Fenglian Bai

Improving the efficiency of solution processable organic photovoltaic devices by a star-shaped molecular geometry

A new solution processable star-shaped organic molecule S(TPA-BT) has been synthesized for application in organic solar cells (OSCs). The properties and structures of S(TPA-BT) and a related linear molecule L(TPA-BT) were studied, including UV-visible spectroscopy, hole charge mobility, and theoretical calculated geometry and electronic properties. S(TPA-BT) film shows a broader and stronger absorption band in the range of 440–670 nm, lower band gap of 1.86 eV, higher hole mobility of 4.71 × 10−5 cm2V−1 s−1 and better film-forming properties compared with those of L(TPA-BT) film. ITO/PEDOT:PSS/S(TPA-BT) or L(TPA-BT):PCBM/Ba/Al bulk-heterojunction OSCs were fabricated with S(TPA-BT) or L(TPA-BT) as donor material. The power conversion efficiency (PCE) of an OSC based on a blend of S(TPA-BT) and PCBM (1 : 3, w/w) reached 1.33% under A.M. 1.5 illumination, 100 mW cm−2, with a short-circuit current density (JSC) of 4.18 mA cm−2, an open circuit voltage of 0.81 V, and a fill factor of 39%. The PCE of 1.33% and Jsc of 4.18 mA cm−2 are among the highest values reported so far for solution processable OSCs.

The photo- and electroluminescence of some novel light emitting copolymers

Abstract A new class of novel high-efficiency light-emitting nitrogen-containing PPV-related copolymers which have hole transfer moieties such as triphenylamine (TPA) and alkyl-carbazole units and conjugated aromatic units such as 4,4′-biphenylene, 1,4-phenylene, 1,4- or 1,5-naphthylene and 9,10-anthrylene, was designed and synthesized by the well-known Wittig–Hornor reaction. The optical and electrical-properties were examined. The resulting alternating copolymers were highly soluble in common organic solvents, with good film-forming properties and high fluorescence quantum yields and also emit blue/green lights when used as the active layer of LEDs.

Localized Emitting State and Energy Transfer Properties of Quadrupolar Chromophores and (Multi)Branched Derivatives

In order to better understand the nature of intramolecular charge and energy transfer in multibranched molecules, we have synthesized and studied the photophysical properties of a monomer quadrupolar chromophore with donor-acceptor-donor (D-A-D) electronic push-pull structure, together with its V-shaped dimer and star-shaped trimers. The comparison of steady-state absorption spectra and fluorescence excitation anisotropy spectra of these chromophores show evidence of weak interaction (such as charge and energy transfer) among the branches. Moreover, similar fluorescence and solvation behavior of monomer and branched chromophores (dimer and trimer) implies that the interaction among the branches is not strong enough to make a significant distinction between these molecules, due to the weak interaction and intrinsic structural disorder in branched molecules. Furthermore, the interaction between the branches can be enhanced by inserting π bridge spacers (-C═C- or -C≡C-) between the core donor and the acceptor. This improvement leads to a remarkable enhancement of two-photon cross-sections, indicating that the interbranch interaction results in the amplification of transition dipole moments between ground states and excited states. The interpretations of the observed photophysical properties are further supported by theoretical investigation, which reveal that the changes of the transition dipole moments of the branched quadrupolar chromophores play a critical role in observed the two-photon absorption (2PA) cross-section for an intramolecular charge transfer (ICT) state interaction in the multibranched quadrupolar chromophores.

Synthesis and spectroscopic properties of a series of hyperbranched conjugated molecules with 1,3,5-triphenylbenzene as cores

A series of conjugated hyperbranched molecules with 1,3,5-triphenylbenzene rings as cores and with different connecting groups have been synthesized. Their structures were characterized by FTIR, 1H-NMR and 13C-NMR. These molecules exhibit good solubility and film-forming ability. In particular, they have high thermal stability (TID > 350 °C). The spectroscopic properties of these molecules were studied both in solution and as spin-coated films. The twisted intramolecular charge transfer (TICT) state was observed in hyperbranched molecules containing triphenylamine as the connecting unit. Light emitting devices (ITO/PEDOT/hyperbranched molecule/Ba/Al) have been fabricated.

Design, synthesis and photophysical properties of a hyperbranched conjugated polymer

Abstract The conjugated dendrimer, poly(3,5-bisvinylic)benzene (HS1), was synthesized by two routes. Elementary analysis, FTIR, NMR and GPC spectra were used to characterize the dendrimer. HS1 has good solubility in common organic solvents although it does not contain long alkyl side chain. The photophysical properties in solution and net solid film are inspected. Absorption maxima were determined at 290 and 310 nm with a molar absorption coefficient in the order of 104, and the emission maxima were at 359 and 369 nm. The excitation spectrum is similar to that of the absorption. A mirror image relationship existed between the absorption and emission spectra characteristic of the rigid structure of the polymer. The Stokes shift (70 nm) is large, which means the large difference in the energy level between the ground and singlet excited state. The fluorescence quantum yield is about 36%. The net film of HS1 has a broad emission at 446 nm. The marked difference between the solution and net solid film in photophysical properties suggested the large change in intermolecular interaction from solution to the solid film state. Molecular mechanical calculations are used to aid the design and simulation of the structure of HS1. In generation 1–2, the optimized geometry shows a relatively good planarity. While the torsion appeared at the interference of the branches in generation 3 and the optimized geometry feels a zeolite structure. The calculation result is used to explain the experimental observation. The good photophysical properties and processing ability could make HS1 an ideal candidate in electroluminecence device as a blue emitter.

Photoconduction and application of oxotitanium phthalocyanine dual-layered thin films

Abstract The photo-conduction of oxotitanium phthalocyanine (TiOPc) and their application as photo-generation carrier materials in dual layered photoreceptors were studied. The conductivities of the photo/dark conductions were 1.54×10 −12 and 1.23×10 −13 S/cm in the ohmic region (slope≈1), and 6.73×10 −10 and 1.31×10 −12 S/cm in non-ohmic regions obtained from the photo/dark J–V characteristics at room temperature, respectively. The ratio of photo/dark conduction increases with increasing field, from more than one order of magnitude at field 1.30×10 2 V/cm to near three orders of magnitude at 1.30×10 5 V/cm. The photoelectron properties of TiOPc as photo-generation carrier material in the dual-layered photoreceptor were characterized by the use of corona dark charging and photo-discharging. The obtained data are listed in the following: the saturated surface dark potential V 0 =630 V; dark decay rate R d =6.5 V/s, measured by average potential decreases in the initial 2 s after dark decay; light sensitivity E 1/2 =2.5 erg/cm 2 , calculated from illuminating power E L =5 μW and half photodecay time t 1/2 =0.5 s; surface residual potential V r =25 V.

Light-emitting diodes based on an alternating copolymer containing triphenylamine and phenylene units

The optical and electroluminescence properties of a light-emitting poly(arylene vinylene) derivative comprising triphenylamine as the arylene unit (TPA–PPV) are demonstrated. TPA–PPV is soluble in common organic solvents, and has a high photoluminescent (PL) quantum yield (ΦPL=0.94 in benzene). A single-layer light-emitting diode Indium–tin–oxide/TPA–PPV/Al utilizing blue–green light-emitting TPA–PPV as the emissive layer is fabricated, and luminance in excess of 612 cd/m2, turn-on voltages of 1.5 V, and high luminous efficiency (3.0 lm/w) are reported.

Synthesis and properties of high efficiency light emitting hyperbranched conjugated polymers

A novel kind of hyperbranched conjugated light emitting polymers with 2,5-dimethyloxyl substituted phenylene vinylene as the connecting units and different hole or electron transporting groups such as dimethylaniline phenylene or pyridyl phenylene as the terminal groups were synthesized by modified one step Wittig polymerization reaction. The photophysical properties in solution and in films of hyperbranched conjugated polymers were studied in details. The oxidation potentials were measured by cyclic voltammetry. It was found that the hyperbranched conjugated polymers were highly soluble in common organic solvents, with good film-forming properties. In particular, the thermal stability of HPVs is extremely higher than that of linear polyphenylene vinylene derivatives. In addition, the LED, LEC devices using hyperbranched polymer as active layer were investigated. All results show that these novel hyperbranched conjugated polymers could be good candidates for the fabrication of high performance LED and other optoelectronic devices.

Electroluminescence enhancement by blending PVK with an alternating copolymer containing triphenylamine and phenylene units

The PL and EL properties of a novel light-emitting alternating copolymer composed of triphenylamine (TPA) and phenylene vinylene derivate (MEHPPV), that is, TPA-MEHPPV and its blend with PVK were demonstrated. A considerable enhancement of the EL intensity was observed by using the polymer blends as the emission layer in the organic light-emitting devices (OLEDs). The energy transfer process in the polymer blends was also discussed.

Synthesis and characterization of a high‐efficiency light‐emitting alternating copolymer

A new high-efficiency light-emitting alternating copolymer of triphenylamine and pure PPV (TPA–PPV) has been designed and synthesized. The copolymer was highly soluble in common solvents. It could be spin cast onto various substrates to give highly transparent homogeneous thin films without heat treatment. The fluorescence quantum yield in benzene is almost up to 1.00. The maximum fluorescence wavelength for this alternating copolymer appeared around 470 nm. The fluorescence of TPA–PPV solution quenched by C60 was examined, and the result indicated that a strong interaction exists between TPA–PPV and C60 at the exited state. A primary single-layer LED based on ITO/TPA–PPV/Al has been fabricated, and the onset voltage is only 1.5 V and a bright green light was observed. The electroluminescence spectrum gives a peak at 510 nm when the operating voltage of 17 V was applied. The photoluminescence spectrum also appeared at the same wavelength as the electroluminescence, indicating that the same excited states are involved in the two processes.