Zhiqun He

Anomalously large interface charge in polarity-switchable photovoltaic devices: an indication of mobile ions in organic–inorganic halide perovskites

It is proved that a large amount of mobile ions exist in organic–inorganic halide perovskites. The accumulated ions at the interface change the band bending of the semiconductor, leading to polarity-switchable photovoltaic devices. The interface charge significantly influences the function and performance of perovskite devices. The discovery of the interface charge has important implications for current–voltage hysteresis in perovskite solar cells.

Blue electroluminescence of novel pyrazoloquinoline and bispyrazolopyridine derivatives in doped polymer matrices

Blue electroluminescence has been demonstrated using novel materials 1,3-diphenyl-4-methyl-1H-pyrazolo[3,4-b]quinoline (PAQ4) and 4-phenyl-1,3,5,7-tetramethyl-1,7-dihydrodipyrazolo[3,4-b;4′,3′-e]pyridine (PAP1) for the first time. The peak maxima of their electroluminescent spectra were at 442 nm (ca. 20 nm shift from its photoluminescent maximum) for PAQ4 and 425 nm (within the same spectral range as the photoluminescence) for PAP1.

A Mesogenic Triphenylene−Perylene−Triphenylene Triad

A straightforward synthesis of triphenylene-perylene-triphenylene triad structures has been achieved by using versatile triphenylene intermediates bearing a single oxyalkyl amine side chain. Among these, PBITP(10) showed a stable columnar mesophase implying favorably matched core-core separations in the structure. Importantly, the triad can be used as a vehicle for doping columnar triphenylene matrices with functional but incompatible perylene units and a mixture of hexahexyloxytriphenylene matrix doped with 0.1% PBITP(10) is homogeneous and liquid crystalline.

Electroluminescence from novel pyrazole-based polymer systems

A novel class of electroluminescent pyrazole-based polymers have been synthesised and their electroluminescent properties have been demonstrated. These materials show bright electroluminescence and promising properties for electroluminescent applications.

Synthesis of crown ether-linked discotic triphenylenes.

Novel triphenylene dimers linked by a central crown ether core have been synthesized and characterized. The crown ether is most conveniently formed as a final step to permit purification and isolation of ion-free material, and extension of the protocol permits synthesis of triad structures linked though a 27-crown-9 macrocycle. The latter compounds present a new discotic motif that supports mesophase formation.

Efficient organic solar cells using copper(I) iodide (CuI) hole transport layers

We report the fabrication of high power conversion efficiency (PCE) polymer/fullerene bulk heterojunction (BHJ) photovoltaic cells using solution-processed Copper (I) Iodide (CuI) as hole transport layer (HTL). Our devices exhibit a PCE value of ∼5.5% which is equivalent to that obtained for control devices based on the commonly used conductive polymer poly(3,4-ethylenedioxythiophene): polystyrenesulfonate as HTL. Inverted cells with PCE >3% were also demonstrated using solution-processed metal oxide electron transport layers, with a CuI HTL evaporated on top of the BHJ. The high optical transparency and suitable energetics of CuI make it attractive for application in a range of inexpensive large-area optoelectronic devices.

Electron drift mobility in pyrazolo[3,4-b]quinoline doped polystyrene layers

Pyrazolo[3,4-b]quinoline derivatives are found to have electron transport properties. A field dependent electron drift mobility, in the range of 10−6–3×10−5 cm2/(V s) at an electric field of (1–7)×105 V/cm, has been found from the pyrazolo[3,4-b]quinoline doped polystyrene specimen, which was measured using a conventional time-of-flight method. The results are fit well into a correlated disorder model for a dipole medium when energetic features of the transport molecules are also taken into account.

Expanded Porphyrin-like Structures Based on Twinned Triphenylenes

Triphenylene twins are intriguing structures, and those bridged through their 3,6-positions by dipyrromethene units give a new class of macrocycles that can be viewed as rigid, expanded porphyrin derivatives in which coplanarity is enforced in a formally antiaromatic π system. Somewhat surprisingly, however, macrocyclization leads to significant overall stabilization of the dipyrromethene chromophores.

Homeotropic alignment through charge- transfer-induced columnar mesophase formation in an unsymmetrically substituted triphenylene derivative*

An unsymmetrically substituted triphenylene, with two adjacent chloroethoxyethyl lateral flexible chains, was synthesized and characterized. Although this compound showed no mesomorphic behavior, it formed a donor–acceptor charge-transfer complex with 2,4,7-trinitrofluorenone (TNF). The resulting 1:1 complex has been investigated using UV–vis and IR spectroscopy, optical microscopy, thermal analysis, and X-ray diffraction. A columnar mesophase with hexagonal symmetry was found. More interestingly, this charge-transfer complex can be easily aligned on a glass surface in a homeotropic orientation, which is stable at room temperature (RT) and over a wide temperature range.

Spatially separated charge densities of electrons and holes in organic-inorganic halide perovskites

Solution-processable methylammonium lead trihalide perovskites exhibit remarkable high-absorption and low-loss properties for solar energy conversion. Calculation from density functional theory indicates the presence of non-equivalent halogen atoms in the unit cell because of the specific orientation of the organic cation. Considering the 〈100〉 orientation as an example, I1, one of the halogen atoms, differs from the other iodine atoms (I2 and I3) in terms of its interaction with the organic cation. The valance-band-maximum (VBM) and conduction-band-minimum (CBM) states are derived mainly from 5p orbital of I1 atom and 6p orbital of Pb atom, respectively. The spatially separated charge densities of the electrons and holes justify the low recombination rate of the pure iodide perovskite. Chlorine substitution further strengthens the unique position of the I1 atom, leading to more localized charge density around the I1 atom and less charge density around the other atoms at the VBM state. The less overlap of charge densities between the VBM and CBM states explains the relatively lower carrier recombination rate of the iodine-chlorine mixed perovskite. Chlorine substitution significantly reduces the effective mass at a direction perpendicular to the Pb-Cl bond and organic axis, enhancing the carrier transport property of the mixed perovskite in this direction.