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Recent numerical modeling and experimental work have shown that appropriate conduction band alignment at the emitter/absorber interface is critically important for high efficiency CdTe solar cells. To date, however, the properties of the transparent conducting oxide (TCO)/emitter interface have not been thoroughly investigated. Here, we use numerical modeling to determine the role of the conduction band alignment at the TCO/emitter interface. Our findings are increasingly important to device performance as efforts are made to widen the bandgap of the absorber. We also investigate the relative importance of the barrier caused by poor front contact versus the barrier at the back contact. It is well known that a barrier at the back contact can reduce the open circuit voltage of the device and produce rollover in the current density-voltage but is not well appreciated that a barrier at the front of the device can play a completely analogous role. We also show that for optimum device performance at any absorber bandgap, the conduction band of the TCO must be no more than 0.3 eV below the conduction band of the emitter.Recent numerical modeling and experimental work have shown that appropriate conduction band alignment at the emitter/absorber interface is critically important for high efficiency CdTe solar cells. To date, however, the properties of the transparent conducting oxide (TCO)/emitter interface have not been thoroughly investigated. Here, we use numerical modeling to determine the role of the conduction band alignment at the TCO/emitter interface. Our findings are increasingly important to device performance as efforts are made to widen the bandgap of the absorber. We also investigate the relative importance of the barrier caused by poor front contact versus the barrier at the back contact. It is well known that a barrier at the back contact can reduce the open circuit voltage of the device and produce rollover in the current density-voltage but is not well appreciated that a barrier at the front of the device can play a completely analogous role. We also show that for optimum device performance at any absorbe...

Thin film photovoltaic (PV) devices and modules prepared by commercial processes can be severely compromised by through-device low resistance electrical pathways. The defects can be due to thin or missing semiconductor material, metal diffusion along grain boundaries, or areas containing diodes with low turn-on potentials. We report the use of single wall carbon nanotube (SWCNT) layers to enable both protection against these defects and back contact formation for CdTe PV devices. Samples prepared with a SWCNT back contact exhibited good efficiency and did not require shunt protection, while devices prepared without shunt protection using a standard metal back contact performed poorly. We describe the mechanism by which the SWCNT layer functions. In addition to avoiding the need for shunt protection by other means, the SWCNT film also provides a route to higher short circuit currents.

Hot wire chemical vapor deposition (HWCVD) has been employed for the continuous generation of carbon single-walled nanotube (SWNT) materials. Interestingly, transmission electron microscopy analyses revealed only the presence of isolated SWNTs, rather than nanotubes existing in bundles. An analysis of the growth mechanism explaining the production of isolated SWNTs is provided. Also, the Raman radial breathing modes (RBMs) of the isolated HWCVD-generated nanotubes are compared to the RBMs of small bundles of nanotubes deposited by a conventional CVD technique having a similar diameter distribution.

A simple method for producing a Au:p‐n‐CdTe Schottky barrier is described. The shallow p‐n junction is formed by photoelectrochemical surface oxidization of n‐CdTe. X‐ray photoelectron spectroscopy, Auger electron spectroscopy depth profiling, and electron‐beam‐induced‐current measurements provide important insight into the underlying causes of the formation of the p‐type layer. Current‐voltage and capacitance‐voltage measurements show that the thin p‐layer enhances the effective barrier height relative to that of a traditional Au:n‐CdTe junction. These results account for the Au:p‐n‐CdTe cells exhibiting higher open‐circuit photovoltages and higher photoconversion efficiencies than do Au:n‐CdTe Schottky‐barrier cells. From temperature dependence studies of the current‐voltage characteristics, detailed information on the charge‐transport mechanism of the junction was obtained. Photocurrent spectra of Au:p‐n‐CdTe as a function of temperature reveal that exciton excitation in CdTe contributes to the photocu...

We report a treatment that enhances and stabilizes the photoluminescence (PL) from porous Si films. Films prepared by anodization in a 50% HF/ethanol solution were annealed at 450 °C in vacuum, exposed to air, and then exposed to a remote‐hydrogen plasma. Infrared absorption spectroscopy revealed that the concentration of oxygen, rather than hydrogen, was increased by the processing steps, and that silicon dihydride species had been eliminated from the surface. The PL from a treated film was initially ∼30 times more intense than from the as‐etched films. The PL intensity increased with illumination time in air until a steady‐state intensity was reached.

A fast, sensitive, automated Fourier transform (FT) photoluminescence (PL) spectrometer with tunable excitation has been developed for analyzing carbon nanotube suspensions over a wide spectral range. A commercially available spectrometer was modified by the addition of a tunable excitation source, custom collection optics, and computer software to provide control and automated data collection. The apparatus enables excitation from 400to1100nm and detection from 825to1700nm, permitting the analysis of virtually all semiconducting single-walled nanotubes (SWNTs), including those produced by the high pressure carbon monoxide conversion and laser processes. The FT approach provides an excellent combination of high sensitivity and fast measurement. The speed advantage exists because the entire emission spectrum is collected simultaneously, while the sensitivity advantage stems from the high optical throughput. The high sensitivity is demonstrated in the measurement of very dilute SWNT suspensions and the observation of novel spectral features, and the speed is demonstrated by measuring the real-time changes in the SWNT PL during rebundling. This contribution describes the assembly of components, the methods for automating data collection, and the procedures for correcting the wavelength-dependent excitation intensity and the interferometer and detector responses.

A design for a scanning tunneling microscope that is well suited for electrochemical investigations is presented. The construction of the microscope ensures that only the tunneling tip and the sample participate in electrochemical reactions. The design also allows rapid replacement of the tip or sample, and enables facile introduction of auxiliary electrodes for use in electrochemical experiments. The microscope utilizes stepper motor driven approach mechanics in order to achieve fully remote operation and to allow reproducible coarse control of tip/sample spacings for electrochemical experiments. Highly ordered pyrolytic graphite images at atomic resolution in air and aqueous solutions can be obtained with this microscope.