Development of New Polymer Systems and Quantum Dots - Polymer Nanocomposites for Flexible OLED Display Applications

 Development of New Polymer Systems and Quantum Dots Polymer Nanocomposites for Flexible OLED Display Applications Lihua Zhao, Zhang-Lin Zhou, Zengshan Guo, Jian Pei, Samuel Mao HP Laboratories HPL-2011-68 Semi-interpenetrating polymer networks, Emissive polymers, Quantum Dots conjugated oligomer/polymers nanocomposites, OLEOs Recently, tremendous progress has been made toward application of organic (small molecule/polymer) light-emitting diodes (OLEDs) in full color flat panel displays and other devices. However, with current technologies, OLEDs are still struggling with high manufacturing costs which really limit the size of OLEDs panels and with life time, especially differential aging of colors. To be more cost-effective for fabricating OLEDs, we believe solution-processing would be an attractive path due to its simplicity and highly reduced equipment costs. This proceeding paper discusses our recent progress in development of new polymer systems that are highly solvent-resistant but maintaining their photophysical properties and hybrid quantum-dots (QDs)-polymer nanocomposites for their use in multicolor and multilayer OLEDs pixels through solution-processing. Our new polymer systems are named conductive semi-interpenetrating polymer networks (C-Semi-IPNs) served in different layers of OLEDs devices, containing an inert polymer network and conducting polymer(s) including hole transport and emissive materials. Since these do not require complicated chemical modification or introduction of reactive moieties to OLED materials, many state-of-the-arts emissive polymers can be utilized to achieve RGB and white OLEDs. The research findings on hybrid QDoligomer nanocomposite as a good analogue lead to the successful design and synthesis of QDpolymer nanocomposites which were used to build proof-of-the-concept devices showing a good promise in providing excellent color purity and stability as well as device robustness. External Posting Date: May 21, 2011 [Fulltext] Approved for External Publication Internal Posting Date: May 21, 2011 [Fulltext] Copyright 2011 Hewlett-Packard Development Company, L.P. Development of New Polymer Systems and Quantum Dots Polymer Nanocomposites for Flexible OLED Display Applications Lihua Zhao, Zhang-Lin Zhou, Zengshan Guo, Jian Pei, Samuel Mao Hewlett-Packard Labs, Hewlett-Packard Company, 1501 Page Mill Rd, Palo Alto, CA, 94304, U.S.A. 2 The Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China 3 Lawrence Berkeley National Laboratory and Department of Mechanical Engineering, University of California, Berkeley, Mail Code 1740, Berkeley, CA, 94720, U.S.A. ABSTRACT Recently, tremendous progress has been made toward application of organic (small molecule/polymer) light-emitting diodes (OLEDs) in full color flat panel displays and other devices. However, with current technologies, OLEDs are still struggling with high manufacturing costs which really limit the size of OLEDs panels and with life time, especially differential aging of colors. To be more cost-effective for fabricating OLEDs, we believe solution-processing would be an attractive path due to its simplicity and highly reduced equipment costs. This proceeding paper discusses our recent progress in development of new polymer systems that are highly solvent-resistant but maintaining their photophysical properties and hybrid quantum-dots (QDs)-polymer nanocomposites for their use in multicolor and multilayer OLEDs pixels through solution-processing. Our new polymer systems are named conductive semi-interpenetrating polymer networks (C-Semi-IPNs) served in different layers of OLEDs devices, containing an inert polymer network and conducting polymer(s) including hole transport and emissive materials. Since these do not require complicated chemical modification or introduction of reactive moieties to OLED materials, many state-of-the-arts emissive polymers can be utilized to achieve RGB and white OLEDs. The research findings on hybrid QDoligomer nanocomposite as a good analogue lead to the successful design and synthesis of QDpolymer nanocomposites which were used to build proof-of-the-concept devices showing a good promise in providing excellent color purity and stability as well as device robustness.Recently, tremendous progress has been made toward application of organic (small molecule/polymer) light-emitting diodes (OLEDs) in full color flat panel displays and other devices. However, with current technologies, OLEDs are still struggling with high manufacturing costs which really limit the size of OLEDs panels and with life time, especially differential aging of colors. To be more cost-effective for fabricating OLEDs, we believe solution-processing would be an attractive path due to its simplicity and highly reduced equipment costs. This proceeding paper discusses our recent progress in development of new polymer systems that are highly solvent-resistant but maintaining their photophysical properties and hybrid quantum-dots (QDs)-polymer nanocomposites for their use in multicolor and multilayer OLEDs pixels through solution-processing. Our new polymer systems are named conductive semi-interpenetrating polymer networks (C-Semi-IPNs) served in different layers of OLEDs devices, containing an inert polymer network and conducting polymer(s) including hole transport and emissive materials. Since these do not require complicated chemical modification or introduction of reactive moieties to OLED materials, many state-of-the-arts emissive polymers can be utilized to achieve RGB and white OLEDs. The research findings on hybrid QDoligomer nanocomposite as a good analogue lead to the successful design and synthesis of QDpolymer nanocomposites which were used to build proof-of-the-concept devices showing a good promise in providing excellent color purity and stability as well as device robustness. INTRODUCTION Organic (Small molecule/polymer) light-emitting diodes (OLEDs) can be a great candidate to bring a new information display concept to our future life. Recently, tremendous progress has been made toward application of OLEDs in full color flat panel displays and other devices. However, with current technologies, OLEDs are still struggling with high manufacturing costs which really limit the size of OLEDs panels and with life time, especially differential aging of colors. Besides the cost, there is still quite a long way to go for OLEDs display being really flexible. Our goal is to create a display that is light weight, flexible, thin, extremely low cost, video capable, brilliant color as well as low power consumption. We believe that plastic and roll-to-roll (R2R) manufacturing will be key enablers toward this goal. Of course, a lot of components are associated for fabricating a flexible OLEDs display. Two important components are active matrix thin film transistor (TFT) backplane and OLEDs based frontplane. Hewlett-Packard Labs as the world-class high tech research center have developed Self-Aligned Imprint Lithography (SAIL) method [1] so as to be the first to demonstrate lowcost fabrication of an amorphous silicon TFT backplane containing very high (sub-micro) resolution and aspect ratio with a fully R2R process on a roll of 50 um thick and 1km long plastic substrates. With our highly flexible R2R processed active matrix TFT backplanes, for the OLEDs based frontplane fabrication to be compatible with plastic and R2R manufacturing, solution-processing appears more attractive and more cost-effective due to its simplicity and highly reduced equipment costs than vacuum deposition of small molecules by which the production cost increases super-linearly with the area to be coated, especially for most advanced OLEDs using multilayer structures [2-6]. One of the important issues related to solution-processing is multilayer capability. Therefore, it is of crucial importance that previously deposited layers are resistant against the solvent used to deposit an additional layer. Two different approaches are currently applied to such device fabrication: 1. use of “orthogonal” solvents and a change in the polarity/solubility of the materials, 2. introduction of reactive moieties. However, these approaches have their limitations, such as limited solvent selection or synthetic compatibility, and complication. In the first part of this proceeding paper, we describe a general method for developing new polymer systems, named conductive semi-interpenetrating polymer networks (C-semi-IPNs) that contain an inert, low-cost polymer network and conductive polymers serving different purposes in OLEDs stack. Such C-semi-IPNs can be easily incorporated into solution-processed multilayer OLEDs devices. Due to the nature of IPNs in which polymer networks are at least partially interlaced on a molecular scale, the conductive polymer(s) included in these C-semi-IPNs can be protected from subsequent layer depositions where a solvent would otherwise attack the unprotected underlying film. In addition, the formation of these C-semi-IPNs avoids the complexity of introducing reactive moieties to OLED materials. Our first development embedding a hole transporting material (HTM) into an inert cross-linked polymer network to protect the HTM while processing the subsequent layer has been reported[7,8], and a brief summary is included below. This technique has now been expanded to the synthesis of emissive C-semi-IPNs (referred to E-semi-IPN below), serving as emitting layer in OLEDs devices. Our results included in this paper demonstrate that this synthetic method, with its broad choices for making the suitable polymer networks, provides great opportunities to incorporate any state-ofthe-art emissive polymeric materials to achieve RGB and white OLEDs. Moreover, these materials may be suitable for fabricating both bottom-emitting and top-emitting device structures that are more easily integrated onto flexible roll-to-roll printed backplanes. As mentioned above, besides cost, the other challenge OLEDs are facing is differential aging of colors. Organic emitting materials including both small molecules and polymers for OLEDs not only emit lights with a broad band but also age on a different pace with different emissive colors. We had our interest to explore alternative materials to address this problem. Emissive colloidal quantum dots (QDs) are well-known for their good color purity, differential stability and tunability as well as robustness by nature. Recently, hybrid nanocomposites based on quantum dots (QDs) and conjugated polymers have attracted substantial attention due to their applications in flexible electronics, light-emitting displays [9], and photovoltaic [10]. In most of the previous QD-polymer nanocomposite materials, the QDs are merely physically mixed with the polymer matrix rather than chemically attached (through chemical bonds or chemical complexes) to the polymer material. Physically mixing CdSe/ZnS QDs with the blue-emitting electroluminescent polymer, PFO, cannot produce an expected homogenous and uniform dispersion for good energy transfer from PFO to QDs, therefore, the development of integrated polymer/QDs composite systems by molecular engineering is essential. In the second part of this proceeding paper, we first present in detail on our design, synthesis and characterizations of hybrid QD-oligomer nanocomposites, where emitting oligomers are coordinately bonded to QDs surfaces, as a good analogue leading to a successful development of nanocomposites of CdSe/ZnS based QDs and amine-functionalized poly(9,9-dihexylfluorene) (PFH) derivatives. A series of characterization studies including photoluminescence (PL), photoluminescence excitation (PLE) and electroluminescence (EL) measurements revealed that the efficient Förster energy exchange from the PFH derivative to the QDs results in an efficient red emission purely from the QDs. Their use in hybrid QD-polymer LEDs show a good promise in providing excellent color purity and stability as well as device robustness.