The study of thermal silicon dioxide electrets formed by corona discharge and rapid-thermal annealing

A silicon dioxide (SiO₂) electret passivates the surface of crystalline silicon (Si) in two ways: (i) when annealed and hydrogenated, the SiO₂–Si interface has a low density of interface states, offering few energy levels through which electrons and holes can recombine; and (ii) the electret’s quasipermanent charge repels carriers of the same polarity, preventing most from reaching the SiO₂–Si interface and thereby limiting interface recombination. In this work, we engineer a charged thermal SiO₂electret on Si by depositing corona charge onto the surface of an oxide-coated Si wafer and subjecting the wafer to a rapid thermal anneal (RTA). We show that the surface-located corona charge is redistributed deeper into the oxide by the RTA. With 80 s of charging, and an RTA at 380 °C for 60 s, we measure an electretcharge density of 5 × 10¹² cm⁻², above which no further benefit to surface passivation is attained. The procedure leads to a surface recombination velocity of less than 20 cm/s on 1 Ω-cm n-type Si, which is commensurate with the best passivation schemes employed on high-efficiency Si solar cells. In this paper, we introduce the method of SiO₂electret formation, analyze the relationship between charge density and interface recombination, and assess the redistribution of charge by the RTA

Near-infrared free carrier absorption in heavily doped silicon

Free carrier absorption in heavily doped silicon can have a significant impact on devices operating in the infrared. In the near infrared, the free carrier absorption process can compete with band to band absorption processes, thereby reducing the number of available photons to optoelectronic devices such as solar cells. In this work, we fabricate 18 heavily doped regions by phosphorus and boron diffusion into planar polished silicon wafers; the simple sample structure facilitates accurate and precise measurement of the free carrier absorptance. We measure and model reflectance and transmittance dispersion to arrive at a parameterisation for the free carrier absorption coefficient that applies in the wavelength range between 1000 and 1500 nm, and the range of dopant densities between ∼10¹⁸ and 3 × 10²⁰ cm⁻³. Our measurements indicate that previously published parameterisations underestimate the free carrier absorptance in phosphorus diffusions. On the other hand, published parameterisations are generally consistent with our measurements and model for boron diffusions. Our new model is the first to be assigned uncertainty and is well-suited to routine device analysis

Bulk Incorporation with 4‐Methylphenethylammonium Chloride for Efficient and Stable Methylammonium‐Free Perovskite and Perovskite‐Silicon Tandem Solar Cells

Methylammonium (MA)-free perovskite solar cells have the potential for better thermal stability than their MA-containing counterparts. However, the efficiency of MA-free perovskite solar cells lags behind due to inferior bulk quality. In this work, 4-methylphenethylammonium chloride (4M-PEACl) is added into a MA-free perovskite precursor, which results in greatly enhanced bulk quality. The perovskite crystal grains are significantly enlarged, and defects are suppressed by a factor of four upon the incorporation of an optimal concentration of 4M-PEACl. Quasi-2D perovskites are formed and passivate defects at the grain boundaries of the perovskite crystals. Furthermore, the perovskite surface chemistry is modified, resulting in surface energies more favorable for hole extraction. This facile approach leads to a steady state efficiency of 23.7% (24.2% in reverse scan, 23.0% in forward scan) for MA-free perovskite solar cells. The devices also show excellent light stability, retaining more than 93% of the initial efficiency after 1000 h of constant illumination in a nitrogen environment. In addition, a four-terminal mechanically stacked perovskite-silicon tandem solar cell with champion efficiency of 30.3% is obtained using this MA-free composition. The encapsulated tandem devices show excellent operational stability, retaining more than 98% of the initial performance after 42 day/night cycles in an ambient atmosphere

Discovery That Theonellasterol a Marine Sponge Sterol Is a Highly Selective FXR Antagonist That Protects against Liver Injury in Cholestasis

Background: The farnesoid-x-receptor (FXR) is a bile acid sensor expressed in the liver and gastrointestinal tract. Despite FXR ligands are under investigation for treatment of cholestasis, a biochemical condition occurring in a number of liver diseases for which available therapies are poorly effective, mice harboring a disrupted FXR are protected against liver injury caused by bile acid overload in rodent models of cholestasis. Theonellasterol is a 4-methylene-24-ethylsteroid isolated from the marine sponge Theonella swinhoei. Here, we have characterized the activity of this theonellasterol on FXR-regulated genes and biological functions. Principal Findings: Interrogation of HepG2 cells, a human hepatocyte cell line, by microarray analysis and transactivation assay shows that theonellasterol is a selective FXR antagonist, devoid of any agonistic or antagonistic activity on a number of human nuclear receptors including the vitamin D receptor, PPARs, PXR, LXRs, progesterone, estrogen, glucorticoid and thyroid receptors, among others. Exposure of HepG2 cells to theonellasterol antagonizes the effect of natural and synthetic FXR agonists on FXR-regulated genes, including SHP, OSTa, BSEP and MRP4. A proof-of-concept study carried out to investigate whether FXR antagonism rescues mice from liver injury caused by the ligation of the common bile duct, a model of obstructive cholestasis, demonstrated that theonellasterol attenuates injury caused by bile duct ligation as measured by assessing serum alanine aminostrasferase levels and extent of liver necrosis at histopathology. Analysis of genes involved in bile acid uptake and excretion by hepatocytes revealed that theonellasterol increases the liver expression of MRP4, a basolateral transporter that is negatively regulated by FXR. Administering bile duct ligated mice with an FXR agonist failed to rescue from liver injury and downregulated the expression of MRP4. Conclusions: FXR antagonism in vivo results in a positive modulation of MRP4 expression in the liver and is a feasible strategy to target obstructive cholestasis

High efficiency IBC solar cell with oxide-nitride-oxide passivation

This thesis considers the optimisation of an SiO2-SiNx-SiOx (ONO) dielectric coating for silicon solar cells comprising thermal SiO2, plasma-enhanced chemical vapour deposition PECVD SiNx and SiOx, together with the development and fabrication of high efficiency interdigitated back contact (IBC) solar cells. The three-layer ONO dielectric stack provides excellent surface passivation and anti-reflection properties. The initial thermal SiO2 layer provides chemical surface passivation. The second layer of the PECVD SiNx hydrogenates the underlying thermal SiO2 and provides a field-effect passivation with positive charges. The third layer of the PECVD SiOx improves the overall anti-reflection coating (ARC) of an ONO stack in air. Positive corona charging of the ONO stack further improves the surface passivation, and annealing at 400 C traps the charges and renders it stable for at least two years. An optimised ONO stack for surface passivation has achieved lifetimes above previous parameterised Auger limits. The n-type wafers with resistivities of 0.5, 1.07, 1.77 and 100 ohms.cm achieved lifetimes of 3.7, 15.1, 25.5 and 170 ms, respectively. The surface passivation of an ONO stack was fine-tuned for both phosphorus and boron diffusions for integration into IBC solar cells. A large contribution of positive charges within the stack was mainly from the thermal SiO2 and PECVD SiNx. Post-oxidation annealing (POA) of the thermal SiO2 and high SiNx refractive index reduced the overall positive charge of the ONO stack. A lower positive charge improved the passivation on the boron diffused surface while retaining excellent passivation on the phosphorus diffused surface. The cell design and fabrication processes for IBC solar cells were improved from previous fabrication processes at the Australian National University (ANU) by utilising ONO surface passivation. The refinement for IBC solar cell design was based on detailed simulations. The improvements to the cell processing included permanently eliminating low bulk lifetime defects on high resistivity float zone (FZ) wafers, gettering of contaminants, and consistent texturing results. Implementing optimised ONO stacks for surface passivation and an ARC onto the IBC solar cell achieved a certified efficiency of 25.0%. Non-ideal recombination contributed to a relatively low fill factor for the champion cell, and disregarding the non-ideal recombination in the simulation results revealed that an efficiency of 25.2% is achievable

Improved Diffused-Region Recombination-Current Pre-Factor Analysis

Photoconductance (PC) measurements of the diffused-region recombination-current pre-factor J0d make the approximation of excess-carrier Δn density uniformity. That is, from the front illuminated surface to the rear the Δn is constant. Kane and Swanson

Capturing the attributes of reducible oxides for steam reforming and methanation reactions

Catalyst supports are known for their influence on the behaviour and performance of the metal phase supported. This is especially applicable to reducible oxides which interchangeable oxidation states offer many interesting surface chemistry attributes. Titanium dioxide (TiO2) is one of the most studied metal oxides for its ‘strong metal-support interaction’ properties while oxides of cerium have also garnered significant attention. Both these oxides have therefore been chosen as the support material for nickel catalysts and their performance for steam reforming of methane (SRM) and methanation investigated. For this purpose, TiO2-supported nickel was studied for its SRM performance at 400 – 500 oC. Different TiO2-supported-nickel species were found to have different roles in low temperature SRM. The more weakly interacting species contribute to hydrogen production via the water-gas shift reaction whereas the more strongly interacting fractions are crucial for the low temperature activation of methane. This was confirmed when a performance comparison was made against silica-supported nickel which provided a system with weaker metal-support interaction. Following these findings, the addition of cerium to titanium oxide supports was investigated. In this section of work, the support oxides were synthesised with varied compositions. Two synthesis techniques (sol-gel and flame spray pyrolysis) were used to manipulate the support features and their impact on low temperature SRM. It was found that while the Ni-loaded sol-gel oxides displayed improved methane conversion upon enriching the support with cerium, the effect differed for the flame-made equivalents. Despite a higher support surface area and better dispersion of surface nickel, the poor support crystallinity of the flame-made catalysts ultimately translated to poor stability during SRM. The last section of the investigation explored the use of sol-gel-synthesised ceria-titania mixed oxide supported nickel catalysts for their proficiency in photothermal CO2 methanation. The findings revealed that the support plays an instrumental role in maintaining the reduced state of supported nickel with the metallic state being important for photothermal activation. Moreover, the relative stability of the reduced surface cerium states in the presence of titanium oxide also facilitated adsorption of the CO2 reactants and contributed to improved CO2 conversions

REMOVAL OF HYDROGEN AND DEPOSITION OF SURFACE CHARGE DURING RAPID THERMAL ANNEALING

The submission of a hydrogenated oxide-passivated silicon wafer to a rapid thermal anneal (RTA) leads to a complicated change in effective lifetime τeff. Within seconds, τeff decreases rapidly before increasing to near its initial level, and then decre