Dianming Sun

Carbazole-based polysiloxane hosts for highly efficient solution-processed blue electrophosphorescent devices

The efficient host materials containing carbazole moieties linked to the backbone of polysiloxane through a phenyl bridge have been synthesized and characterized. They exhibit good film forming ability, high thermal decomposition temperatures and suitable 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. In addition, typically, the host PCzMSi film displays good mechanical performance by a nanoindentation technique to meet the practical application. These results demonstrate that the design of polysiloxane-based host materials is a promising approach to realize high performance solution-processed blue phosphorescent polymer light emitting diodes (PhPLEDs).

A versatile hybrid polyphenylsilane host for highly efficient solution-processed blue and deep blue electrophosphorescence

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 lm 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 using FIr6 and FCNIrpic as dopants, with high efficiencies of 11.3 cd A−1 and 8.6 cd A−1, respectively.

Arylsilanes and siloxanes as optoelectronic materials for organic light-emitting diodes (OLEDs)

Organic light emitting diodes (OLEDs) are currently receiving much attention for applications in new generation full-colour flat-panel and flexible displays and as sources for low energy solid-state lighting. Arylsilanes and siloxanes have been extensively studied as components of OLEDs, mainly focusing on optimizing the physical and electronic properties of the light-emitting layer and other functional layers within the OLED architecture. Arylsilanes and siloxanes display the advantages of good solubility in common organic solvents and excellent resistance to thermal, chemical and irradiation degradations. In this review, we summarize the recent advances in the utilization of arylsilanes and siloxanes as fluorophore emitters, hosts for phosphor emitters, hole and exciton blocking materials, and as electron and hole transporting materials. Finally, perspectives and challenges related to arylsilanes and siloxanes for OLED applications are proposed based on the reported progress and our own opinions.

Synthesis of Dibenzothiophene‐Containing Ladder Polysilsesquioxane as a Blue Phosphorescent Host Material

A ladder polysilsesquioxanes with side chain of dibenzothiophene groups (BS-LPSQ) was successfully synthesized. The ladder structure of BS-LPSQ was characterized by MALDI-TOF MS, XRD, and (1)H NMR spectroscopy. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), atomic force microscopy (AFM), and spectroscopic analyses revealed that the BS-LPSQ has good film-forming ability, high thermal and morphological stability, and good miscibility to the dopant iridium bis(4,6-difluorophenyl)pyridinato-N,C(2)-picolinate (FIrpic), high triplet energy, and a wide bandgap. In addition, compared with the ringed polysiloxane BS-PSQ phosphorescent host material reported previously, the ladder structure of BS-LPSQ has not only a higher thermal resistance, but also could prevent molecular aggregation and effectively avoid quenching of fluorescence. Thus, the BS-LPSQ may be used as a better host for the blue-light-emitting iridium complex FIrpic. The performance of the electrophosphorescent device, based on the ladder BS-LPSQ as the active layer, is superior to that of ringed BS-PSQ and any other polyhedral oligomeric silsesquioxane (POSS)-based or polymer host materials.

Synthesis and properties of siloxane modified perylene bisimide discotic liquid crystals

A series of symmetric and asymmetric 1,6,7,12-tetrachloroperylene bisimides (PBICls) were synthesized and modified by siloxane substituents at the imide nitrogen atom. Siloxane substitutions do not apparently affect the electronic properties of PBICIs as demonstrated by CV experiments. They display both thermotropic and lyotropic liquid crystalline behaviors. The effect of different siloxane substituents on their liquid crystal structures was investigated in detail. Small angle X-ray scattering indicates that PBICls adopt hexagonal columnar packing in thermotropic liquid crystals. In addition, PBICls exhibit good optical properties, good solubility and film-forming ability. Thus the oriented films of PBICl liquid crystals could be easily fabricated by mechanical shear, which show anisotropic properties in UV-vis absorption spectra.

Synthesis of well-defined poly(phenylcarbazole-alt-triphenylphosphine oxide) siloxane as a bipolar host material for solution-processed deep blue phosphorescent devices

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. In this work, we synthesized an efficient alternating copolysiloxane-based host material poly(phenylcarbazole-alt-triphenylphosphine oxide) siloxane (PCzPOMSi) with phenylcarbazole and triphenylphosphine oxide moieties linked to the backbone of polysiloxane. PCzPOMSi exhibits a high decomposition temperature (Td = 437 °C) and glass transition temperature (Tg = 118 °C), and it can form a stable amorphous state. The silicon–oxygen linkage of PCzPOMSi disrupts its conjugation and results in a sufficiently high triplet energy level (ET = 3.0 eV). A 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, a maximum external quantum efficiency of 4%, and a maximum current efficiency of 8.5 cd A−1. These results demonstrate that a well-designed alternating copolysiloxane-based host is a promising approach to realize high performance solution-processed deep blue PhPLEDs.

A dual-fluorescent composite of graphene oxide and poly(3-hexylthiophene) enables the ratiometric detection of amines

A composite prepared by grafting a conjugated polymer, poly(3-hexylthiophene) (P3HT), to the surface of graphene oxide was shown to result in a dual-fluorescent material with tunable photoluminescent properties. Capitalizing on these unique features, a new class of graphene-based sensors that enables the ratiometric fluorescence detection of amine-based pollutants was developed. Moreover, through a detailed spectroscopic study, the origin of the optical properties of the aforementioned composite was studied and was found to be due to electronic decoupling of the conjugated polymer from the GO. The methodology described herein effectively overcomes a long-standing challenge that has prevented graphene based composites from finding utility in sensing and related applications.

Synthesis of triphenylamine based polysiloxane as a blue phosphorescent host

In this work, a triphenylamine based polysiloxane (PTPAMSi) has been successfully synthesized. The PTPAMSi exhibits a high decomposition temperature (Td = 377 °C) and glass transition temperature (Tg = 63 °C). It also displays 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, a maximum external quantum efficiency of 3.8%, and a maximum current efficiency of 7.6 cd A−1. These results demonstrate that using polysiloxane to modify triphenylamine is a promising approach to improve the physical properties of triphenylamine while maintaining its photophysical and electrochemical properties.

Anisotropic highly-conductive films of poly(3-methylthiophene) from epitaxial electropolymerization on oriented poly(vinylidene fluoride)

Electrochemical polymerization of 3-methylthiophene on highly oriented poly(vinylidene fluoride) (PVDF) film was achieved by cyclic voltammetry to yield well-ordered poly(3-methylthiophene) (P3MT) thin films with anisotropic structural and conductivity properties. The conductivity of P3MT along the direction perpendicular to the chain direction of PVDF, after electrochemical dedoping, is 59 ± 3 S cm−1, while that along the PVDF chain direction is 1.2 ± 0.4 S cm−1. The high conductivity of the P3MT is attributed to the well-ordered structure with its flat-on single crystals as confirmed by electron diffraction and Reflection Absorption Infra-red Spectroscopy (RAIRS). The data are consistent with P3MT chains aligned with π–π stacking perpendicular to the chain direction of the PVDF substrate. Epitaxial electropolymerization is an unusual method of preparing highly ordered thin films of organic semiconductors.

Oligosiloxane Functionalized with Pendant (1,3-Bis(9-carbazolyl)benzene) (mCP) for Solution-Processed Organic Electronics

A new oligosiloxane derivative (ODCzMSi) functionalized with the well-known 1,3-bis(9-carbazolyl)benzene (mCP) pendant moiety, directly linked to the silicon atom of the oligosiloxane backbone, has been synthesized and characterized. Compared to mCP, the attachment of the oligosiloxane chain significantly improves the thermal and morphological stabilities with a high decomposition temperature (Td =540 °C) and glass transition temperature (Tg =142 °C). The silicon-oxygen linkage of ODCzMSi disrupts the backbone conjugation and maintains a high triplet energy level (ET =3.0 eV). A phosphorescent organic light-emitting diode (PhOLED) using iridium bis(4,6-difluorophenyl)pyridinato-N,C(2) picolinate (FIrpic) as the emitter and ODCzMSi as the 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) . The overall performance of this device is competitive with the best reported solution-processed blue PhOLEDs. Memory devices using ODCzMSi as an active layer exhibit non-volatile write-once read-many-times (WORM) characteristics with high stability in retention time up to 10(4)  s and a low switch on voltage. This switching behaviour is explained by different stable conformations of ODCzMSi with high or low conductivity states which are obtained under the action of electric field through a π-π stacking alignment of the pendant aromatic groups. These results with both PhOLEDs and memory devices demonstrate that this oligosiloxane-mCP hybrid structure is promising and versatile for high performance solution-processed optoelectronic applications.