Recent progress of multilayer composite transparent conductive film
2017
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 SnO 2 (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, MoO 3 , WO 3 , TiO 2 , 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.
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