The low photovoltaic efficiency of iron pyrite-based solar cells is often related to the presence of sulfur deficiencies. In this paper surfur-rich iron pyrite nanocrystals (FeS₂ NCs) are synthesized by the hot injection method and deposited using layer by layer deposition. Optical absorption measurements show substantial sub-bandgap absorption, which is attributed to a sulfur-rich, thin surface layer. Microwave photoconductance measurements show very little signal of films with the original long ligands, while an approximately 100-fold higher signal is observed for films treated with FeCl₂ and 1,2-ethanedithiol (EDT) solutions. In mesoporous hybrid systems of FeS₂/SnO₂ both sub-band-gap and above-band-gap photons lead to electron injection from FeS₂ into the SnO₂ conduction band. We explain these findings by proposing that pinning of the Fermi level by the surface layer leads to a downward band bending in the direction of the surface within the FeS₂ NC. Hence, photoexcited electrons will first move toward the shell where they relax into empty surface states. As the holes remain behind in the core of the nanocrystals, this results in a charge-separated state with a long lifetime. Interestingly, these surface electrons are able to migrate in the FeS₂ NP layer by interparticle tunneling and can still decay by injection into SnO₂. Hence, our results indicate that SnO₂ is a suitable electron acceptor for FeS₂. The long-lived charge-separated electrons and holes could be exploited efficiently in photodetectors.

(Graph Presented) A new method to deposit perovskite thin films that benefit from the thickness control and conformality of atomic layer deposition (ALD) is detailed. A seed layer of ALD PbS is place-exchanged with PbI₂ and subsequently CH₃NH₃PbI₃ perovskite. These films show promising optical properties, with gain coefficients of 3200 ± 830 cm^-1.

Colloidal quantum dots have grown in interest as materials for light amplification and lasing in view of their bright photoluminescence, convenient solution processing and size-controlled spectral tunability. To date, lasing in colloidal quantum dot solids has been limited to the nanosecond temporal regime, curtailing their application in systems that require more sustained emission. Here we find that the chief cause of nanosecond-only operation has been thermal runaway: the combination of rapid heat injection from the pump source, poor heat removal and a highly temperature-dependent threshold. We show microsecond-sustained lasing, achieved by placing ultra-compact colloidal quantum dot films on a thermally conductive substrate, the combination of which minimizes heat accumulation. Specifically, we employ inorganic-halide-capped quantum dots that exhibit high modal gain (1,200 cm^-1) and an ultralow amplified spontaneous emission threshold (average peak power of ∼50 kW cm^-2) and rely on an optical structure that dissipates heat while offering minimal modal loss.