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Solution-Processed Organosilicon Host Materials For Blue Phosphorescent Organic Light Emitting Diodes

Posted on:2017-03-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:D M SunFull Text:PDF
GTID:1108330491460338Subject:Materials Science and Engineering
Abstract/Summary:PDF Full Text Request
Phosphorescent organic light emitting diodes (PhOLEDs) that harvest both singlet and triplet excitons, leading to an internal quantum efficiency of 100%, have been realized by using phosphorescent dyes. In general, high efficient PhOLEDs are based on the host-guest strategy where a triplet emitter is dispersed homogeneously in a suitable host matrix to prevent self-quenching and triplet-triplet annihilation. Thermal evaporation under high vacuum is widely used to fabricate PhOLEDs. However, this process is expensive because a high vacuum is required, meanwhile most materials are wasted during this process. In contrast, solution processing techniques such as spin-coating, ink-jet printing, silk-screen printing and roll-to-roll are cost-effective and convenient for simple device configurations, allowing large area devices such as flat panel displays to be fabricated. So it remains meaningfull to develop blue-emitting hosts for solution processing.The main contents and results are described as follows:1. Chapter 1 first reviews the recent development process of OLEDs, basic process of electroluminance and main parameters to characterize performance of OLEDs. And then the basic concept and mechanism are illustrated. Heavy metal complex for blue phosphorescence and design for host-guest structure are introduced. In addition, calculation of frontier molecular orbitals and determination of triplet energy are described. Recent progress of organosilicon host materials are reviewed. Finally, the main design strategies of this thesis are outlined.2. In chapter 2, two efficient host materials containing carbazole moiety linked to the backbone of polysiloxane through a phenyl bridge has been synthesized and characterized. They exhibit good film forming ability, high thermal decomposition temperatures and proper glass transition temperatures, so as to form stable amorphous states. Moreover, the silicon-oxygen linkage disrupts their conjugation and results in a sufficiently high triplet energy level (3.0 eV). Iridium bis (4,6-difluorophenyl)pyridinato-N,C2 picolinate (FIrpic)-based devices using them as hosts show good overall performance with low efficiency roll-off. The device using PCzMSi as the host demonstrates the best performance with a maximum current efficiency of 22.8 cd A-1, a maximum power efficiency of 9.4 lm W-1 and a maximum external quantum efficiency of 11.9% at a practical luminance of 1165 cd m-2. Even at a brightness of 5000 cd m-2 level, the external quantum efficiency (EQE) still remains as high as 10%, suggesting a gentle roll-off of device efficiency at high current density.3. In chapter 3, a triphenylamine based polysiloxane (PTPAMSi) is successfully synthesized. PTPAMSi exhibits a high decomposition temperature (Td=377℃) and glass transition temperature (Tg=63℃). It also dispalys good film-forming ability, high morphological stability and good miscibility with the dopant FIrpic as revealed by atomic force microscopy (AFM). The silicon-oxygen linkage of PTPAMSi disrupts its conjugation and results in a sufficiently high triplet energy level (ET= 2.9 eV). A FIrpic-based device using PTPAMSi as a host shows a turn-on voltage of 6.8 V, maximum external quantum efficiency of 3.8%, maximum current efficiency of 7.6 cd A-1.4. In chapter 4, we synthesized an efficient alternating copolysiloxane-based host material (PCzPOMSi) with phenylcarbazole and triphenylphosphine oxide moieties being linked to the backbone of polysiloxane. Alternating copolymers with both hole and electron transporting side groups as bipolar hosts are of great interest for deep blue phosphorescent devices due to the uniform distribution of electrons and holes within the emitting layer. PCzPOMSi exhibits a high decomposition temperature (Td= 437℃) and glass transition temperature (Tg=118℃), It exists in stable amorphous state. The silicon-oxygen linkage of PPCzMSi disrupts its conjugation and results in a sufficiently high triplet energy level (Et=3.0 eV). An bis((3,5-difluoro-4-cyanophenyl)pyridine) iridium picolinate (FCNIrpic)-based device using PCzPOMSi as a host shows a turn-on voltage of 7.7 V, maximum external quantum efficiency of 4%, and maximum current efficiency of 8.5 cd A-1.5. In chapter 5, an efficient functionalized oligosiloxane (PDCzMSi) containing a well-known 1,3-bis(9-carbazolyl)benzene (mCP) moiety, which linked to the silicon atom of oligosiloxane backbone directly has been synthesized and characterized. Compared to mCP, the incorporation of oligosiloxane mainchain significantly improves the thermal and morphological stabilities with a high decomposition temperature (Td= 540 ℃) and glass transition temperature (Tg=142℃), while the silicon-oxygen linkage of PDCzMSi disrupts its conjugation and maintains a sufficiently high triplet energy level (ET=3.0 eV). An iridium bis (4,6-difluorophenyl)pyridinato-N,C2 picolinate (FIrpic)-based device using PDCzMSi as a host shows a relatively low turn-on voltage of 5.0 V for solution processed PhOLEDs, maximum external quantum efficiency of 9.2%, and maximum current efficiency of 17.7 cd A-1.In addition, an efficient polysiloxane derivative (PmCPSi) containing the well-known 1,3-bis(9-carbazolyl)benzene (mCP) moiety, linked as a pendant unit to the polysiloxane backbone has been synthesized and characterized. Compared to mCP, the mCP- polysiloxane hybrid has significantly improved thermal and morphological stabilities with a high decomposition temperature (Td=523℃) and glass transition temperature (Tg=194℃), while the silicon-oxygen linkage of PmCPSi disrupts its conjugation and maintains a high triplet energy level (ET=3.0 eV).On the basis of PmCPSi host, blue phosphorescent organic light emitting devices (PhOLEDs) show the effective confinement of triplet excitons and efficient energy transfer to the guest emitter, resulting in a relatively lower turn-on voltage of 5.8 V, a higher maximum external quantum efficiency of 9.24%, and maximum current efficiency of 18.93 cd A-1, compared with analogous poly(vinylcarbazole) (PVK) based devices (7.7 V,6.76%,12.29 cd A-1). Moreover, a two-component warm white device with a maximum current efficiency of 10.4 cd A-1 is obtained.6. In chapter 6, a universal hybrid polymeric host (PCzSiPh) for blue and deep blue phosphors has been designed and synthesized by incorporating electron-donating carbazole as pendants on a polytetraphenylsilane main chain. The polymer PCzSiPh (4) has a wide bandgap and high triplet energy (ET) because of the tetrahedral geometry of the silicon atom in the tetraphenylsilane backbone. The distinct physical properties of good solubility, combined with high thermal and morphological stability give amorphous and homogenous PCzSiPh films by solution processing. As a result, using PCzSiPh as host with the guest iridium complex TMP-FIrpic gives blue phosphorescent organic light-emitting diodes (PhOLEDs) with overall performance which far exceeds that of a control device with poly(vinylcarbazole) (PVK) host. Notably, FIrpic-based devices exhibit a maximum external quantum efficiency (EQE) of 14.3%(29.3 cd A-1,10.4 1m W-1) which are comparable to state-of-the-art literature data using polymer hosts for a blue dopant emitter. Moreover, the versatility of PCzSiPh extends to deep blue PhOLEDs based on FIr6 and FCNIrpic with high efficiencies of 11.3 cd A-1 and 8.6 cd A-1, respectively.7. In chapter 7, a series of 3,3’-Bicarbazole (mCP) functionalized tetraphenylsilane derivatives SimCPx, including bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2), tris(3,5-di(9H-carbazol-9-yl)phenyl)methylsilane (SimCP3-CH3), tris(3,5-di(9H-carbazol-9-yl)phenyl)phenylsilane (SimCP3-Ph) and tetrakis(3,5-di(9H-carbazol-9-yl)phenyl)silane (SimCP4), served as bipolar blue hosts for bis [2-(4,6-difluorophenyl) pyridyl-N, C2’] iridium (Ⅲ) (FIrpic) have been synthesized by incorporating different ratios of mCP subunits into central silicon atom. All the SimCPx derivatives have wide bandgaps and high triplet energies (ET) because of the indirect linkage by silicon between each mCP subunit. The good solubility, high thermal and morphological stability of SimCPx are beneficial to forming amorphous and homogenous films through solution processing. Density function theory simulations manifest the better bipolar characteristics for SimCPx with three and four mCP units than the represented bipolar host SimCP2. As a result, SimCP4 presents the best electron transporting ability for charge balance. Consequently, the lowest driving voltage of 4.8 eV and the favorable maximum efficiencies of 14.2% for external quantum efficiency (EQE) (28.4 cd A-1,13.5 lm W-1) are achieved by solution-processed SimCP4-based blue phosphorescent organic light-emitting diodes (PhOLEDs) as the highest performance among SimCPx, in which 32% improved device efficiencies than that of SimCP2 are obtained. It is inspiring to develop efficient bipolar hosts for blue phosphors just by incorporating monopolar carbazole into arylsilanes within two steps.
Keywords/Search Tags:solution-processed organic light emitting diodes, blue phosphorescence, host materials, organosilicon, high triplet energy
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