MIS-stimulated back-surface passivation of interdigitated back-contacts solar cells

The possibility of surface recombination losses reduction on the rear side of interdigitated back-contact solar cells by field-effect passivation is investigated. To provide field-effect passivation, an additional biased metal/insulator/semiconductor (MIS) structure is formed between n⁺⁺ and p⁺-doped regions. The source of the bias is the potential that appears in the solar cell under illumination. Two-dimensional (2D) numerical simulations were performed to determine the best passivation conditions. In particular, the influence of symmetric and asymmetric capture cross sections for electrons and holes at the rear side of the cell is simulated and the advantage of symmetric capture cross section for this type of passivation is discussed. Experimental and calculated data are compared for Si/SiO₂ interface.     

AB based solar cells and concentrating optical elements for space PV modules

This paper presents briefly the results of the development of , and GaSb cells manufactured for tandem solar cells, as well as tandems designed for point and line-focus concentrator modules. The maximum efficiency 23–23.8% (25°C, AM0) under 20–100 suns has been reached in the infrared transparent cells with prismatic cover. The efficiency 27.5% under AM1.5, 140 suns conditions has been reached as well. The bottom cells are based on lattice-matched or GaSb homo-junction Zn-diffused structures. The summation of the highest efficiencies measured in the top and bottom cells gave the values 28.8%–29.4% (AM0, 20–70 suns, 25°C) and 33.2% (AM1.5, 100 suns, 30°C). Two types of concentrator photovoltaic modules employing the reflective optical elements have been developed. The first type is based on compound parabolic concentrators, the second one on line-focus parabolic troughs. The estimated specific parameters of these modules with single-junction solar cells are the following: 230–240 W · m⁻² (AM0) and 3 kg · m⁻². The usage of tandem cells will allow to increase specific power of these modules on the value of 20–25%.     

Enhanced long-term durability of proton exchange membrane fuel cell cathode by employing Pt/TiO2/C catalysts

Pt/TiO₂/C catalysts are employed as the cathode catalysts for proton exchange membrane fuel cell (PEMFC). The comparative studies on the Pt/C and Pt/TiO₂/C catalysts are conducted with the physical and electrochemical techniques.After the accelerating aging test (AAT), the remaining electrochemical active surface area (EAS) of the Pt/TiO₂/C catalysts is 75.6%, which is larger than that of the Pt/C catalysts (42.6%). The apparent exchange current density () of the oxygen reduction reaction (ORR) at the Pt/C catalysts decreases from 3.02 × 10⁻⁹ to 1.32 × 10⁻¹¹ A cm⁻² after the AAT. And the value of of the ORR at the Pt/TiO₂/C catalysts is 2.88 × 10⁻⁹ A cm⁻² before the AAT and 2.51 × 10⁻⁹ A cm⁻² after the AAT. Furthermore, the output performance degradation of the PEMFC using the Pt/TiO₂/C cathode catalysts is also less than that using the Pt/C catalysts. The particle size of the Pt/C catalysts increases significantly from 5.3 to 26.5 nm after the AAT. The mean particle size of the Pt/TiO₂/C catalysts is 7.3 nm before the AAT and 9.2 nm after the AAT. It can be concluded that the long-term durability of the Pt/TiO₂/C catalysts in a PEMFC is much better than that of the Pt/C catalysts.     

Predictions of a critical fuel thickness for flame extinction in a quiescent microgravity environment

A simplified analysis and data acquired in the 4.5 s drop tower in MGLAB, Japan in a quiescent oxygen/nitrogen environment are presented for the prediction of the flammability limit in a quiescent microgravity environment. In the experimental matrix the oxygen level and thickness of PMMA are treated as control parameters. Published data from quiescent microgravity experiments on thin ashless filter paper and thick PMMA are also compared with the prediction of the analysis. Based on scale analysis, it is hypothesized that all fuels—from PMMA to cellulose—behave as thermally thin fuels during steady spread of flames in a quiescent environment. An expression for the spread rate that includes radiative effects is proposed for the first time: η₀ ∼ 1/2 + 1/2 , where η₀ is the spread rate non-dimensionalized by its thermal limit and ₀ is the non-dimensional radiation number. For ₀ > 1/4, which in dimensional terms translates to a critical thickness criterion τ > (F²/4)(ρ_gc_g/ρ_sc_s)(λ_g/εσ)[(T_v − T_∞)/(T_v⁴ − T_∞⁴)], flame extinction occurs irrespective of all other environmental conditions. Based on this prediction, an extinction thickness can be calculated even at 100% oxygen level. The experimental data from the MGLAB agree reasonably well with this prediction. Flammability maps with fuel half-thickness and oxygen level as coordinates are developed for PMMA and cellulosic fuels, which are shown to be consistent with the current and published data.     

Hydrogen production in a microbial electrolysis cell with nickel-based gas diffusion cathodes

Gas diffusion cathodes with Ni alloy and Ni catalysts manufactured by chemical deposition were tested for H₂ production in a microbial electrolysis cell (MEC). In a continuous flow MEC, multi-component cathodes containing Ni, Mo, Cr, and Fe, at a total catalyst load of 1 mg cm⁻² on carbon support demonstrated stable H₂ production at rates of with only 5% methane in the gas stream. Furthermore, a Ni-only gas diffusion cathode, with a Ni load of 0.6 mg cm⁻², demonstrated a H₂ production rate of . Overall, H₂ production was found to be proportional to the Ni load implying that inexpensive gas diffusion cathodes prepared by chemical deposition of Ni can be successfully used for continuous production of H₂ in a MEC.     

Study of electrical properties of oxidized porous silicon for back surface passivation of silicon solar cells

Back surface passivation becomes a key issue for the silicon solar cells made with thin wafers. The high surface recombination due to the metal contacts can be lowered by reducing the back contact area and forming local back surface field (LBSF) in conjunction with the passivation with dielectric layer. About 3×10^-7 m thick porous silicon (PS) layer with pore diameter mostly of 1×10^-8–5×10^-8 m was formed by chemical etching of silicon using the acidic solution containing hydrofluoric acid (HF), nitric acid (HNO₃) and De-ionized water in the volume ratio 1:3:5 at 298 K for which etching time was kept constant for 360 s. Electrical properties of oxidized PS was studied through the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of the metal–insulator–semiconductor (MIS) device in which the oxidized PS was used as an insulating layer and the results were further analyzed. The C–V curves of all the studies MIS devices showed the negative flatband voltage varying from -2 to , confirming that the oxidized layer of PS has fixed positive charge.     

Ionic liquid electrolyte based on S-propyltetrahydrothiophenium iodide for dye-sensitized solar cells

A new ionic liquid S-propyltetrahydrothiophenium iodide (T₃I) was developed as the solvent and iodide ion source in electrolyte for dye-sensitized solar cells. The electrochemical behavior of the /I⁻ redox couple and effect of additives in this ionic liquid system was tested and the results showed that this ionic liquid electrolyte revealed good conducting abilities and potential application for solar devices. The effects of LiI and dark-current inhibitors were investigated. The dye-sensitized solar cell with the electrolyte (0.1 mol L⁻¹ LiI, 0.35 mol L⁻¹ I₂, 0.5 mol L⁻¹ NMBI in pure T₃I) gave short-circuit photocurrent density (J_sc) of 11.22 mA cm², open-circuit voltage (V_oc) of 0.61 V and fill factor (FF) of 0.51, corresponding to the photoelectric conversion efficiency (η) of 3.51% under one Sun (AM1.5).     

Chemical diffusivity and ionic conductivity of GdBaCo2O5+δ

The transport properties of layered perovskite GdBaCo₂O_5+δ (GBCO), which has recently been proposed as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs), are investigated as a function of oxygen partial pressure (OPP) over the oxygen partial pressure range of 10⁻⁴ ≤ pO₂ (atm) ≤ 0.21 at 1073 ≤ T (K) ≤ 1323. The increase in total conductivity with increasing temperature below the low-temperature, order-disorder transition indicates a semiconductor-type behaviour with an activation energy of 0.42 eV. When OPP is increased to air pressure at a fixed temperature, the total conductivity increases with an apparent slope (∂log σ/∂log pO₂) of 1/10 to 1/22. The maximum oxygen ion conductivity, as extracted from the oxygen permeation measurements, is around 0.01 S cm⁻¹ under the nitrogen condition, which strongly supports the potential for cathode application. The chemical diffusion coefficient () and surface exchange coefficient (κ) are also calculated from the d.c. conductivity relaxation measurement and the values are best fitted by the following two equations:

     

Density functional study of the chemisorption of O2 on the zig-zag surface of graphite

The reaction between molecular oxygen and the zig-zag surface of two model graphites has been studied using density functional theory at the B3LYP/6-31G(d) level of theory. Chemisorption, desorption, rearrangement, and surface migration pathways were characterized and kinetic parameters computed in order to provide a mechanistic understanding of the processes occurring during carbon gasification. The chemisorption reaction is barrierless and highly exothermic, releasing . Surface migration reactions were found to occur with a barrier of ∼170 kJ mol⁻¹, while two desorption processes were found to have barriers of 420 and 340 kJ mol⁻¹, with the possibility of a stable intermediate forming in the latter pathway. Loss of CO from this stable intermediate was found to occur with a barrier of ∼170 kJ mol⁻¹. The initial chemisorption products are highly activated due to the exothermicity of their formation and may proceed directly to CO desorption without stabilization, especially at higher temperatures. Once one molecule of CO is lost, surface migration, rearrangement, and desorption reactions of the remaining oxide were found to have barriers from 195, 470, and 400 kJ mol⁻¹, respectively.