Characteristics of flexible indium tin oxide electrode grown by continuous roll-to-roll sputtering process for flexible organic solar cells

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Abstract

The preparation and characteristics of flexible indium tin oxide (ITO) electrodes grown on polyethylene terephthalate (PET) substrates using a specially designed roll-to-roll sputtering system for use in flexible organic solar cells are described. It was found that both electrical and optical properties of the flexible ITO electrode were critically dependent on the Ar/O2 flow ratio in the continuous roll-to-roll sputter process. In spite of the low substrate temperature (<50 °C), we can obtain the flexible ITO electrode with a sheet resistance of 47.4 Ω/square and an average optical transmittance of 83.46% in the green region of 500–550 nm wavelength. Both X-ray diffraction and field emission scanning electron microscopy analysis results showed that all flexible ITO electrodes grown on the PET substrate were amorphous with a very smooth and featureless surface, regardless of the Ar/O2 flow ratio due to the low substrate temperature, which is maintained by a cooling drum. In addition, the flexible ITO electrode grown on the Ar ion-beam-treated PET substrates showed more stable mechanical properties than the flexible ITO electrode grown on the wet-cleaned PET substrates, due to an increased adhesion between the flexible ITO and the PET substrates. Furthermore, the flexible organic solar cell fabricated on the roll-to-roll sputter-grown flexible ITO electrode at an optimized condition exhibited a power conversion efficiency of 1.88%. This indicates that the roll-to-roll sputtering technique is a promising continuous sputtering process in preparing flexible transparent electrodes for flexible solar cells or displays.

Introduction

There is currently considerable interest in flexible organic solar cells for use as a new generation of energy storage devices due to their light weight, robust profile, ability to flex, curve, roll and fold for portability, as well as their easy fabrication using roll-to-roll-based thin film technologies [1], [2], [3], [4], [5], [6], [7]. These flexible 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 flexible organic solar cells, it is necessary to prepare transparent conducting oxide (TCO) with low resistance, high transmittance, superior flexibility, 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], [9], [10], [11], [12]. In addition, the roll-to-roll-based continuous TCO deposition process should be employed to deposit the flexible TCO electrode for low cost and the mass production of flexible solar cells [13], [14]. Until now, amorphous indium tin oxide (ITO) films grown by batch-type direct current (DC) or radio-frequency (RF) sputtering have been widely used as the anode layer in flexible organic solar cells [2], [3], [4], [15], [16]. Although ITO electrode is widely used in OPVs as a TCO material, the high cost of ITO materials makes it difficult 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 difficulty 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 flexible 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 flexible ITO electrode grown by the roll-to-roll sputtering process have not been investigated in detail [6], [16], [17], [18].

In this work, we investigated electrical, optical, structural, mechanical, and the surface properties of flexible ITO electrodes grown on a PET substrate using a specially designed roll-to-roll sputtering system for flexible solar cells. It was found that the resistivity and transmittance of flexible ITO electrodes are critically influenced by the Ar/O2 flow ratio during continuous roll-to-roll sputtering. At an optimized Ar/O2 flow ratio, DC power, working pressure, and rolling speed, we can obtain a flexible ITO electrode with a resistivity of 9.5×10−4 Ω cm, average transmittance of ∼83.46%, and figure of merit value of about 3.4×10−3 Ω−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 flexible ITO electrode with flexible organic solar cells, which demonstrate the possibility of the ITO films grown by continuous roll-to-roll sputtering as a flexible anode layer in the flexible organic solar cells.

Section snippets

Experimental

The flexible ITO electrode with a thickness of 200 nm was deposited on a flexible 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 flexible ITO electrodes. The 200 mm wide PET substrate with a thickness of 188 μm was passed repeatedly over the cooling drum by motion of unwind

Results and discussion

Fig. 2 shows the thickness of a flexible ITO electrode grown on a PET substrate as a function of the DC power and rolling speed. It was shown that the thickness of flexible ITO electrode monotonically increased with increasing DC power. The DC power dependence on the thickness of flexible ITO electrode could be explained by following equation [20], [21]:Sr=Sions+SneutralsTheoretically, the total sputtering rate Sr in a roll-to-roll sputtering can be determined by the sputtering rates due to ion

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 flexible ITO electrode on a PET substrate. It was found that both the electrical and optical properties of the flexible ITO electrode were critically dependent on the Ar/O2 flow ratio during continuous roll-to-roll sputtering. In addition, all flexible ITO electrodes show amorphous structure and a very smooth surface

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.

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