Nanostructured Materials

Nanostructured materials, e.g. semiconductors created by using colloidal nanocrystals as 'artificial atoms', are emerging as tunable, earth-abundant and potentially non-toxic materials for solar energy conversion, light emission and electronic applications. Theory and simulations are used to help interpret a growing body of complex measurements, and to predict optimal systems for harvesting sun light, and materials with desired electronic transport properties.

Buried interfaces in all-inorganic nanocrystalline solids

Controlling the surface chemistry of nano building blocks and their interfaces with ligands is one of the outstanding challenges for the rational design of all-inorganic nanostructured solids. We carried out a combined theoretical and experimental study to characterize, at the atomistic level, buried interfaces in solids of InAs nanoparticles capped with Sn2S64- ligands. These prototypical nanocomposites are known for their promising transport properties and unusual negative photoconductivity. We found that inorganic ligands dissociate on InAs to form a surface passivation layer. A nanocomposite with unique electronic and transport properties is formed, that exhibits type II heterojunctions favourable for exciton dissociation. We identified how the matrix density, sulfur content and specific defects may be designed to attain desirable electronic and transport properties, and we explain the origin of the measured negative photoconductivity of the nanocrystalline solids.

Solids of chalcogenide nanoparticles

Lead chalcogenide nanoparticle solids have been successfully integrated into certified solar cells and represent promising platforms for the design of novel photoabsorbers for photoelectrochemical cells. While much attention has been drawn to improving efficiency and device performance through altering the character of the individual nanoparticles, the role of interactions between nanoparticles is not yet well-understood. Using first-principles molecular dynamics and electronic structure calculations, we investigated the combined effect of temperature and interaction on functionalized lead chalcogenide nanoparticles (NPs). We showed that at finite temperature, interacting NPs are dynamical dipolar systems, with the average values of dipole moments and polarizabilities substantially increased with respect to those of the isolated building blocks. Additionally, research in our group has focused on Intermediate Band (IB) solar cells, where we proposed that colloidal NPs are a viable and efficient platform for the implementation of the IB solar cell concept.

Embedded chalcogenide nanoparticles

We carried out atomistic calculations on chalcogenide nanostructured materials, i.e., PbSe QDs in CdSe matrices and CdSe embedded in PbSe, and we established how interfacial and core structures affect their electronic properties. We showed that defects present at interfaces of PbSe nanoparticles and CdSe matrices give rise to detrimental intragap states, degrading the performance of photovoltaic devices. Instead, the electronic gaps of the inverted system (CdSe dots in PbSe) are clean, indicating that this material has superior electronic properties for solar applications. In addition, our calculations predicted that the core structure of CdSe and in turn its band gap may be tuned by applying pressure to the PbSe matrix, providing a means to engineering the properties of new functional materials.

Embedded Si nanocrystals

Si nanocrystals (NCs) are often synthesized in oxide or nitride matrices. Our coupled classical and quantum simulations of 1-2 nm Si nanoparticles embedded in amorphous-SiO2 has shown that by tuning the density of the oxide , one may form nanoscale heterojunctions with either straddling (type I) or staggered (type II) energy gaps. We have also found that interfacial strain plays a key role in determining the variation of the nanaoparticle gap as a function of size, as well as conduction band offsets. NCs extracted from matrices with their oxide shell have allowed us to study the origin of blinking. In addition to oxides we have studied Si NPs in ZnS matrices and found that upon high temperature amorphization of the host chalcogenide, sulfur atoms are drawn to the NP surface. Sulfur content may be engineered to form a type II heterojunction, with complementary charge transport channels for electrons and holes.

Ligand engineering

Band edge positions of semiconductors determine their functionality in many optoelectronic applications such as photovoltaics, photoelectrochemical cells, and light emitting diodes. Here we show that band edge positions of lead sulfide (PbS) colloidal semiconductor nanocrystals, specifically quantum dots (QDs), can be tuned over 2.0 eV through surface chemistry modification. We achieved this remarkable control through the development of simple, robust, and scalable solution-phase ligand exchange methods, which completely replace native ligands with functionalized cinnamate ligands, allowing for well-defined, highly tunable chemical systems. By combining experiments and ab initio simulations, we establish clear relationships between QD surface chemistry and the band edge positions of ligand/QD hybrid systems. We find that in addition to ligand dipole, inter-QD ligand shell inter-digitization contributes to the band edge shifts. We expect that our established relationships and principles can help guide future optimization of functional organic/inorganic hybrid nanostructures for diverse optoelectronic applications.

Defect states and charge transport in quantum dots

The presence of trap states in the electronic gap of semiconducting limits their usability in solar cells, and developing a universal strategy to remove trap states is a persistent challenge. Using calculations based on density functional theory, we studied defects states in hydrogenated Si dots and in lead chalcogenide nanoparticles. We showed that hydrogen acts as an amphoteric impurity on PbS nanoparticle surfaces, passivating defects arising from ligand imbalance or off-stoichiometric surface terminations. Using constrained density functional theory calculations, we showed that hydrogen treatment of defective nanoparticles is also beneficial for charge transport in films. The same techniques was used to study shallow and deep impurity states in Si nanoparticles.

Novel Silicon Phases

Silicon exhibits a large variety of different bulk phases, allotropes and composite structures, such as e. g. clathrates or nanostructures, at both higher and lower densities compared to diamond-like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, that are non-toxic and earth-abundant, with properties tailored precisely towards specific applications, including solar energy conversion devices.

Organic Photovoltaics

Establishing how the conformation of organic photovoltaic (OPV) polymers affects their electronic and transport properties is critical in order to determine design rules for new OPV materials and in particular to understand the performance enhancements recently reported for ternary blends. We carried out coupled classical and ab initio molecular dynamics simulations showing that polymer linkage twisting significantly reduces optical absorption efficiency, as well as hole transport rates in donor polymers. We predicted that blends with components favoring planar geometries contribute to the enhancement of the overall efficiency of ternary OPVs. Furthermore, our electronic structure calculations for the PTB7–PID2–PC71BM system showed that hole transfer rates are enhanced in ternary blends with respect to their binary counterpart. Our results also pointed at thermal disorder in the blend as a key reason responsible for device voltage losses and at the need to carry out electronic structure calculations at finite temperature to reliably compare with experiments.

Excitation Spectra of Si and Ge Nanoparticles

We carried out density functional and many body perturbation theory calculations of the electronic, optical, and impact ionization properties of Si and Ge nanoparticles (NPs), including core structures based on high-pressure bulk Si and Ge phases. Si particles with a BC8 core structure exhibit significantly lower optical gaps and multiple exciton generation (MEG) thresholds, and an order of magnitude higher MEG rate than diamondlike ones of the same size. Hence BC8 nanoparticles may be promising candidates for MEG-based solar energy conversion. High pressure structures of Ge nanoparticles (ST12) show as well promising MEG properties.

MoS2 Nanoparticles

Bulk MoS2, a prototypical metal dichalcogenide, is an indirect band gap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to a monolayer, a strong photo-luminescence emerges, indicating an indirect to direct band gap transition, as predicted by ab-initio calculations. This observation shows that quantum confinement in layered d-electron materials such as MoS2 may provide new opportunities for engineering the electronic structure of matter at the nanoscale.

Optical Spectra of Si Nanowires

Optical absorption spectra of thin silicon nanowires have been computed using many body perturbation theory, by solving the Bethe-Salpeter equation in the static approximation. We used a technique recently developed in our group, that avoids explicit calculation of empty electronic states, as well as storage and inversion of the full dielectric matrix. Establishing numerical error bars of computed spectra turned out to be critical, in order to draw meaningful comparisons with experiment and between results obtained within different algorithms. The dependence of spectra on surface structure has been analyzed in detail.