Solar Energy Materials & Solar Cells 97 (2012) 71–77
Contents lists available at SciVerse ScienceDirect
Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat
Influence of PC60BM or PC70BM as electron acceptor on the performance of
polymer solar cells
Fujun Zhang a,n, Zuliang Zhuo a, Jian Zhang b,n, Xin Wang a, Xiaowei Xu a, Zixuan Wang a, Yusheng Xin a,
Jian Wang a, Jin Wang a, Weihua Tang c,n, Zheng Xu a, Yongsheng Wang a
a
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, People’s Republic of China
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, 457 Zhongshan Road, Dalian 116023, People’s Republic of China
c
Key Laboratory for Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China
b
a r t i c l e i n f o
abstract
Article history:
Received 21 July 2011
Received in revised form
1 September 2011
Accepted 5 September 2011
Available online 5 October 2011
Two series of P3HT-based polymer solar cell (PSCs) only with different electron acceptors PC70BM or
PC60BM were investigated under the same conditions. The PSCs with PC70BM as the electron acceptor
exhibit a relatively strong and broad absorption in the visible range and high external quantum
efficiency, which results in a relatively high open circuit voltage (Voc) of 0.62 V, short circuit current
density (Jsc) of 11.85 mA/cm2 and power conversion efficiency (PCE) of 3.52%. The PSCs with PC60BM as
the electron acceptor have a relatively low Voc of 0.58 V, Jsc of 10.68 mA/cm2 and PCE of 3.02% due to the
weak absorption of PC60BM in the visible light range. The function of PC70BM instead of PC60BM as
electron acceptor could be summarized as follows: i) the strong absorption PC70BM in the visible range
results in much more photon harvesting; ii) the relatively low electron mobility and relatively big size
of PC70BM molecule influences the charge transporting and phase separation. The ultimate performances of PSCs are codetermined by the photon harvesting, exciton dissociation, charge carrier
transport and collection.
& 2011 Elsevier B.V. All rights reserved.
Keywords:
P3HT
PC70BM
PC60BM
Photon harvesting
Power conversion efficiency
1. Introduction
Polymer solar cells (PSCs) have attracted more and more
attention as a new energy source due to their light weight, ease
of large scale manufacture, compatibility with flexible substrates
and the need to develop an inexpensive clean and sustainable
renewable energy source for satisfying economic development
and human living [1–4]. Krebs et al. have reported a series of
landmark research works in the development of large area, high
stability, high efficiency and different configuration PSCs [5–8].
Recently, some imaginative scientists have already paid much
more attention to the lifetime and degradation mechanism of
PSCs with different cells configurations, especially to the inverted
configuration with high work function metal as the top anode
[9–12]. However, many challenges have to be efficiently
addressed before the vision of large scale manufacture and
widespread usage of low-cost PSCs can be anticipated, including
high efficiency, high stability, low-cost, high speed production,
large-area and environment friendly process. The fundamental
n
Corresponding authors. Fax: þ 86 10 51683933.
E-mail addresses: [email protected] (F. Zhang),
[email protected] (J. Zhang), [email protected] (W. Tang).
0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2011.09.006
issues, such as the interfacial states, the stability/degradation of
devices, the effect of annealing treatment and doping concentration, the balance of charge carriers, are still not very clear [13–17].
Krebs et al. have already developed solvent-free and environmentally friendly solvent processing methodologies [18,19]. The allwater-processing solar cells with inverted structure exhibited a
PCE up to 0.70% [18]. Besides the obvious processing advantage of
the solubility switching, removing the side chains from the bulk
active layer furthermore means removal of non-absorption material. Several studies show enhanced stability of the active layer
towards general degradation [20,21]. The possibility of achieving
aqueous processing and operator safety and avoiding environmentally harmful solvents has been demonstrated, which is a
great step towards the commercialization of PSCs.
In the past years, much more attention was paid to develop
high efficiency polymer donors with strong and broad absorption
in the visible light range [9]. In fact, the electron acceptors are of
the same importance as that of the electron donors for high
performance PSCs. Up to date, [6,6]-phenyl-C-61-butyric acid
methyl ester (PC60BM) is the most commonly investigated electron acceptor for solution processed PSCs. Recently, some new
fullerene derivatives with up-shifted lowest unoccupied molecular orbits (LUMO) energy levels have been developed and
assembled with the most representative donors P3HT, showing
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F. Zhang et al. / Solar Energy Materials & Solar Cells 97 (2012) 71–77
higher open circuit voltage (Voc) and PCE [22,23]. However, the
low absorption of PC60BM and its derivatives in the visible light
range is not conducive to further improving the performance of
PSCs. To address this short fall, [6,6]-phenyl-C-71-butyric acid
methyl ester (PC70BM) and its derivatives are selected as the
electron acceptors to improve light absorption in the visible light
range, leading to more exctions in the active layer [24]. It is worth
mentioning that the performance of PSCs should be codetermined
by the following parameters, including absorption, the extent of
phase separation and the balance of charge carrier transporting.
Recently, Xi et al. also reported that the cells with C70 as the
electron acceptor and copper phthalocyanine (CuPc) as electron
donor show an increased photovoltaic performance compared
with the cells with C60 as the electron acceptor and CuPc as
electron donor [25]. In this paper, two series of P3HT-based PSCs
with different electron donors were investigated under the same
conditions. The effects of PC70BM and PC60BM on the performance
of PSCs were comparatively studied by the absorption spectra, the
extent of phase separation, external quantum efficiency (EQE) and
saturation photocurrent density.
2. Experimental details
The indium tin oxide (ITO) coated glass substrates (sheet
resistance 15 O/&) were cleaned consecutively in ultrasonic
baths containing acetone, ethanol and de-ionized water and dried
by high speed nitrogen gas. The cleaned substrates were treated
by UV–ozone for 10 min to improve the work function of ITO. A
thin layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spin-coated on the substrates at the
speed of 3000 round per minute (RPM) for 40 s. The substrates
coated with PEDOT:PSS thin film were transferred to a hot plate
and annealed at 150 1C for 10 min. Photovoltaic materials P3HT,
PC60BM and PC70BM were purchased from Sigma-Aldrich and
dissolved in chloroform to prepare 10 mg/ml stock solution
without further purification, respectively. Then PC60BM and
PC70BM solutions were mixed with P3HT with the same volume
ratio, respectively. Their blended solutions were spin-coated on
PEDOT:PSS coated ITO substrates and then annealed at 120 1C for
10 min under atmosphere conditions. Subsequently, the substrates coated with different active layers were transferred to a
vacuum chamber to prepare the aluminum (Al) cathode under
5 10 3 Pa. The thickness of Al cathode is about 100 nm and the
active area is about 0.09 cm2 through foursquare shadow mask.
The absorption spectra of all above-mentioned films were
measured on Shimadzu UV-3101 PC spectrometer. The photoluminescence spectra were measured on Perkin Elmer LS55
fluorescence spectrometer. The current density–voltage (J–V) characteristics of PSCs were measured and recorded by Keithley source
meter 2410 in dark and under illumination at 100 mW/cm2 using a
150 W Xenon lamp. The EQE spectra were measured on Zolix Solar
Cell Scan 100. The optical microscope photographs were captured
by a Nikon Eclipse TE2000-S inverted microscope. The morphology
and structure of the blended thin films were investigated with
atom force microscopy (AFM) using a multimode Nanoscope IIIa
operated in tapping mode. All measurements were carried out at
room conditions. The chemical structures of the used photovoltaic
materials and schematic diagram of cells are shown in Fig. 1.
3. Results and discussion
Photon harvesting is the first key issue to obtain high performance PSCs. The absorption spectra of pure PC70BM, pure PC60BM
and P3HT blended with PC70BM or PC60BM thin films after
annealing treatment at 120 1C for 10 min are shown in the
Fig. 2a. It is apparent that the blended P3HT:PC70BM film shows
a stronger absorption in the visible range than P3HT:PC60BM film
due to the contribution from PC70BM molecule. The pure PC70BM
thin films exhibits relatively strong absorption from 400 to
700 nm range and longer wavelength absorption at 362 nm
compared with PC60BM at 338 nm in the UV light range. Recently,
Nicolaidis et al. reported that about 13% of Jsc arises from the
contribution of the fullerene component in the P3HT:PC60BM
(1:1) system under air mass (AM) 1.5 illumination conditions
[26]. It means that the photocurrent generated by light that is
absorbed by the fullerene component needs to be considered
when evaluating the performance of PSCs systems containing
PCBM. Therefore, PC70BM should give more contribution on the Jsc
in the P3HT:PC70BM (1:1) system due to its stronger and broader
absorption range in the visible light range compared with
PC60BM. Fig. 2b shows absorption spectra variation of P3HT:
PC60BM and P3HT:PC70BM thin films before and after annealing
treatment. The absorption P3HT:PC70BM films after annealing
treatment is much stronger than that of P3HT:PC60BM films,
which favors more photon harvesting and the improvement of
based-P3HT:PC70BM PSCs performance. The annealing treatment
on the active layer or the devices has been extensively carried out
in order to improve the performance of PSCs, especially for the
P3HT:PCBM system [20,27–30]. The main contributions of
annealing treatment could be summarized as the following
points: i) increase P3HT crystallization, which may affect its
highest occupied molecular orbit (HOMO) level and particularly
the nature of film morphology nears the film-air interface [29]; ii)
enhance absorption intensity and range of P3HT as well as
improve interchain and intrachain orders of P3HT [31,32]; iii)
reorganize and form a phase segregated 3D structure of donor and
acceptor molecules enhancing the charge transfer efficiency [33];
iv) enhance charge carrier mobility and the photoluminescence
(PL) intensity of P3HT thin films [34,35]; vi) optimize interfacial
contact between the metal electrode and the active layer
Fig. 1. Chemical structure of used photovoltaic materials and the schematic configuration of PSCs.
F. Zhang et al. / Solar Energy Materials & Solar Cells 97 (2012) 71–77
73
Fig. 2. The absorption spectra of active layers (a) after annealing treatment at 120 1C for 10 min, (b) before and after annealing treatment, (c) the photoluminescence
spectra of P3HT, PC60BM, PC70BM and their blended films, in order to visualize exciton dissociation induced by PC60BM or PC70BM, the PL intensity of the blended films was
magnified more than 100 times, (d) the electroluminescence spectra of ITO/PEDOT:PSS/P3HT/Al under different driving voltage, (e) the PL spectra of P3HT:PC60BM and
P3HT:PC70BM before and after annealing treatment and (f) the photograph of pure P3HT and P3HT:PC60BM thin film under room conditions and UV light excitation
conditions. The images of P3HT:PC70BM thin film are very similar to that of P3HT: PC60BM thin film.
resulting in the increase of FF [21]. Recently, Tsoi et al. directly
demonstrated the vertical phase separation for P3HT:PCBM after
annealing treatment using the ultraviolet and angle resolved
X-ray photoelectron spectroscopy techniques [29]. The photoinduced charge transfer from P3HT to PC60BM or PC70BM was
investigated by PL spectra under the excitation wavelength of
500 nm, as shown in Fig. 2c. Polymer P3HT shows a strong red
emission peaking at 637 nm and 696 nm, which are strongly
quenched by doping of electron donors due to the fine mixing of
P3HT polymer chains with the PC60BM or PC70BM molecules.
Fig. 2d shows the electroluminescence (EL) spectra of ITO/PEDOT:PSS/P3HT/Al under different driving voltage. The EL devices
have a low turn-on voltage of 5 V. However, a smooth EL spectra
of P3HT:PCBM could hardly be obtained, which should be
attributed to the interpenetrating network of P3HT with PC60BM
or PC70BM, resulting in the exciton dissociation and emission
quenching. In order to further demonstrate the effect of annealing
treatment on the luminescent characters of active layer, the PL
spectra of the blended thin films were measured before and after
annealing treatment under the same conditions and are shown in
the Fig. 2e. It is apparent that PL intensity of blended thin films
was slightly increased after annealing treatment, which should be
attributed to the increase of P3HT phase at the film-air interface.
The photographs of blended thin films and pure P3HT thin film
were captured before and after annealing treatment at room
conditions and UV-light excitation conditions, as shown in the
Fig. 2f. The photographs of all blended thin films were black under
the UV-light excitation conditions, the detailed variations were
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F. Zhang et al. / Solar Energy Materials & Solar Cells 97 (2012) 71–77
investigated by optical microscopy before and after annealing
treatment. Optical microscopy was used to monitor the surface
morphology of the blended films before and after annealing
treatments at room conditions and green light excitation conditions, as shown in the Fig. 3. It is known that the P3HT could be
effectively excited by green light according to the absorption of
P3HT thin films. Microscope images were collected using a CCD
camera with different exposure time. Figs. 3a and 3d are microscopy images of P3HT:PC60BM and P3HT:PC70BM under room
conditions. The microscopy images of the blended thin films
before annealing treatment were measured under green light
excitation and are shown in the Figs. 3b and 3f. The microscopy
images are completely dark with 0.5 s exposure time. However,
there are some faint red emission points on the microscope
images of blend thin films after annealing treatment under green
light excitation with 0.5 s exposure time, as shown in the Figs. 3c
and 3g. In order to directly observe the effect of annealing
treatment on surface luminescence of the blended films, the
Fig. 3. The optical microscopy of P3HT:PC60BM (a–d) and P3HT:PC70BM (e–f);
images a and e are under white light, other images are under green light
excitation; Un-before annealing treatment, An-after annealing treatment, 0.5 s
and 8 s are the exposure times. (All the optical microscopes were measured under
the same conditions except the exposure time.)
exposure time was increased from 0.5 s to 8 s. And then apparent
red emission from P3HT was observed and the microscopy images
are shown in the Figs. 3d and 3h. There are some relatively large
holes on the P3HT:PC60BM film surfaces compared with
P3HT:PC70BM film surfaces, resulting in the larger surface roughness of P3HT:PC60BM film. The phenomenon is identical to the
experimental results according to their AFM images after annealing treatment.
In order to confirm the nanostructure change of films induced
by the annealing treatment on the blended thin films, the
morphology of P3HT:PC60BM or P3HT:PC70BM was measured by
AFM, as shown in Fig. 4. The root mean square roughnesses of the
blended thin films were increased from 0.27 nm to 0.99 nm and
from 0.28 nm to 0.84 nm for P3HT:PC60BM and P3HT:PC70BM
after annealing treatment, respectively. The increase of surface
roughness could be understood that the PC60BM molecular could
easily penetrate into the bulk of active layer and left some
hollows on the film surface due to the smaller size of PC60BM
compared with PC70BM. Meanwhile, the diffuse of PC70BM molecular may be limited by the P3HT network-like structure during
annealing treatment due to its relatively large size of PC70BM
molecular [28,36]. According to the microscopy images and AFM
images, the relatively small roughness of P3HT:PC70BM thin films
may be induced by the limited diffusion of PC70BM molecular
during annealing treatment. He and Li. reported that with
increasing the size of the fullerene cage, the solubility of the big
size fullerene derivative becomes poorer, and the miscibility of
the methanofullerenes with polymer donors clearly diminishes
from PC60BM to PC70BM, and then to PC84BM [28]. The very
similar phenomenon in P3HT with PC60BM or PC70BM solutions
was also observed in our experiments.
It is apparent that the enhanced phase separation extent for
two kinds of films was observed after annealing treatment, which
could be attributed to the additional force for rearrangement of
organic molecular during annealing treatment, as shown in Fig. 5.
The phase separation extent of P3HT:PC60BM is different from
Fig. 4. (a, b) Morphology of P3HT:PC60BM and P3HT:PC70BM thin films before annealing treatment and (c), (d) after annealing treatment.
F. Zhang et al. / Solar Energy Materials & Solar Cells 97 (2012) 71–77
75
Fig. 5. (a, b) The phase images of P3HT:PC60BM and P3HT:PC70BM thin films before annealing treatment and (c), (d) after annealing treatment.
that of P3HT:PC70BM due to PC60BM’s relatively long diffusion
distance. The increase of P3HT phase near the film–air interface
could be demonstrated by the increase of PL intensity of P3HT
after annealing treatments, as shown in the Fig. 2e. The vertical
phase separation of P3HT:PCBM system intensity of this diffraction peak at 5.31 associated with the (100) reflection of the P3HT
could be increased by annealing treatment, indicating an
improvement of the degree of crystallization of P3HT at the
interfaces between the blended thin films and air [29,37].
After comprehensive understanding of the above investigations on the absorption, photoluminescence and morphology of
P3HT doped with PC60BM or PC70BM films before and after
annealing treatment, the two series of P3HT:PC60BM and P3HT:
PC70BM devices were investigated under the same conditions. The
J–V characteristic curves of two kinds of PSCs were measured in
dark and under illumination, as shown in Fig. 6. The Voc, Jsc, fill
factor (FF), and PCE of each cell and their averaged values are
summarized in Table 1. The maximum PCE of PSCs with PC70BM
or PC60BM as electron acceptor reaches 3.55% and 3.13%, respectively. The statistical experimental results show that the devices
based on PC70BM as electron acceptor have relatively high Voc
of 0.62V and Jsc of 11.37 mA/cm2, and a relatively low FF
of 48.03%, resulting in a PCE of 3.37%. And the devices with
PC60BM as the electron acceptor have a relatively low Voc of 0.60 V
and Jsc of 9.84 mA/cm2, and a relatively high FF of 50.98%,
resulting in a PCE of 3.00%. The devices with PC70BM as the
electron acceptor have a relatively high Jsc, which is attributed
likely to the broad and strong absorption in the visible light range.
In order to further confirm the contribution from PC70BM absorption in the visible range, the EQE spectra of two kinds of PSCs
were measured.
Fig. 7 shows the comparison of EQE spectra of the devices with
P3HT:PC60BM or P3HT:PC70BM as the active layer. It is evident
that the EQE spectra reflect an identical trend with the absorption
spectra in the entire range, especially in the range from 350 nm to
Fig. 6. The J–V characteristic curve of PSCs in dark and under illumination
100 mW/cm2; detailed key parameters corresponding to the selected curves are
compared in the inset table.
Table 1
The key parameters of polymer solar cells with different electron donors.
Cell No.
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
Averaged values
1
2
3
4
0.62
0.62
0.61
0.62
11.61
10.88
11.14
11.85
49.26
48.41
46.50
47.93
3.55
3.27
3.16
3.52
Voc ¼0.62
Jsc ¼11.37
FF¼48.03
PCE¼3.37
5
6
7
8
061
0.60
0.59
0.60
9.96
9.08
10.68
9.64
51.51
53.13
47.94
51.32
3.13
2.89
3.02
2.97
Voc ¼0.60
Jsc ¼9.84
FF¼50.98
PCE¼3.00
Remark: 1–4 for PC70BM; 5–8 for PC60BM; the averaged values are listed in the last
column.
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F. Zhang et al. / Solar Energy Materials & Solar Cells 97 (2012) 71–77
Fig. 7. EQE spectroscopes of the PSCs based on P3HT:PC60BM or P3HT:PC70BM as
the active layer.
520 nm. The EQE of PSCs with PC70BM as electron donor is higher
than that of PSCs with PC60BM as the electron donor, which is
strongly supported by their absorption spectra. In order to further
confirm this phenomenon, the photocurrent density (Jph) dependence on the effective voltage was plotted according to the J–V
characteristic curves in dark and under illumination, as shown in
Fig. 8a.
Here, Jph is defined as the difference between the current density
JL under illumination and the current density JD in dark, thus
Jph ¼ JL JD. The V0 is defined as the voltage when Jph ¼0 mA/cm2.
Obvious, the photocurrent density nicely saturates at high effective
voltage for the two kinds of PSCs, indicating that at sufficiently large
reverse bias almost all photo-generated bound pairs are dissociated
and the ratio charge carriers recombination in the active layer to
their extraction from the devices arrives to a kinetic balance. In the
BHJ cells, the photocurrent generation is governed by two main
factors [38,39]: i) the fractional number of absorbed photons in the
active layer and ii) the EQE defined by the fraction of collected
carriers per absorbed photon. According to the photocurrent density
curve in the saturation range, the Jph of PSCs with PC70BM as
electron acceptor is 1.1 time larger than that of PSCs with PC60BM
as electron acceptor, as shown in Fig. 8b. The much more photons
could be harvested in P3HT:PC70BM active layer according to their
absorption spectra. However, the exciton dissociation and charge
carrier transport and collect will also affect the Jph of PSCs with
PC70BM as electron acceptor. The function of PC70BM instead of
PC60BM as electron acceptor could be summarized by the following
items: i) the strong absorption PC70BM in the visible range results in
much more photon harvesting; ii) the relatively low electron
mobility and relatively big size of PC70BM molecule influence the
charge transporting and phase separation. Electron donor P3HT is a
workhorse material for organic field effect transistor and PSCs, the
hole mobility of P3HT thin films is larger than 2 10 3 cm2 V 1 s 1
after annealing treatment. The electron motility of PC70BM in
blended films is nearly 6.8 10 4 cm2 V 1 s 1 reported by
Mihailetchi et al. and Zhao et al. [40,41]. Meanwhile, the electron
mobility of PC60BM thin films is 2 10 3 cm2 V 1 s 1 [42]. In the
P3HT:PC70BM system, the unbalanced transporting of charge carriers could lead to space charge build-up and carrier recombination
in the bulk, making the decrease of FF. Both light harvesting
and charge transport of electron acceptors should be considered
when designing and developing new electron acceptors for high
efficiency PSCs.
Fig. 8. (a) the photocurrent density dependence on the effective voltage of
P3HT:PC60BM and P3HT:PC70BM, the inset image is the semi-log J–V characteristic
curves of PSCs in dark and under illumination at 100 mW/cm2; (b) the ratio
of Jph of the cells based on PC70BM to that of cells based on PC60BM in the
saturation range.
4. Conclusions
In conclusion, two series of P3HT-based PSCs with PC70BM or
PC60BM as electron acceptor were investigated under the same
conditions. The PSCs with PC70BM have a relatively high Voc, Jsc
and PCE, which should be attributed to the absorption of PC70BM
in the visible range. However, the PSCs with PC70BM as the
electron acceptor have a relatively low FF induced by the
unbalance of charge carrier transport. It means that a series of
parameters, such as photon harvesting, exciton dissociation,
charge carrier transport and collection should be simultaneously
optimized to obtain the high efficiency PSCs.
Acknowledgments
The authors express our thanks to National Natural Science
Foundation of China under Grant nos.(10804006, 20904057 and
21074055); Basic Research Foundation of the Central Universities
(2011JBM123); the 111 Project (B08002) and NUST Research
Funding (no. 2010ZDJH04). F. Zhang thanks the support from
the ‘‘Double Hundred Talents Plan’’ of Beijing Jiaotong University.
J. Zhang acknowledges financial support by 100 Talents Program
of The Chinese Academy of Science.
F. Zhang et al. / Solar Energy Materials & Solar Cells 97 (2012) 71–77
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