Characteristics of GaAs solar cells on Ge substrate with a preliminary grown thin layer of AlGaAs

Characteristics of GaAs solar cell on Ge substrate with a new buffer layer structure is reported. The buffer layer structure, which consisted of a preliminarily grown thin layer of A1_xGa_1−xAs and a 1 μm thick GaAs layer, was designed to obtain a high quality GaAs layer on Ge substrate by metalorganic chemical vapor deposition (MOCVD). Performance of a GaAs solar cell fabricated on Ge substrate with the buffer layer structure was compared with that fabricated on Ge substrate with a conventional GaAs buffer layer and also that fabricated on GaAs substrate. A conversion efficiency of 23.18% (AM1.5G) was successfully obtained for the cell fabricated on Ge substrate with the new buffer layer structure, while it was 20.92% for the cell fabricated on Ge substrate with the conventional GaAs buffer layer. Values of V_oc and J_sc, for the cell fabricated on Ge substrate with the new buffer layer structure were approximately comparable to those of a 25.39% efficiency GaAs solar cell fabricated on GaAs substrate.     

Characterization of antiphase domains on GaAs grown on Ge substrates by conductive atomic force microscopy for photovoltaic applications

The role of antiphase domains formed on GaAs grown on Ge is analyzed by means of conductive atomic force microscopy. The correlation of the derivative topography scans with the conductive scans shows a constant current value in most of the surface under study; although at certain locations high current leaks occur causing an inhomogeneous conductivity through the GaAs layer as the density of antiphase domains increases. This result implies that the existence of antiphase domains decreases the parallel resistance of solar cells, helping to understand the impact of these defects on the electrical behavior of these devices     

Low-energy proton irradiation effects on GaAs/Ge solar cells

This paper reports the low-energy proton irradiation effects on GaAs/Ge solar cells for space use. The proton irradiation experiments were performed with a fluence of 1.2×10¹³ cm⁻², energies ranging from 0.1 to 3.0 MeV. The results obtained demonstrate that the irradiation with a proton energy of 0.3 MeV gives rise to the most degradation rates of I_sc, V_oc and P_max of the solar cells with no coverglass, which is related to the proton irradiation-induced vacancies near the pn junction in GaAs/Ge cells. The degradation rates of I_sc, V_oc and P_max of the solar cells with coverglass increase as the proton energy increases due to the cascade ions induced by collision processes. It is found that the coverglass has an obvious protection effect against the irradiation with the proton energy below 0.5 MeV.     

Measurement of silicon and GaAs/Ge solar cells ac parameters

The ac parameters (cell capacitance and cell resistance) of Silicon (Si) and Gallium Arsenide (GaAs/Ge) solar cells are measured at different temperatures using time domain technique. The cell capacitance is calculated from the Open circuit voltage decay (OCVD) and the cell resistance from solar cell I–V characteristics measured under dark condition. It is observed that the solar cell capacitance increases whereas the cell resistance decreases with increase in temperature.     

Measurement of silicon and GaAs/Ge solar cell device parameters

The device parameters (carrier lifetime, ideality factor), and physical parameters (built-in voltage, doping concentration) of silicon (Si) and gallium arsenide (GaAs/Ge) solar cells are measured at different temperatures using time domain technique. Carrier lifetime is calculated from open circuit voltage decay (OCVD). Built-in voltage and doping concentration are derived from the cell capacitance measured at different bias voltages. Ideality factor is derived from the I–V characteristics of solar cell. Carrier lifetime increases while built-in voltage decreases with increase in temperature. Ideality factor of the solar cell decreases with temperature.     

Catalytic hydrogenation for improvement of GaAs/Ge solar cell efficiency

Hydrogen passivation on MOCVD grown p-GaAs epilayers on Ge substrate have been studied by plasma and catalytic hydrogenation and the results were compared. The conversion efficiency of the GaAs/Ge solar cells was found to increase by 10% after catalytic hydrogenation at AM1.5. This increase in efficiency is probably due to passivation of surface dangling bonds.     

5–20 MeV proton irradiation effects on GaAs/Ge solar cells for space use

This paper reports the high-energy proton irradiation effects on GaAs/Ge space solar cells. The solar cells were irradiated by protons with energy of 5–20 MeV at a fluence ranging from 1×10⁹ to 7×10¹³ cm⁻², and then their electric parameters were measured at AM0. It was shown that the I_sc, V_oc and P_max degrade as the fluence increases, respectively, but the degradation rates of I_sc, V_oc and P_max decrease as the proton energy increases, and the degradation is relative to proton irradiation-induced defect Ec−0.41 eV in irradiated GaAs/Ge cells.     

The influence of the surface on charge carrier transport in GaAs films

Contactless transient photoconductivity measurements in n-doped GaAs films with different doping concentrations are presented. It is shown that at low excess carrier densities the signal is due to excess charge carriers separated in the space charge region. These signals are characterized by a long decay time not related to the bulk properties of the films. At moderate excess carrier densities surface recombination takes place still hampered by the space charge field giving way to a high surface recombination at high excitation densities.     

Influence of carrier recombination in the space charge region on minority carrier lifetime in the base region of solar cells

In this paper the minority carrier lifetime (τ) in the base region of an n⁺/p silicon solar cell is calculated. The open circuit voltage decay method is employed. The influence of carrier recombination in the space charge region is considered through an interface recombination velocity, S_i. An analytical expression for τ is obtained and its value for one particular case is reported.     

Life cycle assessment and evaluation of energy payback time on high-concentration photovoltaic power generation system

In this study, the environmental load of photovoltaic power generation system (PV) during its life cycle and energy payback time (EPT) are evaluated by LCA scheme. Two hypothetical case studies in Toyohashi, Japan and Gobi dessert in China have been carried out to investigate the influence of installation location and PV type on environmental load and EPT. The environmental load and EPT of a high-concentration photovoltaic power generation system (hcpV) and a multi-crystalline silicon photovoltaic power generation system (mc-Si PV) are studied. The study shows for a PV of 100 MW size, the total impacts of the hcpV installed in Toyohashi is larger than that of the hcpV installed in Gobi desert by 5% without consideration of recycling stage. The EPT of the hcpV assumed to be installed in Gobi desert is shorter than EPT of the hcpV assumed to be installed in Toyohashi by 0.64 year. From these results, the superiority to install PV in Gobi desert is certificated. Comparing with hcpV and mc-Si PV, the ratio of the total impacts of mc-Si PV to that of hcpV is 0.34 without consideration of recycling stage. The EPT of hcpV is longer than EPT of mc-Si PV by 0.27 year. The amount of global solar radiation contributing to the amount of power generation of mc-Si PV is larger than the amount of direct solar radiation contributing to the amount of power generation of hcpV by about 188 kW h/(m² year) in Gobi desert. Consequently, it appears that using mc-Si PV in Gobi desert is the best option.     

Influence of hydrogen passivation on majority and minority charge carrier mobilities in ribbon silicon

Multicrystalline silicon materials and ribbons in particular contain a higher amount of defects as compared to monocrystalline silicon, which have to be passivated during solar cell processing in order to reach satisfactory cell efficiencies. Within the solar cell process, this is usually carried out via the deposition of a hydrogen-rich SiN_x layer and a following firing step. During passivation, the electronic properties of the materials (conductivity, mobility) can change which might have an influence on the optimised parameters like emitter sheet resistance and grid geometry. This paper deals with the impact of hydrogen passivation on the electronic properties of majority and minority charge carriers in ribbon silicon materials. Majority charge carrier mobilities resulting from Hall measurements are strongly increasing after hydrogenation especially at temperatures below 300 K. Even at room temperature, changes in mobility up to a factor of 2 have been observed. For the determination of minority charge carrier mobilities in processed solar cells, a new method is presented based on spatially resolved internal quantum efficiency and lifetime measurements. It allows the calculation of mapped mobilities especially in materials showing small diffusion lengths. The same reductions in mobility of a factor 2–3 as compared to monocrystalline silicon for both majority and minority charge carriers could be detected in RGS silicon.     

An evaluation on the life cycle of photovoltaic energy system considering production energy of off-grade silicon

In this study, single-crystalline silicon (c-Si) photovoltaic (PV) cells and residential PV systems using off-grade silicon supplied from semiconductor industries were evaluated from a life cycle point of view. Energy payback time (EPT) of the residential PV system with the c-Si PV cells made of the off-grade silicon was estimated at 15.5 years and indirect CO₂ emission per unit electrical output was calculated at 91 g-C/kWh even in the worst case. These figures were more than those of the polycrystalline-Si and the amorphous-Si PV cells to be used in the near future, but the EPT was shorter than its lifetime and the indirect CO₂ emissions were less than the recent average CO₂ emissions per kWh from the utilities in Japan. The recycling of the c-Si PV cells should be discussed for the reason of the effective use of energy and silicon material.     

Effect of holding time in brazing cooling process on creep life of the solid oxide fuel cell with bonded compliant seal

In present paper, effect of holding time at 600 °C during the brazing cooling process on creep life of solid oxide fuel cell (SOFC) with bonded compliant seal (BCS) is investigated by the finite element method. The research indicates that creep crack initiation time in BCS structure increases significantly with the holding time increasing. Compared with that the traditional cooling method during the brazing process, the creep crack initiation time can be prolonged more than twice by the holding time of 150 h with the operating temperature of 600 °C, it increases from 14,949 h to 31,911 h. When the operating temperature is 800 °C, the creep crack initiation time of SOFC can hardly be affected if the holding time exceeds 10 h. Based on the creep damage analysis and considering the cost of the SOFC manufacturing process, it is recommended that the holding time should not be exceeded 300 h if the operating temperature is below 750 °C. And when the operating temperature is 800 °C, the recommended holding time should not be longer than 10 h. The research of the present paper can provide theoretical guidance for the long life manufacturing and reliability operation of SOFC.     

The influence of degradation effects in proton exchange membrane fuel cells on life cycle assessment modelling and environmental impact indicators

Although proton exchange membrane fuel cell (PEMFC) systems are expected to have lower environmental impacts in the operational phase, compared to conventional energy conversion systems, there are still certain economic, operational, and environmental setbacks. Durability under a wide range of operating conditions presents a challenge because degradation processes affect the PEMFC efficiency. Typically, life cycle assessment (LCA) of PEMFC systems do not include performance degradation. Thus, a novel semi-empirical PEMFC model is developed, which includes degradation effects caused by different operational regimes (dynamic and steady-state). The model is integrated into LCA through life cycle inventory (LCI) to achieve a more realistic and accurate evaluation of environmental impacts. Verification of the model clearly showed that the use of existing LCI models underestimates the environmental impacts. This is especially evident when green hydrogen is used in PEMFC operational phase, where manufacturing phase and maintenance (stack replacements) become more influential. Input parameters of the model can be modified to reflect technological improvements (e.g. platinum loading or durability) and evaluate the effects of future scenarios.     

Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on “ideal” seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)-oriented diodes consistently exhibit a higher V_oc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10⁻³ to 10³ Suns. We determine 7 μm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3 μm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10 μm.     

Harmonising methodological choices in life cycle assessment of hydrogen: A focus on acidification and renewable hydrogen

The environmental sustainability of hydrogen energy systems is often evaluated through Life Cycle Assessment (LCA). In particular, environmental suitability is usually determined by comparing the life-cycle indicators calculated for a specific hydrogen energy system with those of a reference system (e.g., conventional hydrogen from steam methane reforming, SMR-H₂). In this respect, harmonisation protocols for comparative LCA of hydrogen energy systems have recently been developed in order to avoid misleading conclusions in terms of carbon footprints and cumulative energy demand. This article expands the scope of these harmonisation initiatives by addressing a new life-cycle indicator: acidification. A robust protocol for harmonising the acidification potential of hydrogen energy systems is developed and applied to both SMR-H₂ and a sample of case studies of renewable hydrogen. According to the results, unlike other energy systems, there is no correlation between acidification and carbon footprint in the case of hydrogen energy systems, which prevents the estimation of harmonised acidification results from available harmonised carbon footprints. Nevertheless, an initial library of harmonised life-cycle indicators of renewable hydrogen is now made available.     

臭氧对CH4/空气稀混合气滞燃期影响规律研究

为提高气体机稀薄燃烧时的燃烧性能,解决天然气发动机在稀薄燃烧情况下点火能量高以及火焰传播速度慢的问题,提出利用强氧化性的臭氧对燃料进行改质,进而提高天然气燃烧性能的思路。通过Chemkin软件研究臭氧添加对甲烷滞燃期的影响,并对改善燃烧的化学机理进行了初步探索。试验结果表明添加臭氧后,某些重要基元反应的温度敏感性、自由基和中间物质的浓度和出现时间发生较大变化,进而改善了甲烷的点火特性。     

The effect of ignition delay time on the explosion behavior in non-uniform hydrogen-air mixtures

The purpose of this study is to examine the explosion characteristics of non-uniform hydrogen-air mixtures with turbulent mixing. In the experiment, hydrogen is first filled into a 20 L spherical chamber to a desired initial pressure, then air is introduced into the same chamber through a fast response solenoid valve, by adjusting the ignition delay time (t_d), i.e., the time period between the end of air injection and the action of ignition, the turbulent mixing strengthen (or called uniformity of hydrogen-air mixture) is then changed. The experimental results show that the explosions are overall enhanced as t_d decreases, which indicates that turbulence plays a leading role in enhancing the explosion behaviors. In addition, it is found that the effect of turbulence on p_max is more prominent in end-wall ignition than that in center ignition. This is because the heat loss per unit time is higher in end-wall ignition due to the flame front continuously contacts with inner wall of the chamber throughout the explosion process, although the explosion duration time t_e for both ignition cases is reduced when turbulence is introduced, heat loss reduction for end-wall ignition is generally larger than that in center ignition. Lately, a systematical analysis of the turbulent effect associated with various equivalence ratios on the explosion characteristics is conducted in end-wall ignition. Those experimental results illustrate that the turbulence-enhancing influence is more noticeable when hydrogen-air mixtures move toward the lower explosion limit. However, no significant influence of turbulence on explosion process can be found as combustible mixtures tend to the fuel-rich side. This is mainly because that when hydrogen-air mixtures tend to fuel-rich side, τ_e reduction caused by the presence of turbulence is relatively weak as compared with that under quiescent condition, resulting in heat loss during explosion process changes slightly, hence there is no significant impact on explosion parameters.     

Influence of the pressure holding time on strain generation in fuel injection lines

An influence of the pressure holding time on residual strain generation during the autofrettage process was studied experimentally for the first time in the present work. It is the state of the art that fuel injection lines are held at the autofrettage pressure for only a few seconds in an industrial production. In doing so, it is assumed that a desirable residual stress–strain pattern is generated. However, the results of the experimental investigations outlined in this work indicated that completion of the plastic deformation caused by the autofrettage process and generation of the desirable stress–strain pattern require a much longer period. As shown, a third-order polynomial equation best described the interdependence between the time required for the completion of the process, the corresponding autofrettage pressure and the generated strain state. The method presented can be used as a tool for the determination of the optimal autofrettage process parameters in industrial production of fuel injection lines.