The Transmittance Modulation of ZnO/Cu/ZnO Transparent Conductive Electrodes Prepared on Glass Substrates.

Dooho Choi

doi:10.3390/ma13183916

Abstract

With the explosive development of optoelectronic devices, the need for high-performance transparent conductive (TCE) electrodes for optoelectronic devices has been increasing accordingly. The two major TCE requirements are (1) visible light average transmittance higher than 80% and (2) sheet resistance lower than 10 Ω/sq. In this study, we investigated the critical role of the top and bottom ZnO thicknesses for the ZnO/Cu/ZnO electrodes prepared on glass substrates. It was shown that the required Cu thickness to meet the conductivity requirement is 8 nm, which was fixed and then the thicknesses of the top and ZnO layers were independently varied to experimentally determine the optimized conditions for optical transparency. The thicknesses of the top and bottom ZnO layers were both found to significantly affect the peak transmittance as well as the average visible light transmittance. The ZnO/Cu/ZnO electrode exhibits peak and average transmittance of 95.4% and 87.4%, excluding the transmittance of glass substrates, along with a sheet resistance of 9.7 Ω/sq, with a corresponding Haacke’s figure of merit () of 0.064, which exceeds the reported value for the ZnO/Cu/ZnO electrodes, manifesting the need of experimental optimization in this study.

Highlights

With the explosive development of optoelectronic devices such as light emitting diodes and solar cells, the need of highly transparent and conductive transparent conductive electrodes (TCE)is increasing [1,2,3] The general TCE requirements include visible light transmittance of higher than 80% and the sheet resistance in the of lower than 10 Ω/sq [4,5] The conventional TCE material satisfying these requirements is indium tin oxide (ITO), but such properties can be obtained only via an annealing process at temperatures higher than 250 ◦ C, [6] which significantly increases the fabrication cost and time in addition to the inherent incompatibility with heat sensitive flexible polymer substrates such as polyethylene terephthalate (PET) films
This study reports the dramatic modulation of the TCE transmittance with the independent variations of the top and bottom oxide layers, which results in the maximum and average transmittance for the optimized ZnO (40 nm)/Cu (8 nm)/ZnO TCE
As the sheet resistance of the ZnO layers exceeds the limit (2 MΩ/sq.) of the four point probe system, the conductivity of the OMO electrodes can be considered to be determined by the Cu layers, whose sheet resistance values were found to be independent of the bottom ZnO thickness

Summary

Introduction

With the explosive development of optoelectronic devices such as light emitting diodes and solar cells, the need of highly transparent and conductive transparent conductive electrodes (TCE). Volmer–Weber 3D thin film growth mode so that the continuous layer formation occurs at metal layer thicknesses as thin as possible These approaches utilized various novel growth techniques including surface modification of polymer substrates before metal layer deposition [14], trace-level gas (O2 and N2 ) addition during metal sputtering [15] and impurity cosputtering [16]. While Ag is the primary material of choice for the OMO electrodes due to its low resistivitiy (1.6 μΩ-cm) and high visible-light transmittance in the form of an ultrathin (~10 nm) layer, TCEs are one of the most expensive components in optoelectronic devices and it is desirable to replace Ag with low cost electrode materials [17] Cu has been recently considered as a promising alternative due to its low resistivity (1.7 μΩ-cm) that is comparable to that of Ag and its low material cost that is only ~1%.

Experiment

Results and Discussion

Conclusions