Interfacial Chemistry Effects in the Electrochemical Performance of Silicon Electrodes under Lithium-Ion Battery Conditions.

Understanding the solid electrolyte interphase (SEI) formation and (de)lithiation phenomena at silicon (Si) electrodes is key to improving the performance and lifetime of Si-based lithium-ion batteries. However, these processes remain somewhat elusive, and, in particular, the role of Si surface termination merits further consideration. Here, scanning electrochemical cell microscopy (SECCM) is used in a glovebox, followed by secondary ion mass spectrometry (SIMS) at identical locations to study the local electrochemical behavior and associated SEI formation, comparing Si (100) with a native oxide layer (SiOx /Si) and etched with hydrofluoric acid (HF-Si). HF-Si shows greater spatial electrochemical heterogeneity and inferior lithiation reversibility than SiOx /Si. This is attributed to a weakly passivating SEI and irreversible lithium trapping at the Si surface. Combinatorial screening of charge/discharge cycling by SECCM with co-located SIMS reveals SEI chemistry as a function of depth. While the SEI thickness is relatively independent of the cycle number, the chemistry - particularly in the intermediate layers - depends on the number of cycles, revealing the SEI to be dynamic during cycling. This work serves as a foundation for the use of correlative SECCM/SIMS as a powerful approach to gain fundamental insights on complex battery processes at the nano- and microscales.

Stable chemical enhancement of passivating nanolayer structures grown by atomic layer deposition on silicon.

Incorporation of carrier-selective passivating contacts is on the critical path for approaching the theoretical power conversion efficiency limit in silicon solar cells. We have used plasma-enhanced atomic layer deposition (ALD) to create ultra-thin films at the single nanometre-scale which can be subsequently chemically enhanced to have properties suitable for high-performance contacts. Negatively charged 1 nm thick HfO2 films exhibit very promising passivation properties - exceeding those of SiO2 and Al2O3 at an equivalent thickness - providing a surface recombination velocity (SRV) of 19 cm s-1 on n-type silicon. Applying an Al2O3 capping layer to form Si/HfO2/Al2O3 stacks gives additional passivation, resulting in an SRV of 3.5 cm s-1. Passivation quality can be further improved via simple immersion in hydrofluoric acid, which results in SRVs < 2 cm s-1 that are stable over time (tested for ∼50 days). Based on corona charging analysis, Kelvin probe measurements and X-ray photoelectron spectroscopy, the chemically induced enhancement is consistent with changes at the dielectric surface and not the Si/dielectric interface, with fluorination of the Al2O3 and underlying HfO2 films occurring after just 5 s HF immersion. Our results show that passivation is enhanced when the oxides are fluorinated. The Al2O3 top layer of the stack can be thinned down by etching, offering a new route for fabrication of ultra-thin highly passivating HfO2-containing nanoscale thin films.

Room Temperature Enhancement of Electronic Materials by Superacid Analogues.

Treatment with the superacid bis(trifluoromethanesulfonyl)amide (sometimes known as TFSA, TFSI, or HNTf2) enhances the properties of a wide range of optoelectronic materials, resulting in longer effective carrier lifetimes and higher photoluminescence quantum yields. We have conducted a multimaterial study treating both crystalline silicon and transition metal dichalcogenide (TMDC) monolayers and few-layer flakes with solutions formed from TFSA and a range of compounds with related chemical structures with different Lewis acidities, in order to elucidate the factors underpinning the TFSA-related class of enhancement treatments. We adopt dichloromethane (DCM) as a common solvent as it provides good results at room temperature and is potentially less hazardous than TFSA-dichloroethane (DCE) heated to ∼100 °C, which has been used previously. Kelvin probe experiments on silicon demonstrate that structurally similar chemicals give passivating films with substantially different charge levels, with the higher levels of charge associated with the presence of CF3SO2 groups resulting in longer effective lifetimes due to an enhancement in field-effect passivation. Treatment with all analogue solutions used results in enhanced photoluminescence in MoS2 and WS2 compared to untreated controls. Importantly we find that MoS2 and WS2 can be enhanced by analogues to TFSA that lack sulfonyl groups, meaning an alternative mechanism to that proposed in computational reports for TFSA enhancement must apply.

Atomic level termination for passivation and functionalisation of silicon surfaces.

Chemical treatments play an essential role in the formation of high quality interfaces between materials, including in semiconductor devices, and in the functionalisation of surfaces. We have investigated the effects of hydrogen and fluorine termination of (100)-orientation silicon surfaces over a range of length scales. At the centimetre scale, lifetime measurements show clean silicon surfaces can be temporarily passivated by a short treatment in both HF(2%) : HCl(2%) and HF(50%) solutions. The lifetime, and hence surface passivation, becomes better with immersion time in the former, and worse with immersion time in the latter. At the nanometre scale, X-ray photoelectron spectroscopy and atomic force microscopy show treatment with strong HF solutions results in a roughened fluorine-terminated surface. Subsequent superacid-derived surface passivation on different chemically treated surfaces shows considerably better passivation on surfaces treated with HF(2%) : HCl(2%) compared to HF. Lifetime data are modelled to understand the termination in terms of chemical and field effect passivation at the centimetre scale. Surfaces passivated with Al2O3 grown by atomic layer deposition behave similarly when either HF(2%) : HCl(2%) or HF(50%) are used as a pre-treatment, possibly because of the thin silicon dioxide interlayer which subsequently forms. Our study highlights that chemical pre-treatments can be extremely important in the creation of high quality functionalised surfaces.

Light Enhanced Hydrofluoric Acid Passivation: A Sensitive Technique for Detecting Bulk Silicon Defects.

A procedure to measure the bulk lifetime (>100 µsec) of silicon wafers by temporarily attaining a very high level of surface passivation when immersing the wafers in hydrofluoric acid (HF) is presented. By this procedure three critical steps are required to attain the bulk lifetime. Firstly, prior to immersing silicon wafers into HF, they are chemically cleaned and subsequently etched in 25% tetramethylammonium hydroxide. Secondly, the chemically treated wafers are then placed into a large plastic container filled with a mixture of HF and hydrochloric acid, and then centered over an inductive coil for photoconductance (PC) measurements. Thirdly, to inhibit surface recombination and measure the bulk lifetime, the wafers are illuminated at 0.2 suns for 1 min using a halogen lamp, the illumination is switched off, and a PC measurement is immediately taken. By this procedure, the characteristics of bulk silicon defects can be accurately determined. Furthermore, it is anticipated that a sensitive RT surface passivation technique will be imperative for examining bulk silicon defects when their concentration is low (<10(12) cm(-3)).