The first report of a transparent conducting oxide (TCO) was published in 1907, when Badeker reported that thin films of Cd metal deposited in a glow discharge chamber could be oxidized to become transparent while remaining electrically conducting. Since then, the commercial value of these thin films has been recognized, and the list of potential TCO materials has expanded to include, for example, Al-doped ZnO, GdInOx, SnO2, F-doped In2O3, and many others. Since the 1960s, the most widely used TCO for optoelectronic device applications has been tin-doped indium oxide (ITO). At present, and likely well into the future, this material offers the best available performance in terms of conductivity and transmissivity, combined with excellent environmental stability, reproducibility, and good surface morphology. The use of other TCOs in large quantities is application-specific. For example, tin oxide is now widely used in architectural glass applications.
Transparent conducting oxides (TCOs) are an increasingly important component of photovoltaic (PV) devices, where they act as electrode elements, structural templates, and diffusion barriers, and their work function controls the open-circuit device voltage. They are employed in applications that range from crystalline-Si heterojunction with intrinsic thin layer (HIT) cells to organic PV polymer solar cells. The desirable characteristics of TCO materials that are common to all PV technologies are similar to the requirements for TCOs for flat-panel display applications and include high optical transmissivity across a wide spectrum and low resistivity. Additionally, TCOs for terrestrial PV applications must use low-cost materials, and some may require device-technology-specific properties. We review the fundamentals of TCOs and the matrix of TCO properties and processing as they apply to current and future PV technologies.
Surface characterization of indium−tin oxide (ITO) thin films has been carried out with monochromatic
X-ray photoelectron spectroscopy (XPS) following various solution pretreatments, RF air plasma etching
or high-vacuum argon-ion sputtering. Commercially available ITO films show high concentrations of
hydrolyzed oxides and oxy-hydroxides in the near-surface region, along with stoichiometric oxide (In2O3,
SnO2) and variable levels of oxygen defect sites. XPS revealed that solution and vacuum treatments changed
both the relative surface coverage of the hydroxides and, to a lesser extent, the concentration of oxide defect
sites in the near-surface region. These pretreatments have a significant effect on both the coverage and
electron-transfer rates for chemisorbed ferrocene dicarboxylic acid (Fc(COOH)2), with the air-plasma-etched ITO showing the highest surface coverage of Fc(COOH)2 and an RCA treatment showing the highest
electron-transfer rates.
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