High-Performance All-Polymer Solar Cells Achieved by Fused

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High Performance All-Polymer Solar Cells Achieved by Fused Perylenediimide-Based Conjugated Polymer Acceptors Yuli Yin, Jing Yang, Fengyun Guo, Erjun Zhou, Liancheng Zhao, and Yong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03603 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Materials & Interfaces

High Performance All-Polymer Solar Cells Achieved by Fused Perylenediimide-Based Conjugated Polymer Acceptors Yuli Yin,1, # Jing Yang,2# Fengyun Guo,1 Erjun Zhou,2* Liancheng Zhao1, Yong Zhang1* 1

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

China 2

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in

Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China #Both authors contributed equally to this work.

KEYWORDS: fused perylene diimide, polymeric electron acceptors, all-polymer solar cells, power conversion efficiency, conjugated polymer

ABSTRACT We report three n-type polymeric electron acceptors (PFPDI-TT, PFPDI-T and PFPDI-Se) based on the fused perylene diimide (FPDI) and thieno[3,2-b]thiophene, thiophene or selenophene units for all-polymer solar cells (all-PSCs). These FPDI-based polymer acceptors exhibit the strong absorption between 350 nm and 650 nm with the wide optical badgaps of 1.86~1.91 eV, showing good absorption compensation with the narrow bangap polymer donor. The lowest unoccupied molecular orbital (LUMO) energy levels were located at around -4.11 eV, which are comparable with the fullerene derivatives and other small molecular electron acceptors. The conventional all-PSCs based on the three polymer acceptors and PTB7-Th as polymer donor gave the remarkable power conversion efficiencies (PCEs) 1

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of over 6%, and the PFPDI-Se-based all-PSC achieved the highest PCE of 6.58 % with a short-circuit current density (Jsc) of 13.96 mA/cm2, an open-circuit voltage (Voc) of 0.76 V, and a fill factor (FF) of 62.0%. More interestingly, our results indicate that the photovoltaic performances of the FPDI-based polymer acceptors are mainly determined by the FPDI unit with the small effect from the comonomers, which is quite different from the others reported rylene diimide-based polymer acceptors. This intriguing phenomenon is speculated as the huge geometry configuration of FPDI unit, which minimizes the effect of the comonmer. These results highlight a promising future for the application of the FPDI-based polymer acceptors in the high efficient all-PSCs.

INTRODUCTION In recent years, all-polymer solar cells (all-PSCs), which compose of a p-type conjugated polymer as the donor and an n-type conjugated polymer as the electron acceptor, have been one of the most intensively investigated optoelectronic devices in the academic communities because the use of a polymer electron acceptor in polymer solar cell (PSC) can confer multiple potential benefits over fullerene or small molecular electron acceptor, including high absorption coefficients, better morphological stability and superior mechanical properties.1-7 With the efforts of the researchers, all-PSCs have been developed rapidly in the past years,8-14 for example, a remarkable PCE of 10.1% has been recently reported with a wide-bandgap polymer donor (PTzBI-Si) and the polymer acceptor (N2200).15 Nevertheless, the current 2


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device performances of all-PSCs are still lagged far behind the state-of-the-art polymer:fullerene or polymer:non-fullerene small molecular acceptor based PSCs.6, 16 One of the main reasons is due to the lack of suitable n-type conjugated polymer acceptors that match well with the p-type conjugated polymer donors (i. e. PTB7-Th).17 Currently, only few polymer electron acceptors, such as N2200,18-21 PNDT-T10,22 NDP-V,23 and PZ1,24 etc, could achieve the remarkable photovoltaic performance in all-PSCs due to the challenges that ideal polymer acceptors should simultaneously possess the properties of strong light harvest abilities, proper energy levels as well as high electron affinity and high electron mobility.25-27 To further improve the photovoltaic performance of all-PSCs, the appropriated strategy is to design and synthesize the novel n-type conjugated polymer acceptors with these aforementioned essential properties. Recently, perylenediimide (PDI)-based polymers have received much attention for the promising potentials in various optoelectronic applications because of their advantages such as high molar absorption efficiency, superior electron mobility, appropriate molecular energy and easy chemical modiﬁcation.28-32 It is well acknowledged that the extended aromatic units with the large conjugated coplanar area are very important in determining the molecular orientations and resulting good performance in optoelectronic device.33-37 Recently, we introduced the fused perylenediimide (FPDI) unit into the polymer acceptors and synthesized a series of donor-acceptor (D-A) and acceptor-acceptor (A-A) typed FPDI-based polymer acceptors with bithiophene and benzothiadiazole units as the comonomers, 3



respectively. The all-PSCs under conventional device structures with PTB7-Th as the polymer donor obtained the remarkable PCE of up to 6.39%, which is significantly higher than that of the PDI-based counterparts under same conditions.38-39 Zhao and Yan et al. also synthesized the FPDI and vinylene-based polymer acceptor and achieved an impressive PCE of up to 8.59% in the inverted all-PSCs.23 It has indicated the FPDI unit is a very promising building block in constructing the high efficient polymer acceptors. Compared to the richness of other type polymer acceptors, however, the family of FPDI-based polymeric acceptors are highly needed to expand and the understanding of its structure-performance relationship, which is of great significance for improving the efficiency of all-PSCs, is also extraordinary desirable. It is believed that the further developments of FPDI-based polymer electron acceptors through the synergy of molecular engineering strategy will promote the photovoltaic performance of all-PSCs significantly. Herein, we design and synthesize three new FPDI-based polymeric acceptors (PFPDI-TT, PFPDI-T and PFPDI-Se) with the various electron-donating units (thieno[3,2-b]thiophene, thiophene and selenophene). Considering their analogical molecular configurations, these polymeric acceptors displayed the similar absorption covering from 300 to 650 nm, which possess the good absorption compensation with the polymer donor PTB7-Th in the long-wavelength region. Similarly, the lowest unoccupied molecular orbital (LUMO) energy levels of PFPDI-TT, PFPDI-T and PFPDI-Se were found to be around -4.11 eV, which is comparable with fullerene derivatives. The photovoltaic performances of these polymer acceptors in all-PSCs 4


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were investigated. When combined with the widely used donor material PTB7-Th, all-PSCs with PFPDI-TT, PFPDI-T or PFPDI-Se as the electron acceptors exhibited the promising PCEs of over 6%, and the PFPDI-Se-based all-PSC showed the best performance with the remarkable PCE of 6.58%, a high Jsc of 13.96 mA/cm2 and an FF of 62.0%, indicating that the FPDI-based polymer acceptor is significantly superior to the monomeric PDI-based counterpart, and showing a great potential as a promising acceptor in all-PSCs. More interestingly, it is also found that the photovoltaic performances of these FPDI-based polymer acceptors are mainly manipulated by the FPDI unit with the small effect of the comonomers, which is quite different from the widely known rylene diimide systems, such as PDI,40 NDI,6, 8, 41 NDTI42 or DTCDI43 systems. RESULTS AND DISCUSSION Synthesis.

Scheme 1. The synthetic routes of polymer PFPDI-TT, PFPDI-T and PFPDI-Se. 5



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The synthetic routes of PFPDI-TT, PFPDI-T and PFPDI-Se were shown in Scheme 1. The

fused

PDI

monomer

dimethylstannylthieno[3,2-b']thiophene

(FPDI-Br), (1),

the

donor

dimethylstannylthiophene

units

of

(2)

and

dimethylstannylselenophene (3) were synthesized according to the reported procedures.23,

39, 44-45

The polymer electron acceptors PFPDI-TT, PFPDI-T and

PFPDI-Se were synthesized by Pd-catalyzed Stille polymerization of FPDI-Br with compounds 1, 2 and 3, respectively, in the yields of 70-80% after the standard purifications. The molecular weights of the polymers were measured by the high temperature gel permeation chromatography (GPC) with 1,3,5-trichlorobenzene at 150 oC as the eluent. The number-average molecular weights (Mns) were found to be 20.1, 22.8 and 20.5 kDa for PFPDI-TT, PFPDI-T and PFPDI-Se, respectively, with the polydispersity index of 2.23, 2.10 and 3.46. These polymers can be easy to be dissolved into the chlorinated solvents such as chloroform, chlorobenzene and o-dichlorobenzene with the assistant of the alkyl chains on the repeat unit. In addition, the satisfactory molecular weight is desirable because it will help to improve the film-forming ability and thus potentially benefit their photovoltaic performance. The thermal stabilities of these polymers were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere. These polymer electron acceptors all displayed a decomposition temperature (Td, 5% weight loss) exceeding 380 °C (Fig. S1a), and also have no apparent glass transition observed (Fig. S1b). Optical and Electrochemical Properties. 6


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Figure 1. The absorption spectra of polymer acceptors PFPDI-TT, PFPDI-T and PFPDI-Se in solution (a) and thin film states (b). The absorption spectra of PTB7-Th were also shown for comparisons.

Fig. 1 shows the absorption spectra of PFPDI-TT, PFPDI-T and PFPDI-Se in the diluted chlorobenzene solution and thin film states. The corresponding optical data were listed in Table 1. It can find that PFPDI-TT, PFPDI-T and PFPDI-Se exhibit similar absorption features in either solution or film state with the absorption covering from 300 nm to 650 nm, which are resulted from the similar polymer backbones in the three polymers. It can also see that the absorption of three polymers possess the good absorption compensation with the polymer donor PTB7-Th in the long-wavelength region (Fig. 1), that will benefit for the higher current density in all-PSCs by covering and absorbing more sunlight. As shown in Fig. 1a, there are two main absorption bands in the absorption spectra of PFPDI-TT, PFPDI-T and PFPDI-Se with the molar coefficient of up to 1.0 × 105 M-1 cm-1 (Fig. 1a). The higher energy absorption band with the peak of ~400 nm comes from the characteristic absorption of FPDI, which is the result of electron transition from the ground state (π) of sub-PDI unit to the excited state (π*) of the bridging C=C bonds in FPDI.35,

39

The absorption band

between 450 nm and 650 nm is believed to be contributed by the π-π* transitions of the FPDI moiety with the peak of 550 nm and the intramolecular charge-transfer (ICT) 7



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transition band between FPDI and the donor unit of thieno[3,2-b']thiophene, thiophene and selenophene, in which the ICT band is peaked at ~600 nm and is also very weak due to the limited long-range interactions since FPDI moiety possesses the large and torsion configuration.23,

38-39, 46

In thin film states, the three polymer

acceptors exhibited the identical absorption features compared to that in solution. The small differences in the absorption spectra of solution and film states may imply the strong intermolecular aggregations even in solution states for the three polymers. The absorption edges in thin films are found at 667 nm, 650 nm and 650 nm for PFPDI-TT, PFPDI-T and PFPDI-Se, corresponding to the optical band gaps of 1.86 eV, 1.91 eV and 1.91 eV, respectively. From these results, it can find that the absorption of PFPDI-TT, PFPDI-T and PFPDI-Se in either solution or film states are dominantly contributed from the FPDI moiety, which imply that the donor unit, such as thieno[3,2-b']thiophene, thiophene or selenophene, plays a minor role in determining the absorption of the resulted polymer unless there is strong intramolecular charge-transfer interactions with FPDI unit.23, 38-39 Table 1. Properties of PFPDI-TT, PFPDI-T, PFPDI-Se. Mn

Polydispersity

(KDa)

index

PFPDI-TT

20.1

PFPDI-T

22.8

PFPDI-Se

20.5

3.46

Polymer

a

λabs (nm)

λonseta

Egopt

LUMO

HOMO

(nm)

(eV)

(eV)a

(eV)b

Solution

Film

2.23

401/548

400/544

667

1.86

-4.11

-5.97

2.10

401/548

400/544

650

1.91

-4.11

-6.02

401/548

400/544

650

1.91

-4.12

-6.03

b

Measuredd from cyclic voltammetry. Calculated from the LUMO and band-gap.

8


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Figure 2. The cyclic voltammetry of PFPDI-TT, PFPDI-T and PFPDI-Se films (a) and the energy levels of the materials used in all-PSCs (b).

The electrochemical properties of these polymer acceptors were measured by using cyclic voltammetry (CV) in acetonitrile, and the CV curves and energy levels of the three polymer acceptors were shown in Fig. 2 and Table 1. As shown in Fig. 2a, it can see that PFPDI-TT, PFPDI-T and PFPDI-Se exhibit the reversible reduction characteristic peaks with the corresponding potentials at -0.33 V, -0.33 V and -0.32 V versus Fc/Fc+, implying strong electron-accepting abilities (Fig. 2a). The LUMO energy levels of PFPDI-TT, PFPDI-T and PFPDI-Se were calculated to be -4.11 eV, -4.11 eV and -4.12 eV, respectively, which are comparable with other FPDI polymers and PCBM derivatives.23, 38, 47 The HOMO energy levels, calculated from the LUMO energy levels and the optical band-gaps, were found at -5.97 eV, -6.02 eV and -6.03 eV, respectively (Fig. 2b). The very similar HOMO and LUMO energy levels for the three polymers are due to their very similar structures. It is known that that the LUMO energy level in the donor-acceptor polymer is mainly dominant by the acceptor unit, which is the FPDI moiety in this case, and the HOMO energy level is determined mainly by the donor unit. Therefore, it can understand the reason for the slighter 9



high-lying HOMO energy level of PFPDI-TT in comparison with that of PFPDI-T and PFPDI-Se. Fig. 2b shows the diagram of the three polymer acceptors and the polymer donor PTB7-Th used in all-PSCs. It can find that the differences of LUMO energy levels between PTB7-Th and the polymer acceptors are ~0.48-0.49 eV, where the offsets of HOMO energy levels are ~0.77-0.83 eV. The large offsets indicate that there will be enough driving forces for the efficient exciton dissociations in all-PSCs based on PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se. Density Functional Theory calculations.

Figure 3. DFT optimized molecular geometries and frontier molecular orbitals of PFPDI-TT (a), PFPDI-T (b) and PFPDI-Se (c).

To further understand the configurational and electronic properties of these polymer acceptors, density functional theory (DFT) calculations using the Gaussian package B3LYP/3-61G* were performed to evaluate the geometries and frontier molecular orbitals of the dimer of PFPDI-TT, PFPDI-T and PFPDI-Se. In order to reduce the calculation time, the bulky alkyl chains were replaced with the methyl group, and the 10


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simplifications don't affect the results. Fig. 3 shows the optimized molecular geometries and orbitals of the three polymers. It can find that the three polymer acceptors exhibited similar backbone geometries and orbital topologies as a result of their similar chemical structures. As shown in Fig. 3, the three-dimensional molecular geometries of PFPDI-TT, PFPDI-T and PFPDI-Se could be found due to the large torsion angel in both FPDI moiety and FPDI-donor segments. The dihedral angles between FPDI moiety and thieno[3,2-b]thiophene, thiophene and selenophene units were calculated to be 119°, 117° and 111°, respectively, resulting in the larger twisted geometry for the three FPDI-based polymer acceptors. In addition, Fig. 3 also displays the frontier orbital energies of these polymer acceptors and the calculated results show that the LUMO energy levels are exclusively delocalized on FPDI moiety and have very small orbital overlap with the donor unit, which therefore can understand the similar LUMO energy levels from the CV measurements. However, the HOMO energy levels of three polymer acceptors were mainly localized at thieno[3,2-b']thiophene, thiophene and selenophene units, but also extended to the FPDI moiety, which means that there are certain degrees of intramolecular charge interactions in the ground states of FPDI moiety and the donor unit (Fig. 3). This also can explain the slight difference on the HOMO energy levels of the three polymers. The calculated HOMO and LUMO energy levels for PFPDI-TT, PFPDI-T and PFPDI-Se are at -5.85/-3.66 eV, -5.92/-3.66 eV and -5.95/-3.64 eV, respectively, which are correlated well with the electrochemical measurements. Photovoltaic Properties. 11



Figure 4. The J-V curves (a) and EQE curves (b) of PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se based all-PSCs.

To investigate the photovoltaic properties of these polymer acceptors, all-polymer solar cell with FPDI-based polymers as electron acceptors were fabricated under the conventional configuration of ITO/PEDOT:PSS/PTB7-Th:polymer acceptor/Ca/Al, in which polymer PTB7-Th was selected as the electron donor since its good absorption complementary with the three polymer acceptors (Fig. 1). The processing solvent for the all-PSCs is chlorobenzene with a total concentration of 15 mg/mL. The detailed optimization procedures and the photovoltaic parameters were summarized in the Supporting Information.

12


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Initially, the different weight ratios of PTB7-Th:PFPDI-TT (PFPDI-T or PFPDI-Se) were optimized and it found that the best weight ratio of PTB7-Th:polymer acceptor was 1:1, which is same with our previous reports with PTB7-Th as the electron donor in all-PSCs.38-39 Further optimization on the thermal annealing of the active layer revealed that the best condition was at 120 oC for 10 minutes. Under such conditions, the PTB7-Th:PFPDI-TT-based all-PSC give a PCE of 5.01% with a Voc of 0.77 V, a Jsc of 9.32 mA/cm2 and an FF of 59%, while a significant improvement on the photovoltaic performance of PTB7-Th:PFPDI-T was observed with a PCE of 5.82%, a Voc of 0.75 V, a Jsc of 11.24 mA/cm2 and an FF of 58%. For PTB7-Th:PFPDI-Se based all-PSC, the best PCE was found to be 6.07% with a Voc of 0.75 V, a Jsc of 11.45 mA/cm2 and an FF of 59% (see Supporting information). Further optimizations on the morphologies of the active layers in all-PSCs based on the three polymer acceptors were performed by applying the various solvent additives.48-49 The results found that the photovoltaic performances of PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se devices are not sensitive to the solvent additives of 1,8-diiodooctane (DIO) and diphenyl ether (DPE), and the PCEs only show the similar or slighter improvement compared to that without the addition of solvent additives. However, while selecting 1-chloronaphthalene (CN) as the solvent additive, the photovoltaic performances exhibit large changes with the amount of CN used. It was found that the photovoltaic performances become deteriorate at small amount of CN (0.5%-1%), whereas the enhanced performances were observed when the ratios of CN were increased to 3-5% compared to that without CN. The best ratio of CN to 13



chlorobenzene was at 5% for all three devices, further increasing the amount of CN will decrease the performances. Fig. 4a shows the optimized current density-voltage (J-V) curves of PFPDI-TT, PFPDI-T and PFPDI-Se based all-PSCs under the best conditions and the device parameters are summarized in Table 2. It can find that the PCE of PTB7-Th:PFPDI-TT based active layer (108 nm) display an improved PCE of 6.28% with 0.77 V, a Jsc of 12.74 mA/cm2 and an FF of 64% compared to the PCE of 4.88% without CN as the solvent additive. Similarly, the PCE of PTB7-Th:PFPDI-T active layer (107 nm) was also improved to 6.49% with a Voc of 0.76 V, a Jsc of 14.00 mA/cm2 and an FF of 61%, while the all-PSC based on PTB7-Th:PFPDI-Se active layer (106 nm) shows the highest PCE of 6.58% with a Voc of 0.76 V, a Jsc of 13.96 mA/cm2 and an FF of 62% (Table 2). It is found that the significant improved PCEs with CN as the solvent additive compared to that without solvent additive for all three all-PSCs are mainly due to the increased Jsc and FF, which are believed to be the improvements on the desirable morphologies of the active layers with solvent additive.50-51 In Table 2, we also listed the photovoltaic performances of PDI-based polymer acceptors PPDI-TT, PPDI-T and PPDI-Se with thieno[3,2-b']thiophene, thiophene and selenophene as the comonomers, respectively, in inverted all-PSCs. As it can be seen from Table 2, the optimized PCEs for PTB7-Th:PPDI-TT, PTB7-Th:PPDI-T and PTB7-Th:PPDI-Se were only 2.50%, 5.13% and 3.72%, which are far below the FPDI-based counterparts.40 The significant improvements on the photovoltaic performances are mainly attributed to the torsion configuration of FPDI unit, which forms a helical structure and can prevent the strong aggregation of PDI 14


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unit with a more favorable molecular packing behavior and morphology while blending with polymer donor. In addition, from these results and also our previous reported all-PSCs based on FPDI-polymer acceptors, we can observe that FPDI-based polymeric acceptors regardless the comonomer units all displayed the promising photovoltaic performances with PCE of over 6% in all-PSCs, which are much higher than that of the monomeric PDI-based devices.13, 38-39, 52 Table 2. The photovoltaic performances of all-PSCs based on PFPDI-TT, PFPDI-T and PFPDI-Se as the electron acceptors. Active layer

Voc

Jsc

FF 2

PCE

µe

µh

2

2

(cm /(V s))

(cm /(V s))

Reference

6.28 (6.05)

4.92×10-5

2.22×10-4

This work

61

6.49 (6.19)

1.68×10-5

2.13×10-4

This work

13.96

62

6.58 (6.36)

3.68×10-5

2.42×10-4

This work

PTB7-Th:PPDI-TTb 0.80

7.07

43

2.50 (2.45)

-

-

Ref. 40

PTB7-Th:PPDI-Tb

0.79

12.35

52

5.13 (5.13)

-

-

Ref. 40

PTB7-Th:PPDI-Seb 0.77

10.28

46

3.72 (3.66)

-

-

Ref. 40

(mA/cm )

(%)

(%)

PTB7-Th:PFPDI-TTa 0.77

12.74

64

PTB7-Th:PFPDI-Ta 0.76

14.00

PTB7-Th:PFPDI-Sea 0.76

a

c

(V)

Under conventional device structures. bUnder inverted device structures. cThe average values were obtained from

over 10 cells.

The photoresponses of all-PSCs based on PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se under optimal conditions were investigated by the external quantum efficiency (EQE) covering the wavelength range from 300 to 800 nm. The calculated Jsc values from the integration of EQE spectra for PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se devices are 12.64 mA/cm2, 13.47 mA/cm2 and 13.58 mA/cm2, respectively, which are in line well with the measured Jsc within 5% deviation (Fig. 4b). As shown in Fig. 4b, the EQE curves of the three 15



all-PSCs all display a broad response in the entire visible light region of 300 nm to 800 nm. The short wavelength photoresponses in the range of 300-550 nm are mainly attributed to the polymer acceptors and the photoresponses in the long-wavelength range of 600-800 nm are mainly corresponding to the absorption of the polymer donor PTB7-Th. PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se devices show the similar broad photoresponses with the maximum values of ~63% at 550 nm, while the EQE spectrum of PTB7-Th:PFPDI-TT exhibit relatively weaker photoresponses in the 450-580 nm with the maximum value of ~61% at 640 nm. The photoresponses of all three all-PSCs are in good agreement with the absorption spectra of PTB7-Th and the polymer acceptors as shown in Fig. 1.

16


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Figure 5. Dependence of Voc (a) and Jsc (b) on illuminated light intensity (P) of all-PSCs based on PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se.

To

gain

more

insight

into

the

charge-recombination

mechanism

within

PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se-based devices, we investigate the Jsc and Voc as a function of light intensity (P), which was varied from 1 to 100 mW cm-2, and the corresponding fitting results were shown in Fig. 5. Typically, in order to judge the degree of bimolecular or trap-assisted recombinations in the device, the Voc as a function of illumination intensity in logarithmic scale was carried out, and the corresponding slope is described by kBT/q, where kB is the Boltzmann constant, T is the temperature, and q is the elementary charge.27 Generally, the dominating mechanism is the bimolecular recombination if the slope is equal to 1 (S=1×kBT/q), whereas a slope of 2×kBT/q implies that the dominating recombination becomes trap-assisted or Shockley-Read-Hall recombination.53-54 As shown in Fig. 5a, the slopes of PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se devices were estimated to be 1.190, 1.284 and 1.157, respectively, which demonstrate that the dominating charge recombination mechanism in these devices is the bimolecular recombination, but the trap-assisted recombination also plays a role in it. In addition, the Jsc as a function of illuminated light intensity was also performed to further understand the effect of the recombination mechanisms, and the corresponding equation is described as Jsc∝Pα, where α is the slope of the lines on the log-log plot (Fig. 5b).55-56 If α is equal to or approximates to 1, it indicates there is the negligible bimolecular recombination in the device at the short-circuit condition.20 As shown in 17



Fig. 5b, the values of PFPDI-TT, PFPDI-T and PFPDI-Se-based devices were estimated to be ~0.95, it implies that the light-intensity-dependent recombination is also occurred via the bimolecular recombination, and also means that most of the free carriers are collected at the electrodes prior to bimolecular recombination. On the basis of the results, it demonstrates, whether in the open-circuit condition or short-circuit condition, that the PFPDI-TT, PFPDI-T and PFPDI-Se-based all-PSCs exhibited the restrained unfavorable recombination behavior, which is beneficial for achieving high Jsc and FF.

Figure 6. Hole mobilities for the hole-only devices (a) and electron mobilities for the electron-only devices (b) based on PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se. 18


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To understand the charge transport properties, the space-charge-limited-current (SCLC) method was employed to measure the charge carrier mobilities of PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se based all-PSCs. Fig. 6 shows the J-V curves of the hole-only and electron-only devices with the device structures of ITO/PEDOT:PSS/PTB7-Th:polymer acceptor/Au and ITO/TiOx/ PTB7-Th:polymer

acceptor/Al,

respectively,

for

PTB7-Th:PFPDI-TT,

PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se. As shown in Fig. 6, the Ohmic and SCLC regions for all devices can be clearly observed. The hole mobilities calculated using the Mott-Gurney equation were 2.22 ×10-4 cm2/(V s), 2.13 × 10-4 cm2/(V s) and 2.42

×10-4

cm2/(V

s)

for

PTB7-Th:PFPDI-TT,

PTB7-Th:PFPDI-T

and

PTB7-Th:PFPDI-Se hole-only devices, respectively, and the calculated electron mobilities are 4.92 ×10-5 cm2/(V s), 1.68 × 10-5 cm2/(V s) and 3.68 ×10-5 cm2/(V s) for PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se hole-only devices, respectively. Consequently, benefiting from the reasonably balanced carrier transport (µh/µe= 4.51, 12.6 and 6.58 for PFPDI-TT, PFPDI-T or PFPDI-Se) in the active layer, PTB7-Th: PFPDI-T, PFPDI-TT or PFPDI-Se-based all-PSCs all display superior FF (over 60%) in comparison with that of the monomeric PDI-based devices (below 55%).12, 40, 57

19



Page 20 of 29

Figure 7. AFM height and TEM images of PTB7-Th:PFPDI-TT (a, d), PTB7-Th:PFPDI-T (b, e) and PTB7-Th:PFPDI-Se (c, f) blend films.

The

surface

morphologies

of

PTB7-Th:PFPDI-TT,

PTB7-Th:PFPDI-T

and

PTB7-Th:PFPDI-Se blend films were investigated by the atomic force microscopy (AFM) and transmission electron microscopy (TEM) and the images were shown in Fig. 7. As shown in Fig. 7a-c, it can be found that all blend films exhibit the quite smooth features with no obvious density fluctuations, the root-mean-square roughness (Rq) of 2.25 nm, 2.08 nm and 1.71 nm were observed for PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se blend films, respectively. The TEM images also provided more direct evidence to support the promising photovoltaic performances of PTB7-Th:PFPDI-TT, PTB7-Th:PFPDI-T and PTB7-Th:PFPDI-Se based all-PSCs (Fig. 7d-f). It can be seen that the three blend films displayed an apparently uniform morphology with the well-distributed nanofibrillar structures, which is beneficial for efficient exciton dissociations and charge transfer in the device. 20


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It is well known that the effective exciton diffusion length is typically is 10 nm or less, which means that the small or too large domains in the active layer are not favorable for the efficient exciton diffusion and charge separation.58-59 As a results, with the 5 vol % CN additive treatment, PTB7-Th: PFPDI-TT, PFPDI-T and PFPDI-Se blend films all show the appropriate sizes of ca. 8-9 nm within the range of exciton diffusion length (Fig. 7d-f). The AFM and TEM results indicate that the vast majority of excitons will be able to diffuse to and dissociate at the donor/acceptor interfaces, and therefore, resulting in the improved photovoltaic performances in the all-PSCs.

CONCLUSION In summary, three polymer acceptors, PFPDI-TT, PFPDI-T and PFPDI-Se, were designed and synthesized by coupling the fused electron-deficient building block (FPDI) with the different comonomers thieno[3,2-b]thiophene, thiophene and selenophene, respectively. These polymer acceptors possess the wide optical badgaps of 1.86~1.91 eV covering the strong absorption from 350 nm to 650 nm. The LUMO energy levels of around 4.10 eV were also observed for the three polymer acceptors. In addition, these polymer acceptors also showed the large HOMO and LUMO energy offsets (∆EHOMO and ∆ELUMO) indicating enough driving forces for efficient exciton dissociations in all-PSCs. The all-PSCs with PTB7-Th as the polymer donor were fabricated in the conventional device structures. The all-PSC based on PTB7-Th:PFPDI-TT displayed a PCE of 6.28%, and the PCE of PTB7-Th:PFPDI-T based device was increased to 6.49%. In the case of PTB7-Th: PFPDI-Se all-PSCs, the highest PCE of 6.58% was achieved with a Jsc of 13.96 mA/cm2, a Voc of 0.76 V, 21



and an FF of 62.0%. Moreover, we also found that the photovoltaic performances of all-PSCs based FPDI polymer acceptors are mainly determined by the FPDI unit, but have a little effect from the comonomers. These results further underscore the importance of FPDI-based polymer acceptors as the promising candidates to explore the potential of the high performance all-PSCs.

EXPERIMENTAL SECTION Materials. Monomer FPDI-Br, 1, 2 and 3 were synthesized according to our and others previous work.23, 39-40 PTB7-Th (Mw: 93000, PDI: 2.5) was purchased from 1-Materials Inc. All other commercial reagents and solvents were obtained commercially and used without further purification unless stated otherwise.

Synthesis of PFPDI-T. In a 25-mL flask, FPDI-Br (200 mg, 0.126 mmol), 1 (51.8 mg, 0.126 mmol), Pd2(dba)3 (5.0 mg, 0.0055 mmol) and P(o-tol)3 (10.0 mg, 0.033 mmol) were added. The flask was then refilled with nitrogen, and toluene (6 mL) was added using a syringe under nitrogen. The reaction mixture was evacuated and refilled with nitrogen for three times, and then was heated to 110 oC for 72 hours. For end-capping, 2-(tributylstannyl)thiophene (0.2 mL, 0.61 mmol) was added into the mixture and stirred for 12 hours, 2-bromothiophene (0.5 mL, 5.1 mmol) was then added and stirred for 12 hours. After cooling to the room temperature, the mixture was poured into methanol. The precipitation was collected and dissolved into cholorbenzene. The chlorobenzene solution was concentrated and poured into acetone. The precipitation was collected and dissolved into cholorbenzene again and the solution was concentrated and poured into hexane. The collected solid was then dried over vacuum to give PFPDI-2T (145 mg, 77%). 1H NMR (400 MHz, CDCl3) δ 10.34 (s, 3H), 22


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9.71-8.63 (m, 5H), 7.83-7.25 (m, 4H), 5.47-5.20 (m, 4H), 2.63-2.17 (m, 7H), 1.96 (dd, J = 40.5, 25.8 Hz, 9H), 1.55-1.09 (m, 48H), 0.85 (dd, J = 31.6, 5.5 Hz, 24H).

Synthesis of PFPDI-TT. The PFPDI-TT was synthesized by following the above method in the yield of 78%. 1H NMR (400 MHz, CDCl3) δ 10.34 (s, 3H), 9.47-8.71 (m, 5H), 7.95-7.25 (m, 5H), 5.35 (t, J = 17.2 Hz, 4H), 2.62-2.17 (m, 7H), 1.95 (dd, J = 32.8, 27.6 Hz, 9H), 1.34 (dd, J = 37.4, 29.6 Hz, 48H), 0.97-0.63 (m, 24H).

Synthesis of PFPDI-Se. The PFPDI-Se was synthesized by following the above method in the yield of 76%. 1H NMR (400 MHz, CDCl3) δ 10.34 (s, 3H), 9.71-8.63 (m, 5H), 7.83-7.25 (m, 4H), 5.47-5.20 (m, 4H), 2.63-2.17 (m, 7H), 1.96 (dd, J = 40.5, 25.8 Hz, 9H), 1.55-1.09 (m, 48H), 0.85 (dd, J = 31.6, 5.5 Hz, 24H).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:*******.

1

H NMR spectrum, UV-visible absorption spectra,

all-polymer solar cell device fabrication, the photovoltaic performance parameters according to the optimization procedure.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Yong Zhang). *E-mail: [email protected] (Erjun Zhou).

ORCID Yuli Yin: 0000-0002-8179-4185 23



Erjun Zhou: 0000-0003-1182-311X Yong Zhang: 0000-0002-9587-4039

Author Contributions #Y.L.Y. and J.Y. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (21644006, 51403044, 51473040, 51673048, and 21602040), the National Natural Science Foundation of Beijing (2162045), and the Chinese Academy of Sciences (QYZDB-SSW-SLH033). Y. Zhang thanks the support from the Fundamental Research Funds for the Central Universities (Harbin Institute of Technology).

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Table of Content High Performance All-Polymer Solar Cells Achieved by Fused Perylenediimide-Based Conjugated Polymer Acceptors Yuli Yin, Jing Yang, Fengyun Guo, Erjun Zhou,* Liancheng Zhao, Yong Zhang*

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High-Performance All-Polymer Solar Cells Achieved by Fused

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