Chemistry Reference
In-Depth Information
These products have one thing in common; they need to combine opposing
material properties, namely, optical transparency and metallic conductivity, in order
to produce
1. Spectral-selective characteristics
2. Transparent electrodes
Usually, transparency and metallic conductivity are mutually exclusive. There
are two possibilities to overcome this contradiction. Conductive and transparent
coatings can either be a multilayer arrangement based on thin metal layers or a
homogeneous TCO coating [378]. In contrast to metallic multilayer, TCO films
exhibit good mechanical, chemical, and thermal stability.
8.2.4.2.3.1 Physical Properties of TCO Materials
TCOs are degenerately doped wide-gap semiconductors. An important feature to
characterize a TCO layer is the resistivity. To achieve a low resistivity, either the
thickness of the TCO, the carrier concentration, or the electron mobility can be
increased (n-type). Thickness and electron concentration have a direct effect on the
opticalpropertiesofthematerial.TheabsorptionintheTCOincreaseswithincreasing
layer thickness (Beer's law).
The electron density influences the absorption of the TCO in two ways. The
effective optical band gap of a degenerate semiconductor is widened with increas-
ing electron concentration because of the high occupation of the lowest states in
the conduction band by electrons excited from the shallow donor states, so that
absorption into these conduction band states is forbidden by the Pauli principle
(Burstein-Moss shift [379]). This gives a lower transparency limit, which is in the
order of 350 nm.
For long wavelength light, the absorption by free carriers limits the transmit-
tance of TCOs. The absorption and reflection of an electron gas increases, and the
transmission decreases for frequencies lower than the plasma frequency
ω
p
2π
2
·
c
n
e
e
2
ε
HF
ε
0
m
∗
ω
2
p
=
=
,
(8.74)
λ
p
where
ε
HF
is the high-frequency dielectrically constant
m
∗
is the effective mass,
n
e
is the carrier density
Equation 8.74 describes the change from the dielectrical behavior in the visible region
to the metallic characteristics in the near IR. This so-called plasma edge for TCOs
usually lies in the near-infrared region (800-1 100 nm). A higher electron density
allowing a higher conductivity at the same time shifts the plasma edge to shorter
wavelengths.
Thus, increasing the electron mobility is the only way to achieve a higher con-
ductivity of the TCO material without reducing its long wavelength transmittance.
In general, apart from the effective mass resulting from the band structure of the
material, the mobility of TCOs is limited by scattering at grain boundaries in the
