Optics and Photonics Journal, 2013, 3, 222-226
doi:10.4236/opj.2013.32B052 Published Online June 2013 (http://www.scirp.org/journal/opj)
Effects of Annealing Conditions on ZnO Buffer
Layer for Inverted Polymer Solar Cells
Chuan Liu, Lihua Zheng, Zhiyang Gao, Yuhui Gan, Jian Zhang*, Chuannan Li*
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering,
Jilin University, Qianjin Street, Changchun, China
Email: *zhangjian@jlu.edu.cn, *licn@jlu.edu.cn
Received 2013
ABSTRACT
A solution-processed zinc oxide (ZnO) thin film as the buffer layer with optimized processes especially the annealing
conditions for inverted polymer solar cells (PSCs) has been demonstrated. Firstly the thickness of ZnO buffer layer was
optimized, and different annealing conditions including temperature and time have also been taken into consideration.
And the best Power Conversion Efficiency (PCE) 3.434% was observed when the ZnO buffer layer was spin–coated at
1500 rpm and annealed at 275 for 5 min, and AFM results showed that morphology of this ZnO film has the best
uniformity which was beneficial to form high quality polymer composite active layer.
Keywords: Polymer Solar Cells (PSCs); Zinc Oxide; Buffer Layer; Annealing; Morphology
1. Introduction
Polymer solar cells (PSCs) offer a potentially low-cost,
lightweight, flexible and scalable source of renewable
energy. However, before PSCs can become a marketable
energy technology, further improvements on efficiency
and stability are required. And the Power conversion
efficiencies (PCEs) of PSCs have been improved through
such as designs of the device structure, careful controls
of morphology and the applications of low band gap ma-
terials [1-3]. And there exists significant interests in
polymer bulk heterojunction solar cells by solution proc-
ess due to their low temperature and high efficiency. In
the regular bulk heterojunction PSCs structure, transparent
indium tin oxide (ITO) usually is used as the anode and
low work function metal as the cathode. Thus it brings
the problem of low stability and efficiency. Therefore
highly efficient polymer solar cells using an inverted
structure, in which the positions of anode and cathode are
reversed, have been demonstrated [4]. The low work
function metal e.g. calcium used as the cathode in the
regular structure, is replaced by a relatively nonreactive
electron collection layer and transparent ITO film, and
the stable metal such as Ag or Al can be used as the top
anode of inverted PSCs, this significantly improves the
air stability of the polymer solar cells. Furthermore, ver-
tical phase separation in polymer blends has proven to be
advantageous in the inverted structure [5].
One of the important keys to achieving high perform-
ance inverted PSCs is the selection of the electron collec-
tion layer between the transparent cathode and the active
polymer composite layer. The purpose of the electron
collection layer is to provide hole blocking capability and
a low resistive pathway for efficiently electron extraction.
Inorganic compounds CsCO3 [6-7], TiO2 [8,9] and ZnO
[10] have been demonstrated as the effective electron
collection materials. Particularly, solution-processed ZnO
is an attractive candidate because it easily forms a nanos-
tructure film with more efficient charge extraction and
transporting capability, at the same time the solution-
processed ZnO film reveals a strong hole blocking capa-
bility too [11]. During the fabrication processes of ZnO
buffer layer, the rotating speed and annealing conditions
greatly affect its electron collection and hole blocking
characteristics, the formation of polymer composite active
layer on ZnO film, and the performance of PSCs thereby
[12,13].
Here we demonstrate the fabrication of inverted bulk
heterojunction PSCs utilizing a ZnO interlayer as buffer
layer between the ITO and active layer. The ZnO film is
fabricated by solution processing, the morphology, IV
characteristics with different annealing condition is
thoroughly analyzed, and different annealing conditions
on the ZnO buffer layer are optimized. And we also ex-
plore the best rotation speed of ZnO precursor solution
for ZnO film.
2. Experiments
PSCs in these experiments were fabricated on patterned
*Corres
p
ondin
g
author.
Copyright © 2013 SciRes. OPJ
C. LIU ET AL. 223
ITO substrates which were ultrasonicated in acetone and
isopropyl alcohol, followed by a 10 min UV-O3 treat-
ment before spin-coating. The ZnO precursor solution,
consisting of 0.5 M zinc acetate dihydrate and 0.5 M
monoethanolamine in 2-methoxyethanol, was first spin
coated onto ITO substrates at different rpm for 40 sec-
onds. Subsequently ZnO films are annealed at different
conditions. Then the ZnO film was ultrasonicated in
acetone and isopropyl alcohol for 5 min and dried at 200
for 5 min in order to remove the organic residuals of
the ZnO film. The active layer for PSCs was spin coated
from a 20 g/l solution of P3HT: PCBM 1:1 by weight in
dichlorobenzene at 600 rpm for 1 min. The P3HT:
PCBM active layer on ITO/ZnO combined film was an-
nealed using a hot plate at 110 for 10 min inner a ni-
trogen atmosphere, its thickness is about 230 nm. Finally
a modified layer MoO3 (4 nm) and top anode metal alu-
minum (80 nm) was vacuum-deposited onto the P3HT:
PCBM active layer. The configuration of PSCs device
structure is shown in Figure 1.
Devices were measured under simulated illumination
at AM 1.5 G, 100 mW/cm2 with a Keithley 2400 source
meter controlled by a computer. The solar simulator was
calibrated using a reference Si solar cell and all electrical
measurements were carried out in air at room tempera-
ture. The surface morphologies of the photoactive layers
were measured by atomic force microscopy (AFM). The
AFM images were obtained using a Veeco Dimension
Icon system with tapping-mode.
3. Results and Discussions
Firstly, solar cells were fabricated with different rotation
speed of ZnO, and then annealed under the two tempera-
tures 200℃ and 275, their current density-voltage (J-V)
characteristics are shown in Figures 2(a) and (b). And
the performances of the solar cells are summarized in
Table 1. For both two kinds of annealing conditions, the
device shows a remarkable improvement with 1500 rpm
of ZnO film compared to the devices with other spin-
coating speeds. For device annealed at 200 for 1 hour,
Al
GLASS
ITO
P3HT:PCBM
MoO
3
ZnO
Figure 1. Configuration of the inverted PSCs device struc-
ture.
(a) (b)
Figure 2. J-V characteristics of devices with ZnO film by different spin-coating speeds and annealing at (a) 200 for 1 h and
(b) 275 for 5 min.
Table 1. Summarized performance characteristics of polymer solar cells with different spin-coating speeds and annealing
temperatures.
Rotating speed of ZnO (rpm) Annealing Condition VOC (V) JSC (mA/cm2) FF (%) PCE (%)
1000
1500
2000
2500
1000
1500
2000
2500
200, 1h
200, 1h
200, 1h
200, 1h
275, 5min
275, 5min
275, 5min
275, 5min
0.6
0.60
0.61
0.60
0.61
0.62
0.62
0.63
8.203
7.567
6.999
7.343
7.342
9.152
7.771
7.450
54.69
59.84
59.04
54.03
61.42
60.53
62.03
62.94
2.692
2.717
2.501
2.380
2.751
3.434
2.964
2.954
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C. LIU ET AL.
224
the short circuit current density (JSC) is 7.567 mA/cm2,
the open circuit voltage (VOC) is equal to 0.60 V, and the
filling factor (FF) is 59.84%, resulting in a PCE of 2.717%.
At the same time when the annealing condition is 275
for 5 min, the JSC, VOC, FF and PCE is 9.152 mA/cm2,
0.62 V, 60.53% and 3.434% respectively.
Figure 3(a) is the AFM height images of the ZnO film
spin coated at 1500 rpm and annealed at 200 for 1h.
And Figure 3(c) shows the height image of ZnO film
annealed at 275 for 5 min. The r.m.s. roughness of
Figure 3(a) is 3.39 nm, and Figure 3(c) shows an r.m.s.
roughness of about 1.36 nm, which is much smoother
than the former. Figure 3(b) and (d) show the AFM
phase image of the ZnO buffer layer annealed at 200
for 1h and 275 for 5 min respectively. It also shows
that annealing at 275 for 5 min can form a homoge-
neous ZnO film evenly distributing on the ITO. All these
provide a better condition to deposit a high quality poly-
mer composite active film, and enhance their combina-
tion of the ZnO buffer layer and the polymer composite
active layer. As a result, it improves the efficiency of the
device by means of increasing the photogenerated elec-
trons transmitted to the electrode through ZnO buffer
layer.
Then the ZnO film is annealed at 200 and 275
for different times in order to optimize the annealing
condition.
The performances of the solar cells are summarized in
Table 2. Figures 4(a) and (b) show the dark characteris-
tics and J-V characteristics of the devices with ZnO
buffer layer annealed at 200 for different times (20
min marked as device A, 40 min as device B, 60min as
device C, 80 min as device D). From Figure 4(a), the
current density under forward biases of device B and C is
much higher than device A and D, this means device B
and C have a better injection capability. But the current
density at reversed biases from device C is lower than the
device B, which implies a lower leakage current, and
when applied voltage located from 0 V to 0.4 V, device
C has a largest current density. Therefore device C has
the largest RSH (1368.3 Ω·cm2), shown in Table II. That
indicates device C has the largest JSC and good FF which
can be seen in Figure 4(b). So when the annealing tem-
perature is 200 and annealing time is 40 min, devices
have the highest efficiency which is 2.955%.
Figure 4(c) and (d) show the J-V characteristics and
dark characteristics of the devices with ZnO buffer layer
annealed at 275 for different times (5 min marked as
device E, 10 min as device F, 15 min as device G, and 20
min as device H). From Fig. 4c, the current density at
forward biases decreases with the increase of annealing
time, which shows the device E has the best injection
characteristic. So the device E shows the largest RSH
(1976 Ω·cm2) and the lowest RS (1.996Ω·cm2).
Therefore when the annealing time is 5 min, the device E
has the highest efficiency which is 3.434%.
(a) (b) (c) (d)
Figure 3. (a) AFM height image and (b) phase image of ZnO film annealed at 200 for 1 h, (c) AFM height image and (d)
phase image of ZnO film annealed at 275 for 5 min.
Table 2. Summarized performance characteristics of polymer solar cells with different annealing time.
Annealing temperature () Time (min) VOC (V) JSC (mA/cm2)FF (%) PCE (%) RS(·cm2) RSH(·cm2)
200
200
200
200
275
275
275
275
20 (A)
40 (B)
60 (C)
80 (D)
5 (E)
10 (F)
15 (G)
20 (H)
0.59
0.60
0.59
0.60
0.62
0.62
0.60
0.61
8.490
8.536
7.567
7.252
9.152
7.857
7.653
7.318
57.95
57.70
59.84
36.99
60.53
58.93
58.55
58.29
2.693
2.955
2.717
1.610
3.434
2.871
2.733
2.602
3.989
6.169
2.315
36.53
1.996
6.163
5.975
4.906
243.9
1368.3
264.3
207.5
1976
519.2
460.9
304.8
Copyright © 2013 SciRes. OPJ
C. LIU ET AL. 225
B
C
A
D
(a) (b)
(c) (d)
Figure 4. Dark current characteristics and J-V characteristics of the Devices with ZnO annealing at 200 for different time
(a, b), and the devices with ZnO annealing at 275for different time (c,d).
5. Conclusions
Inverted PSCs with a ZnO buffer layer between the ITO
cathode and the polymer composite P3HT:PCBM was
fabricated. Firstly the ZnO buffer layer was spin coated
using ZnO precursor solution at different rotation speeds
in order to form ZnO film with optimized thickness, and
the PCE of the PSCs reached 3.434% with the rotation
speed 1500 rpm. The various annealing temperature and
times were studied, and the devices annealed at a higher
temperature 275 showed a higher PCE than that an-
nealed at 200. Moreover, when the annealing tem-
perature is 275 the PCE of devices decreased with the
increase of annealing time. Consequently, the device
with the ZnO film spin coated at 1500 rpm and annealed
275 for 5 min showed the highest PCE 3.434%, owing
to the formation of the optimal ZnO buffer layer film.
6. Acknowledgements
This work was supported by the National Natural Sci-
ence Foundation of China (NSFC) under Grant 61177025
and National Training Programs of Innovation and En-
trepreneurship for Undergraduates of China under Grant
2012A51130.
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