N-doped graphite has been reported to provide enhanced catalytic properties as a support material for Pt catalysts in fuel cell applications. With use of a combined experimental and modeling approach, this work identifies the potential fundamental mechanisms for this enhancement effect. To ensure a well-defined experimental system, this work employs highly oriented pyrolitic graphite (HOPG) as a model analogue of the graphite support commonly used in fuel cell applications. Undoped, Ar-doped, and N-doped HOPG substrates have been investigated via electrochemical capacitance and X-ray photoelectron spectroscopy (XPS) measurements. The results indicate that doping, especially N-doping, induces significant modification to the electronic structure of the HOPG surface. A simplified model of the doping effects and band structures for the doped graphite surfaces are proposed to explain these results. When Pt nanoparticles are grown on top of these dopant-modified HOPG surfaces, the resulting Pt/surface-defect interactions significantly impact the Pt nanoparticle nucleation, growth, and catalytic activity.

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Back contacts can significantly limit CdTe solar cell performance, reducing both open circuit voltage (V oc) and fill factor (FF). Copper is an essential component of effective back contacts, but its presence in the CdTe absorber creates detrimental recombination centers. Rapid thermal processing (RTP) is demonstrated as a highly effective approach for reducing back contact barriers in CdTe solar cells contacted with ZnTe:Cu buffer layers, substantially improving both FF (>73%) and V oc (>850 mV). Current density and quantum efficiency remain essentially unchanged, but a five-fold increase in minority carrier lifetime is observed which is attributed to passivation of recombination sites in the back contact region. Quantitative analysis of secondary ion mass spectrometry shows that the majority of Cu segregates to the Au metallization layer and that the ZnTe buffer appears to inhibit the Cu diffusion into CdTe. 3D imaging of the back contact region using atom probe tomography shows that optimized devices are characterized by preferential segregation of copper to both the Au|ZnTe and CdTe|ZnTe interfaces, perhaps in the form of Cu x Te. With its low thermal budget the RTP process has been successfully applied to multiple device architectures. including devices with certified efficiencies in excess of 16%.

The use of ZnTe buffer layers at the back contact of CdTe solar cells has been credited with contributing to recent improvements in both champion cell efficiency and module stability. To better understand the controlling physical and chemical phenomena, high resolution transmission electron microscopy (HR-TEM) and atom probe tomography (APT) were used to study the evolution of the back contact region during rapid thermal processing (RTP) of this layer. After activation the ZnTe layer, initially nanocrystalline and homogenous, transforms into a bilayer structure consisting of a disordered region in contact with CdTe characterized by significant Cd-Zn interdiffusion, and a nanocrystalline layer that shows evidence of grain growth and twin formation. Copper, co-evaporated uniformly within ZnTe, is found to dramatically segregate and aggregate after RTP, either collecting near the ZnTe|Au interface or forming Cu x Te clusters in the CdTe layer at defects or grain boundaries near the interface. Analysis of TEM images revealed that Zn accumulates at the edge of these clusters, and three-dimensional APT images confirmed that these are core-shell nanostructures consisting of Cu 1.4 Te clusters encased in Zn. These changes in morphology and composition are related to cell performance and stability.

The epitaxy of Ti on Si͑001͒ exhibits a profound intermixing of Ti and Si atoms giving rise to the formation of titanium silicide. This phenomenon differs considerably from typical epitaxial growth and is not understood. Using first-principles total-energy calculations we examined the reaction of a Ti adatom with a Si͑001͒ surface. We found that the penetration of the Ti adatom into a near-surface interstitial site and the subsequent ejection of its neighboring surface Si atoms onto a terrace is kinematically favored with respect to the ''normal'' hopping diffusion on a Si surface. These reactive processes provide the microscopic mechanism of an initial stage of transition-metal silicidation. ͓S0163-1829͑98͒01332-0͔