Wei-Chieh Lin

Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine Substituents and Fabrication of OLEDs†

Organic light-emitting diodes (OLEDs) based on heavy transition-metal complexes are playing a pivotal role in next generation of, for example, flat panel displays and solid-state lighting. The readily available, Os-, Pt-, and in particular Ir-based phosphorescence complexes grant superior advantage over fluorescent materials. This is mainly due to heavyatom-induced spin–orbit coupling, giving effective harvesting of both singlet and triplet excitons. However, tuning of phosphorescence over the entire visible spectrum still remains a challenge. Particularly, designing new materials to show higher energy, such as deep-blue emission—with an ideal CIEx,y coordinate (CIE = Commission Internationale de L Eclairage) of (0.14, 0.09)—encounters more obstacle than the progress made for obtaining green and red colors. Representative blue phosphors are a class of Ir complexes possessing at least one cyclometalated 4,6-difluorophenyl pyridine {(dfppy)H} ligand, known as FIrpic, FIr6, FIrtaz, and others. The majority of blue phosphors showed inferior color chromaticity with a sum of CIEx+y values being much greater than 0.3 or with single CIEy coordinate higher than 0.25. Such inferior chromaticity, in part, has been improved upon adoption of carbene-, triazolyl-, and fluorine-substituted bipyridine (dfpypy) based chelates. The above urgency prompted us to search for better and new blue phosphors. We produced a class of 2-pyridylazolate chelates possessing very large ligand-centered p–p* energy gap, as evidenced by the blue-emitting Os complexes. Subsequently, room-temperature blue phosphorescence was also visualized for the respective heteroleptic Ir complexes, particularly for those dubbed “nonconjugated” ancillary chelate(s). The nonconjugated ligands so far comprise a benzyl substituted pyrazole, an N-heterocyclic carbene, phosphines, and other ingenious molecular designs. Herein, we report the preparation of a novel class of heteroleptic Ir complexes by incorporation of tripodal, facially coordinated phosphite (or phosphonite), denoted as the P^C2 chelate, for serving as the ancillary, together with the employment of 2-pyridyltriazolate acting as blue chromophore. The reaction intermediate, which possesses an acetate chelate, was isolated and characterized to establish the synthetic pathway. The tridentate P^C2 ancillary chelate offers several advantages: 1) Good stabilization of complex and necessary long-term stability in application of for example, emitting devices. 2) The strong bonding of phosphorous donors is expected to destabilize the ligand field d–d excited state, thus minimizing its interference to the radiative process from the lower lying excited state. 3) P^C2 inherits profound and versatile functionality (see below) capable of fine-tuning the electronic character. As a result, highly efficient blue phosphorescence is attained with good OLED performance. Treatment of a mixture of [IrCl3(tht)3] (tht = tetrahydrothiophene) with an equimolar amount of triphenylphosphine (PPh3), triphenylphosphite {P(OPh)3}, and an excess of sodium acetate resulted in a high yield conversion (> 80%) into [Ir(P^C2)(PPh3)(OAc)] (1a); P^C2 = tripodal dicyclometalated phosphite (Scheme 1). Subsequent replacement of acetate in 1a with chelating 3-tert-butyl-5-(2-pyridyl)triazo-

Microcavity‐Embedded, Colour‐Tuneable, Transparent Organic Solar Cells

In this work microcavity-capped colour-tuneable SMOSCs are evaluated. By adopting a microcavity-structured cathode with optical spacer layers of different thicknesses fabricated in a Ag/NPB/Ag structure, the transmission spectra of complete devices can be tuned over the entire visible-light region (400-750 nm). The fabricated semitransparent colour-tuneable solar cells show an average efficiency of 4.78% under 1-sun illumination.

Epitaxial Growth of Transition Metal Silicides on Silicon

Recent progresses in the epitaxial growth of refractory metal suicides, FeSi 2 and manganese suicides on silicon are reviewed. The formation and structures of epitaxial suicides are described. Factors affecting the suicide epitaxy are examined. The lattice match criteria for the growth of epitaxial suicides are assessed. The effects of anharmonicity in the interatomic force of overlayer on the heteroepitaxial growth and pseudomorphism are discussed. The properties and possible applications of epitaxial suicides are summarized. Prospects for the study of epitaxial suicides are addressed.

Pyridine-based electron transporting materials for highly efficient organic solar cells

The effect of organic electron transporting layers (ETLs) on organic solar cells (OSCs) has been evaluated. Thermally deposited small molecule OSCs utilizing the electron transporting material 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and the pyridine-containing derivatives 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB), bis-1,2-(3,5-di-3-pyridyl-phenyl)benzene (B3PyPB) and diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) as ETLs have been fabricated and studied. Impedance spectroscopy and current density–voltage characteristics show that the device performance is closely related to the intrinsic electron mobilities of the ETLs. It was demonstrated that the compound TmPyPB, which possesses good thermal properties, high transparency, high refractive index, and low ionization potential, as well as high electron mobility, is a promising material as an ETL in OSCs, resulting especially in lower series resistance and higher shunt resistance. The devices adopting TmPyPB as an ETL show exceptional characteristics with high fill-factor (FF) and very high power conversion efficiency (PCE) up to 53% and 6.3%, respectively, which are great enhancements compared to cells using conventional BCP as an ETL material.

Efficient inverted quasi-bilayer organic solar cells fabricated by using non-halogenated solvent processes

Here we demonstrate the fabrication of novel, “quasi-bilayer” inverted organic photovoltaic devices using halogen-free solvents. The inferior solubility of pristine fullerene in non-halogenated solvents was used to control the interpenetration of upper polymeric donor layers with bottom fullerene layers. Notably, island-like nano-morphologies were revealed by AFM, SEM, TEM, cross-sectional TEM images and PL quenching measurement. Correlation between device performance, thin-film nano-morphology and ac impedance was observed. High efficiencies of 6.55% and 7.15% were observed for PBDTTT-C-T and PTB7 cells, respectively. These results demonstrate that this novel process not only offers an effective new method to control the morphology of solar active layers but, more importantly, could also be applied to a wide range of current material systems to produce efficient devices that comply with the non-toxic halogen-free requirement.

Novel oxygen sensor based on terfluorene thin-film and its enhanced sensitivity by stimulated emission

Ter(9,9-diarylfluorene) (TDF) neat film featuring oxygen sensing with advantages of fast response, reversibility and high efficiency is reported. The fast O2 fluorescence quenching process (4.0 × 1010 M−1 s−1) is unique and is mainly due to intact amorphous morphology of the TDF film, providing ample porous sites in nature so that O2 can travel therein freely. This, together with a large S1–T1 energy gap of TDF, leads to efficient O2 sensitization, i.e. the O2 induced S1–T1 intersystem crossing. The sensitivity can be further enhanced up to ∼10-fold and ∼20-fold in amplified spontaneous emission (ASE) and lasing action, respectively. The work thus demonstrated a new class of organic materials suited for high-speed, high-sensitivity oxygen sensing.