The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells

doi:10.4236/msa.2011.212226

Paper Menu >>

Journal Menu >>

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.

E-mail: *[email protected]

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

-0.5 -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

Metal

Oxide

Time

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.

REFERENCES

[1] J.-C. Wang, W.-T. Weng, M.-Y. Tsai, M.-K. Lee, S.-F.

Horng, T.-P. Perng, C.-C. Kei, C.-C. Yuc and H.-F.

Mengd, “Highly Efficient Flexible Inverted Organic Solar

Cells Using Atomic Layer Deposited ZnO as Electron

Selective Layer,” Journal of Materials Chemistry, Vol.

20, No. 29, 2010, pp. 862-866. doi:10.1039/b921396a

[2] S. K. Hau, H.-L. Yip, H. Ma and A. K.-Y. Jen, “High

Performance Ambient Processed Inverted Polymer Solar

Cells through Interfacial Modification with a Fullerene

Self-Assembled Monolayer,” Applied Physics Letters,

Vol. 93, No. 23, 2008, pp. 233-304.

doi:10.1063/1.3028094

[3] F. Zhang , X. Xu, W. Tang, J. Zhang, Z. Zhuo, J. Wang, J.

Wanga, Z. Xu and Y. Wanga, “Recent Development of

the Inverted Configuration Organic Solar Cells,” Solar

Energy Materials and Solar Cells, Vol. 95, No. 7, 2011,

pp. 2758-2761. doi:10.1016/j.solmat.2011.02.002

[4] J.-S. Huang, C.-Y. Chou, M.-Y. Liu, K.-H. Tsai, W.-H.

Lin and C.-F. Lin, “Solution-Processed Vanadium Oxide

as an Anode Interlayer for Inverted Polymer Solar Cells

Hybridized with ZnO Nanorods,” Organic Electronics,

Vol. 10, No. 3, 2009, pp. 1060-1065.

doi:10.1016/j.orgel.2009.05.017

[5] W.-H. Baek, I. Seo, T.-S. Yoon, H. H. Lee, C. M. Yun

and Y.-S. Kim, “Hybrid Inverted Bulk Heterojuntion So-

lar Cells with Nanoimprinted TiO2 Nanopores,” Solar

Energy Materials and Solar Cells, Vol. 93, No. 9, 2009,

pp. 1587-1591. doi:10.1016/j.solmat.2009.04.014

[6] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray

and L. Yu, “For the Bright Future-Bulk Heterojunction

Polymer Solar Cells Eith Power Conversion Efficiency of

7.4%,” Advanced Material, Vol. 22, No. 20, 2010, pp. 1-4.

doi:10.1002/adma.200903528

[7] A. K. K. Kyaw, X. W. Sun, C. Y. Jiang, G. Q. Lo, D. W.

Zhao and D. L. Kwong, “An Inverted Organic Solar Cell

Employing a Sol-Gel Derived ZnO Electron Selective

Layer and Thermal Evaporated MoO3 Hole Selective

Layer,” Applied Physics Letters, Vol. 93, No. 22, 2008,

Article ID: 221107. doi:10.1063/1.3039076

[8] M. Jørgensen, K. Norrman and F. C. Krebs, “Stability/

Degradation of Polymer Solar Cells,” Solar Energy Mate-

rials and Solar Cells, Vol. 92, No. 7, 2008, pp. 686-714.

doi:10.1016/j.solmat.2008.01.005

[9] K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D. D. C.

Bradley and J. R. Durrant, “Degrasation of Organic Solar

Cells Due to Air Exposure,” Solar Energy Materials and

Solar Cells, Vol. 90, No. 20, 2006, pp. 3520-3530.

doi:10.1016/j.solmat.2006.06.041

[10] F. C. Krebs, J. E. Carle, N. Cruys-Bagger, M. Andersen,

M. R. Lilliedal, M. A. Hammond and S. Hvidt, “Life-

times of Organic Photovoltaics: Photochemistry, Atmos-

phere Effects and Barrier Layers in ITO-MEHPPV,” So-

lar Energy Materials and Solar Cells, Vol. 86, No. 4,

2005, pp. 499-516. doi:10.1016/j.solmat.2004.09.002

[11] J. Boucle, P. Ravirajanac and J. Nelson, “Hybrid Poly-

mermetal Oxide Thin Films for Photovoltaic Applica-

tions,” Journal of Materials Chemistry, Vol. 17, No. 30,

2007, pp. 3141-3153. doi:10.1039/b706547g

[12] S. Yodyingyong, X. Zhou, Q. Zhang, D. Triampo, B.

Limketkai and G. Z. Cao, “Enhanced Photovoltaic Per-

formance of Nanostructured Hybrid Solar Cell Using

Highly Oriented TiO2 Nanorods,” Journal of Physics

Chemistry, Vol. 114, 2010, pp. 21851-21855.

[13] R. Zhu, C. Y. Jiang, B. Liu and S. Ramakrishna, “Hyghly

Efficient Nanoporous TiO2-Polythiophene Hybrid Solar

Cells Based on Interfacial Modification Using a Metal-

Free Organic Dye,” Advanced Materials, Vol. 21, No. 9,

2009, pp. 994-1000. doi:10.1002/adma.200802388

[14] J.-S. Huang, C.-Y. Chou and W.-H. Lin, “Enhancing Per-

formance of Organic-Inorganic Hybrid Solar Cells Using

a Fullerene Interlayer from All-Solution Processing,” So-

lar Energy Materials and Solar Cells, Vol. 94, No. 10,

2010, pp. 182-186. doi:10.1016/j.solmat.2009.08.019

[15] M. N. Shan, S. S. Wang, Z. Q. Bian, J. P. Liu and Y. L.

Zhao, “Hybrid Inverted Organic Photovoltaic Cells Based

on Nanoporous TiO2 Films and Organic Small Mole-

cules,” Solar Energy Materials and Solar Cells, Vol. 93,

No. 9, 2009, pp. 1613-1617.

doi:10.1016/j.solmat.2009.04.017

[16] T. Yang, W. Cai, D. Qin, E. Wang, L. Lan, X. Gong, J.

The Role of Oxide Thin Layer in Inverted Structure Polymer Solar Cells

1701

Peng and Y. Cao, “Solution-Processed Zinc Oxide Thin

Film as a Buffer Layer for Polymer Solar Cells with an

Inverted Device Structure,” Journal of Physics Chemistry,

Vol. C114, 2010, pp. 6849-6853.