XPS AND UPS investigation of NH/sub 4/OH-exposed Cu(In,Ga)Se/sub 2/ thin films
Photoelectron spectroscopy was used to determine the compositional and electronic changes occurring in Cu(In,Ga)Se/sub 2/ thin films as a result of immersion in aqueous ammonia solution. We find that NH/sub 4/OH-treated CIGS surfaces are preferentially etched of indium and gallium, resulting in the formation of a thin layer of a degenerate Cu-Se compound that we tentatively identify as Cu/sub 2/Se. The work function of ammonia-treated samples is found to increase by 0.6 eV relative to as-grown CIGS thin films. The uniformity of chemical bath effects (etching & deposition) was found to be improved by the addition to the bath of a non-ionic surfactant. Initial device results show that the new surfactant-based chemical bath deposition (CBD) method may lead to better and thinner CdS buffer layers.
High surface recombination velocity limits Quasi-Fermi level splitting in kesterite absorbers
Kelvin Probe Force Microscopy, Photoluminescence imaging and numerical simulations are used to study the surfaces of Cu2ZnSnSe4 absorber layers. In particular, the effect of NH4OH and annealing under ambient conditions is investigated. We observe drastic changes in the measured quasi Fermi-level splitting (QFLs) after chemical cleaning of the absorber surface with NH4OH, which is traced back to a removal of the surface inversion. Air annealing recovers surface inversion, which reduces the recombination current at the surface. Annealing above 200 °C leads to a permanent change in the work function which cannot be modified by NH4OH etching anymore. This modification makes the QFLs insensitive to surface cleaning and explains why air annealing in Cu2ZnSnSe4 is important. From numerical simulations we find that a large surface recombination velocity needs to be present in order to describe the experimental observations.
Philip Roth’s Great Books: A Reading of The Human Stain
Philip Roth belongs to the first generation of American novelists for whom a university education in the liberal arts was the norm. This essay charts Roth’s developing acquaintance with classical literature, the starting point for Great Books courses in the 1950s, and the implications for his fiction of his understanding of the difference in scope and purpose between the two representative classical genres. It focuses on The Human Stain (2000), the novel in which he reflects most directly upon what tragedy and epic once did, and what might happen to novels if they became more like the one or the other.