Accurate measurements of bulk minority carrier lifetime are essential in order to determine the true limit of silicon's performance and to improve solar cell production processes. The thin film which forms when silicon wafers are dipped in solutions containing superacids such as bis(trifluoromethane)sulfonimide (TFSI) has recently been found to be effective at electronically passivating the silicon surface. In this paper we first study the role of the solvent in which TFSI is dissolved for the passivation process. We study ten solvents with a wide range of relative polarities, finding TFSI dissolved in hexane provides improved temporal stability, marginally better passivation and improved solution longevity compared to dichloroethane which has been used previously. Sample storage conditions, particularly humidity, can strongly influence the passivation stability. The optimised TFSI-hexane passivation scheme is then applied to a set of 3 Ω cm n-type wafers cut from the same float-zone ingot to have different thicknesses. This enables the reproducibility of the scheme to be systematically evaluated. At 1015 cm−3 injection the best case effective surface recombination velocity is 0.69 ± 0.04 cm/s, with bulk lifetimes measured up to the intrinsic lifetime limit at high injection and >43 ms at lower injection. Immersion of silicon in superacid-based ionic solutions therefore provides excellent surface passivation, and, as it is applied at room temperature, the effects on true bulk lifetime are minimal.
Electrochemical double layer capacitors are well known for their high power output. However, pseudocapacitors have recently demonstrated the ability to compete here, with hybrid devices being a compromise between high power and high energy, by way of combined battery and supercapacitor technologies. Metal oxides have shown promising performance within battery and supercapacitor operation. However, cost and low conductivity have limited the performance of many of these materials and has led to the need for doping the material and refining particle shapes and sizes. 1 Molybdenum dioxide is a promising material, having many properties required for pseudocapacitance, including its electrical conductivity (atypical for a metal oxide.) 2,3 Further to this it has a theoretical specific capacity of 838 mAhg -1 when undergoing a four electron redox reaction. The various niobium oxides have multiple oxidation states, chemical stability and a high potential window making them suitable for energy storage. 4 As niobium pentoxide has a theoretical capacity of 200 mAhg -1 , which is higher than lithium titanate, (a common anode 5 ), these materials have been tested in batteries and supercapacitors. Yet there remains considerable room for investigation surrounding mixed and doped versions of these materials. Facile one-pot hydrothermal synthesis was used to make nanostructured pseudocapacitive materials. Molybdenum dioxides and niobium oxides were systematically synthesised with varied reaction conditions to gain shape and size control. Molybdenum dioxide nanoparticles were synthesised with the addition of a shape director to form spherical secondary particles. Upon doping the molybdenum dioxide with niobium, crystalline nanoparticles were formed with an average diameter of ~150 nm. Undoped niobium oxides form ‘flower-like’ particles, with the addition of a molybdenum precursor the reaction yields molybdenum dioxide/niobium oxide composite where molybdenum dioxide nanoparticles decorate the niobium oxide particles. All materials were manufactured into electrodes using conventional processing methods and activated carbon electrodes were made for asymmetric devices. The materials showed varying electrochemical properties directly influenced by characteristics gained through the synthesis conditions. Electrochemical testing was carried out on these materials vs. Li’Li + in half-cells and in asymmetric supercapacitor cells to assess their suitability for hybrid devices. Y. Gogotsi and R. M. Penner, ACS Nano , 12 , 2081–2083 (2018). Y. Liu, H. Zhang, P. Ouyang, and Z. Li, Electrochim. Acta , 102 , 429–435 (2013) http://dx.doi.org/10.1016/j.electacta.2013.03.195. X. Li et al., J. Power Sources , 237 , 80–83 (2013) http://dx.doi.org/10.1016/j.jpowsour.2013.03.020. P. Arunkumar et al., RSC Adv. , 5 , 59997–60004 (2015) http://xlink.rsc.org/?DOI=C5RA07895D. E. Lim et al., ACS Nano , 8 , 8968–8978 (2014). Figure 1
The surface properties of many inorganic electronic materials (e.g., MoS2, WSe2, Si) can be substantially modified by treatment with the superacid bis(trifluoromethane)sulfonimide (TFSI). Here we find more generally that solutions based on molecules with trifluoromethanesulfonyl groups, including TFSI, give rise to excellent room temperature surface passivation, with the common factor being the presence of CF3SO2 groups and not the solution's acidity. The mechanism of passivation comprises two effects: (i) chemical passivation; and (ii) field effect passivation from a negatively charged thin film likely to be physically adsorbed by the surface. Degradation of surface passivation is caused by de-adsorption of the thin film from the surface, and occurs slowly in air and rapidly upon vacuum exposure. The air stability of the passivation is enhanced by the presence of droplets at the surface which act to protect the properties of the film. The finding that nonacidic solutions can provide excellent electrical passivation at room temperature opens up the possibility of using them on materials more sensitive to an acidic environment.
Minimizing carrier recombination at interfaces is of extreme importance in the development of high-efficiency photovoltaic devices and for bulk material characterization. Here, we investigate a temporary room temperature superacid-based passivation scheme, which provides surface recombination velocities below 1 cm/s, thus placing our passivation scheme amongst state-of-the-art dielectric films. Application of the technique to high-quality float-zone silicon allows the currently accepted intrinsic carrier lifetime limit to be reached and calls its current parameterization into doubt for 1 Ω·cm n-type wafers. The passivation also enables lifetimes up to 65 ms to be measured in high-resistivity Czochralski silicon, which, to our knowledge, is the highest ever measured in Czochralski-grown material. The passivation strategies developed in this work will help diagnose bulk lifetime degradation under solar cell processing conditions and also help quantify the electronic quality of new passivation schemes.
Abstract A composite of Nb 2 O 5 and MoO 2 was synthesised using a hydrothermal reaction (225 °C) followed by a short heat‐treatment step (600 °C) to achieve a high‐capacity, high‐rate anode for lithium‐ion battery applications. The composite was shown via powder X‐ray diffraction and electron microscopy to be an intimate mix of individual oxide particles rather than an atomically mixed oxide material, and shown by X‐ray fluorescence spectroscopy (XRF) to contain a 45 : 55 ratio of Nb : Mo. This material is demonstrated to show notable rate capability in lithium (Li) half‐cell cycling and rate tests. When cycled at 100 °C the material achieved over 100 mAh g −1 even after 400 cycles and shows a stable reversible capacity of 514 mAh g −1 (at 1 C), realising its theoretical capacity. The composite shows electrochemical results comparable to Nb 2 O 5 :C composites yet achieves far higher capacities at low‐rate due to the MoO 2 content.
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