Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells

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A Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells Reaching 9.4% Efficiency Shutao Xu, Liuliu Feng, Jun Yuan, Zhi-Guo Zhang, Yongfang Li, Hongjian Peng, and Yingping Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 22, 2017

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ACS Applied Materials & Interfaces

A Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells Reaching 9.4% Efficiency Shutao Xu†, Liuliu Feng†, Jun Yuan†, Zhi-Guo Zhang‡, Yongfang Li‡, Hongjian Peng†, Yingping Zou*†§ †

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083,

China * E-mail: [email protected](Y.Zou) ‡

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of

Sciences, Beijing 100190, China §

State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China

ABSTRACT Through introducing six fluorine atoms onto quinoxaline (Qx), a new electron acceptor unit-hexafluoroquinoxaline (HFQx) is firstly synthesized. Based on this unit, we synthesize a new donor-acceptor (D-A) copolymer (HFQx-T), which is composed of benzodithiophene (BDT) derivative donor block and an HFQx accepting block. The strong electron-withdrawing properties of fluorine atoms increase significantly the open-circuit voltage (Voc) by tuning the highest occupied molecular orbital (HOMO) energy level. In addition, fluorine atoms enhance the absorption coefficient of the conjugated copolymer and change the film morphology, which implies an increase of the short-circuit current density (Jsc) and fill factor (FF). Indeed, the HFQx-T:ITIC blended film achieves an impressive power conversion efficiency (PCE) of 9.4% with large short-current density (Jsc) of 15.60 mA/cm2, high Voc of 0.92 V and FF of 65% via two step annealing (thermal annealing (TA) and solvent vapor annealing (SVA) treatments). The excellent

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results obtained, show that the new copolymer HFQx-T synthesized could be a promising candidate for organic photovoltaics. KEYWORDS: hexafluoroquinoxaline, photovoltaic polymer, high efficiency, thermal annealing, nonfullerene

INTRODUCTION In the last few decades, polymer solar cells (PSCs) have attracted broad attention because of their clean, renewable, low-cost, lightweight and flexibility.1-2 The bulk heterojunction (BHJ) were highly efficient structure in PSCs, which based on a p-type conjugated semiconductor as the donor and an n-type semiconductor as the acceptor.3 The donor-acceptor (D-A) combination, is now known to be the most efficient structure of polymers for photovoltaic performances4. Up to now, fullerenes derivatives acceptor materials such as [6,6]-phenyl-C61/C71-butyric acid methyl ester (PC61BM/ PC71BM) are commonly used due to their high electron mobility, efficient electron transfer and transport properties.5,6 The PCEs based on organic materials: fullerene BHJ (simple junction) device reach now more than 10%.7 The fullerene derivatives present also several disadvantages such as a weak absorption in visible-near infrared region, difficulty to tune their optical properties and the energy levels and a relative high cost, hindering thus the development in the future of fullerene-based PSCs’ further development. Hence, a series of small molecular acceptor

materials

such

as

perylene

diimide

(PDI),

naphthalene

diimide

indacenodithiophene (IDT) and its derivatives have been successfully developed in PSCs.

(NDI), 8

These

materials are low cost, have a broad absorption range and could be tuned easily.9-12 Colin Nuckolls and his coworkers reported a helical PDI dimer with a PCE of 6.1%.13 Li’s group achieved a PCE

2


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of 8.27% with a high FF of 70% for all PSCs by using a low bandgap NDI based polymer as an acceptor material.14 In 2016, Hou’s group designed an IEICO acceptor in PSCs and achieved a PCE of 8.40%.15 Zhan et al reported for the first time an acceptor material, named ITIC (indacenodithieno[3,2-b]thiophene

(IDT)

as

central

donor

unit

and

2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) as acceptor end groups) (shown in Figure 1a). This n-type material blended with a low-bandgap p-type polymer (PTB7-Th), achieved a PCE of 6.80%. 16Then, many highly efficient fullerene-free devices based on ITIC and derivatives were fabricated. Up to now, the performances have reached more than 10%.17-19 Many researches have shown that introducing fluorine atom in conjugated polymers is a highly effective method for modulating optical and electrochemical properties to improve the photovoltaic performance.20 As well known, fluorination can significantly decrease the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) energy levels due to the high electronegativity of the fluorine atom.21 The fluorination of p-type materials could be an interesting approach to increase the Voc. 22-23 The fluorine atom has a very small atomic radius and a strong interactions with carbon and hydrogen atoms,24 and thus, the insertion of fluorine atom in π-conjugated polymers could facilitate the molecular planarity and intermolecular assembly. 25-26 In addition, the hydrophobicity and adjust the polarity of polymers lead to improve the charge mobility of the polymers, control the interfacial interaction between the donors and the acceptors.

27-29

These

unique features can also improve the film morphology by reducing the phase domains and by increasing interfacial areas, which imply an exciton dissociation more efficiently and thus a higher Jsc and FF values. 30 Quinoxaline (Qx) is a strong electron acceptor unit, widely used as building block for

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optoelectronic applications.31 The structure of unit can be tuned easily in order to modify the optoelectronic properties of the resulting polymer.32-33 The Qx based polymers with fluorine-free blend with fullerenes derivatives as n-type materials only achieved a PCE of 5.0%.34 This performance increased up to 7.39% by adding two fluorine atoms in the structure.35 Our group further synthesized a tetrafluoroquinoxaline based polymer (PBDTT-ffQx), which achieved high PCE of 8.60% when blended with PC71BM as acceptor material and 8.47% when blended with ITIC, a small n-type molecule (shown in Figure 1a).

36-37

In order to improve photovoltaic performance, except materials synthesis, active layer treatment during device fabrication is also important. The device performances are sensitive to solvent additives such as 1,8-diiodoactane (DIO) because of D:A ratio variation and the slow drying process of the residual high-boiling-point additive, implying an undesirable morphological change. In addition, residual DIO greatly accelerates the photo-oxidative degradation phenomenon by acting as radial initiator, thus decreasing the lifetime of the device.38 Building on successful strategies, we firstly introduced six fluorine atoms on Qx unit in order to obtain an ideal acceptor unit -HFQx. Thereafter we have explored the relationship between the structure and the properties of HFQx based polymer and investigated the additive-free device photovoltaic performance. In

this

work,

we

used

2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b’]dithiophene (BDT) as an electron donating unit, hexafluoroquinoxaline as electron accepting unit to synthesize a new D-A conjugated polymer, named HFQx-T. HFQx-T has a higher Voc value compared to the PBDTT-ffQx, due to a lower HOMO energy level. Therefore, an outstanding PCE of 9.4% with Voc of 0.92V, Jsc of 15.6 mA/cm2 and FF of 65% was achieved without any additive when HFQx-T was

4


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blended with ITIC. These results suggest that HFQx is a promising accepting unit in constructing high performance photovoltaic polymers.

RESULTS AND DISCUSSION Synthesis and Characterization The synthetic procedures of the monomer and HFQx-T are presented in Scheme 1 and experimental details are shown in the supporting information. All the reaction steps are simple and classical reactions. Compound 2 was obtained by reduction reaction using NaBH4 with a yield of 68%. Through simple one step Williamson reaction under argon, colorless oil product 4 was got at 150 oC with a yield of 82%. Using classical Grignard reaction, light yellow viscous liquid compound 5 was obtained with a yield of 21%. Compound 6 was synthesized by Schiff-base reaction using glacial acetic acid as solvent between compound 2 and compound 5 with 79% yield. By a Stille coupling reaction using PdCl2(PPh3)2 as catalyst, an orange solid compound 7 was prepared between compound 6 and tributyl(thiophene-2-yl)- stannane at 110oC with a yield of 91%. The solid red monomer (M1) was obtained by bromination with 81% yield. Through a typical Stille Coupling reaction, the copolymer HFQx-T was synthesized with Pd(PPh3)4 as catalyst in toluene at 110 oC. All intermediates were purified by silica gel column chromatography, their structures were well characterized by NMR spectroscopy. The copolymer was carefully purified by continuous Soxhlet extractions with methanol, hexane, acetone and chloroform (CF) successively. The CF fraction was concentrated under vacuum and then it was filtered by flash column chromatography using chloroform as eluent. Finally, the HFQx-T was precipitated in methanol and collected by filtration. The polymerization yield of 53% was obtained. The molecular weight of HFQx-T was

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determined

by

high

temperature

gel-permeation

chromatography

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(HT-GPC)

using

1,2,4-trichlorobenzene (TCB) as eluent and polystyrene as the standard. HFQx-T has a number average molecular weight (Mn) of 18 kDa with a polydispersity index (PDI: Mw/Mn) of 2.4. The thermal property of the copolymer was investigated by thermogravimetric analysis (TGA). The TGA graph was shown in Figure S8. Under nitrogen, the onset temperature of 5% weight loss is 402 oC for HFQx-T which demonstrated that the thermal stability of the copolymer is high enough for device applications.

39

The related data including the molecular weights and TGA of HFQx-T

are summarized in Table 1.

Figure 1. a) Chemical structures of ITIC, PBDTT-ffQx and HFQx-T; b) Molecular energy-level alignment of the materials involved in the PSCs; c) Device structure used for PSCs fabrication, ETL used is ZrAcac with HFQx-T:PCBM or PDINO with HFQx-T:ITIC BHJ layer.

6


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Table 1. Molecular weights and thermal properties of the HFQx-T copolymer

Mn (kDa)

Mw (kDa)

PDI

Td (oC)

HFQx-T

18

43.6

2.4

402

Scheme 1. Synthetic Routes of Monomer and HFQx-T 7



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Optical and Electrochemical Properties Ultraviolet-visible (UV-Vis) normalized absorption spectra of HFQx-T in dilute chloroform solution and HFQx-T, ITIC, HFQx-T:ITIC blend as a thin film cast from chloroform solution on a quartz substrate are shown in Figure 2a. The optical data, including the maximum absorption peak

wavelengths (λmax), absorption onset wavelengths (λonset), and the optical band gap ( ) are summarized in Table 2. The absorption in the wavelength range of 300-500 nm is originated from the localized π-π* transition, whereas a relatively broad and intense absorption in the range of 500-700 nm is attributed to the intramolecular charge transfer (ICT) between the strong electron-accepting HFQx and the electron-donating BDT unit.40 The maximum absorption peaks for HFQx-T in CF and thin film state are 604 nm and 601 nm, respectively. The absorption spectra in solution and film are almost same. From the Figure 2a, we also find that the ITIC can efficiently make up the absorption of HFQx-T in 500-800 nm, which is the main range of visible light so that the HFQx-T:ITIC device have high Jsc. Fluorine atom not only has powerful electronegativity but also has little steric hindrance due to its’ small atom radius.41 The six fluorine atoms on Qx may lead to the strong intermolecular interactions between polymer chains both in solution and the film absorption spectra without obvious shift. In the long wavelength, the film absorption has weak vibronic shoulder, which implied that an ordered aggregation and strong π-π stacking interactions in the polymer film.32 Many investigations have proved that the conjugated polymer showed temperature-dependent optical properties because of the strong aggregation of polymer in solution. 42

Therefore, we explored the UV-Vis absorption spectroscopy dependent on temperature under

20-90oC to explore the aggregation effect of HFQx-T in solution state (Figure S9 a). The polymer

8


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main absorption peak showed slight blueshift from 604 nm to 592 nm with increasing temperature. For a conjugated polymer solution, inter-chain interaction can be reduced by increasing temperature, which means HFQx-T has some aggregations in solution because of inter-chain interaction.43

HFQx-T exhibits absorption edge of 705 nm, which corresponds to the optical bandgap ( = 1240/λedge) of 1.76 eV. Importantly, HFQx-T has better light harvest capacity compared to tetrafluoroquinoxaline counterpart. As shown in Figure 2b, the absorption coefficients of HFQx-T and PBDTT-ffQx are 0.74×105 cm-1 and 0.67×105 cm-1, respectively, which means further introducing more fluorine atoms onto Qx can efficiently improve the absorption coefficient to benefit Jsc. Cyclic voltammetry (CV) was applied to measure the HOMO and LOMO energy level of HFQx-T, which was measured under argon by using tetra-n-butylammoniumhexafluorophosphate (n-Bu4NPF6, 0.1 M in acetonitrile) as the supporting electrolyte.44 We use the ferrocene/ferrocenium (Fc/Fc+) redox couple as the standard, the CV curves and data are displayed in Figure 2c and Table

) and oxidation potential ( ) calculated from the onset 2. The onset reduction potential (

points of CV curve are -0.86V and 1.09V for HFQx-T, the HOMO and LUMO energy levels can be deduced from the equations of (1) and (2) are -5.45 eV and -3.50 eV.

45

EHOMO= -(4.80-E1/2, Fc/Fc++ ) (eV)

(1)

ELUMO= -(4.80-E1/2, Fc/Fc++ ) (eV)

(2)

The band offset between LUMO energy level of HFQx-T and LUMO energy level of PC71BM, named △ELUMO, is over 0.3 eV, which has enough driving force to conquer the excition binding energy.46 The band offset between LUMO energy level of PC71BM or ITIC and HOMO energy level of polymer are △Eoffset1=1.45 eV and △Eoffset2=1.63 eV, respectively. It is well known that the 9



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Voc is proportional to the band offset between LUMO of the acceptor and HOMO of donor. △Eoffset2 is larger than △Eoffset1 of 0.18 eV, this is the reason why the Voc using ITIC as an acceptor are higher than those from PC71BM as an acceptor, which indicates that the energy levels of HFQx-T and ITIC are more matched than that of PC71BM. Photoluminescence (PL) quenching tests were further aimed to investigate the exciton dissociation in the blend film. The PL spectra are shown in Figure 3, it is clear that the spectral shape of pure HFQx-T and ITIC are broad. When excitation wavelength at 600 nm, PL emission peak of the donor appears in the range of 620-890 nm centered at 734 nm. For the HFQx-T:ITIC film, its emission is almostly quenched by 72%. After thermal annealing (TA) and solvent vapor annealing (SVA) treatment, the blend films are effectively quenched by 94%, implying efficient electron transfer from HFQx-T to ITIC for excitons generated in donor phase. When excitation wavelength at 700 nm, PL emission peak of ITIC appears in the range of 720-900 nm centered at 754 nm. For the blend film, using the photo excitation at 700 nm, the blend films is effectively quenched by 99.2%. After TA and SVA treatment, the blend films are effectively quenched by 99.90% suggesting effective hole transfer from ITIC to HFQx-T. The quenching experiments can partially confirm that the efficient charge transfer between the HFQx-T and the ITIC.

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Normalized Absorption (a.u.)

a) HFQx-T solution HFQx-T film

1.0

ITIC film HFQx-T:ITIC blend film

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm)

Absorption Coefficient (cm-1)

b) 8x10

4

7x10

4

6x10

4

5x10

4

4x10

4

3x10

4

2x10

4

1x10

4

PBDTT-ffQx HFQx-T

0 300

400

500

600

700

800

900

Wavelenghth (nm)

c) HFQx-BDT + Fc/Fc

Intensity (a.u.)

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


-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential (V) vs Ag/AgCl 11



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Figure 2. a) Normalized absorption spectra of HFQx-T in dilute CF solution and HFQx-T, ITIC, HFQx-T:ITIC (1:1.125, w:w) in film states; b) Absorption coefficients of HFQx-T and PBDTT-ffQx film; c) CV curves of HFQx-T in 0.1M Bu4NPF6 acetonitrile solution.

a) HFQx-T @600 nm

HFQx-T:ITIC @600 nm HFQx-T:ITIC (TA+SVA) @600 nm

650

700

750

800

850

Wavelength (nm)

b) TIIC @700 nm HFQx-T:ITIC @700 nm HFQx-T:ITIC (TA+SVA) @700 nm

720 740 760 780 800 820 840 860 880 900

Wavelength (nm)

Figure 3. PL spectra of a) pure HFQx-T, HFQx-T:ITIC blend in as-cast film and HFQx-T:ITIC blend films under optimized conditions; b) pure ITIC, HFQx-T:ITIC blend in as-cast film and HFQx-T:ITIC blend films under optimized conditions. Table 2. Optical and Electrochemical Data of HFQx-T 12


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Absorption spectra

Cyclic voltammetry

Sola

Filmb

p-doping

n-doping

λmax

λmax

λonset

/HOMOd

/LUMOd

(nm)

(nm)

(nm)

(eV)

(V)/(eV)

(V)/(eV)

604

601

705

1.76

1.09/-5.45

-0.86/-3.50

Polymer

HFQx-T a

C

Measured in chloroform solution. bCast from chloroform solution. cBandgap estimated from the

onset wavelength of the optical absorption.

d

HOMO= -(4.80-E1/2,Fc/Fc++Eox) (eV); LUMO=

-(4.80-E1/2,Fc/Fc++Ered) (eV) using Ag/AgCl as the reference electrode. Photovoltaic Properties In order to further investigate how the six fluorine atoms affect the photovoltaic properties of HFQx-T, PSC devices with configuration of ITO/PEDOT:PSS/HFQx-T:PC71BM/ZrAcac/Al or ITO/PEDOT:PSS/HFQx-T:ITIC/PDINO/Al were fabricated. The photovoltaic data are summarized in Table 3, the standard deviation values are also listed in Table 3 and more related data are shown in Table S1. The electron transporting layer (ETL) materials, D/A weight ratio, solvent additives, TA and SVA have been carefully optimized in order to obtain the highest photovoltaic performance. When using PC71BM as an acceptor, we find that the 0.5 vol% DIO additive has an increased PCE from 2.35% (Voc of 0.89 V, Jsc of 6.00 mA/cm2 and FF of 44%) to 7.72% (Voc of 0.85 V, Jsc of 13.42 mA/cm2 and FF of 67%) with D/A weight ratio of 1:1. The Jsc was enlarged by 2.24 times and FF was enlarged by 1.53 times so that the PCE was improved by 329%. DIO additive can efficiently modify the film morphology. The optimal D/A weight ratio is found to be 1.5:1 and the best PCE is 7.76% with Voc of 0.87 V, Jsc of 13.40 mA/cm2 and FF of 66%. When using ITIC as an acceptor, without any treatment, the PCE can reach 8.0% with Voc of 0.95 V, Jsc of 13.42 mA/cm2 and FF of 13



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62% with D/A weight ratio of 1:1. To further improve the photovoltaic performance, the TA and SVA were adopted. Using 1:1.125 D/A weight ratio, the best PCE can reach 9.4% with large Jsc of 15.60 mA/cm2, high Voc of 0.92 V and FF of 65% without any additive via TA and SVA treatment. The corresponding J-V curves are depicted in Figure 4b. Moreover, we observe that the best PCE of HFQx-T (9.4%) is higher than PBDTT-ffQx (8.47%) in ITIC based PSCs. But in PC71BM based PSCs, the best PCE of HFQx-T (7.76%) is lower than PBDTT-ffQx (8.60%). This result suggests that HFQx-T matches better with ITIC whereas PBDTT-ffQx matches better with PC71BM. The external quantum efficiency (EQE) spectra in Figure 4b was performed to verify the Jsc value obtained from J-V measurement (Figure 4a). The integrated Jsc value of best device is 14.87 mA/cm2, and the Jsc obtained from the J-V is 15.59 mA/cm2, the deviation between the Jsc and the integral of the EQE is 4.6% which is less than 5%. The Jsc obtained from J-V curve is valid. As shown in Figure 4b, the EQE curves have a wide response from 300 to 800 nm and three obvious peaks around 50%-72% at approximately 350 nm-750 nm. The maximum EQE of 72% is observed at 601 nm. From the UV-Vis spectra (in Figure S9b), we find that the pure PC71BM main absorption ranges from 200 nm to 500 nm and the pure ITIC main absorption ranges from 500 nm to 800 nm. The ITIC can efficiently make up the absorption of HFQx-T in 500-800 nm, which is the main range of visible light (390-780 nm) so that the HFQx-T:ITIC device has high Jsc which is consistent with the EQE curves.46 From EQE curves, we can obviously find that high and broad photoresponses from 500 nm to 800 nm. Meanwhile, PC71BM can make up the absorption of HFQx-T in 300–500 nm, which is consistent with the EQE of HFQx-T:PC71BM device from 300-500 nm. The ITIC is more efficient to improve the absorption of blend film than the PC71BM therefore to affect the EQE of devices. We also can verify the conclusion from the absorption

14


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coefficient. As show in Figure S9c, The HFQx-T:ITIC blend film have much higher absorption coefficient than that of HFQx-T:PC71BM blend film, which is beneficial to the Jsc. ITIC can more efficiently improve the EQE values than PC71BM.

47

The mobility is one of the most important factors upon PCEs to assess the charge transport and testify if the charge abided by non-geminate recombination mechanism, the hole and electron mobility (µh and µe) were measured by the space-charge limited current (SCLC) method. The device of ITO/PEDOT:PSS/active layer/Au and ITO/ZnO/active layer/PDINO/Al were employed to fabricate the hole-only and electron-only diodes, respectively. The SCLC is calculated in this law:

#

J ≅ ε exp (0.89"$ &)/)* %

(3)

J is the current density, ε0 is the permittivity of the free space, εr is the dielectric constant of the blend film, which is normally 2-4 for organic semiconductor, and we used a dielectric constant of 3, µ0 is the carrier mobility, L is the thickness of the device and V is the internal voltage. V = Vapp -Vbi, Vapp is the applied potential and Vbi is the built-in potential, Vbi values usually are 0.2 V and 0 V in the hole-only and the electron-only devices.48 The hole mobility and electron mobility of HFQx-T and PC71BM blends without any treatment can be calculated to be 0.37×10-4 cm2·V-1·s-1, and 0.23×10-4 cm2·V-1·s-1, respectively, while 2.73×10-4 cm2·V-1·s-1, 2.73×10-4 cm2·V-1·s-1 with 0.5 vol% DIO treatment. Both the hole and electron mobility all get improved over 10 times after DIO treatment, which suggests DIO can optimize the film to form the interpenetrating networks to realize the separation and transport of exciton.49 When using ITIC as an acceptor using 1:1.125 of D/A weight ratio without any treatment, the hole mobility and electron mobility of blends are 0.19×10-4 cm2·V-1·s-1 and 0.76×10-4 cm2·V-1·s-1, respectively. After TA treatment, the hole mobility and electron mobility of blends reach 0.42×10-4 cm2·V-1·s-1 and 1.27×10-4 cm2·V-1·s-1, respectively.

15



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Further treatment by SVA, the hole mobility and electron mobility of blends can be reached at 1.42×10-4 cm2·V-1·s-1 and 4.40×10-4 cm2·V-1·s-1, respectively. Through TA and SVA, the hole mobility and electron mobility of blends get improved at 7.5 times and 5.8 times, which imply that two-step annealing is an efficient method to enhance the hole and electron mobility. Moreover, the balanced hole and electron mobility will decrease recombination and form space charge density to benefits the PCE.50 After TA and SVA, the µh/µe of HFQx-T:ITIC blend increases from 0.25 to 0.32. Generally, the higher and more balanced hole and electron mobility has the better PCE. The hole and electron mobilities of HFQx-T:ITIC are shown in Figure 5, HFQx-T:PC71BM are shown in Figure S10 and related data are listed in Table S2. Commonly, the geminate recombination is dependent of charge carrier density and light intensity, for further understanding charge carrier recombination, the Jsc and light intensities was performed. The correlation between Jsc and light intensity (Plight) can be described by an empirical / where power-law component α should be linearity (≈1) when geminate formula of Jsc ∝+,-.

recombination in device can be ignored.51 In the HFQx-T:PC71BM device, the α value is estimated to be 0.914. When adding 0.5 vol% DIO, the α value gets increased to 0.967. In the HFQx-T:ITIC device, TA and SVA also make the α value increase from 0.957 to 0.976 (Figure 6). The geminate recombination of the HFQx-T:ITIC device is less compared with the HFQx-T:PC71BM device. This also agrees well with the result of a relatively higher Jsc observed for the HFQx-T:ITIC device. The photon energy loss (Eloss) in PSCs is defined as Eloss=Eg-eVoc where Eg is the lowest optical band gap. There is an empirical relationship of Eloss = 0.6 eV, which is often applied to obtain the

012 by a specific BHJ blend.52-53 In practice, the reported Eloss is typically 0.70-1.00 eV. Figure 7a provides a diagram of Voc vs Eg for a series of copolymers reported with PCE＞5% (see Table

16


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S3). Through absorption spectroscopy, ITIC film shows a value for Eg of 1.57 eV (788 nm). For the HFQx-T:PC71BM blend, the Voc is 0.87 V corresponding to Eloss = 0.89 eV while the Voc of the HFQx-T:ITIC blend is 0.92 V with the Eloss is 0.65 eV, which is approaching the minimum Eloss (0.60 eV). From the Table S3 and Figure 7a and 7b, we find that Eloss of HFQx-T: PC71BM is bigger than HFQx-T:ITIC blend, which can explain that the HFQx-T:ITIC blend have a better PCE than HFQx:PC71BM blend.

a) HFQx-T HFQx-T TA HFQx-T TA+SVA

Current Density(mA/cm2)

15 10 5 0 -5 -10 -15 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Voltage(v)

b) 90 80

HFQx-T HFQx-T TA HFQx-T TA+SVA

70 60

EQE(%)

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


50 40 30 20 10 0 300

400

500

600

700

800

Wavelength(nm) Figure 4. a) The J-V curves of PSCs based on HFQx-T:ITIC under the illumination of AM 1.5G, 17



100 mW/cm2; b) EQE curves of the corresponding devices.

a) 160 HFQx-T:ITIC(1:1.125) hole mobility HFQx-T:ITIC(1:1.125) hole mobility TA HFQx-T:ITIC(1:1.125) hole mobility TA+SVA

140

J1/2/A1/2m-1

120 100 80 60 40 20 0

2

4

6

8

V (V) b) 160 140

HFQx-T:ITIC(1:1.125) electron mobility TA+SVA HFQx-T:ITIC(1:1.125) electron mobility TA HFQx-T:ITIC(1:1.125) electron mobility

120

J1/2/A1/2m-1

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

100 80 60 40 20 0

2

3

4

5

6

V (V) Figure 5. a) The hole mobilities and b) electron mobilities of the HFQx-T:ITIC blend.

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a)

-2 Current Density (mA cm )

10

HFQx-T:PC71BM(1:1) a=0.914 HFQx-T:PC71BM(1:1) with 0.5 DIO a=0.964 HFQx-T:PC71BM(1.5:1) with 0.5 DIO a=0.967

1

0.1

1

10

100

Light Intensity (mW cm-2)

b) Current Density (mA cm-2)

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


HFQx-T:ITIC(1:1.125) a=0.957 HFQx-T:ITIC(1:1.125) TA a=0.967 HFQx-T:ITIC(1:1.125) TA+SVA a=0.976

10

1

1

10

100

Light Intensity (mW cm-2) Figure 6. Light intensity dependence of the Jsc density based on a) HFQx-T:PC71BM and b) HFQx-T:ITIC.

19



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Table 3. Summary of the Photovoltaic Characteristics of the HFQx-T:PC71BM and HFQx-T:ITIC Blend Films BHJ system

Ratio

(ETL used)

Additivea

Annealing

Voc

Jsc

FF

(%)

(oC)

(V)

(mA/cm2)

(%)

None

None

0.89

6.00

44

±0.004

±0.09

±0.67

None

110

0.88

6.94

41

±0.005

±0.17

±1.08

0.5

None

0.85

13.42

67

±0.003

±0.20

±1.89

0.5

None

0.87

13.40

66

±0.006

±0.25

1.02

0.95

13.42

62

±0.007

±0.15

±0.92

0.90

14.89

66

±0.005

±0.27

±1.12

0.84

3.06

32

±0.008

±0.18

±1.23

0.95

14.80

63

±0.002

±0.16

±0.79

0.92

15.59

65

±0.003

±0.16

±0.79

HFQx-T: 1:1 PC71BM (ZrAcac)

1.5:1

1:1

None

None

None

150

0.5

None

None

150

None

150(CF)

HFQx-T:ITIC (PDINO)

1:1.125

a

PCE(%) Maxb 2.35

2.53

7.72

7.79

7.99

9.07

0.83

8.54

9.40

Avec 2.32

±0.03 2.53

±0.01 7.63

±0.25 7.66

±0.09 7.78

±0.19 9.02

±0.05 0.82

±0.03 8.39

±0.06 9.25

±0.10

The additive is 1,8-diiodooctane. bThe maximum PCE value of device. cThe average PCE values of 20 devices.

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a) 1.4 eV

polymer:fullerene

1.3

0. 6

polymer:nonfullerene

1.2

E lo ss =

HFQx-T:PC71BM HFQx-T:ITIC

1.1

Voc (V)

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3

1.0

1.2

1.4

1.6

1.8

2.0

0.8

0.9

1.0

Eg (eV)

b) polymer:fullerene

0.8

polymer:nonfullerene HFQx-T:PC71BM HFQx-T:ITIC

EQEmax

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


0.6

0.4

0.5

0.6

0.7

Energy loss (eV) Figure 7. a) Eg and Voc distribution of recent high performance PSCs. Solid line is the eVoc of 0.8 eV, and dashed line is the “eVoc=Eg-0.6”; b) Maximum EQE within Eg vs Eloss for PSCs extracted from Figure7a. (Table S3 for the details of a and b).

Morphological Characterization Tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM)

21



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were used to analyze the surface morphology and the internal morphology of the active layer, respectively. The images of both the as-cast and additives, TA, SVA film are shown in Figure 8, Figure 9 and more data are displayed in Figure S11 and Figure S12. Without any additive in HFQx:PC71BM (1:1, w:w) blend film, the AFM images show fibrous features with the root-mean-square (RMS) roughness of 3.19 nm, while a rougher surface is obtained with the RMS roughness of 4.36 nm with 0.5 vol% DIO treatment. At HFQx-T:PC71BM (1.5:1, w:w), the RMS roughness is up to 4.55 nm corresponding with the best PCE of 7.79%. HFQx-T:PC71BM with DIO has clearer phase separation features than without DIO, the stronger phase separation properties is beneficial for exciton dissociation and charge transport in the device to obtain higher Jsc and FF.54 The TEM also verified the results that the better domain aggregation in film after 0.5 vol% DIO process. For HFQx-T:ITIC blend as cast film, the RMS is 2.69 nm. After TA treatment, the RMS increase to 2.89 nm, then we further use SVA treatment, the RMS is up to 3.25 nm, which suggests that TA and SVA can efficiently improve the phase separation and induce more ordered molecular self-aggregations.55 When adding 0.5 vol% DIO, the RMS intensively increased to 8.22 nm, the image showed too rough fibrous surface, which means too intensive aggregation and it is bad to charge separation and transporting. From TEM images, we also find that HFQx-T:ITIC blend film with TA and SVA treatment shows a bicontinuous D/A interpenetrating network and a well-developed and uniform fibrillar morphology compared to the blend film as cast.

22


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Figure 8. Topographic AFM images (height and phase) of HFQx-T:ITIC blend films (a),(e) without TA (1:1.125, w:w); (b),(f) with TA (1:1.125, w:w); (c),(g) with TA & SVA (1:1.125, w:w); (d),(h) with 0.5 vol% DIO without TA (1:1, w:w). Dimension of images: 5x5 µm2.

23



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 9. Topographic TEM images (2*2 µm2) of HFQx-T:ITIC blend films (a) without TA (1:1.125, w:w); (b) with TA (1:1.125, w:w) (c) with TA & SVA (1:1.125, w:w). (d) with 0.5 vol% DIO without TA (1:1, w:w).

X-Ray Diffraction Analysis X-ray diffraction (XRD) was employed to obtain deep insight of the crystallinity of

24


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HFQx-T:ITIC blend film and the effect after TA & SVA treatment for the device. As shown in Figure S14, there is no peak can be found at the 5 to 17 degree, indicating that no ordered laminar packing was formed in HFQx-T:ITIC blend. However, as shown inset from Figure S14, weak diffraction peaks for π-π stacking d-spacing of 4.43 Å were observed, which suggested the HFQx-T:ITIC has a certain coplanarity and crystallinity in film. After TA and SVA, the peaks for π-π stacking was improved, which implied the crystallinity of blend was increased and consistent with the AFM and TEM results.56 The increased crystallinity is beneficial to charge separation and transport which can efficiently improve the charge carrier mobility.57

CONCLUSIONS In summary, via introducing six fluorine atoms onto Qx, we designed and synthesized a new medium bandgap copolymer HFQx-T based on alkylthienyl substituted BDT donor unit and hexafluoroquinoxaline accepting unit, and applied in fullerene and free-fullerene PSCs. Fluorine atom can downshift the HOMO and LUMO levels of donor to improve Voc, increase extinction coefficient to improve Jsc, changing planarity of polymer and tune the films morphology. The HFQx-T shows deep HOMO level of -5.45 eV and LUMO level of -3.50 eV, which well matches with the energy levels of ITIC. HFQx-T:ITIC device has high EQEmax values up to 72%, good hole and electron mobility so that a high PCE up to 9.4% with outstanding Jsc of 16 mA/cm2, high Voc of 0.92 V and FF of 65% is obtained in additive-free PSCs. HFQx-T:ITIC device has less geminate recombination which is accordance with a high Jsc. Moreover, two step annealing are beneficial for exciton diffusion/dissociation and charge transport to increase FF. Our research demonstrates that introducing six fluorine atoms in Qx system may be an efficient way to obtain excellent

25



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Page 26 of 31

photovoltaic polymers and HFQx-T is a promising donor in non-fullerene PSCs. We firmly believe that Qx based polymers hold bright future for tomorrow's photovoltaics.

ASSOCIATED CONTENT Supporting Information The Supporting information (SI) available: experimental details including general measurement, fabrication and characterization of PSC, materials, synthesis, 1H NMR, 13C NMR, HT-GPC, thermal, optical and photovoltaic data, XRD, device structure, related figures, schemes and tables.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGEMENTS This work has been financially supported by the National Natural Science Foundation of China (51673205,

51173206),

Science

Fund

for

Distinguished

Young

Scholars

of

Hunan

Province (2017JJ1029), Project of Innovation-driven Plan in Central South University, China (2016CX035), State Key Laboratory of Powder Metallurgy, Central South University, China and the Fundamental Research Funds for the Central Universities of Central South University (2016zzts023).

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Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells

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