A polycrystalline, thin-film CuxS/CdS solar cell was exposed to air saturated with water vapor at temperatures between 27 and 67 °C for up to 6 1/2 h. The short-circuit current decreased progressively from 11.7 to 1.02 mA/cm2. Subsequent heating in hydrogen at 150 °C for 680 h and in 170 °C air for 5 h restored the short-circuit current to 14.4 mA/cm2. Modeling of the measured quantum yield indicates that the degradation could be quantitatively explained by two effects: (1) the CuxS minority carrier electron diffusion length decreasing from 0.23 to 0.02 μm (±20%) and (2) the CuxS optical band gap increasing from 1.16 to 1.46 eV (±3%). The recovery was quantitatively modeled by the CuxS diffusion length increasing back to 0.24 μm and the CuxS band gap returning to 1.16 eV. A Burstein–Moss analysis shows the band-gap shift is due to the Fermi level penetration of the valence band as the measured hole concentration increased from 1.03(1020) cm−3 to 4.62(1021) cm−3 during degradation. A new band structure is proposed with six equivalent, indirect, valence band maxima located at 1.16 eV (±3%) below the conduction band edge followed by two direct maxima located at 1.28 and 1.8 eV (±3%) below the conduction band edge. A density-of-states effective mass ratio for holes of 2.0 (±30%) is found. The sharp decrease and recovery in diffusion length is explained by a transition between phonon-assisted and direct minority carrier recombination with changes in CuxS hole concentration.
X-ray photoelectron spectroscopy spectra measured on copper sulfide (CuxS) films showed that a thin surface reaction product containing Cu in the +2 valence state was formed on CuxS films exposed to air for 46 h at 40 °C and 90% relative humidity. An entirely different CuxS surface reaction product layer was formed in dry air at 170 °C for 30 min and it contained sulfur in the +6 valence state. The copper (Cu) valence state in CuxS was not found to be +2 even when the x value was less than 1.9. When the argon sputter-cleaned surface of CuxS or CuxS/CdS films was exposed to room-temperature air for 10 min, cadmium (Cd) atoms appeared on the CuxS surface. X-ray powder diffraction patterns showed that CuO and CdS reacted at 500 °C in flowing nitrogen to form Cu2S and CdO. This cation exchange between CdS and copper oxide may explain the surface Cd on the CuxS films. The standard free energy of reaction between CuO and CdS is positive while that between Cu2O and CdS is negative. These results indicate a method for stabilizing CuxS/CdS solar cells against degradation.
Purpose: In image-guided radiotherapy, an artifact typically seen in axial slices of x-ray cone-beam computed tomography ͑CBCT͒ reconstructions is a dark region or "black hole" situated below the scan isocenter. The authors trace the cause of the artifact to scattered radiation produced by radiotherapy patient tabletops and show it is linked to the use of the offset-detector acquisition mode to enlarge the imaging field-of-view. The authors present a hybrid scatter kernel superposition ͑SKS͒ algorithm to correct for scatter from both the object-of-interest and the tabletop. Methods: Monte Carlo simulations and phantom experiments were first performed to identify the source of the black hole artifact. For correction, a SKS algorithm was developed that uses separate kernels to estimate scatter from the patient tabletop and the object-of-interest. Each projection is divided into two regions, one defined by the shadow cast by the tabletop on the imager and one defined by the unshadowed region. The region not shadowed by the tabletop is processed using the recently developed fast adaptive scatter kernel superposition ͑fASKS͒ method which employs asymmetric kernels that best model scatter transport through bodylike objects. The shadowed region is convolved with a combination of slab-derived symmetric SKS kernels and asymmetric fASKS kernels. The composition of the hybrid kernels is projection-angle-dependent. To test the algorithm, pelvis phantom and in vivo data were acquired using a CBCT test stand, a Varian Acuity simulator, and a Varian On-Board Imager, all of which have similar geometries and components. Artifact intensities and Hounsfield unit ͑HU͒ accuracies in the reconstructions were assessed before and after the correction. Results: The hybrid kernel algorithm provided effective correction and produced substantially better scatter estimates than the symmetric SKS or asymmetric fASKS methods alone. HU nonuniformities in the reconstructed pelvis phantom were reduced from 220 to 50 HU ͑i.e., 22%-5%͒. In the in vivo scans, the black hole artifact was reduced by up to 147 HU, a 73% improvement, and anatomical details in the prostate and rectum areas were made considerably more visible. Conclusions: Radiotherapy tabletops, which are generally flatter and larger than those used for diagnostic CT, can produce significant scatter-related artifacts. The proposed hybrid SKS algorithm accurately estimates scatter from both the object-of-interest and the patient tabletop, and resulting image uniformities and HU accuracies are greatly improved.
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