Towards Greatly Improved Efficiency of Polymer Light Emitting Diodes
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Towards Greatly Improved Efficiency of Polymer Light Emitting Diodes Zhang-Lin Zhou*, Xia Sheng, Lihua Zhao, Gary Gibson, Sity Lam, K. Nauka and James Brug Hewlett Packard Labs, 1501 Page Mill Road Palo Alto 94304, CA 94304, U.S.A. ABSTRACT Polymer light-emitting diodes (PLEDs) show great promise of revolutionizing display technologies. The archetypical multilayer PLED heterostructure introduces numerous chemical and physical challenges to the develoment of efficient and robust devices. These layered structure are formed from solution based spin-casting or printing with subsequesnt removal of the solvent. However, solvent from the freshly deposited film may dissolve or partially dissolve the underlying layer resulting in loss of the desired structure and corresponding device functionality. Undesirable changes in the morphology and interfaces of the polymer films are another detrimental effect associated with solvent removal. Herein, we demonstrated that by embedding hole transporting materials (HTLs) in a cross-linked polymer matrix, the total luminance and external quantum efficiency were greatly improved over devices without this HTL layer. INTRODUCTION The growth and proliferation of electronic devices has created a significant industry-wide demand for new, low-power, light and low-cost display technologies. This demand underlies a current display development initiative within display industries. In the last decade, a number of intensive studies have been made to achieve efficient polymer light-emitting diodes (PLEDs). Chemistry and chemical principles have played a crucial role in the evolution of efficient PLEDs. As a result of extensive multidisciplinary efforts, modern PLEDs offer substantial benefits over conventional cathode ray tubes (CRT) and liquid crystal display (LCD). PLEDs display provides superior brightness and color purity, markedly lower power consumption, as well as full viewing angle without compromising image quality. Compared with small molecule organic LEDs (OLEDs), PLEDs use solution-based processes, which offer the potential for lower cost and roll-to roll processing on flexible substrates. To realize these favorable advantages, significant chemical and physiochemical challenges must be addressed. These challenges include (i) excellent solution-processable multilayer structures, (ii) improved efficiency via balanced charge carrier injection and leakage current reduction, (iii) better thermal stability and (iv) increased operational lifetime [1]. An efficient PLED device typically consists of a stack of organic/polymeric thin layers, each one of them performing a specific function aimed at improving the device performance or achieving the desired device functionality. In many cases, these layered structures are formed from the polymer solution by spin-casting or printing with subsequent removal of the solvent. However, solvent from the freshly deposited film frequently dissolve or partially dissolve the underlying layer, resulting in loss of the desired structure and correspon
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