Effect of copper diffusion on photovoltaic characteristics of CuGaSe2–GaAs cells

CuGaSe₂–GaAs heterojunctions were fabricated by fast evaporation of polycrystalline CuGaSe₂ from a single source on n-type GaAs substrates. The best CuGaSe₂–GaAs photocell (without an antireflective coating) exhibited an efficiency of 11.5%, J_sc=32 mA/cm², V_oc=610 mV and FF=0.60. The spectral distribution of photosensitivity of CuGaSe₂–GaAs junctions extends from 400 to 900 nm. The CuGaSe₂ films were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. XRD analysis indicated that the thin films were strongly oriented along the (1 1 2) plane. SEM studies of CuGaSe₂ films showed nearly stoichiometric composition with grain size about 1–2 μm. The energy dispersive X-ray spectroscopy (EDX) analysis of Cu concentration distribution in n-type GaAs showed that Cu diffused from the film into n-type GaAs during the growth process resulting in formation of the latent p–n homojunction in substrate. The diffusion coefficient of Cu in GaAs at growth temperature (520°C) estimated from EDX measurements was 6×10⁻⁸ cm²/s.     

Partitioning of rare earths and some major elements in the Kizildere geothermal field, Turkey

Rare earth elements and yttrium (REY), Na⁺, K⁺, Cl⁻, and Ca²⁺ were determined in water, steam, mineral scale and rock samples from the Kizildere geothermal field, Turkey. The CO₂-rich parent fluid originates from a sequence of mica schists with marble intercalations. The chemistry of the parent fluid varies with location and time. The average REY composition of the fluids is derived by extrapolation to the lowest Ca concentrations. The apparent vapor–liquid partitioning factors for REY at 145 °C and 5 bar total absolute pressure are about 0.2, whereas for Ca²⁺, Na⁺, K⁺ and Cl⁻ they are <0.05, about 0.0005, 0.0005 and about 0.02, respectively. Apparent scale-liquid distribution coefficients for REY at 145 and 190 °C are about 0.15 and 0.55, whereas at 100 °C they increase from 0.3 (La) to 1.5 (Lu).     

Fermentative hydrogen production by two novel strains of Enterobacter aerogenes HGN-2 and HT 34 isolated from sea buried crude oil pipelines

Present study investigated fermentative hydrogen production of two novel isolates of Enterobacter aerogenes HGN-2 and HT 34 isolated from oil water mixtures. The two isolates were identified as novel strains of E. aerogenes based on 16S rRNA gene. The batch fermentations of two strains from glucose and xylose were carried out using economical culture medium under various conditions such as temperature, initial pH, NaCl, Ni⁺/Fe⁺⁺, substrate concentrations for enhanced fermentation process. Both the strains favoured wide range of pH (6.5–8.0) at 37 °C for optimum production (2.20–2.23 mol H₂/mol-glucose), which occurred through acetate/butyrate pathway. At 55 °C, both strains favoured 6.0–6.5 and acetate type fermentation was predominant in HT 34. Hydrogen production by HT 34 from xylose was highly pH dependant and optimum production was at pH 6.5 (circa 1.98 mol-H₂/mol-xylose) through acetate pathway. The efficiency of the strain HGN-2 at pH 6.5 was 1.92–1.94 mol-H₂/mol-xylose, and displayed both acetate and butyrate pathways. At 55 °C, very low hydrogen production was detected (less than 0.5 m mol/mol-xylose).     

Electrosynthesis and characterization of GaAs in acid solutions by potentiostatic method

Gallium arsenide (GaAs) is one of the important materials used for the fabrication of light emitting diodes, solar cells, microwave devices, etc. In the present work, electrodeposition of GaAs was successfully carried out potentiostatically from an aqueous solution mixture of gallium chloride (GaCl₃) and arsenic oxide (As₂O₃). The optimum deposition potential, pH and bath temperature to synthesize GaAs thin films are found to be −0.8 V versus SCE, 2.0±0.1 and 60 ^°C, respectively. The effects of solution pH, bath temperature and deposition potential on the gallium content of GaAs films are studied. Photoelectrochemical (PEC) solar cells using n-GaAs photo-anode in a polysulphide electrolyte is constructed and I–V, C–V studies are carried out. Various semiconductor parameters such as, flat-band potential, band bending, donor density, depletion layer width are evaluated and the results are discussed.     

The study on high efficient AlxGa1−xAs/GaAs solar cells

The investigation of Al_xGa_1−xAs/GaAs solar cells is carried out by means of both metalorganic chemical vapor deposition (MOCVD) and liquid-phase epitaxial (LPE) technique. The measurements of illuminated I–V characteristics, dark I–V characteristics and quantum efficiencies were performed for the GaAs solar cells made in author's laboratory. The measuring results revealed that the quality of materials in GaAs solar cell's structures is the key factor for getting high-efficient GaAs solar cells, but the effect of post-growth technology on the performances of GaAs solar cells is also very strong. The 21.95% (AM0, 2×27 cm², 25°C) high conversion efficiency in a typical GaAs solar cell has been achieved owing to improving the quality of materials as well as optimizing the post-growth technology of devices.     

Current–voltage characteristics of electrovoltaic and electrophotovoltaic cells

Electrovoltaic (EV) effect provides a way of generating voltage across an unbiased junction under dark. Electrovoltaic (EV) cell in its simplest form is a device based on n⁺–p–n⁺ (or p⁺–n–p⁺) like structure in which if one p–n junction is subjected to an external forward bias, then, a voltage is developed across the other p–n junction such that the n-side gets a negative polarity with respect to the p-side. Connecting to a load across one of the n⁺–p junctions a bipolar transistor can be operated as a three-terminal EV cell. A new device henceforth known as electrophotovoltaic (EPV) cell wherein EV and PV effects could be expected to work cooperatively was also realized. It is based on a structure which is a combination of n⁺–p–n⁺ EV and n⁺–p–p⁺ photovoltaic (PV) cell structures having a common n⁺–p junction and is able to operate in EV, PV and EPV modes. We have developed one-dimensional physical models of EV and EPV cells and have applied them to explain the observed I–V characteristics of an n–p–n silicon bipolar transistor 2N3055 in EV mode and the EPV cell in EV, PV and EPV modes. While the photovoltaic efficiency η_PV decreases slowly with d/L, where d is the thickness and L is the diffusion length of minority carriers in the base region, the electrovoltaic efficiency η_EV has a strong dependence on d/L and decreases sharply with increase in d/L. Transistor 2N3055 with d/L=0.7 demonstrated η_EV>60%, whereas, our EPV cell with d/L>2.7 had η_EV<3%. However, in the EPV cell, the PV and the EV effects were indeed found to work cooperatively and the output power was enhanced in the EPV mode over the PV mode value although the efficiency η_EPV was less than 4.5%. To achieve substantially high values of efficiencies in EV and EPV modes the EPV cell should be designed to have d/L1.     

The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis

The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO₂·xH₂O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO₂ structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.     

Synthesis, photophysics of two new perylene bisimides and their photovoltaic performances in quasi solid state dye sensitized solar cells

Two new symmetrical compounds A and P based on perylene-anthracene and perylene-pyrene, respectively, were synthesized and characterized by FT-IR, ¹H NMR, TGA and TMA. These compounds contained tert-butyl groups which enhanced their solubility, decomposed above 400 °C and gave char yields of 46–65% at 800 °C in N₂. Compound A showed significantly higher glass transition temperature (124 °C) than P (75 °C). Their absorption spectra were broad with longer wavelength absorption at 467–525 nm and optical band gap of 2.05 eV. The solutions of the compounds emitted green-yellow light with maximum at 555 nm, while their films were not photoluminescent. The compound A shows better photovoltaic response than compound P. Quasi solid state dye sensitized solar cells (DSSCs) have been fabricated employing compound A as sensitizer and polymer sol gel as electrolyte and characterized through the current–voltage characteristics in dark as well as under illumination and electrochemical impedance spectra. We found that the Al₂O₃ modification of TiO₂ layer significantly improves the dye absorption resulting in enhancement of power conversion efficiency (PCE) (from 1.15 to 2.13%) which is attributed to the increase in electron lifetime and reduction in back transfer of electrons. Finally, the TiO₂ has been incorporated into the polymer electrolyte gel to improve the power conversion efficiency (3.42%) of the quasi solid state DSSC. The faster electron diffusion in the device, the high ionic conductivity and the low activation energy of the polymer electrolyte are also responsible for enhanced PCE, when TiO₂ nano-particles are incorporated in the polymer electrolyte.     

Contact resistivities of sputtered TiN and Ti---TiN metallizations on solar-cell-type-silicon

Contact resistivities of TiN and Ti---TiN contacts on a shallow junction solar-cell-type silicon substrate have been investigated. The contact materials were sputter-deposited. The method of the transmission line model was applied for contact resistivity measurements. The contact resistivity of the n⁺Si---TiN contact system was 2 × 10⁻³ Ωcm² ± 50 per cent and remained constant after annealing up to 700°C for 30 min. For the n⁺Si---Ti---TiN system, the contact resistivity of 9 × 10⁻⁴ Ωcm² ± 50 per cent was measured. A heat treatment of 700°C. 30 min decreases this value by one order of magnitude and the interposed Ti fully reacts with Si and forms a TiSi₂ layer. The voltage drop caused by the n⁺Si---TiN contact system in a standard non-concentrator solar cell is negligible. The n⁺Si---TiSi₂---TiN contact system should be acceptable for Si solar cells used at up to 100 times solar concentration.     

Development of an emitter for industrial silicon solar cells using the doped oxide solid source diffusion technique

The aim of this paper is to demonstrate for the first time the feasibility of fabricating large-area screen-printed monocrystalline silicon solar cells using the Doped Oxide Solid Source (DOSS) diffusion technique. This process was applied to form the n⁺p emitter junction from highly doped sources prepared in a POCl₃ ambient. The diffusions were performed under a pure nitrogen flow in the temperature range 900–1050°C. In this investigation attention was devoted to the influence of the source doping level on the fill factor. The solar cells were fabricated on industrial quality 4-inch Cz wafers using a simple processing sequence incorporating screen-printed contacts and a TiO₂ antireflection coating deposited by spin-on. Fill factors as high as 79% were obtained. The potential benefit of retaining for passivation purposes the thin residual oxide grown during phosphorus diffusion was evaluated. These first experiments led to a cell efficiency close to 10%.     

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.     

Electrochromic properties of Nb2O5 sol–gel coatings

Sol–gel niobium oxide coatings are promising electrochromic materials. The sols have been prepared by a sonocatalytic mixing of NbCl₅ powder, butanol and acetic acid. Thermal analysis (DTA/TG) coupled to mass spectrometry has been performed on Nb₂O₅ precipitates to quantitatively analyse the effluents. Transparent and defect-free single and multilayers coatings have been deposited on ITO-coated glass by a dip-coating process and then calcined between 400°C and 600°C. The coatings structure change from amorphous to crystalline (TT form) and the later ones are highly textured. The films present a reversible and fast insertion/extraction kinetics for Li⁺ ions. After insertion the amorphous coatings present a grey-brown color, while the crystalline ones are dark blue. The maximum charge density exchanged with a three-layer 200 nm thick coating sintered at 600°C was 16 mC/cm² with a corresponding spectral transmission change practically wavelength-independent varying from 80% to 20%. The coloring efficiency determined at λ=600 nm was 22 cm²/C.     

Performance of solar driven methanol–water combined ejector–absorption cycle in the Athens area

The performance of a solar driven CH₄O-H₂O combined ejector– absorption unit, operating in conjunction with intermediate temperature solar collectors in Athens, is predicted along the five months (May–September) in case of the unit working as heat pump in an industrial area. The operation of the unit and the related thermodynamics are simulated by suitable computer codes and the required local climatological data are determined by statistical processings over a considerable number of years. It is found that the heat gain factor varies in the range from 2.1330 to 2.4481 for the above period of time. The maximum HGF of about 2.4481 is obtained in July at 14.25 hrs with corresponding specific heat gain power 915 W/m². The maximum Q_gain of about 1086 W/m² is obtained in June at 12.54 hrs with corresponding HGF 2.3572. Also the maximum value of HGF was estimated by correlation of three temperatures: generator temperature (85.0°C–97.2°C), condenser temperature (43.3°C–47.6°C) and evaporator temperature (12.6°C–25.4°C).     

New Li ion-conducting ormolytes

The preparation and characterization of two new families of lithium-conducting solid-state electrolytes is reported. Both systems are silica (SiO₂) – polyethyleneglycol (PEG_n) hybrid materials with (type I) or without (type II) covalent organic-inorganic chemical bonds. Their electrical conductivity has been studied by complex impedance spectroscopy between 20°C and 100°C in the frequency range 1 Hz to 10 MHz as a function of the polymer chain length (200<n<1900), polymer concentration and lithium concentration (4<[O]/[Li]<80). The highest room-temperature ionic conductivity (σ6×10⁻² S cm⁻¹) has been found for type II material for ratios [O]/[Li]=15 and PEG₃₀₀/TEOS=1.0. The effect of the chain length on the polymer mobility has been studied by nuclear magnetic resonance by measuring the Li⁺ line widths and the spin-lattice relaxation time T₁ between -100°C and +100°C. The bonded chain mobility increases with the chain length (type II) while the opposite occurs with unbonded chain material (type I). Both types of materials present high ionic conductivity at room temperature and are adequate as Li⁺-conducting electrolyte in all solid-state electrochemical devices.     

CdTe/CdS Solar cells on flexible molybdenum substrates

Development of CdTe/CdS solar cells on flexible metallic substrates is highly interesting due to the light weight and flexible nature of the solar modules. We have deposited CdTe films onto flexible molybdenum substrates using close-spaced sublimation technique and the CdTe/CdS junction was developed by depositing a thin layer of CdS onto the CdTe substrate from a chemical bath. The devices were characterized by Current–voltage (I–V) and photocurrent spectroscopy techniques. Prior to the deposition of the transparent conducting layer, the devices were annealed in air at different temperatures and found that the devices annealed at 400°C have better photovoltaic parameters. The efficiency of a typical device under 60 mW cm⁻² illumination was estimated as 3.5%.     

Phosphorus-doped, silver-based pastes for self-doping ohmic contacts for crystalline silicon solar cells

Undoped and phosphorus-doped Ag-based pastes were applied as circular contacts to the (1 1 1) surface of dendritic web n-type Si. Current–voltage characteristics of as-deposited contacts and contacts annealed at 780°C for 10 min, 950°C for 5 min, 1000°C for 10 min were measured and compared. Annealing above the Ag–Si eutectic temperature (835°C) yielded Si precipitation within the Ag matrix, resulting in increased current across the metal/semiconductor interface. The contact resistivity was significantly lower for P-doped (<0.04 Ω cm²) than for undoped (1.90 Ω cm²) Ag contacts, both of which were annealed at 1000°C. As supported by secondary ion mass spectrometry analyses, these results are attributed to an enhanced P doping level in the Si substrate after annealing the P-doped contacts. A p–n junction diode was demonstrated by alloying the Ag–P paste with p-type Si at 1000°C. The contact resistance was inferred from diode I–V data to be 0.013 Ω cm², a value which is comparable to the 0.010 Ω cm² target value for solar cell contacts.     

Back contact formation using Cu2Te as a Cu-doping source and as an electrode in CdTe solar cells

Cu₂Te was utilized as a Cu source for p⁺ doping in CdTe and as a primary back contact material in CdTe solar cells. A 60 nm-thick Cu₂Te layer was deposited on CdTe film by evaporating Cu₂Te and the samples were annealed at various temperatures. An amorphous layer was found at the Cu₂Te/CdTe interface, while the Cu₂Te has both orthorhombic and hexagonal phases. Annealing at 200°C completely crystallized the amorphous interlayer and enhanced the transformation of orthorhombic phase into hexagonal phase that has a coherent interface with CdTe. A good p⁺ contact was formed at 180°C annealing, where the series resistance of CdTe cells was a minimum of 0.5 Ω·cm² and the fill factor and open-circuit voltage were significantly improved. With the good p⁺ contact, it is possible to determine the exact dopant profile at the CdS/CdTe junction.     

Advances in the development of an AlGaAs/GaAs cascade solar cell using a patterned germanium tunnel interconnect

In this paper, we discuss various aspects of the development of an inverted-grown AlGaAs/GaAs cascade solar cell incorporating a patterned germanium tunnel junction. Topics include the development of the Al_0.37Ga_0.63As top cell, the growth of the GaAs bottom cell over the patterned germanium tunnel junction, and a technique for selective removal of thin AlGaAs/GaAs heterostructures after lattice-matched growth of germanium substrates. The problems to be overcome for the achievement of around 30% efficiencies in the AlGaAs/GaAs cascade cell under concentrator applications are also discussed.     

Assessment of reservoir temperatures of thermal springs of the northern areas of Pakistan by chemical and isotope geothermometry

Chemical and isotope geothermometers, i.e. the Na–K, K–Mg, quartz and δ¹⁸O(SO₄–H₂O), have been applied to estimate the reservoir temperature of the thermal springs in the northern areas of Pakistan. The chemical types of the thermal waters and the effects of mixing of shallow cold water with the thermal end-members are discussed. These waters are neutral to slightly alkaline and have low dissolved contents. Sodium is the dominant cation in almost all the cases. In terms of anions, the hot waters of Budelas are of the SO₄ type, those of Tatta Pani are of mixed character (SO₄ and HCO₃), and the waters from the remaining areas show HCO₃ domination. An absence of tritium in Tatta Pani and Tato thermal springs indicates that they do not have any contribution of shallow young water. In the case of the Murtazabad springs, the wide range of tritium concentrations, negative correlations with surface temperature and Cl, and positive correlation between Na and Cl show that the shallow cold groundwater is mixing with thermal water in different proportions. For the mixed water of Murtazabad thermal springs, ‘isochemical modelling’ using the Na–K, K–Mg and quartz geothermometers indicates an equilibrium temperature in the range 185–200 °C. The δ¹⁸O(SO₄–H₂O) geothermometer gives relatively low temperatures for three springs, whereas two samples are close to the 185–200 °C temperature interval. The reservoir temperatures of Tatta Pani springs (100–120 °C), determined by Na–K and quartz geothermometers, are in good agreement. The δ¹⁸O(SO₄–H₂O) geothermometer gives a relatively higher range (140–150 °C) for most of the Tatta Pani springs. For Tato spring, the isotope and chemical geothermometers (except for the K–Mg) agree on an equilibrium temperature of about 170 °C. Reservoir temperatures of the remaining minor fields are not conclusive due to the lack of sufficient data.     

High-temperature optical properties and stability of AlxOy–AlNx–Al solar selective absorbing surface prepared by DC magnetron reactive sputtering

Al_xO_y–AlN_x–Al selective absorbing surface was prepared by DC magnetron reactive sputtering with aluminum alloy (LY13)¹ in air and argon. The studies were carried out to access the high-temperature (400°C–600°C) optical properties and stability of the coatings. The coatings were found to withstand heating at 600°C for 30 min in 4.5×10⁻³ Pa vacuum with absorptance 0.94 and emittance 0.07 after annealing. After heating at 450°C for 10 h, the specimen still had good performance whose absorptance and emittance was 0.93 and 0.07, respectively. Auger electron spectroscopy was used to analyse the structure of the solar selective surface before and after annealing.