Silver nanoparticle films as a light scattering medium for optical extraction enhancement in organic light-emitting diodes

Thesis (Ph. D.)--University of Rochester. Department of Chemistry, 2016.This work focuses on the development and implementation of silver nanoparticle (AgNP) scattering thin-films into organic light-emitting diodes (OLED). It is commonly accepted that as much as 80% of the light generated within typical OLED devices is lost to various modes of optical waveguiding which prevents energy from leaving the device as useful radiation. It is the subject of this dissertation to address this substantial loss. By incorporating scattering centers within the device structure, trapped light is outcoupled from the device into air. There are numerous methods being explored as a means to increasing the overall outcoupling efficiency in OLEDs. Such methods include modification of the standard device by incorporating micro-lens arrays, high refractive index substrates, aperiodic dielectric mirrors and metal-oxide scattering layers. These methods have achieved various levels of success but there is still substantial room for improvement. When the size of an AgNP is properly tuned, theory predicts that a single particle can scatter light with a resonant wavelength more efficiently than any known metal-oxide structure. The scattering cross section can be as large as 10 to 20 times the geometric cross section. This makes Ag among the strongest known scattering materials in the visible region of the electromagnetic spectrum. Within this thesis, a procedure for fabricating AgNP which match theoretically specified size conditions is demonstrated. The optical properties of the particles are investigated and compared to theoretical Mie solutions to Maxwell’s equations. The particles are then incorporated into OLED devices as a means for disrupting total internal reflection of both the substrate and organic stack waveguide modes. For an AgNP scattering film prepared on the external side of a device substrate, the external quantum efficiency (EQE) of a phosphorescent yellow emitting device operated at 3 mA/cm² was improved to 18.6% compared to an otherwise optically and electronically equivalent improved to 18.6% compared to an otherwise optically and electrically equivalent control sample with an EQE of 14.6% (enhancement factor of 1.34x). For an AgNP film prepared on the internal side of a device substrate, the EQE of a phosphorescent yellow emitting device operated at 3 mA/cm² was improved to 22.9% compared to a control sample with an EQE of 14.4% (1.59x). Theoretical evidence is provided which indicates the system is far from optimized and by further tuning key parameters it is predicted that greater efficiency enhancement is possible