Studies of Solution-Processed Organic Light-Emitting Diodes and All-Polymer Solar Cells

Abstract

Organic electronics, commonly referred as plastic electronics, is an emerging technology for addressing many challenges that our society is facing. Over the past two decades, organic electronics has gained enormous attention due to its many advantages, including low power consumption, low-cost, scalable, and flexible design. Development of high-performance devices by designing new materials and related device engineering is crucial to the future advances in organic electronics. This dissertation focuses on studies of two classes of optoelectronic devices, organic light-emitting diodes (OLEDs) and all-polymer solar cells, fabricated by solution-processing of organic semiconductors, and aims to better understand structure-property-performance relationships resulting from solution-based fabrication. Study of bisindenoanthrazolines and dendritic oligoquinolines were found to be promising new electron-transport materials (ETMs) for high-performance phosphorescent OLEDs (PhOLEDs). Solution-processed multilayered blue PhOLEDs with orthogonal solution-processed ETMs were found to have the highest efficiency (luminous efficiency = 28.3 cd/A and external quantum efficiency = 15.5 %) observed to date among polymer-based devices. The surface morphology and charge-transport properties of the ETMs were successfully tuned by solution-deposition, which made it possible to eliminate the need for interfacial materials and low work function metals commonly used as cathode materials in OLEDs. High-performance solution-processed PhOLEDs using commercial ETMs were also demonstrated using orthogonal solution-processing. The bulk conductivity and charge transport properties of ETMs were enhanced by a novel solution n-doping with alkali metal salts. Sulfone- and dibenzosuberane-based materials were demonstrated as promising new classes of ETMs that possess high triplet energies (> 2.8 - 3.0 eV). Multilayered PhOLEDs with a solution-processed blue triplet emission layer using high triplet energy ETMs as an electron-transport layer were found to have significantly improved performance, including high current efficiency and external quantum efficiency (~ 20 %). Finally, solution-processed polymer/polymer blend solar cells using new naphthalene diimide-based acceptor copolymers were investigated and found to be the most efficient all-polymer solar cells reported to date. Controlling polymer blend morphology by solution-processing from a co-solvent system also led to further enhancement of device performance.