Fluorene Hybrid Template for the

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A Spiro-Shaped cis-Stilbene/Fluorene Hybrid Template for the Fabrication of Small Molecule Bulk Heterojunction Solar Cells Chien-Tien Chen, Fang-Yuan Tsai, Chun-Ying Chiang, and ChihPing Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05528 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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The Journal of Physical Chemistry

A Spiro-Shaped cis-Stilbene/Fluorene Hybrid Template for the Fabrication of Small Molecule Bulk Heterojunction Solar Cells

Chien-Tien Chen,*† Fang-Yuan Tsai,† Chun-Ying Chiang‡ and Chih-Ping Chen*‡

†

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan E-mail: [email protected] ‡

Department of Materials Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan.

ACS Paragon Plus Environment


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Abstract A spiro core appended with trithiophene arms and α-cyanoacrylate end groups was used for the first time in bulk heterojunction solar cell applications. The resulting device performance was evaluated by its blending with PC61BM, leading to a short circuit current density (Jsc) of 3.55 mA cm-2 which was 2.4 and 6.6 times better than those of the open form and non-spiro core analogs. The 1,8-diiodooctane solvent additive suppressed its microcrystalline self aggregation, leading to a well-defined blend film morphology with reduced domain size and thus enhanced Jsc to 6.14 mA cm-2 and fill factor to 67.2% by 70% and 135% increase, respectively. The film domain size can be further reduced to 50 nm by its blending with PC71BM, leading to a PCE of 4.87% with an improved Jsc to 7.93 mA cm-2, a Voc of 0.97 V, a fill factor of 64.1%.


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Introduction The need for sustainable energy generation has promoted substantial research on solar cell technologies in recent decades.1 Despite the dominance by silicon-based solar panels, organic material-based, third generation solar cells like dye-sensitized solar cell (DSSC), and bulk-heterojunction organic solar cells promise the advantage of fabrication simplicity. Specifically, bulk-heterojunction organic solar cell fabricated by use of solution deposition methods provides benefits including low cost, light weight, easy mass production, and a high degree of mechanical flexibility.2-6 Power conversion efficiency (PCE) has exceeded 10%,7-10 the best of 11.7%11 and 10.6%12 for a tandem device with a conjugated polymer donor component. Additionally, small molecule-based bulk-heterojunction (SMBHJ) solar cells hold further advantages on the basis of their ease of synthesis and purification by conventional methods. Their composition and subsequent film-forming reproducibility can be readily analyzed using conventional characterization tools. Recently, SMBHJ solar cells have achieved PCEs of greater than 9%13-15 and 10.1%16 for tandem devices and 12%17 for non-fullerene acceptor materials. Spiro compounds bearing a common sp3-C between two planar extended aromatic units have been adopted in organic optoelectronics applications18 because of their tunable charge mobilities in conjunction with suitable π-spacers, donors, or acceptors.19-21 The resulting materials have been applied in organic light-emitting diodes,22, 23 DSSCs,24-26 and organic photovoltaic cells.27-32 Spirally configured, three-dimensional (3D) p-type materials with well-defined intermolecular alignment in the BHJ solid film can increase OPV performance because of their more evenly distributed nano-domains and better intermolecular interactions with fullerene acceptors.28 Ambipolar oligothiophenes with acceptor end groups have been developed for SMBHJ solar cells33 and used to aim at high performance devices.16, 34-36



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In this contribution, we have designed three materials based on a unique cis-stillbene central unit with or without its confinement within a seven-membered ring (Scheme 1). The three compounds possess the same trithiophene spacer (T3) and α-cyanoacrylate (CA) end groups. The two materials in the former cases bear a C5 spiro-fluorene

and a C5 methylene linker, respectively (i.e., CAT3STIF and

CAT3DBS in Scheme 1). The control material CAT3STB, which does not contain an sp3-C in the core template, is included in the study in order to probe the effects of ring confinement and/or possible spiro-π-conjugation on photophysical and morphological properties and subsequent device performance.

Experimental Section Materials and Characterization. The synthetic pathways of compounds are shown in the Supporting Information. Materials and Equipment for Small Molecular Solar Cells All the bulk-heterojunction photovoltaic cells were prepared using the same preparation procedures and device fabrication procedure referring as following: The glass-indium tin oxide (ITO) substrates (obtained from Sanyo, Japan (8Ω/□)) were first patterned by lithograph, then cleaned with detergent, and ultrasonicated in acetone and isopropyl alcohol, and subsequently dried on hot plate at 120 °C for 5 min, and finally treated with oxygen plasma for 5 min. Poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS, Baytron P- VP AI4083) was filtered through a 0.45 µm filter before being deposited on ITO with a thickness around 30 nm by spin coating at 3000 rpm in the air and dried at 150 °C for 30 min inside glove box. The devices were fabricated using the small molecules concentration of 20 mg mL-1, a spin rate of 2000-3000 rpm for 30 s, and chlorobenzene as the solvent. The optimal thickness of the active layers obtained under these conditions was ~ 100 nm. Subsequently the device was completed by coating 30 nm thickness of Ca and an 80 nm


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thickness of Al in < 10-6 torr pressure respectively. The active area of the device is 5 mm2. Finally the cell was encapsulated using UV-curing glue (purchased from Nagase, Japan). During the encapsulation process, the UV-glue was dispensed onto the edge of a piece of glass in the air. The UV-glue coated glass was transferred to the glove box to cover of the OPV device. The device was then sealed by pressing the UV-glue coated glass on top of the device and the device underwent UV curing (254 nm) for 2 min. After encapsulation using UV-curing glue, we measured the I-V characteristics in air. I-V curves of the OPV devices were measured using a computer-controlled Keithley 2400 source measurement unit (SMU) equipped with a Peccell solar simulator under AM 1.5 G illumination (100 mW cm–2). The illumination intensity was calibrated using a standard Si photodiode detector equipped with a KG-5 filter. The output photocurrent was adjusted to match the photocurrent of the Si reference cell to obtain a power density of 100 mW cm–2. After encapsulation, all devices measurements were operated in an ambient atmosphere at 25 °C. The efficiency of 3.5% of a P3HT/PC61BM reference cell measured under illumination in our laboratory was verified to be 3.4% under AM 1.5 G conditions (100 mW cm–2) in National Institute of Advanced Industrial Science and Technology (AIST, Japan).The morphologies of the materials films were analyzed using a VEECO DICP-II atomic force microscope (AFM) operated in the dynamic force mode at ambient temperature and an etched Si probe operated under a resonant frequency of 131 kHz and a spring constant of 11 N m-1.

Results and discussion The three core templates with pinacolatoboron substituents were first coupled to 5’’-(4-bromo)-3,3’’-di-n-hexyl-2,2’:5’,2’’-terthiophene-5-carbaldehyde37 as catalyzed by Pd(PPh3)4 in basic aqueous media at 90 °C in 18h. The resulting dialdehydes, obtained in 52-89% yields, were then condensed with n-octyl-α-cyanoacetate in the presence of triethylamine as a catalyst to produce the target materials, CAT3STIF, CAT3DBS, and



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CAT3STB 62-90% yields (Scheme 1) Br-T3-CHO C6H13 C6H13 Br

A

O B O

B O O

S S

O

cat. Et3N, CHCl3 r.t., 18 hr

89% 89% 52%

CH2 H, H

C8H17

O

cat. Pd(PPh3)4, Na2CO3 DME/H2O:10/1, 90 C

A=

O

NC

S

90% 90% 61%

130 o

S S O C8H17O

S

S

S C6H13

C6H13

C6H13 CAT3STIF

CN

S O

C6H13 NC

OC8H17 CA

T3 C8H17O2C

T3 CAT3DBS

CN

CO2C8H17 NC

125 o

T3 C8H17O2C CN

T3 CAT3STB

CO2C8H17 NC

Scheme 1. The chemical structures of CAT3STIF, CAT3DBS and CAT3STB.

Figure 1a shows the absorption spectra for CAT3STIF, CAT3DBS and CAT3STB in chloroform (10-5 M). In solution, CAT3STB has the largest extinction coefficient (8.5×104 M-1 cm-1) at the longest wavelength, π-π* transition band (λmax, 479 nm) because of the better extended conjugation due to better bond angles (~125°) between the C=C and the two flanking

phenyl

rings.

The

spiro-fluorene

moiety

in

CTA3STIF

exerts

some

cross-conjugation to the central seven-membered, dibenzosuberene (DBS) template. The absence of cross-conjugation in CAT3DBS may be responsible for the reduced ε and the slight blue shift of λmax (473 nm). One observes shifts of 15-50 nm for all compounds in the film state, along with increased vibronic definition that suggests increased rigidity and π-coplanarity (Figure 1b). The extent of increased conjugation follows the order of CAT3STIF (50 nm) > CAT3DBS (33 nm) > CAT3STB (15 nm). The most pronounced vibronic shoulder at 578 nm in CAT3STIF may be due to the unique cross-conjugation exerted by the perpendicular fluorene moiety.


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1.0E+05

(a)

CAT3STIF CAT3DBS CAT3STB

8.0E+04

1.0


(b)

0.8

Intensity (a.u.)

Absorbtion coefficient (M -1cm -1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6.0E+04

4.0E+04

2.0E+04

0.6

0.4

0.2

0.0E+00 350

400

450

500

550

600

650

700

0.0 400

450

500

Wanelength (nm)

550

600

650

700

Wavelength (nm)

Figure 1. (a) A stacked plot of absorption spectra of CAT3STIF, CAT3DBS, and CAT3STB in chloroform solution; (b) A stacked plot of absorption spectra of CAT3STIF, CAT3DBS, and CAT3STB as films.

Electrochemical properties were measured by using cyclic voltammetry38-40 in CH2Cl2 (10-4 M). The potentials were internally calibrated by using the ferrocene/ferrocenium (Fc/Fc+) redox couple (4.88 eV below the vacuum level). Figure 2 shows their CV stacked plots. Among them, only CAT3STIF shows a quasi-reversible oxidation. Notably, both CAT3STIF and CAT3DBS have similar oxidation onsets and thus similar HOMO energy levels in view of the same DBS core and T3- spacers. The oxidation potential for CAT3STB is lower by 0.09 eV may be attributed to the easy cis-to-trans isomerization of the incipient radical cations and thus increased stabilization thru improved conjugation.




0.70

0.49

-1.54

-1.56 0.66

0.76

Current (A)

0.484

-1.73

0.47

-1.59

-1.63

0.39

-1.85

0.52 0.68

0.40

-1.56

-1.45 -1.65

-2.0

-1.5

0.0

0.5

1.0

Potential (V vs Fc/Fc+)

Figure 2 Cyclic voltammograms of CAT3STIF, CAT3DBS and CAT3STB in dichloromethane solution of 0.1 mol L-1 Bu4NClO4 with a scanning rate of 100 mV s-1 on a carbon electrode

-5.2

-5.28 -5.37 -5.35

Al -4.3

PC71BM

-5.5

ITO

-5.0

-3.7

PC61BM

-4.7

CAT3STB

-4.5

CAT3DBS

-4.0

-3.32 -3.25 -3.43

CAT3STIF

-3.5

PEDOT:PSS

-3.0

Energy Level (eV)

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-4.3

-6.0 -6.1

-6.5

-6.1

Figure 3. Device configuration and relative energy levels

Table 1

Optical and electrochemical properties of CAT3STIF, CAT3DBS and CAT3STB compounds. λmax Sol.a

ε × 103 Sol.b

λmax film c

Egopt filmd

EgCV Sol.e

HOMOf

HOMOg

LUMOg

(nm)

(M-1 cm-1)

(nm)

(eV)

(eV)

(eV)

(eV)

(eV)

CAT3STIF

477

85.0

524

1.94

2.04

−5.38

−5.37

−3.32

CAT3DBS

473

67.2

506

1.97

2.13

−5.25

−5.35

−3.25

CAT3STB

479

92.3

494

1.97

1.85

−5.31

−5.28

−3.43

Dilute solution in CHCl3. b Molar absorption coefficient. c Spin-coated from chlorobenzene solution onto glass surface. d Solid-state optical band gap: Egopt = 1240/λedge. e Electrochemical band gap: EgCV = Eox/oneset – Ered/oneset. f HOMO Measured by AC2. g Electrochemical measurements were carried out in CH2Cl2 (1 mM) in the presence of 0.1 M Bu4NClO4 using glassy carbon as the working electrode and Pt counter electrode at scan rate of 10 to 50 mVs-1. Potentials are referenced relative to an internal ferrocene standard (E1/2 = 113 mV vs. Ag/AgCl). gEHOMO/ELUMO = [−(Eonset − Eonset (FC/FC+ vs. Ag/Ag+)) − 4.88] eV.

a


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None of the compounds exhibits reversible reduction waves. The ease of reduction follows the trend of STB > STIF > DBS. The DBS-based system, CAT3STIF showed a lower LUMO because of its intrinsic cross conjugation. The lower reduction potential by 0.10-0.18 eV for CAT3STB is also attributed to the facile cis-to-trans isomerization of the incipient radical anions. OPV devices were fabricated by using PEDOT-PSS as the hole transport layer and PC61BM and PC71BM as the acceptor (n-type) semiconductors. ITO and Al were used as the anode and cathode electrodes, respectively. The device configuration and relative energy levels (determined by using the electrochemical measurements) are shown in Figure 3. The device performance by using CAT3STIF, CAT3DBS and CAT3STB as donor materials and PC61BM or PC71BM at a 1:0.8 wt ratio are summarized in Table 2. The trend of open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF) follows the order of STIF > STB > DBS (entries 1-3). CAT3STIF gave the best device performance in terms of Voc (1.06 V), Jsc (3.6 mA cm−2), and FF (28.4%) presumably due to wider light harvesting range and more balanced charge carrier transport. The poorest device performance for CAT3DBS provides indication that the spirofluorene unit in CAT3STIF is essential to achieving desirable device characteristics. Improvements both in Jsc (by 73-152%) and FF (by 42-137%) were observed in the cases of STIF (6.14 mA cm−2, 67.2%) and DBS (1.36 mA cm−2, 32.7%) when 0.125% and 0.5% 1,8-diiodooctane (DIO) was added during the film deposition step (entries 4 and 5). The results strongly indicate that more evenly distributed and aligned domains, smaller domain size, and better film morphology can be attained (vide infra). It has been noted that the ideal domain size is 10-20 nm for optimal charge separation and transport41. The inferior performance for the case of STB may be due to the interrupting isomerization by DIO during the film forming process (entry 6). Further optimization in Jsc from 6.14 to 7.93 mA cm−2 can be achieved by switching the n-type materials from PC61BM to PC71BM due to increased



light harvesting from 350 to 500 nm (Figure 4b) and thus Jsc increase by 29% (Figure 4a). The PCE of the device can reach 4.87 %. The external quantum efficiency (EQE) of the optimal device improved from 27 to 45% and from 45 to 55% at 400 and 450 nm, respectively (Figure 4b), since PC71BM possessed a higher absorption range from 350-500 nm. Notably, the uniformly higher conversion efficiency from 450 to 700 nm in PC71BM with the highest EQE (conversion efficiency) of 55% at 500 nm is due to the further improved morphology of the CAT3STIF active layer (vide infra).

0

(a)

60

(b)

CAT3STIF PC61BM w/DIO CAT3STIF PC71BM w/DIO

50

2

Current Density (mA/cm )

2

EQE (%)

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4

40

30

20

6 CAT3STIF PC61BM

10

CAT3STIF PC61BM w/DIO

8

CAT3STIF PC71BM w/DIO 0.0

0.2

0.4

0.6

0.8

1.0

0 300

1.2

400

500

600

700

Wavelength (nm)

Voltage (V)

Figure 4 (a) Current density-voltage characteristics of the SM BHJ devices; (b) External quantum efficiency (EQE) curves of the devices. Table 2 Photovoltaic characteristics of CAT3STIF, CAT3DBS and CAT3STB DIO additive Entry

Active layera

Hole mobility Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%) (µh, cm2 V-1 s-1)

(% v/v) 1

CAT3STIF/PC61BM

0

1.06

3.55

28.4

1.03

1.06 × 10−5

2

CAT3DBS/PC61BM

0

0.94 (0.92±0.02)

0.54 (0.50±0.04)

23.4 (23.3±0.3)

0.12 (0.11±0.02)

3.18 × 10−6

3

CAT3STB/PC61BM

0

1.00 (1.00±0.01)

1.50 (1.45±0.07)

27.5 (28.4±0.9)

0.41 (0.41±0.01)

6.46 × 10−7

4

CAT3STIF/PC61BM

0.125

1.00 (1.00±0.01)

6.14 (5.98±0.16)

67.2 (67.9±0.5)

4.13 (4.06±0.06)

4.02 × 10−5

5

CAT3DBS/PC61BM

0.5

0.90 (0.90±0.01)

1.36 (1.22±0.14)

32.7 (31.4±1.8)

0.39 (0.34±0.05)

1.67 × 10−5

6

CAT3STB/PC61BM

0.5

0.65

0.18

18.9

0.02

2.65 × 10−6

7

CAT3STIF/PC 71BM

0.3

0.97 (0.97±0.01)b

7.93 (7.88±0.05)

64.1 (63.0±0.11)

4.87 (4.81±0.10)

———

a

Active layer thin films cast from 2 %w/v chlorobenzene solutions with varying weight ratios of Materials/PCBM=1:0.8 (w/w). b Data in parentheses are average results for 3-4 devices.


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Figure 5 displays a series of tapping-mode atomic-force microscopic (AFM) images for the surface morphology of the active layer, as well as height and phase images, for different CAT3STIF/fullerene blends. The root-mean-square (RMS) roughness values of CAT3STIF/PC61BM, CAT3STIF/PC61BM with DIO, and CAT3STIF/PC71BM with DIO are 0.8, 0.5, and 0.36 nm, respectively. The blended film without additive showed a larger surface roughness (Figure 5a) and bigger domains of fullerene and donor aggregates (Figure 5b) with diverse distribution of domain size in the range of 50 to 100 nm, responsible for a decrease in charge mobility as a lower FF and Jsc. In marked contrast, the AFM phase images in CAT3DBS and CAT3STB blended films show rough amorphous features (RMS 0.85-0.89) with way separate donor or acceptor aggregates (Figure S3d and S3c). In the cases of the active layer with 0.125% DIO additive, their AFM phase images reveals more evenly distributed fullerene aggregates leading a better phase separation with a more uniformed domain size distribution of about 70 nm (Figure 5d). The finer phase separation with more interface contact between n-type and p-type domains leads to more favourable electron-hole pair dissociation. Therefore, the excitons within the BHJ layers of interpenetrating network have better separate transport and collection abilities, leading to the Jsc and FF enhancements42 of 70% and 135%, respectively. Finally, the active layer blended with PC71BM and 0.3% DIO additive also exhibits an interpenetrating network with even smaller domain of 50 nm (Figure 5f), leading to a further 30 % enhancement of Jsc. Presumably, CAT3STIF possessing a spiro framework with unique cross conjugation tends to form more regular alignments by intermolecular π-π stacking between STIF templates and with n-dopant. Therefore, OPV devices optimized with DIO additive and PC71BM led to a smaller size domain and longer percolating length and thus a higher FF (67.2%) with a more balanced charge transport.



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The stacked plot of the absorption spectra of three different devices based on CAT3STIF and PC61BM are shown in Figure S4. It reveals that the λmax peak of the active layer after annealing is blue shifted from 502 to 456 nm. On the other hand, the λmax peak for the device with film formation with DIO additive is blue shifted only 11 nm. We surmise that the annealing tends to destroy the aggregation of blending and thus reduced intermolecular π-π stacking. Nevertheless, the DIO additive suppresses large self-aggregation and crystallization, leading to a more uniformly distributed domain and smaller domain size responsible for a higher Jsc and fill factor in the optimal device.

Figure 5. Tapping-mode atomic-force microscopic image height (left) and phase (right) image of blend film. (a, b) Films of CAT3STIF/PC61BM (1 × 1 µm). (c, d) Films of CAT3STIF/PC61BM with 0.125% DIO as additive (1 × 1 µm). (e, f) Films of CAT3STIF/PC71BM with 0.3% DIO as additive (1 × 1 µm). Red square shows 200 × 200 nm domain.


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The hole mobilities of the blended films were measured by space charge limited current (SCLC) method (Figure S5). The hole mobilities were 1.06 × 10−5, 3.18 × 10−6, 6.46 × 10−7 cm2 V−1 s−1 for CAT3STIF, CAT3DBS, and CAT3STB, respectively. In the films with DIO treatment, their hole mobilities were 4.02 × 10−5, 1.67 × 10−5, 2.65 × 10−6 cm2 V−1 s−1, which were improved by 4-5 times. In particular, the DIO additive effect is the most beneficial to the case of CAT3STIF which exhibited a higher FF due to much more balanced charge transports.

Conclusion: In conclusion, we have documented a unique central structural unit based on cis-stilbene with or without a spiro linker, which was integrated with tri-thiophene arms and α-cyanoacrylate acceptor end groups to form three new p-type materials. It was found that the spiro-containing system−CAT3STIF can achieve greater order with better light harvesting range than its planar counterpart−CAT3DBS in BHJ layer. The changes in morphology translate to larger Jsc and FF due to improved phase separation in the bulk heterojunction blend. The device made of CAT3STIF and PC71BM with DIO additive processing exhibited more uniformed and optimal domains with better percolation length in the film, leading to the highest PCE of 4.87% and a high FF of 64.1% under illumination of AM 1.5 at 100 mW cm-2

Supporting Information Available General procedures, materials and equipment for small molecular solar cells, compounds characterizations, density functional simulation details, AFM images of CAT3DBS and CAT3STB, absorption spectrum of CAT3STIF active layer, details for SCLC measurements and NMR spectra, are provided in Supporting Information. This materials is available free of



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charge via the Internet at http://pubs.acs.org.

Acknowledgments Financial support from the Ministry of Science of Technology of Taiwan (MOST 104-2113-M-007-002-MY3) and National Center for High-Performance Computing (u32ctc01) of Taiwan provided computing time were greatly acknowledged.

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Fluorene Hybrid Template for the

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