| PhD Viva

Name of the Speaker: Mr. Dhruvajyoti Barah (EE17D035)
Guide: Dr. Debdutta Ray
Online meeting link: https://meet.google.com/caa-onei-bav
Date/Time: 3rd June 2025 (Tuesday), 10:00 AM
Title: Device and process engineering of OLEDs for display pixels and solid-state lighting

Abstract :

Organic Semiconductors have been of great interest because of their many advantages over inorganic semiconductors in terms of processing and applicability. Among the devices realized with organic semiconductors, organic light emitting diode (OLED) has been the most successful. Currently, the OLED technology is so mature for commercialization that it has started replacing the existing displays available in the market. Rapid advancement in the OLED technology is enabling the paradigm shift in displays from rigid to flexible and transparent displays. Simultaneously, OLEDs are also being developed as white light sources for lighting applications.

In the first part of the thesis, we address some of the key problems associated with blue OLED that is one of the primary building blocks of the pixels in active matrix OLED (AMOLED) displays. Unlike red and green OLEDs, phosphorescent blue OLEDs are not yet successful for commercial application due to the poor stability of wide band gap phosphorescent materials. Hence, blue OLEDs suffer from poor efficiency when designed using fluorescent materials. Obtaining a pure blue OLED as per National Television Standards Committee (NTSC) standard is still challenging. In this work, we fabricate a fluorescent blue OLED which employs a host-guest matrix of 4,4’-Bis(N-carbazolyl)- 1,1’-biphenyl (CBP), and 4,4’-bis[(N-carbazole) styryl] biphenyl (BSBCz or BSB4) in the emissive layer of the device. Here, using CBP: BSB4 (6 wt%) host-guest matrix, a pure blue emission satisfying the NTSC standards with CIEy < 0.1 is demonstrated. Charge balance engineering is performed by optimizing the device structure with various material and thickness variations of electron and hole transport layers and a maximum efficiency near to the theoretical limit of external quantum efficiency (EQE) for fluorescent OLED is achieved. For enhanced out-coupling of the generated light, the effect of optical interference is circumvented by exercising optical simulation of the device structure using transfer matrix method (TMM). For further enhancement of the EQE, we embed a thermally activated delayed fluorescent (TADF) blue dopant 10, 10’ - ( 4, 4’ – Sulfonylbis ( 4, 1-phenylene) ) bis (9, 9 – dimethyl - 9, 10 - dihydroacridine (DMAC-DPS) in the emissive layer and carry out optimization of the OLED structure. We also present a detailed analysis of the results obtained after characterization of the fabricated TADF blue OLEDs.

Conventional white OLEDs are designed with the incorporation of multiple colour emitting dopants in the device structure either in the same host matrix or by tandem stacking of individual colour emitting sub-OLEDs. However, this introduces a lot of structural and process complexity and reliability problems. In the second part of the thesis, we present a dopant-free colour tunable OLED using a planar heterojunction of 1,1-bis ((di-4-tolylamino) phenyl) cyclohexane (TAPC) and Bis [2-(diphenylphosphino) phenyl] ether oxide (DPEPO) that yields pure white emission at 8 V. Instead of the excitonic emissions from these materials, the device harvests the electromer and electroplex emissions to produce the red, yellow, and blue emissions simultaneously. By tuning the relative intensities of these three colours of emissions, the white emission is obtained with colour rendering index (CRI) of 89.

All of the present day AMOLED displays are fabricated with vacuum thermal evaporation (VTE) technique where fine metal masks (FMM) are used to define the pixels. While FMM has been successful in scaling up and are currently used in Gen 10 factories, the inherent slow growth rate of VTE leads to the requirement of economies of scale which in turn requires enormous capex. The material utilization in VTE is low which results in materials wastage. In the final part of the thesis, we address these two issues of VTE. In search of a cost effective and rapid fabrication process for the OLEDs, we present a novel method, named close space sublimation (CSS), which has a number of advantages over the conventional VTE process. We experiment the process with two different material transfer techniques and present a detailed discussion with the obtained results. The use of thin film of organic material on the source substrate in the CSS process, unlike powders in a crucible in VTE, ensures advantages in terms of uniform and easier heat flow required for rapid sublimation. We show that the CSS process offers nearly 100% material utilization, better energy efficiency, much faster deposition of organic films, and elimination of co-evaporation for the formation of host-guest matrix in the emissive layer. The fast deposition can be utilized to realize economies of speed where the factory sizes can be smaller without compromising on the takt times of device fabrication.