Flexible IZO/Ag/IZO/Ag multilayer electrode grown on a polyethylene terephthalate substrate using roll-to-roll sputtering
© Kim and Lim; licensee Springer. 2012
Received: 9 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
We investigated the optical, electrical, structural, and surface properties of roll-to-roll [R2R] sputter-grown flexible IZO/Ag/IZO/Ag [IAIA] multilayer films on polyethylene terephthalate substrates as a function of the top indium zinc oxide [IZO] thickness. It was found that the optical transmittance of the IAIA multilayer was significantly influenced by the top IZO layer thickness, which was grown on identical AIA multilayers. However, the sheet resistance of the IAIA multilayer was maintained between the range 5.01 to 5.1 Ω/square regardless of the top IZO thickness because the sheet resistance of the IAIA multilayer was mainly dependent on the thickness of the Ag layers. Notably, the optimized IAIA multilayer had a constant resistance change (ΔR/R0) under repeated outer bending tests with a radius of 10 mm. The mechanical integrity of the R2R-sputtered IAIA multilayer indicated that hybridization of an IZO and Ag metal layer is a promising flexible electrode scheme for the next-generation flexible optoelectronics.
Flexible transparent conducting oxide [FTCO] electrodes with superior flexibility have recently attracted much attention as key components in flexible displays, photovoltaics, and touch panels because device performances are critically dependent on optical, electrical, and mechanical properties of the FTCO [1–3]. In particular, the FTCO electrode should have mechanical robustness against substrate bending without resultant changes in its optical and electrical properties. Until now, amorphous indium tin oxide [ITO] electrodes have been widely employed as an FTCO material in flexible optoelectronic devices due to their high conductivity and transparency in the visible spectral range . However, easy formation and propagation of cracks in the brittle amorphous ITO electrode have been considered as critical drawbacks of the amorphous ITO electrode in flexible optoelectronics . To solve these critical drawbacks of the ITO electrode, flexible oxide-metal-oxide [OMO] multilayer electrodes, such as IZO/Ag/IZO, ITO/Ag/ITO, IZTO/Ag/IZTO, GZO/Ag/GZO, AZO/Ag/AZO, NTO/Ag/NTO, and ZTO/Ag/ZTO schemes, have been suggested due to their very low resistivity and high transparency [6–11]. Another promising candidate for FTCO is an inverted multilayer electrode, the metal-oxide-metal [MOM] multilayer structure. However, the existence of opaque metal layers could reduce the optical transparency of the MOM multilayer even though the MOM multilayer has a very low sheet resistance comparable to that of a metal electrode. Therefore, an additional thin oxide layer on the MOM multilayer is required to enhance the optical transparency of the MOM multilayer. However, the thickness effect of the top oxide layer on the electrical and optical properties of the MOM multilayer has not been investigated in detail.
In this work, we investigated the characteristics of flexible indium zinc oxide [IZO]/Ag/IZO/Ag multilayer electrodes grown on polyethylene terephthalate [PET] substrates using a specially designed roll-to-roll [R2R] sputtering system at room temperature. The electrical, optical, and mechanical properties of the IZO/Ag/IZO/Ag/PET samples were examined as a function of the top IZO thickness to decide the optimum thickness of the top IZO layer. In addition, surface morphology of the top IZO layer sputtered on the Ag/IZO/Ag multilayer was also investigated as a function of the top IZO thickness. Furthermore, mechanical stability of the optimized IZO/Ag/IZO/Ag multilayer under a repeated bending stress was examined using lab-made bending test system.
Results and discussion
In summary, we investigated the characteristics of IAIA multilayer electrodes grown on PET substrates using a specially designed lab-scale R2R sputtering system at room temperature. In two steps, we prepared the IAIA multilayer as a function of the top IZO thickness. It was found that the optical transmittance of the IAIA multilayer was significantly influenced by the thickness of the top IZO layer, which was grown on identical AIA multilayers. However, the sheet resistance of the IAIA multilayer was maintained between the range 5.01 to 5.10 Ω/square regardless of the top IZO thickness because the sheet resistance of the IAIA multilayer was mainly dependent on thickness of the Ag layers. In addition, all IAIA multilayers show an amorphous structure and a very smooth surface due to the low substrate temperature which is maintained by the cooling drum. Furthermore, the high failure strain of the inserted Ag layer improved the robustness of the IAIA multilayer electrode. These results indicated that the R2R sputter-grown IAIA multilayer electrode is a promising candidate to replace conventional amorphous ITO electrodes.
This work was mainly supported by the Samsung Mobile Display Research Center Program.
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