Christopher Buurma

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.

MWIR imaging with low cost colloidal quantum dot films

Suspensions of HgTe colloidal quantum dots (CQD) are readily synthesized with infrared energy gaps between 3 and 12 microns. Infrared photodetection using dried films of these CQDs has been demonstrated up to a cutoff wavelength of 12 microns. The synthesis of CQDs and the fabrication of detector devices employ bench-top chemistry techniques, leading to the potential for the easy manufacture of infrared photon detecting imagers at low cost. Recent electrical and optical measurements of these CQD films are discussed. Recent successful prototypes of complete focal plane arrays from CQD films and commercially-available ROICs are also described.

Simulation of Current Transport in Polycrystalline CdTe Solar Cells

Polycrystalline thin-film CdTe solar cells have demonstrated laboratory efficiency exceeding 17% and are nowadays a commercial technology (albeit with somewhat lower efficiencies). The standard process features a poorly understood recrystallization step, obtained by annealing with a source of chlorine. This study uses two-dimensional numerical modeling to investigate current transport inside the polycrystalline CdTe absorber with and without recrystallization effects [increase of grain size and donor ClTe states at grain boundaries (GBs)]. Solving the Poisson equation and the drift–diffusion model for transport with Fermi statistics, while treating the optical problem by the one-dimensional transfer matrix method and complex refractive indexes, this study shows that: (i) in a columnar absorber (i.e., one where only vertical GBs exist), the presence of ClTe donor traps at GBs results in a dip in the band profiles that effectively serves as an electron collector, significantly increasing the short-circuit current and efficiency compared with nondecorated GBs; (ii) while the same dip acts as a hole barrier and thus can be expected to block holes from flowing when horizontal GBs are present, under illuminated conditions electron collection at GBs reduces the dip enough to allow substantial hole flow, and the cell performance is only moderately affected.

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.

Effects of Inductively Coupled Plasma Hydrogen on Long-Wavelength Infrared HgCdTe Photodiodes

Bulk passivation of semiconductors with hydrogen continues to be investigated for its potential to improve device performance. In this work, hydrogen-only inductively coupled plasma (ICP) was used to incorporate hydrogen into long-wavelength infrared HgCdTe photodiodes grown by molecular-beam epitaxy. Fully fabricated devices exposed to ICP showed statistically significant increases in zero-bias impedance values, improved uniformity, and decreased dark currents. HgCdTe photodiodes on Si substrates passivated with amorphous ZnS exhibited reductions in shunt currents, whereas devices on CdZnTe substrates passivated with polycrystalline CdTe exhibited reduced surface leakage, suggesting that hydrogen passivates defects in bulk HgCdTe and in CdTe.

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.

Influence of Hydrogenation on Electrical Conduction in HgCdTe Thin Films on Silicon

HgCdTe is the standard state-of-the-art infrared detector material for space applications. HgCdTe-based infrared photon detector performance can be hindered due to the presence of bulk crystal defects and dangling bonds at surfaces or interfaces. Passivation of such bulk defects and surfaces can potentially improve detector performance by saturating dangling bonds in dislocation cores and at surfaces. Indeed, results showing improvement of HgCdTe current–voltage characteristics after hydrogenation have been reported. Here we use multiple-carrier fitting of Hall-effect data, acquired under variable magnetic field strengths and sample temperatures, to investigate the physical influence of hydrogenation, as a passivation procedure, on HgCdTe crystalline thin films on Si(211) substrates. We find: (1) evidence of multiple active electrical carrier species in all samples, (2) evidence of surface electrical conduction before and after hydrogenation, and (3) changes in carrier concentration and mobility induced by hydrogenation.

Advances in low-cost infrared imaging using II-VI colloidal quantum dots (Conference Presentation)

II-VI colloidal quantum dots (CQDs) have made significant technological advances over the past several years, including the world’s first demonstration of MWIR imaging using CQD-based focal plane arrays. The ultra-low costs associated with synthesis and device fabrication, as well as compatibility with wafer-level focal plane array fabrication, make CQDs a very promising infrared sensing technology. In addition to the benefit of cost, CQD infrared imagers are photon detectors, capable of high performance and fast response at elevated operating temperatures. By adjusting the colloidal synthesis, II-VI CQD photodetectors have demonstrated photoresponse from SWIR through LWIR. We will discuss our recent progress in the development of low cost infrared focal plane arrays fabricated using II-VI CQDs.

First principles modeling of grain boundaries in CdTe

A fundamental understanding of the role of vacancies, interstitials, dislocations and grain boundaries on the electronic structure of CdTe may lead to efficiency improvements. Atomistic-level characterization, including microscopy and first principles modeling, is crucial in developing such a fundamental understanding. In the present work, we built atomistic grain boundary and dislocation core models directly from the STEM images using image analysis methods and crystallographic information at the interface. Grain boundaries are modeled using first principles density functional theory (DFT) calculations. Electronic structures of large-scale grain models are also computed with an accurate hybrid functional (HSE06). We report the electronic density of states (DOS) and electrostatic potential profiles of different CdTe grain boundaries to understand charge carrier interactions. Thermodynamics of point defects and pairs of point defects that can exist on or near grain boundaries are studied and pertaining changes in electronic structure are reported. The implications of these electronic structure changes at grain boundaries on photovoltaic performance, and corresponding strategies to improve performance, are discussed.