Reversible photoinduced bi-state polymer solar cells based on fullerene derivatives with azobenzene groups

Article abstract of DOI:10.1016/j.orgel.2015.03.046

[6,6]-Phenyl-C61-butyric acid-4′-hydroxyl-azobenzene ester (PCBAb) was synthesized and used as the acceptor in the fabrication of reversible UV-VIS response bi-state polymer solar cells (PSCs) based on the photoinduced cis-trans isomerization of PCBAb. Th

Full text of DOI:10.1016/j.orgel.2015.03.046

Organic Electronics 23 (2015) 1–4
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Reversible photoinduced bi-state polymer solar cells based on fullerene
derivatives with azobenzene groups
Shengbo Ma a, Hungkit Ting a, Lipei Zhang a, Yingzhuang Ma a, Lingling Zheng a, Lixin Xiao a,b,
,
⇑
Zhijian Chen a,b,
⇑
a State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, People’s Republic of China
b Haixi Collaborative Innovation Center for New Display Devices and Systems Integration, Fuzhou University, Fuzhou 350002, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
[6,6]-Phenyl-C61-butyric acid-40-hydroxyl-azobenzene ester (PCBAb) was synthesized and used as the
acceptor in the fabrication of reversible UV–VIS response bi-state polymer solar cells (PSCs) based on
the photoinduced cis–trans isomerization of PCBAb. The device can be switched between ‘‘active’’ and
‘‘sleep’’ by the irradiation of UV and visible light, respectively. The active device has a PCE of 2.0%.
With UV irradiation, the device goes to ‘‘sleep’’ with a lowered PCE (0.4%), and simultaneously decreased
Jsc, Voc and FF, while after visible light treatment, the device is made ‘‘active’’ again. The mechanism of the
bi-state process involves the different electron mobilities of the isomers.
Article history:
Received 7 March 2015
Received in revised form 30 March 2015
Accepted 30 March 2015
Available online 31 March 2015
Keywords:
Polymer solar cells
Photo-isomerization
Reversible bi-state solar cell
Fullerene derivatives
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
changes in the physical properties. As a result, azobenzene deriva-
tives are widely researched in fields that require strong optical
Polymer solar cells (PSCs) have attracted considerable attention
due to their flexible nature, low weight, low cost, ease of prepara-
tion, and potential applications in building integrated photovoltaic
(BIPV) devices [1–3]. Recently, the power conversion efficiency
(PCE) of PSCs was demonstrated to exceed 10% based on con-
siderable effort by several research groups [4,5]. However, new
materials are still required for the development of PSCs because
practical applications require further improvement of the photo-
voltaic properties. For example, PSCs used on the windows of
buildings or agricultural greenhouses may require a self-regulation
function in response to the different characteristics of the solar
spectrum during different times of a day, which is similar to the
‘‘smart glass’’ changing its transmittance according to the sunlight
intensity [6,7]. Such functionality requires new photovoltaic
materials combined with photo-sensitized groups.
responses, such as photovoltaics [9–11], photoswitches [12], opti-
cal data storage [13] and organic field-effect transistors (OFETs)
[14]. These applications often require good electron acceptors with
high electron mobilities, which promote exciton dissociation, such
as fullerene derivatives. Fullerene derivatives are widely used elec-
tron acceptors with high electron mobilities of 10ꢀ3 cm2 Vꢀ1 sꢀ1
,
high LUMO levels of 4.3 eV (able to accept up to 6 electrons), and
deep HOMO levels of 6.1 eV, responsible for efficient photoinduced
hole transfer to the donor [15]. Therefore, in this work, combining
the fullerene derivatives with azobenzene groups, a new material
PCBAb is synthesized and applied in PSCs. PCBAb, as the electron
acceptor in PSCs, has a notable electron mobility response to UV
and VIS light, which can be used to control the PCE of PSCs, thereby
switching the solar cell between ‘‘active’’ and ‘‘sleep’’ states.
Azobenzene derivatives are well-known photochromic materi-
als due to their reversible transformation between two photo-iso-
mers [8]. The photo-isomerization of azobenzene derivatives
occurs when treated with ultraviolet (UV) or visible (VIS) light.
The conversion between the states is usually accompanied by
2. Experiment
After synthesizing PCBA as reported in [16], PCBAb was synthe-
sized through esterification of PCBA and 4-hydroxyazobenzene in
dry CH2Cl2 in the presence of EDC and DMAP (Fig. 1a).
The bulk heterojunction solar cells of ITO/PEDOT:PSS/
P3HT:PCBAb/LiF/Al were fabricated. The PEDOT:PSS used for hole
collection was spin-coated onto an ITO-coated glass substrate.
The P3HT:PCBAb solvent was dissolved 1:1 w/w in dichloroben-
zene with a total concentration of 26 mg/ml. Next, the active layer
⇑
Corresponding authors at: State Key Laboratory for Artificial Microstructures
and Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871,
People’s Republic of China.
E-mail addresses: lxxiao@pku.edu.cn (L. Xiao), zjchen@pku.edu.cn (Z. Chen).
http://dx.doi.org/10.1016/j.orgel.2015.03.046
1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

2
S. Ma et al. / Organic Electronics 23 (2015) 1–4
(a)
OMe
OH
O
O
O
N
O
EDC, DMAP/ CH2Cl2
N
HCl/HOAc
Chlorobenzene
OH
N N
PCBM
PCBA
PCBAb
(b)
O
O
O
N
N
N
O
UV
Vis
N
trans-PCBAb
cis-PCBAb
Fig. 1. (a) Synthesis of PCBAb. (b) Diagram of PCBAb photo-isomerization.
was spin-coated to a thickness of approximately 150 nm and then
annealed at 150 °C for 1 h. Finally, an Al electrode with a LiF buffer
layer for electron collection was thermally deposited through a 2-
mm diameter mask.
absorption difference). The peak at 350 nm is the absorption peak
of the C60 groups [17]. The photo-isomerization leads to the differ-
ences in the absorption of the thin films.
Current density–voltage (J–V) curves were obtained using a
Keithley 2611 power source meter under AM 1.5G illumination
(100 mW/cm2) using a 300-W Newport-Oriel AM 1.5G light source.
The UV treatment involves irradiation of the thin films under a UV
light source (2.5 W/cm2) and the VIS treatment involves the use an
AM 1.5G light source (100 mW/cm2). All measurements were per-
formed in air without encapsulation.
3.2. J–V characteristics
Considering the cis–trans photo-isomerization of the azoben-
zene groups, the device characteristics under UV and VIS treat
were compared. The current density vs. voltage (J–V) plot of the
solar cells is shown in Fig. 3. The device characteristics, such as
the short circuit current (Jsc), open circuit voltage (Voc), fill factor
(FF), PCE, and series and shunt resistances (Rs and Rsh, respectively)
are summarized in Table 1. Under the illumination of 100 mW/cm2
(AM 1.5G), the original PCBAb device, without any irradiation
treatment, had a PCE of 2.0%, and the Voc, Jsc and FF of the original
device were 0.58 V, 7.0 mA/cm2 and 0.49, respectively, which
stayed in the ‘‘active’’ state. When irradiated under UV light with
3. Results and discussion
3.1. UV–VIS absorption spectroscopy
PCBAb can be switched between its trans and cis forms by
irradiation of UV and visible light, as shown in Fig. 1b. The struc-
tural isomerization exhibits different optical absorption. The
absorption spectra of a PCBAb film were measured before and after
irradiation of UV light, as shown in Fig. 2 (the insert shows the
different treatment times of 1, 2, 3 and 6 min, the values of Voc
,
Jsc, PCE, and FF all decreased simultaneously. For the 6-min UV
device, the Jsc, Voc, PCE and FF were 2.3 mA/cm2, 0.45 V, 0.42%
and 0.40, respectively. The Jsc and PCE were notably reduced to
1/3 and 1/5 of their original values, respectively. The decreased
parameters indicated that the device entered the ‘‘sleep’’ state
gradually under UV treatment. Next, the ‘‘sleeping’’ device was
irradiated with visible light for 1, 2 and 5 min, which made it ‘‘ac-
tive’’ again, with the parameters regained gradually. Because the
VIS source had a lower intensity than the UV source, the VIS
response process was considerably slower than that of the UV
response process. Because the trans conformation of azobenzene
is more stable than that of the cis isomer, in the dark at equi-
librium, trans is the dominant isomer (99.99%) [14]. Therefore,
the device was kept in the dark for 12 h to allow the trans isomer
to re-establish the equilibrium. The process is reversible.
3.3. SCLC modeling
The Jsc exhibits an obvious decrease when treated with UV light
(Table 1), as is also shown in the incident photon to converted cur-
rent efficiency spectra (IPCE, Fig. 4a) of the devices. Jsc is greatly
influenced by the absorption of the active layer and the charge
transport properties of the device. The absorption spectra revealed
no significant difference before and after UV treatment. To
Fig. 2. Absorption spectra of PCBAb thin films before and after UV irradiation.
(Insert) Absorption differences between the PCBAb thin film before and after UV
irradiation. The arrows show the remarkable peaks of the absorption differentials.

S. Ma et al. / Organic Electronics 23 (2015) 1–4
3
Fig. 4. (a) IPCE spectra of a device without any treatment, and with 1 min, 2 min,
and 3 min of UV treatment; (b) J–V2 characteristics and the fitting results of an
electron-only device with no treatment, 2 min of UV treatment and 8 min of visible
light treatment in sequence.
Fig. 3. The ‘‘active’’ and ‘‘sleep’’ processes of
treatments in sequence. (a) sleep process under 0–6 min of UV treatment; (b) active
process under 0–5 min VIS and dark treatments.
a solar cell under UV and VIS
measure the mobility change of PCBAb as an acceptor, an electron-
only device was fabricated with the structure of Al/P3HT:PCBAb/
LiF/Al. The electron mobility can be calculated by fitting the J–V
characteristics of the electron-only device to the model of space-
charges-limited current (SCLC) [18]. The SCLC model is given by
J ¼ 9e0er
l
V2=8L3, where J is the current density, L is the thickness
is the electron mobility, er is the relative
of the active layer,
l
dielectric constant of the transport medium, e0 is the permittivity
of free space, and V is the internal voltage in the device. In addition,
V is corrected as follows: V = Vapp ꢀ Vsr ꢀ Vbi, where Vapp is the
applied voltage to the device, Vsr is the voltage drop due to the con-
tact resistance and the series resistance across the electrodes, and
Vbi is the built-in voltage due to the relative work function differ-
ence of the two electrodes. Fig. 4b shows the J–V2 curves of the
Table 1
The photovoltaic performances of the device following UV and VIS treatments.
Fig. 5. Rs and Rsh dependence on the UV and VIS treatments.
Treatment
(min)
Jsc (mA/
Voc
(V)
PCE
(%)
FF
Rs
Rsh
cm2)
(X
cm2)
(X
cm2)
electron-only device with both the original data and the linear fit-
tings under different treatments of no treatment, 2-min UV treat-
ment, and 8-min VIS treatment. The electron mobility of the
device is related to the slopes of the fitting lines at high voltage
[19]; therefore, the data with V2 in the range of 5.0–8.5 is chosen
to be fitted. The slope of original device is much higher than that
of the UV treated device, and the VIS treated device has a slightly
restored slope compared to the UV treated device. This result
implies that UV treatment leads to an electron mobility decrease
of the device and that VIS treatment restores the electron mobility.
Before treat
UV 1
UV 2
UV 3
UV 6 (VIS 0)
VIS 1
VIS 2
VIS 6
Overnight
(dark)
7.0
5.6
4.8
3.9
2.3
2.4
2.5
2.6
3.3
0.58
0.51
0.50
0.48
0.45
0.46
0.47
0.47
0.51
1.99
1.11
0.89
0.72
0.42
0.44
0.48
0.50
0.72
0.49 24
0.39 35
0.37 42
0.38 45
0.40 60
0.41 60
0.41 60
0.42 50
0.43 39
743
154
478
435
329
433
612
551
456

4
S. Ma et al. / Organic Electronics 23 (2015) 1–4
Fig. 5 shows how Rs and Rsh change under different times of UV
and VIS treatment, which is calculated from the J–V characteristics.
In UV region Rs increases from 24 to 60
cm2 under UV treatment,
and Rsh also exhibits a decreasing trend from 743 to 329
cm2. An
11074016, 11121091, 61275034 and 61106123) and the National
Basic Research Program of China (2013CB328700).
X
X
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charge mobility after UV treatment leads to a higher rate of recom-
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A UV–VIS response switchable polymer solar cell was fabricated
based on the photo-isomeric material PCBAb. The electron mobility
of the device is dependent on UV and VIS treatments, which leads
to the change of device performance between ‘‘active’’ and ‘‘sleep’’
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This study was partly financially supported by the National
Natural Science Foundation of China (10934001, 61177020,