Materials Sciences and Applicatio ns, 2011, 2, 1697-1701
doi:10.4236/msa.2011.212226 Published Online December 2011 (http://www.SciRP.org/journal/msa)
1697
The Role of Oxide Thin Layer in Inverted
Structure Polymer Solar Cells
Orawan Wiranwetchayan1,2, Zhiqiang Liang1, Qifeng Zhang1, Guozhon g Cao1*, Pisith Singjai2
1Department of Materials Science and Engineering, University of Washington, Seattle, USA; 2Department of Physics and Material
Science, Faculty of Science, Chiang Mai University, Chiang Mai, Thailand.
Received September 19th, 2011; revised October 24th, 2011; accepted November 6th, 2011.
ABSTRACT
The role of wide band gap oxide thin layer in inverted structure polymer solar cells was investigated by employing ox-
ide films of TiO2 and Nb2O5 approximately 10 nm in thickness deposited onto FTO substrates. The experimental re-
sults demonstrated that the thin oxide layer serving to separate the electron collecting electrode and the photoactive
film of a blend of poly(3-hexylthioph ene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) was necessary to
promote the formation of continuous uniform PCBM film to block holes in P3HT from being recombined with electrons
in collecting electrode. A use of TiO2 buffer layer leads to power conversion efficiency as high as 2.8%. As for Nb2O5, in
spite the fact that its conduction band is higher than the LUMO level of PCBM polymer acting as electron transport
material, a power conversion of 2.7%, which was only slightly different from the 2.8% achieved for the cell employing
TiO2. These experimental results suggest a tunneling mechanism for the electrons to transport from the PCBM to col-
lecting electrode over the oxide film, instead of a diffusion through the oxide film arising from either energy or concen-
tration difference of the photogenerated electrons.
Keywords: Polymer Solar Cell, Oxide Thin Film, TiO2 Thin Film, Nb2O5 Thin Film
1. Introduction
Photovoltaic cells based on conjugated polymer and full-
erene bulk heterojuction composites have attracted much
attention for renewable energy due to their promising
properties such as low production cost, their lightness,
light weight, mechanical flexibility and the possibility of
fabricating them on large area [1-5]. Recent the power
conversion efficiency (PCE) of the bulk heterojunction
polymer solar cells has reached as high as 7.4% in con-
ventional device structures [6]. Conventional organic
photovoltaics, OPVs generally consist of an active layers
sandwiched by a high work function and transparent
metal oxide as the anode such as PEDOT:PSS and a low
function metal as the cathode such as Al [7]. In spite of
high PCE, the conventional OPVs can suffer from deg-
radation of the cathode due to their sensitivity to oxygen
and moisture in air. Therefore, the devices in this struc-
ture exhibit short lifetime [8-10]. In order to overcome
these problems, the inverted device structures is an alter-
ative solution to improve the durability, because it uses a
more air-stable high work function electrode such (Ag,
Au) as back contact to collect holes while using an inor-
ganic semiconductor for buffer layer to collect electrons.
The usage of inorganic semiconductor embedded into the
conjugated polymer have several attributes as electron
acceptors, including relatively high electron mobility,
high electron affinities, hole-blocking ability and good
physical and chemical stability [11-16].
In this work we report the fabrication of inverted
polymer solar cells and investigate the role of thin dense
metal oxide films in the inverted polymer solar cells us-
ing dense film of TiO2 and Nb2O5. These two oxides
were purposely selected in view of the bottom of con-
duction band of TiO2 lower than the LUMO level of
PCBM and that of Nb2O5 higher than the LUMO level of
PCBM. It was found that such a very thin metal oxide
films between the electron collecting electrode and active
layers was necessary to promote the formation of con-
tinuous uniform PCBM film and thus block the holes in
P3HT from being recombined with the electrons in col-
lecting electrode.
2. Experiment Details
2.1. Preparation of TiO2 Sol and Nb2O5 Sol
TiO2 sol was prepared by hydrolyzing titanium alkoxide
in an acidic aqueous. Typically, 1 mL of titanium iso-

The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells
1698
propoxide was added to 20 mL of distilled water con-
taining 0.5 mL of hydrochloric acid. A white precipitate
appeared in the beginning. After stirring for about 30 min,
the precipitate completely dissolved, resulting in the
formation of a TiO2 sol in light yellow color. The fabric-
cation of Nb2O5 sol was adopted a similar process and
recipe, except the replacement of titanium isopropoxide
with niobium ethoxide.
2.2. Fabrication of Solar Cell Devices
The FTO glass substrate was cleaned with DI water,
acetone and isopropanol. The sol of TiO2 or Nb2O5 was
employed to form a thin film on the FTO glass substrate
through a spin-coating method. The substrates with TiO2
or Nb2O5 thin film then suffered a thermal treatment at
450˚C for 1 h to convert the TiO2 or Nb2O5 to crystalline
phase. A chlorobenzene solution of P3HT: PCBM (1:0.8
by weight) containing 20 mg/ml P3HT and 16 mg/ml
PCBM was prepared by stirring in glovebox at 60˚C for
overnight. The solutions were allowed to cool down to
room temperature and then filtered with a 0.2 μm
polytetrafluoroethylene (PTFE) filter. For the device fab-
rication, the as-prepared P3HT/PCBM blend solution was
spin-coated onto the FTO glass substrate coated with
TiO2 or Nb2O5 thin film (note: at 1000 rmp for 30 s) pre-
treated with oxygen plasma for 10 min. The samples
were then baked at 225˚C for 1 min to allow self-or-
ganization of P3HT, as well as to remove residual sol-
vent and to some extent improve the contact between the
polymer and oxide film. On the P3HT: PCBM blend layer,
a diluted poly (3,4-ethylene-dioxylene thiophene)—poly
(styrene sulfonic acid) (PEDOT:PSS, Clevios P VP AL
4083) solution was spin-coated to form hole-transport
layer. The films were consequently baked at 120˚C for
10 min. Finally, a 100 nm thick silver film was deposited
on the PEDOT:PSS to work as top electrode.
Figure 1 shows a schematic configuration of the as-
fabricated inverted polymer solar cell. The semiconduct-
ing metal oxide TiO2 and Nb2O5 thin films are approxi-
mately 10 nm in thickness characterized by observing the
cross-section SEM image of the films. The thicknesses of
the polymer layer are about 300 nm and 30 nm for
P3HT:PCBM and PEDOT:PSS, respectively.
3. Results and Discussion
Prior to investigation of the as-prepared inverted struc-
ture polymer solar cells, a device with structure of
FTO/P3HT:PCBM/PEDOT/Ag, i.e., not including an
oxide thin film to separate the FTO glass substrate and
polymer layers, was studied as a reference. The result is
shown in Figure 2. One can see that there is no photo-
voltaic response at all. This is in agreement with what is
pointed out in literature that an inverted structure poly-
mer solar cell includes a buffer layer to prevent direct
contact between the FTO and polymer layers.
Shown in Figure 3 are the current density (J)—volt-
age (V) curves of the inverted structure polymer solar
cells, in which a layer of TiO2 or Nb2O5 thin film was
deposited on FTO glass substrate to separate the blend of
P3HT and PCBM polymers and the collecting electrode.
It can be seen that for the substrates using TiO2 and
Nb2O5 dense films the values of short-circuit current
density (Jsc) and open-circuit voltage (Voc) show very
small difference. Power conversion efficiencies (  ) for
the cells with TiO2 and Nb2O5 are 2.8% and 2.7%, re-
spectively. Taking into account the above result shown in
Figure 2, it strongly suggests that a thin metal oxide film
as buffer layer between the electron collecting electrode
and the blend of P3HT and PCBM polymers is necessary
to form inverted structure hybrid solar cells.
A dependence of the solar cell efficiency on the thick-
ness of the buffer layer was also studied. Table 1 sum-
marizes the values of short-circuit current density (Jsc),
open-circuit voltage (Voc), fill factor (FF), and power
conversion efficiency (
 ) of inverted TiO2/P3HT:
PCBM/PEDOT/Ag and Nb2O5/P3HT:PCBM/PEDOT/
Ag solar cell. For TiO2 and Nb2O5 with one-cycle spin- -
coating, the devices present similar efficiencies around
2.7% - 2.8%. However, as increasing the spin-coating cycle,
which leads to an increase in the film thickness, the
Figure 1. Schematic configuration of inverted structure po-
lymer solar cell.
-12
-8
-4
0
4
8
12
-0.5 -0.4-0.3 -0.2 -0.100.10.20.30.40.5
Voltage(V)
Current (mA)
Figure 2. I-V curve for FTO/P3HT:PCBM/PEDOT/Ag.

The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells1699
Figure 3. J-V curves of the hybrid solar cells based on TiO2
and Nb2O5 thin film.
power conversion efficiency decreased from 2.8% to
2.3% in the case of TiO2 and decreased from 2.7% to
even zero in the case of Nb2O5. Meanwhile, the short-
circuit current density and fill factor can be also seen a
significant decrease.
Figure 4 shows schematic picture of the energetic lev-
els of Nb2O5, TiO2, PCBM and P3HT. The conduction
band (CB) of TiO2 is lower than the lowest occupied
molecular orbital (LOMO) energy level of P3HT and
PCBM which allows electrons to transport from LOMO
energy level of PCBM into the CB of TiO2. Note that the
conduction band of Nb2O5 is higher than the LUMO of
PCBM. That means the electron transport from the
LOMO energy level of PCBM into the CB of Nb2O5 is
not possible. However, in our study as the results shown
in Table 1, a power conversion of 2.7% was indeed
achieved while Nb2O5 serves as the buffer layer. More
importantly, the achieved efficiency 2.7% is approxi-
mately equal to that (2.8%) obtained for TiO2 (Table 1).
Such a scenario strongly suggests that the electrons seem
to transfer from the PCBM to collecting electrode through
a tunneling process, as shown in Figure 5. Along with an
increase in the thickness of Nb2O5, it presents decreased
short-circuit current density, open-circuit voltage, fill
factor and power conversion efficiency. The efficiency
even drops to zero in the case of thicker Nb2O5 film.
The above experimental observation that shows poor
performance based on thick film indicates that exces-
sively increased thickness of the dense metal oxide film
beyond tunneling distance plays a negative role in the
photovoltaic process. In other words, the transport of elec-
trons unlikely occurs in a diffusion way in view of the
receival of efficiency in the case of Nb2O5 and a quick
decrease in the efficiency as slightly increased thickness of
the buffer layer. Considering the fact that no photovoltaic
response can be obtained in the absence of oxide buffer
layer, the role of the oxide layer is believed to promote
Figure 4. Schematic drawing of the energetic levels of Nb2O5,
TiO2, PCBM and P3HT.
Figure 5. Energy diagram and charge transfer process of
the as-discussed photovoltaic device with FTO/dense metal
oxide /P3HT:PCBM/PEDOT/Ag structure.
Table 1. Summary of the performance of inverted structure
polymer solar cells with buffer layer of TiO2 and Nb2O5.
Voc J
sc
Metal
Oxide
Time
of
Cycle (V) (mA·cm–2) FF (%)η (%)
1 0.574 8.95 53.9 2.8
TiO2 2 0.602 8.87 43.1 2.3
1 0.602 7.94 55.8 2.7
Nb2O52 0.32 1.08 24.7 0
the formation of continuous uniform PCBM film so as to
prevent the P3HT from touching with the FTO substrates
and as such avoid the holes in P3HT from being recom-
bined with the electrons in the collecting electrode.
Therefore, charge separation in our hybrid solar cell oc-
curs at the interface between P3HT and PCBM.

The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells
1700
4. Conclusions
Our study demonstrates that a thin oxide film coating on
FTO glass substrate is necessary in inverted structure
polymer solar cells to protect P3HT from touching the
FTO substrates and promote the formation of continuous
uniform PCBM film to block holes from being recom-
bined. The electrons transport from the PCBM to the
collecting electrode on FTO glass substrate through a
tunneling process. A thick buffer layer would not allow
the occurrence of electron tunneling and therefore leads
to low efficiency or even no photovoltaic response in the
inverted structure solar cells.
5. Acknowledgements
This work is funded by the US Department of Energy,
Office of Basic Energy Sciences, Division of Materials
Sciences, under Award No. DE-FG02-07ER46467 (Q.F.Z.).
This work is also supported in part by the National Sci-
ence Foundation (DMR 1035196), the Air Force Office
of Scientific Research (AFOSR-MURI, FA9550-06-1-
0326), the University of Washington TGIF grant, the
Royalty Research Fund (RRF) from the Office of Re-
search at University of Washington, the Washington Re-
search Foundation, and the Intel Corporation, Office of
the Higher Education Commission, Ministry of Educa-
tion, Thailand. Orawan Wiranwetchayan was supported
by CHE Ph.D. Scholarship, and partially supported by
the Graduate School, Physics and Material Department,
Faculty of Science, Chiang Mai University.
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