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The Effect of Fluorine Substitution on the Molecular Interactions and Performance in Polymer Solar Cells In-Bok Kim, Soo-Young Jang, Yeong-A Kim, Rira Kang, In-Sik Kim, Do-Kyeong Ko, and Dong-Yu Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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ACS Applied Materials & Interfaces

The Effect of Fluorine Substitution on the Molecular Interactions and Performance in Polymer Solar Cells In-Bok Kima, Soo-Young Jangb,c, Yeong-A Kimc, Rira Kangd, In-Sik Kime, Do-Kyeong Koe, DongYu Kim*,a,c a

Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology (GIST), 123

Cheomdangwagi-ro, Buk-gu, Gwangju, 61002, Republic of Korea b

Department of Chemistry and Centre for Plastic Electronics, Imperial College London, Exhibition Rd, London, SW7 2AZ, UK. c

Heeger Center for Advanced Materials (HCAM), School of Material Science and Engineering (SMSE), Gwangju In-

stitute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju, 61002, Republic of Korea d

Radiation Research Division for Industry and Environment, Korea Atomic Energy Research Institute (KAERI), 29

Geumgu-gil, Jeongeup-si, Jeollabuk-do, 56212, Republic of Korea e

School of Physics and Chemistry, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro,

Buk-gu, Gwangju, 61002, Republic of Korea KEYWORDS: conjugated polymers, polymer solar cells, fluorinated polymers, pre-aggregation, non-additive ABSTRACT: Fluorine (F) substitution on conjugated polymers in polymer solar cells (PSCs) has a diverse effect on molecular properties and device performance. We present a series of three D-A type conjugated polymers (PBT, PFBT and PDFBT) based on dithienothiophene and benzothiadiazole units with different numbers of F atoms to explain the influence of F substitution by comparing the molecular interactions of the polymers and the recombination kinetics in PSCs. The pre-aggregation behavior of PFBT and PDFBT in o-DCB at the UV-Vis absorption spectra proves that both polymers have strong intermolecular interactions. Besides, more closely packed structures and change into face-on orientation of fluorinated polymers are observed in polymer:PC71BM blends by GIXD which is beneficial for charge transport and ultimately, for current density in PSCs (4.3, 13.0 and 14.5 mA cm-2 for PBT, PFBT and PDFBT, respectively). Also, the introduction of F atoms on conjugated backbones affects the recombination kinetics by suppressing bimolecular recombination, thereby improving the fill factor (0.41, 0.68 and 0.69 for PBT, PFBT and PDFBT, respectively). Consequently, the PCE of PSCs reached 7.3% without any additional treatment (annealing, solvent additive, etc) in the polymer containing difluorinated BT (PDFBT) that is much higher than non-fluorinated BT (PBT ~ 1%) and mono-fluorinated BT (PFBT ~ 6%).

Introduction Conjugated polymers with donor (D) - acceptor (A) alternating system have rapidly advanced with the aim of achieving high power conversion efficiencies (PCEs) in polymer solar cells (PSCs).1-5 By combining various D and A units, many types of conjugated polymers have been subsequently synthesized,6-13 and the PCE value has exceeded more than 10% in single-junction photovoltaic devices using D-A polymers in virtue of their enlarged spectral absorption range, tunable energy levels, and enhanced charge transport through conjugated backbones.14-21

ACS Paragon Plus Environment


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Several methods have been attempted in order to obtain D-A polymers with a high PCEs, including introducing fused heterocycles as a monomer unit or attaching various substituents on their conjugated backbones.6-21 The substitution of fluorine (F) atoms on the conjugated backbones, in particular, has recently shown remarkable results in PSCs arising from their ability of efficient charge transport and proper formation of nano-scale morphology with fullerene derivatives with the aid of solvent additives, such as 1,8-diiodoocatane (DIO) or 1-chloronaphthalene (CN).28-35 F atom itself has a small size with a van der Waals radius as 1.35 Å, also the most electronegative property of all the elements with a Pauling electronegativity of 4.0. Such a small-size of F atom enables a greater planarity of the polymer backbones and consequently promotes efficient charge transport, compared with when they are substituted with other atoms.15,16,29,30 An enormous electronegativity value of 4.0 means that the F atom would effectively withdraw the electron density of a conjugated system, therefore, stabilize the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), facilitating an enhanced open circuit voltage (VOC) without introducing steric hindrance along the polymer chains. Furthermore, high electronegativity of the F atom often changes the dipole moment of conjugated polymers and gives rise to strong intermolecular interactions between polymer chains, which can influence the forming of nano-scale morphology such as the polymer crystallization or the orientation of polymer crystallites that are closely related to charge transport and recombination in PSCs.36 Inspired by the above ideas, we designed new D-A type conjugated polymers by introducing dithienothiophene (DTT) as a D unit, and benzothiadiazole (BT) as an A unit with different numbers of F substitution (n=0 for PBT, n=1 for PFBT and n=2 for PDFBT) at the 5 or 6 site. Each of three polymers was synthesized via the Stille polymerization of the DTT and BT units with/without F atoms to investigate the effects of F substitution on the photophysical, electrochemical, and photovoltaic properties of the polymers. As the H atoms at BT unit are replaced with the F atom, the polymers exhibit a gradually stronger intermolecular interaction estimated by pre-aggregation and isosbestic behavior in UV-Vis absorption spectra. Such strong intermolecular interaction leads to dense molecular packing of the polymer chains in the polymer:phenyl-C71-butyric acid methyl ester (PC71BM) blend system which increase charge carrier mobility and suppress charge recombination supported by transient absorption and light intensity dependence of JSC analysis. Overall, nonfluorine polymer (PBT) showed a low PCE of 1%, while the efficient charge transport and highly ordered nano-scale morphology of fluorine-based polymer:PC71BM yielded a PCE of 7.3% for PDFBT and 6% for PFBT without any solvent additives or post-annealing treatment.

Experimental Section Materials All chemicals were purchased from Aldrich, TCI, Alfa Aesar, Acros, or Strem Chemicals, and used without further purification. All the monomers were synthesized according to the previously published literature.25,29,37,38 All air- and watersensitive reactions were conducted under a nitrogen atmosphere. Tetrahydrofuran (THF) was distilled from sodium and benzophenone. Polymerization All of the polymers were synthesized by Stille polycondensation. Poly[(2,6-dithieno[3,2-b:2’,3’-d]thiophene)-alt-(4,7-di(4-octyldodecylthiopen-2-yl)-2,1,3benzo[c][1,2,5]thiadiazole (PBT) Two neck round bottom flask was prepared with 4,7-bis(4-dodecylthiophen-2-yl)-


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[2,1,3]benzothiadiazole (1 eq) and 2,5-Bis(trimethylstannyl) dithieno[3,2-b:2’,3’-d]thiophene (1 eq) and purged with nitrogen and subsequently dissolved in 15 ml of degassed CB. Pd2(dba)3 (2 mol%) and tri(o-tol)phosphine (8 mol%) were added into the flask and refluxed for 48 hours at 120 °C. To the reaction 2-tributylstannylthiophene and 2-bromothiophene were added to end cap the polymer and the mixture was reacted at 120 °C. After the reaction was cooled, the reaction was precipitated into methanol/HCl. Filtered precipitation was re-dissolved in CHCl3 and precipitated into methanol again. The precipitate was filtered by a Soxhlet thimble and purified via Soxhlet extraction for 12 hours with methanol, acetone, hexane and chloroform and chlorobenzene (CB). Concentrated solution by evaporation and precipitated into methanol (150 ml), filtered and dried under vacuum. Yield: 424 mg (65%). NMR spectra of polymers could not be measured because of their insufficient solubility in CDCl3. Mn = 24.7 kg mol−1, Đ = 2.1 (determined by GPC with 1,2,4-trichlorobenzene at 150 °C with polystyrene standards). Elemental Anal. Calcd. For (C54H88N2S3): C, 70.53; H, 8.59; N, 2.65; S, 18.22. Found: C, 70.44; H, 8.47; N, 2.56; S, 18.51. PFBT and PDFBT were synthesized by following the same procedure as PBT. Poly[(2,6-dithieno[3,2-b:2’,3’-d]thiophene)-alt-(5-fluoro-4,7-di(4-octyldodecylthiopen-2-yl)-2,1,3benzo[c][1,2,5]thiadiazole (PFBT): Yield: 554 mg (90%) Mn = 41.8 kg mol−1, Đ = 2.3. Elemental Anal. Calcd. For (C54H87FN2S3): C, 69.35; H, 8.35; N, 2.61; S, 17.91. Found: C, 70.13; H, 8.28; N, 2.52; S, 18.43. Poly[(2,6-dithieno[3,2-b:2’,3’-d]thiophene)-alt-(5,6-difluoro-4,7-di(4-octyldodecyl-thiopen-2-yl)-2,1,3benzo[c][1,2,5]thiadiazole (PDFBT): Yield: 563 mg (90%) Mn = 29.1 kg mol−1, Đ = 2.1. Elemental Anal. Calcd. For (C54H86F2N2S3): C, 68.21; H, 8.12; N, 2.57; S, 17.62. Found: C, 69.21; H, 7.97; N, 2.54; S, 18.11. Measurements 1

H NMR spectra analyses, thermal gravity analyses, scanning calorimetry analyses, cyclic voltammetry measurements,

and absorption spectra were conducted according to the previous literatures.25 Elemental analysis was checked on an EA1112 instrument (CE Instrument). The number-average molecular weight (Mn), weight-average molecular weight (Mw) and Đ values of polymers were carry out using high temperature gel permeation chromatography (Polymer Laboratories PL-GPC 210) with 1,2,4-trichlorobenzene as the eluent (flow rate of 1 mL min-1, at 150 °C) and were calibrated using polystyrene as the standard. The ultrafast transient absorption (TA) dynamics was measured with a home-built TA spectrometer and the methodology was described in previous report.39,40 Crystal characteristics of polymer or polymer:PC71BM were analysed by grazing incident x-ray diffraction (GIXD) using the synchrotron radiation source 3C beam line at the Pohang Accelerator Laboratory (PAL). polymers or polymer:PC71BM were spin-coated on top of Si substrates under optimized active layer condition. Film surface morphology was investigated by atomic force microscopy (1 μm × 1 μm, Digital Instruments Nanoscope IV in tapping mode). The nanoscale morphology of the blend film was studied using transmission electron microscopy (Tecnai TEM (FEI) operated at 200 kV). Fabrication and Characterization of PSCs The patterned indium tin oxide (ITO)-coated glass substrate was cleaned using an ultrasonic-bath with De-ionized water, acetone, and isopropanol. Dried substrate in the oven was exposed to UV-O3 and ZnO was spin-coated onto the substrate as the electron injection layer and was baked at 120 °C for 10 min in air. In nitrogen-filled glove box, the blend solution was prepared with polymer and PC71BM (Nano C) with a weight ratio 1:1 in CB:o-DCB solvent with 1:1 volume ratio and heated at 150 °C for 3 hr before deposition. The hot solution was spin-coated onto the ZnO layer. Finally, MoO3 (2.5 nm) and Ag (100 nm) as an anode electrode were deposited on top of the active layer with a shadow mask using thermal evaporation under vacuum (~10−6 torr), and this metal-deposited region was defined as the active area of the devices as



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4.64 mm2. The photocurrent density-voltage (J-V) curves were measured using a Keithley 236 Source Measurement Unit, under air mass 1.5 global (AM 1.5G) irradiation from a calibrated solar simulator with an irradiation intensity of 100 mW cm-2. The EQE and IQE spectra were simultaneously obtained via an Oriel® IQE-200 QE Measurement System. Hole-only

devices

of

the

polymer:PC71BM

were

fabricated

as

ITO

(5.0

eV)/PEDOT:PSS

(5.2

eV)/polymer:PC71BM/MoO3/Ag (4.7 eV) structures. According to space charges limited to current theory (SCLC), the dark current was measured under forward bias and the device characteristics were calculated by means of the following equation: J = 9/8·εμ(V2/L3) where ε is the dielectric permittivity of the polymer, μ is the hole mobility, and L is the film thickness.

Results and discussion Synthesis and Characterizations of the Polymers Synthesis of monomers and polymers Both monomers, dithienothiophene (DTT) and fluorinated benzothiadiazole (BT)-derivatives, were prepared according to the procedures reported in the literatures, 25,29,37,38 and the synthetic schemes are described in Scheme 1. Three types of BT units containing different numbers of F atoms (1) were reacted with two thiophenes having long-branched alkyl chains, 2-octyldodecyl, in order to improve the solubility of target polymers. The series of (2) was then brominated with N-bromosuccinimide (NBS) to obtain dibromo monomers (3). A distannylated DTT monomer (5) was subsequently prepared from dibromo DTT (4), and the polymers were synthesized via Stille polymerization between the di-stannyl functionalized DTT (5) and each of the three di-bromonated BT units (3) to yield PBT, PFBT and PDFBT, respectively, as shown in Scheme 1. The details of the synthetic method and characterizations of the monomers are described in the ESI.

Scheme 1. Chemical structures and synthetic methods of the monomers and polymers.

The molecular weight of the polymers was achieved via gel permeation chromatography (GPC) analysis using 1,2,4trichlorobenzene as an eluent at 150 °C and the measured data (Mn: the number-average molecular weight, Mw: the weight-average molecular weight, Đ: polydispersity index) are summarized in Table 1. Thermal degradations and thermal transition behavior of the polymers were also identified via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), as shown in Figure S1 and summarized in Table 1. All three polymers, PBT, PFBT and PDFBT exhibited excellent thermal stability by recording decomposition temperatures (Td) of 439, 429 and 436 °C, respectively, at a 5%


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weight loss. The melting (Tm) and crystallization (Tc) temperatures of the polymers showed a tendency to increase gradually upon F substitution which can be an evidence of strong intermolecular interaction of fluorinated polymer chains (PFBT and PDFBT).15,32 Table 1. Summary of physical and thermal properties of polymers Mn

Đ

Td

(kg mol )

(kg mol )

(Mw/Mn)

(°C)

PBT

24.7

52.2

2.1

439

240

264

PFBT

41.8

95.3

2.3

429

287

311

PDFBT

29.1

62.5

2.1

436

314

330

Polymer

Mw -1 a

-1 a

Tc b

(°C)

Tm c

(°C)

c

a

Number-average molecular weight (Mn) and weight-average molecular weight (Mw) were determined by GPC with 1,2,4b trichlorobenzene as an eluent at 150 °C with polystyrene standard. Decomposition temperature (Td) was determined by TGA at c 5% weight-loss. Crystallization (Tc) and melting (Tm) temperature were obtained from the maximum peak by DSC.

Density Functional Theory (DFT) Calculation

Figure 1. Structural simulation of dimers for (a) PBT, (b) PFBT*, and (c) PDFBT. *Note: Fluorine substitution in PFBT could be in random arrangement in real state.

The theoretical calculations of molecular structures and energy levels on the dimer scale of PBT, PFBT and PDFBT were studied using DFT (Gaussian 09, B3LYP/6-311G(d,p)), as shown in Figure 1 and S2 and the data are summarized in Table S1. The torsion angle between dithienothiophene and thiophene in all polymers showed 30° due to the steric hindrance of alky side chain on thiophene. On the other hand, since the size of a F atom is small (van der Waals radius, r = 1.35 Å) and there could be conformation locking between S and F, it is reasonable to assume that steric hindrance between BT and adjacent thiophene units could be reduced by F substitution. Indeed, in the case of the F-substituted polymers (PFBT and PDFBT), the torsion angles (θ2) between the F-substituted BT and adjacent thiophene showed smaller torsion angles (0.6 and 1.6 °, respectively) than that of PBT (6.4 °) and these decreased torsion angles would facilitate a more planar structure with an improved intermolecular interaction compared with a non-fluorinated case.16,41 Also, the C-C bond length between BT and adjacent thiophene decreases (Lbond,PBT: 145.87 pm, Lbond,PFBT: 145.71 pm and Lbond,PDFBT: 145.68 pm) as shown in Figure 1 and Table S1. The shortening of the C-C bond length means an increase of double bond characteristics and that could be causative of more planar conformation of molecular structures. The HOMO and LUMO levels of the three polymers were also calculated and are summarized in Table S1. The calculation results reveal that substitution of the F atom



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stabilizes both the HOMO and LUMO levels in good agreement with the experimental evidence of lowering the HOMO and LUMO levels upon F substitution, as measured by cyclic voltammetry described in the next section.


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Figure 2. (a) Absorption spectra in films and o-DCB at room temperature; solution absorption spectra of (b) PBT, (c) PFBT, and (d) PDFBT at various temperatures.



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UV-Vis Absorption and Cyclic Voltammetry The absorption spectra of PBT, PFBT and PDFBT in a 1,2-dichlorobenzene (o-DCB) solution and film at room temperature are shown in Figure 2a. All three polymers demonstrate similar optical band gaps of ~ 1.6 eV in both the film and solution. Given that bathochromic shift is common in a transformation from the solution to the film, their similar absorption spectra signify that all three D-A type polymers have strong intermolecular interactions even in the solution, though the intensity of the vibronic peak of PBT in the solution is slightly decreased. To confirm the formation and dissolution of the pre-aggregated state in solution, the absorption spectra of the three polymers were measured during a cooling process from 150 to 40 °C, shown in Figure 2b -2d. The polymers depict almost identical absorption spectra with an absorption maximum peak at 540 nm when they are fully dissolved in o-DCB solvent at 150 °C. However, there is a difference in temperature to start aggregation among three polymers; PBT doesn’t fully aggregate until 40 °C while the fluorinated polymers, PFBT and PDFBT, start to aggregate at approximate 70 °C and 90 °C, respectively and an absorption peak move at longer wavelength over 700 nm. The tendency toward an increase in aggregation temperature by introducing F atoms in the polymer backbones shown in Figure S3 provides an explanation of the enhancement in the intermolecular interactions arising from the large dipole moment of C-F bonds and the extended structural planarity in fluorinated systems.27-35 Energy levels of PBT, PFBT and PDFBT were determined via cyclic voltammetry (CV) in Figure S4 and the HOMO/LUMO levels of PBT, PFBT and PDFBT were calculated from each of optical band gap and cyclic voltammograms as −4.88/−3.30, −4.92/−3.35 and −5.12 eV/−3.52 eV, respectively. As previously mentioned, the introduction of F atoms on the BT unit lowers the energy level of the polymers without changes in the band gap, since the strong withdrawing property of the F atom stabilizes the π-electron orbital of the conjugated backbones. For the sake of this feature, F substitution not only strengthens molecular interactions also becomes an advantage to the enhancement in VOC of PSCs. The comprehensive data of optical band gaps and energy levels of the three polymers are summarized in Table 2. Table 2. Summary of photophysical and electrochemical properties of polymers Polymer

λonset

sol

λonset

film

opt

Eg

a

HOMO

LUMO

(eV)

PBT

772

789

1.57

−4.87

−3.30

PFBT

781

795

1.56

−4.91

−3.35

PDFBT

771

780

1.59

−5.11

−3.52

b

(eV)

c

(nm)

a

(eV)

b

(nm)

Optical band gap in the film. HOMO level was calculated from the onset of oxidation potential by cyclic voltammetry. LUMO level was estimated the difference between HOMO level and optical band gap in film.

c

Photovoltaic Properties The photovoltaic properties of the three polymers were estimated using the following device structures: ITO/ZnO/polymer:PC71BM/MoO3/Ag. The current density-voltage (J-V) curves and corresponding external quantum efficiency (EQE) spectrum appear in Figure 3 and the characterization data are summarized in Table 3 including average and standard deviation values. In an optimized film fabricating condition, a binary solvent system was employed using CB and o-DCB to control the morphology of the polymer:PC71BM blends. In contrast to other common solution systems, these fluorinated polymer (PFBT and PDFBT)-based blend solution requires no addition of any solvent additives such as 1,8Diiodooctane (DIO) or 1-Chloronaphthalene (CN), and no post-thermal annealing process is needed to form the finer blend morphology of the active layers. Since F-containing polymers in o-DCB exist in an aggregated state at room temperature, as shown in the results of UV-Vis absorption spectroscopy in Figure 2a, F substituted polymer (PFBT and PDFBT) blend solutions with PC71BM were heated at 150 °C before film casting to prevent polymer aggregation and to


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Figure 3. (a) Current density-voltage (J-V) characteristics and (b) external quantum efficiency (EQE) of polymer:PC71BM PSCs.

form higher-quality films. As expected in the previous section, the VOC values of the three polymers are increased gradually (PBT: 0.57 V, PFBT: 0.67 V and PDFBT: 0.73 V) by substituting F atoms which is proportional to the deepening of the HOMO levels of the polymers (PBT: −4.87 eV, PFBT: −4.91 eV, and PDFBT: −5.11 eV).27-35 Also, the JSC values of PBT, PFBT and PDFBT are increased with F additions of 4.3, 13.0, and 14.5 mA cm−2, respectively, with optimized active layer thickness of 90, 120 and 150 nm, as shown in Figure 3a and in Table 3. It is general that the increase in active layer thickness usually lowers the JSC in polymers with low hole mobility due to a concomitant increase in the space charge and bimolecular recombination. Therefore, the increase in optimized active layer thicknesses along with SCLC hole mobility (PBT: 9.2 × 10−6, PFBT: 2.73 × 10−4, PDFBT: 4.42 × 10−4 cm V−1s−1) of a fluorinated polymer-based system explains that F substitution has an effect on the deterioration of bimolecular recombination.28,30 Such an enhancement of the JSC upon F addition could be attributed to an increase in the intermolecular interactions which is an important factor in determining the mobility of free charge carriers. The evidence of reduction in bimolecular recombination is more clearly found in the increase of FF (PBT: 0.41, PFBT: 0.68 and PDFT: 0.69) which directly relates to charge recombination losses in the active layers.30 Overall, PDFBT which has the highest mobility and the deepest HOMO levels showed the best PCE value (7.3%) compared with the PCEs of other polymers (PBT: 1% and PFBT: 6%), even with neither pre- (solvent additives) nor post-treatment (thermal annealing) in its active layer system, which provide not only convenience in film fabrication, also have the advantage of processibility in flexible-based PSCs. Table 3. Photovoltaic properties and SCLC mobility of devices fabricated from polymer:PC71BM (1:1 w/w) blend processed from CB:o-DCB (1:1 v/v)



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Polymer

a

-2

JSC (mA cm )

Thickness

a

VOC b

(V)

a

FF (%)

Page 10 of 17 PCE (best PCE)

a

(%)

a

μhole 2

-1 -1

(cm V s )

(nm)

Measured

PBT

~ 90

4.23 ±0.13

4.48

0.57 ±0.001

40 ±0.8

0.96 ±0.05 (1.0)

9.22 × 10

PFBT

~ 120

12.15 ±0.79

13.2

0.67 ±0.003

67 ±0.6

5.53 ±0.42 (6.0)

2.73 × 10

PDFBT

~ 150

14.32 ±0.18

14.43

0.73 ±0.003

69 ±0.7

7.15 ±0.17 (7.3)

4.42 × 10

Calculated

-6

-4 -4

Average values and standard deviations of device statistics from at least five devices. b Calculated JSC from a EQE curve.

Charge Recombination Process To study the charge recombination processes of PBT, PFBT and PDFBT in blend films with PC71BM in detail, ultrafast transient absorption (TA)39,40 and the light intensity dependence of JSC42 were measured. Femtosecond TA spectroscopy upon excitation at 400 nm was conducted to inspect the effect of introducing F atoms based on the charge carrier dynamics of polymer:PC71BM blend systems (Figure S5a–S5c). All three systems show broad transmission signals (ΔT/T) from 500 to 750 nm that are attributed to photo bleaching (PB) that includes the information on all excited carrier recombination to ground state. The PB decay signals can be presented in multi-phasic dynamics as shown in previous report.43 But, as depicted in Figure 4a, the PB at 620 nm shows fairly monotonic decay with lack of fast component shorter than 10 ps, making it possible to consider only long-lived excited species; the 620 nm PB decays of both PFBT:PC71BM and PDFBT:PC71BM are much slower than that of PBT:PC71BM. To identify the underlying excited nature, the additional TA kinetic experiments at several wavelengths have been conducted under various excitation fluences and were fitted using the multi-exponential decay function.43 As a result, fluence-independent decay components with a time scale of several hundred picoseconds was observed in each polymer:PC71BM blend and this can be ascribed to monomolecular charge recombination.44 The fluence-independent time constants of PBT:PC71BM, PFBT:PC71BM, and PDFBT:PC71BM were extracted to 405 ps, 475 ps, and 495 ps, respectively, therefore indicating that there is longer-lasting monomolecular species in the F-containing systems than the non-fluorinated blend. The bimolecular recombination of mobile carriers was not measured due to the limitation of time and spectral windows of the TA measurement. Together with TA study, the J-V characteristics of PSCs measured at different light intensities (from 100 mW cm−2 to 0.1 mW cm−2) are depicted in Figure S5d –S5f, then translated into the double-logarithmic plots of photocurrent density under short-circuit conditions as a

Figure 4. (a) Transient bleach kinetics (at 620 nm) of poly-2 mer:PC71BM excited at 400 nm with 7 μJ cm . (b) The doublelogarithmic plot of JSC as a function of the incident light intensity and fitting line according to the power law.

function of the incident light intensity to show how the substitution of F atoms into the polymer backbones affects recombination processes in an active layer. The fitting lines adhere to the formula J = a × Iα, where I is the incident light intensity, and α is an exponent constant which represents bimolecular recombination and is interpreted as the slope along the fitting line, as shown in Figure 4b. Since the α approaching unity represents less probability of bimolecular re-


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combination occurrence, αPFBT = 0.88 and αPDFBT = 0.87 approximate more closely to 1 compared with αPBT = 0.86 and that explains polymers containing F atoms in the polymer backbone have less bimolecular recombination than non-fluorine polymers. Such reduced bimolecular recombination upon fluorine addition at charge extraction state can possibly come from higher charge mobility (table3, SCLC mobilities) or more efficient charge extraction to the electrodes. All these features are consistent with the tendency of JSC and FF enhancement caused by a reduction in the charge recombination process in F substituted polymer-based PSCs.45,46 This study of the correlation between the number of F atoms substitution and the charge-recombination process could help to establish a design theory of the polymer structure for high PCE of PSCs. Morphology

Analysis

The crystal structure and molecular orientation of a polymer:PC71BM system are key parameters that determine the charge-transport properties in PSCs,1,30,36 therefore, we compared the aspect of crystal formation of the pristine polymers and polymer:PC71BM blend systems by conducting the grazing incidence X-ray diffraction (GIXD) analysis, as shown in Figure 5. All three polymers, PBT, PFBT and PDFBT, have a bimodal edge-on and face-on orientation in their pristine film considering the existence of diffraction peaks in both the qxy and qz directions, despite PFBT and PDFBT being weighted toward an edge-on orientation. Strong intermolecular interactions of fluorinated polymers also exercise influence on the lamellar d-spacing distance of PFBT (0.300 Å−1, d=20.9 Å) and PDFBT (0.296 Å−1, d=21.2 Å) in the qz direction which are shorter interlayer distance than that of PBT (0.294 Å−1, d=21.3 Å). The lamellar distance of a polymer in PBT:PC71BM shows 0.213 Å−1 (d=29.4 Å), which is much longer than that of pure polymer. Such change is common in some semi-crystalline polymer:acceptor blend systems that the lamellar d-spacing distance of a polymer increases due to the intermixing of acceptor molecules with polymer crystallites.47 However, the degrees of PC71BM intermixing in PFBT and PDFBT are lessened by exhibiting d=22.4 Å for PFBT:PC71BM (pristine, 20.9 Å) and similar d=21.0 Å is observed in PDFBT:PC71BM (pristine, 21.2 Å), which could be an evidence of the development of compact polymer crystallites in their blend systems. Also, both fluorinated polymers demonstrated highly face-on orientations after blended with PC71BM and such change in molecular orientation is profitable for charge transport in the vertical direction of PSCs. Overall, the maintenance of short lamellar d-spacing distance due to strong intermolecular interactions, along with the change to a face-on orientation, contributes to the enhancement in JSC and FF in fluorinated polymer:PC71BM based PSCs. In case of π-π spacing distance, there is little change upon blending with PC71BM in all systems. PBT, PFBT and PDFBT shows their π-π spacing distance of 3.57 Å, 3.55 Å and 3.59 Å, while each of their blend system indicate 3.55 Å, 3.59 Å and 3.54 Å, respectively.



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Figure 5. Grazing incidence X-ray diffraction (GIXD) images for (a) PBT, (b) PFBT, (c) PDFBT, (d) PBT:PC71BM, (e) PFBT:PC71BM, and (f) PDFBT:PC71BM.

Atomic force microscopy (AFM) was also conducted in order to investigate surface morphology of the active layer. A fibril-like morphology of PFBT and PDFBT pristine films is observed in their phase images in Figure 6a - 6c, which could be an evidence of a strong intermolecular interaction and highly ordered molecular structures of the F substituted polymers. Moreover, a lot of small aggregates with high root mean square (RMS) roughness value of 1.81 nm are present in the height images of PBT film, whereas PFBT and PDFBT depict large grains with RMS roughness values of 1.31 nm and 1.51 nm, respectively, as shown in Figure S6a – S6c. In blends with PC71BM, the small aggregates of PBT diminished and the RMS roughness become much lower (0.90 nm) than its pristine state (1.81 nm). On the other hand, the large grains and fibril-like morphology of PFBT and PDFBT are maintained with the addition of PC71BM, as shown in Figures 6d - 6f and S6d – S6f, with RMS roughness values of 1.98 nm and 1.23 nm, respectively. Such well-maintained polymeric grains of fluorinated polymers even in the blend condition correspond well with the analytical results of GIXD, explained in the previous paragraph, whereby strong intermolecular interactions of PFBT and PDFBT lead to a more dense polymer crystals compared with PBT. The nanoscale morphology of polymer:PC71BM films was also studied via high-resolution transmission electron microscopy (HR-TEM), as shown in Figure 6g - 6i. Large-sized polymer and PC71BM aggregates are observed in PBT:PC71BM films (Figure 6g), which would give cause for a serious monomolecular recombination when we consider the exciton diffusion length (order of 10 nm) in common PSC systems.48 PFBT:PC71BM and PDFBT:PC71BM films, however, show much finer morphology with nano-scale phase separation between the polymer and the PC71BM. We assume that by introducing more F atoms, nano-scale interpenetrating network is more prone to be formed between the polymer and the PC71BM while maintaining the domain crystallinity of the polymer, even without any additional treatments. Such a nano-scale separated morphology has a beneficial influence on JSC and FF, contrary to that of a PBT-based system, and lead to a better photovoltaic performance of the PSCs.


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Figure 6. Atomic force microscopy (AFM) phase images (1 μm × 1 μm) of (a) PBT, (b) PFBT, (c) PDFBT, (d) PBT:PC71BM, (e) PFBT:PC71BM, and (f) PDFBT:PC71BM. Transmission electron microscopy (TEM) images of (g) PBT:PC71BM, (h) PFBT:PC71BM, and (i) PDFBT:PC71BM.

Conclusions Three D-A alternating polymers, PBT, PFBT and PDFBT, consisting of DTT and BT units, where the number of F atoms is controlled by fluorination on the BT unit (n=0 for PBT, n=1 for PFBT and n=2 for PDFBT), were synthesized and used as the donor polymers in PSCs. Various analysis was conducted in order to investigate the effect of F atom substitution on conjugated polymers. Strong intermolecular interactions arising from highly electronegative F atom generate polymer aggregates even in solution, measured via UV-Vis absorption spectroscopy, also the HOMO and LUMO levels are lowered upon F substitution. The F-substituted polymers mixed with PC71BM show closely packed lamellar structures in GIXD that is also probative evidence of strong intermolecular interactions of polymer chains. Beside their high charge carrier mobility, reduced charge recombination of PFBT and PDFBT based blend system enhances JSC and FF in PSCs. Among others, PDFBT shows the best solar cell device performance among the three polymers with a PCE of 7.3% demonstrating enhanced values for all of VOC (0.73 V), JSC (14.5 mA cm-2) and FF (0.69) without thermal treatment nor the addition of solvent additives. We revealed the diverse influences of modifications in intermolecular interactions on polymer crystallization, active layer morphology, charge transport/recombination and finally the PCE in PSCs through F atom substitution in conjugated polymers. These results could explain the relationship between the atomic substitution and molecular properties including blend morphology and recombination kinetics, and, therefore, would contribute to a new design of conjugated polymers for use in high performance PSCs.

ASSOCIATED CONTENT Supporting Information. Supporting Information Available: [Supporting Information of PBT, PFBT, and PDFBT (Materials synthesis, characterization of 1

H NMR and elemental analysis, TGA, DSC, DFT data, relative absorption at various temperature and CV), detailed transient

absorption spectra and additional AFM images] This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D.-Y. Kim)

Author Contributions In-Bok Kim and Soo-Young Jang contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the "GRI (GIST Research Institute)" Project through a grant provided by GIST in 2016. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (NRF2015R1A2A1A10054466). This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20153010012110). This research was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (NRF-2015M2A2A4A03044653 and NRF-2015R1A5A1009962). The GIXD was measured using the synchrotron radiation sources 3C beamline at the Pohang Accelerator Laboratory (PAL). We thank the Korea Basic Science Institute (KBSI) for AFM measurement facilities.

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