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Thin Solid Films 469–4

Electrical and optical properties of TCO–Cu2O heterojunction devices

Hideki Tanakaa,b, Takahiro Shimakawaa, Toshihiro Miyataa, Hirotoshi Satob,

Tadatsugu Minamia,*

aOptoelectronic Device System R&D Center, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, JapanbResearch and Development Center, Gunze Limited, 163 Morikawara, Moriyama, Shiga 524-8501, Japan

Available online 11 September 2004

Abstract

This report describes the electrical and photovoltaic properties in heterojunction devices consisting of a cuprous oxide (Cu2O) sheet

and a transparent conducting oxide (TCO) thin film, such as In2O3, ZnO, In2O3:Sn (ITO), ZnO:Al (AZO) or AZO–ITO (AZITO)

multicomponent oxide, prepared by pulsed laser deposition (PLD). Undoped In2O3–Cu2O heterojunctions prepared by PLD exhibited

ohmic current–voltage (I–V) characteristics. The ZnO–Cu2O and AZO–Cu2O devices exhibited better rectifying I–V characteristics and

photovoltaic properties than the ITO–Cu2O devices. It was found that the obtainable I–V characteristics and photovoltaic properties were

considerably affected by the TCO film deposition conditions. An open-circuit voltage (VOC) of 0.4 V, a short-circuit current density ( JSC)

of 7.1 mA/cm2, a fill factor (F.F.) of 0.4 and an energy conversion efficiency (g) of 1.2% were obtained in an AZO–Cu2O device under

AM2 solar illumination. The VOC, JSC, F.F. and g obtained in AZITO–Cu2O heterojunctions increased as the Zn/(Zn+In) atomic ratio was

increased.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Cu2O; Cuprous oxide; Pulsed laser deposition; Solar cell; Heterojunction; Transparent conducting oxide; Thin films; Oxide semiconductor; AZO;

ZnO; Multicomponent oxide

1. Introduction

Cuprous oxide (Cu2O) is an attractive material for

photovoltaic devices because it is a direct-gap semi-

conductor with a bandgap energy of 2.0 eV [1,2]. There

are many reports on solar cells based on a thick Cu2O

sheet [1–6] with a thin film [7,8] prepared by various

techniques. For example, various metal-Cu2O solar cells

consisting of a Cu2O sheet and a metal element such as

Cu, Yb, Mg, Mn, Al and Tl have been reported [1–4,6,9].

In addition, transparent conducting oxide (TCO)–Cu2O

heterojunction solar cells consisting of a combination of a

Cu2O thin film or sheet with a TCO thin film, such as

ZnO, In2O3, SnO2 or CdO, have been prepared by various

deposition methods such as magnetron sputtering

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.06.180

* Corresponding author. Tel./fax: +81 76 294 0695.

E-mail address: minami@neptune.kanazawa-it.ac.jp (T. Minami).

[1,5,10,11]. Although the theoretical energy conversion

efficiency of a Cu2O solar cell is on the order of 20%

under AM1 solar illumination, with the exception of Cu–

Cu2O solar cells (maximum efficiency (g) of 1.76%), an gover 1% has yet to be attained in any reported Cu2O solar

cells [1]. The g obtained in solar cells fabricated using

Cu2O sheets prepared by oxidizing copper at a high

temperature was always found to be higher than that

obtained when using Cu2O thin films [1,8]. The above

reports suggest that the photovoltaic properties of Cu2O

solar cells are significantly affected by the surface treat-

ment and the crystallinity of Cu2O [1,3,5]. In particular,

the deposition method and conditions are important when

depositing a thin film on Cu2O sheets.

In this paper, we describe the electrical and optical

properties of photovoltaic heterojunction devices fabricated

by depositing various TCO thin films on a Cu2O sheet using

a pulsed laser deposition (PLD) method. An energy

conversion efficiency above 1% was obtained in an Al-

70 (2004) 80–85

Fig. 1. I–V characteristics of IO– and ITO–Cu2O heterojunctions.

H. Tanaka et al. / Thin Solid Films 469–470 (2004) 80–85 81

doped ZnO (AZO)–Cu2O heterojunction solar cell under

AM2 solar illumination.

Fig. 2. J–V and P–V characteristics of an ITO–Cu2O solar cell under AM2

illumination.

2. Experimental

Solar cells were fabricated by forming TCO–Cu2O

heterojunctions on the surface of Cu2O sheets with an Au

ohmic back side electrode; the Cu2O sheets function as the

active layer as well as the substrate in the devices. N-type

semiconducting TCO films such as In2O3, ZnO, In2O3:Sn

(ITO), ZnO:Al (AZO) or ZnO–In2O3 (ZIO) multicompo-

nent oxides [12] were deposited by PLD using an ArF

excimer laser. The ArF excimer laser (193 nm) beam

(repetition rate, 20 Hz; pulse width, 20 ns; and fluence,

350 mJ/cm2) was focused onto a rotating target using a

lens. The deposition was carried out under the following

conditions: atmosphere, vacuum (below 1.0�10�4 Pa) or

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

ature to 300 8C; target–substrate (T–S) distance, 40 mm;

and target, sintered oxide pellet. The oxide pellet targets

were prepared by cold pressing a mixture of oxide

powders followed by sintering in air at 1300 to 1400

8C. The ITO and AZO pellets were prepared by sintering

a mixture of In2O3 and SnO2 (5 wt.%) powders and a

mixture of ZnO and Al2O3 (2 wt.%), respectively. In order

to evaluate the electrical and optical properties of the TCO

films, they were also simultaneously deposited on OA-2

glass (Nippon Electric Glass) substrates. The Cu2O sheets

were prepared by oxidizing approximately 0.2-mm-thick

copper (Cu) sheets with a heat treatment carried out in air

at a temperature of 1000 8C for 2–3 h [1,13]. The

prepared Cu2O sheets were a polycrystalline p-type

semiconductor with a hole concentration of approximately

4�1014 cm�3 and a Hall mobility of approximately 90

cm2/V s. The photovoltaic properties of the solar cells

were measured under AM2 solar illumination with a

power of 100 mW/cm2.

3. Results and discussion

3.1. In2O3–Cu2O heterojunctions

Fig. 1 shows current–voltage (I–V) characteristics of

heterojunctions prepared by depositing various In2O3 thin

films (area of 3.14�10�2 cm2) on Cu2O sheets and

measured without illumination. The I–V characteristics of

undoped In2O3(IO)–Cu2O heterojunctions using IO films

deposited at an O2 pressure of 5 and 12 Pa and a deposition

temperature of 150 8C are shown in (a) and (b), respectively:

their resistivities (carrier concentrations) were 2.3�10�3

(1.1�1020) and 2.1 V cm (2.7�1018 cm�3), respectively.

Fig. 1c shows the I–V characteristic of an Sn-doped In2O3

(ITO)–Cu2O heterojunction using an ITO film deposited at

an O2 pressure of 0.1 Pa and a temperature of 150 8C using

an ITO pellet. The resistivity and carrier concentration of the

ITO film were 2.9�10�4 V cm and 5.9�1020 cm�3,

respectively. It should be noted that the IO–Cu2O junctions

exhibited only an ohmic I–V characteristic as well as no

photovoltage; not even the ITO–Cu2O heterojunction

exhibited a good rectifying I–V characteristic. Fig. 2 shows

typical output power–voltage (P–V) and current density

( J)–V characteristics of an ITO–Cu2O heterojunction cell

under AM2 solar illumination. This device exhibited poor

photovoltaic properties: open-circuit voltage (VOC) of 0.034

V, short-circuit current density ( JSC) of 3.7 mA/cm2, fill

factor (F.F.) of 0.26 and g of 0.03% under AM2 solar

illumination. Only poor rectifying I–V characteristics and

photovoltaic properties were obtained in all ITO–Cu2O

heterojunctions deposited by the PLD method used in this

work.

On the contrary, many photovoltaic cells have been

previously demonstrated using heterojunctions prepared by

depositing an ITO thin film on Cu2O substrates [1,5] or by

depositing a Cu2O film on ITO coated glass substrates [7,8].

The photovoltaic properties and I–V characteristics of these

previously reported heterojunctions were considerably

Fig. 3. O2 pressure dependence of I–V characteristics for ZO–Cu2O

heterojunctions.

H. Tanaka et al. / Thin Solid Films 469–470 (2004) 80–8582

dependent on the deposition conditions as well as the

deposition methods of the ITO films. In addition, it also has

been reported that the degradation in photovoltaic properties

of CuInSe2-based solar cells is caused by plasma irradiation

and/or particle bombardment when TCO films are deposited

by magnetron sputtering, the most popular TCO film

deposition method [14,15]. In contrast, a PLD method that

is free of particle bombardment could result in less damage

during deposition than d.c. and/or r.f. magnetron sputtering.

It is also well known that the electrical and optical properties

of TCO thin films are affected by not only the deposition

conditions but also the deposition methods of the TCO films

[16]. In addition to damage-free depositions, various TCO

films recently prepared by the PLD method, such as ITO

and AZO, featured a low resistivity on the order of 10�5 V

cm [17,18]. However, IO– or ITO–Cu2O heterojunctions

prepared by depositing In2O3 films on Cu2O sheets by PLD

have yet to be reported, so far as we know.

As mentioned above, however, it has been previously

reported that ITO–Cu2O heterojunctions prepared using ITO

films deposited by other methods such as sputtering exhibit

good rectifying I–V characteristics [1,5,7,8]. Therefore, the

fact that heterojunctions consisting of undoped In2O3 or ITO

films deposited on p-type semiconducting Cu2O by PLD do

Fig. 4. J–V and P–V characteristics of the ZO–Cu2O

not exhibit a good rectifying I–V characteristic may be

explained by the low level of damage occurring during the

PLD film depositions. The ohmic I–V characteristics

obtained in IO–Cu2O heterojunctions, as shown in Fig. 1,

may be attributed to the fact that the work function of

undoped In2O3 (approximately 4.8 to 5 eV) is usually larger

than that of ITO [19]. It has been reported that the work

function of TCO films with a carrier concentration was

increased [19,20]. The smaller work function of ITO relative

to that of IO may be attributed to a larger Fermi energy

resulting from the carrier concentration of ITO films being

higher than that of the IO films used, as described above.

3.2. ZnO–Cu2O heterojunctions

All ZnO–Cu2O heterojunctions prepared by depositing

undoped ZnO(ZO) and AZO films on Cu2O sheets exhibited

rectifying I–V characteristics, irrespective of the deposition

conditions. For example, Fig. 3 shows the I–V character-

istics of ZO–Cu2O heterojunctions prepared using ZO thin

films (area of 3.14�10�2 cm2) deposited at different O2

pressures and measured without illumination. The ZO–

Cu2O devices shown in (a), (b) and (c) were prepared with

ZO films deposited at a temperature of 150 8C and an O2

pressure of 1, 5 and 12 Pa, respectively; the resistivities

(carrier concentrations) were 1.3�10�2 (1.3�1019),

2.6�10�1 (3.8�1018) and 6.2 V cm (1.0�1018 cm�3),

respectively. The increase of resistivity was attributed to

decreases of both carrier concentration and Hall mobility. As

can be seen in Fig. 3, the obtained rectifying I–V character-

istics improved as the O2 pressure was increased. Fig. 4

shows the photovoltaic properties of the ZO–Cu2O devices

shown in Fig. 3. It should be noted that the photovoltaic

properties measured under AM2 solar illumination also

improved as the O2 pressure was increased. In addition, a

high efficiency of 0.9% was obtained in a ZO–Cu2O

heterojunction cell prepared with a high resistivity ZnO

film.

Herion et al. [10] have reported an efficiency of 0.21% in

a ZnO–Cu2O heterojunction solar cell prepared using an

undoped ZnO thin film deposited by sputtering. The

photovoltaic property of the device was attributed to ZnO

reducing Cu2O to copper at the boundary [1,10]. Never-

solar cells shown in Fig. 3: AM2 illumination.

Fig. 6. Electrical properties of Cu2O sheets as a function of heat treatment

temperature.

H. Tanaka et al. / Thin Solid Films 469–470 (2004) 80–85 83

theless, the O2 pressure dependence of the ZO–Cu2O

devices described above is unlikely to have been a result

of the ZO film deposition reducing the Cu2O to copper at

the boundary.

The photovoltaic properties of AZO–Cu2O heterojunc-

tion cells had not been previously reported, to the best of our

knowledge. Fig. 5 shows typical I–V characteristics of

AZO–Cu2O heterojunctions prepared with AZO thin films

(area of 3.14�10�2 cm2) deposited on Cu2O sheets at a

temperature of RT, 150 and 300 8C and measured without

illumination. The AZO films were deposited in vacuum

using an AZO pellet. Resistivities (carrier concentrations) of

the AZO films deposited at RT, 150, 200 and 300 8C were

1.0�10�3 (6.6�1020), 3.2�10�4 (8.1�1020), 1.9�10�4

(6.9�1020) and 4.2�10�4 V cm (6.5�1020 cm�3), respec-

tively. The deposition temperature dependence of resistivity

of the AZO films is mainly related to that of Hall mobility;

the improvement of mobility is attributed to the fact that

crystallinity improved as the temperature was increased. As

can be seen in Fig. 5, the rectifying I–V characteristics of the

AZO–Cu2O devices were also affected by the deposition

temperature. The series resistance of devices increased

markedly as the deposition temperature was increased. This

was caused by the resistivity increase within the Cu2O

sheets since the resistivity of AZO films showed a tendency

to decrease as the deposition temperature was increased, as

described above. In addition, the resistivity of Cu2O sheets

was found to have increased after they were heated in

vacuum to a temperature in the range from about 150 to 300

8C, as shown in Fig. 6. The resistivity (q), Hall mobility (l)and carrier concentration (n) indicated in Fig. 6 were

measured at RT after the Cu2O sheets were heated to each

treatment temperature for 75 min. The initial resistivity of

all Cu2O sheets used was on the order of 2�10�2 V cm. The

increase in resistivity is mainly attributed to a decrease of

carrier concentration. As a result, the photovoltaic properties

of AZO–Cu2O devices were considerably affected by the

Fig. 5. Deposition temperature dependence of I–V characteristics for AZO–

Cu2O heterojunctions.

deposition temperature of AZO films, as shown in Fig. 7.

The degradation of photovoltaic properties of AZO–Cu2O

devices fabricated with an AZO film deposited at 300 8C is

Fig. 7. VOC, JSC, F.F. and g as functions of deposition temperature for

AZO–Cu2O solar cells under AM2 illumination.

H. Tanaka et al. / Thin Solid Films 469–470 (2004) 80–8584

ascribed to the marked increase of resistivity of the Cu2O

sheet.

It should be noted that the maximum VOC of 0.4 V, JSC of

7.1 mA/cm2, F.F. of 0.44 and g of 1.2% were obtained in

AZO–Cu2O devices prepared using AZO films deposited at

a deposition temperature of 150 to 200 8C and measured

under AM2 solar illumination. In comparison with In2O3–

Cu2O heterojunctions, the better rectifying I–V character-

istics and photovoltaic properties obtained in ZO– and

AZO–Cu2O heterojunctions may be attributed to the differ-

ence of work function between ZnO and In2O3; the work

function of ZnO is slightly smaller than that of In2O3

[19,20], as described above.

3.3. ZIO–Cu2O heterojunctions

As can be seen in Fig. 7, when heterojunctions are

prepared by depositing thin films on the surface of Cu2O, it

is necessary to conduct the depositions at temperatures

below 300 8C. Recently, TCO thin films using multi-

component oxides composed of ZnO and In2O3 have been

newly developed [20]. In particular, in ZnO–In2O3 (ZIO)

films with a Zn content (Zn/(Zn+In) atomic ratio) in the

range from 10 to 20 at.%, a low resistivity can be obtained

Fig. 8. VOC, JSC, F.F. and g as functions of Zn content for AZITO–Cu2O

solar cells under AM2 illumination.

even in a low temperature deposition [21,22]. Thus, we

prepared ZIO–Cu2O and AZITO–Cu2O heterojunctions by

depositing ZnO–In2O3 and AZO–ITO (AZITO) multicom-

ponent oxide thin films, respectively, on Cu2O sheets. As an

example, photovoltaic properties as functions of the Zn

content in deposited films are shown in Fig. 8 for AZITO–

Cu2O heterojunctions measured under AM2 solar illumina-

tion. The AZITO thin films were deposited with an O2

pressure of 0.1 Pa at a deposition temperature of 150 8Cusing AZITO pellets. The AZITO pellets were prepared by

sintering a powder mixture of SnO2 (5 wt.%) added to In2O3

and Al2O3 (1 wt.%) added to ZnO. The resistivity of AZITO

films showed a tendency to increase as the Zn content was

increased up to about 80 at.% before decreasing with a

further increase of the Zn content. In contrast, the VOC, JSC,

F.F. and g obtained in AZITO–Cu2O heterojunctions

increased as the Zn content was increased. The Zn content

dependence of the electrical properties of AZITO films was

not correlated to that of the photovoltaic properties obtained

in AZITO–Cu2O devices. Nevertheless, it should be noted

that the increase in VOC as the Zn content was increased

from 0 to 100 at.% suggests that the Zn content dependence

of the photovoltaic properties in AZITO–Cu2O devices is

mainly attributable to that of the work function of AZITO

films.

4. Conclusions

Various transparent conducting oxide (TCO)–cuprous

oxide (Cu2O) heterojunctions were prepared by depositing

TCO thin films on Cu2O sheets. The electrical and

photovoltaic properties were measured on these hetero-

junction devices prepared by a pulsed laser deposition

(PLD) method with TCO films such as In2O3, ZnO,

In2O3:Sn (ITO), ZnO:Al (AZO) and AZO–ITO (AZITO)

multicomponent oxides. Neither a good rectifying current–

voltage (I–V) characteristic nor a high photovoltage were

obtained in the In2O3–Cu2O and ITO–Cu2O devices. The

ZnO–Cu2O and AZO–Cu2O devices exhibited better I–V

characteristics and photovoltaic properties than the ITO–

Cu2O devices. This improvement may be attributed to the

difference of the work function between ZnO and In2O3. It

was found that the obtainable I–V characteristics and

photovoltaic properties were considerably affected by the

TCO film deposition conditions. An open-circuit voltage

of 0.4 V, a short-circuit current density of 7.1 mA/cm2, a

fill factor of 0.4 and an energy conversion efficiency of

1.2% were obtained in an AZO–Cu2O device prepared

with an AZO film deposited at 150 8C and measured under

AM2 solar illumination. The I–V characteristics and

photovoltaic properties in AZITO–Cu2O heterojunctions

changed as the Zn content was varied. The Zn content

dependence of the photovoltaic properties in AZITO–Cu2O

devices is mainly attributed to that of the work function of

AZITO films.

H. Tanaka et al. / Thin Solid Films 469–470 (2004) 80–85 85

Acknowledgments

The authors wish to acknowledge Mr. K. Suzuki, E. Iida

and G. Sato for their technical assistance in the experiments.

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