Light-Emitting Devices Enabled by Large-Area Correlated-Electron Transparent Conductors

Light-Emitting Devices Enabled by Large-Area Correlated-Electron Transparent Conductors The efficiency and capabilities of light-emitting electronic devices are crucial in application areas spanning optical communication, solid-state lighting, and digital displays. These devices include organic light emitting diodes (OLEDs), a highly impactful and rapidly evolving device technology, that can potentially combine low-cost, high efficiency, and novel capabilities. At the core of the development of OLEDs is the formation of interfaces between light-emitting organic molecules and the inorganic solids that bring electrical power to the devices. These interfaces are particularly challenging because they require inorganic materials with properties that usually do not coexist: electrical conductivity and optical transparency. These inorganic electrode materials are called transparent conductors. These transparent conductors need to form interfaces that have precisely controlled electronic properties when in contact with the organic light-emitting molecules. A new generation of inorganic materials termed correlated-electron transparent conductors has the potential to change how these inorganic/organic interfaces are formed and to bring new directions in the design of OLEDs. This work seeks to develop OLEDs enabled by the innovations in correlated-electron transparent conductors, to probe the efficiency and properties of the OLEDs produced using these materials, and to probe how the properties of the organic/correlated-electron transparent conductor interfaces can be controlled and ultimately improved. The work contributes to the professional preparation of early-career researchers and involves the development of demonstrations of novel transparent conductors to disseminate the concepts underpinning this research to the broadest possible audience. The design, synthesis, and performance of organic light emitting diodes (OLEDs) can be significantly advanced by incorporating a recently developed generation of transparent conductors. Correlated-electron transparent conductors are complex metal oxides that have several advantages in comparison with the present generation of transparent conductor solutions, facilitating directions in OLED design and processing that take advantage of favorable work functions, lower processing time and materials costs, improved mechanical properties, and reduced toxicity. OLEDs fabricated with correlated electron transparent conductors have the potential for more efficient light emission through the creation of reduced-band-offset contacts, improved chemical stability, and greater freedom in device design due to the use of oxides at interfaces with electron-transporting materials. Correlated electron transparent conductors, including SrVO3 and several related compounds, have excellent optical and electronic properties and can be synthesized on technologically relevant large-area amorphous substrates. The work function of SrVO3 depends on which metal oxide component forms the surface termination, indicating that the electronic properties of the interface can be varied by changing the surface preparation. Appropriate surface termination can yield a low work function for contacts to the electron-transporting component of OLEDs or a higher work function for hole-transporting materials. Correlated electron transparent conductors can be synthesized through seeded crystallization on non-epitaxial amorphous large-area substrates, including fused quartz. The scope of correlated-electron conductors thus extends to the large areas used in applications, far beyond the limits of single-crystalline complex-oxide or silicon substrates. This project creates and characterizes correlated-electron transparent conductor/organic interfaces relevant to OLEDs to test the hypothesis that the band offset, a quantity closely linked to the work function, can be controlled using the surface termination. OLEDs are fabricated using correlated-electron transparent conductors to probe both electron-transporting and hole-transporting contacts. The properties of the resulting OLEDs are compared to devices formed with present-generation transparent conductors. The work begins by probing fundamental properties using epitaxial transparent conductors and systematically progresses to more structurally complex non-epitaxial contacts. The factors that influence device stability, including chemical and structural defects arising in large-area crystallization are being probed. This work demonstrates optoelectronic device design with correlated-electron transparent conductors, develops connections between the chemical preparation of the correlated-electron transparent conductor/organic interface to provide a new route to low-band-offset contacts. 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.