Manipulating Refractive Index In Organic Light Emitting Diodes and Direct Triplet Exciton Formation by Charge Injection.
Year: 2018
Type: article
Abstract: SALEHI, AMIN. Manipulating Refractive Index In Organic Light Emitting Diodes and Direct Triplet Exciton Formation by Charge Injection. (Under the direction of prof. Franky So).This work is a study on two major separate aspects of Organic Light Emitting Diodes (OLEDs).In the first half of this thesis, chapters two and chapter three, refractive index and the role of refractive index of constituting layers of a conventional OLED stack on light out-coupling efficiency is studied.It is shown that about 70% of the radiated power is lost to bound optical modes.Role of each mode and the effect of refractive index of each layer on each mode is studied.It is illustrated, that the transverse magnetic (TM) polarized bound modes, i.e. the transverse magnetic waveguided (TMWG) mode and the surface plasmon polariton (SPP) mode have the largest loss contributions.Further it is manifested that the TM-polarized modes are mainly distributed in the electron transporting layer (ETL) in a conventional OLED stack and reducing the refractive index of ETL can tremendously increase optical out-coupling at the expense of suppressing SPP and TMWG modes.It is displayed that transverse electric waveguide (TEWG) mode is mainly distributed in the transparent anode and the hole transporting layers (HTL) in a conventional OLED stack and reducing the refractive index of the transparent anode and the HTL can increase optical outcoupling at the expense of less TEWG loss.Refractive index of most of common organic thin films used in OLED devices is measured and reported.It is shown that majority of organic thin films show an in-plane negative birefringence due to anisotropic molecular geometry.Role of birefringence on light out-coupling is discussed and it is indicated that extraordinary (ordinary) refractive index mainly affects the TM (TE) polarized modes.Furthermore, the refractive index of amorphous thin films deposited using physical vapor deposition can be reduced by pore-inclusion using oblique angle deposition (OAD).In OAD, a substrate is positioned at an oblique angle with respect to a collimated incident vapor flux.Upon formation of initial islands, a self-shadowing lead to a porous columnar growth.The porosity is controlled by the angle of incident and, the refractive index of the thin film can be reduced in a controlled manner.In this work, refractive index of a common ETL, Alq3, is reduced using OAD, and the low-index ETL is incorporated in an OLED stack, resulting in a 30% increase in efficiency.The second half of this thesis, chapter four, presents a major discovery which changes the understanding of exciton formation in organic semiconductors.According to the conventional understanding of organic semiconductors, electrons and holes can only be injected into the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of an organic semiconductor, and the LUMO-HOMO gap is analogues to the bandgap in inorganic semiconductors, and the emitted photons have energies close to the HOMO-LUMO gap.Therefore, to form a radiative exciton a voltage equivalent to the HOMO-LUMO gap is required.Chapter four contradicts this understanding and shows evidence, for the first time, that charge carriers can readily be injected into the sub-bandgap (sub HOMO-LUMO gap) excitonic states, such as triplet states and then excite a band-gap exciton through bimolecular excitonic annihilation, such as triplet-triplet annihilation (TTA), and result in band-gap radiation.Based on the discovery, an efficient hyper-fluorescent blue OLED is made with sub-bandgap operating voltages.Although fluorescent OLEDs have a 25% limit for internal quantum efficiency, which results in a maximum author papers.
Author: Amin Salehi
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2921155178