Doron Azulay

Electrical-thermal switching in carbon-black-polymer composites as a local effect.

Following the lack of microscopic information about the intriguing well-known electrical-thermal switching mechanism in carbon-black-polymer composites, we applied atomic force microscopy in order to reveal the local nature of the process and correlated it with the characteristics of the widely used commercial switches. We conclude that the switching events take place in critical interparticle tunneling junctions that carry most of the current. The macroscopic switched state is then a result of a dynamic-stationary state of fast switching and slow reconnection of the corresponding junctions.

Direct Evaluation of the Quantum Confinement Effect in Single Isolated Ge Nanocrystals

To address the yet open question regarding the nature of quantum confinement in Ge nanocrystals (Ge NCs) we employed scanning tunneling spectroscopy to monitor the electronic structure of individual isolated Ge NCs as a function of their size. The (single-particle) band gaps extracted from the tunneling spectra increase monotonically with decreasing nanocrystal size, irrespective of the capping ligands, manifesting the effect of quantum confinement. Band-gap widening of ∼1 eV with respect to the bulk value was observed for Ge-NCs 3 nm in diameter. The picture emerging from comparison with theoretical calculations and other experimental results is discussed.

Electronic structure and self-assembly of cross-linked semiconductor nanocrystal arrays

We studied the electronic level structure of assemblies of InAs quantum dots and CdSe nanorods cross-linked by 1,4-phenylenediamine molecules using scanning tunneling spectroscopy. We found that the bandgap in these arrays is reduced with respect to the corresponding ligand-capped nanocrystal arrays. In addition, a pronounced sub-gap spectral structure commonly appeared which can be attributed to unpassivated nanocrystal surface states or associated with linker-molecule-related levels. The exchange of the ligands by the linker molecules also affected the structural array properties. Most significantly, clusters of close-packed standing CdSe nanorods were formed.

Where does photocurrent flow in polycrystalline CdS

We have studied the local photoconductance in polycrystalline CdS films using conductance atomic force microscopy under illumination, and found that photoconductivity along the grain boundaries is excited at photon energies significantly smaller than the CdS band gap, Eg, whereas phototransport through the grains is detected only above Eg. In addition, we observed a rather strong persistent photoconductivity effect at both conduction channels. The implications of these findings regarding the band tails in CdS films and photovoltaic applications are discussed.

Annihilation of structural defects in chalcogenide absorber films for high-efficiency solar cells

In polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron–hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of – partially electronically active – planar defects in the {112} planes. Real-time X-ray diffraction reveals that these faults are nearly completely annihilated during an intermediate Cu-rich process stage with [Cu]/([In] + [Ga]) > 1. Moreover, correlations between real-time diffraction and fluorescence analysis during Cu–Se deposition reveal that rapid defect annihilation starts shortly before the start of segregation of excess Cu–Se at the surface of the Cu(In,Ga)Se2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism.

Anomalous photovoltaic effect in nanocrystalline Si∕SiO2 composites

We have observed an anomalous photovoltaic effect in films of Si nanocrystals embedded in SiO2. Using conductive-probe atomic force microscopy and global transport measurements, we found, close to the percolation threshold of the Si crystallite phase, a large photovoltaic effect of up to about 7V. Following the dependence of this effect on the size of the Si nanocrystals and on the relative tip position with respect to the counterelectrode, we suggest a model based on charge separation of excited electron-hole pairs governed by the size-dependent quantum confinement and charging energies.

Grafting Poly(3-hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3-hexylthiophene) (SiNC@P3HT) by applying surface-initiated Kumada catalyst transfer polycondensation (SI-KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor-acceptor interfaces. Systematic characterization provides evidence of a core-shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type-II heterostructure.

Validation of the tunneling percolation staircase model in granular metals

In this Letter, we provide conclusive evidence for a lattice like arrangement of the silver grains in the nanogranular Ag-Al2O3 metal. The evidence for the presence of clearly separated first and second near neighbor grains was derived from the electrical conductivity dependence on the metallic content in these composites. The data were analyzed in light of the tunneling percolation staircase model that we have recently suggested for lattices or systems with discrete well-defined interparticle distances in the continuum.

Can we use time-resolved measurements to get steady-state transport data for halide perovskites?

Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. Howev-er, as yet no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steady-state phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state pa-rameters. To answer the latter question, we revisited pulsed excitation, and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We per-formed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed TR-MC and TR-PL measurements as function of excitation intensity on the same samples of dif-ferent MAPbI3 thin films, and find good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of photo-transport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity.