High LUMO energy pyrrolidinofullerenes as promising electron-acceptor materials for organic solar cells

Article abstract of DOI:10.1039/c5tc02509e

We report the synthesis and investigation of four novel pyrrolidinofullerenes bearing electron donating alkoxyphenyl substituents. Cyclic voltammetry studies revealed that the designed fullerene derivatives exhibit reduced electron affinity compared to th

Full text of DOI:10.1039/c5tc02509e

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High LUMO energy pyrrolidinofullerenes as promising
electron-acceptor materials for organic solar cells
A. V. Mumyatov,a F. A. Prudnov,a L. N. Inasaridze,a O. A. Mukhacheva,a and P. A. Troshina
Received 00th January 20xx,
Accepted 00th January 20xx
We report the synthesis and investigation of four novel pyrrolidinofullerenes bearing electron donating alkoxyphenyl
substituents. Cyclic voltammetry studies revealed that the designed fullerene derivatives have reduced electron affinity
compared to the conventional material [60]PCBM. The organic bulk heterojunction solar cells based on the
pyrrolidinofullerenes and P3HT revealed impressive open-circuit voltages of 700-780 mV and enhanced light power
conversion efficiencies with respect to the reference PCBM/P3HT devices. The designed materials demonstrated also
reasonably good compatibility and decent photovoltaic performances in combination with the carbazole-thiophene-
benzothiadiazole copolymer PCDTBT.
DOI: 10.1039/x0xx00000x
www.rsc.org/
biscycloadducts work well only in combination with crystalline
polymers such as P3HT, PBDTBDD and other.3,7-13 The
crystallization of the polymer during the film formation induces
also ordering of the fullerene phase. Furthermore, different
isomers of biscycloaddutcs have rather different LUMO
energies which induce energetic disorder in the material
represented by a complex isomer mixture.14,15
Introduction
Organic solar cells have been intensively investigated during
the last decade in many research laboratories all over the world.
Conjugated polymers (p-type) and fullerene derivatives (n-type)
are usually used as semiconductor materials in organic solar
cells. Design of novel electron donor conjugated polymers
resulted in the increase in the solar light power conversion
efficiency up to 10% and beyond.1 In contrast to the conjugated
polymers, much less attention is paid to the design of fullerene-
based materials with improved electronic properties.
Theoretical calculations have shown that the application of
fullerene derivatives with reduced electron affinity can increase
the open-circuit voltage (VOC) and, consequently, the efficiency
of organic solar cells based on P3HT and vast majority of other
existing conjugated polymers.2 The electron affinity of the
fullerene sphere can be reduced by opening two or several
double bonds in the carbon cage and attaching appropriate
number of addends. Indeed, organic solar cells based on
biscycloadducts (e.g. bis-PCBM, ICBA) show 150-200 mV
higher VOC’s as compared to the reference devices comprising
classical [60]PCBM as an electron acceptor component.3-5 The
main disadvantage of the fullerene derivatives with two
attached cyclic organic addends is their complex isomeric
composition. The formation of complicated mixtures of ~20
regio- and stereoisomers of biscycloadducts based on C60 and
C70 creates a considerable disorder in their thin films inhibiting
charge transport and affecting badly the photovoltaic
performance of these materials.6 It is notable that
Therefore, it is not surprising that vast majority of highly
promising low band gap conjugated polymers give insufficient
solar cell performances in combination with biscycloadducts.16
Another family of the fullerene-based materials with reduced
electron affinity is represented by endohedral compounds such
as Lu3N@C80-PCBH. Application of such materials in P3HT-
based organic solar cells improved their open-circuit voltages
and light power conversion efficiency.17 However, further
development and practical implementation of functionalized
endohedral fullerene derivatives is limited by their extremely
poor availability.18-20
The most promising and, at the same time, the least explored
approach to lowering the electron affinity of the fullerene core
is the attachment of a single organic addend loaded with the
electron donating substituents. Preparation and investigation of
functionalized derivatives of [60]PCBM comprising multiple
RO- or RS- substituents in the phenyl ring was reported
previously.21 The synthesized compounds demonstrated an
increase in the VOC by ~50 mV, while the efficiency of the
photovoltaic cells was not improved. Furthermore, some
examples of diphenylmethanofullerenes showed improved VOC
in organic solar cells as compared to the conventional
[60]PCBM.22-24 A number of silicon-containing 1,4-adducts of
C60 and Diels-Alder-type cycloadducts with reduced electron
affinity were also reported.25-27 Pyrrolidinofullerenes represent
other promising family of fullerene derivatives with reduced
electron affinity demonstrating enhanced open circuit voltages
and power conversion efficiencies in organic solar cells.28-29
a.Institute for Problems of Chemical Physics of Russian Academy of Sciences,
Semenov Prospect 1, Chernogolovka, 142432, Russia.
E‐mail: troshin2003@inbox.ru; Fax: +7 496515‐5420; Tel: +7 496522 1418.
Electronic Supplementary Information (ESI) available: NMR and MS spectra of F1‐
F4, results of the PL quenching experiments, photovoltaic characteristics of the
devices . See DOI: 10.1039/x0xx00000x
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was
The presented examples strongly suggest that fullerene The precursor 2-bromo-2-(2-methoxyphenyl)acetiVciewacAirdticle Online
2
monocycloadducts with the reduced electron affinity represent synthetized by bromination of 2-(2-methDoOxIy: p10h.e10n3y9l/)Ca5cTeCti0c25a0c9idE
a promising group of n-type materials for organic solar cells. with NBS in the presence of catalytic amount of AIBN.
However, their potential is not explored sufficiently by now. In Pyrrolidinofullerenes F1-F4 and also the reference compound
particular, up to the best of our knowledge, none of the reported F5 were synthesized using a standard reaction based on the
fullerene derivatives with reduced electron affinity showed [3+2]cycloaddition of azomethine ylides to the fullerene cage.33
decent photovoltaic performances with the low crystalline or Compounds F1
-F5 were isolated and purified by column
amorphous conjugated polymers, e.g. PCDTBT.30
chromatography. High purity of the obtained products was
In the present work we pursued the design and investigation of confirmed by HPLC.
a novel group of the fullerene derivatives whose electron The compositions and molecular structures of the synthesized
affinity is lowered considerably due to the electron donating fullerene derivatives were revealed using the 1D and 2D NMR
nature of a single organic addend attached to the fullerene cage. spectroscopy and mass spectrometry (Figs. S1-S12, Electronic
supplementary information, ESI). The NMR spectra revealed
the formation of two stereoisomers of pyrrolidinofullerenes F1
Results and discussion
and F3-F5 due to the hindered nitrogen inversion.
Pyrrolidinofullerene F2 was isolated as a mixture of 5
stereoisomers formed due to the cis-trans isomerism of the
addends in 2- and 5- positions of the pyrrolidine ring, their
restricted rotation due to the proximity of the bulky fullerene
cage and also the hindered nitrogen inversion. These
stereoisomers showed very similar characteristics which did not
allow us to permit their separation and spectroscopic
characterization as individual species. We would like to
emphasize that each of the synthesized pyrrolidinofullerenes
Here we explored a potential of the pyrrolidinofullerene core
structure in the design of promising n-type semiconductor
materials with lowered electron affinity as compared to the
conventional [60]PCBM. First of all, it is known that nitrogen
atom donates its electron density to the fullerene cage thus
increasing its LUMO energy.31 Moreover, there are indications
in the literature that substituents in the positions 2 and 5 of the
pyrrolidine ring can interact directly with the fullerene cage.32
Additionally, a successful application of the pyrrolidine type
fullerene derivatives in organic solar cells was reported.28-29
F1-F5 represented a single regioisomer with 58 -electron
system in the carbon cage (one double bond is opened and
functionalized on the C60 sphere). Therefore, these compounds
are isoelectronic with conventional [60]PCBM and differ
significantly from biscycloadducts such as bis[60]PCBM or
[60]ICBA.3,11-13,34-36
The synthesis of the target fullerene derivatives F1-F5 required
the preparation of N-alkylamino acids
3
-
4
by treatment of
appropriate bromoacetic acids
hexylamine as shown in Scheme 1.
1 and 2 with 2-ethyl-1-
The cyclic voltammetry measurements were used for
investigation of the electrochemical properties of the designed
fullerene derivatives. The cyclic voltammograms of
pyrrolidinofullerenes F1-F5 exhibited three reversible
reduction waves similar to that of [60]PCBM (Fig. 1, Fig. S13,
ESI). However, the first reduction (half-wave) potentials were
cathodically shifted with respect to [60]PCBM. The magnitudes
of this shift were different for 1,2-disubstituted
pyrrolidinofullerenes F1, F3, F4, F5 (20-70 mV) and 1,2,5-
trisubstituted pyrrolidinofullerene F2 (80 mV, Table 1).
200
100
0
-100
[60]PCBM
F1
F2
-200
bis[60]PCBM
-300
-400
-500
-2800 -2400 -2000 -1600 -1200
-800
-400
0
400
Potential vs. Fc+/Fc, mV
Scheme 1 Synthesis of the fullerene derivatives F1-F5
Figure 1. Cyclic voltammograms of pyrrolidinofullerenes F1, F2, [60]PCBM and
bis[60]PCBM. The cyclic voltammograms of pyrrolidinofullerenes F3 and F4 are
provided in Fig. S13, Electronic Supplementary Information (ESI).
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It is known that charge transport propertiesVieowfArtioclregOannlinice
DOI: 10.1039/C5TC02509E
semiconductors affect significantly their photovoltaic
Table 1 Electrochemical and charge transport characteristics of some fullerene
derivatives
performance in devices. In particular, a vast majority of the
fullerene bisadducts bearing two cyclic addends randomly
distributed on the fullerene cage demonstrate inferior electron
transport characteristics. On the contrary, the fullerene
derivatives designed in this work comprise a single pyrrolidine-
type addend with the well-defined addition pattern to the
fullerene cage. However, existence of some stereoisomerism in
the structures of F1-F4 might also affect their charge transport
characteristics. The SCLC mobility measurements were
performed for the designed compounds and the reference
fullerene derivatives [60]PCBM and bis[60]PCBM using
electron-only diode devices ITO/Ca/fullerene derivative/Ca/Ag.
The obtained results (Table 1) suggest that the fullerene
derivatives F1-F4 have rather good electron mobilities which
are similar (F2-F3) or even considerably higher (F1, F4) as
compared to [60]PCBM. On the contrary, fullerene
biscycloadducts (e.g bis[60]PCBM) represented by complicated
mixtures of isomers demonstrate one order of magnitude lower
mobility with respect to [60]PCBM. These results proved that it
is possible to achieve the desired reduction in the electron
affinity of the fullerene cage while maintaining good charge
transport characteristics of the resulting materials.
Compound
E1
,
E2
,
E3
,
1/2
LUMO,b
eV
µe,c
1/2
Va
1/2
Va
Va
cm2V-1s-1
1.1×10-4
4.3×10-4
1.2×10-4
1.7×10-4
8.0×10-4
n/a
[60]PCBM
-1.01
-1.08
-1.09
-1.05
-1.04
-1.03
-1.13
-1.49
-1.55
-1.57
-1.52
-1.51
-1.51
-1.61
-2.08
-2.18
-2.2
-4.08
-4.02
-4.01
-4.05
-4.06
-4.06
-3.97
F1
F2
F3
-2.14
-2.14
-2.15
-2.29
F4
F5
bis-
1.07×10-5
[60]PCBM
aHalf-wave reduction potentials are given vs. Fc+/Fc. bLUMO energies were
estimated from the onsets of the first reduction waves using Fermi energy of −5.1
eV for the Fc+/Fc redox couple. SCLC mobility was determined in electron-only
devices.
c
It is seen from the Table 1 that fullerene derivatives F1-F4 have
considerably higher LUMO energies with respect to
[60]PCBM. Comparing the energy levels of some
representative conjugated polymers and fullerene derivatives
(Fig. 2) suggests that the solar cells comprising F1-F4 instead
of [60]PCBM can show enhanced open circuit voltages and
overall light power conversion efficiencies.
We believe that the observed decrease in the electron affinity of
The designed pyrrolidinofullerenes F1-F4 were investigated as
n-type components of organic bulk heterojunction solar cells in
combination with the conjugated polymers P3HT and PCDTBT
(Fig. 3a). We would like to emphasize that we utilized inverted
device configuration (Fig. 3b) to exclude any possible
unfavorable interactions between the pyrrolidinofullerenes and
the acidic hole transport layer PEDOT:PSS. The inverted solar
cell architecture comprised semitransparent ITO electrode
modified with a thin (3 nm) layer of ytterbium collecting the
electrons, a photoactive fullerene-polymer blend (60-150 nm)
and MoO3 (3 nm)/Ag (100 nm) hole-collecting electrode. The
photoactive composite films were deposited by spin-coating
from chlorobenzene (in the case of P3HT) and 1,2-
dichlorobenzene (when using PCDTBT) solutions at different
spinning frequencies. The polymer : fullerene derivative ratios
was also varied from 1:1 to 1:4 in order to find optimal device
fabrication conditions. The obtained results are presented in
Table 2.
the designed fullerene derivatives F1-F4 is related to the
through-space electronic interactions between the lone electron
pair of the pyrrolidine nitrogen atom and the fullerene cage as
well as similar interactions involving the alkoxy groups (lone
electron pairs of O) introduced in the phenyl substituents in a
close proximity to the fullerene cage. For instance, reference
compound F5 comprising no methoxy groups demonstrates
lower electron affinity as compared to PCBM (effect of the
pyrrolidine nitrogen) but still higher in comparison with F1-F4
loaded with different number of methoxy groups. It is notable
that compounds F1 and F2 with the lowest electron affinity
possess two and three methoxy groups at the positions 2’- and
6’- of the phenyl substituents in the pyrrolidine ring,
respectively.
a
b
Figure 2. Energy level diagram for P3HT, PCDTBT, F2 and [60]PCBM showing the
highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of the
materials.
Figure 3. Molecular structures of conjugated polymers P3HT and PCDTBT (a)
and schematic layout of the Inverted solar cell architecture (b)
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devices (724 mV, see ref. 3,). The solar cells comprising
Table 2 Photovoltaic performance of F1-F4 in combination with P3HT and PCDTBT
View Article Online
DOI: 10.1039/C5TC02509E
pyrrolidinofullerenes F1-F4 and P3HT as a donor polymer
demonstrated enhanced power conversion efficiencies in
comparison with the reference [60]PCBM/P3HT cells.
Ratio
*
Annealing
conditions
VOC
mV
,
JSC
,
FF,
%
ɳ,
%
Composite
mA/cm2
P3HT/
[60]PCBM
P3HT/
F1
165 0C,
1:0.5
599
8.5
60
3.0
We have also shown that pyrrolidinofullerenes can be used
successfully as electron donor components in solar cells with
push-pull donor-acceptor copolymers such as PCDTBT. In
particular, the devices comprising F3 and F4 in combination
with PCDTBT showed high open circuit voltages of 960-980
mV and reasonable power conversion efficiencies exceeding
4% (Table 2). It should be emphasized that
pyrrolidinofullerenes F3 and F4 represent virtually the first
fullerene derivatives with the reduced electron affinity
showing decent photovoltaic performances in combination
with the amorphous conjugated copolymers such as
PCDTBT. The data provided in Table 2 suggest that
dramatically reduced fill factor limits the photovoltaic
performance of the F3/PCDTBT and F4/PCDTBT blends. It
is possible that unbalanced fullerene-polymer blend
morphology is responsible for the observed low fill factors.
The AFM images shown in Fig. 5 do not reveal strong phase
separation between donor and acceptor components of the
blend. Moreover, the F3/PCDTBT and F4/PCDTBT
composite films showed rather featureless morphology,
which is an indication of too strong intermixing between the
donor and acceptor components. Therefore, it is very likely
that insufficient phase separation between the fullerene
derivatives and PCDTBT prevents the formation of efficient
percolation pathways required for delivering photogenerated
charges towards the respective electrodes. We believe that
morphology problems are even more severe in the case of
F1/PCDTBT and F2/PCDTBT blends delivering lower
photovoltaic performances (Table S1, ESI).
3 min
90 0C,
15 min
140 0C,
3 min
747
772
7.7
8.6
59
55
3.4
3.7
1:0.7
P3HT/
F2
PCDTBT/
[60]PCBM
PCDTBT/
F3
1:4
1:2
1:3
871
983
957
7.8
8.9
8.3
60
47
52
4.1
4.1
4.1
90 0С, 15 min
PCDTBT/
F4
* Optimal polymer to fullerene w/w ratio is provided
The light-on J-V characteristics of the fabricated solar cells
based on different material combinations are shown in Fig. 4.
a
5
4
3
[60]PCBM / P3HT
F1 / P3HT
F2 / P3HT
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
-200
0
200
400
600
800
Voltage, mV
a
b
b
6
5
[60]PCBM / PCDTBT
F3 / PCDTBT
F4 / PCDTBT
4
3
2
1
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
c
d
-200
0
200
400
600
800
1000
1200
Voltage, mV
Figure 4. J-V curves of the solar cells based on the composites of the fullerene
derivatives F1- F2 with P3HT (a) and the fullerene derivatives F3-F4 with PCDTBT (b)
measured under simulated AM1.5 (100 mW/cm2) illumination
It is notable that organic solar cells comprising the fullerene
derivatives F1-F4 demonstrate strongly increased open
Figure 5. AFM images of thin films of F1/P3HT (a), F2/P3HT (b), F3/PCDTBT (c)
and F4/PCDTBT (d) blends
circuit voltages as compared to the reference devices with
[60]PCBM. We would like to emphasize that the open circuit
voltage of 772 mV achieved for the F2/P3HT system is even
higher than the value obtained for the bis[60]PCBM/P3HT
At the same time, we have shown that pyrrolidinofullerenes
F1-F4 quench the photoluminescence of PCDTBT in thin
films with virtually the same efficiency as [60]PCBM (Fig.
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S15-S16, ESI). This observation suggests that lowered
electron affinity of F1 F4 does not prevent efficient charge
platinum wire as a counter electrode and a standard Ag/Ag+
View Article Online
pseudo reference electrode. The scan rateDwOaI:s150.010m39V/C/s5.TCA0205.109E
-
separation in their blends with PCDTBT. Therefore,
inefficient charge collection due to unbalanced active layer
morphology seems to be the main factor limiting the
performance of the pyrrolidinofullerene/PCDTBT solar cells.
Our future research efforts will be aimed to overcome the
morphology issues revealed here for the pyrrolidinofullerenes
F1-F4. In particular, designing novel compounds capable of
efficient self-assembling in thin films and/or undergoing
stronger segregation from the polymer will be pursued.
M solution of Bu4NPF6 was used as a supporting electrolyte.
Fabrication of inverted photovoltaic devices
The cleaned and patterned ITO glass (see ref. 37) was
modified with 3 nm of Yb deposited by thermal evaporation
in vacuum. P3HT (15 mg) or PCDTBT (6 mg) and the
corresponding amount of the fullerene derivative (following
the optimal weight ratios given in Table 2) were dissolved
together in
1 mL of chlorobenzene (P3HT) or 1,2-
dichlorobenzene (PCDTBT) with addition of 11 mg of silica
(ACROS Organics 40-60 A). The resulting polymer-fullerene
blend solutions were filtered through a 0.45 µm PTFE
syringe filter and spin-coated at 1200 rpm for 40 sec (P3HT)
or at 800 rpm for 140 sec (PCDTBT) on the Yb-modified
ITO slides. The deposited films were annealed in argon glove
box under the conditions specified in Table 2. The top
electrode comprising 3 nm of MoO3 and 100 nm of Ag was
deposited by thermal evaporation.
Conclusions
Four novel 1,2-disubstituted and 1,2,5-trisubstituted
pyrrolidinofullerenes loaded with the electron donor methoxy
groups have been synthesized and systematically
investigated. It has been shown that introduction of methoxy
groups in a close proximity to the carbon cage results in a
noticeable decrease in its electron affinity presumably due to
direct through-space electronic interactions between the lone
electron pairs of the oxygen atoms and the fullerene
system.
-
Characterization of photovoltaic devices.
The current-voltage (I-V) characteristics of the devices were
obtained in dark and under the simulated 100 mW/cm2
AM1.5 solar irradiation provided by a KHS Steuernagel solar
simulator integrated in MBraun glove box. The intensity of
the illumination was checked every time before the
measurements using a calibrated silicon diode with a known
spectral response. The I-V curves were recorded in inert
atmosphere using Keithley 2400 source-measurement unit.
The active areas of all devices were measured with a good
accuracy just after the J-V measurements to estimate the short
circuit current densities.
The application of the designed pyrrolidinofullerenes as
electron acceptor components of organic solar cells resulted in
a significant improvement of the device open circuit voltages as
compared to the reference systems comprising conventional
[60]PCBM. In particular, the VOC of 772 mV achieved for the
F2/P3HT blends represents one of the highest values ever
reported for the fullerene monocycloadducts. These findings
prove that pyrrolidinofullerenes undoubtedly have a big
potential as promising n-type materials with reduced electron
affinity.
The light power conversion efficiencies of the solar cells
comprising pyrrolidinofullerenes were somewhat higher or SCLC mobility measurements
equal to the parameters of the reference [60]PCBM-based
The electron-only diodes were fabricated using the following
devices. Unfortunately, enhanced VOC was counterbalanced by
reduced fill factors related, most probably, to the unbalanced
active layer morphology. Overcoming this obstacle by
modifying the molecular structures of the pyrrolidinofullerenes
and tailoring their physical properties (e.g. solubility) might
lead to the development of a novel generation of the fullerene-
based n-type materials for highly efficient organic
photovoltaics.
procedure. Thin film (15 nm) of Yb was evaporated on the
cleaned ITO glass. Afterwards, a solution of fullerene
derivative in PhCl-CHCl3 (1:4 v/v) was spin-coated at
different spinning rates (from 100 to 1600 rpm) in order to
provide a set of film thicknesses ranging typically from 80 to
700 nm. The diode structure was completed by evaporation
of 100 nm of Ca and 100 nm of Al on the top of the fullerene
layer. The current-voltage characteristics of the fabricated
diodes were recorded inside a nitrogen glove box (MBraun)
using Keithley 2400 source-measurement unit. The thickness
of the films was determined by making scratches near the
corresponding electrodes and profiling them using NTEGRA
PRIMA microscope (NT-MDT, Russia) operating in a
contact mode. The obtained data were analysed according to
the standard space charge limited current (SCLC) model. The
SCLC mobility depended on the active layer thickness (due
to interface effects, in particular) approaching the maximal
values at 200-400 nm. The highest reproducible values are
provided in Table 1 above.
Experimental
Synthesis
The synthetic procedures and spectral data are provided in
Electronic Supplementary Information (ESI).
Cyclic voltammetry measurements
Cyclic voltammetry measurements were performed for ~1.10-
3
M solutions of the fullerene derivatives in a 1,2-
dichlorobenzene/DMF (3:2) solvent mixture using a cell
equipped with a glassy carbon working electrode (2 mm2),
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Acknowledgements
View Article Online
DOI: 10.1039/C5TC02509E
We thank Dr. D. V. Novikov for support with the
electrochemical measurements. The main part of this work
was funded by Lanxess Germany. Synthesis and
characterization of some samples was also supported by
Russian President Science Foundation (project No. МК-
5260.2014.3) and Research Program No. 24 of the Presidium
of Russian Academy of Sciences.
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC02509E
TOC entry
Appropriately designed pyrrolidinofullerenes loaded with electron donor alkoxy groups showed
enhanced VOC and power conversion efficiencies in organic solar cells