the neat films are shown in the Figure 2d and Table 1. Compared with the solution samples, both solid films exhibit red-shifted absorption bands, implying the closer packing in solid states. In addition, the maximum absorption peak of polymer PBTZ-C4T is red-shifted by 10 nm than PBTZ-4TC counterpart. More interestingly, compared to the PBTZ-4TC, PBTZ-C4T shows much more pronounced shoulder peak at ~600 nm, which is in accordance with their temperature-dependent aggregation properties. The optical bandgap (Eg) is calculated to be 1.91 for PBTZ-4TC and 1.90 eV for PBTZ-C4T. Therefore, both polymers are wide-band-gap polymers, which possess good complimentary absorption with low-bandgap NFAs ITIC-Th1 (seen Support Information Figure S2). The solution UV of the neat polymers in Supporting Information Figure S3a and Figure S3b displays the UV spectra of the polymers with ITIC-Th1, showing a wide absorption range from 400-800 nm. The maximum film absorption coefficients of PBTZ-4TC and PBTZ-C4T are 2.5×104 and 3.3×104 cm-1 (seen Figure 2e), respectively. The PBTZ-4TC exhibits broader absorption range than PBTZ-C4T, but the higher absorption coefficient of PBTZ-C4T is beneficial for achieving higher Jsc in the device. Electrochemical properties of the copolymers were measured by cyclic voltammetry (CV) using three electrode electrochemical cell. The details energy levels are provided in the Figure 2f and cyclic voltammetry curves are displayed in 6


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Supporting Information Figure S4. The LUMO levels of PBTZ-4TC, PBTZ-C4T and ITIC-Th1 are calculated to be -3.68, -3.65 and -4.01 eV, and the HOMO levels of the PBTZ-4TC, PBTZ-C4T and ITIC-Th1 are -5.56, -5.52 and -5.74 eV (seen Table 1), which are deeper than most of WBG polymer donors (usually 5.2-5.4 eV).26-30 This result demonstrates that A1-π-A2 construction concept is very valid in reducing the HOMO level of the molecule, which will help to obtain a high Voc in the device. The corresponding bandgaps of PBTZ-4TC and PBTZ-C4T are calculated to be 1.88 and 1.87 eV, respectively. The CV bandgaps are consistent with the optical Eg. The PSCs photovoltaic properties was fabricated with an inverted device structure of ITO/ZnO/polymer:ITIC-Th1/MoO3/Ag. We changed different solvents,, the polymers donor/the SMs acceptor (D/A) ratio, different spin-coating speed, the thermal annealing temperature and time, and different addition solvent to optimize the performance of the devices (detail information seen in Supporting Information Table S1、Table S2 and Table S3) and the current density-voltage (J-V) curves of the best performance device are shown in the Figure 3a. Without DIO additive, two polymers exhibit relatively low photovoltaic performance, a PCE of 7.68%, Voc of 0.887 V, Jsc of 14.61 mA cm-2 and FF of 59.27% for PBTZ-4TC, and a PCE of 8.11%, Voc of 0.84 V, Jsc of 14.70 mA cm-2 and FF of 65.43% for PBTZ-C4T. Addition of 0.5% DIO remarkably boosts the efficiency of both polymers based devices. For PBTZ-4TC: ITIC-Th1, the PCE of 8.75% is obtained with a Voc of 0.885 V, Jsc of 15.72 mA cm-2 and FF of 62.62%; For PBTZ-C4T: ITIC-Th1, the PCE is up to 9.34% with a Voc of 0.844 V, Jsc of 16.59 mA cm-2 and FF of 66.6% (seen Table 2). Therefore, with and without DIO additive, PBTZ-C4T devices exhibit better device performance with higher Jsc, FF and slightly reduced Voc than the PBTZ-4TC counterpart. The external quantum efficiency (EQE) spectra of the PBTZ-4TC and PBTZ-C4T are displayed in Figure 3b. All the devices show a wide respond range from 300-800 nm. Compared to the additive-free PBTZ-4TC -based device, additive-free PBTZ-C4T-based shows higher EQE value. After addition of DIO additive, PBTZ-C4T also shows the highest EQE value among these devices. The enhanced EQE of PBTZ-C4T-based device 7



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should be ascribed to the higher absorption coefficient and more favorable bulk heterojunction morphology than PBTZ-4TC (discuss later). The Jsc values of PBTZ-4TC and PBTZ-C4T-based devices calculated by integration of EQE spectra are consistent with the J-V measurement. Dissociation of charge-transfer states of bulk heterojunction is critical for photocurrent conversion. Thus, the charge dissociation was measured by steady-state photoluminescence (PL) spectroscopy. Seen Figure 4a-b, the PL spectra of the PBTZ-4TC, PBTZ-C4T and ITIC-Th1 three neat films show the emission maxima peak at 647 nm, 655 and 787 nm, respectively. For the PBTZ-4TC: ITIC-Th1 and PBTZ-C4T: ITIC-Th1 blend films, the PL emission of the two neat films (PBTZ-4TC and PBTZ-C4T) under the excitation wavelength at 560 nm are completely quenched by ITIC-Th1, implying that the photoexcited electrons of both polymers can well transfer from the donors to ITIC-Th1. However, the PL emission of the acceptor ITIC-Th1 at 660 nm is not completely quenched by PBTZ-4TC and PBTZ-C4T but quenches ~90% relative to the PL intensity of the neat films. The results suggest that the holes do not transfer so efficiently from acceptor ITIC-Th1 to polymer donors as the electrons transfer. And from the energy levels (Figure 2f) we can see that the LUMO level difference (ΔELUMO) and the HOMO level difference (ΔEHOMO) between the polymer donors and small molecule acceptor are 0.33 eV and 0.18 eV for PBTZ-4TC:ITIC-Th1, 0.35 and 0.22 eV for PBTZ-C4T: ITIC-Th1. The ΔELUMO values are larger the theoretical value (0.3 eV), which can provide enough driving force for electron transfer. However, the ΔEHOMO for both blends are much lower than 0.3 eV, leading to the insufficient hole transfer. The time-resolved photoluminescence (TRPL) of the blend film was also measured in Figure 4c. The ﬂuorescence lifetime (τ)

of

the

PBTZ-4TC:ITIC-Th1

blend

film

is

661

ps,

while

that

of

PBTZ-C4T:ITIC-Th1 is shorter (651 ps) implying the higher proportional photoinduced charges has been transferred.31 Seen from Figure 3 and Table 2, PBTZ-C4T: ITIC-Th1-based devices exhibit slightly reduced Voc than PBTZ-4TC: ITIC-Th1 counterparts. The lower HOMO level 8


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of PBTZ-C4T than PBTZ-4TC should account for the difference in the Voc. In fact, apart from the effect of energy level of each component, the energy loss during the non-radiation and radiation decay also exerts great influence on the Voc value. Figure 4d compares the PL emission of the PBTZ-4TC and PBTZ-C4T. When both polymers are excited by 550 nm light, the emission peak of PBTZ-C4T is red-shifted than that of PBTZ-4TC, that is, the Stoke shift of PBTZ-C4T is larger than that of PBTZ-4TC. The larger stoke shift of PBTZ-C4T means more energy is lost during the radiative decay, which probably PBTZ-C4T has a higher vibration relaxation than that of PBTZ-4TC.32 The more emission energy loss also results in the reduced Voc of devices with PBTZ-C4T than the ones with PBTZ-4TC. To study the two polymers at the macroscopic scale, atomic force microscopy (AFM) and transmission electron microscopy (TEM) were performed in the phase separation. As shown in Figure 5, compared to the sample without additive, the PBTZ-4TC: ITIC-Th1 film with 0.5% DIO additive displayed a rougher surface. This is because the DIO additive could inhibit compatibility between PBTZ-4TC and ITIC-Th1 appropriately. Excessive compatibility between donor and acceptor would increase the probability of exciton recombination, thus the FF and Jsc values of PBTZ-4TC: ITIC-Th1 blend film are lower than that of 0.5% DIO as additive. The same phenomena

also

emerged

for

PBTZ-C4T:ITIC-Th1

blend

film,

as

the

root-meansquare (RMS) roughness of blend film without additives and additives prepared under the optimized device fabrication conditions is 2.37 and 2.77 nm, respectively. Notably, the PBTZ-C4T: ITIC-Th1 blend film with 0.5% DIO as additive exhibit obvious bi-continuously interpenetrating nano-scale networks, which helps to promote the exciton dissociation and charge transport, thus a relatively higher Jsc and higher PCE are obtained. Phase separation can also be observed from the TEM images. Additives prepared blend show more homogeneous morphology than those without additive. Among those blends, the PBTZ-C4T:ITIC-Th1 blend film with 0.5% DIO shows the most homogeneous fibril-like nano-phase separation with the thinnest fibrils, which help to increase the percentage of excitons reaching the 9



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interface of PBTZ-C4T and ITIC-Th1 where charge transfer occurs. The results of TEM images are also in accordance with the AFM. The morphology always impacts greatly on the only hole or only electron mobility. The space charge-limited current (SCLC) method was thereby used to evaluate the charge carrier mobility. The corresponding features are seen in Supporting Information Figure S5a-b and the results were summarized Table2. The PBTZ-C4T: ITIC-Th1 shows higher hole motility (μh) of 2.510-5 cm2V-1s-1 and higher electron motility (μe) value of 2.910-5 cm2V-1s-1 than PBTZ-4TC: ITIC-Th1 (μh=2.110-5 cm2V-1s-1, μe=2.510-5 cm2V-1s-1). After with DIO additive, both of the blend films exhibit improved μh of 2.310-5 cm2V-1s-1 and μe of 2.710-5 cm2V-1s-1 for PBTZ-4TC:ITIC-Th1, and μh of 3.410-5 cm2V-1s-1, μe of 3.810-5 cm2V-1s-1 for PBTZ-C4T:ITIC-Th1. The improved mobilities lead to the enhanced Jsc values. Moreover, all the electron mobilities of the blend are higher than their corresponding hole mobilities, which is consistent with the PL results. More importantly, the more balance μe/μh values of PBTZ-C4T: ITIC-Th1 than those of PBTZ-4TC: ITIC-Th1 for both with and without additive films (Table 2), accounting for the improved FF.

CONCLUSIONS In

summary,

two

copolymers

PBTZ-4TC

and

PBTZ-C4T

based

on

acceptor1-acceptor2 (A1-π-A2) architecture are developed. Due to the double acceptor blocks with high electron affinity, both A1-π-A2 type copolymers possess the low HOMO levels of 5.52-5.56 eV, which are much lower than most D-A type donors. Intriguingly, subtle optimizing the position of the carboxylate-substitute causes a significantly difference on the properties of the A1-π-A2 type copolymers. PBTZ-C4T with more planar geometry is demonstrated with better light absorption, higher crystallinity, more pronounced temperature-dependent aggregation effect and favorable bulk heterojunction morphology, but with slightly higher HOMO level and more emission energy loss relative to the PBTZ-4TC. Thus, the PBTZ-C4T exhibits the higher PCE of 9.34% than the PBTZ-4TC one (8.75%). These results indicate that 10


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concept of A1-π-A2 type copolymers not only can obtained the deep HOMO levels by tuning the energy levels, but also can provide a facial strategy to greatly enrich the types of polymer donor materials for high performance OSCs.

EXPERIMENTAL SECTION PSCs devices were fabricated with the inverted structure of Glass/ITO/ZnO/active layer/MoO3/Ag. Specific process is as follows: the etched ITO substrate was ultrasonically cleaned in detergent, water (twice), acetone and isopropanol for 2 hours. After drying, the etched ITO substrate process in UV ozone for 15 minutes and then ZnO was spin-casted onto the ITO. Afterwards, the ITO glass was thermally annealed at 205 ℃ for 50 minutes. Next, the active layer was spin-casted on the ZnO. Finally, 3 nm MoO3 and 50 nm Ag layer were deposited by evaporation under a pressure of ca. 4×10-4 Pa. The ZnO was spin-coated at 4000 r.p.m. for 1 min. The concentration of PBTZ-4TC:ITIC-Th1 and PBTZ-C4T:ITIC-Th1 of chlorobenzene solution

were

20

mg/ml

(added

0.5%

1,8-diiodooctane

additive).

The

polymers:ITIC-Th1 were spin-coated at 2500 r.p.m. for 1 min. Except for the spin coating of the ZnO layer onto the ITO substrate, all photovoltaic device fabrication processes were carried out in nitrogen glove box with oxygen and humidity of less than 4 ppm. The test for this work was still measured in nitrogen glove box. The current−voltage (J−V) characteristics were performed by a Keithley 2400 Source Meter (100 mW/cm2, AM 1.5 G) and all devices area are 4 mm2. The external quantum efficiency (EQE) tests were based on an Oriel Cornerstone monochromator, which was still performed in the nitrogen glove box.

ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the https://pubs.acs.org. Detailed synthesis steps of the polymers, characterization instrument and other experiment results (such as TGA curve, UV-spectra curve, cyclic voltammetry curve and the hole only or electron devices plots of the polymers), 1HNMR spectra and 11



different conditions of the polymers on device performance of the polymers.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +86 791 83968830. Author Contributions H. Guo, B. Huang and L. Zhang contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS L.C. thanks for the support from the National Natural Science Foundation of China (NSFC) (51673092, and 21762029). Y.C. thanks for support from National Science Fund for Distinguished Young Scholars (51425304).

REFERENCES (1) Cnops, K.; Rand, B. P.; Cheyns, D.; Verreet, B.; Empl, M. A.; Heremans, P. 8.4% Efficient Fullerene-Free Organic Solar Cells Exploiting Long-Range Exciton Energy Transfer. Nat. Commun. 2014, 5, 3406. (2) Yao, Z.; Liao, X.; Gao, K.; Lin, F.; Xu, X.; Shi, X.; Zuo, L.; Liu, F.; Chen, Y.; Jen, A. K. Dithienopicenocarbazole-Based Acceptors for Efficient Organic Solar Cells with Optoelectronic Response Over 1000 nm and an Extremely Low Energy Loss. J Am Chem Soc. 2018, 140, 2054-2057. (3) An, Y.; Liao, X.; Chen, L.; Yin, J.; Ai, Q.; Xie, Q.; Huang, B.; Liu, F.; Jen, A. K. Y.; Chen, Y. Nonhalogen Solvent-Processed Asymmetric Wide-Bandgap Polymers for Nonfullerene Organic Solar Cells with Over 10% Efficiency. Adv. Funct. Mater. 2018, 28, 1706517. (4) Bin, H.; Gao, L.; Zhang, Z. G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, S.; Xue, L.; Yang, C.; Xiao, M.; Li, Y. 11.4% Efficiency Non-Fullerene Polymer Solar Cells with Trialkylsilyl Substituted 2D-Conjugated Polymer as Donor. Nat. Commun. 2016, 7, 12


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Scheme 1. Molecular structures of PBTZ-4TC and PBTZ-C4T; i) dry-toluene, 2.0 mol% Pd2(dba)3, 8% P(o-tolyl)3 and 100 °C reflux.

17



Figure 1. Optimized conformations of the polymers based on DFT calculations at the B3LYP/6-31g (d, p) level and the side chains were replaced with methyl groups.

18


Page 18 of 25

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Figure

2.

a)

The

X-Ray

diffraction

of

PBTZ-4TC

and

PBTZ-C4T;

Temperature-dependent UV-vis absorption spectra of (b) PBTZ-4TC and (c) PBTZ-C4T solutions (0.02mg/ml in CB); (d) UV-vis absorption spectra of PBTZ-4TC, PBTZ-C4T, and ITIC-Th1 films; (e) UV-vis absorption coefficient of PBTZ-4TC and PBTZ-C4T film; f) the energy level of the PBTZ-4TC, PBTZ-C4T.

19



Page 20 of 25

Table 1. Molecular weights, optical parameters, thermal and physical-chemical properties, frontier energy levels, electron and hole mobilities of the copolymers. Polymer

PDIa

Mn (kDa)a

Td

Film λmax

Film λedg

α

Eg

EHOMO

ELUMO

(℃)b

(nm)c

(nm)d

(104cm-1)e

(eV)f

(eV)g

(eV)g

PBTZ-4TC

14.8

2.05

380

548

648

2.5

1.91

-5.56

-3.68

PBTZ-C4T

38.9

1.97

375

558

652

3.3

1,90

-5.52

-3.65

a)The

result measured by GPC; b) 5% weight-loss temperature measured by TGA; c, d)

absorption of polymer films on quartz plate cast; polymer films; g)calculate

f)

e)

absorption coefficient of the

calculate the optical band gap of the polymer film: Eg = 1240/λedg;

according:

EHOMO/LUMO=-e(Eox/red+4.8)

electrochemically relative to the Fc/Fc+ reference.

20


(eV)

and

determined

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) J-V curves of polymer donors:ITIC-Th1 (1:1) with 0.5% DIO and without additive, under the illumination of AM1.5G, 100 mW/cm2; (b) The EQE spectra of the PSCs with additive or without.

21



Page 22 of 25

Table 2. The best Photovoltaic parameters of the polymer donor:ITIC-Th1 (1:1) under the illumination of AM1.5G, 100 mW/cm2 Vocc

Jscc

donor

(V)

(mA/cm2)

PBTZ-4TCa

0.88±0.004

PBTZ-4TCb PBTZ-C4T a PBTZ-C4Tb

Polymer

a)

FFc

PCEc

µe

µh

(%)

(%)

10−5 cm2v−1s−1

10−5 cm2v−1s−1

14.61±0.22

59.27±0.17

7.68±0.03

2.5

2.1

0.84

0.88±0.004

15.72±0.53

62.62±0.24

8.72±0.03

2.7

2.3

0.85

0.84±0.003

14.70±0.03

65.43±0.13

8.11±0.02

2.9

2.5

0.86

0.84±0.003

16.59±0.11

66.65±0.21

9.34±0.02

3.8

3.4

0.89

µh/µe

As-cast solid film; b) with 0.5% DIO; c) The best performance parameters are shown

with the average value from 18 devices.

22


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Figure 4. PL spectra of (a) PBTZ-4TC, ITIC-Th1 and their blends; (b) PBTZ-C4T, ITIC-Th1 and their blends; (c) TRPL spectra of optimal devices based on PBTZ-4TC: ITIC-Th1 and PBTZ-C4T: ITIC-Th1 and (d) normalized PL spectra of PBTZ-4TC and PBTZ-C4T.

23



Figure 5. AFM and TEM images of the copolymer: ITIC-Th1 blend films; a) PBTZ-4TC: ITIC-Th1 without additive. b) PBTZ-4TC: ITIC-Th1 with 0.5% DIO; c) PBTZ-C4T: ITIC-Th1 without add additive; d) PBTZ-C4T: ITIC-Th1 with 0.5% DIO; e-h) the corresponding phase images; i-l) the corresponding TEM images.

24


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Table of Contents Graphic

25


Double Acceptor Blocks-Containing Copolymers with Deep HOMO

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