Radiation resistant AlGaAs/GaAs concentrator solar cells with internal Bragg reflector

The problem of increasing efficiency, reliability and radiation resistance of solar cells based on AlGaAs/GaAs heterostructures can be solved by using an internal Bragg reflector. The Bragg reflector as a back surface reflector and as a back surface potential barrier which allows to conserve the high photosensitivity in the long- and middle-wavelength parts of the spectrum after electron and proton irradiation. The effect of base doping and base thickness on the radiation resistance of AlGaAs/GaAs solar cells with the internal Bragg reflector has been investigated. Concentrator solar cells efficiency and related parameters before and after 3 MeV electron irradiation at the fluence up to 3×10¹⁵ cm⁻² are represented. A base doping level of 1×10¹⁵ cm⁻³ and base thickness in the range 1.1–1.6 μm give an EOL AM0 efficiency of 15.8% (BOL–22%) at 30 Suns concentration after exposure to 1×10¹⁵ cm⁻² electron fluence. This EOL efficiency is among the highest reported for GaAs single-junction concentrator cells under AM0 conditions. Making the base doping level lower and the base thinner allows retaining a j_EOL/j_BOL ratio of 0.96 upon exposure up to 3×10¹⁵e/cm² 3 MeV electron fluence. These results are additionally supported by the modeling calculations of the relative damage coefficient.     

Development of both-side junction silicon space solar cells

This paper reports the recent results of improving the radiation hardness of silicon solar cells, which is SHARP and NASDA's project since 1998 (Tonomura et al., Second World Conference on Photovoltaic Solar Energy, 1998, pp. 3511–3514). Newly developed 2×2 cm² Si solar cells with ultrathin substrates and both-side junction (BJ) structure showed 72.0 mW (13.3% efficiency) maximum output power at AM0, 28°C after 1 MeV electron irradiation up to 1×10¹⁵ e/cm² and the best cell showed 72.5 mW (13.4%) maximum output power. These solar cells have p–n junctions at both front and rear surfaces and showed less radiation degradation and better remaining factor than previous solar cells.     

Radiation resistance of high-efficiency InGaP/GaAs tandem solar cells

Radiation resistance of high-efficiency InGaP/GaAs tandem solar cells with a world-record efficiency of 26.9% (AM0, 28°C) has been evaluated by 1 MeV electron irradiation. Degradation in tandem cell performance has been confirmed to be mainly attributed to large degradation in the GaAs bottom cell. Similar radiation resistance with GaAs-on-Ge cells has been observed for the InGaP/GaAs tandem cell. Moreover, recovery of the tandem cell performance has been found due to minority-carrier injection under light illumination or forward bias, which causes defect annealing in InGaP top cells. The optimal design of the InGaP base layer thickness for current matching at end of life (EOL) (after irradiation with 10¹⁵ electrons cm⁻²) has been examined.     

Inoculation procedures and characterization of membrane electrode assemblies for microbial fuel cells

Membrane electrode assemblies were prepared following procedures adopted in the fabrication of polymer electrolyte membrane (PEM) fuel fells and used in microbial fuel cells (MFCs) with Shewanella oneidensis MR-1 as a single culture and sodium lactate as the electron donor. Improved inoculation procedures were developed and fuel cell performance with the biofilm density of microbes over the anode is discussed. A novel procedure to condition the membrane is also presented. Polarization measurements were carried out and power density plots were generated. Power density values of 300 mW m⁻² are typically obtained while a maximum value of 600 mW m⁻² is demonstrated indicating good performance for a single cell culture.     

Progress toward high-efficiency (>24%) and low-cost multi-junction solar cell production

TECSTAR has made significant progress in improving the efficiency and yield of multi-junction (Cascade^®) solar cells. TECSTAR has produced 27.0% efficient 2 cm×2 cm Cascade^® cells through improvements in material quality and cell structures. Radiation performance of these cells has also improved. The remaining power after 1E15 e/cm², 1 MeV electron irradiation is greater than 80%. In addition, TECSTAR has successfully developed an integral bypass diode, grown monolithically with the Cascade^® cell. No degradation in the diode performance after subjected to 1E15 e⁻/cm², 1 MeV electron irradiation and no degradation in the CIC assemblies after subjected to 3000 cycles of dark reverse test.     

p-doped μc-Si:H window layers prepared by hot-wire CVD for amorphous solar cell application

We report on boron-doped μc-Si:H films prepared by hot-wire chemical vapor deposition (HWCVD) using silane as a source gas and trimethylboron (TMB) as a dopant gas and their incorporation into all-HW amorphous silicon solar cells. The dark conductivity of these films was in the range of 1–10 (Ω cm)⁻¹. The open circuit voltage V_oc of the solar cells was found to decrease from 840 mV at low hydrogen dilution H-dil=91% to 770 mV at high H-dil =97% during p-layer deposition which can be attributed to the increased crystallinity at higher H-dil and to subsequent band edge discontinuity between μc-Si:H p- and amorphous i-layer. The short circuit current density J_sc and the fill factor FF show an optimum at an intermediate H-dil and decrease for the highest H-dil. To improve the conversion efficiency and the reproducibility of the solar cells, an amorphous-like seed layer was incorporated between TCO and the bulk p-layer. The results obtained until now for amorphous solar cells with and without the seed layer are presented. The I–V parameters for the best p–i–n solar cell obtained so far are J_sc=13.95 mA/cm², V_oc=834 mV, FF=65% and η=7.6%, where the p-layers were prepared with 2% TMB. High open circuit voltages up to 847 mV could be achieved at higher TMB concentrations.     

Epi-n-IZO thin films/1 0 0 Si, GaAs and InP by L-MBE––a novel feasibility study for SIS type solar cells

High quality epitaxial indium zinc oxide (heavily indium oxide doped) (epi-n-IZO) thin films were optimized by laser-molecular beam epitaxy (L-MBE) i.e., pulsed laser deposition (PLD) technique for fabricating novel iso- and hetero-semiconductor–insulator–semiconductor (SIS) type solar cells using Johnson Matthey “specpure”- grade 90% In₂O₃ mixed 10% ZnO (as commercial indium tin oxide (ITO) composition) pellets. The effects of substrate temperatures, substrates and heavy indium oxide incorporation on IZO thin film growth, opto-electronic properties with 1 0 0 silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP) wafers were studied. As well as the feasibility of developing some novel models of iso- and hetero-SIS type solar cells using epi-IZO thin films as transparent conducting oxides (TCOs) and 1 0 0 oriented Si, GaAs and InP wafers as base substrates was also studied simultaneously. The optimized films were highly oriented, uniform, single crystalline approachment, nano-crystalline, anti-reflective (AR) and epitaxially lattice matched with 1 0 0 Si, GaAs and InP wafers without any buffer layers. The optical transmission T (max) 95% is broader and absolute rivals that of other TCOs such as ITO. The highest conductivity observed is σ=0.47×10³ Ω⁻¹ cm⁻¹ (n-type), carrier density n=0.168×10²⁰ cm⁻³ and mobility μ=123 cm²/V s. From opto-electronic characterizations, the solar cell characteristics and feasibilities of fabricating respective epi-n-TCO/1 0 0 wafer SIS type solar cells were confirmed. Also, the essential parameters of these cells were calculated and tabulated. We hope that these data be helpful either as a scientific or technical basis in semiconductor processing.     

Electrooxidation study of pure ethanol/methanol and their mixture for the application in direct alcohol alkaline fuel cells (DAAFCs)

Aliphatic alcohol mainly, ethanol, methanol and their mixture were subjected to electrooxidation study using cyclic voltammetry (CV) technique in a three electrodes half cell assembly (PGSTAT204, Autolab Netherlands). A single cell set up of direct alcohol alkaline fuel cell (DAAFC) was fabricated using laboratory synthesized alkaline membrane to validate the CV results. The DAAFC conditions were kept similar as that of CV experiments. The anode and cathode electrocatalysts were Pt-Ru (30%:15% by wt.)/Carbon black (C) (Alfa Aesar, USA) and Pt (40% by wt.)/High Surface Area Carbon (C_HSA) (Alfa Aesar, USA) respectively. The CV and single cell experiments were performed at a temperature of 30 °C. The anode electrocatalyst was in the range of 0.5 mg/cm² to 1.5 mg/cm² for half cell CV analysis. The cell voltage and current density data were recorded for different concentrations of fuel (ethanol or methanol) and their mixture mixed with different concentration of KOH as electrolyte. The optimum electrocatalyst loading in half cell study was found to be 1 mg/cm² of Pt-Ru/C irrespective of fuel used. The single cell was tested using optimum anode loading of 1 mg/cm² of Pt-Ru/C which was found in CV experiment. Cathode loading was kept similar, in the order of 1 mg/cm² Pt/C_HSA. In single cell experiment, the maximum open circuit voltage (OCV) of 0.75 V and power density of 3.57 mW/cm² at a current density of 17.76 mA/cm² were obtained for the fuel of 2 M ethanol mixed with 1 M KOH. Whereas, maximum OCV of 0.62 V and power density of 7.10 mW/cm² at a current density of 23.53 mA/cm² were obtained for the fuel of 3 M methanol mixed with 6 M KOH. The mixture of methanol and ethanol (1:3) mixed with 0.5 M KOH produced the maximum OCV of 0.66 V and power density of 1.98 mW/cm² at a current density of 11.54 mA/cm².     

Investigation of Cu metallization for Si solar cells

For application of copper metallization to silicon solar cells, electrical resistivity of the electroplated Cu was investigated for different annealing conditions: the rapid thermal annealing (RTA) and the vacuum annealing at various temperatures. The characteristics of Ti as the diffusion barrier were also observed. The specific contact resistance between Si and Ti/Cu was measured using Kelvin test pattern. For 8-min electroplated sample, the lowest resistivity of 2.1 μΩ cm was obtained at 300°C RTA condition. For Cu with Ti barrier, 400°C 2 min vacuum-annealed sample showed etch pits whereas 400°C RTA showed no etch pits. A vacuum annealing at 450°C for 30 min reduced the specific contact resistance to 7.2×10⁻⁶ Ω cm².     

A liquid–gas phase mixed-reactant fuel cell with a RuSeW cathode electrocatalyst

Some data in mixed-reactant fuel cells (MRFC) at Newcastle using formic acid, methanol and ethanol are reported. The importance of using a fuel-tolerant selective cathode catalyst has been identified. The influence of fuel and oxidant conditions and feeding patterns has been evaluated. The cell performance using air, oxygen and hydrogen peroxide is reported. The highest peak power density of 16 mW cm⁻² was obtained with formic acid. The MRFC gave power densities approximately half those of a conventional, un-mixed-reactant fuel cell.     

Direct dimethyl-ether proton exchange membrane fuel cells and the use of heteropolyacids in the anode catalyst layer for enhanced dimethyl ether oxidation

In this study, polarization and impedance experiments were performed on a direct dimethyl ether fuel cell (DMEFC). The experimental setup allowed for independent control of water and DME flow rates. The DME flow rate, backpressure, and water flow rate were optimized. Three heteropolyacids, phosphomolybdic acid, H₃PMo₁₂O₄₀. (HPMo), phosphotungstic acid, H₃PW₁₂O₄₀, (HPW), and silicotungstic acid, H₄SiW₁₂O₄₀, (HSiW) were incorporated into the anode catalyst layer in combination with Pt/C. Both HPW-Pt and HSiW-Pt showed higher overall performance than the Pt control. Anodic polarizations were also performed, at 30 psig, Tafel slopes of 67 mV dec⁻¹, 72 mV dec⁻¹, and 79 mV dec⁻¹ were found for HPW-Pt, HSiW-Pt and the Pt control, respectively. At 0 psig, the Tafel slopes were 56 mV dec⁻¹, 58 mV dec⁻¹, and 65 mV dec⁻¹ for HPW-Pt, HSiW-Pt and the Pt control. The trends in the Tafel slope values are in agreement with the polarization data and the electrochemical impedance spectroscopy results. The addition of phosphotungstic acid more than doubled the power density of the fuel cell, compared to the Pt control. When the maximum power density obtained using the HPW-Pt MEA is normalized by the mass of Pt used, the optimal result, 78 mW mg⁻¹ Pt, the highest observed at 30 psig and 100 °C to date.     

Direct methanol fuel cell operation with pre-stretched recast Nafion

Direct methanol fuel cell operation with uniaxially pre-stretched recast Nafion^® membranes (draw ratio of 4) was investigated and compared to that with commercial (un-stretched) Nafion^®. The effects of membrane thickness (60–250 μm) and methanol feed concentration (0.5–10.0 M) on fuel cell power output were quantified for a cell temperature of 60 °C, ambient pressure air, and anode/cathode catalyst loadings of 4.0 mg cm⁻². Pre-stretched recast Nafion^® in the 130–180 μm thickness range produced the highest power at 0.4 V (84 mW cm⁻²), as compared to 58 mW cm⁻² for Nafion^® 117. MEAs with pre-stretched recast Nafion^® consistently out-performed Nafion^® 117 at all methanol feed concentrations, with 33–48% higher power densities at 0.4 V, due to a combination of low area-specific resistance (the use of a thinner pre-stretched membrane, where the conductivity was the same as that for commercial Nafion^®) and low methanol crossover (due to low methanol solubility in the membrane). Very high power was generated with a 180-μm thick pre-stretched recast Nafion^® membrane by increasing the cell temperature to 80 °C, increasing the anode/cathode catalyst loading to 8.0 mg cm⁻², and increasing the cathode air pressure to 25 psig. Under these conditions the power density at 0.4 V for a 1.0-M methanol feed solution was 240 mW cm⁻² and the maximum power density was 252 mW cm⁻².     

Planar air breathing PEMFC with self-humidifying MEA and open cathode geometry design for portable applications

In this paper, planar air breathing PEMFCs without the need for endplates are proposed for low power portable applications. PEMFCs with 3 different cathode designs (parallel slit, circular open and oblique slit) with the same opening ratio and employing self-humidifying MEAs were investigated. Performance and stability tests were conducted in hydrogen dead-end operation under both self-breathing and forced convection condition. It was found that rib geometry and hydraulic diameter have significant impact on oxygen transportation. It was concluded that circular opening design yields the best performance and highest limiting current. This is because this design provides the shortest rib distance and smallest hydraulic diameter. However, fuel cell instability was observed under self-breathing and forced convection condition. This is due to the water accumulation that could not be removed by natural-evaporation at the opening cathode. Overall, our proposed air breathing PEMFC achieves a specific power of 150 W kg⁻¹ and a power density of 347 mW cm⁻³.     

Thermogalvanic conversion of heat to electricity

A thermogalvanic (nonisothermal) cell was constructed for carrying out power conversion efficiency measurements. The design departed from that of traditional thermogalvanic cells which have largely been used only for studies of open-circuit voltage. The cell was used to obtain temperature coefficients, ∂E/∂T, of the open circuit voltage and power conversion efficiencies, Φ, for an interelectrode temperature difference, ΔT, of 20 K, using various redox couples. The values obtained were the following: Cu²⁺/Cu (1.0 mol dm⁻³), ∂E/∂T = 785 μV K⁻¹; Zn²⁺/Zn (1.0 mol dm⁻³), ∂E/∂T = 790 μV K⁻¹; Fe phen(CN)₄⁻/Fe phen(CN)₄²⁻ (10⁻³ mol dm⁻³), ∂E/∂T = 1046 μV K⁻¹, Φ = 4.17 × 10⁻⁵%; Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ (0.07 mol dm⁻³), ∂E/∂T = 1600 μV K⁻¹, Φ = 1.4 × 10⁻²%. More detailed studies of the latter system when [Fe(CN)₆³⁻] = [Fe(CN)₆⁴⁻] = 0.26 mol dm⁻³ and [KCl] = 0.80 mol dm⁻³, using platinum electrodes, with ΔT = 20 K, gave a current density of 1.45 mA cm⁻² and a power conversion efficiency, Φ, of 2.8 × 10⁻²%. This approaches 0.5% of the maximum theoretical efficiency of a Carnot engine operating across the same temperature difference.     

Semi-continuous co-digestion of solid slaughterhouse waste, manure, and fruit and vegetable waste

The potential of semi-continuous mesophilic anaerobic digestion (AD) for the treatment of solid slaughterhouse waste, fruit-vegetable wastes, and manure in a co-digestion process has been experimentally evaluated. A study was made at laboratory scale using four 2 L reactors working semi-continuously at 35 °C. The effect of the organic loading rate (OLR) was initially examined (using equal proportion of the three components on a volatile solids, VS, basis). Anaerobic co-digestion with OLRs in the range 0.3–1.3 kg VS m⁻³ d⁻¹ resulted in methane yields of 0.3 m³ kg⁻¹ VS added, with a methane content in the biogas of 54–56%. However, at a further increased loading, the biogas production decreased and there was a reduction in the methane yield indicating organic overload or insufficient buffering capacity in the digester.In the second part of the investigation, co-digestion was studied in a mixture experiment using 10 different feed compositions. The digestion of mixed substrates was in all cases better than that of the pure substrates, with the exception of the mixture of equal amounts of (VS/VS) solid cattle–swine slaughterhouse waste (SCSSW) with fruit and vegetable waste (FVW). For all other mixtures, the steady-state biogas production for the mixture was in the range 1.1–1.6 L d⁻¹, with a methane content of 50–57% after 60 days of operation. The methane yields were in the range 0.27–0.35 m³ kg⁻¹ VS added and VS reductions of more than 50% and up to 67% were obtained.     

Wind characteristics and energy potentials for Owerri, Nigeria

Wind characteristics and wind energy resource potentials for Owerri, Nigeria are presented. These were evaluated using routine wind data measurements at a height of 10 m above ground level at the Lake Nwebere Campus, Federal University of Technology, Owerri between 1988 and 1992. The most prevailing wind is from the Southwest and the average wind speed and its variation are 2.80 and 0.81 m s⁻¹, respectively.Accordingly, the maximum annual mean power density exploitable from the wind at this site is 7.66 ± 0.15 W m⁻² out of the estimated available annual mean wind power density of 12.91 ± 0.26 W m⁻². The annual mean energy density available in the wind was found to be 60.29 kW h m⁻². Thus, the potential for year-round wind energy utilization in Owerri, Nigeria is rather low.     

Performance improvements of crystalline silicon by iterative gettering process for short duration and with the use of porous silicon as sacrificial layer

In this work, we demonstrate that an efficient purification method of silicon wafers where iterative sequences were used. Each sequence consists of forming porous silicon (PS) on both sides of the samples, followed by thermal annealing in an infrared furnace under N₂/SiCl₄ ambient. Improvements of the electronic parameters were obtained by optimizing the heat treatments temperatures and the number and duration of the iteration sequences. Best results were obtained for temperatures below 980 °C and for three sequences of 20 min each one. After three sequences the mobility of the majority carrier improved from 94 cm² V⁻¹ s⁻¹ (for untreated wafer) to about 374 cm² V⁻¹ s⁻¹. The observed results were explained taking into account the transport properties of the impurities in the porous media and their concentration at the walls at each iteration. It was found that short iterative sequences give almost the same results than one long sequence duration. Silicon solar cells based on iterative gettered silicon wafers exhibit an increase in the short-circuit current and the open-circuit voltage. This fact seems to be important to ameliorate solar grade silicon (SGS) based solar cells performances.     

Estimating monthly mean valuesof daily total solar radiation for Australia

Monthly mean values of daily total solar radiation were obtained for the widest possible network acrossAustralia. Bureau of Meteorology sources yielded 11 stations with long term records of both measured daily total solar radiation and sunshine hour values. Monthly modified Angstrom equations were developed from these data and used to estimate radiation values for a further 90 stations in the Bureau of Meteorology network that had sunshine hour data. Measured daily total solar radiation data were obtained from a variety of sources mostly outside the Bureau of Meteorology network for an additional 33 stations. Finally, estimates of solar radiation from detailed cloud cover data were used for a further 12 stations, selected because they filled in significant gaps in coverage. These various sources yielded a total of 146 sets of monthly mean values of daily total solar radiation. For each month optimal surfaces, which were functions of position only, were fitted to this network of values using Laplacian smoothing splines with generalized cross validation. Residuals from the fitted surfaces at the data points were acceptably low. Fitted surfaces which included, in addition to position variables, a cloudiness index based on a transform of mean monthly precipitation further reduced these residuals. The latter fitted surfaces permit estimation of monthly mean values of total daily solar radiation at any point on the continent with a root mean square predictive error of no more than 1.25 MJ m⁻² day⁻¹ (5.2 per cent of the network mean) in summer and 0.74 MJ m⁻² day⁻¹ (5.5 per cent of the network mean) in winter.     

Electrochemical characteristics of an La0.6Sr0.4Co0.2Fe0.8O3–La0.8Sr0.2MnO3 multi-layer composite cathode for intermediate-temperature solid oxide fuel cells

An La_0.6Sr_0.4Co_0.2Fe_0.8O₃–La_0.8Sr_0.2MnO₃ (LSCF–LSM) multi-layer composite cathode for solid oxide fuel cells (SOFCs) was prepared on an yttria-stabilized zirconia (YSZ) electrolyte by the screen-printing technique. Its cathodic polarization curves and electrochemical impedance spectra were measured and the results were compared with those for a conventional LSM/LSM–YSZ cathode. While the LSCF–LSM multi-layer composite cathode exhibited a cathodic overpotential lower than 0.13 V at 750 °C at a current density of 0.4 A cm⁻², the overpotential for the conventional LSM–YSZ cathode was about 0.2 V. The electrochemical impedance spectra revealed a better electrochemical performance of the LSCF–LSM multi-layer composite cathode than that of the conventional LSM/LSM–YSZ cathode; e.g., the polarization resistance value of the multi-layer composite cathode was 0.25 Ω cm² at 800 °C, nearly 40% lower than that of LSM/LSM–YSZ at the same temperature. In addition, an encouraging output power from an YSZ-supported cell using an LSCF–LSM multi-layer composite cathode was obtained.     

Outdoor performance of amorphous silicon and polycrystalline silicon PV modules

The daily watt-hour efficiency (η_Wh) and daily integrated output power (P_Wh) of the a-Si and poly-Si module have been used to examine the performances of both modules on the basis of two years' data accumulated at outdoor conditions. Results from the analysis of experimental data taken under incident solar energy higher than 3.0 kWh/m² per day show that the annual average of η_Wh of the a-Si module is about 95% and 92.5% of its efficiency at STC condition at the first and second year, respectively, while the values are nearly unchanged at about 89% for the poly-Si module. During a one year period, the average P_Wh of the a-Si and poly-Si module was about 60% and 56%, respectively, of their calculated output power at STC condition, so that the P_Wh for each watt-peak (W_p) of the maximum power of a-Si module is about 11% higher than that of the poly-Si module.