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

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 Keyword(s):

Semi-interpenetrating polymer networks, Emissive polymers, Quantum Dots - conjugated oligomer/polymers nanocomposites, OLEOs

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.

External Posting Date: May 21, 2011 [Fulltext] Internal Posting Date: May 21, 2011 [Fulltext]

Approved for External Publication

Copyright 2011 Hewlett-Packard Development Company, L.P.

Development of New Polymer Systems and Quantum Dots - Polymer Nanocomposites for Flexible OLED Display Applications 1* Lihua Zhao , Zhang-Lin Zhou1, Zengshan Guo2, Jian Pei2, Samuel Mao3 1 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.

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. EXPERIMENTS Synthesis of C-Semi-IPNs Chemicals were purchased and used as received. For a proof-of-concept demonstration, the preparation of C-semi-IPNs containing ADS132GE as HTM, referred to HTM C-semi-IPN below and pre-polymer formulations can be found in our previous reports [7, 8]. For E-semiIPNs, we have formulated a mixture of N-vinylpyrrolidone (20%), ethoxylated bisphenol A dimethylacrylate (EBAD) (40%), trimethylolpropane trimethylacrylate (35%) and a thermal initiator or UV initiator (5%). Toluene, as solvent, was added to the above mixture to form a solution with a concentration of 5%. Emissive polymers with many different emission colors (e.g. red, green, blue and white) can be dissolved or swollen within the solution. For example, poly(9.9-dioctylfluorenyl-2,7-diyl) (PFO, ADS129BE, American Dye Source) with a concentration of 6 mg/mL in toluene. The well-mixed solution undergoes polymerization to give a cross-linked network to provide E-semi-IPNs as expected. Synthesis of functionalized oligofluorene, polyfluorene derivatives and corresponding hybrid nanocomposites with CdSe/ZnS core-shell QDs Starting materials were purchased and used as received. The key intermediate 6 p-2-(7Bromo-9,9-di-n-octylfluorene)benzyl-Di-n-Octylphosphine Oxide (Br-DOF-Bn-DOPO) was synthesized from 2,7-dibromo-9,9-di-n-octylfluorene via four key steps with an overall yield of about 42%. Functionalized oligofluorene (trimer: Bis-DOPO-Bn-tri-Dioctylfluorene (BDBTFO)) was then synthesized via coupling reaction between Br-DOF-Bn-DOPO (0.270 g, 0.324 mmol) and 9,9'-Dioctylfluorene-2,7-Bis(trimethylene Borate) (0.082 g, 0.147 mmol) in toluene in the presence of degassed K2CO3 (0.082g, 0.59 mmol) aqueous solution (1.5 mL H2O) under Argon at room temperature using Pd(PPh3)4 (8.5 mg, 0.074 mmol) as catalyst. After purification of the cruel material obtained from the recation by column chromatography (EtOAc: Hexane = 2:1 to 2% MeOH in EtOAc), the product BDBTFO was given as a white solid (0.140 g, 50%). This BDBFTO was then added to a solution of pyridine-capped CdSe/ZnS QD (5 mg/mL) in THF. After stirring overnight at 65 ºC in the glovebox, MeOH was added to precipitate out QD-BDBTFO nanocomposite. The collected precipitate was washed with MeOH and then dissolved into 1 mL of CHCl3. The resulting QD-BDBTDO solution was used for further study. Detailed synthetic experiments and materials characterization of oligofluorene

(BDBTFO) and QD-BDBTFO hybrid nanocomposite will be reported elsewhere. Aminefunctionalized polyfluorene derivatives and corresponding QD-polymer nanocomposites were synthesized, fully characterized by 1H and 13C NMR, UV-vis and photoluminescent spectroscopy, gel permeation chromatography, elemental analyses and reported in our published paper [11]. Physical Characterization Absorption spectra were recorded on Cary 6000i UV-vis-NIR spectrometer. PL and EL spectrum was recorded on JobinYvon Triax 550 Spectrometer. PLE measurements were also taken on Cary 6000i UV-vis-NIR spectrometer with some customized modification. Transmission electron microscopy (TEM) was performed on a Philips CM300FEG operating at 300 keV using zero loss energy filtering with Gatan CCD for images. Scanning electron microscopy (SEM) was carried out using a Hitachi S-4500 cold field emission microscope. Device Fabrication For a proof-of-the-concept demonstration, we have chosen a very simple OLEDs device configuration of ITO-PET or glass/PEDOT-PSS or DuPont Buffer (DB)/(C-semi-IPNs as hole transport layer, optional)/polymer, E-Semi-IPNs or hybrid nanocomposite/Al. The ITO-coated glass or PET substrates was cleaned with O2 and CF4 plasma. PEDOT:PSS or DB was coated by spin casting and dried at 100 °C for an hour to give a smooth film of 45 nm. The films (60-80 nm) of polymers, C-semi-IPNs, or hybrid nanocomposites were coated by spin casting from solutions on top of PEDOT or DB, respectively. Then Al film (~120 nm) as cathode was thermally evaporated in a vacuum chamber under a pressure of 4 ×10-5 Torr through shadow masks to give a finished OLEDs stack. DISCUSSION Semi-Interpenetrating Polymer Networks A general methodology for the synthesis of C-semi-IPNs is developed and shown in Figure 1 via a sequential method. X is a mixture of the cross-linkable monomers, oligomers or polymers containing cross-linking agent(s) and initiator or initiators. Cross-linking agents can be 2-branch, 3-branch, or 4-branch cross-linkers that are initiated with energy appropriate to either thermal or photo initiators. Conductive polymer(s) including hole transport materials and emissive materials with many different emission colors (e.g. red, green, blue and white) (referred to herein as "A" based polymers) can be dissolved or swollen within the X-solution.

X X A A XA A A X XA X X A Cross-linking X A AA X polymerization X X AX AA A XA X X A A XA X A XA X A X A AA X X AX A X AA XA X

XA X X X

Semi-IPN

Figure 1. Formation of C-semi-IPN via sequential method In our previous report [7, 8], ADS132GE as hole transport materials (HTM) interpenetrated into a cross-linked polymer network shown in Figure 1a. This cross-linked

polymer network mostly preserved the desired optical and electrical properties of the HTM and maintains the morphology of the embedded polymer film after the solvent (which is used for additional layer deposition) washing. This is verified by corresponding PL measurement and AFM images shown in Figure 1b and 1c.

n N

n-C4 H9

HTL: ADS132GE

Mix with commercially available curing agent

a. b. c. Figure 2. a. Scheme of synthesis HTM C-semi-IPNs; b. PL spectra of HTM C-semi-IPNs before and after solvent washing; c. AFM images of HTM C-semi-IPNs.

HTL uniformly distributed within cross-linked polymer matrix

A number of OLED devices were fabricated and tested using HTM C-semi-IPNs as hole transport layer (HTL) with subsequent layer of ADS129BE as emissive layer (EML) in OLED stack. Advantages offered by this HTL were clearly seen that the electroluminescence efficiency is improved greatly, with a significant reduction of leakage current, by more balanced electron and hole injection in the devices, which demonstrate the effectiveness of HTM C-semi-IPNs [8]. With the inspiration of these research findings, this technique has now been expanded to the preparation of emissive-semi-IPNs (referred as E-semi-IPN below) in which conductive polymers are emitting conjugated polymers. The resulting materials are used to serve as emitting layers in OLEDs devices. For a proof-of-concept demonstration, the formulated solution with ADS129BE as emissive material example undergoes thermally-initiated polymerization to give a cross-linked network to provide E-semi-IPNs as expected. This set of materials does not necessarily represent the best formulation, but it suffices for demonstrating the advantages of Esemi-IPNs for OLED applications. PFO-only thin films and devices are used as control for evaluating blue-emitting E-semi-IPNs. The solvent resistances of thin films of PFO-only and PFO-based E-semi-IPN on DB coated ITO glass were characterized through UV-Vis absorption and photoluminescence (PL) spectroscopy, as shown in Figure 3. After toluene washing, the UV-Vis absorbance of E-semiIPN film remains over 85% of its original film value while the UV-Vis absorbance of PFO-only film decrease by over 95% (Figure 3a). Even more importantly, the PL intensity of E-semi-IPN film remains the same as before toluene washing, compared with almost none emission from the toluene-washed PFO-only film. The over 10% loss of UV-vis absorption from PFO based Esemi-IPN film after toluene washing may attribute to the removal of PFO that was staying on the surface of interpenetrating polymer networks but not interlaced with those highly crosslinked polymer networks. Moreover, E-semi-IPN film shows a higher PL intensity than PFO-only films even though the PFO absorbance (both peak and integral) of E-semi-IPN films is lower than PFO only films (Figure 3b). This indicates that E-semi-IPNs have better internal emission efficiency, which is one of their important advantages over polymer-only emitters when used in OLEDs. Compared with PFO-only films, PFO based E-semi-IPN films appear to have red-shifted absorption and emission which result from the formation of a PFO -phase component. Increased -phase content has also been linked to an increase in the ratio of singlet to triplet excitons, a decrease in photobleaching, and improved hole mobility [12, 13].

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a. b. Figure 3. a. UV-Vis spectra; b. photoluminescence (PL) spectra of PFO-only and PFO based Esemi-IPN films before and after toluene washing. To determine if E-semi-IPNs as emission layers affect EL performance, OLED devices were fabricated with the simple configuration of ITO-glass/DB/PFO-only or PFO-based E-semiIPN/Al. These simple structures do not represent the optimal situation for a best device performance, but they are sufficient to demonstrate the concept. These sample OLED devices were evaluated by current-voltage (I-V) characteristics and electroluminance (EL) spectroscopy shown in Figure 4. Compared with the control device using PFO-only as the emissive layer, the PFO based E-semi-IPN OLED device showed a significantly reduced current leakage, and its luminance efficiency also improved as a factor of two. These results may be attributed to better energy transfer and charge trapping induced by -phase content in E-semi-IPN films, leading to a better balance of charges and more efficient exciton formation [14]. The EL spectrum of PFO based E-semi-IPN OLED device, similar to the PL study discussed above, also showed a red shift of 10 nm compared with the EL spectrum of the PFO-only device, which further support the existence of PFO -phase content in E-semi-IPN films and its influence on device performance. As shown in the inset of Figure 4b, the OLED device built with a toluene-washed PFO based Esemi-IPN layer exhibited bright blue emission. This strongly indicates that the inert polymer networks introduced in E-semi-IPN based OLED devices do not negatively impact device function but help improve the device performance with improved device robustness and solvent resistance. This provides opportunities to incorporate a broader pool of emissive polymers or even possible QD-polymer nanocomposites into the network, optimize the OLED device architecture by multiple layer structures, and build multi-color devices in the fashion of either bottom or top emitting device structures.

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Figure 4. a. I-V characteristics of PFO-only and PFO based E-semi-IPNs OLED devices; b. PL and EL spectra (Inset: EL images of sample E-semi-IPNs OLEDs which was built with the toluene-washed PFO based E- semi-IPN layer). Preliminary results on multi-color E-semi-IPN based OLED devices are shown in Figure 5. Red, Green and Blue (RGB) pixels built on different chips, the same chip (Figure 5a) were achieved with poly[2-methoxy-5-(-ethylhexyloxy)-phenylene-vinylene] (MEH-PPV), poly[9dioctyl-fluorene-co-benzothiadiazole)] (F8BT), and PFO derivative based E-semi-IPNs, respectively, deposited either directly on an ITO/DB surface or on top of other E-semi-IPNs layers, RGB on separated chips were also demonstrated, shown at up-left corner of Figure 5a. White emission OLED devices in a bottom-emitting structure (Figure 5b, Glass/ITO/DB/E-semiIPN/Al, emitting light through ITO glass) and a top-emitting structure (Figure 5c, Glass/Al/Esemi-IPN/DB/Ag (15 nm), emitting light through semi-transparent Ag film) were fabricated with an E-semi-IPN layer with a mixture of RGB polymers in the same polymer networks. Moreover, a white emission OLED panel (1 x1.5 cm2) with an "HP"-logo color filter presents a simpler way to achieve great RGB colors shown in Figure 5d; and two demo devices built on PET flexible substrates integrated with R2R processed segmented backplane on plastic.

(a)

(b)

(c)

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(e)

Figure 5. a. RGB pixels on the same chip prepared from E-semi-IPNs with R, G, B emissive polymer respectively; b. Bottom emitting white pixels from an E-semi-IPN with R, G, B emissive polymers; c. Top emitting white pixels from the same material in b; d. Bottom emitting white panel with color filters; e. demo devices on PET integrated with R2R processed segmented backplane on plastic. Hybrid QD-oligomer/polymer nanocomposite As we discussed above, the inert polymer networks introduced in E-semi-IPN based OLED devices do not negatively impact device function but help improve the device performance with improved device robustness and solvent resistance. It is noteworthy that a similar scheme could be used to incorporate emissive species like, for example, semiconducting quantum dots (QDs) or their mixtures with the appropriate polymer agents to form a multilayer structure since QDs as emitters provide unique properties including narrow emission band which gives good color purity, great color tenability and differential stability. In this part of the paper, we describe our research findings in developing hybrid QDOLED pixels where QD-polymer hybrid nanocomposite. One critical challenge of the hybrid

pixel development is to develop functionalized conjugated conducting polymers that can chemically bind or attach to QDs so as to achieve optimal distribution of emissive inorganic QDs within these polymers so as to enhance the efficiency of Föster energy transfer, thereby increasing device efficiency and improving color purity by quenching the remaining polymer emission. Our first design of these novel materials is to incorporate functional groups such as organo phosphine oxide to QDs surfaces into conducting oligomers or polymers in order to achieve direct attachment of QDs to oligomers or polymer matrix. A number of new organic building blocks have been designed and synthesized. These building blocks are used either as capping agents or monomers to build novel functionalized oligomers/polymers shown in Figure 6 via multi-step organic syntheses.

C8 H17 C8 H17 P O O P C8H17 C 8H 17

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Figure 6. UV-Vis Spectra of functionalized Oligo-/Polyfluorenes Bis-functional oligomer BDBTFO with three fluorene units is selected as the first candidate of hybrid nanocomposites for detailed study to prove the concept since this oligomer with reasonable chromophore length can be synthesized in a high purity which provides relatively easier control for preparing hybrid nanocomposite. The synthesis of this new material is shown in Scheme 1. First, the key intermediate 6 p-2-(7-Bromo-9,9-di-n-octylfluorene)benzylDi-n-Octylphosphine Oxide (Br-DOF-Bn-DOPO) was synthesized from 2,7-dibromo-9,9-di-noctylfluorene via four key steps with an overall yield of about 42%. Key intermediate 6 was treated with 9,9'-Dioctylfluorene-2,7-Bis(trimethylene Borate) via palladium-mediated Suzuki coupling reaction to afford bis-functional oligoflurene trimer BDBTFO with a yield of 50%. Subsequently, oligofluorene BDBTFO reacted with pyridine-capped CdSe/ZnS core-shell QDs in THF via ligand exchange to form hybrid QD- BDBTFO nanocomposite. QD-BDBTFO nanocomposite was evaluated by transmission electron microscopes (TEM), UV-vis and PL measurements. Resulting image and spectra are shown in Figure 7. TEM image (Figure 7a) shows that CdSe/ZnS core-shell QDs are very well separated without aggregation after BDBTFO ligand exchange process, and distances between QDs are in the range of oligomer BDBTFO's chemical lengh. UV-Vis, PL and PLE characterization results indicated that the direct attachment of QDs with conducting oligomers dramatically improves the Föster energy transfer (close to 100%) from oligomer BDBTFO to QDs and give a pure color from quantum confined CdSe/ZnS nanoparticles. This interesting result provides the strong theoretical support to EL devices with improved Föster energy transfer.

Scheme 1. Synthesis of oligofluorene BDBTFO and QD-BDBTFO nanocomposite

Br O Br n-BuLi TMSCl Br TMS n-BuLi O i-Pr O B O 3 4 B O TMS C8 H 17 O P C8H17 Br TMS 2 C 8 H17 C8 H17 Br 2 P O NaOAc THF Pd(PPh3 )4

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c. Figure 7. a. TEM image of QD-BDBTFO nanocomposite; b. UV-vis spectra; c. PL spectra; d. PLE spectra of thin films of BDBTFO, QD-BDBTFO, and QD-BDBTFO with additional BDBTFO. Even though hybrid OLED devices built with this material could not successfully give us convincing EL performance due to poor film forming of QD-BDBTFO nanocomposite, the

research findings of QD-BDBTFO nanocomposite are still encouraging, which leads us to a further development on this subject. We have reported a few series of functionalized polyfluorene derivatives, which might have great utility to prepare QD-polymer nanocomposite shown in Scheme 2 [11, 15, 16]. Amine-functionlized poly(9,9-dihexylfluorene) (PFH) derivatives are presented here as an example. Five amine-functionaized conjugated PFH copolymers were synthesized with different composition, which were fully characterized and showed great thermal stability up to 220 ºC. The detailed study of their photophysical and electrochemical properties has been reported [11]. From the electrochemical data, it was estimated that band gap of these functionalized polymers was around 2.70 to 3.07 ev for P1-P5, which made them good candidates as host materials for QD-polymer hybrid nanocomposites. Scheme 2. Design and synthesis of QD-polymer hyrid nanocomposites

y x

y

x N N + N N N R R R R

H2N

NH 2

39:1 PFH-NH 2F-39-1 (P1) 19:1 PFH-NH2 F-19-1 (P2) x:y= x y 17:3

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PFH-NH 2F-9-1 (P3) PFH-NH2F-17-3 (P4) PFH-NH2 F-4-1 (P5)

Carton structure of Ligand-exchange Product

For proof-of-concept, polymer PFH-NH2F-39-1 (P1) was mixed with CdSe/ZnS coreshell QDs with pyridine ligands, in which pyridine ligand was exchanged by the amine functional groups on P1 to form hybrid QD-polymer nanocomposite. Although there was no strong chemical information collected as evidence of polymers directly attaching to QDs, SEM images shown in Figure 8 revealed that this amine functionalized copolymer helps QDs well dispersed throughout the thin film, compared with an example of the simple physical mixture of trioctylphosphine oxide (TOPO)- covered CdSe/ZnS in poly(9,9-dioctylfluorene) (PFO).

a. b. Figure 8. SEM Images of QD-P1 Composites and QD-TOPO in PFO on Si substrates. Thin films of QD-P1 nanocomposite were deposited by spin casting from choloform solutions on quartz substrates for PL and PLE study. The PL intensity of P1 in this nanocomposite shown in Figure 9a significantly decreases (to <10% of the pure polymer's emission). Note that the PL emission from the QDs may result not only from the expected energy transfer from the polymer but also from direct excitation. As illustrated in Figure 9b, PLE spectrum of the nanocomposite thin film recorded at the maximum of the QD emission (626 nm) exhibits the absorption characteristic features (as shown in Figure 9a inset). The

y

x

4:1

correspondence between the absorption by the polymer and increased emission by the QDs confirms that when exciting at ~380 nm, the majority of the QD emission results from energy transfer from blue-emitting polymer P1 to red-emitting QDs.

1.0 PFH-NH2F-39-1 QD-PFH-NH2F-39-1

1.0 PFH H -N 2F-39-1 Q -PFH D -NH2F-39-1

1.0 PFH-NH2F-39-1 QD-PFH-NH2F-39-1 QD-Pyridine

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PFH-NH2F-39-1 QD-PFH-NH2F-39-1

0.8

A bs or bance

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0.0 300

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35 0

400 W av e l ength (nm)

45 0

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@ 4V

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0.0 350 400 450 500 550 600 650 700

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b.

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@ 6V

Figure 9. a. PL spectra of P1 and QD-P1 nanocomposite thin films on quartz substrates (inset shows the absorption of both thin films); b. PLE spectra of thin films of P1, QD-P1 and pure QDs on quartz substrate. c. EL spectra of ITO/DB/P1/Al (black square) and ITO/DB/QD-P1/Al (red dot) collected at 6 V (Inset: EL picture when the device was operating at 3 V). Therefore, a simple proof-of-concept hybrid OLED device was fabricated with the simplest structure configuration of ITO-glass/DB/P1 or QD-P1/Al to learn how effective hybrid nanocomposite works in actual OLED devices. The EL behavior shown in Figure 9c is very similar to their PL behavior, in fact EL spectrum of QD-P1 showed almost none of P1 emitting characteristics. The greatly QD-dominated emission with current efficiency of 0.36 Cd/A from such a simple structure was consistent with efficient energy transfer from P1 to QDs as shown in Figure 9c inset. The QD-only device with the same device structure and fabrication technique was not even operated long enough for EL characterization. The use of hybrid QD-polymer nanocompsite in this simple hybrid OLED device shows a good promise in providing excellent color purity and stability as well as device robustness. Optimization using more sophisticated device architectures, for example, including C-Semi-IPN as HTL described in the first part of the paper, and incorporating QD-P1 into Semi-IPN to form QD-E-Semi-IPNs so as to be able to deposit additional ETL layers, is currently in progress and more detailed characterization of optimized devices will be published in due course. CONCLUSIONS In summary, we have presented 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 polymer(s) such as hole transport materials and emissive polymers. Our C-semi-IPN films including both HTM C-semi-IPNs and E-semi-IPNs showed good solvent resistance and retaining of original photophysical properties. In addition, a PFO based E-semi-IPN OLED device also showed good EL preservation without any negative impact from incorporated inert polymer networks. This may provide an environment for the formation of a -phase of PFO which helps improve the charge balance and device performance. We have also showed preliminary results of achieving E-semi-IPNs based multi-color devices with different device structures by a facile solution-processing technique without complicated chemical modification or introduction of reactive moieties to OLED material. And also we have developed hybrid nanocomposite materials to take advantages from both organic polymer and

quantum dots region. Our research findings from QD-oligomer BDBTFO supported chemically bonding together between QDs and this functional oligomer provides good energy transfer (close to 100%) from oligomer BDBTFO to QDs and give a pure color from quantum confined CdSe/ZnS nanoparticles. Such a chemical system served as a good analogue directly leaded to a development of functionalized conducting polymers and their corresponding hybrid QD-polymer composites. Our selected QD-P1 hybrid material showed a good promise in providing excellent color purity and stability as well as device robustness. More detailed work on further understand basics, optimizing materials formulation and device structures and improving device performance are currently underway. ACKNOWLEDGMENTS This work was supported by Hewlett-Packard Company and National NSF of China. Thanks to colleagues of Information Surface Lab at HP Labs for great support and valuable discussion. REFERENCES 1. Thin Film Devices and Methods for Forming the Same. US Patent 7541227. 2. Klaus M_llen, Ullrich Scherf, Organic Light Emitting Devices: Synthesis, Properties and Applications, Wiley-VCH, Verlag GmbH & Co. KGaA, Weinheim, 2006, pg. 151. 3. T. R. Herbner, C. C. Wu, D. Marcy, M. H. Lu, J. C. Strum, Applied Physics Letter, 72, 519 (1998). 4. G. Gustafsson, Y. Gao, G. M. Treacy, F. Klavetter, N. Colaneri, A. J. Heeger, Nature, 357, 477, (1992). 5. S. R. Forrest, Nature, 428, 911, (2004). 6. R. Kiebooms, R. Menon, and K. Lee, Handbook of Advanced Electronic and Photonic Materials and Devices, edited by H. S. Nalwa _Academic, San Diego, 2001,Vol. 8, pg. 1. 7. Z. L. Zhou, X. Sheng, K. Nauka, L. Zhao, G. Gibson, S. Lam, C. Yang, J. Brug, and R. Elder, Applied Physics Letter, 96, 013504, (2010) and references therein. 8. Z. L. Zhou, X. Sheng, L. Zhao, G. Gibson, S. Lam, K. Nauka, J. Brug MRS Symposium B Proceedings Paper, 1154, 1154-B10-108, (2009) and references therein. 9. (a) V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Nature, 370, 354, (1994). (b) B. O. Dabbousi, M. G. Bawendi, O. Onitsuka, M. F. Rubner, Appl. Phys. Lett., 66, 1316, (1995). (c) T.-W. F. Chang, S. Musikhin, L. Bakueva, L. Levina,; M. A. Hines, P. W. Cyr,; E. H. Sargent, Appl. Phys. Lett. 84, 4295, (2004). 10. (a) W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science, 295, 2425 (2002). (b) S. A. McDonald, G. Konstantatos, S. G. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina, E. H. Sargent, Nat. Mater., 4, 138, (2005). (c) J. Liu, T. Tanaka, K. Sivula, A. P. Alivisatos, J. M. Frechet, J. Am. Chem. Soc., 126, 6550 (2004). 11. Z. Guo, L. Zhao, J. Pei, Z. L. Zhou, G. Gibson, S. Lam, J. Brug, S. S. Mao Macromolecules, 43(4), 1860-1866 (2010) and references therein. 12. J. Peet, E. Brocker, Y. Xu, G. C. Bazan, Advanced Materials, 20, 1882, (2008). 13. M. Ariu, M. Sima, M. D. Rahn, J. Hill, A. M. Fox, D. G. Lidzey, M. Oda, J. CabanillasGonzalez, D. D. C. Bradley, Physics Review. B, 67, 195333 (2003). 14. H. H. Lu, C. Y. Liu, C. H. Chang, S. A. Chen, Advanced Materials, 19, 2574 (2007). 15. Z. Guo, J. Pei, Z. L. Zhou, L. Zhao, G. Gibson, S. Lam, and J. Brug Polymer, 50(20), 4794-4800(2009). 16. Z. Guo, D. Liu, C. Wang, J. Pei, Z. L. Zhou, L. Zhao, G. Gibson, J. Brug, S. Lam, S. Mao Science China Chemistry, 54(4), 678-684 (2011).

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