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Solar Energy Materials & Solar Cells 93 (2009) 1248–1255


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Solar Energy Materials & Solar Cells 93 (2009) 1248–1255

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Solar Energy Materials & Solar Cells

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Characteristics of ?exible indium tin oxide electrode grown by continuous roll-to-roll sputtering process for ?exible organic solar cells
Kwang-Hyuk Choi a,c, Jin-A Jeong b, Jae-Wook Kang c, Do-Guen Kim c, Jong Kuk Kim c, Seok-In Na d, Dong-Yu Kim d, Seok-Soon Kim e, Han-Ki Kim b,?
School of Advanced Materials and Systems Engineering, Kumoh National Institute of Technology (KIT), Gumi 730-701, South Korea Department of Display Materials Engineering, Kyung Hee University, 1 Seocheon-dong, Yongin-si, Gyeonggi-do 446-701, South Korea c Department of Material Processing, Korea Institute of Material Science (KIMS), 66 Sangnam-dong, Changwon-si, Gyeongnam 641-831, South Korea d Heeger Center for Advanced Materials, Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, 1 Oryoung-dong, Gwangju 500-712, South Korea e School of Materials Science & Chemical Engineering, Kunsan Natioanl Unviersity, Kunsan, Chonbuk 753-701, South Korea
b a

a r t i c l e in f o
Article history: Received 3 July 2008 Received in revised form 17 January 2009 Accepted 23 January 2009 Available online 23 February 2009 Keywords: Indium tin oxide Roll-to-roll sputter PET Organic solar cells Electrode

a b s t r a c t
The preparation and characteristics of ?exible indium tin oxide (ITO) electrodes grown on polyethylene terephthalate (PET) substrates using a specially designed roll-to-roll sputtering system for use in ?exible organic solar cells are described. It was found that both electrical and optical properties of the ?exible ITO electrode were critically dependent on the Ar/O2 ?ow ratio in the continuous roll-to-roll sputter process. In spite of the low substrate temperature (o50 1C), we can obtain the ?exible ITO electrode with a sheet resistance of 47.4 O/square and an average optical transmittance of 83.46% in the green region of 500–550 nm wavelength. Both X-ray diffraction and ?eld emission scanning electron microscopy analysis results showed that all ?exible ITO electrodes grown on the PET substrate were amorphous with a very smooth and featureless surface, regardless of the Ar/O2 ?ow ratio due to the low substrate temperature, which is maintained by a cooling drum. In addition, the ?exible ITO electrode grown on the Ar ion-beam-treated PET substrates showed more stable mechanical properties than the ?exible ITO electrode grown on the wet-cleaned PET substrates, due to an increased adhesion between the ?exible ITO and the PET substrates. Furthermore, the ?exible organic solar cell fabricated on the rollto-roll sputter-grown ?exible ITO electrode at an optimized condition exhibited a power conversion ef?ciency of 1.88%. This indicates that the roll-to-roll sputtering technique is a promising continuous sputtering process in preparing ?exible transparent electrodes for ?exible solar cells or displays. & 2009 Elsevier B.V. All rights reserved.

1. Introduction There is currently considerable interest in ?exible organic solar cells for use as a new generation of energy storage devices due to their light weight, robust pro?le, ability to ?ex, curve, roll and fold for portability, as well as their easy fabrication using roll-to-rollbased thin ?lm technologies [1–7]. These ?exible organic solar cells could offer the possibility for the low cost fabrication of large area solar cells for harvesting energy from sunlight. To fabricate high performance and low cost ?exible organic solar cells, it is necessary to prepare transparent conducting oxide (TCO) with low resistance, high transmittance, superior ?exibility, and a smooth surface on polymer substrates, such as polyethylene terephthalate (PET), polycarbonate (PC), polyimide polyethersulfone (PES), or polyethylene naphthalate (PEN) for effective extraction of hole carriers from active organic layers [8–12]. In addition, the roll-to-

? Corresponding author. Tel.: +82 31 201 2419; fax: +82 31 204 8114.

E-mail address: imdlhkkim@khu.ac.kr (H.-K. Kim). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.01.015

roll-based continuous TCO deposition process should be employed to deposit the ?exible TCO electrode for low cost and the mass production of ?exible solar cells [13,14]. Until now, amorphous indium tin oxide (ITO) ?lms grown by batch-type direct current (DC) or radio-frequency (RF) sputtering have been widely used as the anode layer in ?exible organic solar cells [2–4,15,16]. Although ITO electrode is widely used in OPVs as a TCO material, the high cost of ITO materials makes it dif?cult to envisage very high volume OPV production. Furthermore, considering roll-type polymer substrates, the batch-type DC or RF sputtering process is unsuitable due to dif?culty of the continuous roll-to-roll process. For those reasons, the roll-to-roll sputtering technique has been the subject of considerable attention as a continuous TCO deposition process in the mass production of ?exible organic solar cells. Although the electrical and optical properties of an ITO electrode deposited on various polymer substrates using the batch-type DC or RF sputtering have been reported, the characteristics of a ?exible ITO electrode grown by the roll-to-roll sputtering process have not been investigated in detail [6,16–18].

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In this work, we investigated electrical, optical, structural, mechanical, and the surface properties of ?exible ITO electrodes grown on a PET substrate using a specially designed roll-to-roll sputtering system for ?exible solar cells. It was found that the resistivity and transmittance of ?exible ITO electrodes are critically in?uenced by the Ar/O2 ?ow ratio during continuous roll-to-roll sputtering. At an optimized Ar/O2 ?ow ratio, DC power, working pressure, and rolling speed, we can obtain a ?exible ITO electrode with a resistivity of 9.5 ? 10?4 O cm, average transmittance of $83.46%, and ?gure of merit value of about 3.4 ? 10?3 O?1, despite its preparation at room temperature. In addition, the effective surface treatment of the PET substrate using an Ar ion beam enables us to make mechanically stable ITO electrode with good robustness, due to an improved adhesion between the ITO and the PET substrate. Furthermore, we show the integration of a ?exible ITO electrode with ?exible organic solar cells, which demonstrate the possibility of the ITO ?lms grown by continuous roll-to-roll sputtering as a ?exible anode layer in the ?exible organic solar cells.

2. Experimental The ?exible ITO electrode with a thickness of 200 nm was deposited on a ?exible PET substrate using a specially designed roll-to-roll sputtering system. Fig. 1 shows the schematics of the roll-to-roll sputtering system equipped with a rewind roller, unwind roller, cooling drum, and a cold cathode-type ion gun system for the deposition of high-quality ?exible ITO electrodes. The 200 mm wide PET substrate with a thickness of 188 mm was passed repeatedly over the cooling drum by motion of unwind and rewind roller for the deposition of ITO electrodes during the rollto-roll sputtering process. The rolling speed of the PET substrate can be exactly controlled by the motor speed for unwind and rewind roller. The ITO (10 wt% SnO2+90 wt% In2O3) target and ion gun were placed at a distance of 100 mm from a PET substrate. Before sputtering of the ITO electrode on the PET substrate, the surface of the PET substrate was pretreated by the irradiation of an Ar ion beam at a DC pulsed power of 200 W to remove surface contaminations and improve adhesion between the ITO electrode and PET substrate. Subsequently, the ITO ?lm was sputtered on the rolling PET substrate, which is mechanically attached on cooling drum to maintain the substrate temperature below 50 1C, using DC power as a function of the Ar/O2 ?ow ratio. The thickness

of the ?exible ITO electrode was measured by a pro?lometer. The electrical properties of ?exible ITO were measured by means of a four-point probe and Hall measurement at room temperature. The optical transmittance was measured in the wavelength range 250–800 nm by a UV–visible spectrometer. The surface morphology of the ?exible ITO electrode was analyzed using a ?eldemission scanning electron microscope (FESEM) as a function of the Ar/O2 ratio. To investigate the structural properties of ?exible the ITO electrode, both X-ray diffraction (XRD) and high resolution electron microscope (HREM) examinations were performed. The dependence of binding energy for the elements of a ?exible ITO electrode on the Ar/O2 ?ow ratio was examined by an XPS (PHI5200) carried out by an Al Ka source in an ultra-high-vacuum system with a base pressure of $10?10 Torr. In addition, the interfacial reaction between the ?exible ITO and PET substrate was examined using an Auger electron spectroscopy (AES) depth pro?le with 10 keV energy and 0.0236 mA current of electron beam. Furthermore, the ?exibility of the ?exible ITO electrode was analyzed using a laboratory-made bending test system. The ?exible ITO/PET samples were clamped between two semicircle parallel plates. One plate was mounted to the shaft of a motor, while the other was ?xed to a rigid support. The distance of the stretched mode was 80 mm and that of the bent position was 30 mm. The bending radius was approximated to 8 mm and the bending frequency was 1 Hz. During the bending test, the resistance of the ?exible ITO/PET samples was measured using a multi-meter. Finally, we prepared the ?exible organic solar cells on the roll-to-roll sputter-grown ITO/PET sample, which was prepared at optimized roll-to-roll sputtering conditions. After cleaning the ITO electrode with various organic solvents and oxygen plasma treatment, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS, Baytron P VPAI 4083) was spin-coated onto the ?exible ITO electrode with a thickness of $20 nm, followed by drying at 80 1C for 30 min in air. For the fabrication of the photoactive layers composed of interconnected networks of electron donor and acceptor, blend of poly(3hexylthiophere)(P3HT, Rieke Metals) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C61(PCBM, Nano-C) in chlorobenzene with a ratio 1:0.5 was spin-coated on top of the PEDOT:PSS layer. Then annealed at 80 1C for 10 min in N2 atmosphere to enhance the degree of P3HT ordering [19]. Finally, a Ca (20 nm)/Al (100 nm) cathode layer was patterned on P3HT-PCBM active layer using a shadow metal mask. The photocurrent density–voltage (J–V) curves were measured with a Keithley 4200 source measurement

Fig. 1. Schematic diagram of a specially designed roll-to-roll sputtering system.

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Sheet resistance [ohm/sq]

300

Rolling speed: 0.2 cm/sec

8 6

3. Results and discussion Fig. 2 shows the thickness of a ?exible ITO electrode grown on a PET substrate as a function of the DC power and rolling speed. It was shown that the thickness of ?exible ITO electrode monotonically increased with increasing DC power. The DC power dependence on the thickness of ?exible ITO electrode could be explained by following equation [20,21]: Sr ? Sions ? Sneutrals Theoretically, the total sputtering rate Sr in a roll-to-roll sputtering can be determined by the sputtering rates due to ion (Sions) and neutral bombardment (Sneutrals). Therefore, the increased DC power during the roll-to-roll sputtering of ITO electrodes could result in an increase of Sions, which critically affected the deposition rate of the ITO electrode. However, the increase of rolling speed lead to a decrease in thickness of the ITO electrode, because the exposure time of the PET substrate at the sputtering region of ITO target linearly decreased with the increasing rolling speed. The sample grown at the optimized condition (DC power of 800 W, working pressure of 3 mTorr, and rolling speed of 0.2 cm/s) shows a thickness of 202 nm and a sputtering rate of 105.3 nm/min. Fig. 3 shows the electrical properties of ?exible ITO electrodes grown on a PET substrate at constant DC power of 800 W, working pressure of 3 mTorr, and rolling speed of 0.2 cm/s as a function of the Ar/O2 ?ow ratio. Both the sheet resistance and resistivity of the ?exible ITO electrode grown at the Ar/O2 ?ow ratio of 30/ 1 sccm are lower than those of ?exible ITO electrodes grown at the Ar/O2 ?ow ratio of 20/1 sccm as shown in Fig. 3(a). The minimum sheet resistance of 47.4 O/square and resistivity of 9.5 ? 10?4 O cm were obtained at the Ar/O2 ?ow ratio of 30/1 sccm. However, the increase of Ar ?ow rate in the mixture of the Ar/O2 gas above 30 sccm resulted in an abrupt increase of sheet resistance and resistivity. Fig. 3(b) shows a dependence of carrier mobility and a concentration in the ?exible ITO electrode on the Ar/O2 ?ow ratio. It was shown that the carrier concentration of the ?exible ITO

200 4 100
Sheet resistance Resistivity

2 0

0 20:1 30:1 40:1 Ar/O2 flow rate [sccm] 50:1

40
Mobility Carrier concentration

5x1020 Carrior concentration [cm-3]

Mobility [cm2/V-s]

30

4x1020

20

3x1020

10

2x1020

0 20:1 30:1 40:1 Ar/O2 flow rate [sccm] 50:1

1x1020

Fig. 3. Variation of resistivity, sheet resistance, Hall mobility, and carrier concentration as a function of Ar/O2 ?ow ratio from 20/1 to 50/1 sccm for the ?exible ITO ?lm grown on a PET substrate using continuous roll-to-roll sputtering at constant DC power of 800 W, working pressure of 3 mTorr and rolling speed of 0.2 cm/s: (a) sheet resistance and resistivity and (b) Hall mobility and carrier concentration.

500

Rolling speed 1.0 cm/sec 0.5 cm/sec 0.3 cm/sec 0.2 cm/sec 0.1 cm/sec

400 Thickness [nm]

300

200

100

0 200 400 600 DC power [W] 800 1000

Fig. 2. The thickness of the ?exible ITO electrode grown on a PET substrate using roll-to-roll sputtering system as a function of a DC power and rolling speed.

electrode increased, while the mobility of the ?exible ITO electrode decreased with the increasing Ar ?ow rate in the mixture of Ar/O2 gas. Tahar et al. [15] reported that increasing oxygen partial pressure resulted in decrease in carrier concentration and enhanced Hall mobility due to the dissipation of oxygen vacancies. Therefore, the low resistivity of ?exible ITO ?lms grown at the Ar/O2 ?ow ratio of 30/1 sccm can be explained by enhanced carrier mobility resulting from the dissipation of oxygen vacancy. However, the ?exible ITO ?lms grown at the Ar/O2 ?ow ratio of 20/1 sccm exhibited little increased resistivity due to decreased oxygen vacancies at high oxygen partial pressure. Fig. 4 shows optical transmittance spectra for the ?exible ITO ?lms grown on PET substrate as a function of the Ar/O2 ?ow ratio. All ?exible ITO ?lms show similar optical transparency regardless of the Ar/O2 ?ow ratio. It was believed that the transparency of ?exible ITO electrodes is not sensitive to the Ar/O2 ?ow ratio in the range between 20/1 and 50/1 sccm. It is noteworthy that the transmittance of ?exible ITO ?lms in the green region (500–550 nm) is higher than in the UV region (400–450 nm). The sharp absorption edges in the transmittance spectra are caused by an extrinsic bandgap of ITO ?lm in the range 3.8–4.0 eV [22]. Surface smoothness and morphology of the ?exible ITO electrodes grown on PET substrates as a function of the Ar/O2 ?ow ratio were analyzed by the FESEM examination. Fig. 5 shows that the surface images of all ?exible ITO electrodes are very smooth and featureless without defects such as pinholes, cracks,

Resistivity [10-3ohm-cm]

unit. Cell performance was measured under 100 mW/cm2 illumination intensity from 1 kW Oriel solar simulator with an AM 1.5G ?lter in N2-?lled glove box. For accurate measurement, light intensity was calibrated by a radiant power meter and a reference silicon solar cell certi?cated from national renewable energy laboratory (NREL).

400
DC power: 800 W Working pressure: 3 mTorr

10

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and protrusion regardless of the Ar/O2 ?ow ratio due to the low substrate temperature during continuous roll-to-roll sputtering. In general, the temperature of PET substrate depends on how much thermal energy is absorbed per unit area and how much heat is transferred or lost to the cooling drum during the rolling. Therefore, the smooth surface of the ?exible ITO electrode indicates that the temperature of PET substrate is maintained below 50 1C due to the effective heat transfer of a cooling drum. The smooth surface of the anode layer is very important for ?exible solar cells, because anode spikes can cause breakdown or shorting of ?exible solar cells.

100

80 Transmittance [%]

60

40

20

20:1 30:1 40:1 50:1 PET 300 400 500 600 Wavelength [nm] 700 800

0

Fig. 4. Optical transmittance of the ?exible ITO electrodes grown on a PET substrate using roll-to-roll sputtering as a function of Ar/O2 ?ow ratio from 20/1 to 50/1 sccm.

To obtain an optimized ?exible ITO electrode with lower resistance and higher transmittance, we carried out roll-to-roll sputtering of ?exible ITO electrodes at the narrow Ar/O2 ?ow ratio region between 30/0 and 30/3 sccm. Fig. 6 shows electrical properties of the ?exible ITO electrode ?lm grown on the PET substrate at constant DC power of 800 W, working pressure of 3 mTorr, and rolling speed of 0.2 cm/s as a function of the Ar/O2 ?ow ratio. It was shown that the increase of the Ar/O2 ?ow ratio above 30/2 sccm led to a signi?cant increase in sheet resistance and resistivity. In particular, both sheet resistance and resistivity of ?exible ITO electrode grown at the Ar/O2 ?ow ratio of 30/3 sccm cannot be measured using the Hall measurement and four-point probe due to the high-resistivity-like insulating ?lm. It is believed that the incorporation of lots of oxygen atoms into the ITO electrode during roll-to-roll sputtering led to the change of the ITO electrode from conductor to insulators. Fig. 6(b) shows the dependence of carrier concentration and Hall mobility on the Ar/ O2 ?ow ratio from 30/0 to 30/3 sccm. As expected from Fig. 3(b), the increase of partial oxygen ?ow ratio from 0 to 2 sccm resulted in a decrease in carrier concentration and an increase in carrier mobility due to the incorporation of oxygen atoms into oxygen vacancies. Fig. 7 shows optical transmittance spectra for the ?exible ITO ?lm grown at the narrow Ar/O2 ?ow ratio region of 30/0–30/3 sccm. It was noteworthy that the introduction of oxygen gas into the Ar ambient leads to signi?cant improvement of optical transmittance. Compared to the transmittance of ?exible ITO electrodes grown at pure Ar ambient (30/0 sccm), those of ?exible ITO electrodes grown at oxygen ?ow ratios above 1 sccm showed much higher optical transmittance due to the optimization of stoichiometry in the ?exible ITO electrodes. Although the electrical properties of the ?exible ITO electrode grown at the Ar/O2 ?ow ratio of 30/3 sccm cannot be measured due to the high resistivity of the ITO ?lm, it shows high optical transmittance of 88.99% at a wavelength of 550 nm. This demonstrates that the resistivity of roll-to-roll sputter-grown

Fig. 5. Surface FESEM images of the ?exible ITO electrodes grown on a PET substrate using roll-to-roll sputtering system as a function of Ar/O2 ?ow ratio from 20/1 to 50/ 1 sccm.

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?exible ITO electrodes is more sensitive to the Ar/O2 ?ow ratio than optical transmittance. The surface FESEM images (not shown here) of all ?exible ITO electrodes grown at the Ar/O2 ?ow ratio of 30/0–30/3 sccm exhibit very smooth and featureless surface of ITO

electrode regardless of the Ar/O2 ?ow ratio similar to the surface images shown in Fig. 5, due to effective cooling of PET substrate. From the electrical and optical properties of the ?exible ITO electrodes grown at an optimized condition, we can calculate the ?gure of merit (fTC) suggested by Haacke [23]

fTC ?
400 Sheet resistance [ohm/sq.]
DC power 800 W Working pressure 3 mTorr Rolling speed 0.2 cm/sec

10 8 6 Resistivity [10-3 ohm-cm]

T 10 Rsh

300

200 4 100 2 0 30:0 30:1 30:2 Ar/O2 flow rate [sccm] 30:3

0

40 35 Mobility [cm2/V-s] 30 25 20 15 30:0 30:1 30:2 Ar/O2 flow rate [sccm] 30:3

3.5x1020 3.0x1020 2.5x1020 2.0x1020 1.5x1020 1.0x1020 5.0x1019 0.0 Carrier concentration [cm-3]

Fig. 6. Variation of resistivity, sheet resistance, Hall mobility, and carrier concentration as a function of Ar/O2 ?ow ratio from 30/0 to 30/3 sccm for the ?exible ITO ?lm grown at constant DC power of 800 W, working pressure of 3 mTorr and rolling speed of 0.2 cm/s: (a) sheet resistance and resistivity and (b) hall mobility and carrier concentration.

where T is the transmittance and Rsh is the sheet resistance of the transparent conducting oxide. A maximum fTC value of 3.46 ? 10?3 O?1 could be obtained from the ?exible ITO electrode grown at DC power of 800 W, with a rolling speed of 0.2 scm/s, working pressure of 3 mTorr, and the Ar/O2 ?ow ratio of 30/ 1 sccm, which is an optimized condition in our roll-to-roll sputter system. Bender et al. [24] showed that a maximum fTC value of an ITO–Ag–ITO layer with 10–12 nm thick metal layer on a glass substrate is 24.7 ? 10?3 O?1. The comparable fTC value of the ?exible ITO electrode indicates the possibility of the roll-to-roll sputter-grown ?exible ITO electrode as an electrode for ?exible organic solar cells or organic light-emitting diodes. Fig. 8 shows an XPS wide scan data obtained from the surface of the ?exible ITO electrodes grown at an optimized condition. The XPS wide scan data clearly shows O 1s, In 3d, and Sn 3d peaks, which are indicative of bare ITO electrodes. Compared O 1s and In 3d peaks, the Sn 3d peak shows weaker peak intensity due to low a doping concentration of SnO2 (10 wt%) in In2O3. The detailed binding energy of ?exible ITO electrodes grown on PET substrate as a function of the Ar/O2 ?ow ratio was analyzed by a core-level spectra of O 1s, In 3d, and Sn 3d, as shown in Fig. 9. It was found that there is no change in the binding energy of both In 3d and Sn 3d peaks with increasing oxygen ?ow rate. However, the intensity of the two binding energies, 530.85 and 531.35 eV, in O 1s corelevel spectra, which is caused by the two types of O2? ions in the ?exible ITO electrode, is changed with increasing oxygen ?ow ratio as shown in Fig. 9(c). The lower binding energy peak (OII) is from the O2? ions, which have neighboring indium atoms with their full complement, and the higher binding energy peak (OI) corresponds to oxygen-de?cient regions [25]. Because the increase of oxygen ?ow ratio leads to the incorporation of the oxygen atom into the ITO electrode, the ?exible ITO electrode grown at the Ar/O2 ?ow ratio of 30/3 shows lowest OII peak intensity, which is related to oxygen vacancies [26].

100

DC power: 800 W Working pressure: 3 mTorr Ar:O2 = 30:1 sccm Intensity [arb. unit] In3d O1s Sn3d Rolling speed: 0.2 cm/sec

80 Transmittance [%]

60

40

20

30:0 30:1 30:2 30:3 PET 300 400 500 600 Wavelength [nm] 700 800

In3p

0 1000 800 600 400 Binding energy [eV] 200 0

Fig. 7. Optical transmittance of the ?exible ITO electrodes grown on a PET substrate using roll-to-roll sputtering system at narrowAr/O2 ?ow ratio range from 30/0 to 30/3 sccm.

Fig. 8. XPS wide scan data obtained from the ?exible ITO electrode grown at optimized roll-to-roll sputtering condition (DC power of 800 W, working pressure of 3 mTorr, rolling speed of 0.2 cm/s, and Ar/O2 ?ow ratio of 30/1 sccm).

C1s

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In3d In3d5/2 In3d3/2

30:3 sccm 30:2 sccm 30:1 sccm

458

456

454

452 450 448 446 Binding energy [eV]

444

442

440

Sn3d

Fig. 10 shows the XRD plots obtained from the ?exible ITO electrodes as a function of Ar/O2 ?ow ratio. All XRD plots of the ?exible ITO electrodes show only the intense PET substrate peak at the region $25.841. Due to the low substrate temperature during the roll-to-roll sputtering process, all ?exible ITO electrodes show amorphous structure regardless of the Ar/O2 ?ow ratio. This indicates that the substrate temperature was effectively maintained at low temperature, below 50 1C, by a cooling drum as expected from FESEM results in Fig. 5. To investigate the microstructure of the ?exible ITO electrode grown at an optimized condition in detail, the HREM examination was employed. Fig. 11 shows a cross-sectional HREM image obtained from the ITO electrode grown on a Si substrate. Due to dif?culty in the HREM sample preparation of ?exible ITO electrodes grown on a PET substrate, the ITO electrode was intentionally grown on a Si substrate at identical conditions with an optimized ?exible ITO growth condition. The uniform contrast of the ITO ?lm in Fig. 11 indicates that the structure of the ITO ?lm on a Si substrate is amorphous, as expected from the XRD results. However, some nanocrystallines, as indicated by arrows are embedded in the amorphous ITO matrix, due to the low amorphous/polycrystalline transformation temperature. Even if an ITO electrode is prepared at room temperature, nanocrystallines could be easily formed at the amorphous ITO matrix at a low homogeneous temperatures (T/Tmo0.19–150 1C) [26,27].

Intensity [arb. units]

Intensity [arb. units]

Sn3d3/2

Sn3d5/2

30:3 sccm 30:2 sccm 30:1 sccm

PET substrate 25.84

498

496

494

492 490 488 486 Binding energy [eV]

484

482

480

O1s

1s 531.35 eV 530.85 eV 531.1 eV

Intensity [arb. units]

Ar:O2 = 30:3 sccm Ar:O2 = 30:2 sccm Ar:O2 = 30:0 sccm Ar:O2 = 50:1 sccm Ar:O2 = 40:1 sccm

Intensity [arb.units]

30:3 sccm 30:2 sccm

Ar:O2 = 30:1 sccm
30:1 sccm

Ar:O2 = 20:1 sccm
540 538 536 534 532 530 Binding energy [eV] 528 526 524

20

30

Fig. 9. XPS spectra of (a) In 3d, (b) Sn 3d, and (c) O 1s obtained from the ?exible ITO electrode grown on a PET substrate as a function of Ar/O2 ?ow ratio from 30/1 to 30/3 sccm.

40 2θ

50

60

Fig. 10. XRD plots obtained from the ?exible ITO electrodes grown on PET substrates as a function of Ar/O2 ?ow ratio at constant DC power of 800 W, working pressure of 3 mTorr, and rolling speed of 0.2 cm/s.

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100 Ar+ Ion treated 200nm ITO/PET untreated 200nm ITO/PET ΔR/R0 10

1

0
Fig. 11. Cross-sectional HREM image obtained from the ?exible ITO electrode grown on a Si substrate at optimized roll-to-roll sputtering condition.

200

400 600 Bending cycle

800

1000

Fig. 13. Normalized resistance change after repeated bending as a function of the number of cycles for ?exible ITO electrode grown on untreated and Ar+ ion-treated PET substrate before ITO sputtering.

100 PET substrate Atomic concentration [%] 80 60 40 20 Sn 0 0 5 10 15 20 25 Thickness [nm] 30 35 40 O ITO In C
substrate, the ?exible ITO electrode grown on the Ar ion-treated PET substrate showed much more stable resistance. The robustness of the ?exible ITO electrode grown on the Ar ion-treated PET substrate is attributed to the improvement of adhesion between ?exible ITO ?lms and the PET substrate. Ratchev et al. [28] reported that the ion-beam treatment of PC substrates caused substantial improvement in adhesion strength between Al ?lm and PC substrates. In addition, Zaporojtchenko et al. [29] suggested that the improvement in Cu ?lm–PC substrate adhesion, by low-energy ion irradiation, is attributed to the creation of a large density of new adsorption sites. Therefore, the Ar ion beam treatment of the PET substrate during roll-to-roll sputtering process could result in the improvement of the adhesion between the ITO electrode and PET substrate and creation of new adsorption sites. To investigate the possibility of roll-to-roll sputter-grown ?exible ITO ?lm, as a ?exible anode in ?exible solar cells, we fabricated ?exible organic solar cells on the roll-to-roll grown ITO electrode. Fig. 14 shows current density–voltage (J–V) curves of ?exible organic solar cells fabricated on a roll-to-roll sputtergrown ?exible ITO electrode, at optimized conditions. For fabrication of the ?exible organic solar cells, we cut the ITO/PET sample size of 1.5 ? 1.5 cm2 from large-area ITO/PET sample (200 mm width ? 2000 mm length). Sample 1 and 2 are ?exible solar cells fabricated from identical ?exible ITO electrodes. Samples 1 shows VOC ? 0.54 V, ISC ? 6.31 mA cm?2, FF ? 55.2%, and a power conversion ef?ciency of ZAM1.5 ? 1.88%. In addition, similar performance of sample 2 as sample 1 (VOC ? 0.54 V, ISC ? 6.18 mA cm?2, FF ? 55.3%, and ZAM1.5 ? 1.84%) indicates that our large area ITO/PET electrodes prepared by roll-to-roll sputtering system can be applied to the mass production of solar cells with reproducible ef?ciency values. Although the samples show relatively low power-conversion ef?ciency compared to the previous reports (1.5–3%), the performance might be improved after further optimization of fabrication process including the annealing process, thickness of active layers, and surface treatment of ?exible ITO electrodes [2,30,31].

Fig. 12. AES depth pro?les of a roll-to-roll sputtering grown ?exible ITO/PET substrate grown at optimized condition.

Fig. 12 shows the AES depth pro?le obtained from the ?exible ITO electrode grown on a PET substrate at an optimized condition. It was shown that the ?exible ITO layer is well de?ned on a PET substrate, indicating the absence of signi?cant interfacial reactions between the ITO and the PET substrates due to the low substrate temperature. In addition, the uniform atomic concentrations of In, O, and Sn elements throughout the ITO electrode shows constant deposition of ITO ?lms during continuous roll-toroll sputtering. To investigate the ?exibility of the optimized ?exible ITO electrode prepared on a PET substrate, a laboratory-made bending test system was employed. For comparison, ?exible ITO electrodes were grown on both wet-treated and the Ar ion-treated PET substrates at the same optimized growth condition. The change in resistance was expressed as DR ? R?R0, where R0 is the initial resistance and R is the measured resistance after bending. Fig. 13 shows changes in the resistance of the ?exible ITO electrode grown on wet-treated and the Ar ion-treated PET substrate at a DC pulsed power of 200 W. It was noteworthy that the DR/R0 value of the ?exible ITO electrode grown on wet-treated PET substrates increased remarkably at initial bending cycles, due to the generation and propagation of cracks. However, the ?exible ITO electrode grown on the Ar ion-treated PET substrates showed a fairly constant DR/R0 value throughout the bending test. Compared to the ?exible ITO electrode grown on the untreated PET

4. Conclusion In summary, this study has demonstrated the applicability of a roll-to-roll sputtering technique as an alternative to conventional RF or DC sputtering for continuous sputtering of ?exible ITO

ARTICLE IN PRESS
K.-H. Choi et al. / Solar Energy Materials & Solar Cells 93 (2009) 1248–1255 1255

7 Current density [mA/cm2] 6 5 4 3 2 1 0 0.0

Sample1 : 1.88% (Jsc:6.31, Voc: 0.54, FF:55.2%) Sample2 : 1.84% (Jsc:6.18, Voc: 0.54, FF:55.3%)

0.1

0.2

0.3 Voltage [V]

0.4

0.5

0.6

Fig. 14. Current density–voltage characteristics (air mass 1.5 G condition with incident light power intensity of 100 mW cm?2) of ?exible organic solar cell fabricated on the roll-to-roll sputter-grown ?exible ITO electrode prepared at optimized condition.

electrode on a PET substrate. It was found that both the electrical and optical properties of the ?exible ITO electrode were critically dependent on the Ar/O2 ?ow ratio during continuous roll-to-roll sputtering. In addition, all ?exible ITO electrodes show amorphous structure and a very smooth surface area regardless the of Ar/O2 ?ow ratio, due to the low substrate temperature which is maintained by the cooling drum. Even though the ?exible ITO electrode was prepared at room temperature, we can obtain the ?exible ITO electrode with a sheet resistance of 47.4 O/square and average optical transmittance of 83.46% in the green region between 500 and 550 nm wavelengths. In addition, it was found that the Ar ion treatment of the PET substrate could improve the ?exibility of the ITO electrode due to effective removal of surface contamination and increase in adhesion of the ITO ?lm with the PET substrate. Furthermore, the J–V characteristics of the ?exible organic solar cell prepared on a ?exible ITO electrode grown at optimized conditions show typical current density–voltage characteristics with a conversion power ef?ciency of 1.84–1.88%. This indicates that the roll-to-roll sputtering technique is a promising continuous sputtering process for continuous production of low cost ?exible solar cells.

Acknowledgement This work was supported by the Korea Foundation for International Cooperation of Science & Technology (KICOS) through a grant provided by the Korean Ministry of Education, Science & Technology (MEST) in K20701010289-07B0100-07511. References
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