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Applied Surface Science 244 (2005) 568–572

Effect of AZO film deposition conditions on the photovoltaic

properties of AZO–Cu2O heterojunctions

Hideki Tanakaa, Takahiro Shimakawab, Toshihiro Miyatab,Hirotoshi Satoa, Tadatsugu Minamib,*

aResearch and Development Center, Gunze Limited, 163 Morikawara, Moriyama, Shiga 524-8501, JapanbOptoelectronic Device System R&D Center, Kanazawa Institute of Technology,

7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, Japan

Received 31 May 2004; accepted 4 October 2004

Available online 8 January 2005

Abstract

This report describes the effect of ZnO:Al (AZO) film deposition conditions on the photovoltaic properties of AZO–Cu2O

heterojunction devices. The devices were fabricated by depositing a transparent conducting AZO thin film on a Cu2O sheet that

functions as the active layer as well as the substrate. The photovoltaic properties of these devices were considerably dependent

on deposition temperature, irrespective of the deposition method used to fabricate them. Maximum efficiencies of 1.2 and 1.0%

measured under AM2 solar illumination were obtained in AZO/Cu2O devices fabricated using AZO films deposited at a

temperature around 200 8C by pulsed laser deposition (PLD) and r.f. magnetron sputtering (r.f.MS), respectively. The

improvement in properties resulting from an increase in the deposition temperature up to about 200 8C is attributed to an

improvement of crystallinity in the AZO films; the degradation resulting from an increase over 250 8C is related to an increase of

resistivity in Cu2O.

# 2004 Elsevier B.V. All rights reserved.

PACS: 72.40+w

Keywords: ZnO; AZO; Cu2O; Photovoltaic; Solar cell

1. Introduction

Cuprous oxide (Cu2O) is an attractive material for

photovoltaic devices because the theoretical energy

* Corresponding author. Tel.: +81 76 294 0695;

fax: +81 76 294 0695.

E-mail address: minami@neptune.kanazawa-it.ac.jp

(T. Minami).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2004.10.121

conversion efficiency of a Cu2O solar cell is on the

order of 20% [1]. Although Cu2O heterojunction solar

cells have been extensively investigated, there are no

reports of any exhibiting an energy conversion

efficiency over 1% because of the difficulty of

fabricating an n-type semiconducting Cu2O [1–4].

In addition, there are many reports on metal–

semiconductor (Schottky barrier contact, SBC) solar

cells using Cu2O as the active layer [1,5–7]: 1.76%,

.

H. Tanaka et al. / Applied Surface Science 244 (2005) 568–572 569

the highest energy conversion efficiency reported in

Cu–Cu2O SBC solar cells fabricated by depositing a

Cu thin film on Cu2O sheets prepared by oxidizing

copper at a high temperature [5]. Nevertheless,

practical applications are difficult to achieve because

the devices are unstable due to the use of Cu thin films

as the transparent electrode. In addition, it has been

widely reported in the literature that it is difficult to

prepare a real heterojunction or real SBC on Cu2O

because a Cu metal thin film can easily be created at

the interface by reducing the Cu2O surface [1,2,8].

In this paper, we describe the relationship between

deposition conditions and the resulting photovoltaic

properties of AZO–Cu2O heterojunction devices

fabricated by depositing a transparent conducting

ZnO:Al (AZO) film on a Cu2O sheet. The AZO–Cu2O

heterojunction solar cells can be fabricated using

inexpensive oxide materials. In addition, the fabrica-

tion of AZO–Cu2O heterojunction solar cells that

exhibits efficiency above 1% is demonstrated.

2. Experimental

Photovoltaic devices were fabricated by forming an

AZO–Cu2O heterojunction on the surface of Cu2O

sheets, functioning as the active layer as well as the

substrate, and an Au ohmic electrode on the back side

of the device. The Cu2O sheets were prepared by

oxidizing approximately 0.2 mm thick Cu sheets

with a 2–3 h heat treatment conducted in air at a

temperature of 1000 8C [1,9]. The prepared Cu2O

sheets were polycrystalline p-type semiconductors

with a hole concentration of approximately 4 �1014 cm�3 and a Hall mobility of approximately

90 cm2/Vs. An AZO thin film (area of 3.14 �10�2 cm2) was deposited on the Cu2O sheets by

pulsed laser deposition (PLD) [10] or r.f. magnetron

sputtering (r.f.MS) [11]. PLD using an ArF excimer

laser (repetition rate, 20 Hz) was carried out under the

following conditions: excitation fluence, 350 mJ/

cm�2; atmosphere, vacuum (below 1.0 � 10�4 Pa)

or O2 gas (0.1–12 Pa); deposition temperature, room

temperature (RT) to 300 8C; target–substrate (T–S)

distance, 40 mm and target, sintered AZO (Al2O3

content of 1 wt.%) pellet. The r.f.MS deposition was

carried out under the following conditions: sputter

pressure, 0.8 Pa; atmosphere, pure Ar; r.f. power,

30 W; deposition temperature, RT to 300 8C; T–S

distance, 30 mm and target, AZO (Al2O3 content of

2 wt.%) powder. The Cu2O substrates in these AZO

film depositions were set parallel to the target surface.

In order to evaluate the electrical and optical

properties of the AZO films, they were also

simultaneously deposited on glass substrates. The

photovoltaic properties of the solar cells were

measured under AM2 solar illumination [12] with a

power of 100 mW/cm2.

3. Results and discussion

It was found that all AZO–Cu2O heterojunction

devices fabricated using AZO thin films deposited by

either PLD or r.f.MS exhibit rectifying current–

voltage (I–V) characteristics, irrespective of the film

deposition conditions. Beyond this basic rectifying

characteristic, however, the obtained I–V character-

istics were dependent on not only the deposition

method used but also the deposition conditions.

Fig. 1(a and b) shows a comparison of I–V

characteristics between AZO/Cu2O heterojunction

devices fabricated using AZO thin films deposited

at various deposition temperatures by PLD and by

r.f.MS, respectively. The I–V characteristics of devices

fabricated with the two different deposition methods

exhibited a similar dependence on the deposition

temperature; in general, increasing the deposition

temperature from RT to 200 8C resulted in improving

the rectifying characteristic and increasing device

resistance. The variation found in the I–V character-

istics can be caused by changes in the electrical

properties of the n-AZO layer and the p-Cu2O layer as

well as the interface region between these layers. The

improvement shown in the rectifying characteristic

may be assigned to a reduction of defects near the

interface between the AZO and Cu2O layers and in the

AZO thin-film layer itself, resulting from an

improvement of crystallinity in the AZO layer. This

improvement of AZO film crystallinity, evidenced

from X-ray diffraction analyses, also correlates to the

increase of Hall mobility of the AZO films seen in

Fig. 2.

Resistivity, carrier concentration and Hall mobility

as functions of the deposition temperature are shown

in Fig. 2(a and b) for AZO thin films deposited on

H. Tanaka et al. / Applied Surface Science 244 (2005) 568–572570

Fig. 1. I–V characteristics of devices fabricated using AZO films deposited at various deposition temperatures by (a) PLD or (b) r.f.MS.

glass substrates by PLD and r.f.MS, respectively. The

resistivity of all these AZO films decreased as the

deposition temperature was increased from RT,

reached a minimum at approximately 200 8C and

then increased as the temperature was increased

further. This result seems to show that the variation of

resistivity relative to the deposition temperature seen

Fig. 2. Resistivity, carrier concentration and Hall mobility as fu

in AZO films cannot be related to the change found in

device resistance described previously. Thus, the

increase of device resistance is mainly attributed to an

increase of resistivity in the Cu2O sheet resulting from

the heating during AZO film deposition. This is also

evidenced by the fact that annealing in vacuum at

temperatures up to 300 8C increased the resistivity of

nctions of the deposition temperature for AZO thin films.

H. Tanaka et al. / Applied Surface Science 244 (2005) 568–572 571

Fig. 3. J–Vand P–V characteristics in devices fabricated using AZO films deposited at various deposition temperatures by (a) PLD or (b) r.f.MS.

Fig. 4. Obtained VOC, JSC, F.F. and h as functions of the deposition

temperature for devices fabricated using AZO films.

Cu2O from approximately 2 � 102 to 1 � 104 V cm;

the hole concentration decreased, but the Hall mobility

was unchanged.

Fig. 3(a and b) shows a comparison of current

density–voltage (J–V) and output power (P)–V

characteristics of the same AZO/Cu2O devices shown

in Fig. 1 fabricated with PLD and r.f.MS, respectively,

and measured under AM2 solar illumination. The

photovoltaic properties of all these devices, while

considerably dependent on the deposition tempera-

ture, exhibited relatively the same behavior in relation

to increases of the deposition temperature. The

obtained open-circuit voltage (VOC), short-circuit

current density (JSC), fill factor (F.F.) and energy

conversion efficiency (h) as functions of the deposi-

tion temperature are shown in Fig. 4(a and b) for AZO/

Cu2O devices fabricated with PLD and r.f.MS,

respectively. As a result of the obtained photovoltaic

properties such as VOC, JSC and F.F. generally

exhibiting a maximum value at a deposition tempera-

ture around 200 8C, irrespective of the deposition

method, the maximum efficiency was obtained in

devices fabricated at temperatures around 200 8C. The

improvement found by increasing the deposition

temperature up to about 200 8C is attributed to the

improvement of AZO film crystallinity; the degrada-

tion seen as the deposition temperature is increased

over 250 8C can be related to the increase of Cu2O

resistivity.

It should be noted that maximum efficiencies of 1.2

and 1.0% were obtained in AZO/Cu2O devices

fabricated using AZO films deposited by PLD and

r.f.MS, respectively. The obtained maximum VOC of

H. Tanaka et al. / Applied Surface Science 244 (2005) 568–572572

approximately 0.4 V may be explained by the work

function of ZnO (approximately 4.5 eV) usually being

smaller than that of Cu2O (approximately 5 eV). The

fact that the obtainable VOC in devices fabricated using

AZO films deposited by PLD was slightly higher than

that in devices fabricated by r.f.MS may be also

explained by the difference in the work function

between these AZO films; AZO films deposited by

PLD should have a smaller work function, resulting

from a larger carrier concentration, than that of AZO

films deposited by r.f.MS, as shown in Fig. 2.

4. Conclusion

The relationship between deposition conditions

and photovoltaic properties was investigated in AZO–

Cu2O heterojunction devices fabricated by depositing

a transparent conducting ZnO:Al (AZO) film on Cu2O

sheets by pulsed laser deposition (PLD) or r.f.

magnetron sputtering (r.f.MS). The photovoltaic

properties were considerably dependent on the

deposition temperature of AZO thin films, irrespective

of the deposition method used. It was found that the

maximum open-circuit voltage, short-circuit current

density and fill factor were obtained in devices

fabricated using AZO films deposited at a temperature

around 200 8C. High energy conversion efficiencies

above 1% measured under AM2 solar illumination

were obtained in devices fabricated using AZO films

deposited at 200 8C. The improvement of photovoltaic

properties resulting from an increase of the deposition

temperature up to about 200 8C is attributed to an

improvement of AZO film crystallinity, whereas the

degradation seen with an increase of deposition

temperature over 250 8C is attributed to an increase

of Cu2O resistivity.

Acknowledgments

The authors wish to acknowledge Mr. T. Arai, Y.

Araki, K. Kawabe, K. Suzuki, E. Iida and G. Sato for

their technical assistance in the experiments.

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