Electron transport and ultrafast spectroscopic studies of new methanofullerenes bearing a heteroatom in the exohedral chain

Article abstract of DOI:10.1039/c9nj04188e

Fullerene derivatives (C60 and C70) have been widely used in excitonic solar cells due to their exceptional electron accepting properties and low reorganization energies. In recent years, a wide variety of fullerene-based n-Type materials have been developed, showing tremendous potential in the field of organic photovoltaics. However, only a few studies have been performed on heteroatom bearing methanofullerenes and their opto-electronic properties for use as n-Type organic semiconductor materials. In the present study, we report the synthesis of two mono-substituted methanofullerene derivatives, i.e., C60-Th (Product 1) and C60-TPA (Product 2), with a heteroatom (S and N, respectively) in the exohedral chain attached via an ethylene linker to the cyclopropane ring, and a comprehensive study of their photophysical and electrochemical properties. The methanofullerene derivatives have been synthesized using an amine-Assisted cycloaddition (AACA) reaction method. The structures of the synthesized products were established via different spectroscopic techniques. Reasonable quenching efficiencies were observed with respect to the fluorescence emission in mixtures with the donor polymer P3HT using both methanofullerenes. The electron transport properties were evaluated through fabricating electron-only devices, and 10 and 6 times higher mobilities were found compared to PC61BM for Products 1 and 2, respectively. Finally, the charge transfer properties were evaluated in mixtures with P3HT via transient absorption spectroscopy to study the ultrafast charge separation and formation of long-lived charge-separated states. The study suggests that both the products show excellent electron transport properties and the formation of longer-lived charge-separated states in mixtures with the donor polymer for use as n-Type materials for organic solar cells.

Full text of DOI:10.1039/c9nj04188e

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Electron Transport and Ultrafast Spectroscopic Studies of New
Methanofullerenes bearing Heteroatom in exohedral chain
a#
b#
b#
a#*
Samya Naqvi, Nikita Vasistha, Mahesh Kumar and Rachana Kumar
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Advanced Materials and Devices Metrology Division
a
Photovoltaic Metrology Group
Photonics Materials Metrology Group
b
CSIR-National Physical Laboratory
Dr. K. S. Krishnan Marg, New Delhi
India-110012
#
Academy of Scientific and Innovative Research, New Delhi
*
Corresponding Author
Advanced Materials and Devices Metrology Division
Photovoltaic Metrology Group
CSIR-National Physical Laboratory
Dr. K. S. Krishnan Marg, New Delhi
India-110012
E-mail: rachanak.npl@nic.in, rachanasinghchem@gmail.com
Tel:+91-11-4560-8577
Fax : +91-11-4560-9310

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Abstract
Fullerene derivatives (C60 and C70) have been widely used in excitonic solar cells due to their
exceptional electron accepting properties and low reorganization energy. In the recent years, a wide
variety of fullerene based n-type materials have been developed showing tremendous potential in the
field of organic photovoltaics. However, only a little study has been performed on heteroatom
bearing methanofullerenes for opto-electronic properties to be used as n-type organic semiconductor
material. In the present study, we report synthesis of two mono substituted methanofullerene
derivatives, i.e., C60-Th (Product 1) and C60-TPA (Product 2), having heteroatom (S & N
respectively) in exohedral chain attached by ethylene linker on cyclopropane ring and
comprehensively studied their photophysical and electrochemical properties. Methanofullerene
derivatives have been synthesized using our amine assisted cycloaddition (AACA) reaction method.
The structure of synthesized products was established by different spectroscopic techniques. Efficient
quenching efficiency was observed in fluorescence emission in mixture with donor polymer P3HT
for both the methanofullerenes. Electron transport properties were evaluated by fabricating electron
only devices and found 10x and 6x higher than PC61BM for product 1 and 2 respectively. Finally,
the charge transfer properties were evaluated in mixture with P3HT by transient absorption
spectroscopy to establish ultrafast charge separation and formation of long lived charge separated
states. The study suggests both the products show excellent electron transport properties and
formation of longer lived charge separated states in mixture with donor polymer to be used as n-type
material for organic solar cells.
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Key words. Methanofullerene, heteroatom, AACA, transient absorption, spectroscopy, solar cells

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1
. Introduction
The development of new organic functional materials is drawing the considerable attention of
researchers in the area of organic photovoltaics (OPV) to harvest maximum solar energy and to
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accomplish the cheaper source of energy demands. Fullerene chemistry has been evolved
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tremendously since its large scale production reported in 1990 introducing different types of
reactions and synthesizing medley of fullerene derivatives with variety of functional groups
3
attached exohedrally. Among all the reactions, cycloaddition reactions are the most prevalently
used for synthesis of methanofullerene derivatives for different applications including organic
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solar cells. The modified fullerenes maintain the anomalous properties of pristine fullerenes like
low reorganization energy and high electron affinity.5-11 For example, attachment of suitable
organic moieties over fullerenes results in tuning of energy levels, better molecular interactions
as well as their solubility. Introduction of electron donating groups on exohedral addends in
methanofullerenes, results in improved solubility as well as increase in open circuit voltage of
12
solar cell devices. These advantages enable modified fullerenes to be used as n-type material in
for organic electronics.13-15 For the synthesis of methanofullerene derivatives, cyclopropanation
by 1,3 dipolar cycloaddition reaction of diazo compounds generated from different precursors,
has been exploited extensively. Phenyl[C61]butyric acid methyl ester (PC61BM) and [6,6]
Phenyl[C71]butyric acid methyl ester (PC71BM) are the most conventional acceptor materials
synthesized by 1,3 dipolar cycloaddition of diazo compound and has been used widely in organic
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photovoltaics (OPV). We have recently reported the amine assisted cycloaddition (AACA)
reaction for the synthesis of [6,6]PC61BM monoadduct and several other methanofullerenes
catalyzed by amine under total aerobic conditions for their applications in organic electronics.17
Although, among myriad of fullerene derivatives several heteroatom bearing fullerene products

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are reported (Prato product, fused fullerene etc.) using amines and sulfur derivatives, only a
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few methanofullerene derivatives are reported with heteroatom in exohedral addend. Recently
thiophene containing methanofullerenes were developed and used as electron transport layer in
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Perovskite solar cells as organic Lewis base to passivate the uncoordinated lead ions.20,21 There
is still lack of reports on heteroatom containing methanofullerenes with comprehensive study of
their opto-electronic and charge transfer properties. More research is required on preparation of
methanofullerenes specially using low cost amines and thiophene derivatives for cost effective
bulk scale synthesis for large area application.
In this context, in the current work we have synthesized two new methanofullerene derivatives
having heteroatom in exohedral addend. In product 1 (C60-Th), thiophene ring end unit is
attached on fullerene exohedrally via cyclopropane ring and ethylene linker. The other Product
2
,(C60-TPA), contains triphenylamine end group attached to fullerene exohedrally in same
manner as Product 1 (figure 1). Thiophene and triphenylamine both are electron rich groups and
22
act as energy absorbing centers. Both the molecules have been synthesized by AACA process
from their diazo intermediate. The synthesized molecules have been characterized by UV-vis,
FTIR, NMR spectroscopy and MALDI-TOF mass spectrometry for the establishment of their
structure. HOMO-LUMO energy levels of products have been calculated by cyclic voltammetry.
Thermal stability was confirmed by thermal gravimetric analysis (TGA). The electron mobility
of the products was also evaluated by making electron only devices. Photoluminescence and time
resolved fluorescence study was performed to establish efficient charge transfer interactions
between methanofullerene and donor P3HT polymer. Finally to further support the efficient
charge transfer and generation of charge separated states, we performed transient absorption

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spectroscopy in mixture with P3HT. Both the materials show excellent acceptor properties with
ultrafast charge separation.
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Figure 1. Chemical structure of Product 1 (C60-Th) and Product 2 (C60-TPA).
2
2
. Experimental Section
.1. Materials and Characterization
Thiophene-3-Carboxaldehyde, 4-(Diphenylamino) benzaldehyde, p-toluenesulfonyl hydrazide,
triethylamine and Poly(3-hexyl)thiophene were purchased from SigmaAldrich and used without
further purification. [60]Fullerene was purchased from AlfaAesar. All solvents were used after
drying and were purchased from Alfa Aesar. Fourier Transform Infrared spectroscopy (FTIR)
was done on Perkin Elmer FTIR Spectrum 2. The sample was ground with anhydrous KBr and
-1
pallets were prepared to record the spectrum from 4000 to 500 cm . A background in air was
done before samples. UV-vis absorption spectroscopy measurement was performed on Shimadzu
UV-vis spectrophotometer (UV-1800) for 25 M solutions and spin coated films. Products were
1
also characterized for H NMR on Jeol 400 MHz spectrometer in deuterated chloroform (CDCl )
3
1
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using tetramethylsilane (TMS) as internal standard. We were not able to record C NMR due to
low concentration of products. MALDI-TOF-TOF mass spectrometry AB SCIEX used to
determine the molecular weight of the products using α-cyano 4-hydroxy cinnamic acid matrix.
Three electrode standard configuration was used for cyclic voltammetry (CV) measurements

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using a platinum wire as counter electrode, Ag wire as reference electrode and Pt-disc as
working electrode. 0.1 M TBAPF (tetra-n-butylammonium hexafluorophosphate) was used as
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supporting electrolyte and 1 mM solution of methanofullerene in o-dichlorobenzene (o-DCB)
was used to perform the experiment. Current vs Voltage was measured on an Autolab
potentiostate. Fluorescence measurements were performed on Varian (CARY eclipse)
Fluorescence Spectrophotometer in 25 M solution and also in films. A solid sample holder
assembly was used to record the fluorescence in films. Horiba Jobin Yvon (Fluorohub) was used
to record time resolved fluorescence in 25 M toluene solution but were not able to measure in
films due to instruments limitation. Ultrafast transient absorption spectroscopy was performed
using optical pulse from Ti:Sapphire laser amplifier (35 fs, 4 mJ/pulse, 1 KHz, 800 nm). A beam
splitter was used to split the pulse into two beams. An optical parametric amplifier (TOPAS,
Light Conversion) was employed on the high intensity beam (pump) to vary the wavelength from
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90 nm to 2600 nm. The weak intensity beam (probe) was propagated through a CaF crystal to
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generate white light continuum (WLC). A computer-controlled delay stage was used to optically
delay the probe beam with respect to pump beam. In the current study we used 450 nm
wavelength as a pump beam at normal incidence and the changes in absorption was detected by
using a gated CMOS detector. The time resolved study was performed using HELIOS (Ultrafast
systems) spectrometer and life times were estimated by global fitting.
2.2. General method of synthesis of Chalcone 1a and Chalcone 2a.17a Acetophenone (2 mM)
and Thiophene-3-carboxaldehyde for 1a (2 mM), or 4-(Diphenyl)amino benzaldehyde for 2a (2
mM) were dissolved in 20 mL ethanol. After dissolution 40 % NaOH solution (10 mL) was
added with continuous stirring at room temperature and the progress of reaction was monitored
by TLC. Finally the reaction mixture was precipitated using dilute HCL (20%). Crude product

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was filtered and washed many times with water and purified by column chromatography using
ethyl acetate: n-hexane (70:30) as eluent.
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Yield: ~90 %. 1a: FTIR (cm ) 3086, 2927, 1662, 1592, 1443, 1278, 1208, 1011, 919, 718, 623;
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H NMR (CDCl , δ) (ppm) 7.25 (1H), 7.29 (1H), 7.3 (1H), 7.4 (2H), 7.5 (2H), 7.7 (1H), 7.9
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2H). 2a: FTIR (cm ) 3054, 1670, 1578, 1503, 1327, 1202, 1010, 753, 704; H NMR (CDCl ,
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00 MHz) δ (ppm) 6.9 (2H), 7.1 (8H), 7.4 (7H), 7.7 (2H), 7.9 (2H).
.3 General method of synthesis of Hydrazone (1b) and Hydrazone (2b) of chalcone (1a &
1
7a
a) : The tosyl hydrazone was prepared by refluxing a solution of chalcone 1a or chalcone 2a
(0.5 mM), p-toluene sulfonyl hydrazide (1.1 equivalent) in methanol (50 mL) with continuous
stirring for ~6 hour. Crystals of hydrazone precipitate out of solution on cooling and dried after
washing with cold methanol.
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Yield: ~75 %. 1b: FTIR (cm ,) 3203, 3021, 1616, 1310, 1194, 1044, 751; H NMR (CDCl ,
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400 MHz) δ (ppm) 2.36 (3H), 6.22 (1H), 6.88 (1H), 7.01 (2H), 7.06 (1H), 7.19 (1H), 7.27 (3H),
-1
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.40 (2H), 7.45 (2H), 7.7 (2H). 2b: FTIR (cm ,) 3192, 3059, 2921, 1677, 1585, 1489, 1268,
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150, 1010, 759; H NMR (CDCl , 400 MHz) δ (ppm) 2.3 (3H), 6.7 (1H), 6.9 (2H), 6.9-7.11
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(10H), 7.25-7.30 (9 H), 7.6 (2H), 7.7 (2H).
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.4 General method for synthesis of methanofullerenes, C60-Th (1) & C60-TPA (2) :
Hydrazone, 1b or 2b (0.25 mM) was dissolved in dichloromethane (10 mL) and cooled down to
0oC. A catalytic amount of triethyl amine (0.9 mL) was added and stirred for three hours at this
temperature. A solution of C (0.5 equivalent) in o-dichlorobenzene (20 mL) was added and
6
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o
temperature was raised to 85-90 C. Stirring was continued for 8 hours followed by precipitation
with methanol. Precipitate was collected by centrifugation. Crude product was loaded on silica
gel column (200 mm x 18 mm) for purification using toluene/hexane (70:30) mixture as eluent.

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Unreacted fullerene was isolated as the first fraction followed by purple colored monoadduct i.e.,
5,6] fulleroid. Refluxing [5,6] isomers in o-dichlorobenzene for 5 hours converts totally to wine
[
colored [6,6]methanofullerene by thermal isomerisation.
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Product 1: Yield: ~30 %; FTIR (cm ,) 2972, 1454, 1314, 1268, 1014, 794; H NMR (CDCl ,
3
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00 MHz), δ (ppm) 6.75 (1H), 6.86 (1H), 6.96 (1H), 7.12 (2H), 7.3-7.8 (5H); HR‐MS
(MALDI‐TOF): m/z for C73H10S, calculated 918.8312; found 917.8234 (M-1).
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Product 2: Yield: ~32 %; FTIR (cm ,) 2916, 1591, 1487, 1257, 1018, 729; H NMR (CDCl ,
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00 MHz), δ (ppm) 6.21 (1H), 6.84 (1H), 6.93-7.16 (6H), 7.26-7.30 (4H), 7.37-7.54 (4H), 7.72-
.94 (5H); HR‐MS (MALDI‐TOF): m/z for C87H21N, calculated 1080.1014; found 1079.0936
(M-1).
2
.5. Fabrication of electron only devices17c
2.5.1. Preparation of zinc oxide precursor solution
The solution of zinc oxide (ZnO) has been prepared by dissolving 1 g (4.5 mM) zinc acetate
dihydrate (Zn(CH CO ) .2H O) in 10 mL methoxyethanol with 0.254 mL monoethanolamine
3
2 2
2
o
(MEA) as stabilizing agent at room temperature. The mixture solution was then stirred at 60 C
for about 2 hours until a clear and homogenous solution was obtained.
2
.5.2. Current–voltage characteristics.
Electron only devices with the ITO/ZnO/Product 1 or 2/Al architecture were fabricated on a
cleaned ITO coated glass substrate with sheet resistance of 20 Ohm/sq under ambient conditions.
Substrates were ultrasonically cleaned with detergent, distilled water, acetone and isopropyl
alcohol and dried in vaccum oven. The ZnO precursor solution was spin coated onto the
o
substrates at 3000 rpm followed by annealing at 250 C for 30 minutes. Methanofullerene
solution (20, 30 & 40 mg/mL) in chlorobenzene were prepared and spin coated at 1000 rpm on

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top of the electron transport layer followed by annealing at 120 C for 15 min. Finally, the
cathode material i.e., a 100 nm Al layer was deposited on top of active layer under high vaccum
to complete the device structure. Film thickness was measured using calibrated Ellipsometer (J A
Woolam, Model: Vase). The J–V characteristics were recorded using a calibrated computer
controlled Keithley 236 source meter under dark.
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2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
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5
5
5
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5
5
6
0
1
2
3
4
5
6
7
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0
1
2
3
4
5
6
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8
9
0
1
2
3
4
5
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7
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
. Results and Discussion
The most accepted approach for synthesis of methanofullerene is via diazoalkane intermediate
from tosyl hydrazone precursor in pyridine in presence of sodium methoxide under inert
atmosphere.23,24 This process is very hazardous and causes high impact on environment due to
disposals during material synthesis. Amine assisted cycloaddition reaction has been found less
hazardous and more energy efficient over other methods for methanofullerene synthesis as the
reaction is performed under aerobic conditions without using pyridine and sodium methoxide.25
AACA method makes bulk synthesis of methanofullerene feasible at very cheap prices required
for commercialization of OPV. In the present work, we have synthesized two different
methanofullerene derivatives bearing heteroatoms (Figure 1) by our AACA method.
For the synthesis of methaonofullerene, chalcone was prepared in first step via Claisen-Schmidt
condensation of acetophenone with respective ketone (Scheme 1). The reaction of thiophene-3-
carboxaldehyde and 4-(Diphenyl)amino benzaldehyde with acetophenone forms enone 1a and 2a
respectively where, ,  double bond is present on phenyl ketone. Hydrazones (1b and 2b) were
prepared from their respective chalcone in methanol using p-toluenesulfonyl hydrazide. After
o
tosylation, diazo intermediate was generated at 0 C in presence of tertiary amine as catalyst
under aerobic conditions. The fullerene (C60) solution in o-DCB was added insitu and reaction

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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
o
was carried out for 8 hrs at 90 C as shown in Scheme 1 (A & B) and the progress of reaction
was monitored by thin layer chromatography (TLC). The reaction mixture was precipitated using
methanol and subjected to column chromatography for purification. Kinetically favorable
product, [5,6] open fulleroid is converted into thermodynamically stable [6,6] methanofullerene
by refluxing in o-DCB for 5 hours.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
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1
2
3
4
5
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7
8
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0
1
2
3
4
5
6
7
8
9
0
A)
B)
Scheme 1. Synthesis of (A) Product 1 (C60-Th) and (B) Product 2 (C60-TPA) (i) ethanol, 40% NaOH
solution, rt; (ii) p-toluenesulfonyl hydrazide (1.2 eq.), methanol, reflux, 24 hours; (iii) dichloromethane,
o
o
triethyl amine, 0 C, 3 hours; (iv) C in o-dichlorobenzene, 90 C, 8 hours; (v) o-DCB, reflux, 5 hours.
6
0
The structure of product 1 & 2 has been analyzed by different spectroscopic techniques. The
-1
FTIR spectra of fullerene product 1 clearly shows aromatic C-H stretching at 3096 cm and C=C
-1
bond stretching at 1621 cm . While the FTIR spectra of product 2 shows aromatic C-H
-
1
-1 1
stretchings at 3057, 3024 cm and C=C bond stretching at 1627 cm . H NMR spectroscopy
1
further confirm the purity and formation of [6,6] methanofullerenes. The H NMR spectrum of
product 1 shows the appearance of ethylene protons at 6.75 and 6.86 ppm and other aromatic

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protons appear between 6.96-7.8 ppm. Product 2 shows the chemical shift at 6.2 and 6.8 ppm for
ethylene protons with other aromatic protons between 7.0-7.7 ppm (19H) being indicative for the
2
6
formation of product. Mass analysis further confirmed the formation of monoadduct for both
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
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4
4
4
4
5
5
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5
5
5
5
5
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0
1
2
3
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6
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8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
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5
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7
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9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
the products (See SI).
3
.1. Steady State Absorption Spectroscopy
Absorption spectroscopy is a widely used technique to confirm the conversion of [5,6] fulleroid
into [6,6] methanofullerene. The peak at ~ 697 nm confirms the formation of [6,6] adduct which
23
is not present in [5,6] adduct. Figure 2A and S15 show the comparative absorption spectra of
product 1 & 2 in chloroform solution (25 M) in visible range. Both the products show similar
absorption to PC61BM, the widely used acceptor material in organic solar cells. Absorption
spectra shows clear appearance of peaks at 430 and 697 nm for product 1 and at 433 and 702 nm
for fullerene adduct 2 confirming the [6,6] addition on fullerene core and formation of
methanofullerene. The optical band gap has been calculated from absorption onset and the values
estimated to be 1.77 and 1.76 eV for product 1 and 2 respectively. Absorption spectra were also
recorded in thin films (spin coated 25 M chloroform solution at 1000 rpm) which show broad
absorption in the whole visible region and extended absorption up to 800 nm due to aggregate
formation in solid state. Both the products show higher absorption than reference PC61BM in
films.

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1
1
1
1
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1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
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3
3
3
4
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0
1
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0
1
2
3
4
5
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9
0
1
2
3
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5
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Comparative UV-visible absorption spectra of Product 1 (C60-Th) and Product 2 (C60-TPA)
and PC61BM in (A) chloroform solution and (B) Spin coated films.
3.2. Thermal properties
The thermogravimetric analysis was performed to study the thermal stability of materials for
optimization of annealing temperature of films during device fabrication. The TGA thermograms
o
are shown in Figure 3. Both the methanofullerene derivatives show no weight loss up to 150 C
o
17a
and only 10% weight loss up to 200 C as observed in earlier cases. Both the materials show
o
~
50% weight loss up to 900 C. For optimization of annealing temperature of films, the thermal
stability information is prerequisite and here we find that the films of both the products can be
o
annealed at 120 C.

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1
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2
2
2
2
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0
1
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0
1
2
3
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0
1
2
3
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1
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3
4
5
6
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8
9
0
1
2
3
4
5
6
7
8
9
0
-1
o
Figure 3. TGA of Product 1 (C60-Th) and Product 2 (C60-TPA) under N flow of 20 mL min at 10 C
2
-1
min temperature ramping.
3
.3. Electrochemical Analysis
Electrochemical studies of products were evaluated by cyclic voltammetry (CV) to determine the
redox potentials and HOMO-LUMO levels of the synthesized methanofullerene derivatives.
Figure 4 shows the cyclic voltammogram of methanofullerene derivatives and data are
summarized in Table 1. Both the products show well defined quasi-reversible reduction
1
7
potentials indicating their n-type semiconductor nature. Product 1 showed three clear reduction
peaks at -1.29, -1.66 and -2.10 V whereas in product 2, two reversible reductions at -1.31 and -
1
.68 V were observed (Figure 4 A & B) for fullerene reduction. Product 2 also showed one
electron oxidation of triphenylamino group at ~0.46 V. HOMO-LUMO energy levels were
calculated using the empirical formula EHOMO/LUMO= -(Eonset(red/ox)+4.8 eV). LUMO energy levels
were estimated to be -3.7 and -3.68 eV respectively for product 1 & 2 using the onset reduction
potential which is lower than PC61BM.27 From oxidation onset, HOMO energy level was
calculated to be 5.59 eV and 5.13 eV for product 1 and product 2 respectively. Using HOMO-
LUMO energy levels, the electrochemical band gap was calculated to be 1.89 eV and 1.45 eV for

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1
1
1
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1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
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5
5
5
5
5
5
5
5
5
5
6
product 1 & 2 respectively. These values are within the error range compared to optical band gap
2
8
mentioned earlier. Although LUMO energy levels are same for both the products, HOMO has
uplifted in product 2 due to presence of electron rich triphenylamine group causing a low
oxidation potential, therefore, and thus lowering the band gap of product 2.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
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5
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 1. Electrochemical data of Product 1 (C60-Th) and Product 2 (C60-TPA).
redonset red-1 red-2 red-3 Oxonset ox-1 LUMO HOMO Electrochem
band Gap
(
V)
(V)
(V)
(V)
(V)
0.79
0.33
(V)
(eV)
(eV)
-5.59
-5.13
(
eV)
C60-Th (1)
-1.10
-1.12
-1.29 -1.66 -2.10
-
-3.7
1.89
1.45
C60-TPA (2)
-1.31 -1.68
-
0.46
-3.68
+
Figure 4. Cyclic voltammetry curve of (a) C60-Th and (b) C60-TPA (in o-dichlorobenzene (v/s Fc/Fc )
-1
with 0.1 M n-Bu NPF as a supporting electrolyte at a scan rate of 50 mV s .
4
6

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3
.4. Electron Transport Studies
To ascertain the potential of new methanofullerenes for their applications in organic solar cells,
we have fabricated electron only devices at variable thickness to investigate the charge carrier
mobility. The solution of methanofullerene products (1 & 2) was prepared in chlorobenzene with
varying concentration from 20-40 mg/mL followed by spin coating on top of the ZnO coated
ITO substrates under inert conditions. The schematic of device and energy level diagrams are
shown in Figure 5.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
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4
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4
4
4
5
5
5
5
5
5
5
5
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5
6
0
1
2
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
1
2
3
4
5
6
7
8
9
0
3
.7 3.7
4
.2
4
.3
Al
4
.8
Al as
Cathode
ITO
5.13
5.6
Electron-only Devices
7.4
Figure 5. Schematic of fabricated electron only device with energy level diagram of
corresponding layers.
Electron mobility of materials was estimated by the space charge limited current (SCLC) model.
m
At applied voltage above 0.1 V, the curves demonstrate power law dependence i.e. J α V where
2
9
m takes different values over different voltage regimes. After the ohmic region, the second
region i.e., trap-controlled space charge limited conduction (SCLC) has been observed at the
gradient value of 2 at a single energy level inside the band gap; this mechanism can be
represented by the following Mott-Gurney equation 1:30
(푉 ― 푉퐵푖)2
9
퐽푆퐶퐿퐶 = 휇 휀 휀
……Eq. 1
0 0 푟
퐿3
8

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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
where, JSCLC stands for space charge limited current density, µ is the mobility of organic
0
semiconductor, ε is the vacuum permittivity and ε permittivity of medium, L is thickness of
0
r
organic semiconductor layer and V is applied potential and VBi is the built-in-potential.
Generally charge carrier mobility in organic materials is low due to structural disorder in the
films. I-V data was recorded under dark condition. Current density-voltage curves for different
concentrations are shown in Figure S16 and S17. At low applied voltage the dependence of J
over V is linear and behavior is ohmic (Slope 1). On increasing the voltage the charge carriers
are injected and current becomes space charge limited with value of m=2 (Slope 2). Electron
mobility values have been calculated by fitting the experimental J-V in the given equation 1. The
electron mobility for new methanofullerenes with variable thickness has been summarized in
Table 2. The film thickness is an important parameter for the determination of charge carrier
mobility. The decreased layer thickness limits the injection of charge carriers into the bulk of
material that leads to the reduced charge carrier collection rate resulting in decreased charge
carrier mobility. The highest mobility for synthesized acceptors was achieved with 200 nm
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-3
2
-3
2
thickness and the values are 1.02 x 10 cm /V.s and 0.6 x 10 cm /V.s for product 1 and 2
-3
2
17c
respectively which is 10x and 6x higher than PC61BM (0.1 x 10 cm /V.s).
Table 2. Electron mobility values for Product 1 (C60-Th) and Product 2 (C60-TPA)
Thickness, nm
Mobility
(cm /V.s)
2
(uncertainty in
nm)
C60-Th
C60-TPA
0.93 x 10-4
0.77 x 10-5
1
1
2
00 (10)
50 (26)
00 (33)
7.1 x 10-4
1.12 x 10-4
1.02 x 10-3
0.6 x 10-3

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3
.5 Fluorescence Spectroscopy Study
The fluorescence spectra of mixture of product 1 or 2 with polymer (P3HT) (1:1 weight ratio)
have been measured to analyze the quenching efficiency using equation 2,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
q = (I -I)/I
…….Eq. 2
o
o
where, I is the PL intensity of pure P3HT with same concentration and I is the PL intensity of
o
P3HT:product mixture. P3HT shows strong fluorescence at ~580 nm on excitation with 450 nm
wavelength (Figure 6A). In mixture of P3HT:product 1 the fluorescence peak of P3HT is
quenched by 60% while in P3HT:product 2 90% quenching was observed (Figure 6A). The
decrease in the intensity of emission bands in P3HT: Product (1&2) mixture as compared to neat
P3HT shows efficient intermolecular charge transfer and dissociation of photogenerated excitons
in P3HT on excitation (Figure 3). We also recorded the fluorescence spectra of neat P3HT and
P3HT: Product (1&2) blend films (1:1 weight ratio) using 450 nm excitation wavelength (Figure
6B). The films showed red shifted emission between 600-850 nm for the corresponding red
shifted absorption in films. We observed quenching efficiency of ~50-65 % for product 1 & 2,
however, 50 weight% PC61BM/P3HT blend films have shown fluorescence quenching
3
1
efficiency above 95% following charge transfer process. Further work in underway to optimize
the P3HT/Product 1or 2 weight ratio and annealing temperature to obtain maximum quenching
efficiency.
Time correlated single photon counting (TCSPC) was performed to estimate the lifetime of
singlet excited state of P3HT and in mixture with product 1 & 2 (1:1 weight ratio) (Figure S18).
As reported earlier by us, the study was performed in solution to avoid improper estimation of
life time of excited states due to traps in film.17a The singlet excited state relaxation of P3HT at
584 nm showed lifetime of ~0.85 ns.17a The mixture with product 1 and product 2 showed further

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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
reduced decay time to be 0.78 ns and 0.72 ns due to non radiative charge transfer relaxation of
excited P3HT molecules. The observed decay time is slightly higher than what we have observed
with other methanofullerenes having electron withdrawing groups on exohedral addends.17a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
0
1
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(A)
(B)
Figure 6. Fluorescence emission spectra of P3HT and mixture with Product 1 (C60-Th) and Product 2
C60-TPA) (1:1 weight ratio) in (A) chloroform solution and (B) in films using 450 nm excitation
(
wavelength.
We further calculated the electron transfer rate constant for charge separation (kET(CS)) using the
lifetime estimated by TCSPC using the equation 3.
1
1
푘퐸푇(퐶푆)
=
―
…….Eq. 3
휏(푚푖푥푡푢푟푒)
휏(푃3퐻푇)
8
8 -1
Product 1 and 2 showed kET(CS) 1x10 and 2.1x10 s respectively in mixture with P3HT which is

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-1 32
in the same order of P3HT:PCBM mixture in o-DCB (~4.6x10 s ). At this point we need
deeper study to understand the lower quenching efficiency and lower rate of electron transfer in
these heteroatom bearing methanofullerene compared to our previously reported
methanofullerenes in 1:1 weight ration mixture with donor polymer.17c Therefore, we performed
transient absorption spectroscopy for neat and mixture with P3HT for both the products to study
the excited state dynamics.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
.6 Transient Absorption Spectroscopy (TAS) Study
Transient absorption spectroscopy was performed for neat P3HT, product 1 & 2 and also in
mixture of product 1 or 2 with P3HT in chloroform (1:1 weight ratio). Transient absorption
spectra give direct information about the generation of excited states and it also specifies the
spectral regions for further electron transport process. The TAS spectra were recorded both in
visible and NIR region on excitation (pump) with 480 nm wavelength. For neat P3HT, ground
state absorption (GSA) bleaching was observed between ~520-620 nm and stimulated emission
appeared at >620 nm after ~500 fs (Figure S19). After 1 ps delay, along with GSA, transient
absorption appeared at ~740 nm for absorption by photogenerated charges in P3HT. Neat
product 1 showed transient absorption at ~714, 741 and 767 nm appearing after 5 ps delay time
in visible region (Figure S20) while transient absorption at ~ 885 nm and 1030 nm appeared at 2
and 5 ps delay time respectively in NIR region (Figure S21). The transient absorption bands in
visible region are due to singlet-singlet transitions and absorption at 885 nm is due to excited
singlet states absorption of product 1. Absorption for photogenerated charges appear at ~1030
33
nm. Neat product 2 shows two transient absorption bands at ~712 nm and ~756 nm for singlet-
singlet transitions after 5 ps delay time (Figure S22). Similar to product 1, product 2 also shows
transient absorption at 885 and 1030 nm for excited singlet state and photogenerated charge

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absorption respectively (Figure S23). An interesting observation in TAS spectra of product 1 is
the appearance of an extra transient absorption at 767 nm compared to product 2. This transient
absorption is similar to what we observed in polymer P3HT for photogenerated charges on
thiophene ring. Intramolecular charge transfer interactions between thiophene ring and fullerene
facilitated by ethylene linker causes the formation of charged species within the molecule
showing transient absorption at ~767 nm. This also causes lower quenching efficiency of P3HT
emission in P3HT:Product1 mixture. In TAS of P3HT: Product 1 mixture strong stimulated
emission appears at 600 nm, as seen in fluorescence emission spectra, with a lifetime of 249 ps
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(Figure 7). After 1 ps delay time, the strong transient absorption appears between 700-770 nm
+
.
34
with maxima at 766 nm for P3HT cation radical (P3HT ) with a lifetime of 1.6 ns (Figure 7A).
Simultaneously, within 1 ps a transient absorption appears at ~1037 nm for fullerene anion
-.
33
radical (C60 ) formation (Figure 7B) with a lifetime of 1.5 ns.

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(
B)
Figure 7. Transient absorption spectra of Product 1 mixture with P3HT in (A) UV-visible region and (B)
NIR region on excitation with 480 nm wavelength. The inset shows the kinetics at 766 and 1037 nm for
+
.
-.
P3HT and C60 charge transfer species respectively.

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Figure 8. Transient absorption spectra of product 2 mixture with P3HT in (A) UV-visible region and (B)
+
.
-.
NIR region. The inset shows the kinetics at 767 and 1037 nm for P3HT and C60 charge transfer species
respectively.
On the other hand P3HT:Product 2 mixture shows strong photo bleaching at 530 nm for ground
+
.
state absorption and ~580 nm for stimulated emission. The absorption for P3HT cation radical
-.
was observed at 767 nm with lifetime of 1.2 ns as shown in Figure 8A and C60 anion radical
appeared at 1037 nm with lifetime of 1.5 ns (Figure 8B). In both the cases the generation of
+
.
-.
charge separated states i.e. P3HT and C60 were observed within short delay time of ~ 2 ps.
Although the quenching efficiency was 90% in these fullerene derivatives on mixing with

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P3HT, ultra fast charge transfer with formation of long lived charge separated states has been
observed as required for application in OPV devices.
4
. Conclusions
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In conclusion, we have synthesized two different methanofullerene derivatives having
heteroatoms in exohedral chain via our cost effective AACA process, establishing the versatile
application of this process for generation of diazoalkane intermediates bearing different types of
groups. The products have been synthesized in good yield and high purity. Electrochemical data
shows tuning of LUMO as well as HOMO levels and therefore band gap due to presence of
electron donating groups. Both the products show electron transfer rate constant in the same
order to P3HT:PC61BM mixtures and generation of charge separated states with nanosecond
lifetimes, thus confirming the efficient charge separation on mixing with P3HT. Both the
products show higher electron mobility than PC61BM, making them potential candidate to be
used as n-type material in organic electronic devices. Also this work shows the tuning of HOMO
levels in methanofullerenes by easy substitution with electron donating groups in exohedral
chain. Uplifted HOMO levels in acceptor will make them suitable to be used with low band gap
polymer with lower HOMO energy levels.
Compliance with Ethical Standards:
The authors declare that they have no conflict of interest.

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