Chunxiu Zhang

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.

Nanotubes and Columnar Phase Formation from a Polymer/Discotic Molecule Composite Induced by Geometric Confinement

Composite nanotubes of poly(methyl methacrylate) and a discotic liquid crystalline material have fabricated on an ordered porous anodic alumina template. The nanotubes formed by the template have a diameter distribution from 150 ∼200 nm and the thickness of the walls of the nanotube is 20 ∼ 30 nm. The x-ray diffraction studies showed that the discotic liquid crystalline material formed oriented column in the nanotubes due to spatial confinement. The results showed that the geometric confinement can promote the columnar growth of the discotic liquid crystalline molecules along the long axes of the nanotube.

Organic photovoltaic devices from discotic materials

Disk-shaped molecules can be self-assembled into columns, which may exhibit one-dimensional charge transporting properties within well-oriented domains. They are promising materials to choose in fabrication of organic photovoltaic device and to improve photovoltaic responses. In this work, a discotic triphenylene derivative 2,3,6,7,10,11-hexapentyloxytriphenylene (HAT5) was investigated and used in fabrication of organic photovoltaic devices. The device have a double-layered structure of ITO/HAT5/PTCDI-C13/Al, in which HAT5 acts as an acceptor and hole transporting layer and N,N-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) acts as a donor and electron transporting layer. The device demonstrated an open circuit voltage up to 0.7V, which depended strongly on the thickness of the PTCDI-C13 layer. Function of organic/organic interface to the generation of photovoltaic effect was observed experimentally through a comparison of device made with HAT5 layers of different molecular arrangement. A higher ordered morphology obtained through annealing led to a substantial improvement in the fill factor of the device. A distinct change in open circuit voltage was also observed upon the molecular orientation though it was not what we anticipated. This work demonstrated a useful route to tune the performance of photovoltaic response of organic devices through manipulating molecular orientation and organization in the device.