Among solution-processed nanocrystals containing environmentally benign elements, bismuth sulfi de (Bi 2 S 3 ) is a very promising n-type semiconductor for solar energy conversion. Despite the prompt success in the fabrication of optoelectronic devices deploying Bi 2 S 3 nanocrystals, the limited understanding of electronic properties represents a hurdle for further materials developments. Here, two key materials science issues for light-energy conversion are addressed: bandgap tunability via the quantum size effect, and photocarrier trapping. Nanocrystals are synthesized with controlled sizes varying from 3 to 30 nm. In this size range, bandgap tunability is found to be very small, a few tens of meV. First principles calculations show that a useful blueshift, in the range of hundreds of meV, is achieved in ultra-small nanocrystals, below 1.5 nm in size. Similar conclusions are envisaged for the class of pnictide chalcogenides with a ribbon-like structure [Pn 4 Ch 6 ] n (Pn = Bi, Sb; Ch = S, Se). Time-resolved differential transmission spectroscopy demonstrates that only photoexcited holes are quickly captured by intragap states. Photoexcitation dynamics are consistent with the scenario emerging in other metal-chalcogenide nanocrystals: traps are created in metal-rich nanocrystal surfaces by incomplete passivation by long fatty acid ligands. In large nanocrystals, a lower bound to surface trap density of one trap every sixteen Bi 2 S 3 units is found.

In this work we study the morphology and electronic properties of Bi 2 S 3 nanostructures by means of atomistic simulations. We focus on elongated nanoribbons that are the building blocks of the corresponding crystal structures, and we study saturated and unsaturated nanocrystals of finite size in comparison with one-dimensional infinite ones. By means of (time-dependent) density functional theory calculations we provide evidence that the optical gap can be tuned through quantum confinement with sizable effects for ribbons smaller than three nanometers. By a comparison with Sb 2 S 3 , we conclude that Bi 2 S 3 nanostructures have similar tunability of the bandgap and a better tendency of passivating defects at the (010) surfaces through local reconstructions.

Zinc oxide is typically used as an electron acceptor in the preparation of hybrid solar cells. Such hybrids are, in many cases, synthesized from solutions. In this work, we investigate the possible persistence of solvent tetrahydrofuran molecules at the interface with zinc oxide by means of atomistic simulations based on model potentials and density functional theory calculations. We found a strong interaction between the solvent and the zinc oxide that leads to the formation of an ordered layer of tetrahydrofuran molecules bound to the zinc oxide substrate

Ultrathin crystalline Bi 2 S 3 nanostructures are studied by first-principles atomistic modeling and supported by experiments. Consistent with previous findings, the theoretical analysis shows that nanowires with lateral sizes as small as a few nanometers are energetically possible. Also, we were able to synthesize ultrathin nanowires by means of a low-cost, nontoxic colloidal route. Transmission electron microscopy data reveal a coherence length of the nanowires exceeding 30 nm. The simulations show that surfaces affect the electronic structure of the material inducing peculiar 1D-like electronic states on the nanowire edges that are located 300 meV above the valence band. Sulfur vacancies are instead responsible for localized states a few hundred millielectronvolts below the conduction band. The possibility of eliminating the surface-induced intragap states is theoretically investigated by passivating the surfaces of the nanowires with carboxylic and amine groups that are commonly employed in colloidal synthesis. Small molecules methylamine and acetic acid are expected to fully passivate the surfaces of the nanowires, removing the edge states and restoring a clean band gap. Present results suggest a possible route for improving optoelectronic properties of Bi 2 S 3 nanostructures by tuning the size of the ligand molecules.