Recent progress of multilayer composite transparent conductive film

Abstract

Transparent conductive films (TCFs), which transmit light and conduct electrical current simultaneously, are widely used as transparent electrodes across technical fields such as flat-panel displays, touch screens, solar cells and light-emitting devices. In present, TCO films such as tin-doped indium oxide (ITO), aluminum-doped zinc oxide (AZO) and fluorine doped SnO2 (FTO) are the most widely used transparent electrodes in these devices. However, the manufacturing of ITO film requires precious raw materials indium and other TCO films still have to improve their conductivity and transmittance in visible region. Moreover, due to the inherent brittleness of oxide materials, the flexibilities of TCO films are poor, which could not meet the application requirements of flexible electronics. More and more researchers are focusing on finding new transparent conductive materials as substitutes. Multilayer composite TCFs based on ultrathin metal film exhibit high conductivity, good transparency and excellent flexibility. In this feature article, we review the recent progress of multilayer composite TCFs with dielectric/metal/dielectric (DMD) structure by describing the basic principles, the materials and thickness selection of each layer, the structural types, the methods of photoelectric performance improvement, and other important properties of multilayer composite TCFs with the DMD structure. Due to the good conductivity and ductility of metal layers, multilayer composite TCFs with the DMD structure show low sheet resistance and excellent mechanical flexibility. Some metals such as Ag, Au, Cu, Al, Pd, Pt, Mo, Ni, In and metal alloys have been used as the metal interlayer of DMD electrodes. Among them, the most frequently used metals are Ag, Cu and Au. The two dielectric layers in DMD electrodes may be utilized to improve the overall transmittance in the visible spectral range by optical interference within the multilayer structure, while their thickness can be chosen as a function of the device properties requested. As the metal provides a high lateral conductivity, the dielectric layers in the DMD electrodes do not require being highly conductive, therefore they can be selected in a wide range of materials such as ITO, AZO, FTO, MoO3, WO3, TiO2, ZnS, etc. Experimental data have shown that the same transmittance and sheet resistance values could be obtained with DMD structures composed of different dielectric and metal combinations. In addition, high temperature deposition and annealing are not required to achieve good electrical conductivity, in spite of the higher resistivity achieved by dielectric films prepared without heating. Therefore, DMD electrodes are suitable to deposit onto unheated plastic substrates by continuous roll-to-roll techniques. The efficiencies of DMD electrodes are seriously constrained by a deleterious trade-off between the optical transmittances and electrical conductivities of the metal layers. An improvement in the electrical conductivity requires an increase in the thickness of the metal layer, but the increase of thickness seriously reduces the transmittance. Moreover, due to the three dimensional island growth mode of metal layers deposited using vacuum coating techniques, the metal film usually has a threshold thickness that is the minimum possible thickness for forming a continuous layer to provide sufficient electrical paths. Below the threshold thickness, both the electrical resistivity and the optical absorption rapidly increase. The conductivity and transmittance of a thin metal layer are normally optimized at or near the percolation threshold thickness, since a high optical transmission is required in most cases. Therefore, reducing the percolation threshold thickness of metal layers is a key to improve their conductivity and transmittance simultaneously. In the latest years, some impressive improvements have been achieved by controlling the underlay material, seed layer, dopants and deposition rate at the deposition of metal layer. So far, multilayer composite TCFs with the DMD structure have primarily been applied in solar cell and light-emitting devices, where multilayer composite TCFs may be used as cathodes or anodes, even intermediate electrodes due to the electrode work function easily adjusted by the selection of dielectric layer material. When the multilayer electrodes are applied in solar cell, higher power conversion efficiencies have been achieved compared with devices fabricated on single-layer TCO electrodes. Although there are some challenges yet to overcome to optimize the processing and performance of multilayer composite TCFs, multilayer composite TCFs with the DMD structure remain a highly suitable candidate for various flexible electronic applications in the near future.