The band-edge potentials of p-GaN in aqueous solutions were examined with photocurrent measurements, and those of n-GaN were examined with both photocurrent measurements and impedance spectroscopy. The measured band-edge potentials were different for both the different materials and the different measurement techniques. These differences are attributed to differences in the interface charging due to slow charge-transfer kinetics at the interface between the semiconductor and the solution. Using photocurrent measurements, the conduction band-edge potential was Φnormalc,normals=false(−1.092−0.063×normalpHfalse) V vs. a standard calomel electrode (SCE) for p-GaN and Φnormalc,normals=false(−0.538−0.046×normalpHfalse) V SCE for n-GaN. Using impedance spectroscopy, the conduction band-edge potential for n-GaN was Φnormalc,normals=false(−0.816−0.047×normalpHfalse) V SCE. © 2003 The Electrochemical Society. All rights reserved.

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%.

A systematic study of tin-catalyzed vapor−liquid−solid (VLS) growth of silicon nanowires by plasma-enhanced chemical vapor deposition at temperatures ranging from 300 to 400 °C is presented. Wire structure, morphology, and growth rate are characterized as a function of process variables. The nanowires are observed to have a crystalline core with a polycrystalline shell due to simultaneous VLS axial growth and vapor−solid radial growth. Axial and radial growth rates are controllable through hydrogen dilution of the plasma which affects the concentration of silane radicals in the plasma. In addition, wire length is observed to saturate with increasing growth time. Post growth chemical analysis suggests this is due to etching and disappearance of tin seeds in the hydrogen plasma which occur in parallel with wire growth. This opens up the possibility of a unique in situ approach to fabricating metal-free nanowire arrays for device applications.

Articles you may be interested inThe effects of high temperature processing on the structural and optical properties of oxygenated CdS window layers in CdTe solar cells

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.