| PhD Seminar

Name of the Speaker: Mr. Dhruvajyoti Barah (EE17D035)
Guide: Dr. Debdutta Ray
Online meeting link: https://meet.google.com/bxx-qvpe-ojc
Date/Time: 22nd August 2023 (Tuesday), 3:00 PM
Title: Device and Process Engineering of OLEDs for Display Pixels and Solid-state Lighting

Abstract

Organic light emitting diodes (OLEDs) have gained tremendous attention in display and lighting technologies due to their attractive features such as 1) colour reproducibility due to a wider colour gamut, 2) thin and lightweight, 3) ease of fabrication, 4) power efficient and 5) feasibility to realize on flexible substrates.

In the first part of the talk, we will present an optimized device structure of a host-guest matrix-based pure blue fluorescent OLED with a Commission Internationale de l’Eclairage (CIE) coordinate of (0.148, 0.091) using BSB4 as the fluorescent emitter. A pure blue emission suitable for display application is achieved by the blue shift of the emission spectrum of the host-guest matrix from that of the pristine guest molecules. The OLED structure is optimized by considering (i) charge balance in the emissive layer for high exciton density and (ii) optical interference of generated light in the organic layers for increased light out-coupling. With the optimized device structure, we obtain a maximum EQE of > 4%, that is close to the theoretical EQE limit of 5% for fluorescent OLEDs and overcome the maximum reported EQE (=2%) for OLEDs with BSB4 as a light-emitting material. Similarly, we design and fabricate a TADF blue OLED by using a host-guest matrix of DPEPO and DMAC-DPS in the emissive layer and demonstrate a maximum EQE of 17%, where the theoretical limit of EQE for a TADF OLED is 20% taking light out-coupling effects into consideration.

In the second part of the talk, a dopant-free OLED will be presented that offers colour tunability with varying applied voltage and yields a white-light emission at 8 V with colour rendering index (CRI) of 89. With the proposed device structure, we overcome the difficulties in realizing conventional white OLEDs, which generally requires either a tandem stacking of red, green, and blue emitting units by charge generation layers (CGL) or incorporation of the red, green, and blue dopants in the same host matrix. The dopant-free OLED utilizes the electromer and electroplex emissions from a planar heterojunction of two electron and hole transport materials to yield a white light that bears a full-width half maxima (FWHM) of 330 nm with the three major peaks corresponding to three colours - red, yellow, and blue. The analysis of the features in the electroluminescence (EL) spectrum of the emitted white light will be presented.

Finally, we will discuss the aspects of the close space sublimation (CSS) process to fabricate OLEDs and present the results obtained after experimenting with two different heating techniques. The CSS process has the advantage of OLED fabrication with nearly 100% material utilization, energy efficient and fast deposition of organic films, elimination of co-evaporation for the formation of host-guest matrix etc.