Organic Electronic Devices

Transcription

1 Organic Electronic Devices Week 5: Organic Light-Emitting Devices and Emerging Technologies Lecture 5.2: Design Considerations for OLEDs Bryan W. Boudouris Chemical Engineering Purdue University 1

2 Lecture Overview and Learning Objectives Concepts to be Covered in this Lecture Segment Introduction to Important Properties of Molecules Used in Organic Lightemitting Devices (OLEDs) Discussion of Common OLED Semiconducting Materials and the Deposition Techniques Used for These Materials White Light OLED Geometries and Passive and Active Matrix Display Designs Learning Objectives By the Conclusion of this Presentation, You Should be Able to: 1. List at least four important design considerations for molecules to be used in OLED devices. 2. Explain why the introduction of heavy metal atoms to the molecular structure of organic materials increases OLED device performance. 3. Sketch a schematic diagram of how OLED displays can be created to emit white light.

3 Desired Properties of OLED Semiconducting Materials There are a number of important properties associated with OLED semiconducting materials. In general, these work better for small molecule organic semiconductors as opposed to polymeric semiconductors. Specifically, the materials must: Be of very high purity. This is because there is a large amount of current that is to be passed through the active layer of the devices. Have high thermal and oxidative stability. Have a high luminescence quantum yield in the solid state. Have high charge transport mobility values. Form quality thin films in order to prevent pinhole defects. Have good color purity in order to be in the appropriate range for the CIE coordinate diagram.

4 The First Generation of OLED Materials Were Fluorescent Common Fluorescent Organic Semiconducting Materials Blue Green Red 4,4 -bis(2,2 -diphenylvinyl)- 1,1 -biphenyl (DPVBi) Tris(8-hydroxyquinolato) aluminum(iii) (Alq 3 ) + 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) These Materials Have a Maximum Quantum Yield of 25%

5 Next-Generation OLED Materials are Phosphorescent Common Phosphorescent Organic Semiconducting Materials Blue Green Red Bis[2-(4,6- difluorophenyl)pyridinato- C 2,N](picolinato)iridium(III) (F 2 Irpic) Tris[2-phenylpyridinato- C 2,N]iridium(III) [Ir(ppy) 3 ] Platinum(II) 2,3,7,8,12,13,17,18- octaethyl-21h,23hporphyrin (PtOEP) These Materials Have a Much Higher Quantum Yield Due to the Presence of Heavy Metal Atoms in the Molecular Structure

6 Heavy Metals Facilitate Intersystem Crossing Image Reproduced From: Inspire Website.

7 Device Fabrication Equipment Evaporator Schematic Organic Evaporation Chamber Heating Source Quartz Crystal Monitor (QCM) Evaporation Shutter Image Reproduced From: Innovative Technologies Website. Further Reading: Geffroy, B.; le Roy, P.; Prat, C. Polym. Int. 2006, 55, 572.

8 Examples of Artistic Solid State Lighting Through WOLEDs

9 Strategy for Producing White Light Emission Image Reproduced From: Cambridge Display Technologies.

10 Passive Matrix versus Active Matrix Displays Passive Matrix Display Active Matrix Display Further Reading: Geffroy, B.; le Roy, P.; Prat, C. Polym. Int. 2006, 55, 572.

11 Summary and Preview of the Next Lecture Organic light-emitting devices (OLEDs) must be designed from the molecular level through the model (e.g., wide screen television level). Through continuous breakthroughs in science and engineering, high-impact applications are being developed for small molecule-based OLED devices currently. These materials are made in manners that allow for the large-area coverage of devices, and allow for multiple colors to be observed on a single system. Next Time: Polymer-based Thermoelectric Devices The overlay of red, green, and blue light-emitting sub-cells allow for the emission of just about any color of light that is of interest. Because OLEDs operate by emitting light directly instead of filtering certain wavelengths of light from a white-light source, the images appear sharper and brighter to the human eye. The application of active matrix displays to OLED technologies has allowed them to breakthrough in large-area, high-resolution applications.