Tadas Paulauskas

Effect of substrate on the atomic structure and physical properties of thermoelectric Ca 3 Co 4 O 9 thin films

The incommensurately layered cobalt oxide Ca(3)Co(4)O(9) exhibits an unusually high Seebeck coefficient as a polycrystalline bulk material, making it ideally suited for many high temperature thermoelectric applications. In this paper, we investigate properties of Ca(3)Co(4)O(9) thin films grown on cubic perovskite SrTiO(3), LaAlO(3), and (La(0.3)Sr(0.7))(Al(0.65)Ta(0.35))O(3) substrates and on hexagonal Al(2)O(3) (sapphire) substrates using the pulsed laser deposition technique. X-ray diffraction and transmission electron microscopy analysis indicate strain-free growth of films, irrespective of the substrate. However, depending on the lattice and symmetry mismatch, defect-free growth of the hexagonal CoO(2) layer is stabilized only after a critical thickness and, in general, we observe the formation of a stable Ca(2)CoO(3) buffer layer near the substrate-film interface. Beyond this critical thickness, a large concentration of CoO(2) stacking faults is observed, possibly due to weak interlayer interaction in this layered material. We propose that these stacking faults have a significant impact on the Seebeck coefficient and we report higher values in thinner Ca(3)Co(4)O(9) films due to additional phonon scattering sites, necessary for improved thermoelectric properties.

A new silicon drift detector for high spatial resolution STEM-XEDS: performance and applications.

A newly designed, 100 mm2, silicon drift detector has been installed on an aberration-corrected scanning transmission electron microscope equipped with an ultra-high resolution pole piece, without requiring column modifications. With its unique, windowless design, the detectors active region is in close proximity to the sample, resulting in a dramatic increase in count rate, while demonstrating an increased sensitivity to low energy X-rays and a muted tilt dependence. Numerous examples of X-ray energy dispersive spectrometry are presented on relevant materials such as Al x Ga1-x N nanowires, perovskite oxides, and polycrystalline CdTe thin films, across both varying length scales and accelerating voltages.

Atomic-resolution characterization of the effects of CdCl2 treatment on poly-crystalline CdTe thin films

Poly-crystalline CdTe thin films on glass are used in commercial solar-cell superstrate devices. It is well known that post-deposition annealing of the CdTe thin films in a CdCl2 environment significantly increases the device performance, but a fundamental understanding of the effects of such annealing has not been achieved. In this Letter, we report a change in the stoichiometry across twin boundaries in CdTe and propose that native point defects alone cannot account for this variation. Upon annealing in CdCl2, we find that the stoichiometry is restored. Our experimental measurements using atomic-resolution high-angle annular dark field imaging, electron energy-loss spectroscopy, and energy dispersive X-ray spectroscopy in a scanning transmission electron microscope are supported by first-principles density functional theory calculations.

Atomic and electronic structure of Lomer dislocations at CdTe bicrystal interface.

Extended defects are of considerable importance in determining the electronic properties of semiconductors, especially in photovoltaics (PVs), due to their effects on electron-hole recombination. We employ model systems to study the effects of dislocations in CdTe by constructing grain boundaries using wafer bonding. Atomic-resolution scanning transmission electron microscopy (STEM) of a [1–10]/(110) 4.8° tilt grain boundary reveals that the interface is composed of three distinct types of Lomer dislocations. Geometrical phase analysis is used to map strain fields, while STEM and density functional theory (DFT) modeling determine the atomic structure at the interface. The electronic structure of the dislocation cores calculated using DFT shows significant mid-gap states and different charge-channeling tendencies. Cl-doping is shown to reduce the midgap states, while maintaining the charge separation effects. This report offers novel avenues for exploring grain boundary effects in CdTe-based solar cells by fabricating controlled bicrystal interfaces and systematic atomic-scale analysis.

Density functional theory modeling of twin boundaries in CdTe as informed by STEM observations

CdTe is one of the most promising photovoltaic materials, currently second only to Si in market share. However, the practical efficiencies of CdTe photovoltaic cells are still significantly below the theoretical limit, indicating possible room for improvement. One aspect in which a fundamental understanding may lead to efficiency improvements is grain boundaries. Atomistic-level characterization, including microscopy and first principles modeling, is crucial in developing such a fundamental understanding.

Creation and analysis of atomic structures for CdTe bi-crystal interfaces by the grain boundary genie

Grain boundaries (GB) in poly-CdTe solar cells play an important role in species diffusion, segregation, defect formation, and carrier recombination. While the creation of specific high-symmetry interfaces can be straight forward, the creation of general GB structures in many material systems is difficult if periodic boundary conditions are to be enforced. Here we describe a novel algorithm and implementation to generate initial general GB structures for CdTe in an automated way, and we investigate some of these structures using density functional theory (DFT). Example structures include those with bi-crystals already fabricated for comparison, and those planning to be investigated in the future.

Chemical analysis with single atom sensitivity using aberration-corrected STEM

The last few years have seen a paradigm change in scanning transmission electron microscopy, STEM, with unprecedented improvements in both spatial and spectroscopic resolution being realized by aberration correctors, cold-field emission guns and monochromators. The successful correction of lens aberrations has greatly advanced the ability of the STEM to provide direct, real space imaging at atomic resolution. Very complementary to reciprocal space methods, this is especially advantageous for aperiodic systems, nanostructures, interfaces and point defects. Aberration-correction has also enabled the development of new imaging techniques, such as incoherent annular bright field (ABF) imaging, which enables the direct visualization of light atoms, such as hydrogen or lithium. While these instrumentation developments have brought notable successes in materials analysis, in particular for hetero-interfaces, catalysis and thin-film studies, they have also challenged the established experimental protocols and our fundamental understanding of both imaging and spectroscopy in the STEM. Aberration correction also allows increased flexibility in choosing the appropriate electron energy to minimize beam induced damage while maintaining atomic-resolution (e.g. 60 keV electrons for studying graphene with 1.3 Å resolution).[1]

Decay of high-energy electron bound states in crystals

High-energy electrons that are used as a probe of specimens in transmission electron microscopy exhibit a complex and rich behavior due to multiple scattering. Among other things, understanding the dynamical effects is needed for a quantitative analysis of atomic-resolution images and spectroscopic data. In this study, state-correlation functions are computed within the multislice approach that allow to elucidate behaviors of transversely bound states in crystals. These states play an important role as a large fraction of current density can be coupled into them via focused electron probes. We show that bound states are generically unstable and decay monoexponentially with crystal depth. Their attenuation is accompanied by a resonant intensity transfer to Bessel-like wavefunctions that appear as Laue rings in the far-field diffraction patterns. Behaviors of bound states are also quantified when thermal effects are included, as well as point defects. This approach helps to bridge the Bloch wave and multisliced electron propagation pictures of dynamical scattering providing new insights into fundamental solutions of the wave equation, and may assist in developing quantitative STEM/TEM imaging techniques.

Atomic — Scale study of model CdTe grain boundaries

Poly-crystalline CdTe-based thin film photovoltaic (PV) devices are the forerunners in commercialized solar cell technology. Despite the commercial success, best laboratory cells achieve ~21.5% power conversion efficiency and hence are still ~10% short of theoretical limit. In this collaborative research project we investigate effects of the grain boundaries via wafer-bonded CdTe bicrystals. Lifetime measurements are carried out using two-photon absorption, while atomic resolution imaging and first-principles calculations are used to correlate the results. Here we present fundamental atomic-scale studies of several model grain boundaries.

A fundamental study of the effects of grain boundaries on performance of poly-crystalline thin film CdTe solar cells

Poly-crystalline CdTe-based thin film photovoltaic devices have shown a great potential and are commercially used for large-scale energy conversion applications. Despite this success conversion efficiency of CdTe has achieved very minor improvements over the last 20 years. To overcome this stagnation and further drive cost-per-watt of the modules, better atomic-scale understanding of native dislocation structures and grain boundaries is needed. In this collaborative study we systematically investigate effects of grain boundaries using ultra-high-vacuum bonded CdTe bi-crystals with pre-defined misorientation angles.