New Electron Acceptor Derived from Fluorene: Synthesis and

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New Electron Acceptor Derived from Fluorene: Synthesis and Its Photovoltaic Properties Puvvala Nagarjuna, Anirban Bagui, Jianhui Hou, and Surya Prakash Singh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03768 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 1, 2016

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The Journal of Physical Chemistry

New Electron Acceptor Derived from Fluorene: Synthesis and its Photovoltaic Properties P. Nagarjuna,1, 2 Anirban Bagui1, Jianhui Hou,3 Surya Prakash Singh*1,2 1

Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal

Road, Tarnaka, Hyderabad-500007, India, Email: [email protected] Ph.:+91-40-27191710 2

Academy of Scientific and Innovative Research (AcSIR), New Delhi, India

3

State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular

Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

Abstract Two new fullerene derivatives N3 and N6, derived from flourene, have been synthesized for the organic photovoltaic applications. The molecule N3 is a diarylmethano fullerene derivative, whereas N6 contains a chromophore 4-nitro-cyaostilbene (NCS). Their optical, electrochemical and structural properties were systematically studied by 1H-NMR and NMR, FT-IR and UV-visible absorption spectroscopy.

13

C-

A series of bulk heterojunction

polymer solar cells was constructed from these newly developed acceptors using PBDTTTC-T and PTB7 as electron donors with different donor-to-acceptor weight ratios along with the variation in the volume percentage of DIO additive. The best solar cells made from PTB7:N3 and PTB7:N6 blends exhibited power conversion efficiencies of 4.1% and 3.6%, the cells made from PBDTTT-C-T:N3 and PBDTTT-C-T:N6 blends exhibited power conversion efficiencies of 2.3% and 1.4% respectively with very high open circuit voltage of over 0.85V. 1. INTRODUCTION Organic solar cells (OSC) have intensively been investigated as a renewable energy source because electricity can be generated utilizing this technology at very low cost. Use of environment-friendly organic semiconductors instead of conventional silicon to build solar cells has attracted immense scientific and industrial interest because organic photovoltaic (OPV) technology offers low cost, light weight and flexible large area applications.1-2 The most successful structure in OPV so far is bulk hetero-junction (BHJ) device comprising ptype conjugated polymer as donors and phenyl-C61-butyric acid methyl ester (PC60BM) or 1

ACS Paragon Plus Environment


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phenyl-C71-butyric acid methyl ester (PC70BM) as an electron acceptors and their power conversion efficiency has gradually increased to around 10%.3-4 However, limited photoabsorption of these molecules in the UV-visible region of solar spectrum and lack of donoracceptor compatibility compels the researchers to invent new donor and acceptor molecules in order to achieve high PCEs.5 A great research effort has been done to invent novel fullerene acceptors and several donor copolymers in last few years.6-10 The PCE has reached over 10% by structural modification of low band gap polymers.11 In solution process technique benzo(1,2-b:4,5-b′)dithiophene BDT based donor polymer derivative12 showed good device performance due to their unique chemical structure and planarity brings to form ideal morphology at donor-acceptor interphase, support to π-π stacking which promote the good charge transport in the devices, and also have low HOMO energy levels which helps to increase the open circuit voltage (Voc).13 Qiao et al. reported a high PCE of 8.34% using a polymer solar cell made from a BDT based donor poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2′-ethyl-hexanoyl)-thieno[3,4b]thiophen-4,6-diyl]} (PBDTTT-C-T) blended with PCBM.14 Hou et al. also reported that thienyl-substituted BDT polymers showed good potentials for organic photovoltaic applications.15 Another low band gap polymer poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4b]thiophenediyl}) (PTB7) also appeared to be a promising candidate as an electron donor for OPV applications. A high efficiency of 7.4% from PTB7:PC71BM has been reported in normal structure.16 The PCE was further improved to 8.7% by Lu et al. by incorporating silver and gold metal nano-particles in the blend photoactive layer.17-18 Zhicai He et al. reported 9.2% PCE from PTB7:PC71BM solar cell in inverted structure using poly [(9,9bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene) (PFN) as a buffer layer.19 Although great efforts have been made on the development of new polymeric donor materials, only limited research has been carried out to develop new promising acceptors. The buckminsterfullerene is well known as an n-type semiconductor in the field of organic electronics. However, it’s very low solubility in the organic solvents compels the researchers to make their derivatives highly soluble in low boiling solvents in order to use them in the inexpensive solution processed technique.20-22 Some research efforts have been made in the development of new fullerene acceptors using triphenylamine,23 fluorene,24 thienyl,25 and 2


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furan.26 Hummelen et al. reported on improving the compatibility of fullerene acceptors with fluorine-containing donor-polymers in organic photovoltaic device.27 In organic electronic devices, controlling the film morphology is also critically important. The substituents of fullerene play major role in controlling the physical properties, such as solubility and crystallinity. O C10H 21

NH2 NH

C10H21 C10H 21

O

C10H21

O

DCM, 0 - rt Overnight

C10H 21 C 10H 21

C10H21 C10H 21

O

O

1) C60, pyridine, NaOMe, ODCB Ref lux-80 0C, 24h

N

O

2) Isomerization Toluene, Ref lux 110 0C, 24h

NH S

MeOH Reflux-65 0C Overnight

O

Cl

O

O

O

O

0C

O

S

O

AlCl 3

O

N3

C10H21 C10H 21

C10H21

OH

C10H21 O O

OH

HCl / ACOH Clorobenzene

CN

O

EDCl/DMAP DCM, 0 0C - rt 12h

Reflux- 110 0C 12h

NO2 C10H 21

F10CBA

C10H 21

NC NO 2 O O

N6

Scheme 1. Synthesis and chemical structures of N3 and N6 fullerene acceptors.

3



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Here in, we report designing and synthesis of two new fullerene derivatives, e.g., N3 and N6 for OPV applications.

We follow two principles to functionalized fullerene i)

28

functionalization of PCBM

and ii) diarylmethanofullerene.29-30 The molecular structures

and synthesis steps are demonstrated in Scheme 1.The cyclopropane ring in N3 is having two aryl rings one aryl ring contains fluorine with long alkyl chain length, which increases the solubility of the molecule in organic solvents and another aryl ring has an electron withdrawing group (EWG) group, which increases the electron accepting nature of the fullerene. The other molecule N6, which contains NO2 and CN groups, is synthesized from 9,9-didecyl-9H-fluoren-2-yl-C61-butyric-acid methyl ester converted to acid and followed by EDCl & DMAP coupling as shown in Scheme 1. Here NO2 and CN act as EWG and the attachment of dodecyl chain to the fluorene moiety increases the solubility. Both N3 and N6 are soluble in low boiling solvents like CHCl3, CH2Cl2 providing an advantage for solution processed BHJ solar cells. It also raises the lowest unoccupied molecular orbital (LUMO) energy level, which enhances the open circuit voltage (Voc) of the solar cells made from both PBDTTT-C-T and PTB7.31-32 2. EXPERIMENTAL SECTION 2.1 General: Commercially available chemicals were purchased and used without further purification, Fullerene (C60) is purchased from Sigma-Aldrich, 5-oxo-chloro velarate is purchased from Alfa Aesar, Methyl-(4-chloroformyl) benzoate is purchased from TCI and all the solvents were purified and distilled according to the known adapted procedures. 2.2 Material Characterization: The both derivatives (N3 and N6) were prepared following by optimized procedures and characterised by recording the H1-NMR, 13C-NMR, FT-IR, and MALDI-TOF spectra. UV- visible absorption spectra were taken on a CARY 5000 UVVisible NIR Spectrophotometer. The electrochemical cyclic voltammetry is conducted on CH instrument.

Synthesis

of

N3:

Methyl

4-(9,9-didecyl-9H-fluorene-2-yl)(2-

tosylhydrazono)methyl)benzoate (170 mg, 0.219 mmol) was dissolved in dry pyridine (3.0 mL) under nitrogen and sodium methoxide (11.85 mg, 0.21 mmol) was added. The mixture was stirred for 30 min. at room temperature. A solution of (95 mg, 0.131 mmol) of C60 in (10 mL) of dry 1, 2-dichlorobenzene (ODCB) was added. The resulting mixture was stirred at 80 0

C for 24 h. cooling to room temperature was followed by removal of solvent by rotary 4


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evaporator and purified by column chromatography (silica gel; toluene). First fraction was unreacted C60 and second fraction containing brown colour was collected and solvent was evaporated. This compound dissolved in toluene and refluxed for 24h for isomerisation. The resulting brown solution was concentrated through rotary evaporator and pure compound was collected through column chromatography using (silica gel, ethyl acetate: hexane) as a eluent yielded 60 mg (20%) of the title compound.1H-NMR (ppm)CDCl3,500(MHz):- 8.30-8.23 (d, 2H), 8.22-8.14 (d, 2H), 8.11-8.02 (m, 2H), 7.83-7.76 (d, 1H), 7.75-7.67 (d, 1H), 7.39-7.28 (m, 3H), 3.96-3.91 (s,3H), 2.09-1.93 (m, 4H), 1.29-0.97 (m, 32H) 0.88-0. Synthesis of N6: F10CBA 35mg (0.027 mmol) and 4-nitro-4’-hydroxy-α-cyanostilbene 9.5mg (0.036 mmol) was taken in to 50 ml round bottom flask and add 10ml of dry DCM cooled to 00C under nitrogen atmosphere, to this solution, EDCl (0.554 mmol) and DMAP (0.0194 mmol) was added and stir the reaction at same temperature for 1h and then allow to room temperature for overnight. The crude compound subjected to silica gel column chromatography

using

ethyl

acetate:

hexane

(1:10)

as

eluent.

1

H-NMR

(ppm)CDCl3,300(MHz):- 8.34-8.30 (d, 2H), 7.98-7.93 (m, 2H), 7.86-7.83 (d, 2H), 7.38-7.35 (m, 3H), 7.25-7.10 (m, 4H), 5.35, (s, 1H), 3.13-3.06 (bs, 2H), 2.90-2.79 (t, 2H), 2.49-2.40 (m,2H), 2.08-1.98 (m, 4H), 1.23-1.04 (m, 32H), 0.84-0.82 (t, 6H), 1.32-1.24 (solvent peaks). 2.3 Device Fabrication: The ITO-coated glass substrates were first rinsed thoroughly with detergent and then cleaned by ultrasonic treatment in deionized water, acetone and isopropyl alcohol, consecutively for 15 minutes in each case. After an overnight long drying process inside thermal oven, substrates were subjected to UV-ozone treatment for 30 min. Normal structure OSC: These solar cells were fabricated with the device structure-ITO/ poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)

(PEDOT:PSS)/PBDTTT-C-T:N3

(or, N6)/Mg/Al, where ITO/PEDOT:PSS acts as anode and Al acts as a cathode. First, 35nm thick hole transport layer of PEDOT:PSS was spin coated on top of ozonised ITO. The PBDTTT-C-T:N3 and PBDTTT-C-T:N6 blend active layers were coated from the orthodichlorobenzene (o-DCB) solution of concentration 10mg/ml and subsequently annealed at o

150 C for 30 min. The additive 1,8 diiodooctane (DIO) was used to increase the solubility of acceptors further33. A detailed study on device performance using different weight ratios of donor and acceptor molecules along with the variation in volume percentage of DIO additive have been carried out to get the optimized device. The donor-to-acceptor weight ratio in the 5



active layer was varied as 1:1, 1:2 and 1:3 and the volume percentage of DIO was also varied from 1-5% in order to get the optimal possible device. The thicknesses of PBDTTT-C-T:N3 and PBDTTT-C-T:N6 blended photo-active layers were measured as 128 and 99 nm, respectively. As the molecule N6 has less solubility compared to N3, so it compacts very less in the active layer resulting in less thickness. Finally, 20 nm thick Mg and 100 nm Al were deposited consecutively to form the top electrode, i.e. cathode.34-36

100 nm Al 20 nm Mg 128/99 nm PBDTTT-C-T:N3/N6 35 nm PEDOT:PSS 150 nm ITO

-3.0

-3.6

-6.0

-4.3 Al

-3.5 -3.6

-4.8 ITO

-5.1

-5.1

-4.2

-4

Energy level (eV)

-5.5

-3

-3.7

-4.3 Ag

-4.8

-5

-5.2

ITO

PTB7

-5.0

-2.3

Mg

PBDTTT-C-T

Energy level (eV)

-2

-4.0 -4.5

(b)

-3.2

-3.5

90 nm 3 nm 85/70 nm 30 nm 150 nm

Ag MoOx PTB7:N3/N6 ZnO ITO

(a)

PEDOT:PSS

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

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-6

-5.9 N3/N6

-7

-6.5

-8

-5.9 N3/N6

-5.3 MoO3

-7.5 ZnO

Distance (a.u)

Distance (a.u)

(c)

(d)

Figure 1. The constituent layers of the solar cells with respective thicknesses in (a) normal and (b) inverted structure; Energy band diagram of (c) PBDTTT-C-T based normal structure OSC and (d) PTB7-based inverted solar cell. All values are in eV unit.

Inverted structure OSC: In case of inverted structure solar cell, PTB7 was used as the donor material. Firstly, 30 nm thick ZnO electron transport layer37 was spin coated on top of o

ozonised ITO and subsequently annealed at 240 C for 15 minutes. The ZnO solution was prepared by sol-gel method. An amount of 0.384 gm zinc acetate dihydrate was first mixed in 4.895 ml 2-methoxy ethanol and then 0.105 ml of MEA was added. The solution was stirred overnight at room temperature before spin coating. On top of the ZnO, the photo-active layers of PTB7:N3 and PTB7:N6 with a weight ratio of 2:3 were spin coated from 25 mg·ml-1 o-DCB solution. In order to improve the solubility of the molecules further, 3% (volume) di6


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iodooctane (DIO) was mixed to the solution as an additive. The measured film thicknesses are around 85 and 70 nm for PTB7:N3 and PTB7:N6 composite films, respectively. The o

active layer films were then dried at 40 C for one hour. Finally, 3 nm thick MoO3 and 90 nm thick Ag was deposited by physical vapour deposition at a rate of 0.1- 0.2 Å˖s-1 and 0.5-1.0 Å˖s-1, respectively in a vacuum deposition system with base pressure below 3×10-6 mbar. The device structure and energy band diagrams are shown in Figure 1(a) and (b). 2.4 Device Characterization: A xenon arc lamp based solar simulator from Newport's Oriel was used as a light source for solar cell testing. The current–voltage characteristics of the devices were measured with a Keithley 2400 semiconductor parameter analyser.

The

incident photon to collected electron efficiency (IPCE) was measured with a lock-in detector after illuminating the cell area with monochromatic light from a tungsten lamp. Devices were encapsulated inside a nitrogen filled glove box before electrical characterization. All the measurements were carried out in ambient conditions. 3.

RESULTS AND DISCUSSION

3.1

UV-Visible absorption Spectra

The absorption spectra of newly synthesised fullerene derivatives, e.g. N3 and N6 are measured along with PC60BM at a concentration of 10-5 M in dichloromethane (DCM) solution and displayed in Figure 2(a). As it is seen from the figure, N3 and N6 have stronger absorption in the UV region compared to that of PC60BM. Moreover, a new peak at 310 nm arises for both N3 and N6 along with the already existed peaks at 260 and 328 nm. This certainly ensures higher light coupling with solar spectra for the newly developed acceptors.38-39 Figure 2(b) shows UV-visible absorption spectra of PTB7:N3/N6 blend films coated on ITO glasses. It is obvious from the graph that PTB7:N3 film absorbs more photon than PTB7:N6 composite films almost throughout the spectral range of interest.

7



0.5

0.7 N3 N6 PCBM

0.6

PTB7:N3 (2:3) PTB7:N6 (2:3)

0.4

Absorbance (a.u.)

0.5

Absorbance (a.u.)

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

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0.4 0.3 0.2

0.3

0.2

0.1 0.1 0.0 250

300

350

400 450 Wavelength (nm)

500

550

600

0.0 300

400

500 600 Wavelength (nm)

700

800

Figure 2. (a) UV-vis absorption spectra of N3, N6 and PCBM in DCM solution, (b) Absorption spectra of PTB7:N3 and PTB7:N6 blend thin films coated on ITO glass.

3.2

Electrochemical properties

The electrochemical properties of fullerene derivative acceptors were evaluated by the cyclic voltammetry. The cyclic voltgrams of both acceptors are shown in Figure 3, which shows three similar quasi-reversible reduction waves in the potential range of 0 to -2.2. HOMO and LUMO energy levels of these two derivatives are calculated from the CV and differential pulse voltammetry (DPV) by taking their onset first oxidation-reduction potentials and ferrocene as an internal standard (Potential V vs. Fc/Fc+) with following equation.40 LUMO = −e (E1red + 4.80)

(1)

The calculated values are tabulated in Table 1. As per the results it is observed that the molecule N3 has marginally higher LUMO level which enhances the open circuit voltage (Voc) of the device. Table-1 HOMO and LUMO values of N3 and N6 Fullerene

E1red (V)

E1ox (V)

LUMO (eV)

HOMO (eV)

N3

0.61

1.629

-3.58

-5.88

N6

0.63

1.629

-3.62

-5.88

8


-2

(a)

-1

0 N3

8

CV DPV

0 CV DPV

0

4

2 2

4

N6

1

8 6

4

-1

0

-1 -2

-2

-3

-4 -4

0

-2

-1

-6

-5

0 -2

2

-6

0

DPV Current (µA)

Current (µ A)

6

-2

(b)

DPV Current ( µA)

-2

Potential (V)

-1 Potential (V)

-8

0

Figure 3. CV and DPV figures of (a) N3 and (b) N6. [The onset reduction potential (Ered1) is measured in o-DCB solvent, potential V vs. Fc/Fc+ (internal standard) tetra butyl ammonium hexafluoro phosphate (TBAPF6, 0.1 M) as the supporting electrolyte, reference electrode: saturated calomel electrode, counter electrode: platinum wire and working electrode: glassy carbon]

3.3

Light J-V characteristics

5

(b)

PBDTTT-C-T:N3 (1:2), DIO - 1% PBDTTT-C-T:N6 (1:1), DIO - 5%

5 PTB7:N3 (2:3), DIO - 3% PTB7:N6 (2:3), DIO - 3%

2

2

Current density (mA/cm )

(a) Current Density (mA/cm )

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


Current (µA)

Page 9 of 18

0

-5

0

-5

-10

0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 4. Light J-V characteristics of the polymer solar cells made from newly developed acceptors N3 and N6 blended with (a) PBDTTT-C-T in normal structure and (b) PTB7 in inverted structure. The donor to acceptor weight ratio and DIO additive content for these optimised devices are mentioned in the legend.

The current density-voltage (J-V) measurements of the solar cells were carried out under standard illumination of the AM 1.5G spectrum at 1 sun light intensity. The light J-V characteristics of the best performing solar cells built from the PBDTTT-C-T:N3/N6 and PTB7:N3/N6 blends in normal and inverted structure are shown in Figure 4(a) and (b), respectively and the results are summarised in Table 2. In case of PBDTTT-C-T based 9



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Page 10 of 18

conventional OSCs, firstly the donor-to-acceptor (D/A) weight ratio was varied to get best possible results from each set of devices. Then the volume percentage of DIO additive was varied to get the optimum device keeping the best D/A ratio fixed. The results have been represented in Figure S1 (a-d) of supporting information. The D/A weight ratio of 1:2 was found to be optimum for PBDTTT-C-T:N3 based solar cells as seen from Figure S1(a). Moreover, the BHJ solar cells made from the same blend showed best performance when DIO content in the active layer solution was only 1% (Figure S1(b) and a PCE of 2.3% was achieved thereof. The device showed an open circuit voltage of 0.87V, a short-circuit current density of 6.41 mA/cm2 and a fill factor of 44.1%. On the other hand, PBDTTT-C-T:N6 based devices showed best performance when the donor-to-acceptor ratio was kept at 1:1 with 5 vol% of DIO content. The device showed a Jsc of 5.0 mA/cm2, Voc of 0.83V and a FF of 33.9%. A PCE of 1.4% was achieved from the cell which is comparatively less than the first set of devices. As seen from the Figure 4(a), higher value of Voc and Jsc basically enhanced the photovoltaic performance in the PBDTTT-C-T:N3 based solar cell compared to that of N6 based devices. Table 2: Photovoltaic properties of PBDTTT-C-T:N3 and PBDTTT-C-T:N6 solar cells with different donor-acceptor weight ratio and volume percentage of additive Active layer

D/A ratio

PBDTTT-C-T:N3

1:1 1:2 1:3

w/o

1:2

1% 3% 5%

1:1 1:2 1:3

w/o

PBDTTT-C-T:N6

1:1 PTB7:N3 PTB7:N6

2:3 2:3

Additive (DIO)

1% 3% 5% 3% 3%

Voc (V)

Jsc FF (mA/cm2) (%)

PCE (%)

0.868 0.878 0.899 0.876 0.875 0.876 0.738 0.617 0.500 0.837 0.831 0.835 0.812 0.805

3.98 3.79 3.68 6.41 5.53 5.45 2.46 2.42 1.62 3.99 4.14 5.00 9.73 9.06

1.03 1.04 0.99 2.30 2.15 2.09 0.48 0.40 0.18 1.14 1.27 1.41 4.12 3.64

29.7 31.3 32.0 41.1 44.4 43.7 26.6 26.5 23.3 43.1 36.8 33.9 52.1 50.2

In case of PTB7 based inverted structure solar cells, donor-to-acceptor weight ratio were kept at 2:3 and 3 vol% of DIO additive was mixed in o-DCB solvent. These values were 10


Page 11 of 18

adopted from the literature for optimized PTB7 devices in inverted structure.41 A PCE of 4.1% was achieved from PTB7:N3 based BHJ solar cell, whereas PTB7:N6 solar cell showed a maximum efficiency of 3.6%. This is mainly because of the lower short circuit current density in the latter case. Marginally higher value of Voc was also observed in case of PTB7:N3 based devices than the latter one. The shunt resistances are calculated to be 272 and 222 Ω·cm2 for N3 and N6 based devices, respectively.

Poor shunt resistance in

PTB7:N6 solar cells indicate more recombination of charge carriers in those devices. Less solubility of N6 in o-DCB may cause reduced donor-acceptor interface for excitons to be broken up into charges and in-sufficient percolation path for charges to get collected to electrode in PTB7:N6 films than PTB7:N3 resulting in higher recombination. On the other hand, series resistances are found to be 16.9 and 13.7 Ω·cm2 for PTB7:N3 and PTB7:N6, respectively. Higher value of series resistance in the PTB7:N3 device is possibly because of higher thickness of the film (85 nm) compared to that of PTB7:N6 blend films (70 nm). 3.4

EQE measurement 50 40

60

PBDTTT-C-T:N3 PBDTTT-C-T:N6

PTB7:N3 PTB7:N6

50

30

40 EQE (%)

EQE(%)

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


20

30 20

10

10

0 300

400

500 600 Wavelength(nm)

700

800

0

400

500

600 Wavelength (nm)

700

800

Figure 5. EQE curves of photovoltaic devices using fullerene derivative acceptors N3 and N6 blended with PBDTTT-C-T in normal structure and PTB7 in inverted structure.

The external quantum efficiency (EQE) spectra of both the devices made from two different fullerene derivatives have been shown in Figure 5. The PBDTTT-C-T:N6 based solar cell exhibited a maximum EQE of 25% at 700 nm with a shoulder peak at 650 nm. It has two more weak peaks at 350 and 450 nm. On the other hand, PBDTTT-C-T:N3 device showed the highest peaks at 650 and 700 nm with around 40% of EQE. Here also two more peaks are present, but at 350 and 490 nm. Similarly, PTB7:N3 based solar cell showed a maximum IPCE of 57.6% at 632 nm and PTB7:N6 device exhibited highest IPCE of 51.7% at 616 nm. Higher value of EQE in the PBDTTT-C-T:N3 and PTB7:N3 based devices clearly 11



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indicates a more efficient photon-to-electron conversion process in those devices compared to that in N6 based devices.42 3.5

AFM images

Nano-scale film morphology in the lateral direction of the PTB7:N3/N6 composite layers have been visualized using AFM in tapping mode. Figure 6 shows the topological images of the blend films as obtained from atomic force microscopy on 5 × 5 µm2 scanning area. The three-dimensional view of the AFM images clearly indicates that the PTB7:N6 blend film (Figure 6d) has rougher surface than PTB7:N3 film (Figure 6c). The root-mean-square (RMS) roughness for PTB7:N3 and PTB7:N6 films are found to be 8.9±1.2 nm and 20.7±2.3 nm, respectively. Moreover, a very prominent nano-structure like pattern, similar to fine knitting fabric, is observed in the phase diagram of PTB7:N3 film (Figure 6e), whereas larger agglomerations are found in case of PTB7:N6 blend films (Figure 6f). This suggests that the N3 molecule might have better solubility in o-DCB and more miscibility with the donor molecule than N6.

Figure 6. (a,b) AFM height images; (c,d) 3D view of the surface topology; and (e,f) phase diagrams of the PTB7:N3 (left) and PTB7:N6 (right) blend films.

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The poor performance of the PBDTTT-C-T:N6 and PTB7:N6 based devices is probably because of the less solubility of N6 molecule in o-DCB solvent which leads to rough morphology. On contrary, as the molecule N3 has high solubility because of the presence of long di-alkyl chain and at the same time, high electron acceptance property due to presence of an electron withdrawing group, it is expected to aggregate properly on the substrate surface to produce a uniform layer. 4. CONCLUSION In summary, we have designed and synthesized two new fullerene derivatives to be used as electron acceptors in solution processed polymer solar cells. The BHJ solar cells made from these acceptors using the low band gap polymer PBDTTT-C-T as donor, showed high Voc above 0.85V, which is mainly due to the wide energy gap between HOMO level of the donor and the LUMO level of acceptor. The presence of long alkyl chain in diarylmethano fullerene (N3) increases the solubility of this fullerene derivative and the presence of electron withdrawing group enhances the electron accepting nature of the acceptor, which resulted in better device performance for PBDTTT-C-T:N3 and PTB7:N3 composites. This work is an extension to our research program on the development of new soluble fullerene acceptors. However, we believe that these are not the best results in terms of the device performance and our current efforts are being focussed on the optimization of device further using these acceptors. Because of better solubility of these molecules in organic solvents and higher open circuit voltage in case of the device, using these newly developed acceptors over other conventional acceptors would be surely a wise choice for fabrication of solution processed BHJ solar cells. ASSOCIATED CONTENT Supporting Information Optimization of PBDTTT-C-T:N3/N6 solar cells, cyclic voltammetry of standard reference ferrocene, absorption coefficients for N3, N6 and PC61BM, characteristic J-V curve of PTB7:PC61BM based inverted structure solar cells and corresponding EQE curves, experimental details on synthesis of fullerene acceptors and structural characterization by 1HNMR, 13C-NMR (Hz), ESI-MS, MALDI-TOF.

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ACKNOWLEDGEMENTS PN thanks to CSIR for providing senior research fellowship. SPS thanks to XII FY CSIRINTELCOAT (CSC0114) for financial support. We also gratefully acknowledge DST-UK (APEX-Phase-II) for their support.

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New Electron Acceptor Derived from Fluorene: Synthesis and

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