Collaborative Research: Predictive theory, synthesis and characterization of a new type of transparent conductor without doping

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Non-technical description: Technologies such as photovoltaic solar cells, smart windows, light emitting diodes, touch-screen sensors, electronic papers, and flat panel displays require for their operation a critical component that is both an electrical conductor and optically transparent, the so-called transparent conductors. These compounds are unique, as transparency and conductivity are generally mutually exclusive properties of compounds. Indeed, optical transparency (as in window glass) is generally associated with electrical insulation, whereas electrical conductivity (such as in copper or gold) is generally associated with optically opaque metals. Known transparent conductors such as indium oxide doped with Sn are made by instilling conductivity in transparent insulators. The research team aims to develop a novel family of transparent conductors - metallic ceramics - by designing transparency in metals. This presents a new method to design optical properties as distinct from electronic properties. The approach has an exciting intellectual impact as it suggests a general approach for inverse design - starting from science-based design principles used as 'filters' for computational material selection, followed by identification of representative examples and then laboratory validation. The graduate students and postdocs of CU Boulder and Northwestern Universities are given exciting opportunities to participate in this cutting edge research effort. Technical description: Transparent conducting oxides are paradoxical materials that form the basis of modern day technologies, such as touch screens, flat panel displays, solar panels, etc. The current method of choice for discovering new transparent conductors have relied on heavily doping transparent insulators until they become conducting. Unfortunately, wide band gap materials are intrinsically resistant to doping of charges because of the existence of so-called 'doping bottlenecks'. Namely, the introduction of a high concentration of free carriers into insulators generally leads to the spontaneous formation of structural defects with polarity that compensate that of the intentional doping. Consequently, the progress in finding and optimizing such technologically critical transparent conducting oxide materials has been frustratingly slow. The research team proposes an opposite, and likely more fruitful alternative of designing transparency in metals by looking for compounds that obey a set of 'design principles' with specific band structures. The design rules for such intrinsic transparent conductors are: (i) identify a compound with metallic band structure; such that (ii) it has low plasma frequency and (iii) low inter-band optical absorption across the Fermi level, and (iv) apply synthesis and characterization to the 'best of class' compounds, here Ba-Nb-O. The most visible advantage of metallic transparent conductors is that their conductivity comes from their high, indigenous carrier concentration. The complex theory-experimental approach in the proposal enables more efficient progress as alternative to the previously used trial-and-error method.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.