20.1: Invited Paper: High Performance Top‐Emission Inkjet Printed OLEDs with Printed Electron Transport Layer

Abstract

High performance top‐emission inkjet printed (IJP) OLEDs have been successfully developed with printed electron transport layer (ETL). The major challenge of multilayer IJP OLEDs is that the solution deposition of the upper layer will cause penetration, dissolution, or intermixing of the under‐layer film. Through optimization of ETL ink formulation and employing the solvent‐resistant EML materials, the damage of the ink solvent to the EML film can be well alleviated.

Advances in Solution‐Processed OLEDs and their Prospects for Use in Displays

“…2023, 35,2207454 SOLEDs produced by IJP is estimated to be ≈25% lower than those of currently used white EOLEDs produced by LG Display Co. [30] However, SOLEDs still have big challenges that must be solved before they are suitable for industrial uses. The most important considerations are regarding solution processing, such as liquidstate processability of materials, [24][25][26]32,35,39,40] intermixing of layers or redissolution of underlying layers in OLEDs, [41,42] limited efficiencies and their severe rolloff, [20] and operational instability caused by molecular aggregation and lessdensely packed solutionprocessed organic film and their SOLEDs [16] compared to the thermally evaporated coun terparts. EOLED devices have been developed to have multiple functional layers including charge injection, transporting, and blocking layers for better charge balance, and therefore have high luminous efficiency and high operational stability.…”

“…In contrast, SOLEDs can use simple structures, like RGB EOLEDs and therefore can easily be fabricated in topemission structures (Figure 2b, right). [24,25,27,[32][33][34][35] The topemission struc tures amplify light intensity and increase luminous efficiency by using the strong resonancecavity effect between two elec trodes. Also, topemission devices emit light in the opposite direction from thinfilm transistors, so the aperture ratio can also be increased (Figure 1d).…”

“…However, SOLEDs use liquid solutions as starting materials, so other physical properties, including viscosity, vapor pressure, and sur face tension, must also be considered. [24][25][26]32,39,40] These prop erties are important to guarantee inkjet applicability and film flatness after vacuum drying (Figure 2c). Also, the underlying layer in the SOLED must have resistance to the solution that is used to process the upper layer; this resistance is described as solution orthogonality.…”

Innovations in thin‐film electronics for the new generation of displays

“…Typical OLEDs have turnon voltages in the 2-4 V range (dependent upon type). 20,21 Two LTPS TFTs each require 1.5-2 V of V DS to support sufficient current for maximum OLED luminance, due to a larger threshold voltage relative to IGZO TFTs. 13 It should be noted that this is not an inherent limitation of LTPS but can arise in complementary subpixel circuits, which require a threshold voltage of a few volts to ensure symmetry of operating voltage between nand p-type LTPS TFTs.…”

“…Figure 1 illustrates this second major power consumption issue with LTPO:2 or 3 TFTs in series between V DD and V SS , requiring V DD in excess of the 3.6‐V battery voltage available in a mobile device. Typical OLEDs have turn‐on voltages in the 2–4 V range (dependent upon type) 20,21 . Two LTPS TFTs each require 1.5–2 V of V DS to support sufficient current for maximum OLED luminance, due to a larger threshold voltage relative to IGZO TFTs 13 .…”