Fabrication and Characterization of Phthalocyanine/C<sub>60</sub> Solar Cells with Inverted Structure

doi:10.4236/aces.2012.24056

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Advances in Chemical Engineering and Science, 2012, 2, 461-464

http://dx.doi.org/10.4236/aces.2012.24056 Published Online October 2012 (http://www.SciRP.org/journal/aces)

Fabrication and Characterization of Phthalocyanine/C60

Solar Cells with Inverted Structure

Kazumi Yoshida1, Takeo Oku1*, Atsushi Suzuki1, Tsuyoshi Akiyama1, Yasuhiro Yamasaki2

1Department of Materials Science, The University of Shiga Prefecture, Hikone, Japan

2Orient Chemical Industries Co. Ltd., Department of New Business, Osaka, Japan

Email: *[email protected]

Received May 31, 2012; revised June 30, 2012; accepted July 10, 2012

ABSTRACT

Photovoltaic and optical properties of fullerene/phthalocyanine heterojunction solar cells with normal and inverted

structures were fabricated and investigated. Aluminum and gallium phthalocyanines were used for the n-type semicon-

ductor. The solar cells with inverted structure had more stability compared to that with normal structure in the air.

Nanostructures of the solar cells were investigated by transmission electron microscopy, and energy levels of the mole-

cules were calculated and discussed.

Keywords: Organic Thin Film Solar Cell; Inverted Structure; Phthalocyanine; Fullerene; PCBM; TiO2; Sol-Gel

1. Introduction

Solar cells are expected to solve problems of environ-

mental pollution and exhaustion of fossil fuel, and de-

velopment and practical use of solar energy are needed.

Organic thin film solar cells have an advantage for re-

newable energy resources because of their low cost,

flexible, light weight and fabricate at low temperatures

by spin-coating and printed method [1-3]. Recently,

polymer /fullerene solar cells have been investigated, and

the conversion efficiency of ~5% was obtained [4-6].

Metal phthalocyanines (MPc) are a group of small

molecules with Q-band absorption in the red to near-IR

range, and they have high optical, light stability, chemi-

cal stability and photovoltaic property. Therefore, they

are used for donor materials of organic thin film solar

cells. The heterojunction solar cells using copper

phthalocyanine and fullerene have been fabricated by

evaporation method, and its conversion efficiency was

~3% [7]. The characteristics such as electron conductiv-

ity and absorption range change by changing a central

metal [8-11].

The inorganic solar cells such as using single crystal

silicon have high stability in air. However, the organic

thin film solar cells with normal structures as shown in

Figure 1 (a), have no stability in air. Al metal has often

been used as the back electrode of the organic solar cells

with normal structures, due to its low work function. The

Al is oxidized to insulator Al2O3 at the Al/organic inter-

face and the diffused Al into the active layer acts as a

recombination site. A acidic poly(3,4-ethylenedioxy-

lenethiophene): poly(4-styrene sulfonic acid) (PEDOT:

PSS) would damage the device performance due to cor-

rosion to indium-tin-oxide (ITO). Both of which make

lifetime of the cell very short. An approach to solve these

problems is to use cells with an inverted structure as

shown in Figure 1(b). The cells with an inverted struc-

ture have a TiO2 layer, which work as electron transport

layer. There are some reports of inverted structure, and

improvement of stability has been reported [12-14].

The purpose of the present work is to fabricate and

characterize heterojunction solar cells with normal and

inverted structures using MPc and fullerene. Gold was

used for the electrode instead of aluminum. TiO2 thin

films were fabricated by sol-gel method, and used as

electron transfer layer. Photovoltaic mechanism, the light

induced charge separation and charge transfer of the so-

lar cells with normal and inverted structures will be dis-

cussed on the basis of light-induced current density volt-

age (J-V) curves, and optical absorption. The energy lev-

PEDOT:PSS

Organic layer

Al (Electrode)

Au (Electrode)

PEDOT:PSS

Organic layer

TiO

ITO ITO

FTO

(a) (b)

Figure 1. Schematic cell structures with (a) normal and (b)

inverted structures.

*Corresponding author.

K. YOSHIDA ET AL.

462

els of the molecules were calculated, and nanostructures

of the solar cells were investigated by transmission elec-

tron microscopy.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

300 400 500 600 700 800

Absorbance

Wavelengh (nm)

GaPcCl/C

Al PcCl/C

2. Experimental Procedures

Solar cells with normal structure were fabricated by the

following process. Indium tin oxide (ITO) grass plates

(Geomatec, ~10 Ω/□) were cleaned by an ultrasonic bath

with acetone and methanol, and were dried by nitrogen

gas. A thin layer of PEDOT:PSS (Sigma Aldrich) was

spin-coated on the ITO substrates. After annealing at

100˚C for 10 min in N2 atmosphere, metal phthalocya-

nine (metal: Al or Ga) and fullerene (C60) layer were

prepared on a PEDOT:PSS layer by vacuum evaporation.

Finally, aluminum (Al) metal contact were evaporated as

a top electrode and annealed at 140˚C for 10 min in N2

atmosphere.

Solar of cells with an inverted structure were fabri-

cated by following process. The TiO2 precursor solutions

were prepared from titanium isopropoxide (TTIP),

2-methoxyethanol and acetylacetone. TTIP (0.46 ml) was

add to 2-methoxyethanol (2.5 ml). After stirred for 1h,

acetylacetone (0.61 ml) as the stabilizer was slowly

added, and stirred for 12h [14]. The TiO2 precursor solu-

tion was spin-coated on fluorine dope tin oxide (FTO)

substrate (Luminescence Technology, ~14 Ω/□). After

annealing at 100˚C for 10 min in N2 atmosphere, solution

of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) in

1 ml chlorobenzene on a TiO2 layer by spin-coat method.

Then, gallium phthalocyanine layer were prepared on a

PCBM layer by evaporation. A PEDOT:PSS was spin-

coated onto the active layer. Gold metal contact were

evaporated as a top electrode and annealed at 140˚C for

10 min in N2 atmosphere.

Current density-voltage (J-V) characteristics (Hokuto

Denko Co. Ltd., HSV-100) of the solar cells were meas-

ured both in the dark and under illumination at 100 mW/

cm2 by using an AM 1.5 solar simulator (San-ei Electric,

XES-301S). The solar cells were illuminated through the

side of the ITO substrates, and the illuminated areas were

0.16 cm2. Optical absorption of the solar cells was inves-

tigated by means of UV-visible spectroscopy (JASCO,

V-670ST). Transmission electron microscope (TEM) ob-

servation was carried out by a 200 kV TEM (Hitachi

H-8100). The molecular structures were optimized by CS

Chem3D (Cambridge Soft) and molecular orbital calcu-

lations using Gaussian 03.

3. Results and Discussion

Figure 2 shows UV-visible absorption spectra of AlPc/

C60 and GaPc/C60 heterojunction solar cells. The meas-

urement region is in the range from 300 to 800 nm. The

optical absorption at 350 nm corresponds to of Soret

Figure 2. UV-vis absorption spectra of GaPc/C60 and

AlPc/C60 thin films.

band of Pc. Absorption in the range of 600 - 700 nm and

630 - 700 nm correspond to Q-band for AlPc and GaPc,

respectively. Absorption at ~400 nm is PCBM. Since the

absorption was observed in the whole region, it is con-

sidered that the sunlight is efficiently absorbable. Meas-

ured J-V characteristic parameters of heterojunction solar

cells with a normal structure under illumination are shown

in Table 1. A solar cell with GaPc/C60 structure provided

a power convergent efficiency (η) of 7.9 × 10–3%, fill

factor (FF) of 0.22, open circuit voltage (VOC) of 0.30 V,

and short-circuit current (JSC) of 0.12 mA/cm2, which is

better than those of an AlPc/C60 device. These solar cells

with a normal structure provided a conversion efficiency

of 0% after 24 h. Ta ble 2 shows GaPc/PCBM solar cells

with an inverted structure have more stability in air than

that with a normal structure. Since PEDOT:PSS would

prevented oxygen diffusion into active layers, active lay-

ers did not oxidized.

Figures 3(a)-(c) show TEM image, electron diffrac-

tion pattern and high-resolution image of TiO2 thin films,

respectively. The particle size of TiO2 is 20 - 50 nm from

the TEM image, and the electron diffraction pattern and

high resolution image show formation of TiO2 anatase

structure by annealing at 450˚C.

Table 1. Experimental parameters of MPc/C60 solar cells

with normal structure.

Sample VOC (V) JSC (mA/cm2) FF η (%)

GaPc/C60 0.30 0.12 0.22 7.9 × 10–3

AlPc/C60 0.26 0.0030 0.23 1.8 × 10–4

Table 2. Experimental parameters of GaPc/PCBM solar

cells with inverted structure.

Sample VOC (V) JSC (mA/cm2) FF η (%)

GaPc/PCBM 0.56 0.44 0.240.059

After 2 months0.64 0.25 0.210.033

K. YOSHIDA ET AL. 463

TiO

101

000

103

202

30 nm

105

5nm

Figure 3. (a) TEM image; (b) Electron diffraction pattern;

and (c) High-resolution image of TiO2 thin films.

An energy level diagram of the heterojunction solar

cells with normal and inverted structures were summa-

rized as shown in Figure 4. Previously reported values

were used for the energy levels of the figures by adjust-

ing to the present work [15-17]. Energy barrier would

exist near the semiconductor/metal interface. In the cells

with a normal structure, electronic charge is transferred

by light irradiation from the ITO or FTO substrate side,

and electrons are transported to an Al electrode, and

holes are transported to an ITO substrate. In the cells

with an inverted structure, electrons are transported to an

FTO substrate, and holes are transported to an Al. When

C60 is used for inverted structure, the energy barrier

would be at the TiO2/C60 interface. To reduce the energy

barrier, PCBM with higher LUMO levels is suitable. Voc

of organic solar cells is related with energy gap between

HOMO of MPc and LUMO of C60 or PCBM, and control

of the energy levels is important to improve the photo-

voltaic performance [15].

-4.8eV

-5.0eV hν

-4.3eV

-4.5eV

-6.2eV

PEDOT

:PSS

ITO MPc C

(a)

TiO

FTO PEDOT:

PSS

PCBM MPc

(b)

-4.8eV

-4.4eV -5.0eV

-3.8eV

-4.2eV

-7.5eV

-6.0eV

hν

Figure 4. Energy level diagram of solar cells with (a) nor-

mal and (b) inverted structures.

4. Conclusion

Phthalocyanine/fullerenen heterojunction solar cells with

normal and inverted structures were fabricated and char-

acterized. A device with inverted cell using GaPc/PCBM

provided Voc of 0.56 V, Jsc of 0.44 mA/cm2, FF of 0.24,

and η of 0.059%. The solar cell with an inverted struc-

ture has more stability in the air than that of a normal

structure. TEM image, electron diffraction, and high-

resolution image confirmed TiO2 formed anatase struc-

tures and polycrystalline. A carrier mechanism of solar

cells with normal and inverted structures was discussed

based on energy diagram.

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