Transparent Conducting Oxides: The Invisible but Essential Component of Solar Cells
Transparent conducting oxides or TCOs combine what are usually mutually exclusive properties—conductivity and optical transparency. They are employed as IR-reflective and anti-static coatings and as transparent electrodes in various applications (flat-panel displays, touch screens, light-emitting diodes, etc.). This talk focuses on their application as top electrodes in solar cells, where the basic requirements are high transparency to the solar spectrum, low sheet resistance, and band-matching with the active elements, which is especially important for organic photovoltaics (OPVs) to avoid losses associated with non-ohmic contacts. We will consider the factors (and tradeoffs) governing high conductivity and low absorption of light, and the defect mechanisms governing carrier generation and Fermi level. Considerable progress has been made on the conductivity side of the equation, with optimized TCO conductivities approaching the "Bellingham limit."
We then consider the factors controlling the work functions of TCOs. Prevailing "myths" will be dispelled (e.g., that there is a single work function for a given oxide or a given oxide surface). Much has been learned in recent years concerning the factors governing the ionization potential of TCOs which, given the Fermi level, determines the work function of a given oxide surface. Important factors include surface orientation, surface termination, and surface modification, all of which can result in surface dipole modification, thereby altering the ionization potential and measured work function. Materials to be considered in the talk include pure and tin-doped indium oxide (In2O3, ITO), pure and Al-doped zinc oxide (ZnO, AZO), pure and antimony-doped tin oxide (SnO2, ATO), and Zn-and-Sn codoped indium oxide (ZITO). Parallelogram plots of work function vs. Fermi level can help identify useful ranges for TCOs in both conventional and inverted OPV cell designs.