Transparent conductive oxides (TCOs) have extensive applications in the diverse optoelectronics applications. However, it is often difficult to identify transparent oxides with high electrical conductivity due to their intrinsically wide band gaps. Furthermore, those that have been identified as offering useful properties often contain high cost and/or toxic components (e.g. In₂O₃) or are difficult to process (e.g. CuAlO₂) or both; leading to challenging commercialization issues.

In comparison, CaO and Al₂O_3, traditional cement components, have long been studied and found to form multiple phases at different stoichiometric ratios. Among these, 12CaO^.7Al₂O₃ (C12A7) in particular has received considerable recent attention because it is a novel type of TCO. C12A7 is a light metal oxide with a unique set of cage structures that can be processed to give transparent, dense films that are typically insulating but that can be easily transformed into a transparent semiconducting films. As formed, C12A7 consists of a set of cage structures wherein charge balance is maintained through incorporation of O^2- ions in these cages. Hosono et al among other groups has discovered that these O^2- ions can be replaced through H₂ treatment with OH^- and H^- ions (C12A7:H). Most important is their finding that UV treatment of (C12A7:H) can be used to generate transparent electrides, (C12A7:e^-) providing materials with conductivities as high as 1 S cm^-1 in pellets or sputtered films. These materials thus offer the potential to process low cost TCOs at commercial scales providing simple processing routes to such materials can be developed.

Traditionally, C12A7 materials have been processed using solid-state reactions followed by pulsed laser deposition (PLD) or floating zone (Fz) crystallization methods as high temperature, high cost approaches to single-phase films. These techniques require a significant number of process steps to generate C12A7:e. We demonstrate here an effective mass production method by producing C12A7 nanopowders (NPs) via liquid-feed flame spray pyrolysis (LF-FSP). Nearly fully dense, single phase, dense, flexible and transparent C12A7 thin films (< 30 m) can be produced by processing these NPs into green films by tape-casting, thermo-compression and then sintering to 1300/3 h/O₂. Subsequent heat treatments in 20% H₂/80% N₂ to replace O^2- ions forming C12A7:H^- followed by UV irradiation provide C12A7:e^- with electrical conductivities of 75 mS cm^-1. C12A7:e^- is expected to belong to a new class of TCOs due to its low materials and processing costs, environmental affinity, and natural abundance when processed efficiently.