Manipulating Polymer Donors Toward a High-Performance Polymer

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Energy, Environmental, and Catalysis Applications

Manipulating polymer donors toward high-performance polymer acceptor based on fused perylenediimide building block with built-in twisting configuration Yuli Yin, Zhi Zheng, Yi Lu, Daoyuan Chen, Ming Liu, Fengyun Guo, Shiyong Gao, Liancheng Zhao, and Yong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07067 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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ACS Applied Materials & Interfaces

Manipulating Polymer Donors toward High-Performance Polymer Acceptor Based on Fused Perylenediimide Building Block with Builtin Twisting Configuration Yuli Yin,a Zhi Zheng,a Yi Lu,a Daoyuan Chen,a Ming Liu,a Fengyun Guo,a Shiyong Gao,a Liancheng Zhao,a Yong Zhanga,b* a

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001,

China b School

of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China

*Corresponding authors (email: [email protected])

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ABSTRACT A novel fused perylenediimide based polymer electron acceptor (PFPDI-BDF) with built-in twisting configuration was constructed for application in all-polymer solar cells. To shed light on the compatibility of FPDI polymer acceptor and to identify suitable polymer donor for device applications, we considered herein to investigate three polymer donors (PBDB-T, PTB7-Th and PCPDTFBT) with different optical, electronic properties, as well as polymer chain packing behavior for comparing the device performance. After fabricated with PFPDI-BDF, polymer donor PBDB-T with the wide band gap showed a decent power conversion efficiency (PCE) of 4.86% with an opencircuit voltage (Voc) of 0.82 V, short-circuit current density (Jsc) of 8.94 mA/cm2 and a recorded fill factor (FF) of 66.3%, which is one of the best FF reported for PDI-based all-polymer solar cells (all-PSCs). The enhanced efficiency of 6.05% was found in the medium band gap polymer PTB7-Th devices due to the more complementary absorption region that makes the photoactive blends absorb more photons, giving rise to an increased Jsc of 12.97 mA/cm2. On the other hand, due to the inferior exciton dissociation/extraction efficiency and unfavorable morphology compatibility, the narrow band gap polymer donor PCPDTFBT/PFPDI-BDF devices exhibited the worst PCE of only 0.71% with a low Jsc of 2.2 mA/cm2 and an FF of 42.4%. KEYWORDS: fused perylenediimide, built-in twisting configuration, compatibility, polymer electron acceptor, power conversion efficiency

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INTRODUCTION Recently, polymeric electron acceptors as the alternative fullerene molecular acceptors have achieved rapid development because of their high mobility, high molar extinction coefficient, tunable absorption and energy levels.1-6 As a result, all-polymer solar cells (all-PSCs), comprised of polymeric donor and electron acceptor, have become one of the research fronts in polymer solar cells because of their advantages on the structure tunability, morphological and mechanical properties, etc., which show a great potential for future large-area production.7-12 However, compared with the success of non-fullerene small molecular alternatives in fullerene-free PSCs, one of the big challenges for non-fullerene polymeric acceptors is the limited availability of efficient polymer acceptors and their unfavorable morphology compatibility with polymer donor.13-15 Up to now, only few polymer acceptors, mainly based on naphthalene diimide (such as N2200),16-18 perylene diimide (such as NDP-V),19-20 and recently developed IDT-typed polymeric acceptors (such as PZ1 and PFBDT-IDTIC),21-23 show the promising power conversion efficiencies (PCEs) over 8% after carefully and timeconsumed

morphological

optimization

through

tuning

polymer/solvent

and

polymer/additive interactions between polymer donor and acceptor. Therefore, designing and developing novel efficient polymer acceptors is highly urgent and challenged to further promote the photovoltaic efficiency in all-PSCs. It is well-known that the trade-off in the self-aggregation and charge transport properties of polymer acceptor is a major obstacle for achieving efficient all-PSCs.24-26 Encouragingly, we recently have developed a series of fused PDI (FPDI)-based 3



polymer acceptors that could provide an alternative choice to address this issue because of the appropriate fixed built-in angel in FPDI unit, which naturally endows a suppressed aggregation behavior and high electron mobility in the resulted polymer acceptors.27-29 For example, we have reported three polymer acceptors (PFPDI-BDT, PPDI-BDT and PNDI-BDT) with FPDI, PDI and NDI building blocks and compared their photovoltaic efficiencies in all-PSCs, which show that the FPDI-based polymer acceptors displayed superior performance than that of the traditional PDI and NDIbased polymer acceptors under the same condition.30 It can find that the built-in twisting structure of FPDI unit plays a significant role in the improvement of photovoltaic performance since the helical structure of FPDI can suppress the strong aggregation of PDI unit, which provide a more favorable packing behavior resulting in a preferred bulk heterojunction morphology with polymer donor.31-33 Little has been done, however, on expanding the family of FPDI-based polymeric electron acceptors for a deeply understanding the relationship of structure and photovoltaic performance, which is significant for enhancing the photovoltaic performance of all-PSCs. On the other hand, the wide range of donor polymers developed in past 20 years provides a rich library for immediate use in all-PSCs, and significant efforts are desired to systematically probe the polymer donor/acceptor compatibility based on FPDI polymer acceptor and available efficient polymer donors, which, however, remains underexplored. In this work, we designed and developed a novel FPDI polymer acceptor (PFPDIBDF) and choose three typical polymer donors (PBDB-T, PTB7-Th and PCPDTFBT) with wide, medium and narrow band-gaps as well as different energy levels (Fig. 1 and 4


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2) to investigate their roles in the photovoltaic performances of all-PSCs. When blended with the polymer acceptor PFPDI-BDF, the medium band-gap polymer donor PTB7Th showed the highest PCE of 6.05% with an impressive Jsc of 12.97 mA/cm2, which was higher than that of wide band-gap polymer donor PBDB-T:PFPDI-BDF-based device (4.86%). More interestingly, an unanticipated low PCE of 0.71% with an unsatisfactory Jsc of 2.2 mA/cm2 and an unfavorable FF of 42.4% were found in the narrow band-gap polymer donor PCPDTFBT:PFPDI-BDF-based device even they have complementary absorption spectra and matched energy levels. The charge generation/extraction, recombination dynamics and film morphology for the three allPSCs were then systematically investigated. RESULTS AND DISCUSSION Synthesis. Polymer Donors: C 4H 9

C 2H 5

C 4H 9

C 2H 5

C 2H 5 S

S

O S



C 4H 9 C 4H 9

O S

C 2H 5

O

O

S S

S

S

C 2H 5

 n

S

S



N

S

C 4H 9

C 4H 9

Wide Band-Gap Polymer

O

C5H11 C5H11 N

O

O

C5H11 N

Narrow Band-Gap Donor

built-in twisting C 4H 9

C 2H 5

C5H11 O

O Br (Br)

C H C5H11 4 9

C5H11 C5H11 N

O

O

N

C5H11

N

O

O

C5H11 C5H11

M1

N

C 2H 5

O O

Pd2(dba)3, P(o-toly)3

O

Sn

O

Sn

O

Toluene

O

O C 2H 5

C5H11

 n

O



O O

C 4H 9

PCPDTFBT

Medium Band-Gap Donor

O

Br

 n

C 2H 5 C 2H 5

PTB7-Th

FPDI-Polymer Acceptor: C5H11

N

F

C 2H 5

PBDB-T

S

S

 n

S C 2H 5 C 4H 9

S



S

S

C 4H 9

F

O

C 4H 9

C5H11

M2

N

O

O

C5H11 C5H11

N

O C5H11

C 2H 5 C 4H 9

PFPDI-BDF

Figure 1 The molecular structures of the polymer donors and the synthetic route of polymer acceptor PFPDI-BDF. 5



The chemical structures of three polymer donors (PBDB-T, PTB7-Th and PCPDTFBT) with different band-gaps, and the synthetic route of polymer acceptor PFPDI-BDF are presented in Fig. 1. Briefly, the target polymer acceptor PFPDI-BDF was obtained by a classical Pd-catalyzed Stille coupling reaction of BDF and FPDI monomers, which were synthesized according to the reported work.29, 34 According to our previous work, it is noted that the built-in dihedral angle (~15o) within FPDI unit plays synergistic role in preventing large undesirable phase aggregation and crystallinity in the photoactive layers, and the large polymer backbone torsion of more than 100o between FPDI and BDF units however contribute a minor role in preventing the stronger tendency of PDI unit.27, 28 The polymer acceptor is characterized by 1H NMR to confirm their structures, and the number-average molecular weight (Mn) of PFPDI-BDF is found to be 21.9 kDa with a polydispersity index of 2.27 (see Supporting Information). With the assistance of the alkyl chains, PFPDI-BDF can be easily dissolved in the chlorinated solvents, including chloroform, chlorobenzene and odichlorobenzene.

Figure 2 UV-vis absorption spectra (a) and energy levels diagram (b) of PFPDI-BDF, PBDB-T, PTB7-Th and PCPDTFBT films. 6


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Optical and Electrochemical Properties. Visible-near-infrared absorption spectra of the polymer acceptor and these polymer donors in solid state are shown in Fig. 2a. PFPDI-BDF shows a strong absorption from 350 to 600 nm with the maximum absorption peak at 402 nm, which is attributed to electron transition from the ground state (π) to the excited state (π*) in FPDI unit. On the other hand, a distinct absorption characteristic peak was observed and located at 550 nm, which is the result of the effective intermolecular interactions between the BDF electron-donor unit and FPDI electron-acceptor unit. The optical band gap of PFPDIBDF was calculated to be 1.70 eV according to the absorption edges (~730 nm). As shown in Fig. 2a, the absorption spectra of polymer donors with variety of band gaps was also provided. In terms of the complementary absorption, the wide-band gap donor PBDB-T displayed a relatively large absorption overlap with the polymer acceptor compared to that of medium band gap donor PTB7-Th, which may result in lower Jsc in all-PSCs. It is worth noting that the PBDB-T-based PSCs easily achieved high Voc and impressive FF because of the low-lying highest occupied molecular orbital (HOMO) and the favorable miscibility in the blend films.35-36 For the narrow band gap donor PCPDTFBT, it shows the best absorption complementary with PFPDI-BDF in the region of 300 to 850 nm, which thus is desirable for enhancing light harvest so as to achieve the increased Jsc of the all-PSCs. Subsequently, the absorption coefficient of three blend film were also measured and found the PBDB-T:PFPDI-BDF blend displayed the highest absorption coefficient of 4.8×104 M-1 cm-1 in the narrowest absorption region. Compared to the PBDB-T-based blend, PTB7-Th- and PCPDTFBT7



based blend film both showed gradually widened absorption spectrum with lower molar extinction coefficient of 3.0×104 M-1 cm-1 at 400 nm, 3.2×104 M-1 cm-1 at 403 nm, respectively, which is also conducive to the generation of photoelectrons from the absorption point of view (Figure S2a). The energy levels of PFPDI-BDF were measured by using cyclic voltammetry (CV), and the energy levels diagram of these materials used in this work are shown in Fig. 2b. According to the onset of reduction potential (Fig. S2b) and the optical band gap of PFPDI-BDF, the lowest unoccupied molecular orbital (LUMO) energy level was found to be -4.07 eV, and the corresponding HOMO energy level was -5.74 eV. Considering the energy levels, there should be more or less energy level offsets between three polymer donors and acceptor, which will result in efficient exciton dissociation and charge transfer.37-38 It can find that both HOMO and LUMO offsets between the polymer donors and that of PFPDI-BDF are higher than 0.4 eV, which indicate that PBDB-T, PTB7-Th and PCPDTFBT are suitable polymer donors to investigate the compatibility of PFPDI-BDF from the energy level point of view. Photovoltaic Properties.

Figure 3 Characteristic J-V curves of PBDB-T:PFPDI-BDF, PTB7-Th:PFPDI-BDF and 8


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PCPDTFBT:PFPDI-BDF-based all-PSCs under AM 1.5 G irradiation (a), and the corresponding EQE curves (b).

To examine their photovoltaic performances, we fabricated all-PSCs with a device structure of ITO/PEDOT:PSS/photoactive layer/PFN-Br/Ag, where PBDB-T, PTB7Th and PCPDTFBT are used as polymer donors, PFPDI-BDF is used as polymer acceptor, and PFN-Br is used as an electron transport layer because it enabled effective electron extraction.39 The detailed optimization procedures are shown in the Supporting Information. In terms of 1-chloronaphthalene (CN) has been demonstrated in the FPDI-based polymer acceptors/PTB7-Th system and could significantly enhance the performance of the resulted devices,27-30 different volume fraction CN-chlorobenzene solution was investigated, and found that 1% CN treated device shows the best photovoltaic performance of 5.37%, with a Voc of 0.76 V, Jsc of 12.01 mA/cm2, and an FF of 58.8% (Tables S1). Moreover, the photovoltaic performance of PTB7-Th:PFPDI-BDF-based devices can be further improved by thermal annealing (TA) at 130 oC for 10 min (Table S2). As shown in Fig. 3a, the best PCE of 6.05% was achieved by PTB7-Th-based blend film (114 nm) with the enhanced Jsc (12.01 mA/cm2) and impressive FF of 61.4% (Table 1). On the other hand, for the PBDB-T/and PCPDTFBT-based all-PSCs devices, different solvent additives (1% CN, 3% CN, 1% DIO and 3% DIO) were processed to optimize the morphologies of the active layers. It can be found that the PCEs of PBDBT/and PCPDTFBT-based devices deteriorate with DIO treatment (1%−3%), whereas highest performances were observed when selecting 1% CN as the solvent additive 9



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(Table S3). In addition, the photovoltaic parameters of these devices with TA-treatment (130 oC) at different donor/acceptor ratio were provided in Table S4 (Supporting Information), and the optimized weight ratio is 1:1. Under optimal conditions, all-PSCs based on PBDB-T:PFPDI-BDF photoactive layer (110 nm) demonstrated a high PCE of 4.86% with an enhanced Voc of 0.82 V, Jsc of 8.94 mA/cm2, and an outstanding FF of 66.3%, which is among the highest FF of PDI-based polymer acceptors reported so far. However, it is unexpected that PCPDTFBT:PFPDI-BDF-based device with the optimized thickness of 112 nm only showed a PCE of 0.77% with a Voc of 0.78 V, a low Jsc of 2.27 mA/cm2, and an unfavorable FF of 43.2%, even they showed the good match on absorption and energy levels. It can be found that the built-in twisting polymer acceptor (PFPDI-BDF) displayed good compatibility with PBDB-T and PTB7-Th, while poor matching can still be observed in the PCPDTFBT:PFPDI-BDF device, probably due to the unfavorable dissociation efficiency and morphology compatibility/or called miscibility problems that are often encountered in the all-PSCs devices. Table 1 The photovoltaic performances of all-PSCs based on PBDB-T, PTB7-Th and PCPDTFBT as polymer donors and PFPDI-BDF as polymer acceptor. Voc

Jsc

FF

PCE

µh

µe

(V)

(mA/cm2)a

(%)

(%)b

(cm2/(V s))

(cm2/(V s))

PBDB-T

0.82

8.94 (8.50)a

66.3

4.86 (4.82)b

9.5×10-5

5.19×10-5

1.83

PTB7-Th

0.76

12.97 (12.6)a

61.4

6.05 (5.97)b

5.58×10-5

1.70×10-5

3.28

PCPDTFBT

0.76

2.2 (2.2)a

42.4

0.71 (0.70)b

1.24×10-4

1.47×10-5

8.44

Donor

a

µh/µe

The values in parentheses are calculated from EQE. b The average values were obtained from over 10 cells.

The photoresponses of these all-PSCs devices were measured using external 10


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quantum efficiency (EQE) to understand the observed Jsc in three all-PSCs. As expected, the device of wide band-gap polymer donor PBDB-T with PFPDI-BDF showed the narrowest response in the region of 300-650 nm with the maximum values of ∼53% at 620 nm (the calculated Jsc of 8.5 mA/cm2), which is corresponded to their absorption overlap as discussed above. As shown in Fig. 3b, the EQE curves of PTB7-Th:PFPDIBDF device displayed a broad and strong response in the wavelength region from 300 to 750 nm, and the calculated Jsc from the integration of EQE spectra is 12.6 mA/cm2 , which is well in line with our previous results. More interestingly, the device of PCPDTFBT:PFPDI-BDF showed the widest photoresponse ranging from 300 to 850 nm, but the extremely low EQE value (below 10%) over this range were observed with the calculated Jsc of 2.2 mA/cm2, which should be attributed to the poor miscibility and compatibility between polymer donor (PCPDTFBT) and PFPDI-BDF. Moreover, contributions from both the polymeric donors and acceptor can be seen in EQE spectra, which closely track the blend absorption curve (Fig. S2a), suggesting that the significant differences in EQE and Jsc are due to changes in the efficiency of charge separation or charge collection.

Figure 4 Photocurrent density (Jph) vs effective voltage (Veff) characteristics (a), and short-circuit 11



current density vs the light density (b) of three all-PSCs devices.

To deep understand and elucidate the reasons for the large variations of photovoltaic performances, the exciton dissociation efficiency, charge carrier recombination, and film morphologies of these all-PSCs were investigated. The dependences of the photocurrent (Jph) on the effective applied voltage (Veff) were performed to understand the charge generation and extraction. It can find that the saturation currents (Jsat) of PBDB-T:PFPDI-BDF-, PTB7-Th:PFPDI-BDF- and PCPDTFBT:PFPDI-BDF-based devices are 8.69, 13.6 and 5.59 mA/cm2, respectively, which is defined as the saturation photocurrent density at larger Veff (typically above 2 V). Generally, the exciton dissociation and charge collection efficiency is proportional to Jph/Jsat ratio, PBDB-Tand PTB7-Th-based devices exhibited the exciton dissociation probabilities of 87.8%, whereas PCPDTFBT:PFPDI-BDF-based device only showed a Jph/Jsat ratio of 40.8%, which results suggest that there are better exciton dissociation and charge extraction efficiencies in PBDB-T- and PTB7-Th-based devices. On the other hand, the maximum exciton generation rate (Gmax), which is more related to photon absorption and excitons generation, can be calculated according to the method reported in the literature,40 and found the device of PTB7-Th:PFPDI-BDF showed the largest Gmax value of 7.73×1027 m-3 s-1 relative to that of the PBDB-T-based devices (4.76×1027 m-3 s-1), which indicates the stronger light absorption efficiency and consistence with the absorption profile. More interestingly, PCPDTFBT:PFPDI-BDF blends exhibited the lowest-efficiency photon capturing in the devices with the poor Gmax value of 3.12×1027 m-3 s-1, which is contrary to its wide and strong absorption characteristics (Fig. 2a), primarily depends 12


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on morphological factors such as exaggerated domain size and undesirable phase separation. In addition, the curve of PCPDTFBT:PFPDI-BDF-based device did not reach saturation even with the applied voltage of exceeding 4V (Fig. 4a), that means very few excitons in the photoactive layer can be successfully dissociated into free carriers and collected at the corresponding electrodes, which may be one of the reasons for its measured low current density and fill factor. To investigate the carrier recombination mechanism, we further measured Jsc values of each all-PSCs under different illumination intensities (Plight), and the relationship between Jsc and Plight could be expressed as a power-law dependence of Jsc ∝ Plightα, where the slope (α) is equal to 1 indicates that there is no bimolecular recombination.41 As shown in Fig. 4b, the α of PBDB-T:PFPDI-BDF, PTB7-Th:PFPDI-BDF devices were 0.986 and 0.961, respectively, while PCPDTFBT:PFPDI-BDF device showed a relatively lower α value of 0.84, which indicates that there are significant bimolecular recombination in PCPDTFBT:PFPDI-BDF device(Fig. 4b). In addition, the slope of Voc against ln (Plight) helps us to estimate the degree of monomolecular/ or trap-assisted recombination in the devices. Generally, a slope of 2.0 kBT/q is observed in the case of trap-assisted recombination, while the slope value would be 1.0 kBT/q when the dominating recombination becomes bimolecular recombination. In our case, the slope values for PBDB-T:PFPDI-BDF, PTB7-Th:PFPDI-BDF devices were equal to 1.20 kBT/q and 1.29 kBT/q, respectively, which indicates that bimolecular recombination plays dominant role in the charge recombination process (Figure S3). The higher slope (1.69 kBT/q) of the PCPDTFBT-based all-PSCs device illustrates that stronger trap13



assisted recombination was occurred, which may be due to the suboptimal morphology in the PCPDTFBT:PFPDI-BDF blend film.

Figure 5 Characteristic J-V curves in the space charge limited region for the hole-only (a) and the electron-only (b) devices.

The charge transport properties were investigated by using the space-charge-limited current (SCLC) measurement, and the characteristic curves are plotted in Fig. 5. The devices of PBDB-T:PFPDI-BDF showed a hole mobility of 9.5 × 10-5 cm2/(V s) and an electron mobility of 5.19 × 10-5 cm2/(V s). The hole and electron mobilities of PTB7Th:PFPDI-BDF blend were 5.58 × 10-5 cm2/(V s) and 1.7 × 10-5 cm2/(V s), respectively. In the case of PCPDTFBT:PFPDI-BDF, it can find the hole mobility was as high as of 1.24 × 10-4 cm2/(V s), while the electron mobility of 1.47 × 10-5 cm2/(V s) was obtained. It is well known that the higher and more balanced hole and electron mobilities are necessary for the higher current density and better fill factor.42-43 From these results, it can see that PBDB-T and PTB7-Th-based devices exhibit the more balanced charge transport (µh/µe=1.83 and 3.28, respectively) than that of PCPDTFBT device (µh/µe=8.44), which then can partly explain the observed lower Jsc and FF in PCPDTFBT:PFPDI-BDF device. 14


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Surface Morphologies.

Figure 6 The AFM height and phase images of PBDB-T:PFPDI-BDF (a and d), PTB7-Th:PFPDIBDF (b and e) and PCPDTFBT:PFPDI-BDF (c and f) films (the inset is the 3D height image).

Besides the charge carrier mobility, the surface morphology of photoactive layer also plays significant role on device performances. The film morphologies of these blends were investigated using atomic force microscopy (AFM). As shows in Fig. 6, PBDB-T and PTB7-Th-based blends showed smooth fiber-like nanostructure and fine phaseseparated domains, whereas large fibrillary aggregates can be observed in PCPDTFBT:PFPDI-BDF film. The root-mean-square (RMS) roughness were found to be 4.3 nm for PBDB-T:PFPDI-BDF and 1.2 nm for PTB7-Th:PFPDI-BDF (Fig. 6a and 6b), which are exhibit more homogeneous phase separation promoting exciton dissociations and charge transfer. In contrast, PCPDTFBT:PFPDI-BDF-based blend film showed large size aggregations and undesirable phase separation (Fig. 6f), the RMS was calculated to be 13.7 nm, resulting a significant restriction on the efficient exciton diffusion and charge separation process and thus leading to a unfavorable Jsc 15



and FF. From a morphological point of view, the good miscibility or morphology compatibility between polymer donor and polymer acceptor, which is critical to obtain optimal nanoscale phase separation or efficient charge transport in the blend, and is the key to obtain a high FF, high Jsc, and the overall PCEs in the all-PSCs devices.44-45 In addition, from the point view of molecular structures, the polymer chain packing behaviors and/or the crystallization of polymer donors as well as their interactions with polymer acceptor may play the important role to achieve the different morphologies, even though there are very prominent match on the absorption (band-gap) and energy levels between polymer donor and acceptor in all-PSCs. CONCLUSION In conclusion, we have synthesized a polymer acceptor PFPDI-BDF based on FPDI unit which has a built-in twisting configuration. Its photovoltaic property in all-PSCs as polymer electron acceptor were investigated by using three polymer donors (PBDBT, PTB7-Th and PCPDTFBT) with different optical and electronic properties to elucidate the importance of polymer compatibility. As a result of the more balanced charge transport mobility and the favorable morphology compatibility, the wide band gap polymer donor (PBDB-T) showed a PCE of 4.86% with a Jsc of 8.94 mA/cm2 and an impressive FF of 66.3%, which is almost the highest FF value reported for PDIbased polymer acceptors to date. As expected, the increased PCE of 6.05% with an enhanced Jsc of 12.97 mA/cm2 could be obtained when using a medium band gap polymer PTB7-Th as donor in conventional device architectures. On the contrary, when using the narrow polymer donor PCPDTFBT as polymer donor, the devices showed the 16


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worst performance of 0.71% with a low Jsc of 2.27 mA/cm2, and an FF of 43.2%, which can be attributed to the poor exciton dissociation probabilities (Jph/Jsat=40.8%), stronger bimolecular recombination (α=0.84), unbalanced charge transport mobility (µh/µe=8.44) and undesirable morphology compatibility (RMS=13.7 nm) in the devices. The significant differences in photovoltaic performances between PBDB-T-, PTB7-Th- and PCPDTFBT-based all-PSCs highlight the importance of identifying suitable polymer donor in the design of efficient FPDI-based all-PSCs. EXPERIMENTAL SECTION Materials. Monomers 1, and 2 were prepared by our previous work.27,

34

Polymer

PBDB-T (Mw: 38000, polydispersity index: 2.5) and PTB7-Th (Mw: 47000, polydispersity index: 2.0) were purchased from Derthon Optoelectronic Materials Science Technology Co LTD., and polymer PCPDTFBT (Mw: 23400, polydispersity index: 1.54) was obtained from our previously synthesized literature.46 All the materials were used as received without purification. Synthesis of PFPDI-BDF. The polymer acceptor (PFPDI-BDF) were synthesized according to a classical Pd-catalyzed Stille coupling reaction. Firstly, M1 (200 mg, 0.126 mmol) and M2 (105.9 mg, 0.126 mmol) were successively added into degassed toluene (10 mL) under argon protection, and resulting mixture was then degassed twice. Then the catalyst, Pd2(dba)3 (5.0 mg) and P(o-tol)3 (10.0 mg), were quickly added to the flask, followed by heating the solution at 110 oC for 72 hours. Subsequently, an excess of 2-(tributylstannyl)thiophene (0.2 mL) was added to the mixture. After 12 hours, 2-bromothiophene (0.5 mL) was added and kept at 110 °C for 12 hours. Then, the reaction was poured into methanol, and the black solid was filtered and further 17



precipitated in acetone and hexane. Finally, the final product (PFPDI-BDF) was collected and dried under vacuum for 12 hours (194 mg, 80%). 1H NMR (500 MHz, CDCl3) δ 10.45 (s, 4H), 9.58 (s, 2H), 9.01 – 8.26 (m, 6H), 6.17 (d, J = 126.5 Hz, 4H), 5.36 (dd, J = 37.9, 23.3 Hz, 4H), 2.85 (s, 4H), 2.43 (s, 8H), 1.94 (d, J = 69.9 Hz, 10H), 1.66 – 1.18 (m, 62H), 1.13 – 0.61 (m, 36H). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:*******. Experimental details on characterization, spectroscopic details, and additional figures or tables.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Yong Zhang). ORCID Yuli Yin: 0000-0002-8179-4185 Yong Zhang: 0000-0002-9587-4039 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge the support from National Natural Science Foundation of China (21644006 and 51403044) and Natural Science Foundation of Heilongjiang Province of China (E2018036). Y. Zhang thanks the support from the Fundamental 18


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Manipulating Polymer Donors Toward a High-Performance Polymer

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