Structural and electronic properties of nanocrystalline silicon thin film solar cells fabricated by hot wire and ECR-plasma CVD techniques

Nanocrystalline silicon has become the material of interest recently, for solar cell applications and also in the fabrication of thin film transistors. The material contains crystalline grains surrounded by amorphous tissues and when used as intrinsic layer in solar cell devices, greatly enhances the device stability against the light induced degradation which is a critical problem with amorphous silicon solar cells. The conventional PECVD techniques used for the deposition of high efficiency devices have a major drawback of very low growth rates. This project deals with a systematic study of structural and electronic properties of nanocrystalline Si:H films and devices fabricated using a relatively new technique called the Hot Wire CVD (HWCVD). In addition, we study the influence of ions on the crystalline ratio, grain size and orientation of the nanocrystalline films. Our apparatus allows us to add plasma ions separately from the primary growth process, which is growth using only radicals that are generated by the thermal dissociation of silane and hydrogen at the hot wire. In this way, we have also deposited the first ever nanocrystalline silicon solar cells by the combined HWCVD and Electron Cyclotron Resonance (ECR) PECVD technique.;While hot wire deposition of nanocrystalline Si:H has been studied in the past, virtually all previous work utilized a close-proximity hot wire deposition condition that creates a varying temperature profile during deposition because of the intense heating of the growing film due to radiation from the filament. In contrast, in this work, we use a remote filament to minimize sample heating, a conclusion verified by experimental measurements of surface temperatures during growth conditions. We have found that low energy ion bombardment, by either inert (helium) or reactive (hydrogen) ions significantly helps in crystallization of the film. We also systematically study the influence of hydrogen dilution on grain size and grain orientation of the film. It is found that higher hydrogen dilution suppresses the \u3c220\u3e grains and leads to more random nucleation. It is found that \u3c220\u3e orientation is the thermodynamically preferred growth direction and \u3c111\u3e grains are created due to random nucleation which is enhanced by increasing the ion bombardment from the plasma source. We have also studied the fragmentation pattern of silane in ECR PECVD using a quadruple mass spectrometer. The study revealed the dominant radicals in both nc-Si and a-Si depositions for varying power and chamber pressures.;In the second part of this work, we focus primarily on the fabrication and analysis of the electronic properties of solar cells using nanocrystalline intrinsic layers. Apart from measuring the regular I-V characteristics and quantum efficiency, we investigate the critical device properties such as the defect densities in the intrinsic layer and the diffusion length of the minority carriers. By correlating the device results with the structural properties of the films, we are able to conclude that the maximum diffusion length and the minimum defect density can only be attained by depositing the intrinsic layers that are close to the transition to amorphous phase. Although few studies have been done on this transition regime of the deposited films, most of them have concentrated only on the film properties such as conductivity ratios and crystalline fractions. This work clearly describes why transition region is ideal for the fabrication of high efficiency solar cells and what are the critical deposition parameters that are involved in their design

Interfacial Void Model for Corrosion Pit Initiation on Aluminum

A model for pit initiation during galvanostatic anodic etching of aluminum in acid chloride-containing solutions was developed. The predictions were compared to experimental potential transients and pit-size distributions. The model presumed that pits initiated from subsurface nanoscale voids, which were exposed by uniform corrosion. Void concentrations fit from potential transients depended on times of caustic and acid exposure before etching, in agreement with prior characterization of the voids by positron annihilation measurements. The model yielded realistic predictions of the effect of applied current density and temperature on the potential transients. The effective void concentration was found to increase with the chloride concentration in the etching solution; this suggested that higher chloride concentrations inhibit passivation of newly exposed voids, enhancing their survival probability. On the whole, the interfacial void model provided a promising quantitative description of pit initiation during anodic etching

Study of the pit initiation mechanism and metal dissolution kinetics during anodic etching of aluminum

The role of subsurface nano-scale voids present near the metal-oxide film interface, on pit nucleation was investigated. Electrochemical processes during the initial stages of etching were thoroughly characterized to evaluate the hypothesis that voids are the primary pit initiation sites. Dissolution rate measurements made in etch pits revealed contradicting trends of constant and potential dependent dissolution current densities. The same trends were also exhibited by the measurements made in etch tunnels, which eliminated the possible role of geometric form of corrosion in such a contradiction. Finally it was concluded that the experimental time-scale during which the dissolution rate measurements were made, was the only factor responsible for such varying trends. A kinetic model similar to the Vetter-Gorn model for metals covered with oxide films was proposed. The model was validated by its ability to predict the observed constant and potential dependent dissolution rates under different experimental-time scales. A mathematical model for pit initiation during the initial stages of galvanostatic anodic etching of aluminum in acid-chloride solutions was developed. The model incorporated all of the electrochemical processes characterized during the initial stages of etching and was based on the interfacial void hypothesis which assumes voids to be the only pit initiation sites. The effect of various experimental conditions such as caustic pretreatment time, applied current density, etchant temperature, chloride concentration in etchant etc. were analyzed using the etching potential transient. The dependence of void concentration on the caustic pretreatment time was determined by fitting the potential transients and the observed trend had a fine agreement with previously established PAS results. Using the fit void concentrations, the model could successfully track the experimental transients for various etchant temperatures and applied current densities. However, the model failed to predict the temperature-dependent pit depths found experimentally, which might be due to the simplified assumptions made in its development. Also the role of chloride ion kinetics in pit initiation mechanism should be clearly understood and incorporated in the model for successfully predicting the experimental results obtained for varied chloride concentrations in the etchant

Electron mobility in nanocrystalline silicon devices

Electron mobility in the growth direction was measured using space charge limited current techniques in device-type nin structure nanocrystalline Si:H and nanocrystalline Ge:H structures. The films were grown on stainless steel foil using either hot wire or remote plasma enhanced chemical vapor deposition techniques. Grain size and crystallinity were measured using x ray and Raman spectroscopy. The size of grains in films was adjusted by changing the deposition conditions. It was found that large ⟨220⟩ grain sizes (∼56nm) role= presentation style= display: inline; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px 2px 0px 0px; margin: 0px; position: relative; \u3e(∼56nm)(∼56nm) could be obtained using the hot wire deposition technique, and the conductivity mobility at room temperature was measured to be 5.4cm2∕Vs role= presentation style= display: inline; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px 2px 0px 0px; margin: 0px; position: relative; \u3e5.4cm2/Vs5.4cm2∕Vs in films with such large grains. The plasma-grown films had smaller grains and smaller mobilities. The mobility was found to increase with increasing grain size and with increasing temperature

Anti-bacterial activity of inorganic nanomaterials and their antimicrobial peptide conjugates against resistant and non-resistant pathogens

This review details the antimicrobial applications of inorganic nanomaterials of mostly metallic form, and the augmentation of activity by surface conjugation of peptide ligands. The review is subdivided into three main sections, of which the first describes the antimicrobial activity of inorganic nanomaterials against gram-positive, gram-negative and multidrug-resistant bacterial strains. The second section highlights the range of antimicrobial peptides and the drug resistance strategies employed by bacterial species to counter lethality. The final part discusses the role of antimicrobial peptide-decorated inorganic nanomaterials in the fight against bacterial strains that show resistance. General strategies for the preparation of antimicrobial peptides and their conjugation to nanomaterials are discussed, emphasizing the use of elemental and metallic oxide nanomaterials. Importantly, the permeation of antimicrobial peptides through the bacterial membrane is shown to aid the delivery of nanomaterials into bacterial cells. By judicious use of targeting ligands, the nanomaterial becomes able to differentiate between bacterial and mammalian cells and, thus, reduce side effects. Moreover, peptide conjugation to the surface of a nanomaterial will alter surface chemistry in ways that lead to reduction in toxicity and improvements in biocompatibility

Structural and electronic properties of nanocrystalline silicon thin film solar cells fabricated by hot wire and ECR-plasma CVD techniques

Nanocrystalline silicon has become the material of interest recently, for solar cell applications and also in the fabrication of thin film transistors. The material contains crystalline grains surrounded by amorphous tissues and when used as intrinsic layer in solar cell devices, greatly enhances the device stability against the light induced degradation which is a critical problem with amorphous silicon solar cells. The conventional PECVD techniques used for the deposition of high efficiency devices have a major drawback of very low growth rates. This project deals with a systematic study of structural and electronic properties of nanocrystalline Si:H films and devices fabricated using a relatively new technique called the Hot Wire CVD (HWCVD). In addition, we study the influence of ions on the crystalline ratio, grain size and orientation of the nanocrystalline films. Our apparatus allows us to add plasma ions separately from the primary growth process, which is growth using only radicals that are generated by the thermal dissociation of silane and hydrogen at the hot wire. In this way, we have also deposited the first ever nanocrystalline silicon solar cells by the combined HWCVD and Electron Cyclotron Resonance (ECR) PECVD technique.;While hot wire deposition of nanocrystalline Si:H has been studied in the past, virtually all previous work utilized a close-proximity hot wire deposition condition that creates a varying temperature profile during deposition because of the intense heating of the growing film due to radiation from the filament. In contrast, in this work, we use a remote filament to minimize sample heating, a conclusion verified by experimental measurements of surface temperatures during growth conditions. We have found that low energy ion bombardment, by either inert (helium) or reactive (hydrogen) ions significantly helps in crystallization of the film. We also systematically study the influence of hydrogen dilution on grain size and grain orientation of the film. It is found that higher hydrogen dilution suppresses the grains and leads to more random nucleation. It is found that orientation is the thermodynamically preferred growth direction and grains are created due to random nucleation which is enhanced by increasing the ion bombardment from the plasma source. We have also studied the fragmentation pattern of silane in ECR PECVD using a quadruple mass spectrometer. The study revealed the dominant radicals in both nc-Si and a-Si depositions for varying power and chamber pressures.;In the second part of this work, we focus primarily on the fabrication and analysis of the electronic properties of solar cells using nanocrystalline intrinsic layers. Apart from measuring the regular I-V characteristics and quantum efficiency, we investigate the critical device properties such as the defect densities in the intrinsic layer and the diffusion length of the minority carriers. By correlating the device results with the structural properties of the films, we are able to conclude that the maximum diffusion length and the minimum defect density can only be attained by depositing the intrinsic layers that are close to the transition to amorphous phase. Although few studies have been done on this transition regime of the deposited films, most of them have concentrated only on the film properties such as conductivity ratios and crystalline fractions. This work clearly describes why transition region is ideal for the fabrication of high efficiency solar cells and what are the critical deposition parameters that are involved in their design.</p

Study of the pit initiation mechanism and metal dissolution kinetics during anodic etching of aluminum

The role of subsurface nano-scale voids present near the metal-oxide film interface, on pit nucleation was investigated. Electrochemical processes during the initial stages of etching were thoroughly characterized to evaluate the hypothesis that voids are the primary pit initiation sites. Dissolution rate measurements made in etch pits revealed contradicting trends of constant and potential dependent dissolution current densities. The same trends were also exhibited by the measurements made in etch tunnels, which eliminated the possible role of geometric form of corrosion in such a contradiction. Finally it was concluded that the experimental time-scale during which the dissolution rate measurements were made, was the only factor responsible for such varying trends. A kinetic model similar to the Vetter-Gorn model for metals covered with oxide films was proposed. The model was validated by its ability to predict the observed constant and potential dependent dissolution rates under different experimental-time scales. A mathematical model for pit initiation during the initial stages of galvanostatic anodic etching of aluminum in acid-chloride solutions was developed. The model incorporated all of the electrochemical processes characterized during the initial stages of etching and was based on the interfacial void hypothesis which assumes voids to be the only pit initiation sites. The effect of various experimental conditions such as caustic pretreatment time, applied current density, etchant temperature, chloride concentration in etchant etc. were analyzed using the etching potential transient. The dependence of void concentration on the caustic pretreatment time was determined by fitting the potential transients and the observed trend had a fine agreement with previously established PAS results. Using the fit void concentrations, the model could successfully track the experimental transients for various etchant temperatures and applied current densities. However, the model failed to predict the temperature-dependent pit depths found experimentally, which might be due to the simplified assumptions made in its development. Also the role of chloride ion kinetics in pit initiation mechanism should be clearly understood and incorporated in the model for successfully predicting the experimental results obtained for varied chloride concentrations in the etchant.</p

Kinetic Model for Aluminum Dissolution in Corrosion Pits

The kinetics of aluminum dissolution in etch pits and tunnels, in a 1 M HCl-3 M H2SO4 solution at 70°C, were investigated. Dissolution current densities during growth of tunnels and pits, at potentials of roughly −0.8 and 1 V vs.Ag/AgCl respectively, were found to be approximately 6 A/cm2. Transient experiments using current step reductions during pitting, or anodic current pulses during tunnel growth, revealed strongly potential-dependent current densities up to 300 A/cm2. The results suggested that the dissolution rate is potential-dependent when measured on times scales of ∼1 ms after potential disturbances, but insensitive to potential in quasi-stationary experiments. A kinetic model was presented assuming a monolayer or multilayer chloride layer on the aluminum surface, including kinetic expressions for transfer of Al+3 and Cl− ions at the film/solution interface, and ionic conduction in the film. In agreement with experiments, the model yields constant or potential-dependent dissolution rates following a Butler-Volmer relation, depending on the time scale of experimental measurements. The large current densities in anodic transient experiments derived from high rates of Cl− incorporation during film growth.This article is from Journal of the Electrochemical Society 151 (2004): B45–B52, doi:10.1149/1.1635386. Posted with permission.</p