Influence of Nanocrystal Size on the Optoelectronic Properties of Thin, Solution-Cast Sn-Doped In2O3 Films

Thin films made from transparent conducting oxide (TCO) nanocrystals are promising alternatives to traditional vacuum-sputtered films. However, the material properties of nanocrystal-derived thin films are dependent upon the doping levels and sizes of the nanocrystal building blocks. To date, a lack of deliberate and precise control over size in TCO nanocrystals has hindered the investigation of how nanocrystal size affects the optoelectronic properties of the resulting thin films. Here, this gap is addressed through the use of a synthetic approach that produces a series of uniform nanocrystals with tunable, well-defined sizes with nanometer resolution. A size ladder of Sn-doped In2O3 (ITO) nanocrystals, containing seven samples ranging from 5 to 21 nm in diameter, was synthesized sequentially under the same reaction conditions in a single slow-injection reaction. The nanocrystals displayed constant dopant levels, homogeneous dopant distributions, and high carrier concentrations (similar to 10(21) cm(-3)) across all sizes produced. The ITO nanocrystals were solution-deposited into thin films and processed under mild conditions. For all nanocrystal sizes, the films were smooth and crack-free and exhibited >95% optical transparency. The resistivities of the thin films decrease over an order of magnitude, from 5.0 X 10(-2) to 4.5 X 10(-3) Omega cm for the 5.3 and 21.5 nm samples, respectively. Larger nanocrystals exhibit lowered thin film resistivities due to decreased coulombic charging energy, decreased electron surface scattering, and reduced interface density.