Victor G. Karpov

Effects of nonuniformity in thin-film photovoltaics

We discuss the physical origin and effects of micrononuniformities on thin-film photovoltaics. The key factors are the large device area and the presence of potential barriers in the grain boundaries (for polycrystalline films) and in device junctions. We model the nonuniformity effects in the terms of random microdiodes connected in parallel through a resistive electrode. The microdiodes of low open circuit voltages are shown to affect macroscopically large regions. They strongly reduce the device performance and induce its nonuniform degradation in several different modes. We support our predictions by experiments, which show that the device degradation is driven by the light-induced forward bias and is spatially nonuniform.

Electrical conduction in chalcogenide glasses of phase change memory

Amorphous chalcogenides have been extensively studied over the last half century due to their application in rewritable optical data storage and in non-volatile phase change memory devices. Yet, the nature of the observed non-ohmic conduction in these glasses is still under debate. In this review, we consolidate and expand the current state of knowledge related to dc conduction in these materials. An overview of the pertinent experimental data is followed by a review of the physics of localized states that are peculiar to chalcogenide glasses. We then describe and evaluate twelve relevant transport mechanisms with conductivities that depend exponentially on the electric field. The discussed mechanisms include various forms of Poole-Frenkel ionization, Schottky emission, hopping conduction, field-induced delocalization of tail states, space-charge-limited current, field emission, percolation band conduction, and transport through crystalline inclusions. Most of the candidates provide more or less satisfact...

Theory of electronic transport in noncrystalline junctions

A theory of electronic transport in noncrystalline junctions is developed and compared to the experimental data. Junction transport is represented as hopping in both real space and energy space, which is dominated by rare yet exponentially effective optimum channels representing favorable configurations of localized states. Our work correlates the current-voltage characteristics of noncrystalline, thin-film devices with material parameters and predicts large ideality factors that increase under light and depend on applied bias. Also, the frequently observed variations in efficiency and degradation between nominally identical devices are a natural consequence of the theory. The theory is shown to be in good qualitative agreement with our measurements extracted from a large set of experimental data on thin-film cadmium telluride/cadmium sulfide solar cells.

Shunting path formation in thin film structures

We present a model for shunt formation in thin films containing small volume fractions of conductive components, below the critical volume fraction of percolation theory. We show that in this regime shunting is due to almost rectilinear conductive paths, which is beyond the percolation theory framework. The criteria of rectilinear paths shunting versus the percolation cluster scenario are established. The time and temperature dependence of shunting statistics is predicted with possible applications in phase change memory and thin oxides.

Hot spot runaway in thin film photovoltaics and related structures

We show that thin film diode structures, such as photovoltaics and light emitting arrays, can undergo zero threshold localized thermal runaway leading to thermal and electrical nonuniformities spontaneously emerging in originally uniform systems. The linear stability analysis is developed for a system of thermally and electrically coupled two discrete diodes, and for a distributed system. These results are verified with numerical modeling that is not limited to small fluctuations. The discovered instability negatively affects the device performance and reliability. It follows that these problems can be mitigated by properly designing the device geometry and thermal insulation.

Surface parameters determining a metal propensity for whiskers

We consider surface parameters responsible for variations in propensity for whisker formation and growth between (1) different metals and (2) different recipes of the same metal. The former is attributed to metal surface tension, while the latter is related to the surface charge density that is sensitive to structure imperfections, stresses, contaminations, etc. We propose a figure of merit combining these two parameters that describes a metal propensity for whiskers and the relative smallness of whisker concentration. We argue that many previously observed correlations between whiskers and stresses, stress gradients, intermetallic compounds, contaminations, etc., can be attributed to the effects of the above two parameters.

Understanding the movements of metal whiskers

Metal whiskers often grow across leads of electric equipment causing short circuits and raising significant reliability issues. Their nature remains a mystery after several decades of research. It was observed that metal whiskers exhibit large amplitude movements under gentle air flow or, according to some testimonies, without obvious stimuli. Understanding the physics behind that movements would give additional insights into the nature of metal whiskers. Here, we quantitatively analyze possible mechanisms of the observed movements: (1) minute air currents; (2) Brownian motion due to random bombardments with the air molecules; (3) mechanically caused movements, such as (a) transmitted external vibrations, and (b) torque exerted due to material propagation along curved whiskers (the garden hose instability); (4) time dependent electric fields due to diffusion of ions; and (5) non-equilibrium electric fields making it possible for some whiskers to move. For all these mechanisms, we provide numerical estimat...

Nucleation of plasmonic resonance nanoparticles

We predict the electromagnetic field driven nucleation of nanoparticles that provide plasmonic oscillations in resonance with the field frequency. The oscillations assume a phase that maximizes the particle polarization and energy gain due to nucleation. We derive closed form expressions for the corresponding nucleation barrier and particle shape vs. field frequency and strength, metal plasma frequency, conductivity, and the host dielectric permittivity. We show that the plasmonic polarization allows for nucleation of particles that would not be stable in zero field.

Understanding and mitigating effects of nonuniformities on reliability of thin film photovoltaics

Thin-film photovoltaics (PV) are sensitive to lateral nonuniformities (LN) that manifest themselves in spatial variations of the device local characteristics and in the variability of the measured parameters between nominally identical devices. LN affect all the aspects of device operations and stability and appear as a hidden cost of the otherwise inexpensive technology. They are omnipresent as originating from multiple factors typical of thin-film PV: deposition geometry, wet and heat treatments, dispersion in grain and amorphous phase parameters, and fluctuations in metal-semiconductor barriers. LN are seen in the device mappings, including that of PL, Voc, OBIC, EBIC, thermography, and electroluminescence. Stresses localized on certain vulnerable spots drive the entire device degradation. We present a general summary of physical processes related to LN, including modeling aspects, characteristic length and variability scales, statistics, degradation mechanisms, and superadditive effects between different device components, such as a negative correlation between the resistive and LN related loss, and a positive correlation between LN and device shunting failures under stress. We then review the known practical techniques of mitigating LN effects patented by different groups from 1970s to nowadays and show how nonuniformity treatments play the key role in the existing technologies.

Whisker growth on Sn thin film accelerated under gamma-ray induced electric field

We report on the growth of tin metal whiskers significantly accelerated under non-destructive gamma-ray irradiation. Sn thin film, evaporated on glass substrate, was subjected to a total of 60 h of irradiation. The irradiated samples demonstrated enhanced whisker development, in both densities and lengths, resulting in an acceleration factor of ~50. We attribute the observed enhancement to gamma-ray induced electrostatic fields, affecting whisker kinetics. These fields are due to the substrate charging under gamma-rays. We propose that gamma-ray irradiation can be a much needed tool for accelerated testing of whisker propensity.