The effects of RbF postdeposition treatment (RbF-PDT) on Cu­(In,Ga)­Se2, CuInSe2, and CuGaSe2 thin films and solar cell devices are comparatively studied. Similar to the effect of the KF postdeposition treatment (KF-PDT), Cu­(In,Ga)­Se2 and CuInSe2 film surfaces show significant pore formation resulting in a rough surface morphology with RbF-PDT, whereas this is not the case for In-free CuGaSe2. The device properties of the In-containing and In-free Cu­(In,Ga)­Se2 solar cells also show contrasting results, namely, Cu­(In,Ga)­Se2 or CuInSe2 devices show an increase in the open circuit voltage (V oc) and fill factor (FF) values and almost constant or a slight decrease in the short-circuit current density (J sc) values with RbF-PDT, whereas CuGaSe2 devices show no significant improvements in the V oc and FF values but a substantial increase in the J sc values. These results suggest that the alkali effects on the Cu­(In,Ga)­Se2 film and device properties strongly depend on the group III elemental composition in the Cu­(In,Ga)­Se2 films as well as alkali-metal species.

Scanning transmission electron microscopy-energy dispersive X-ray spectroscopy measurements performed on Cu(In,Ga)Se2 (CIGS) thin-film devices grown with RbF postdeposition treatment (RbF-PDT) revealed the formation of a RbInSe2 compound at the p-CIGS/n-CdS heterointerface. However, this type of Rb-compound formation was not observed with elemental In-free ternary CuGaSe2 (CGS) devices. The film surface of CIGS grown with RbF-PDT was found to be Ga-depleted; thus, the practical interface structure turned out to be CIGS/(CuInSe2, RbInSe2)/CdS. In contrast to the significant improvements observed in In-containing CIGS photovoltaic device performance with RbF-PDT, no significant improvements have been observed in In-free CGS devices thus far. These results suggest that the presence of elemental In plays a key role in obtaining beneficial alkali Rb effects for enhancing device performance as well as surface modification with RbF-PDT.

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We report a scandium oxyhydride BaScOH prepared by solid state reaction under high pressure. Rietveld refinements against powder synchrotron X-ray and neutron diffraction data revealed that BaScOH adopts the ideal cubic perovskite structure (Pm3̅m), where oxide (O) and hydride (H) anions are disordered. H nuclear magnetic resonance (NMR) spectroscopy provides a positive chemical shift of about +4.4 ppm, which can be understood by the distance to the nearest (and possibly the next nearest) cation from the H nucleus. A further analysis of the NMR data and calculations based on ab initio random structure searches suggest a partial cis preference in ScOH octahedra. The present oxyhydride, if compositionally or structurally tuned, may become a candidate for H conductors.

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Both elemental Rb and Cs are known as effective dopants in enhancing Cu­(In,Ga)­Se2 (CIGS) thin-film photovoltaic device performance. It is, however, found that there are critical differences in the effects of these two alkali metals on CIGS thin-film properties. Namely, CIGS film surfaces show RbInSe2 compound formation after RbF postdeposition treatment (PDT), whereas no such segregated compound formation is observed with CsF-PDT. On the other hand, both Rb and Cs have the effect of increasing effective hole carrier density, photoluminescence intensity, and carrier lifetime in CIGS similar to the effects of the lighter alkali-metals Na and K. The fundamental mechanism behind metastable acceptor formation, which is known as the light-soaking or bias-soaking effect leading to CIGS photovoltaic performance enhancement, is experimentally revealed to be independent of the presence of alkali metals in CIGS, and alkali-metals still have an effect of facilitating metastable acceptor formation.

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