Recently, solution-processing became a viable route for depositing CdTe for use in photovoltaics. Ultrathin (∼500 nm) solar cells have been made using colloidal CdTe nanocrystals with efficiencies exceeding 12% power conversion efficiency (PCE) demonstrated by using very simple device stacks. Further progress requires an effective method for extracting charge carriers generated during light harvesting. Here, we explored solution-based methods for creating transparent Ohmic contacts to the solution-deposited CdTe absorber layer and demonstrated molecular and nanocrystal approaches to Ohmic hole-extracting contacts at the ITO/CdTe interface. We used scanning Kelvin probe microscopy to further show how the above approaches improved carrier collection by reducing the potential drop under reverse bias across the ITO/ CdTe interface. Other methods, such as spin-coating CdTe/A2CdTe2 (A = Na, K, Cs, N2H5), can be used in conjunction with current/light soaking to improve PCE further.

Iridium, platinum, and gold nanoparticles, protected with sulfur, selenium, or tellurium dodecane ligands, were synthesized under ambient laboratory conditions. These nine nanoparticles were characterized by thermogravimetric analysis, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). XPS was used to determine the degree of oxidation present at the metal–chalcogen interface at the time of synthesis and after 1 week of aging under ambient laboratory conditions. Upon synthesis, interfaces involving sulfur atoms were found to have no degree of oxidation and to retain this lack of oxidation over the course of 1 week. In contrast, all interfaces involving tellurium were found to have some degree of oxidation (28, 77, and 76% for Ir, Pt, and Au particles, respectively) at the time of synthesis, and this degree of oxidation increased over the course of 1 week to 38, 83, and 92% for Ir, Pt, and Au, respectively. For interfaces involving selenium, all interfaces initially lacked any oxidation and the iridium and platinum interface was found to be stable over 1 week. On the other hand, the gold interface oxidized over time, reaching 60% oxidation after 1 week under ambient laboratory conditions. Thus, our work shows that IrS, IrSe, PtS, PtSe, and AuS provide metal–ligand interfaces that are stable, with respect to oxidation under ambient conditions.

Using electron spin resonance spectroscopy, we demonstrate that the morphological asymmetries present in small spheroidal metallic nanoparticles give rise to asymmetries in the behavior of electrons held in states near the metal’s Fermi energy. We find that the effects of morphological asymmetries for these spheroidal systems are more important than the effects of size distributions when explaining the asymmetry in electronic behavior. This is found to be true for all the particles examined, which were made from Cu, Ag, Pd, Ir, Pt, and Au, bearing dodecanethiolate ligands. In the case of the Ag particles, we also demonstrate that the same model used to account for morphological effects in the electron spin resonance spectra can be used to account for small asymmetries present in the plasmon spectrum. This result demonstrates that the electronic properties of even small particles are tunable via morphological changes.