10 research outputs found
Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications
Based on the arrangement of two-dimensional 'melon', we construct a unit cell
for polymeric carbon nitride (PCN) synthesized via thermal polycondensation,
whose theoretical diffraction powder pattern includes all major features
measured in x-ray diffraction. With the help of this unit cell, we describe
the process-temperature-induced crystallographic changes in PCN that occur
within a temperature interval between 510 and 610 °C. We also discuss further
potential modifications of the unit cell for PCN. It is found that both
triazine- and heptazine-based g-C3N4 can only account for minor phases within
the investigated synthesis products
Molecularly imprinted conductive polymers for controlled trafficking of neurotransmitters at solid–liquid interfaces
We realize a molecularly imprinted polymer (MIP) which is imprinted with the
retinal neurotransmitter glutamate. The films prepared by electrochemical
deposition have a smooth surface with a granular morphology as observed using
an atomic force microscope. Multiple reflection attenuated total reflection
infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS)
are used to chemically confirm the imprint of a neurotransmitter in the MIP at
the solid–liquid and the solid–air interface, respectively. Fluorescence
spectroscopy using the dye fluorescamine is used to monitor the changes in
neurotransmitter concentration in various solvents induced by application of
voltage to the MIP. By controlling neurotransmitter trafficking across a
solid–liquid interface with voltage, we suggest the possibility of using such
a neurotransmitter imprinted MIP for chemical stimulation of retinal neurons.
The current state of the art approach to restore sight in certain cases of
blindness is the replacement of damaged photoreceptors by a subretinal implant
consisting of light-sensitive photodiodes. Thus a future perspective of our
work would be to chemically stimulate the neurons by replacing the photodiodes
in the subretinal implant by the neurotransmitter imprinted polymer film
An optical quasimonomer
A comprehensive investigation of the luminescent properties of carbon nitride
polymers, based on tri-s-triazine units, has been conducted. Steady-state
temperature- and excitation-power-dependent as well as time-resolved
measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong
photoluminescence, covering the visible spectrum. The spectral, thermal, and
temporal features of the photoluminescence can be satisfactorily described by
the excitation and radiative recombination of molecular excitons, localized at
single tri-s-triazine units. The discussed model is in accordance with the
recently reported absorption features of carbon nitride polymers. Thus, from
the point of view of optical spectroscopy, the material effectively behaves as
a monomer
Large Neutral Barrier at Grain Boundaries in Chalcopyrite Thin Films
The electronic structure of grain boundaries in polycrystalline Cu In; Ga Se2 thin films and their role on solar cell device efficiency is currently under intense investigation. A neutral barrier of about 0.5 eV has been suggested as the reason for the benign behavior of grain boundaries in chalcopyrites. Previous experimental investigations have in fact shown a neutral barrier but only a few 10 meV high, which cannot be expected to have a significant influence on the solar cell efficiency. Here we show that a full investigation of the electrical behavior of charged and neutral grain boundaries shows the existence of an additional narrow neutral barrier, several 100 meV high, which is tunneled through by the majority carriers but is sufficiently high to explain the benign behavior of the grain boundarie
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Grain boundary assisted photocurrent collection in thin film solar cells
The influence of absorber grain boundaries on the photocurrent transport in chalcopyrite based thin film solar cells has been calculated using a two dimensional numerical model. Considering extreme cases, the variation in red response is more expressed than in one dimensional models. These findings may offer an explanation for the strong influence of buffer layer preparation on the spectral response of cells with small grained absorbers
Grain boundary assisted photocurrent collection in thin film solar cells
The influence of absorber grain boundaries on the photocurrent transport in chalcopyrite based thin film solar cells has been calculated using a two dimensional numerical model. Considering extreme cases, the variation in red response is more expressed than in one dimensional models. These findings may offer an explanation for the strong influence of buffer layer preparation on the spectral response of cells with small grained absorbers
Junction formation of Cu3BiS3 investigated by Kelvin probe force microscopy and surface photovoltage measurements
Recently, the compound semiconductor Cu3BiS3 has been demonstrated to have a band gap of ~1.4 eV, well suited for photovoltaic energy harvesting. The preparation of polycrystalline thin films was successfully realized and now the junction formation to the n-type window needs to be developed. We present an investigation of the Cu3BiS3 absorber layer and the junction formation with CdS, ZnS and In2S3 buffer layers. Kelvin probe force microscopy shows the granular structure of the buffer layers with small grains of 20–100 nm, and a considerably smaller work-function distribution for In2S3 compared to that of CdS and ZnS. For In2S3 and CdS buffer layers the KPFM experiments indicate negatively charged Cu3BiS3 grain boundaries resulting from the deposition of the buffer layer. Macroscopic measurements of the surface photovoltage at variable excitation wavelength indicate the influence of defect states below the band gap on charge separation and a surface-defect passivation by the In2S3 buffer layer. Our findings indicate that Cu3BiS3 may become an interesting absorber material for thin-film solar cells; however, for photovoltaic application the band bending at the charge-selective contact has to be increased