Superstrate-type CuInSe2-based thin film solar cells by a low-temperature process using sodium compounds

Superstrate-type solar cells with a Au/CuInSe₂(CIS)/In_xSe_y,/ZnO : Al/glass structure were investigated. The CIS films were deposited by coevaporation method with intentionally incorporated Na₂S at a substrate temperature of 350°C. Even at relatively low substrate temperatures, sodium compounds enhanced the (1 1 2) preferred orientation of the chalcopyrite structure, and also improved the cell performance. The In_xSe_y buffer layers disappeared after CIS deposition by interdiffusion. Preliminary cells yielded an efficiency of 7.5% with V_oc, = 430 mV, J_sc = 29.4 mA/cm² and FF = 0.60. The light soaking and forward bias effects were observed for these cells.     

Novel device structure for Cu(In,Ga)Se2 thin film solar cells using transparent conducting oxide back and front contacts

Cu(In_1−xGa_x)Se₂ (CIGS)-based thin film solar cells fabricated using transparent conducting oxide (TCO) front and back contacts were investigated. The cell performance of substrate-type CIGS devices using TCO back contacts was almost the same as that of conventional CIGS solar cells with metallic Mo back contacts when the CIGS deposition temperatures were below 500 °C for SnO₂:F and 520 °C for ITO. CIGS thin film solar cells fabricated with ITO back contacts had an efficiency of 15.2% without anti-reflection coatings. However, the cell performance deteriorated at deposition temperatures above 520 °C. This is attributed to the increased resistivity of the TCO’s due to the removal of fluorine from SnO₂ or undesirable formation of a Ga₂O₃ thin layer at the CIGS/ITO interface. The formation of Ga₂O₃ was eliminated by inserting an intermediate layer such as Mo between ITO and CIGS. Furthermore, bifacial CIGS thin film solar cells were demonstrated as being one of the applications of semi-transparent CIGS devices. The cell performance of bifacial devices was improved by controlling the thickness of the CIGS absorber layer. Superstrate-type CIGS thin film solar cells with an efficiency of 12.8% were fabricated using a ZnO:Al front contact. Key techniques include the use of a graded band gap Cu(In,Ga)₃Se₅ phase absorber layer and a ZnO buffer layer along with the inclusion of Na₂S during CIGS deposition.     

Annealing effect of Cu2Te---Au contact to evaporated CdTe film on photovoltaic properties of CdS/CdTe solar cell

The annealing effect of an evaporated Cu₂Te---Au contact to CdTe film on the photovoltaic properties of thin-film CdS/vacuum-evaporated CdTe solar cells has been investigated. V_oc and J_sc for the cells with Cu₂Te---Au contact increased greatly with increasing annealing temperature and showed a maximum value at around 250. Photovoltaic properties of the cell with Cu₂Te---Au contact were improved by annealing in a greater extent than those of the cell with Te---Au or Te---Cu contacts. The cells with Cu₂Te---Au contact exhibit a higher conversion efficiency, comparing with the cells with Te---Au or Te---Cu contacts. The cell with Cu₂Te---Au contact showed a conversion efficiency of 10%. Cu₂Te---Au contact acts as the best pseudoohmic contact on vacuum evaporated CdTe film.     

Stability of Cu(In,Ga)Se2 solar cells and evaluation by C–V characteristics

To confirm the long-term reliability of Cu(In,Ga)Se₂, (CIGS) solar cells, we investigated the I–V and C–V characteristics during tests under irradiation or dark condition. Under irradiation, the test samples showed a little increase in efficiency (η) and open-circuit voltage (V_oc) which showed their electrical durability to light irradiation. But the diode factor (n) and series resistance (R_s) showed large changes in value. Also, the built-in voltage (V_b) and density gradient (dN_A/d_x) in the CIGS layer calculated from the C–V characteristics showed distinct changes during the test. After 4 SUN irradiation, two samples in the same fabrication-lot showed new light absorption in the lower-energy range than sun the energy gap of CIGS. We explain the change of C–V characteristics for the samples under strong irradiation with a new model named “Junction retrograde” which can treat defect generation by irradiation to reduce the acceptor density in graded p-n junction. This model for C–V analysis can be used to investigate the long-term reliability of CIGS solar cells under irradiation.     

Cd-free buffer layers for CIGS solar cells prepared by a dry process

ZnSe buffer layers for Cu(ln,Ga)Se₂/buffer/ZnO solar cells have been prepared by metal organic chemical vapor deposition (MOCVD). Using photoassisted MOCVD, deposition temperatures down to 265°C are possible. It is shown that deposition temperatures well below 300°C are essential as well as deposition times not much longer than 3 min. Higher temperatures and longer deposition times lead to absorber degradation. With optimized buffer deposition efficiencies of 11% have been obtained on CIGS absorbers from the Siemens pilot production line.     

Thermal stability of GaAs tunnel junctions using carbon as a p-type dopant grown by metal-organic vapor phase epitaxy

The annealing characteristics of GaAs tunnel junctions using carbon (C) as a p-type dopant were studied through current-voltage measurement and secondary-ion mass spectroscopy (SIMS). After annealing (750°C, 20 min), diodes fabricated with a C-doped p⁺-GaAs/Si-doped n⁺-GaAs tunnel junction showed an excellent tunnel peak current density (J_P) of 114 mA/cm², which is a larger value than ever reported for structures using Be or Zn as a p-type dopant. Degradation of J_P, in diodes after annealing was found to drastically improve when the tunnel junction was sandwiched between p- and n-AlGaAs layers. The inserted n-AIGaAs layer was more effective in suppressing J_P, degradation than the p-AIGaAs layer. SIMS results revealed that Si diffusion, which causes J_P, degradation, was suppressed at the interfaces of the p⁺-GaAs/n⁺-GaAs tunnel junction and the GaAs/AIGaAs heterojunction. An excellent J_P value of 1.7 A/cm² at 55 mV was obtained after annealing (750°C, 20 min) through structural optimization.     

Schottky solar cells with amorphous carbon nitride thin films prepared by ion beam sputtering technique

This paper reports on the successful deposition of amorphous carbon nitride thin films (a-CN_x) and fabrication of ITO/a-CN_x/Al Schottky thin-film solar cells by using the technique of ion beam sputtering. XPS and Raman spectra are used to characterize the deposited thin films. Nitrogen atoms are incorporated into the films in the form of carbon–nitrogen multiple bands. Their optical properties are also investigated using a spectroscopic ellipsometer and UV/VIS/NIR spectrophotometer. The refraction of the carbon nitride thin films deposited lies in the range of 1.7–2.1. The Tauc optical band gap is about 0.6 eV. The photovoltaic values of the device, short-circuit current and open-circuit voltage are 1.56 μA/cm² and 250 mV, respectively, when exposed to AM1.5 illumination (100 mW/cm², 25°C).     

The role of temperature on the corrosion and passivation of type 310S stainless steel in eutectic (Li + K) carbonate melt

The corrosion behavior of type 310S stainless steel was studied in the eutectic Li + K carbonate as a function of temperature by several electrochemical methods. Within the range 600–675 °C the steel passivated spontaneously at OCP condition after a few hours of immersion. Active-passive transition was observed in the polarization curves below 600 °C and above 675 °C indicating a failure to reach a stable passive condition even at prolonged exposure times. Impedance analysis indicates that passivity does not lead to the formation of an impervious barrier layer as denoted by the presence of diffusional components at low frequencies indicating oxide growth. Corrosion rates exhibited a minimum at 675 °C at both OCP and at cathode polarization conditions. A mechanism to explain the active–passive transition has been proposed based on the phase transition from LiFe₅O₈ to LiFeO₂.     

Performance analysis and impedance spectral signatures of high temperature PBI–phosphoric acid gel membrane fuel cells

Polarization curves, i.e., dc performance, and impedance spectral signatures of polybenzimidazole (PBI)–phosphoric acid (H₃PO₄) membrane fuel cells are obtained in the temperature range of 160–180 °C, in an effort to investigate the effect of temperature, anode humidification, various cathode stoichs, and use of oxygen versus air. Thus, in situ electrochemical impedance spectroscopy (EIS) was used to obtain various resistances, ohmic as well as charge-transfer resistances, under these conditions. The results obtained show that PBI–H₃PO₄ gel membrane fuel cells exhibit very good performance in the temperature range of 160–180 °C with an ohmic resistance similar to Nafion. Mass transfer limitations were determined by comparing performance polarization curves with air and oxygen along with EIS. Further EIS was also used to obtain signatures during fuel starvation, and electrical shorting across the cell.     

Numerical and experimental research of the characteristics of concentration solar cells

The development of automatic tracking solar concentrator photovoltaic systems is currently attracting growing interest. High concentration photovoltaic systems (HCPVs) combining triple-junction InGaP/lnGaAs/Ge solar cells with a concentrator provide high conversion efficiencies. The mathematical model for triple-junction solar cells, having a higher efficiency and superior temperature characteristics, was established based on the one-diode equivalent circuit cell model. A paraboloidal concentrator with a secondary optic system and a concentration ratio in the range of 100X–150X along with a sun tracking system was developed in this study. The GaInP/GalnAs/Ge triple-junction solar cell, produced by AZUR SPACE Solar Power, was also used in this study. The solar cells produced by Shanghai Solar Youth Energy (SY) and Shenzhen Yinshengsheng Technology Co. Ltd. (YXS) were used as comparison samples in a further comparative study at different concentration ratios (200X–1000X). A detailed analysis on the factors that influence the electrical output characteristics of the InGaP/lnGaAs/Ge solar cell was conducted with a dish-style concentrating photovoltaic system. The results show that the short-circuit current (I_sc) and the open-circuit voltage (V_oc) of multi-junction solar cells increases with the increasing concentration ratio, while the cell efficiency (η_c) of the solar cells increases first and then decreases with increasing concentration ratio. With increasing solar cell temperature, I_sc increases, while V_oc and η_c decrease. A comparison of the experimental and simulation results indicate that the maximum root mean square error is less than 10%, which provides a certain theoretical basis for the study of the characteristics of triple-junction solar cell that can be applied in the analysis and discussion regarding the influence of the relevant parameters on the performance of high concentration photovoltaic systems.     

The effect of a long post weld heat treatment on the integrity of a welded joint in a pressure vessel steel

The effect of a long post weld heat treatment on the microstructure and mechanical properties of a welded joint in a 0·2%C-1·4%Mn-0·5%Mo pressure vessel steel was studied. Multipass submerged-arc welds were made at a heat input of 1·2 and 4·3 kJ mm⁻¹. Individual microstructural regions observed in the heat-affected zone of the actual weld were simulated. These regions were brittle in the as-simulated condition. Post weld heat treatment for periods of up to 40 h at 620°C resulted in a significant improvement in the Charpy impact toughness. At the same time, a loss of the heat-affected zone and weld metal hardness and transverse weld strenghth occurred. A fracture toughness (J_Ic) of 134 kJ m⁻² was measured in the heat-affected zone of the 4·3 kJ mm⁻¹ welds after prolonged post weld heat treatment. The improvement in weldment toughness with post weld heat treatment was primarily attributed to softening of the structure.     

Defect reduction in electrochemically deposited CdS thin films by annealing in 02

Using the electrochemical deposition method, CdS thin films were deposited from acid solutions (pH = 2.5) containing CdS0₄ and Na₂S₂0₃ on indium-oxide coated glass substrates. These films were annealed in N₂, air, or O₂ atmosphere at 200–500°C for 30 min. Photoluminescence spectra were measured at 77 K. For the films annealed in N₂, the band edge emission became weaker and the luminescence due to defects shifted to longer wavelengths as the annealing temperature was raised above 300°C. However, for the films annealed in air or O₂, the band edge emission was observed strongly irrespective of the annealing temperature and the luminescence due to defects was weak. Thus the O₂ annealing is useful for the defects reduction.     

Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H₂ production based on CeO₂/Ce₂O₃ oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO₂ → Ce₂O₃ + 0.5O₂; (2) hydrolysis, Ce₂O₃ + H₂O → 2CeO₂ + H₂. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO₂), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.     

Bismuth oxide doped scandia-stabilized zirconia electrolyte for the intermediate temperature solid oxide fuel cells

Electrical and structural properties of bismuth oxide doped scandia-stabilized zirconia (ScSZ) electrolyte for solid oxide fuel cells (SOFCs) have been evaluated by means of XRD, TGA, DTA, and impedance spectroscopy. The amount of Bi₂O₃ in the ScSZ was varied in the range of 0.25–2.0 mol%. The original ScSZ samples indicated a rhombohedral crystalline structure that in general has lower conductivity than the cubic phase. However, the addition of Bi₂O₃ to ScSZ electrolyte was found to stabilize the cubic crystalline phase as detected by XRD. Impedance spectroscopy measurements in the temperature range between 350 and 900 °C indicated a sharp increase in conductivity for the system containing 2 mol% of Bi₂O₃ that is attributed to the presence of the cubic phase. In addition, impedance spectroscopy measurements revealed significant decrease of both the grain bulk and grain boundary resistances with respect to the temperature change from 600 to 900 °C and concentration of Bi₂O₃ from 0.5 to 2 mol%. The electrical conductivity at 600 °C obtained for 2 mol% Bi₂O₃ doped ScSZ was 0.18 S cm⁻¹.     

Characteristics of oxygen-blown gasification for combustible waste in a fixed-bed gasifier

With increasing environmental considerations and stricter regulations, gasification of waste is considered to be a more attractive technology than conventional incineration for energy recovery as well as material recycling. The experiment for combustible waste mixed with plastic and cellulosic materials was performed in a fixed-bed gasifier to investigate the gasification behaviour with the operating conditions. Waste pelletized to a diameter of 2–3 cm and 5 cm length, was gasified in the temperature range 1100–1450 °C. The composition of H₂ was in the range 30–40% and CO 15–30% depending upon the oxygen/waste ratio. Gasification of waste due to the thermoplastic property of the mixed-plastic melting and thermal cracking shows a prominent difference from that of coal or coke. It was desirable to maintain the top temperature at 400 °C to ensure the mass transfer and uniform reaction throughout the packed bed. As the bed height was increased, the formation of H₂ and CO was increased, whilst the CO₂ decreased by the char-CO₂ reaction and plastic cracking. From the experimental results, the cold gas efficiency was around 61% and the heating values of product the gases were in the range of 2800–3200 kcal/Nm³.     

Chemical bath deposition of Cds buffer layer for GIGS solar cells

We investigated the chemical bath deposition of US thin flims on the Cu(In,Ga)Se₂ (GIGS) absorber layers and glasses. The process of the chemical bath deposition of US layer affected the performance of the CIGS solar cells. The CdS layers were deposited on the CIGS film from CdI₂, thiourea (NH₂CSNHn₂) and ammonia solutions. The influence of pH on the chemical bath deposition process was studied. The surfaces of the US films were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The compositions of the obtained CdS layers were analyzed by Auger electron spectroscopy (AES). The performance of the CIGS solar cells was discussed on the basis of the characteristics of the chemical bath deposited layer. We have successfully fabricated a high-efficiency CIGS solar cell with an efficiency of 17% using a US layer with stoichiometric composition.     

Membranes based on phosphotungstic acid and polybenzimidazole for fuel cell application

Composite membranes based on phosphotungstic acid (PWA) adsorbed on silica (SiO₂) and polybenzimidazole (PBI) have been prepared and their physico-chemical properties have been studied. The membranes with high tensile strength and thickness of less than 30 μm can be cast. They are chemically stable in boiling water and thermally stable in air up to 400°C. Proton conductivity is influenced by the temperature (range: 30–100°C), relative humidity and PWA loading in the membrane. Maximum conductivity of 3.0×10⁻³ S/cm is obtained at 100% relative humidity and 100°C with membrane containing 60 wt.% PWA/SiO₂ in PBI. Conductivity measurements performed at higher temperatures, in the range from 90°C to 150°C, give almost stable values of 1.4–1.5×10⁻³ S/cm at 100% relative humidity.     

Method of gasification in IGCC system

A coal gasifier is designed to operate at the temperature range of 1200–1300 °C. The 1200 °C sets the lower limit to the carbon reforming efficiency of the high temperature reformer, and the 1300 °C is the lower limit of the fluid temperature of coal slags, below which they may be collected as non-fluid slag. The gasifier is connected to two syngas burners where a portion of product syngas is combusted with O₂ gas and produce ultra hot H₂O and CO₂ gases, these two gases enter into the gasifier and maintain the gasifier temperature at above 1200 °C and reform carbon into syngas. The temperature of the gasifier is controlled by the flow of O₂ gas into the syngas burner, where O₂ gas is completely consumed and none left to enter into the gasifier. This removes any possibility of forming oxidated products, and compressed CO₂ gas spray coal powder into the gasifier column and non-fluid slag is collected at the bottom. A higher level integration of oxidation–reduction cycle is shown for a IGCC system, wherein the exhaust gas of syngas turbine drives the reduction reaction of coal gasification.

A smooth and uniform temperature control within the gasifier assures high efficiency of carbon reforming and quality of product syngas. Conventional Lurgi gasifier relies on its large heat capacity and accumulating coal slag along the inner walls of the gasifier has made the gasifier bigger, lately as large as a three story building. The gasifier of the present design is constructed much smaller in its size, but with greater reforming efficiency.     

Low-temperature catalytic partial oxidation of hydrocarbons (C1–C10) for hydrogen production

The catalytic partial oxidation of hydrocarbons to provide hydrogen for fuel cells, mobile or stationary, requires high temperatures (900°C), multireactors and incurs the highest incremental costs for the gasoline fuel processor. New experimental data between 500°C and 600°C, supported by equilibrium calculations, show that hydrogen with low carbon monoxide concentrations can be produced from liquid and gaseous hydrocarbons, thus simplifying the reactor chain. Low sulphur refinery feeds (C₄–C₆, C₄–C₁₀), simulated natural gas (C₁–C₃) and single compounds are used and safety procedures discussed. Results from laboratory reactors with 1 wt% rhodium on mixed oxide catalysts show that hydrogen rates of 43,000 lH₂/h/l reactor (power density 129 kWth/l reactor) are produced with RON=95 feeds. However, the cost and availability of rhodium limit the catalyst rhodium content to 0.1 wt% when 31,100 lH₂/h/l reactor were measured. Optimisation and reactor scale-up for heat management is in progress.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).