Synthesis and Photovoltaic Properties of Novel Monoadducts and Bisadducts Based on Amide Methanofullerene

Abstract

Four new [6,6]-phenyl-C(61) and C(71) butylsaure n-dibutyl amides (PCBDBA) with mono- and bis-adduction on C(60) and C(70) cages, respectively, have been synthesized as models to study the effect of the mono- and bis-adduction on fullerene cages on device performance when used as electron acceptors with the donor of regioregular P3HT in bulkheterojunction organic photovoltaics (BHJ-OPV). The optoelectronic, electrochemistry, and photovoltaic properties of these mono- and bis-products were fully investigated. The best device performance of these fullerene derivatives were obtained from the two monoadducts with power conversion efficiency (PCE) of 1.77% for C(60) derivative and 1.90% for C(70) derivative, respectively, which are close to PCBM's 2.43%. The results revealed the structure-function relationship among the monoadduct and bisadduct derivatives of C(60) and C(70) with the BHJ-OPV performance.

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Synthesis and Photovoltaic Properties of Novel Monoadducts and
Bisadducts Based on Amide Methanofullerene
Chao Liu,†,‡Shengqiang Xiao,*,§Xiangping Shu,§Yongjun Li,†Liang Xu,†,‡Taifeng Liu,†,‡Yanwen Yu,†,‡
Liang Zhang,⊥Huibiao Liu,†and Yuliang Li*,†
†Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, P. R. China
‡Graduate University of Chinese Academy of Sciences, Beijing 100190, P. R. China
§State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, WUT-USG Joint Laboratory of Advanced
Optoelectronic Materials and Devices, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China
⊥Shandong Normal University, Jinan, 250014, P. R. China
*
S Supporting Information
ABSTRACT: Four new [6,6]-phenyl-C61and C71butylsaure n-dibutyl amides
(PCBDBA) with mono- and bis-adduction on C60and C70cages, respectively,
have been synthesized as models to study the effect of the mono- and bis-
adduction on fullerene cages on device performance when used as electron
acceptors with the donor of regioregular P3HT in bulkheterojunction organic
photovoltaics (BHJ-OPV). The optoelectronic, electrochemistry, and photo-
voltaic properties of these mono- and bis-products were fully investigated. The
best device performance of these fullerene derivatives were obtained from the
two monoadducts with power conversion efficiency (PCE) of 1.77% for C60
derivative and 1.90% for C70derivative, respectively, which are close to PCBM’s
2.43%. The results revealed the structure−function relationship among the
monoadduct and bisadduct derivatives of C60and C70with the BHJ-OPV
performance.
KEYWORDS: fullerene, photovoltaic, solar cell, C60, C70
■INTRODUCTION
To achieve high power conversion efficiency (PCE), great
research efforts have been focused on designing and synthesiz-
ing new polymers for fullerene-polymer blended bulk
heterojunction organic photovoltaics (BHJ-OPVs),1−7and
new materials with PCE of 9.2% were generated.8Among the
acceptor materials used in OPVs, [6,6]-phenyl C61-butyric acid
methyl ester (PC61BM) plays a dominant role due to its
advantages of good solubility in organic solvents, high electron
mobility, high electron affinity, and ease of preparation at low
cost. However, the weak absorption in the visible region and
lower-lying LUMO level are shortcomings. Weak absorption in
the visible region limits its contribution to the light harvest in
the photovoltaic conversion.9Lower-lying LUMO level of an
acceptor material results in lower open circuit voltage (Voc) of
OPVs because Vocis related to the gap between the LUMO of
the acceptor and the HOMO of the donor in a BHJ-OPV
device.10Although many alternative structures to PCBM have
been synthesized and used as acceptors in OPVs, most of their
performances could not outperform PCBM.11−17A few showed
better performance but with limited successful applications with
donor polymers or need additional device fabrication
techniques.9,18−20
PC71BM was an electron acceptor with higher photovoltaic
performance in BHJ-OPVs, attributed to higher molar
extinction coefficient in the visible region due to asymmetric
C70 core.4,9,21Aromatic groups attached to fullerenes by
covalent or conjugated linkage can increase the molar
absorption of fullerene derivatives and decrease the number
of π electrons of the fullerene core which weakens the ability of
accepting electrons and then increases LUMO level.22There-
fore, the LUMO level of [6,6] isomers (methanofullerenes)
with 58 π electrons was higher than that of [5,6] isomers
(fulleroids) with 60 π electrons. Moreover, with fewer π
electrons on fullerene core, the higher LUMO level can be
obtained with more adducts.23,24Multiadducted PCBM
mixtures used as acceptors in BHJ-OPV were first reported
by Padinger et al.;25low performance and Jscwere obtained
using doctor blade technology for device fabrication. Afterward,
Lenes et al.26studied the physical and photovoltaic properties
of several bisadducts of PC61BM-like derivatives and the
triadduct of PC61BM. They found that the higher LUMO level
and Voc of the derivatives can be obtained with more
Received:
Accepted:
Published: January 2, 2012
December 1, 2011
January 2, 2012
Research Article
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adductions, but the Jscdecreased, especially for the triadduct of
PC61BM. They attributed the low Jscto the decreased electron
mobility of derivatives with higher adductions.26,27
Understanding the structure−function relationships that
relate specifically to fullerene derivatives could lead to new
“design rules” for producing benign, high-performance photo-
voltaic devices. Our studies focused on fullerene physics and
chemistry and were dedicated to develop low cost, ease of
preparation, and high performance fullerene materials for
photovoltaic application in recent years.15,28−30In the present
work, we reported our detailed optoelectronic, electro-
chemistry, photovoltaic, and morphology study on a series of
fullerene materials, [6,6]-phenyl-C61or C71butylsaure n-dibutyl
amides (PCBDBA), which were N,N-disubstituted amide
analog of PCBM with higher solubility than PCBM. As a
continuation to our previous work,15C60, C70, and their
bisadducted derivatives, i.e., fullerenes A1, A2, B1, and B2 as
shown in Figure 1, were designed and synthesized . They were
good models to study the influence of the bisadducted fullerene
materials on the performance of bulk heterojunction organic
photovoltaics, to reveal the structure−function relationship of
C60and C70monoadducts and bisadducts on the photovoltaics
performance and provide the logos on how to design fullerene
materials for this purpose.
■EXPERIMENTAL SECTION
General. All solvents were purified and freshly distilled prior to use
according to literature procedures and purification handbook.
Commercially available materials were used as received. Styles of
equipment and reagents were shown in Table S7s and S8 in the
Supporting Information.
Synthesis and Characterization. The PCBDBA series were
synthesized according to the method developed by Hummelen et al.31
(Scheme S1, Supporting Information). General method for a
diazomethane addition to fullerene: A mixture of hydrazone (1
mmol), freshly prepared sodium methoxide (1.2 mmol), and dry
pyridine (15 mL) was placed under nitrogen and stirred at room
temperature for 15−30 min. Then, a solution of fullerene (1 mmol) in
dry 1,2-dichlorobenzene (o-DCB, 50 mL) was added. The mixture was
stirred at 80−120 °C for 12−24 h, and then, the solvent was removed
in vacuo. The crude product was purified by chromatography on silica
gel (200−300 mesh) with 0−50% ethyl acetate in toluene as the
eluent. Every portion of different adducts were collected and then
redissolved in o-DCB and refluxed for 24 h to ensure most of [5,6]
open shell isomers converted to [6,6] close shell methanofullerenes.
After removing o-DCB in vacuo, the product was purified by
chromatograph on silica gel (300−400 mesh) two times to remove
some decomposed compounds and other fullerene impurities. Then,
the product was dissolved in a little dichloromethane and precipitated
with MeOH, centrifuged, and decanted. The remaining pellet was
washed three times with methanol, hexane, or Et2O, respectively, in a
supersonic bath to remove small molecule impurities and silicon gel.
Finally, all materials were dried in vacuo at 100 °C for 24 h, resulting
in as pure [6,6]methanofullerene derivatives as possible.
1H NMR and13C NMR spectra were calibrated using signals from
trimethylsilane (TMS) and reported downfield from TMS. Matrix-
assisted laser desorption ionization time-of-flight mass spectrometry
(MALDI-TOF-MS) gave the M−, according to the calculated value of
m/z of all the C60and C70derivatives. The FT-IR spectra showed
absorption features at 526 cm−1c.a. indicative of the C60core, while
578 cm−1and 534 cm−1for the C70core.
A1. Monoadduct: Yield, 30%;
(ppm): 7.94 (2 H, d, J = 7.4 Hz), 7.53 (2 H, t, J = 7.4 Hz), 7.43 (1 H,
t, J = 7.16 Hz), 3.30 (2 H, t, 7.6 Hz), 3.20 (2 H, t, J = 8.0 Hz), 2.93 (2
H, t, J = 8.0 Hz), 2.50 (2 H, t, J = 7.7 Hz), 2.22 (2 H, m), 1.51 (4 H,
m), 1.29 (4 H, m), 0.92 (6 H, m).13C NMR (100 MHz, CDCl3): δ
(ppm): 171.38, 149.02, 148.14, 146.06, 145.36, 145.30, 145.25, 145.21,
144.96, 144.94, 144.82, 144.65, 144.60, 144.21, 143.94, 143.22, 143.17,
143.15, 143.10, 142.40, 142.33, 142.32, 142.29, 141.16, 140.95, 138.21,
137.79, 136.99, 132.32, 128.62, 128.36, 80.18, 52.44, 48.01, 46.00,
34.23, 33.29, 31.54, 30.26, 23.05, 20.72, 20.63, 14.29, 14.28. IR (KBr)
= v (cm−1): 3297 (w), 2954 (s), 2953 (s), 2864 (m), 1644 (s), 1450
(m), 1426 (m), 1375 (w), 1210 (w), 1184 (w), 1136(w), 752 (m),
710 (m), 577 (w), 551 (w), 527 (m, C60feature). MALDI-TOF CCA
m/z: 1007.3 (M−); calcd. (C79H29NO), 1007.22.
A2. Bisadduct: Yield, 30%;1H NMR (CDCl3, 400 MHz): δ (ppm):
8.26−7.70 (2 H, m), 7.68−7.34 (3 H, m), 3.42−2.86 (4 H, m), 2.65−
1.87 (4 H, m), 1.82−1.03 (10 H, m), 1.03−0.58 (6 H, m). IR (KBr) =
v (cm−1): 3368 (w), 2956 (s), 2900 (s), 2856 (s), 1739 (m), 1645 (s),
1460 (s), 1377 (m), 1289 (w), 1258 (w), 1214 (w), 1142 (w), 1085
(w), 1020 (w), 850 (w), 760 (w), 735 (w), 702 (w), 525 (m), 463 (w,
C60feature). MALDI-TOF CCA m/z: 1007.3 (M−); MALDI-TOF
CCA m/z: 1295.1 (M−); calcd. (C98H58N2O2), 1295.45.
B1. Monoadduct: Yield 20%;
(ppm):1H NMR (400 MHz, CDCl3): δ (ppm): 8.08−7.75 (2 H, m),
7.58−7.46 (2 H, m), 7.45−7.37 (1 H, m) 3.56−2.93 (4 H, m), 2.54−
2.36 (4 H, m), 2.31−1.75 (2 H, m), 1.51 (4 H, s), 1.42−1.11 (4 H, m),
1.03−0.80 (6 H, m). IR (KBr) = v (cm−1): 3445 (w), 2952 (m), 2923
(m), 2855 (m), 1644 (s), 1557 (w), 1539 (w), 1454 (m), 1428 (s, C70
feature), 1374 (w), 1289(w), 1213 (w), 1135 (w), 1073 (w), 841 (w),
795 (w), 727 (w), 674 (w), 643 (w), 578 (m, C70feature), 534 (m, C70
feature), 458 (w). MALDI-TOF CCA m/z: 1007.3 (M−); MALDI-
TOF CCA m/z: Monoadduct, 1127.5 (M−); calcd. (C79H29NO),
1127.22.
B2. Bisadduct: Yield, 15%;1H NMR (600 MHz, CDCl3): δ(ppm):
8.04−7.90 (2 H, m), 7.61−7.32 (3 H, m), 3.39−3.11 (4 H, m), 2.61−
2.29 (4 H, m), 2.27−1.98 (2 H, m), 1.66−1.45 (4 H, m), 1.39−1.16
(m, 4 H), 1.02−0.76 (6 H, m). IR (KBr) = v (cm−1): 3421 (mb), 2958
(s), 2926 (s), 2856 (m), 1647 (s), 1456 (m), 1440 (m), 1425 (m, C70
feature), 1383 (m), 1261 (m), 1095 (m), 1028 (m), 862 (w), 804 (m),
801 (w), 646 (w), 578 (w, C70feature), 535 (w, C70feature), 458 (w),
419 (w). MALDI-TOF CCA m/z: 1007.3 (M−); MALDI-TOF CCA
m/z: 1416.1(M−); calcd. (C98H58N2O2), 1415.45.
All fullerene multiadducts are complicated mixtures of isomers,
while for C70, with a lower symmetry than C60, a mixture of isomers of
monoadduct also formed.26Herein,13C NMR are unable to give exact
information, because there are hundreds of signals from the fullerene
and aromatic moiety to be assigned; the multiadduct can be analyzed
by HPLC on analytical Cosmosil Buckyprep Waters column, i.e., 4.6
mm (ID) × 250 mm column, using toluene as eluent at a flow rate of
0.6 mL/min, to give some information about the minimum number of
isomers; however, it seemed difficult to indicate how many isomers
1H NMR (CDCl3, 400 MHz): δ
1H NMR (400 MHz, CDCl3): δ
Figure 1. Structures of 4 fullerene derivatives.
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there were even though 5PYE or 5PBB analytical column was used.
The resulting chromatograms (320 nm detection) of the four were
showed in Figure 2, and isomer integration was listed in Table S2,
Supporting Information, which suggested that there were at least 3
isomers of A2, 5 isomers of B1, and 5 isomers of B2.
Thermal Analysis. Samples were kept at 100 °C in vacuo for 24 h
before thermal analysis. Thermogravimetric/differential thermal
analysis (TG/DTA) was measured to understand the thermal stability
of the fullerene derivatives as shown in Table S1, Supporting
Information, which indicates that all fullerene derivatives have a
thermal decomposition temperature higher than 200 °C. Differential
scanning calorimetry (DSC) measurement of them showed that there
were no crystallization temperatures (Tc) under 200 °C, but A1 has a
melting temperature (Tm) at 239 °C (peak); B1 has Tgat 128 °C and
Tmat 219 °C (Figure S4, Supporting Information, the peak at 196 °C
was transition phase between the melting phase and glass phase),
which suggested that all four fullerene derivatives are amphorous
materials.32
Photovoltaic Fabrication. The device structure used in this study
was a classic sandwich structure with ITO/PEDOT:PSS as a hole-
collecting electrode and Al as an electron-collecting electrode (Figure
S5, Supporting Information). Indium/tin oxide (ITO, 150 nm thick)
glasses were cleaned by supersonic treatment in acetone, water,
acetone, and i-propanol for at least 10 min, respectively and then by
UV/ozone treatment for 20 min and used as the anode. A thin layer of
poly (ethylene dioxythiophene):polystyrenesulfonic acid (PE-
DOT:PSS, CLEVIOS PH750) was incorporated between the ITO
and the active layer to reduce device leakage, which was 50 nm thick
spin-coated at 2000 rpm for 50 s and baked at 120 °C for 30 min to
dry. Methanofullerenes and P3HT in the equal weight ratio were
mixed and dissolved in o-DCB solution at the concentration of 17
mg·mL−1. The solution was then spin-coated onto the top of
PEDOT:PSS at 900, 1000, and 1100 rpm for 60 s, respectively. For
all devices, the active layer has a thickness of around 100 nm. Time for
solvent annealing (SA) was at least 60 min. Al was then deposited on
the active layer. The deposition rates were usually 0.05−0.10 nm·s−1,
and the thickness of the evaporation layers were monitored by a
thickness/rate meter (MBraun). After cathode was deposited, the
devices were post-thermally annealed (PTA) at 110 °C for 10 min
before cooling down to room temperature under nitrogen in glovebox.
The crossing area of 0.18 cm2between the cathode and the anode was
defined as active device area. All fabrication steps after the deposition
of PEDOT:PSS layer were carried out in a nitrogen glovebox with
H2O < 0.1 ppm and O2<0.1 ppm. The I−V characteristics in the dark
and under illumination were measured. Photocurrent was measured
under a solar simulator with AM 1.5G illumination (100 mW·cm−2) in
glovebox.
■RESULTS AND DISCUSSION
UV−Vis Solution Absorption. The UV−vis spectra of the
fullerene derivatives in 10−5M toluene solution were shown in
Figure 3 and Table 1 in calibration to concentration. The
spectra showed that the absorption of C70compounds were
stronger than its C60homologues, and bisadducts were stronger
than its monoadduct homologues in 400−800 nm, which is
consistent with the literature.17,24The absorption of A1 was
almost the same as that of PC61BM, especially the featured
peaks at 434 and 696 nm. The same absorption characteristics
of A1 and PC61BM indicated that the structure changing from
methyl ester to n-dibutylamide had little effect on the electronic
properties of the whole molecule, which was consistent with its
Ered
A2, B1, and B2 have no distinct and sharp absorption band, but
with broad band instead, there were several isomers in the three
fullerene derivatives convinced by1H NMR,13C NMR, and
HPLC.
Electrochemistry Properties. The electrochemical prop-
erty is one of the most important properties of fullerenes. The
gap between the LUMO level of the donor and acceptor
provides the driving force for the charge dissociation of the
excitons in the polymer donor to overcome the binding energy
of the excitons. The value of the difference between the LUMO
level of the acceptor and the HOMO level of the donor
determines Voc.10,33The value of the difference between the
HOMO levels of the donor and acceptor provides the driving
force for the dissociation of the excitons in the acceptor. The
energy level alignment between four fullerene derivatives and
regioregular P3HT assures that there are good charge-transfer
1onesttested by cyclic voltammetry and our previous work.15
Figure 2. HPLC analysis figures of four fullerene derivatives. LC:
Waters 699; column: Cosmosil Buckyprep Waters 4.6 mm(ID) × 250
mm; eluent: HPLC grade toluene; flow rate: 0.6 mL/min; detection
wavelength: 320 nm; neither recycle nor another special technique was
used; and the retention time of C60in this condition was 13.3 min and
21.7 min for C70.
Figure 3. UV−vis absorption spectra of fullerene solution in toluene at
10−5M, calibrated by concentration. Solid symbols stand for
monoadduct derivatives, and hollow symbols stand for bisadduct
derivatives.
Table 1. UV-Vis Absorbance Peak or Broad Band Dataa
fullerene400−450 (nm)
434
434
433
463
450−600 (nm)>500 (nm)
PC61BM
A1
A2
PC71BM
B1
B2
aUV-vis absorption was tested in toluene solution at RT.
698
699
698
673
497
462, 540
486
497699
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and charge-separate states in the donor/acceptor blends
(Figure S3, Supporting Information).34,35The LUMO level
of all the fullerene derivatives was measured by cyclic
voltammetry (as shown in Table 2 and Figure 4) and estimated
from their onset value of the first reduction (Ered
= −
E
LUMO e(
red
A1, with n-dibutyl amide group instead of methyl ester of
PCBM, has almost the same Ered
group has little effect on the electrochemistry properties of
fullerene, in line with UV comparison of A1 and PC61BM,
which is in agreement with our previous report.15Differential
pulse voltammetry (DPV) is a sensitive and semiquantitative
1onest) by eq 1.
+
+
4.80)(VvsFc/Fc )
1onest
(1)
1onestas PC61BM; n-dibutyl amide
electrochemistry method to determine the electrochemical
properties. Symmetrical peaks of Epaand Epc, equal ipaand ipc,
and three sharp and equal intensity peaks of DPV showed that
A1 was a typical reversible fullerene derivative. The equal Ered
of A1 and B1 as PC61BM and PC71BM was attributed to the
relief of strain and pyracylene-type electronic of [70] fullerene
derivatives.36The Ered
of A2, B1, and B2 were quasi-reversible
while the Ered
indicated by the asymmetric peaks of Epaand Epcand unequal
value of ipaand ipc. which also can be demonstrated by the
existence of double or multipeaks from DPV curves. These
results further confirmed the existence of isomers in these
derivatives.23
Photovoltaic Properties. The PCE of the monoadduct
fullerene based OPVs performed better than the bisadduct
ones, i.e., A1 > A2 and B1 > B2. We had expected that A2 and
B2 would have more excellent performance than their
monoadducts, as both A2 and B2 with LUMO level 0.02 and
0.10 eV higher than that of PCBM (Figure 5 and 6, Table S6,
Supporting Information). Unfortunately, whether SA and PTA
were used or not, the PCE of A2 and B2 based OPVs were very
low. The Vocof the devices based on A2 was slightly lower than
that of A1 while B2’s Vocdrops dramatically compared with
that based on B1. Notably, the Jscof the B1 and B2 based
device were very low, which were similar to that of the
TriPC61BM as reported.26
The number of isomers of A2 and B2 indicated by HPLC
were 5 and 5, respectively, at least, because there were too
broad peaks in HPLC figures to recognize how many there
were (Figure 2). The more addition, the smaller mobility of
multiaddcut had.26A possible explanation for this decrease was
1onest
1
2and Ered
3of them were obviously semireversible as
Table 2. Electrochemistry Properties of Fullerene
Derivativesa
fullereneEred
1onest(V)
−1.09
−1.10
−1.11
−1.14
−1.11
−1.20
LUMO (eV)
−3.71
−3.70
−3.69
−3.66
−3.69
−3.61
Δ (eV)
0.00
0.01
0.02
0.05
0.02
0.10
PC61BM
PC71BM
A1
A2
B1
B2
aMeasurement parameters: Ered
in o-DCB solution; supporting electrolyte, tetrabutyl ammonium
hexafluorophosphate (TBAPF6, 0.1 M); working electrode, glassy
carbon; counter electrode, Pt wire; reference electrode, Ag wire;
internal reference, 0.3 mg c.a. A.R. ferrocene by Soxhlet extracted; scan
rate, 50 mV·s−1; temperature, room temperature 25 °C c.a.; Δ was
compared to PC61BM’s LUMO level.
1onestwas in V vs Fc/Fc+; 10−4to 10−3M
Figure 4. CV and DPV figures of four fullerene derivatives. The parameters of CV were listed in the caption of Table 2. DPV were tested in the same
cell as CV did. The parameters of DPV: amplitude, 0.05 V; pulse width, 0.2 s; sample width, 0.0167 s; pulse period, 0.5 s. (a) A1; (b) A2; (c) B1; (d)
B2.
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that macroscopic mobility was the average of all orientation,
and isomers increased the disorder strongly; thus, A2 and B2 of
at least 5 isomers, respectively, may have very low mobility
leading to very low Jsc.
Surface morphology study by AFM technology was not
consistent with the whole phase situation in 3D structure,
because the polymer/fullerene film was not homogeneous
vertically supported by neutron scattering,37while it is a good
and readily available method to reveal the phase separation to
some content. Phase images showed the scale of phase
separation; it is believed that the scale surrounding 10 nm
had the maximum effect on exciton diffusion and dissociation.38
Moreover, the film roughness indicated by height images
reveals the quality of film with the index of root-mean-square
roughness (rms). The phase images with SA treatment and SA
and PTA treatment were shown in Figures 7 and 8, and rms
was listed in Figure 9.
Although none of the scale of phase separation was smaller
than 10 nm, usually bigger than 50 nm, the phase images were
almost consistent with the device performance. A2 and B2
based devices had a phase separation scale of 80 to 200 nm; the
phase was very nonuniform. On the other hand, A1 and B1
based devices had a phase separation scale of 50 to 70 nm, and
B1’s was much more ordered than A1’s with small continuous
blocks. After PTA treatment, A1 and B1’s phase separation was
decreased about 10 nm, and B1 formed a nanoisland at about
40 to 60 nm. A2’s phase separation was more unordered and at
large scale. Surprisingly, B2 based film showed ordered and
small scale phase separation but low device performance, which
was an analogue with early multiadduct fullerene literature.39
rms showed a similar trend. PTA treatment decreased the film
Figure 5. J−V curves of P3HT/fullerene devices under AM 1.5G illumination at 100 mW·cm−2.(a) SA treatment; (b) SA and PTA treatment. The
J−V curves of B2 with SA treatment; A2 and B2 with SA and PTA treatment were not shown because they were almost lying on the X axis.
Figure 6. Device indices of each fullerene based OPVs in the condition of SA and SA and PTA. (a) power conversion efficiency(PCE); (b) open
circuit voltage (Voc); (c) current density (Jsc); (d) fill factor (FF).
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roughness of A1 and B1 based devices notably, but A2 and B2
have no similar trend; even A2’s rms increased, suggesting that
PTA treatment can improve film quality.
■CONCLUSION
We synthesized four [6,6]-phenyl-C61and C71butylsaure n-
dibutyl amide (PCBDBA) fullerene derivatives, and the
photovoltaic cells were fabricated with P3HT. The hetero-
junction organic photovoltaics showed device performance for
two monoadducts A1 and B1, which have PCE of 1.77% and
1.90%, respectively, near PCBM’s 2.43%. Our results also
indicate that more adducts of fullerenes afford lower efficiency,
even resulting in broken circuit. This is due to the fact that
multiadduct has lower mobility, larger scale of phase separation,
and higher rms than monoadducts. It is demonstrated that the
products were good models to study the structure performance
of the monoadduct and bisadduct fullerene materials for
revealing the relationship between C60and C70monoadduct
and bisadduct derivatives and providing the logos on how to
design fullerene materials for this purpose. It may have great
potential for further applications on optoelectronic devices
based on π-conjugated conducting polymers in various fields.
■ASSOCIATED CONTENT
*
Details of synthesis, NMR, IR, MS, HPLC, TG/DTA,
equipment, and reagents. This material is available free of
charge via the Internet at http://pubs.acs.org.
■AUTHOR INFORMATION
Corresponding Author
*Yuliang Li: fax, 86-10-82616576; tel, 86-10-62588934; e-mail,
ylli@iccas.ac.cn. Shengqiang Xiao: fax, 86-27-87879468; tel, 86-
27-87870537; e-mail, shengqiang@whut.edu.cn.
■ACKNOWLEDGMENTS
This work was supported by the National Nature Science
Foundation of China (21031006, 10874187, 20831160507),
the NSFC-DFG joint fund (TRR61), and the National Basic
Research 973 Program of China. The solar cell fabrication and
test were financially supported by self-determined and
innovative research funds of WUT (2011-II-009). Prof. S.X.
is also thankful for the financial support from the State Key Lab
of Advanced Technology for Materials Synthesis and
Processing (2009-PY-1).
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Figure 7. AFM phase images of four fullerene based OPVs with SA
treatment in tapping mode. (a) A1; (b) A2; (c) B1; (d) B2.
Figure 8. AFM phase images of four fullerene based OPVs with SA
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Figure 9. rms of four fullerene based OPVs with SA and PTA
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