Zhongjie Ren

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.

Nonfullerene-Acceptor All-Small-Molecule Organic Solar Cells Based on Highly Twisted Perylene Bisimide with an Efficiency of over 6%

Two twisted singly linked perylene bisimide (PBI) dimers with chalcogen bridges in the PBI cores, named C4,4-SdiPBI-S and C4,4-SdiPBI-Se, were synthesized as acceptors for nonfullerene all-small-molecule organic solar cells (NF all-SMSCs). A moderate-band-gap small-molecule DR3TBDTT used as the electron donor displayed complementary absorption with C4,4-SdiPBI-S and C4,4-SdiPBI-Se. It was found that solvent-vapor annealing (SVA) played a critical role in the photovoltaic performance in NF all-SMSCs, which improves the crystallinity of the donor and acceptors, promotes the proper phase segregation domain size, and therefore enhances charge transport. The power conversion efficiencies (PCEs) of NF all-SMSC devices based on DR3TBDTT/C4,4-SdiPBI-S and DR3TBDTT/C4,4-SdiPBI-Se increased from 2.52% to 5.81% (JSC = 11.12 mA cm-2, VOC = 0.91 V, and FF = 57.32%) and from 2.65% to 6.22% (JSC = 11.55 mA cm-2, VOC = 0.92 V, and FF = 58.72%), respectively, after exposure to chloroform vapor. The best efficiency of 6.22% is one of the highest PCEs for NF all-SMSC-based PBI acceptors so far. The studies illustrate that highly efficient NF all-SMSCs can be achieved by using a PBI acceptor with a suitable SVA process.

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.

The phase transition behavior of poly(butylene adipate) in the nanoporous anodic alumina oxide

PBA nanotubes with different diameters have been prepared. The crystallization behavior and phase transition behavior have been explored by using X-ray diffraction and DSC. For isothermal melt crystallization, the temperature dependence of the crystal phase and the orientation of PBA crystals in the Anodic Alumina Oxide (AAO) templates are very different from that of the bulk. In the AAO templates, especially in the narrow nanopores, the b-axis of the α phase prefers to adopt the orientation parallel to the long axis of the pore. In addition, an in situ X-ray experiment indicates that some molecular chains cannot pack into the crystal lattice in the AAO template at high crystallization temperature, and they are able to crystallize only after cooling back to room temperature. The core–shell structure of PBA exists in the AAO template, which leads to the incomplete crystallization of the α form at higher crystallization temperature and the formation of β-crystals in the cooled sample. The phase transition behavior of β-crystals in the heating process is also affected by nanoporous confinement. The expansion of the β-crystal unit cell is depressed and the phase transition behavior of β to α is altered in the AAO template. At a slow heating rate, compared to the β-PBA in bulk, β-crystals transit to α-crystals at a slower rate in the templates. At a fast heating rate, less β-crystals transit to α-crystals and more β-crystals prefer to melt directly in the AAO templates.