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Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells
Accepted Manuscript Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells Renlong Li, Gongchu Liu, Baobing Fan, Xiaoyan Du, Xiaofeng Tang, Ning Li, Lei Ying, Christoph J. Brabec, Fei Huang, Yong Cao PII:
S1566-1199(18)30272-6
DOI:
10.1016/j.orgel.2018.05.050
Reference:
ORGELE 4710
To appear in:
Organic Electronics
Received Date: 31 March 2018 Revised Date:
15 May 2018
Accepted Date: 28 May 2018
Please cite this article as: R. Li, G. Liu, B. Fan, X. Du, X. Tang, N. Li, L. Ying, C.J. Brabec, F. Huang, Y. Cao, Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.05.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
0
PBDB-T:IF-TN PBDB-T:IDT-TN
-5
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2
Current Density (mA/cm )
Graphical abstract
PCE = 3.03 % V = 1.01 eV
-10
OC
-15
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Non-radiative VOC loss = 0.23 V
0.2 0.4 0.6 0.8 Voltage(V)
EP
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-0.2 0
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PCE = 5.89 % V OC = 0.97 eV
1
1.2
ACCEPTED MANUSCRIPT
Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells
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Renlong Li,a,1 Gongchu Liu,a,1 Baobing Fan,a,b Xiaoyan Du,b Xiaofeng Tang,b Ning Li,b Lei Ying,*a Christoph J. Brabec,b,c Fei Huang,*a and Yong Caoa
Materials and Devices, South China University of Technology,
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Optoelectronic
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a State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer
Guangzhou 510640, P. R. China. E-mail: [email protected], E-mail: [email protected]
b Institute of Materials for Electronics and Energy Technology (i-MEET), FAU
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Erlangen-Nürnberg, 91058 Erlangen, Germany.
c Bavarian Center for Applied Energy Research (ZAE Bayern), Immerwahrstraße 2,
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91058 Erlangen, Germany.
1
ACCEPTED MANUSCRIPT Abstract We report two novel small-molecule non-fullerene acceptors, IF-TN and IDT-TN, with indenofluorene (IF) and indacenodithiophene (IDT) as their respective central
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electron-rich cores and naphthyl-fused indanone (N) as the electron-withdrawing end-groups. By pairing these non-fullerene acceptors with a widely used polymer PBDB-T, the fabricated polymer solar cells based on PBDB-T:IDT-TN obtain a power
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conversion efficiency up to 5.89%, associated with a relatively high open-circuit
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voltage of 0.97 V and a remarkably low non-radiative open-circuit voltage loss of 0.23 V. These findings provide an effective approach for the rational molecular design of small-molecule acceptors with low non-radiative open-circuit voltage loss. Keywords: Non-fullerene polymer solar cells, electron acceptors, small molecules,
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open-circuit voltage loss
2
ACCEPTED MANUSCRIPT 1. Introduction Over the past two decades, solution-processed polymer solar cells (PSCs) have been considered as promising candidates for harnessing solar energy due to their unique
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advantages, including low cost, light weight, mechanical flexibility, and short energy payback time because they can be fabricated using roll-to-roll processing techniques [1-4]. For the electron-accepting materials used for the light-harvesting layer, the
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recently emerged non-fullerene acceptors (NFAs) have attracted tremendous interests
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due to their outstanding properties, such as intense and broad absorption, easily tunable energy levels, and facile accessibility [5-9]. The specific merits of such small molecule NFAs include well-defined molecular structures and molecular weights and their ability to be obtained with high purity without batch-to-batch variations.
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Typically, small-molecule NFAs with acceptor-donor-acceptor (A-D-A)–type molecular structures have been extensively investigated in PSCs and have achieved high power conversion efficiencies (PCEs) [10-21]. In order to extend the absorption
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of the small molecule acceptors into the near-infrared region to enhance their
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light-harvesting capability to achieve a high short-circuit current (JSC), much effort has been focused on increasing the conjugation size of the molecules, modifying the side chains, and introducing electron-deficient groups [22-25]. However, red-shifting the absorption of the electron-accepting materials may inevitably reduce the lowest unoccupied molecular orbital (LUMO) level and thus leading to a decreased open-circuit voltage (VOC) [24,26]. More to this point, enlarging the backbone size of the acceptors may lead to over-aggregation of molecules, which would lead to a large 3
ACCEPTED MANUSCRIPT phase separation that would be detrimental to the fill factor (FF) [23,27]. In this respect, the development of new NFAs that can address the trade-off between the absorption and the VOC of the resulting devices is highly desired. a
novel
moiety
of
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Recently,
2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene) malononitrile (N) that can
extend
the
conjugation
length
has
been
employed
as
end-capped
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electron-deficient groups for NFAs [28-30]. The electron-deficient N group exhibited
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better planarity than phenyl-based indanone, which not only enhanced the light-harvesting capability and lead to the improvement of JSC, but also enhanced the intermolecular π-π interaction and lead to in optimal film morphology [29]. In this communication, we describe the design and synthesis of two novel NFAs, IF-TN and
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IDT-TN, that incorporate indenofluorene (IF) and indacenodithiophene (IDT) as their respective electron-donating core units (Fig. 1a). The use of the N unit as end-group extended the molecular planarity, which could enhance the intramolecular push-pull
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effect between the electron-donating unit in the center and the electron-withdrawing
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unit in the terminal [29].
4
ACCEPTED MANUSCRIPT 7x104
(b)
IF-TN IDT-TN PBDB-T
4
6x10
5x104 4x104 4
3x10
2x104 1x104 0 300
400
500 600 700 Wavelength (nm)
800
-3.93 -4.03
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-5.33
-4.18 PNDIT-F3N-Br
-5.10
-5.5
IDT-TN
-4.7
IF-TN
-5.0
PBDB-T
-4.5
ITO
SC
-4.0
900
(c)
-3.53
-3.5
PEDOT:PSS
Energy Level (eV)
-3.0
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-1
Absorption Coefficient (cm )
(a)
-4.6 Ag
-5.42 -5.47 -5.80
Fig. 1 (a) The molecular structures and (b) absorption spectra of the acceptors and the
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donor, and (c) energy level diagram.
2. Experimental section
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2.1. Materials and synthesis
All the reactions were carried out under the protection of the inert atmosphere.
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All reagents and solvents were commercially available and used directly without further purification unless otherwise specified. Tetrahydrofuran (THF) and toluene (PhMe) were dried from sodium/benzophenone and freshly distilled before to use. Compounds (1), (2) and N were synthesized according to the reported procedures [29,3133]. The synthetic routes of the two acceptors are described in Scheme 1, and the detailed synthesis procedures are shown as following. Synthesis of IF(C8)-T-CHO 5
ACCEPTED MANUSCRIPT To a 50 mL two-necked round bottom flask, compound (1) (300 mg, 0.31 mmol), compound 3-(2-ethylhexyl)thiophene (286 mg, 0.93 mmol), Pd(PPh3)4 (30 mg, 0.026 mmol) and THF (20 mL) were added. The mixture was deoxygenated with argon for
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30 min. And then K2CO3 (1 M, 1.0 mL) was added and the reaction system was heated at 80 oC overnight. After cooling to room temperature, the reaction was quenched by water and extracted with dichloromethane (DCM). And then purified by
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flash column chromatography on silica gel with dichloromethane/petroleum ether (1:2)
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as the eluent to afford IF(C8)-T-CHO as a yellow solid (252 mg, 70%). 1HNMR (500MHz, CDCl3, δ): 9.89 (s, 2H), 7.81 (d, J = 10 Hz, 2H), 7.66 (d, J = 5.0 Hz, 4H), 7.46, 7.44 (dd, J = 1.5 Hz, J = 1.5 Hz, 2H), 7.42 (d, J = 5.0 Hz, 2H), 2.69 (d, J = 5.0 Hz, 4H), 2.05 (t, J = 10 Hz, 8H), 1.61 (m, 2H), 1.22 (m, 24H), 1.06 (m, 32H), 0.78 (m,
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24H), 0.68 (m, 8H). 13C NMR (125 MHz, CDCl3, δ): 182.90, 151.63, 150.68, 150.29, 141.96, 140.86, 140.33, 139.38, 138.94, 131.99, 128.29, 125.53, 123.78, 119.80, 114.29, 54.98, 40.63, 40.48, 32.82, 32.45, 31.78, 30.32, 30.05, 29.70, 29.31, 29.22,
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28.68, 25.51, 23.89, 22.97, 22.57, 14.05, 14.03, 10.51.
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Synthesis of IDT(C8)-T-CHO
The synthetic route was similar to the reported procedure [29]. To a 50 mL
two-necked round bottom flask compound 6 (250 mg, 0.24 mmol), compound 3-(2-ethylhexyl)thiophene (218 mg, 0.72 mmol), Pd(PPh3)4 (30 mg, 0.026 mmol) and toluene (30 mL) were added. The mixture was deoxygenated with nitrogen for 30 min. The reaction mixture was refluxed for 24 h, and then was cooled down to room temperature. After removing the solvent, the residue was purified using column 6
ACCEPTED MANUSCRIPT chromatography on a silica gel with dichloromethane/petroleum ether (1:3) as an eluent to yield an orange solid (209 mg, 75%). 1HNMR (500MHz, CDCl3, δ): 9.84 (s, 2H), 7.57 (s, 2H), 7.31 (s, 2H), 7.18 (s, 2H), 2.81 (br.s, 4H), 2.04-1.98 (m, 4H),
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1.92-1.86 (m, 4H), 1.75-1.70 (m, 2H), 1.62-1.39 (m, 16H), 1.22-1.88 (m, 8H), 1.12 (br.s, 32H), 0.90-0.86 (m, 16H), 0.82-0.79 (t, J = 7.5 Hz, 16H). 13C NMR (125 MHz, CDCl3, δ): 182.53, 155.86, 153.46, 143.88, 143.08, 139.73, 139.08, 136.38, 135.79,
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125.53, 122.21, 113.49, 54.28, 39.88, 39.08, 33.73, 32.55, 31.80, 30.23, 29.99, 29.34,
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29.24, 28.71, 25.69, 24.28, 23.07, 22.60, 14.11, 14.07, 10.64. MS (MALDI-TOF) calcd for C74H110O2S4, 1158.7389; found, 1159.171.
Synthesis of IF-TN
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IF(C8)-T-CHO (150mg, 0.131mmol), N (96mg, 0.393mmol), and chloroform (30 mL) were added to a two-necked round-bottomed flask, the mixture was deoxygenated with argon for 20 min and pyridine (0.5 mL) was added slowly, and
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then stirred overnight at 65 oC. After cooling to room temperature, the solvent was
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removed under vacuum. The crud product was purified by column chromatography on a silica gel using dichloromethane as an eluent and then recrystallization from chloroform/methanol to afford IF-TN as a dark purple solid (167mg, 80%). 1HNMR (500MHz, CDCl3, δ): 9.22 (s, 2H), 8.97 (s, 2H), 8.41 (s, 2H), 8.12-8.05 (m, 4H), 7. 87 (d, J =5Hz, 2H), 7.81 (s, 2H), 7.73-7.69 (m, 6H), 7.60 (dd, J =5, 5Hz, 2H), 7.54 (s, 2H), 2.75 (d, J =10Hz, 4H), 2.10 (t, J =7.5Hz, 8H), 1.69-1.63 (m, 2H), 1.29-1.10 (m, 56H), 0.85-0.71 (m, 32H);
13
C NMR (125 MHz, CDCl3, δ): 188.31, 160.78, 158.92, 7
ACCEPTED MANUSCRIPT 151.80, 150.92, 148.35, 142.56, 140.85, 140.51, 138.78, 136.26, 135.47, 135.45, 134.72, 132.95, 131.97, 130.68, 130.24, 129.95, 129.67, 128.52, 127.04, 124.66, 124.39, 123.56, 120.02, 115.21, 115.02, 114.47, 68.36, 55.13, 40.70, 40.44, 32.67,
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32.47, 31.80, 30.09, 29.37, 29.26, 28.64, 25.57, 23.96, 23.00, 22.59, 14.07, 14.04, 10.50. MS (MALDI-TOF) calcd for C110H126N4O2S2, 1598.9322; found, 1600.221. Synthesis of IDT-TN
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The synthetic routes of IDT-TN are similar to that of IF-TN. The target product
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was obtained as a deep green solid (171 mg, 82% yield) 1H NMR (500 MHz, CDCl3, δ): 9.18 (s, 2H), 8.88 (br.s, 2H), 8.39 (s, 2H), 8.06-8.04 (m, 4H), 7.72-7.66 (m, br.s, 6H), 7.54 (s, 2H), 7.38 (s, 2H), 2.89 (br.s, 4H), 2.11-2.05 (m, 4H), 2.00-1.94 (m, 4H), 1.87-1.80 (m,2H), 1.47-1.31 (m, 16H), 1.23-1.15 (m, br.s, 40H), 0.96-0.89 (m, 16H),
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0.83-0.80 (t, J =7.5Hz, 16H); 13C NMR (125 MHz, CDCl3, δ): 188.40, 160.18, 154.25, 149.66, 146.62, 137.49, 136.34, 136.22, 135.36, 134.81, 132.90, 130.63, 130.21, 129.80, 129.56, 126.79, 124.40, 123.62, 115.36, 115.22, 113.92, 99.98, 67.59, 54.40,
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39.41, 39.26, 33.87, 32.58, 31.81, 30.02, 29.37, 29.28, 28.70, 25.79, 24.33, 23.11,
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22.62, 14.16, 14.09, 10.67. MS (MALDI-TOF) calcd for C106H122N4O2S4, 1610.8451; found, 1612.150.
8
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ACCEPTED MANUSCRIPT
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Scheme 1. The synthetic routes of the target compounds IF-TN and IDT-TN.
1
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2.2. Characterization and measurement
H and 13C NMR spectra were recorded on a 500 MHz NMR spectrometer with
tetramethylsilane (TMS) as the internal reference. MALDI-TOF-MS spectra was tested on a Bruker Agilent1290/maXis spectrometer. UV-vis spectra was performed
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by using a HP 8453 spectrophotometer. Thermogravimetric analyses (TGA) were recorded on a NETZSCH TG 209 at a heating rate of 10 oC min-1 under a nitrogen flow rate of 20 mL min-1. Cyclic voltammetry (CV) experiments were performed on a
EP
CHI600D electrochemical workstation equipped with an glass carbon electrode as
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working electrode, a platinum wire counter electrode, a Ag/AgNO3 reference electrodes
and
a
0.1
L-1
mol
acetonitrile
solution
of
tetrabutylammoniumhexafluorophosphate (n-Bu4NPF6) as the supporting electrolyte. The
scan
rate
was
100
mV
s-1
under
a
nitrogen
atmosphere.
The
ferrocene/ferrocenium redox couple (Fc/Fc+) was used to calibrate the potential of Ag/AgNO3 reference electrode and its onset potential was measured to be 0.3 V. The HOMO and LUMO levels were calculated according to the equations: 9
ACCEPTED MANUSCRIPT EHOMO=-e(Eoxonset-EFc/Fc++4.8) eV and ELUMO=-e(Eredonset-EFc/Fc++4.8) eV, respectively, where Eoxonset and Eredonset were the onset of oxidation and reduction potentials, respectively. The atomic force microscopy (AFM) measurements were carried out on
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a NanoMan VS microscope with a tapping mode. The geometry of the acceptors were optimized by Density Functional Theory (DFT) method at a B3LYP/6-31G(d) level to optimize the ground state geometries. All the calculations of the acceptor molecules
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were measured using the Gaussian 09 package package40. In this work, the
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straight-chain substituents of the acceptors were replaced with methyl groups, and the branched side-chain substituents were replaced with isopropyl groups for calculations. Grazing-incidence wide-angle X-ray scattering (GIWAXS) experiments were carried out on a Xenocs Xeuss 2.0 system with an Excillum MetalJet-D2 X-ray source
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operated at 70.0 kV, 2.8570 mA, and a wavelength of 1.341 Å. The grazing-incidence angle was set at 0.20°. Scattering pattern was collected with a DECTRIS PILATUS3
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R 1M area detector.
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2.3. Device fabrication and characterization Organic
photovoltaic
devices
with
a
conventional
configuration
of
ITO/PEDOT:PSS/active layer/PNDIT-F3N-Br/Ag were fabricated. Firstly, the ITO glass substrates were pre-cleaned sequentially by using detergent, water, ethanol, acetone, and isopropyl alcohol, each of the process was approximately 15 min under sonication, and then dried in oven at 70 °C for 10 h before to use. The PEDOT:PSS (Heraeus Clevios P VP A 4083) was spin-coated onto the ITO glass at 3000 rpm for 10
ACCEPTED MANUSCRIPT 30 s and then annealed at 150 oC for 10 min in air. Subsequently, the substrates were transferred into a N2-protected glove box for spin-coating the active layer. The donor polymer PBDB-T and the small molecule acceptor, IF-TN or IDT-TN, were dissolved
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in CB solution (with variant blend ratios, the total concentration of the donor polymer and the acceptor is 10 mg mL-1). The mixed solution was spin-coated atop the PEDOT:PSS layer at 2000 rpm for 30 s to form the active layer with a film
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thicknesses approximately 100 nm. Then, the active layers were treated with thermal
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annealing at 80 oC for 5 min. Finally, the interface layer (5 nm) of PNDIT-F3N-Br in methanol (0.5mg mL-1) was spin-coated on the blended films, and then the top electrode silver (100 nm) was deposited onto the interlayer PNDIT-F3N-Br by thermal evaporation though a shadow mask in a vacuum chamber with a base pressure
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of 1×10-6 mbar. The active layer area of the device was around 0.04 cm2. The current density-voltage (J-V) characteristics were recorded under an AM 1.5G solar simulator (Taiwan, Enlitech SS-F5) at a light intensity of 100 mW cm-2 by using a
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computer-controlled Keithley 2400 source meter, which was tested by a calibrated
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silicon solar cell (certified by National Renewable Energy Laboratory) before to test. The PL spectra was performed on a FLS920 spectro-fluorimeter (Edinburgh Instruments) and the EQE spectra was measured on a commercial EQE measurement system (Taiwan, Enlitech, QE-R).
2.4. Fabrication and characterization of single-carrier devices The charge carrier mobilities of PBDB-T: acceptors blend films and the acceptor 11
ACCEPTED MANUSCRIPT neat films were determined from space-charge-limited current (SCLC) devices. The devices structures were ITO/Al/ blend films (or neat film)/Ca/Al for electron-only mobility and ITO/PEDOT:PSS/blend films (neat film)/MoO3/Ag for hole-only
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mobility, respectively. The charge carrier mobilities were determined by fitting the dark J–V current to the model of a single carrier SCLC which were calculated
9 = ε ε μ 8
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according to the following equation:
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where J is the current density, ε0 and εr are the permittivity of free space and relative permittivity of the material, respectively. and µh is the zero-filed mobility, V is the effective voltage and d is the thickness of the organic layer. The effective voltage can be obtained from the equation V = Vappl−Vbi−Vs, where Vapp is the applied voltage, Vbi
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is the offset voltage and Vs is the voltage drop. The carrier mobilities can be calculated from the slope of the J1/2–V curves.
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3. Results and discussion
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3.1. Material synthesis and characterization The detailed synthetic routes of the two acceptors, IF-TN and IDT-TN, are
shown
in
Scheme
1.
The
N
group
was
synthesized
by
using
2,3-naphthalenedicarboxylic anhydride as the starting material [29]. The intermediate compounds IF(C8)-T-CHO and IDT(C8)-T-CHO were synthesized based on palladium-catalyzed Suzuki and Stille coupling reactions, respectively [3133]. By treating IF(C8)-T-CHO and IDT(C8)-T-CHO with an N unit via a Knoevenagel 12
ACCEPTED MANUSCRIPT condensation reaction, IF-TN and IDT-TN were obtained with good yield of 80% and 82%, respectively. The molecular structures of the target NFAs were confirmed by 1H and
13
C nuclear magnetic resonance (NMR) spectroscopy as well as matrix-assisted
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laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) (Fig. S8-S18, SI). These NFAs showed excellent solubility in chloroform, chlorobenzene, and o-dichlorobenzene.
As evaluated by differential scanning calorimetry
SC
measurement from 25 to 240 oC, no obvious thermal transition characteristics can be
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note for both NFAs (Fig. S1b, SI). Thermogravimetric analysis revealed their good thermal stability, with decomposition temperature (Td, 5% weight loss) as 354°C and 353°C for IF-TN and IDT-TN, respectively (see Fig. S1a, SI).
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3.2. Theoretical calculations
The optimal molecular geometries of IF-TN and IDT-TN were simulated by density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level. Fig. 2
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shows that the torsion angles of the optimized geometries of IF-TN and IDT-TN were
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48.5° and 24.9°, respectively. The torsion angle of IF-TN is larger than that of IDT-TN because the phenyl ring has greater steric hindrance than the thienyl ring. IDT-TN has more coplanar backbones than IF-TN, which may facilitate π-electron delocalization and enhance charge mobility.
13
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ACCEPTED MANUSCRIPT
Fig. 2 Geometry-optimized structures and frontier molecular orbitals of (a) IF-TN and
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3.3. Optical and electrochemical properties
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(b) IDT-TN, as determined by DFT calculations at the B3LYP/6-31G(d, p) level.
Fig. 1b displays the ultraviolet-visible (UV-vis) absorption spectra of IF-TN and IDT-TN as thin films. IF-TN film shows two absorption bands of 300-400 nm and 480-680 nm, while IDT-TN film exhibits an evident absorption between 560 to 880
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nm. The maximum absorption spectra of IDT-TN are red-shifted by 11 nm from solution (λmax = 756 nm) to film (λmax = 767 nm), which can be attributed to the strong π–π intermolecular interactions in thin films. For IF-TN, the absorption spectra of the
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core unit of IF at 346 nm is stronger than the ICT absorption spectra at 570 nm, which
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can be attributed to the intense π-π stacking of the large structure core IF. The red-shifted absorption of IDT-TN compared to that of IF-TN can be attributed to the stronger electron-donating capability of IDT than that of the IF unit. From the absorption onset, the optical bandgaps of IF-TN and IDT-TN are calculated to be 1.81 and 1.43 eV, respectively.
The electronic energy levels of IF-TN and IDT-TN were
evaluated via cyclic voltammetry (CV) measurements with ferrocene/ferrocenium (FC/FC+) as the reference (Fig. S2b, SI). The HOMO/LUMO levels of IF-TN and 14
ACCEPTED MANUSCRIPT IDT-TN were estimated to be –5.80/–3.93 and –5.42/–4.03 eV, respectively (Fig. 1c). The LUMO gaps between PBDB-T and IF-TN and IDT-TN are 0.4 and 0.5 eV,
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respectively, which ensures efficient exciton dissociation.
3.4. Photovoltaic properties
To investigate the potential applications of IF-TN and IDT-TN in PSCs as
layer/
PNDIT-F3N-Br/Ag,
and
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ITO/PEDOT:PSS/active
SC
electron acceptors, we fabricated PSCs with a conventional configuration of
poly[(9,9-bis(3’-((N,N-dimethyl)-methylammonium)propyl)-2,7-fluorene)-alt-5,5’-bis (2,2’-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N’-di(2-ethylhexyl)imide]d ibromide (PNDIT-F3N-Br) was used as the interfacial layer to facilitate electron
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extraction [ 34 , 35 ]. The active layers composed of a wide-bandgap polymer PBDB-T:IF-TN or PBDB-T:IDT-TN with a weight ratio of 1:1, were processed from chlorobenzene (CB). Fig. 3a shows the current density–voltage (J–V) curves of the
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optimized devices, with the corresponding photovoltaic parameters listed in Table 1.
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The detailed optimization of the PSCs is summarized in the experimental section Table S3 (SI).
The as-cast device based on PBDB-T:IF-TN showed a PCE of (2.54±0.02)%,
while a slightly higher PCE of (4.34±0.09)% (VOC = 0.973±0.002 V, JSC = 10.88±0.21 mA cm-2, FF = (41.02±0.28)%) was obtained for PBDB-T:IDT-TN-based device. When 2% CN additive was used as additive, the PCEs of IF-TN− and IDT-TN−based devices were elevated to (2.89±0.18)% [VOC = 1.006±0.005 V, JSC = 7.02±0.28 mA 15
ACCEPTED MANUSCRIPT cm-2, FF = (40.95±1.15)%] and (5.77±0.09)% [VOC = 0.967±0.003 V, JSC = 13.22±0.12 mA cm-2, FF = (45.17±0.4)%], respectively. The external quantum efficiency (EQE) spectra of the studied devices based
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PBDB-T and the two acceptors are shown in Fig. 3b. For IF-TN, the EQE shows a broad photoresponse from 320 to 730 nm with a maximum peak up to 42% at 580 nm, whereas for IDT-TN, the EQE exhibits a broader response, from 330 to 880 nm, with
SC
a maximum value of about 49% at 670 nm. The EQE spectra are in good agreement
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with the UV-Vis absorption spectra of the acceptors and the PBDB-T donor (Fig. 1b), suggesting that both of the polymer donor and the small molecule NFAs contribute to the photo-current. The integrated photocurrent from the EQE spectra are 6.02 and 6.86 mA cm-2 for the IF-TN devices, and 10.61 and 12.80 mA cm-2 for the IDT-TN
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devices with and without CN additive, respectively, which agree with those obtained from J-V measurements. The higher JSC were achieved from the devices processed CN additive, which can be attributed to the more effective charge transfer in the
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factors.
EP
photoactive layer, or the enhanced charge carrier mobility, or the combination of both
16
50
-5
-10
0.2 0.4 0.6 0.8 Voltage(V)
1
1.2
(b)
40 30 20 10
-15 -0.2 0
PBDB-T:IF-TN PBDB-T:IF-TN (2% CN) PBDB-T:IDT-TN PBDB-T:IDT-TN (2% CN)
60
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0
70
(a)
PBDB-T:IF-TN PBDB-T:IF-TN (2%CN) PBDB-T:IDT-TN PBDB-T:IDT-TN (2%CN)
EQE (%)
5
2
Current Density (mA/cm )
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0 300 400 500 600 700 800 900 Wavelength (nm)
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Fig. 3 (a) J–V curves and (b) EQE spectra of devices based on PBDB-T, IF-TN, and
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IDT-TN devices without or with CN.
Table 1 Photovoltaic properties of the PSCs based on PBDB-T as donor and IF-TN or IDT-TN as acceptor under AM 1.5 G at 100 mW cm-2. CN
Jsc
FF
PCEa
(mA cm-2)
(%)
(%)
Voc
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Active layer
(vol%)
(V)
w/o
1.000±0.008 6.19±0.06
41.10±0.40
2.54±0.02 (2.57)
PBDB-T:IFTN
2%
1.006±0.005 7.02±0.28
40.95±1.15
2.89±0.18 (3.03)
w/o
0.973±0.002 10.88±0.21 41.02±0.28
4.34±0.09 (4.49)
0.967±0.003 13.22±0.12 45.17±0.47
5.77±0.09 (5.89)
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PBDB-T:IDT-TN PBDB-T:IDT-TN a
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PBDB-T:IF-TN
2%
The PCE values in the parenthesis represent the highest values of the fabricated
devices.
To clarify the higher JSC values for the devices processed with CN additive, we analyzed the photoluminescence (PL) spectra of pristine donor and acceptor films, 17
ACCEPTED MANUSCRIPT and relevant blend films (Fig. 4a and Fig. 4b). The blend films processed with 2% CN additive exhibited more pronounced PL quenching than those without CN, which is favorable for an efficient charge transfer. To investigate the charge recombination, we
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measured the JSC as a function of light intensity (Plight) from 5 to 100 mW cm-2 (Fig. 4c and 4d). The power-law dependence of JSC on illumination intensity can be expressed as JSC ∝ (Plight)α, where α is the exponential factor, which is close to unity
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when the bimolecular recombination in an organic solar cell is weak. When processed
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with CN, the obtained α values for both IF-TN- and IDT-TN-based devices increased (from 0.955 to 0.961 for IF-TN, and from 0.954 to 0.974 for IDT-TN), indicating that
(a)
PBDB-T IF-TN PBDB-T:IF-TN PBDB-T:IF-TN (2% CN)
0.8 0.6 0.4 0.2
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1
excited @550 nm
Normalized PL intensity
1.2
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Normalized PL intensity
the bimolecular recombination was slightly impeded. 1.2
1
0.8
0.2 0
PBDB-T:IDT-TN PBDB-T:IDT-TN (2%CN)
α = 0.961
1
0.1
640 680 720 760 800 840 880 Wavelength (nm)
100
(c) -2
10
excited @610 nm
0.4
Jsc (mA cm )
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-2
Jsc (mA cm )
PBDB-T:IF-TN PBDB-T:IF-TN (2%CN)
α = 0.955
10 100 -1 Light intensity (mW cm )
(b)
0.6
0 600 650 700 750 800 850 900 Wavelength (nm) 100
PBDB-T IDT-TN PBDB-T:IDT-TN PBDB-T:IDT-TN (2% CN)
(d)
10
α = 0.974 1
0.1
α = 0.954
10 100 -1 Light intensity (mW cm )
Fig. 4 (a and b) PL spectra of thin films: PBDB-T, IF-TN and PBDB-T: IF-TN (1:1, 18
ACCEPTED MANUSCRIPT w/w, without and with 2% CN) and PBDB-T, IDT-TN and PBDB-T:IDT-TN (1:1, w/w, without and with 2% CN), respectively; (c and d) JSC as a function of light intensity for the devices based on PBDB-T:IF-TN and PBDB-T:IDT-TN with or
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without CN additives, respectively.
To elucidate the realization of simultaneous low bandgap and high VOC for
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PBDB-T:IDT-TN-based devices, we analyzed the VOC loss in both PBDB-T:IDT-TN
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and PBDB-T:IF-TN devices that processed with 2% CN. As determined from the intersection of the absorption edge and the EQE maximum (Fig. S5, SI) [36], the bandgaps (Eg) of PBDB-T:IDT-TN and PBDB-T:IF-TN were 1.58 and 1.93 eV, respectively (Table S5). The corresponding Schockley-Queisser limit VOC (VOC, SQ)
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[37] was estimated to be 1.32 and 1.64 V, respectively. According to the reciprocal theory [ 38 ], the radiative VOC limit (VOC,
rad)
calculated by combining
Fourier-transform photocurrent spectroscopy (FTPS) and electroluminescence (EL)
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spectroscopy (Fig. 5), were 1.20 and 1.37 V for IDT-TN and IF-TN-based devices,
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respectively. Therefore, the non-radiative VOC loss (∆VOC, nr), which equals to VOC, rad – VOC, meas, is 0.23 V for the PBDB-T:IDT-TN cell and 0.36 V for the PBDB-T:IF-TN cell. The detailed calculation procedures are provided in the supporting information. The remarkably low ∆VOC,
nr
of 0.23 V obtained for the IDT-TN-based device is
among the lowest losses reported for state-of-the-art organic solar cells and is approaching the losses reported for other conventional photovoltaic technologies, such as Si and CIGS [39,40]. It is also noteworthy that the difference between the VOC, 19
ACCEPTED MANUSCRIPT SQ
and the VOC, rad for IDT-TN-based device is only 0.12 V, which contributes to the
simultaneous low bandgap (enhanced JSC) and high VOC for PBDB-T:IDT-TN-based devices. However, it remains unclear the origination of relatively low ∆VOC,nr of the
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PBDB-T:IDT-TN blends. It is well-established that there is an intrinsic link between the ∆VOC,nr and electron-vibration coupling, and the non-radiative VOC loss of the organic solar cells is unavoidable in organic solar cells [ 41 , 42 ]. There is a
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fundamental correlation between the charge transfer (CT) state energy and the
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non-radiative VOC losses, which is associated with an increased wave-function overlap between the relaxed CT state and higher order vibrational modes of the ground state [43]. Similar low ∆VOC,nr has also been realized in other OSCs based on non-fullerene
-2
EQEFTPS
10
φEL/φbb
-5
1.2
1.4
10
-2
10
-3
1.6
PBDB-T:IF-TN
1.8
2.0
2.2
10
-5
10
-6
100
100
10-1
10-1
φEL
EQE 10-2 10
-3
10-2 10-3
EQEFTPS
10-4 10-5
10-4
10-6
φEL/φbb
10-5
10-7 10-8
10
2.4
Energy (eV)
10-7
PBDB-T:IDT-TN
-6
1.0
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1.0
10
10-4
10-3 10-4
φEL
EP
EQE
10
EQE
(b)
EL
10-1
-1
EQE
100
100
EL
(a)
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acceptors[44].
10-8 1.2
1.4
1.6
1.8
2.0
2.2
2.4
Energy (eV)
Fig. 5 Semi-logarithmic plots of measured EQE, FTPS calibrated by EQE (EQEFTPS), and ϕEL/ϕbb fitting as well as normalized EL (ϕEL) as a function of energy for devices based on PBDB-T:IF-TN (1:1, ,wt:wt, 2% CN) (a) and PBDB-T:IDT-TN (1:1, wt:wt, 2% CN) (b). The black arrow represents the charge transfer feature. The ratio of ϕEL/ϕbb was used to plot the EQE in the low energy regime (black line), where ϕEL and ϕbb represent the emitted photon flux and the room-temperature blackbody photon 20
ACCEPTED MANUSCRIPT flux, respectively.
3.5. Charge carrier mobility
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The electron mobilities for the blend films with and without CN were measured by the space-charge limited current (SCLC) (Fig. 6). The electron mobility of the PBDB-T:IF-TN blend was slightly enhanced from 7.78× 10-6 cm2 V-1 s-1 for the
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pristine film to 1.02 × 10-5 cm2 V-1 s-1 for the film processed with CN. A similar trend
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was observed for PBDT:T-IDT-TN blend film (Table S4, SI). We have recently shown that SCLC may significantly underestimated the mobility of photoexcited carriers in a disordered semicomductor [ 45 ]. Although Time of Flight is more correctly determining mobility values, SCLC was chosen here because of the limited film
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thickness. These observations are consistent with the slightly enhanced current density for devices incorporating CN as the solvent additive.
(a)
100 1/2 1/2
80 60 40
1
0
1.5 2 2.5 V -V -V (V) appl
PBDB-T:IF-TN PBDB-T:IF-TN (2% CN) PBDB-T:IDT-TN PBDB-T:IDT-TN (2% CN)
20
10
0.5
(b)
120
J
J
140
(A /m)
20
AC C
1/2
30
1/2
(A /m)
40
PBDB-T:IF-TN PBDB-T:IF-TN (2% CN) PBDB-T:IDT-TN PBDB-T:IDT-TN (2% CN)
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50
bi
3
3.5
-20
0
0.5
1 V
s
1.5 2 -V -V (V)
appl
bi
2.5
s
Fig. 6 (a) Electron-only and (b) hole-only J1/2–V characteristics of PDBT-T:IF-TN and PDBT-T:IDT-TN blend films without or with CN.
21
3
ACCEPTED MANUSCRIPT 3.6. Morphology Atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) were used to probe the thin film morphology, with the
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corresponding images shown in Fig. 7 and Fig. S6-S7 (SI). Both blend films exhibited relatively smooth surface profile with fine fibrous structures across the entire films (Fig. 7a, b). The GIWAXS patterns of the IF-TN and IDT-TN neat films exhibited
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strong (010) diffraction peaks in the out-of-plane (OOP) direction, with corresponding
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q values of about 1.81 and 1.87 Å−1, respectively (Fig. S7a and c, SI), implying the formation of predominant face-on orientation. Similarly, both PBDB-T:IF-TN and PBDB-T:IDT-TN blends showed obvious OOP (010) peaks at approximate 1.8 Å−1 with mirror (100) peak in the in-plane (IP) direction (Fig. 7c, d), indicating the Note that the OOP (010) diffraction peaks
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maintaining of the face-on orientation.
are stronger for both blend films processed from CB containing CN than those without CN (Fig. S7b and d, SI), indicating more phase-separated microstructure
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morphology with more ordered molecular chains. These observations were consistent
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with the improved charge carrier mobilities of devices processed with CN additives.
22
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Fig. 7 AFM height images (5 × 5 µm) and 2D-GIXD patterns of PBDB-T:IF-TN (a
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4. Conclusion
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and c, 2% CN) and PBDB-T:IDT-TN (b and d, 2% CN) blend films.
In summary, we developed two novel non-fullerene small molecule acceptors,
IF-TN and IDT-TN, based on indenofluorene or indacenodithiophene as the core unit, respectively, and end-capped with end-extended conjugation-group indanone derivatives. The acceptor IF-TN exhibited obviously larger bandgap of 1.81 eV, while a small bandgap of 1.43 eV was obtained for IDT-TN. Polymer solar cells based on a wide-bandgap electron-donating copolymer PBDB-T and such two acceptors 23
ACCEPTED MANUSCRIPT exhibited moderate power conversion efficiencies of 3.03% and 5.89%, respectively. Of particular interest is that the device based on IDT-TN exhibited a remarkably low non-radiative VOC loss of 0.23 V, which is one of the lowest values reported for
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state-of-the-art PSCs and is approaching the losses reported for mature PV technologies. These results suggest that extension of the conjugation length of the end-groups is a good strategy for the molecular design that can extend the
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light-absorption spectra and maintain a relatively high open-circuit voltage.
Acknowledgments
This work was financially supported by the Ministry of Science and Technology (No. 2014CB643501), the Natural Science Foundation of China (No. 91633301,
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21490573, 51673069), and the Science and Technology Program of Guangzhou, China (No. 201710010021, 201707020019 and 2017A050503002). N.L. gratefully acknowledges the financial support from the DFG research grant (BR 4031/13-1) and
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the Bavarian Ministry of Economic Affairs and Media, Energy and Technology by
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funding the HI-ERN (IEK11) of FZ Jülich. C.J.B. gratefully acknowledges the financial support through the “Aufbruch Bayern” initiative of the state of Bavaria (EnCN and “Solar Factory of the Future”), the Bavarian Initiative “Solar Technologies go Hybrid” (SolTech) and the SFB 953 (DFG).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://xxxxxx. 24
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Two novel small-molecule non-fullerene acceptors with naphthyl-fused indanone as the end-groups were developed
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Device based on IDT-TN shows a PCE of 5.89% with a high VOC of 0.97 V. Device based on PBDB-T:IDT-TN exhibited a remarkably low non-radiative
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open-circuit voltage loss of 0.23 V.
S1566-1199(18)30272-6
DOI:
10.1016/j.orgel.2018.05.050
Reference:
ORGELE 4710
To appear in:
Organic Electronics
Received Date: 31 March 2018 Revised Date:
15 May 2018
Accepted Date: 28 May 2018
Please cite this article as: R. Li, G. Liu, B. Fan, X. Du, X. Tang, N. Li, L. Ying, C.J. Brabec, F. Huang, Y. Cao, Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.05.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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0
PBDB-T:IF-TN PBDB-T:IDT-TN
-5
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2
Current Density (mA/cm )
Graphical abstract
PCE = 3.03 % V = 1.01 eV
-10
OC
-15
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Non-radiative VOC loss = 0.23 V
0.2 0.4 0.6 0.8 Voltage(V)
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-0.2 0
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PCE = 5.89 % V OC = 0.97 eV
1
1.2
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Non-fullerene acceptors end-capped with an extended conjugation group for efficient polymer solar cells
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Renlong Li,a,1 Gongchu Liu,a,1 Baobing Fan,a,b Xiaoyan Du,b Xiaofeng Tang,b Ning Li,b Lei Ying,*a Christoph J. Brabec,b,c Fei Huang,*a and Yong Caoa
Materials and Devices, South China University of Technology,
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Optoelectronic
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a State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer
Guangzhou 510640, P. R. China. E-mail: [email protected], E-mail: [email protected]
b Institute of Materials for Electronics and Energy Technology (i-MEET), FAU
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Erlangen-Nürnberg, 91058 Erlangen, Germany.
c Bavarian Center for Applied Energy Research (ZAE Bayern), Immerwahrstraße 2,
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91058 Erlangen, Germany.
1
ACCEPTED MANUSCRIPT Abstract We report two novel small-molecule non-fullerene acceptors, IF-TN and IDT-TN, with indenofluorene (IF) and indacenodithiophene (IDT) as their respective central
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electron-rich cores and naphthyl-fused indanone (N) as the electron-withdrawing end-groups. By pairing these non-fullerene acceptors with a widely used polymer PBDB-T, the fabricated polymer solar cells based on PBDB-T:IDT-TN obtain a power
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conversion efficiency up to 5.89%, associated with a relatively high open-circuit
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voltage of 0.97 V and a remarkably low non-radiative open-circuit voltage loss of 0.23 V. These findings provide an effective approach for the rational molecular design of small-molecule acceptors with low non-radiative open-circuit voltage loss. Keywords: Non-fullerene polymer solar cells, electron acceptors, small molecules,
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open-circuit voltage loss
2
ACCEPTED MANUSCRIPT 1. Introduction Over the past two decades, solution-processed polymer solar cells (PSCs) have been considered as promising candidates for harnessing solar energy due to their unique
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advantages, including low cost, light weight, mechanical flexibility, and short energy payback time because they can be fabricated using roll-to-roll processing techniques [1-4]. For the electron-accepting materials used for the light-harvesting layer, the
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recently emerged non-fullerene acceptors (NFAs) have attracted tremendous interests
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due to their outstanding properties, such as intense and broad absorption, easily tunable energy levels, and facile accessibility [5-9]. The specific merits of such small molecule NFAs include well-defined molecular structures and molecular weights and their ability to be obtained with high purity without batch-to-batch variations.
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Typically, small-molecule NFAs with acceptor-donor-acceptor (A-D-A)–type molecular structures have been extensively investigated in PSCs and have achieved high power conversion efficiencies (PCEs) [10-21]. In order to extend the absorption
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of the small molecule acceptors into the near-infrared region to enhance their
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light-harvesting capability to achieve a high short-circuit current (JSC), much effort has been focused on increasing the conjugation size of the molecules, modifying the side chains, and introducing electron-deficient groups [22-25]. However, red-shifting the absorption of the electron-accepting materials may inevitably reduce the lowest unoccupied molecular orbital (LUMO) level and thus leading to a decreased open-circuit voltage (VOC) [24,26]. More to this point, enlarging the backbone size of the acceptors may lead to over-aggregation of molecules, which would lead to a large 3
ACCEPTED MANUSCRIPT phase separation that would be detrimental to the fill factor (FF) [23,27]. In this respect, the development of new NFAs that can address the trade-off between the absorption and the VOC of the resulting devices is highly desired. a
novel
moiety
of
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Recently,
2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene) malononitrile (N) that can
extend
the
conjugation
length
has
been
employed
as
end-capped
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electron-deficient groups for NFAs [28-30]. The electron-deficient N group exhibited
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better planarity than phenyl-based indanone, which not only enhanced the light-harvesting capability and lead to the improvement of JSC, but also enhanced the intermolecular π-π interaction and lead to in optimal film morphology [29]. In this communication, we describe the design and synthesis of two novel NFAs, IF-TN and
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IDT-TN, that incorporate indenofluorene (IF) and indacenodithiophene (IDT) as their respective electron-donating core units (Fig. 1a). The use of the N unit as end-group extended the molecular planarity, which could enhance the intramolecular push-pull
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effect between the electron-donating unit in the center and the electron-withdrawing
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unit in the terminal [29].
4
ACCEPTED MANUSCRIPT 7x104
(b)
IF-TN IDT-TN PBDB-T
4
6x10
5x104 4x104 4
3x10
2x104 1x104 0 300
400
500 600 700 Wavelength (nm)
800
-3.93 -4.03
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-5.33
-4.18 PNDIT-F3N-Br
-5.10
-5.5
IDT-TN
-4.7
IF-TN
-5.0
PBDB-T
-4.5
ITO
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-4.0
900
(c)
-3.53
-3.5
PEDOT:PSS
Energy Level (eV)
-3.0
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-1
Absorption Coefficient (cm )
(a)
-4.6 Ag
-5.42 -5.47 -5.80
Fig. 1 (a) The molecular structures and (b) absorption spectra of the acceptors and the
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donor, and (c) energy level diagram.
2. Experimental section
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2.1. Materials and synthesis
All the reactions were carried out under the protection of the inert atmosphere.
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All reagents and solvents were commercially available and used directly without further purification unless otherwise specified. Tetrahydrofuran (THF) and toluene (PhMe) were dried from sodium/benzophenone and freshly distilled before to use. Compounds (1), (2) and N were synthesized according to the reported procedures [29,3133]. The synthetic routes of the two acceptors are described in Scheme 1, and the detailed synthesis procedures are shown as following. Synthesis of IF(C8)-T-CHO 5
ACCEPTED MANUSCRIPT To a 50 mL two-necked round bottom flask, compound (1) (300 mg, 0.31 mmol), compound 3-(2-ethylhexyl)thiophene (286 mg, 0.93 mmol), Pd(PPh3)4 (30 mg, 0.026 mmol) and THF (20 mL) were added. The mixture was deoxygenated with argon for
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30 min. And then K2CO3 (1 M, 1.0 mL) was added and the reaction system was heated at 80 oC overnight. After cooling to room temperature, the reaction was quenched by water and extracted with dichloromethane (DCM). And then purified by
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flash column chromatography on silica gel with dichloromethane/petroleum ether (1:2)
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as the eluent to afford IF(C8)-T-CHO as a yellow solid (252 mg, 70%). 1HNMR (500MHz, CDCl3, δ): 9.89 (s, 2H), 7.81 (d, J = 10 Hz, 2H), 7.66 (d, J = 5.0 Hz, 4H), 7.46, 7.44 (dd, J = 1.5 Hz, J = 1.5 Hz, 2H), 7.42 (d, J = 5.0 Hz, 2H), 2.69 (d, J = 5.0 Hz, 4H), 2.05 (t, J = 10 Hz, 8H), 1.61 (m, 2H), 1.22 (m, 24H), 1.06 (m, 32H), 0.78 (m,
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24H), 0.68 (m, 8H). 13C NMR (125 MHz, CDCl3, δ): 182.90, 151.63, 150.68, 150.29, 141.96, 140.86, 140.33, 139.38, 138.94, 131.99, 128.29, 125.53, 123.78, 119.80, 114.29, 54.98, 40.63, 40.48, 32.82, 32.45, 31.78, 30.32, 30.05, 29.70, 29.31, 29.22,
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28.68, 25.51, 23.89, 22.97, 22.57, 14.05, 14.03, 10.51.
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Synthesis of IDT(C8)-T-CHO
The synthetic route was similar to the reported procedure [29]. To a 50 mL
two-necked round bottom flask compound 6 (250 mg, 0.24 mmol), compound 3-(2-ethylhexyl)thiophene (218 mg, 0.72 mmol), Pd(PPh3)4 (30 mg, 0.026 mmol) and toluene (30 mL) were added. The mixture was deoxygenated with nitrogen for 30 min. The reaction mixture was refluxed for 24 h, and then was cooled down to room temperature. After removing the solvent, the residue was purified using column 6
ACCEPTED MANUSCRIPT chromatography on a silica gel with dichloromethane/petroleum ether (1:3) as an eluent to yield an orange solid (209 mg, 75%). 1HNMR (500MHz, CDCl3, δ): 9.84 (s, 2H), 7.57 (s, 2H), 7.31 (s, 2H), 7.18 (s, 2H), 2.81 (br.s, 4H), 2.04-1.98 (m, 4H),
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1.92-1.86 (m, 4H), 1.75-1.70 (m, 2H), 1.62-1.39 (m, 16H), 1.22-1.88 (m, 8H), 1.12 (br.s, 32H), 0.90-0.86 (m, 16H), 0.82-0.79 (t, J = 7.5 Hz, 16H). 13C NMR (125 MHz, CDCl3, δ): 182.53, 155.86, 153.46, 143.88, 143.08, 139.73, 139.08, 136.38, 135.79,
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125.53, 122.21, 113.49, 54.28, 39.88, 39.08, 33.73, 32.55, 31.80, 30.23, 29.99, 29.34,
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29.24, 28.71, 25.69, 24.28, 23.07, 22.60, 14.11, 14.07, 10.64. MS (MALDI-TOF) calcd for C74H110O2S4, 1158.7389; found, 1159.171.
Synthesis of IF-TN
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IF(C8)-T-CHO (150mg, 0.131mmol), N (96mg, 0.393mmol), and chloroform (30 mL) were added to a two-necked round-bottomed flask, the mixture was deoxygenated with argon for 20 min and pyridine (0.5 mL) was added slowly, and
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then stirred overnight at 65 oC. After cooling to room temperature, the solvent was
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removed under vacuum. The crud product was purified by column chromatography on a silica gel using dichloromethane as an eluent and then recrystallization from chloroform/methanol to afford IF-TN as a dark purple solid (167mg, 80%). 1HNMR (500MHz, CDCl3, δ): 9.22 (s, 2H), 8.97 (s, 2H), 8.41 (s, 2H), 8.12-8.05 (m, 4H), 7. 87 (d, J =5Hz, 2H), 7.81 (s, 2H), 7.73-7.69 (m, 6H), 7.60 (dd, J =5, 5Hz, 2H), 7.54 (s, 2H), 2.75 (d, J =10Hz, 4H), 2.10 (t, J =7.5Hz, 8H), 1.69-1.63 (m, 2H), 1.29-1.10 (m, 56H), 0.85-0.71 (m, 32H);
13
C NMR (125 MHz, CDCl3, δ): 188.31, 160.78, 158.92, 7
ACCEPTED MANUSCRIPT 151.80, 150.92, 148.35, 142.56, 140.85, 140.51, 138.78, 136.26, 135.47, 135.45, 134.72, 132.95, 131.97, 130.68, 130.24, 129.95, 129.67, 128.52, 127.04, 124.66, 124.39, 123.56, 120.02, 115.21, 115.02, 114.47, 68.36, 55.13, 40.70, 40.44, 32.67,
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32.47, 31.80, 30.09, 29.37, 29.26, 28.64, 25.57, 23.96, 23.00, 22.59, 14.07, 14.04, 10.50. MS (MALDI-TOF) calcd for C110H126N4O2S2, 1598.9322; found, 1600.221. Synthesis of IDT-TN
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The synthetic routes of IDT-TN are similar to that of IF-TN. The target product
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was obtained as a deep green solid (171 mg, 82% yield) 1H NMR (500 MHz, CDCl3, δ): 9.18 (s, 2H), 8.88 (br.s, 2H), 8.39 (s, 2H), 8.06-8.04 (m, 4H), 7.72-7.66 (m, br.s, 6H), 7.54 (s, 2H), 7.38 (s, 2H), 2.89 (br.s, 4H), 2.11-2.05 (m, 4H), 2.00-1.94 (m, 4H), 1.87-1.80 (m,2H), 1.47-1.31 (m, 16H), 1.23-1.15 (m, br.s, 40H), 0.96-0.89 (m, 16H),
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0.83-0.80 (t, J =7.5Hz, 16H); 13C NMR (125 MHz, CDCl3, δ): 188.40, 160.18, 154.25, 149.66, 146.62, 137.49, 136.34, 136.22, 135.36, 134.81, 132.90, 130.63, 130.21, 129.80, 129.56, 126.79, 124.40, 123.62, 115.36, 115.22, 113.92, 99.98, 67.59, 54.40,
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39.41, 39.26, 33.87, 32.58, 31.81, 30.02, 29.37, 29.28, 28.70, 25.79, 24.33, 23.11,
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22.62, 14.16, 14.09, 10.67. MS (MALDI-TOF) calcd for C106H122N4O2S4, 1610.8451; found, 1612.150.
8
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ACCEPTED MANUSCRIPT
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Scheme 1. The synthetic routes of the target compounds IF-TN and IDT-TN.
1
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2.2. Characterization and measurement
H and 13C NMR spectra were recorded on a 500 MHz NMR spectrometer with
tetramethylsilane (TMS) as the internal reference. MALDI-TOF-MS spectra was tested on a Bruker Agilent1290/maXis spectrometer. UV-vis spectra was performed
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by using a HP 8453 spectrophotometer. Thermogravimetric analyses (TGA) were recorded on a NETZSCH TG 209 at a heating rate of 10 oC min-1 under a nitrogen flow rate of 20 mL min-1. Cyclic voltammetry (CV) experiments were performed on a
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CHI600D electrochemical workstation equipped with an glass carbon electrode as
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working electrode, a platinum wire counter electrode, a Ag/AgNO3 reference electrodes
and
a
0.1
L-1
mol
acetonitrile
solution
of
tetrabutylammoniumhexafluorophosphate (n-Bu4NPF6) as the supporting electrolyte. The
scan
rate
was
100
mV
s-1
under
a
nitrogen
atmosphere.
The
ferrocene/ferrocenium redox couple (Fc/Fc+) was used to calibrate the potential of Ag/AgNO3 reference electrode and its onset potential was measured to be 0.3 V. The HOMO and LUMO levels were calculated according to the equations: 9
ACCEPTED MANUSCRIPT EHOMO=-e(Eoxonset-EFc/Fc++4.8) eV and ELUMO=-e(Eredonset-EFc/Fc++4.8) eV, respectively, where Eoxonset and Eredonset were the onset of oxidation and reduction potentials, respectively. The atomic force microscopy (AFM) measurements were carried out on
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a NanoMan VS microscope with a tapping mode. The geometry of the acceptors were optimized by Density Functional Theory (DFT) method at a B3LYP/6-31G(d) level to optimize the ground state geometries. All the calculations of the acceptor molecules
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were measured using the Gaussian 09 package package40. In this work, the
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straight-chain substituents of the acceptors were replaced with methyl groups, and the branched side-chain substituents were replaced with isopropyl groups for calculations. Grazing-incidence wide-angle X-ray scattering (GIWAXS) experiments were carried out on a Xenocs Xeuss 2.0 system with an Excillum MetalJet-D2 X-ray source
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operated at 70.0 kV, 2.8570 mA, and a wavelength of 1.341 Å. The grazing-incidence angle was set at 0.20°. Scattering pattern was collected with a DECTRIS PILATUS3
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R 1M area detector.
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2.3. Device fabrication and characterization Organic
photovoltaic
devices
with
a
conventional
configuration
of
ITO/PEDOT:PSS/active layer/PNDIT-F3N-Br/Ag were fabricated. Firstly, the ITO glass substrates were pre-cleaned sequentially by using detergent, water, ethanol, acetone, and isopropyl alcohol, each of the process was approximately 15 min under sonication, and then dried in oven at 70 °C for 10 h before to use. The PEDOT:PSS (Heraeus Clevios P VP A 4083) was spin-coated onto the ITO glass at 3000 rpm for 10
ACCEPTED MANUSCRIPT 30 s and then annealed at 150 oC for 10 min in air. Subsequently, the substrates were transferred into a N2-protected glove box for spin-coating the active layer. The donor polymer PBDB-T and the small molecule acceptor, IF-TN or IDT-TN, were dissolved
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in CB solution (with variant blend ratios, the total concentration of the donor polymer and the acceptor is 10 mg mL-1). The mixed solution was spin-coated atop the PEDOT:PSS layer at 2000 rpm for 30 s to form the active layer with a film
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thicknesses approximately 100 nm. Then, the active layers were treated with thermal
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annealing at 80 oC for 5 min. Finally, the interface layer (5 nm) of PNDIT-F3N-Br in methanol (0.5mg mL-1) was spin-coated on the blended films, and then the top electrode silver (100 nm) was deposited onto the interlayer PNDIT-F3N-Br by thermal evaporation though a shadow mask in a vacuum chamber with a base pressure
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of 1×10-6 mbar. The active layer area of the device was around 0.04 cm2. The current density-voltage (J-V) characteristics were recorded under an AM 1.5G solar simulator (Taiwan, Enlitech SS-F5) at a light intensity of 100 mW cm-2 by using a
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computer-controlled Keithley 2400 source meter, which was tested by a calibrated
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silicon solar cell (certified by National Renewable Energy Laboratory) before to test. The PL spectra was performed on a FLS920 spectro-fluorimeter (Edinburgh Instruments) and the EQE spectra was measured on a commercial EQE measurement system (Taiwan, Enlitech, QE-R).
2.4. Fabrication and characterization of single-carrier devices The charge carrier mobilities of PBDB-T: acceptors blend films and the acceptor 11
ACCEPTED MANUSCRIPT neat films were determined from space-charge-limited current (SCLC) devices. The devices structures were ITO/Al/ blend films (or neat film)/Ca/Al for electron-only mobility and ITO/PEDOT:PSS/blend films (neat film)/MoO3/Ag for hole-only
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mobility, respectively. The charge carrier mobilities were determined by fitting the dark J–V current to the model of a single carrier SCLC which were calculated
9 = ε ε μ 8
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according to the following equation:
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where J is the current density, ε0 and εr are the permittivity of free space and relative permittivity of the material, respectively. and µh is the zero-filed mobility, V is the effective voltage and d is the thickness of the organic layer. The effective voltage can be obtained from the equation V = Vappl−Vbi−Vs, where Vapp is the applied voltage, Vbi
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is the offset voltage and Vs is the voltage drop. The carrier mobilities can be calculated from the slope of the J1/2–V curves.
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3. Results and discussion
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3.1. Material synthesis and characterization The detailed synthetic routes of the two acceptors, IF-TN and IDT-TN, are
shown
in
Scheme
1.
The
N
group
was
synthesized
by
using
2,3-naphthalenedicarboxylic anhydride as the starting material [29]. The intermediate compounds IF(C8)-T-CHO and IDT(C8)-T-CHO were synthesized based on palladium-catalyzed Suzuki and Stille coupling reactions, respectively [3133]. By treating IF(C8)-T-CHO and IDT(C8)-T-CHO with an N unit via a Knoevenagel 12
ACCEPTED MANUSCRIPT condensation reaction, IF-TN and IDT-TN were obtained with good yield of 80% and 82%, respectively. The molecular structures of the target NFAs were confirmed by 1H and
13
C nuclear magnetic resonance (NMR) spectroscopy as well as matrix-assisted
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laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) (Fig. S8-S18, SI). These NFAs showed excellent solubility in chloroform, chlorobenzene, and o-dichlorobenzene.
As evaluated by differential scanning calorimetry
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measurement from 25 to 240 oC, no obvious thermal transition characteristics can be
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note for both NFAs (Fig. S1b, SI). Thermogravimetric analysis revealed their good thermal stability, with decomposition temperature (Td, 5% weight loss) as 354°C and 353°C for IF-TN and IDT-TN, respectively (see Fig. S1a, SI).
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3.2. Theoretical calculations
The optimal molecular geometries of IF-TN and IDT-TN were simulated by density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level. Fig. 2
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shows that the torsion angles of the optimized geometries of IF-TN and IDT-TN were
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48.5° and 24.9°, respectively. The torsion angle of IF-TN is larger than that of IDT-TN because the phenyl ring has greater steric hindrance than the thienyl ring. IDT-TN has more coplanar backbones than IF-TN, which may facilitate π-electron delocalization and enhance charge mobility.
13
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Fig. 2 Geometry-optimized structures and frontier molecular orbitals of (a) IF-TN and
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3.3. Optical and electrochemical properties
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(b) IDT-TN, as determined by DFT calculations at the B3LYP/6-31G(d, p) level.
Fig. 1b displays the ultraviolet-visible (UV-vis) absorption spectra of IF-TN and IDT-TN as thin films. IF-TN film shows two absorption bands of 300-400 nm and 480-680 nm, while IDT-TN film exhibits an evident absorption between 560 to 880
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nm. The maximum absorption spectra of IDT-TN are red-shifted by 11 nm from solution (λmax = 756 nm) to film (λmax = 767 nm), which can be attributed to the strong π–π intermolecular interactions in thin films. For IF-TN, the absorption spectra of the
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core unit of IF at 346 nm is stronger than the ICT absorption spectra at 570 nm, which
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can be attributed to the intense π-π stacking of the large structure core IF. The red-shifted absorption of IDT-TN compared to that of IF-TN can be attributed to the stronger electron-donating capability of IDT than that of the IF unit. From the absorption onset, the optical bandgaps of IF-TN and IDT-TN are calculated to be 1.81 and 1.43 eV, respectively.
The electronic energy levels of IF-TN and IDT-TN were
evaluated via cyclic voltammetry (CV) measurements with ferrocene/ferrocenium (FC/FC+) as the reference (Fig. S2b, SI). The HOMO/LUMO levels of IF-TN and 14
ACCEPTED MANUSCRIPT IDT-TN were estimated to be –5.80/–3.93 and –5.42/–4.03 eV, respectively (Fig. 1c). The LUMO gaps between PBDB-T and IF-TN and IDT-TN are 0.4 and 0.5 eV,
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respectively, which ensures efficient exciton dissociation.
3.4. Photovoltaic properties
To investigate the potential applications of IF-TN and IDT-TN in PSCs as
layer/
PNDIT-F3N-Br/Ag,
and
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ITO/PEDOT:PSS/active
SC
electron acceptors, we fabricated PSCs with a conventional configuration of
poly[(9,9-bis(3’-((N,N-dimethyl)-methylammonium)propyl)-2,7-fluorene)-alt-5,5’-bis (2,2’-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N’-di(2-ethylhexyl)imide]d ibromide (PNDIT-F3N-Br) was used as the interfacial layer to facilitate electron
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extraction [ 34 , 35 ]. The active layers composed of a wide-bandgap polymer PBDB-T:IF-TN or PBDB-T:IDT-TN with a weight ratio of 1:1, were processed from chlorobenzene (CB). Fig. 3a shows the current density–voltage (J–V) curves of the
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optimized devices, with the corresponding photovoltaic parameters listed in Table 1.
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The detailed optimization of the PSCs is summarized in the experimental section Table S3 (SI).
The as-cast device based on PBDB-T:IF-TN showed a PCE of (2.54±0.02)%,
while a slightly higher PCE of (4.34±0.09)% (VOC = 0.973±0.002 V, JSC = 10.88±0.21 mA cm-2, FF = (41.02±0.28)%) was obtained for PBDB-T:IDT-TN-based device. When 2% CN additive was used as additive, the PCEs of IF-TN− and IDT-TN−based devices were elevated to (2.89±0.18)% [VOC = 1.006±0.005 V, JSC = 7.02±0.28 mA 15
ACCEPTED MANUSCRIPT cm-2, FF = (40.95±1.15)%] and (5.77±0.09)% [VOC = 0.967±0.003 V, JSC = 13.22±0.12 mA cm-2, FF = (45.17±0.4)%], respectively. The external quantum efficiency (EQE) spectra of the studied devices based
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PBDB-T and the two acceptors are shown in Fig. 3b. For IF-TN, the EQE shows a broad photoresponse from 320 to 730 nm with a maximum peak up to 42% at 580 nm, whereas for IDT-TN, the EQE exhibits a broader response, from 330 to 880 nm, with
SC
a maximum value of about 49% at 670 nm. The EQE spectra are in good agreement
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with the UV-Vis absorption spectra of the acceptors and the PBDB-T donor (Fig. 1b), suggesting that both of the polymer donor and the small molecule NFAs contribute to the photo-current. The integrated photocurrent from the EQE spectra are 6.02 and 6.86 mA cm-2 for the IF-TN devices, and 10.61 and 12.80 mA cm-2 for the IDT-TN
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devices with and without CN additive, respectively, which agree with those obtained from J-V measurements. The higher JSC were achieved from the devices processed CN additive, which can be attributed to the more effective charge transfer in the
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factors.
EP
photoactive layer, or the enhanced charge carrier mobility, or the combination of both
16
50
-5
-10
0.2 0.4 0.6 0.8 Voltage(V)
1
1.2
(b)
40 30 20 10
-15 -0.2 0
PBDB-T:IF-TN PBDB-T:IF-TN (2% CN) PBDB-T:IDT-TN PBDB-T:IDT-TN (2% CN)
60
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0
70
(a)
PBDB-T:IF-TN PBDB-T:IF-TN (2%CN) PBDB-T:IDT-TN PBDB-T:IDT-TN (2%CN)
EQE (%)
5
2
Current Density (mA/cm )
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0 300 400 500 600 700 800 900 Wavelength (nm)
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Fig. 3 (a) J–V curves and (b) EQE spectra of devices based on PBDB-T, IF-TN, and
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IDT-TN devices without or with CN.
Table 1 Photovoltaic properties of the PSCs based on PBDB-T as donor and IF-TN or IDT-TN as acceptor under AM 1.5 G at 100 mW cm-2. CN
Jsc
FF
PCEa
(mA cm-2)
(%)
(%)
Voc
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Active layer
(vol%)
(V)
w/o
1.000±0.008 6.19±0.06
41.10±0.40
2.54±0.02 (2.57)
PBDB-T:IFTN
2%
1.006±0.005 7.02±0.28
40.95±1.15
2.89±0.18 (3.03)
w/o
0.973±0.002 10.88±0.21 41.02±0.28
4.34±0.09 (4.49)
0.967±0.003 13.22±0.12 45.17±0.47
5.77±0.09 (5.89)
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PBDB-T:IDT-TN PBDB-T:IDT-TN a
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PBDB-T:IF-TN
2%
The PCE values in the parenthesis represent the highest values of the fabricated
devices.
To clarify the higher JSC values for the devices processed with CN additive, we analyzed the photoluminescence (PL) spectra of pristine donor and acceptor films, 17
ACCEPTED MANUSCRIPT and relevant blend films (Fig. 4a and Fig. 4b). The blend films processed with 2% CN additive exhibited more pronounced PL quenching than those without CN, which is favorable for an efficient charge transfer. To investigate the charge recombination, we
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measured the JSC as a function of light intensity (Plight) from 5 to 100 mW cm-2 (Fig. 4c and 4d). The power-law dependence of JSC on illumination intensity can be expressed as JSC ∝ (Plight)α, where α is the exponential factor, which is close to unity
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when the bimolecular recombination in an organic solar cell is weak. When processed
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with CN, the obtained α values for both IF-TN- and IDT-TN-based devices increased (from 0.955 to 0.961 for IF-TN, and from 0.954 to 0.974 for IDT-TN), indicating that
(a)
PBDB-T IF-TN PBDB-T:IF-TN PBDB-T:IF-TN (2% CN)
0.8 0.6 0.4 0.2
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1
excited @550 nm
Normalized PL intensity
1.2
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Normalized PL intensity
the bimolecular recombination was slightly impeded. 1.2
1
0.8
0.2 0
PBDB-T:IDT-TN PBDB-T:IDT-TN (2%CN)
α = 0.961
1
0.1
640 680 720 760 800 840 880 Wavelength (nm)
100
(c) -2
10
excited @610 nm
0.4
Jsc (mA cm )
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-2
Jsc (mA cm )
PBDB-T:IF-TN PBDB-T:IF-TN (2%CN)
α = 0.955
10 100 -1 Light intensity (mW cm )
(b)
0.6
0 600 650 700 750 800 850 900 Wavelength (nm) 100
PBDB-T IDT-TN PBDB-T:IDT-TN PBDB-T:IDT-TN (2% CN)
(d)
10
α = 0.974 1
0.1
α = 0.954
10 100 -1 Light intensity (mW cm )
Fig. 4 (a and b) PL spectra of thin films: PBDB-T, IF-TN and PBDB-T: IF-TN (1:1, 18
ACCEPTED MANUSCRIPT w/w, without and with 2% CN) and PBDB-T, IDT-TN and PBDB-T:IDT-TN (1:1, w/w, without and with 2% CN), respectively; (c and d) JSC as a function of light intensity for the devices based on PBDB-T:IF-TN and PBDB-T:IDT-TN with or
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without CN additives, respectively.
To elucidate the realization of simultaneous low bandgap and high VOC for
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PBDB-T:IDT-TN-based devices, we analyzed the VOC loss in both PBDB-T:IDT-TN
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and PBDB-T:IF-TN devices that processed with 2% CN. As determined from the intersection of the absorption edge and the EQE maximum (Fig. S5, SI) [36], the bandgaps (Eg) of PBDB-T:IDT-TN and PBDB-T:IF-TN were 1.58 and 1.93 eV, respectively (Table S5). The corresponding Schockley-Queisser limit VOC (VOC, SQ)
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[37] was estimated to be 1.32 and 1.64 V, respectively. According to the reciprocal theory [ 38 ], the radiative VOC limit (VOC,
rad)
calculated by combining
Fourier-transform photocurrent spectroscopy (FTPS) and electroluminescence (EL)
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spectroscopy (Fig. 5), were 1.20 and 1.37 V for IDT-TN and IF-TN-based devices,
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respectively. Therefore, the non-radiative VOC loss (∆VOC, nr), which equals to VOC, rad – VOC, meas, is 0.23 V for the PBDB-T:IDT-TN cell and 0.36 V for the PBDB-T:IF-TN cell. The detailed calculation procedures are provided in the supporting information. The remarkably low ∆VOC,
nr
of 0.23 V obtained for the IDT-TN-based device is
among the lowest losses reported for state-of-the-art organic solar cells and is approaching the losses reported for other conventional photovoltaic technologies, such as Si and CIGS [39,40]. It is also noteworthy that the difference between the VOC, 19
ACCEPTED MANUSCRIPT SQ
and the VOC, rad for IDT-TN-based device is only 0.12 V, which contributes to the
simultaneous low bandgap (enhanced JSC) and high VOC for PBDB-T:IDT-TN-based devices. However, it remains unclear the origination of relatively low ∆VOC,nr of the
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PBDB-T:IDT-TN blends. It is well-established that there is an intrinsic link between the ∆VOC,nr and electron-vibration coupling, and the non-radiative VOC loss of the organic solar cells is unavoidable in organic solar cells [ 41 , 42 ]. There is a
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fundamental correlation between the charge transfer (CT) state energy and the
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non-radiative VOC losses, which is associated with an increased wave-function overlap between the relaxed CT state and higher order vibrational modes of the ground state [43]. Similar low ∆VOC,nr has also been realized in other OSCs based on non-fullerene
-2
EQEFTPS
10
φEL/φbb
-5
1.2
1.4
10
-2
10
-3
1.6
PBDB-T:IF-TN
1.8
2.0
2.2
10
-5
10
-6
100
100
10-1
10-1
φEL
EQE 10-2 10
-3
10-2 10-3
EQEFTPS
10-4 10-5
10-4
10-6
φEL/φbb
10-5
10-7 10-8
10
2.4
Energy (eV)
10-7
PBDB-T:IDT-TN
-6
1.0
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1.0
10
10-4
10-3 10-4
φEL
EP
EQE
10
EQE
(b)
EL
10-1
-1
EQE
100
100
EL
(a)
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acceptors[44].
10-8 1.2
1.4
1.6
1.8
2.0
2.2
2.4
Energy (eV)
Fig. 5 Semi-logarithmic plots of measured EQE, FTPS calibrated by EQE (EQEFTPS), and ϕEL/ϕbb fitting as well as normalized EL (ϕEL) as a function of energy for devices based on PBDB-T:IF-TN (1:1, ,wt:wt, 2% CN) (a) and PBDB-T:IDT-TN (1:1, wt:wt, 2% CN) (b). The black arrow represents the charge transfer feature. The ratio of ϕEL/ϕbb was used to plot the EQE in the low energy regime (black line), where ϕEL and ϕbb represent the emitted photon flux and the room-temperature blackbody photon 20
ACCEPTED MANUSCRIPT flux, respectively.
3.5. Charge carrier mobility
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The electron mobilities for the blend films with and without CN were measured by the space-charge limited current (SCLC) (Fig. 6). The electron mobility of the PBDB-T:IF-TN blend was slightly enhanced from 7.78× 10-6 cm2 V-1 s-1 for the
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pristine film to 1.02 × 10-5 cm2 V-1 s-1 for the film processed with CN. A similar trend
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was observed for PBDT:T-IDT-TN blend film (Table S4, SI). We have recently shown that SCLC may significantly underestimated the mobility of photoexcited carriers in a disordered semicomductor [ 45 ]. Although Time of Flight is more correctly determining mobility values, SCLC was chosen here because of the limited film
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thickness. These observations are consistent with the slightly enhanced current density for devices incorporating CN as the solvent additive.
(a)
100 1/2 1/2
80 60 40
1
0
1.5 2 2.5 V -V -V (V) appl
PBDB-T:IF-TN PBDB-T:IF-TN (2% CN) PBDB-T:IDT-TN PBDB-T:IDT-TN (2% CN)
20
10
0.5
(b)
120
J
J
140
(A /m)
20
AC C
1/2
30
1/2
(A /m)
40
PBDB-T:IF-TN PBDB-T:IF-TN (2% CN) PBDB-T:IDT-TN PBDB-T:IDT-TN (2% CN)
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50
bi
3
3.5
-20
0
0.5
1 V
s
1.5 2 -V -V (V)
appl
bi
2.5
s
Fig. 6 (a) Electron-only and (b) hole-only J1/2–V characteristics of PDBT-T:IF-TN and PDBT-T:IDT-TN blend films without or with CN.
21
3
ACCEPTED MANUSCRIPT 3.6. Morphology Atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) were used to probe the thin film morphology, with the
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corresponding images shown in Fig. 7 and Fig. S6-S7 (SI). Both blend films exhibited relatively smooth surface profile with fine fibrous structures across the entire films (Fig. 7a, b). The GIWAXS patterns of the IF-TN and IDT-TN neat films exhibited
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strong (010) diffraction peaks in the out-of-plane (OOP) direction, with corresponding
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q values of about 1.81 and 1.87 Å−1, respectively (Fig. S7a and c, SI), implying the formation of predominant face-on orientation. Similarly, both PBDB-T:IF-TN and PBDB-T:IDT-TN blends showed obvious OOP (010) peaks at approximate 1.8 Å−1 with mirror (100) peak in the in-plane (IP) direction (Fig. 7c, d), indicating the Note that the OOP (010) diffraction peaks
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maintaining of the face-on orientation.
are stronger for both blend films processed from CB containing CN than those without CN (Fig. S7b and d, SI), indicating more phase-separated microstructure
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morphology with more ordered molecular chains. These observations were consistent
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with the improved charge carrier mobilities of devices processed with CN additives.
22
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Fig. 7 AFM height images (5 × 5 µm) and 2D-GIXD patterns of PBDB-T:IF-TN (a
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4. Conclusion
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and c, 2% CN) and PBDB-T:IDT-TN (b and d, 2% CN) blend films.
In summary, we developed two novel non-fullerene small molecule acceptors,
IF-TN and IDT-TN, based on indenofluorene or indacenodithiophene as the core unit, respectively, and end-capped with end-extended conjugation-group indanone derivatives. The acceptor IF-TN exhibited obviously larger bandgap of 1.81 eV, while a small bandgap of 1.43 eV was obtained for IDT-TN. Polymer solar cells based on a wide-bandgap electron-donating copolymer PBDB-T and such two acceptors 23
ACCEPTED MANUSCRIPT exhibited moderate power conversion efficiencies of 3.03% and 5.89%, respectively. Of particular interest is that the device based on IDT-TN exhibited a remarkably low non-radiative VOC loss of 0.23 V, which is one of the lowest values reported for
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state-of-the-art PSCs and is approaching the losses reported for mature PV technologies. These results suggest that extension of the conjugation length of the end-groups is a good strategy for the molecular design that can extend the
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light-absorption spectra and maintain a relatively high open-circuit voltage.
Acknowledgments
This work was financially supported by the Ministry of Science and Technology (No. 2014CB643501), the Natural Science Foundation of China (No. 91633301,
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21490573, 51673069), and the Science and Technology Program of Guangzhou, China (No. 201710010021, 201707020019 and 2017A050503002). N.L. gratefully acknowledges the financial support from the DFG research grant (BR 4031/13-1) and
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the Bavarian Ministry of Economic Affairs and Media, Energy and Technology by
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funding the HI-ERN (IEK11) of FZ Jülich. C.J.B. gratefully acknowledges the financial support through the “Aufbruch Bayern” initiative of the state of Bavaria (EnCN and “Solar Factory of the Future”), the Bavarian Initiative “Solar Technologies go Hybrid” (SolTech) and the SFB 953 (DFG).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://xxxxxx. 24
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Two novel small-molecule non-fullerene acceptors with naphthyl-fused indanone as the end-groups were developed
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Device based on IDT-TN shows a PCE of 5.89% with a high VOC of 0.97 V. Device based on PBDB-T:IDT-TN exhibited a remarkably low non-radiative
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open-circuit voltage loss of 0.23 V.