Oxidative photoelectrochemical technology with Ti/TiO2 anodes

Rutile and anatase TiO₂ films have been grown on Ti plates by thermal (500–800°C) and anodic oxidation followed by thermal annealing (400–500°C), respectively. The photoelectrochemical efficiency of these photoanodes, evaluated by current density measurements in the photooxidation of 4-methoxybenzyl alcohol in deaerated CH₃CN, has been determined. The photocurrent efficiency increases with the thickness of the TiO₂ rutile film up to 1 μm (the most efficient thickness). At the wavelengths furnished by the irradiation apparatus similar thicknesses of anatase and rutile films show nearly the same efficiencies. Anodic bias produces similar relative increases of current intensity in both crystalline forms.     

Gettering effects by aluminum upon the dark and illuminated I–V characteristics of N–P–P silicon solar cells

Impurity gettering is an essential process step in silicon solar cell technology. A widely used technique to enhance silicon solar cell performance is the deposition of an aluminum layer on the back surface of the cell, followed by a thermal annealing. The aluminum thermal treatment is typically done at temperatures around 600°C for short times (10–30 min). Seeking a new approach of aluminum annealing at the back of silicon solar cells, a systematic study about the effect the above process has on dark and illuminated I–V cell characteristics is reported in this paper. We report results on silicon solar cells where annealing of aluminum was done at two different temperatures (600°C and 800°C), and compare the results for cells with and without aluminum alloying. We have shown that annealing of the aluminum in forming gas at temperatures around 800°C causes improvement of the electrical cell characteristics. We have also made evident that for temperatures below 250 K, the predominant recombination process for our cells is trap-assisted carrier tunneling for both annealing temperatures, but it is less accentuated for cells with annealing of aluminum at 800°C. For temperatures above 250 K, the recombination proceeds through Shockley–Read–Hall trap levels, for cells annealed at both temperatures. Furthermore, it seems from DLTS measurements that there is gettering of iron impurities introduced during the fabrication processes. The transport of impurities from the bulk to the back surface (alloyed with aluminum) reduces the dark current and increases the effective diffusion length as determined from dark I–V characteristics and from spectral response measurements, respectively. All these effects cause a global efficiency improvement for cells where aluminum is annealed at 800°C as compared to conventional cells where the annealing was made at 600°C.     

Effect of porous silicon on the performances of silicon solar cells during the porous silicon-based gettering procedure

In this work we analyse the effect of porous silicon on the performances of multicrystalline silicon (mc-Si) solar cells during the porous silicon-based gettering procedure. This procedure consists of forming PS layers on both front and back sides of the mc-Si wafers followed by an annealing in an infrared furnace under a controlled atmosphere at different temperatures. Three sets of samples (A, B and C) have been prepared; for samples A and B, the PS films were removed before and after annealing, respectively. In order to optimize the annealing temperature, we measure the defect density at a selected grain boundary (GB) using the dark current–voltage (I–V) characteristics across the GB itself. The annealing temperature was optimized to 1000 °C. The effect of these treatments on the performances of mc-Si solar cells was studied by means of the current–voltage characteristic (at AM 1.5) and the internal quantum efficiency (IQE). The results obtained for cell A and cell B were compared to those obtained on a reference cell (C).     

Influence of annealing in different chlorides on the photovoltaic parameters of CdS/CdTe solar cells

This paper investigates CdS/CdTe solar cells treated with various chloride solutions and then annealed in the air before the deposition of the back contact. It was found that the cells with higher efficiency were fabricated using CdCl₂ solution treatment and thermal annealing. Such cells are further examined by measuring their current density vs. voltage (J–V) characteristics at various temperatures, quantum efficiency (QE), and capacitance vs. voltage (C–V) characteristics. In addition, the uniformity of the QE across the cells was examined by using the laser beam induced current (LBIC) technique.     

Investigation of pulsed non-melt laser annealing on the film properties and performance of Cu(In,Ga)Se2 solar cells

Pulsed non-melt laser annealing (NLA) has been used for the first time to modify near-surface defects and related junction properties in Cu(In,Ga)Se₂ (CIGS) solar cells. CIGS films deposited on Mo/glass substrates were annealed using a 25 ns pulsed 248 nm laser beam at selected laser energy density in the range 20–60 mJ/cm² and pulse number in the range 5–20 pulses. XRD peak narrowing and SEM surface feature size increase suggest near-surface structure changes. Dual-beam optical modulation (DBOM) and Hall-effect measurements indicate NLA treatment increases the effective carrier lifetime and mobility along with the sheet resistance. In addition, several annealed CdS/CIGS films processed by NLA were fabricated into solar cells and characterized by photo- and dark-J–V and quantum efficiency (QE) measurements. The results show significant improvement in the overall cell performance when compared to unannealed cells. The results suggest that an optimal NLA energy density and pulse number for a 25 ns pulse width are approximately 30 mJ/cm² and 5 pulses, respectively. The NLA results reveal that overall cell efficiency of a cell processed from an unannealed film increased from 7.69% to 13.41% and 12.22% after annealing 2 different samples at the best condition prior to device processing.     

Microcrystalline materials and cells deposited by RF glow discharge

Recent developments in depositing high quality intrinsic and doped microcrystalline Si at low temperatures, 100–140°C, and high rates of 6 nm/s are reviewed. A new high-pressure depletion deposition method is described that suppresses ion bombardment and yields low defect densities. Passivation of oxygen related donors by hydrogen is discussed as well as the dissociation of passivated B–H dopant atoms by annealing at 200°C. Interface damage effects on superstrate and substrate cells are identified. The influence of the microcrystalline texture formed spontaneously during growth on optimizing optical confinement has been studied.     

Prospects for coal gasification in Pakistan

Pakistan is currently facing serious energy supply problems. Energy demand has been increasing by about 8% per year during the last 12yr and this trend is likely to continue. Since 1980–1981 the oil import bill has been consuming more than 50% of yearly export earning. As there is not much scope for a sizeable increase in the domestic supply of gas, oil, or hydroelectric power, increasing the use of domestic coal is necessary to avoid excessive dependence on imported energy. Coal gasification to produce substitute natural gas (SNG) is not economical at present coal production costs, due to the low cost of indigenous gas and subsidized furnace oil and kerosene and the high SNG production costs from the technology available at present. If domestic prices of gas and liquid fuels are increased to the level of current international oil prices and developments in coal gasification technologies can bring about expected reductions in capital costs and improvements in efficiency, coal gasification may become economical in Pakistan. It is estimated that indigenous coal resources can potentially supply 3–6 million TCE/yr of SNG by 2000—about 10–20% of the substitutable fossil fuels demand for that year—along with meeting about 9% of the electricity demand.     

Spectrally dependent photocurrent generation in aggregated MEH-PPV:PPDI donor–acceptor blends

Films of MEH-PPV and PPDI blends with weight ratio 1:2 have been prepared by spin-coating and annealing between 0 and 60 min at 95 °C. The films were characterized by absorption spectroscopy and atomic force microscopy (AFM). The main emphasis has been on the photon conversion efficiency in the photovoltaic cells as a function of excitation wavelength and applied voltage/electric field. Site selective excitation at wavelengths at which either non-aggregated bulk PPDI or dimers/aggregates of PPDI absorb prove that (i) the rate-limiting process for power conversion is the field-assisted escape of optically generated geminate electron–hole pairs from their mutual coulombic potential and (ii) the photogeneration yield depends on the donor–acceptor topology. A significant difference of the yield has been noted when alkoxy-pendent groups in MEH-PPV are replaced by phenyl-alkoxy groups.     

Degradation of transparent metal–insulator–semiconductor solar cells due to heating effects

All the output parameters of the metal–insulator–semiconductor solar cells are degraded after heating. Also the dark current and the non-ideality factor are increased with heating. A reduction in the built-in potential has been detected. The capacitance–voltage–frequency measurements indicate the presence of interface states. These states are heavily occupied by electrons. Heating will increase the density of these states and consequently reduce the barrier height and the overall cell efficiency.     

Crystallographic analysis of high quality poly-Si thin films deposited by atmospheric pressure chemical vapor deposition

Crystallinity of thin film polycrystalline silicon (poly-Si) grown by atmospheric pressure chemical vapor deposition has been investigated by X-ray diffraction measurement and Raman spectroscopy. Poly-Si films deposited at high temperatures of 850–1050°C preferred to 2 2 0 direction. By Raman spectroscopy, the broad peak of around 480–500 cm⁻¹ belonged to microcrystalline Si (μc-Si) phase was observed even for the poly-Si deposited at 950°C. After high-temperature annealing (1050°C) 3 3 1 direction of poly-Si increased. This result indicates that the μc-Si phase at grain boundary became poly-Si phase preferred to 3 3 1 direction by high-temperature annealing. Effective diffusion length of poly-Si films deposited at 1000°C was estimated to be 11.9–13.5 μm and 10.2–12.9 μm before and after annealing, respectively.     

Thin film CdTe-CdS heterojunction solar cells on lightweight metal substrates

An unconventional thin film CdTe-CdS solar cell device configuration, in which the substrate is molybdenum foil and CdS is deposited on top of CdTe, was developed. A conducting contact between Mo and CdTe was achieved by using thin interlayers. The CdTe–CdS junction process development led to an open circuit voltage of 824 mV. CdCl₂ treatment followed by oxygen annealing greatly improved solar cell performance. An initial study of capacitance–voltage data has shown a depletion layer width of 4.62 μm into CdTe in the dark. Cell efficiency of 5.3% was achieved. Higher cell efficiency is now being sought by improving the top contact to CdS and lowering series resistance.     

Preparation and characterization of novel nickel–palladium electrodes supported by silicon microchannel plates for direct methanol fuel cells

A novel anode structure based on the three-dimensional silicon microchannel plates (Si-MCP) is proposed for direct methanol fuel cells (DMFCs). Ni–Pd nanoparticles produced by electroless plating onto the Si-MCP inner sidewalls and followed by annealing at 300 °C under argon serve as the catalyst. In order to evaluate the electroactivity of the nanocomposites, Ni–Pd/silicon composites synthesized by the same method are compared. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electrochemical methods are employed to investigate the Ni–Pd/Si-MCP anode materials. As a result of the synergetic effects rendered by the MCP and Ni–Pd nanoparticles, the Ni–Pd/Si-MCP nanocomposites exhibit superior electrocatalytic properties towards methanol electro-oxidation in alkaline solutions, as manifested by the negative onset potential and strong current response to methanol even during long-term cyclical oxidation of methanol. This new structure possesses unique and significant advantages such as low cost and integratability with silicon-based devices.     

Device performance characterization and junction mechanisms in CdTe/CdS solar cells

The performance of CdTe/CdS solar cells has been successfully characterized in terms of a device model based on Shockley–Read–Hall (SRH) recombination theory. The model has been applied to a large number of devices from our laboratory in the 10–15% efficiency range and is able to provide key insights into the diode properties of our devices and the fundamental mechanisms that determine performance. Methods for the reliable measurement of key device parameters are presented, and the results are verified by simulating the characterization data in a self-consistent manner. Crossover between the dark and light J–V curves has been identified as a front contact phenomenon arising from the presence of CdS. Junction mechanisms and an observed relationship between reverse saturation current and diode quality factor are discussed. Our techniques indicate that all values of diode quality factor are between 1 and 2 which is consistent with SRH recombination theory and explainable in terms of the location and lifetimes of the recombination centers. It is found that devices with large diode factors are dominated by midgap states. Reduction of midgap states results in a reduction of the diode factor and improved performance.     

Powder profile studies in electrodeposited cuprous oxide films

Thin films of copper oxide have been electrodeposited cathodically on copper substrates at room temperature. The deposited films have been characterized by X-ray diffraction and scanning electron microscopy. XRD showed the formation of crystalline cuprous oxide (Cu₂O). The XRD peaks are found to be shifted towards lower angle with narrowing of the profiles and the lattice parameter increases with annealing temperature. Scanning electron micrographs showed the formation of localized grain growth region which may be due to the non-uniform deposition of the films. The X-ray diffraction line broadening analysis of the as deposited as well as annealed films have been studied in order to evaluate the microstructural parameters which characterize the microstructural changes. The microstructural parameters like coherent domain size, RMS microstrain and dislocation density have been calculated using Warren-Averbach (Fourier) for multiple order, integral breadth (single and multiple line) and Williamson–Hall plot. The results of analysis obtained by different methods have been compared. The coherent domain size and RMS strain are not found to change appreciably with the increase of film thickness (4–13 μm). The optimum pH value of the electrolytic solution to deposit the films is found to lie in the range 9.2–9.3 where the strain variation is small compared to other pH values. The values of crystallite size and strains obtained by Warren–Averbach method and integral breadth method are comparable. However, Williamson–Hall plot overestimates the values of these two parameters. It is found that the crystallite size increases and RMS strain decreases with the increase of annealing temperature. The dislocation density is also found to decrease with annealing temperature.     

2D-numerical analysis and optimum design of thin film silicon solar cells

Device modeling for p–i–n junction μc-Si basis thin film polycrystalline Si solar cells has been examined with a simple model of columnar grain structure and its boundary condition utilizing two-dimensional device simulator. As the simulation results of solar cell characteristics show, open-circuit voltage (V_oc) and curve fill factor (FF) considerably depend on those structural parameters, while short-circuit current density (J_sc) is comparatively stable by courtesy of homogeneous built-in electric field in the i layer. It has also been found that conversion efficiency over 12% could be expected with 1 μm grain size and well-passivated condition with 3 μm thick i-layer.     

High-rate microcrystalline silicon deposition for p–i–n junction solar cells

We have developed a high-rate plasma process based on high-pressure and silane-depletion glow discharge for highly efficient microcrystalline silicon (μc-Si:H) p–i–n junction solar cells. Under high-rate conditions (2–3 nm/s), we find that the deposition pressure becomes the dominant parameter in determining solar-cell performance. With increasing deposition pressure from 4 to 7–9 Torr, short-circuit current increases by 50% due to a remarkable improvement in quantum efficiencies at the visible and near infrared. As a result, the maximum efficiency of 9.13% has been achieved at an i-layer deposition rate of 2.3 nm/s. We attribute the improved performance of high-pressure-grown μc-Si:H solar cells to the structural evolution toward denser grain arrangement that prevents post-oxidation of grain boundaries.     

Energy and exergy analysis of steam cooled reheat gas–steam combined cycle

This paper deals with parametric energy and exergy analysis of reheat gas–steam combined cycle using closed-loop-steam-cooling. Of the blade cooling techniques, closed-loop-steam-cooling has been found to be superior to air-film cooling. The reheat gas–steam combined cycle plant with closed-loop-steam-cooling exhibits enhanced thermal efficiency (around 62%) and plant specific work as compared to basic steam–gas combined cycle with air-film cooling as well as closed-loop-steam cooling. Further, with closed-loop-steam-cooling, the plant efficiency, reaches an optimum value in higher range of compressor pressure ratio as compared to that in film air-cooling. It has also been concluded that reheat pressure is an important design parameter and its optimum value gives maximum plant efficiency.Component-wise inefficiencies of steam cooled-reheat gas–steam combined cycle based on the second-law-model (exergy analysis) have been found to be the maximum in combustion-chamber (≈30%), followed by that in gas turbine (≈4%).     

Battery behavior prediction and battery working states analysis of a hybrid solar–wind power generation system

Lead–acid batteries used in hybrid solar–wind power generation systems operate under very specific conditions, and it is often very difficult to predict when the energy will be extracted from or supplied to the battery. Owing to the highly variable working conditions, no battery model has achieved a good compromise between the complexity and precision. This paper presents a simple mathematical approach to simulate the lead–acid battery behaviors in stand alone hybrid solar–wind power generation systems. Several factors that affect the battery behaviors have been taken into account, such as the current rate, the charging efficiency, the self-discharge rate, as well as the battery capacity. Good agreements were found between the predicted results and the field measured data of a hybrid solar–wind project. At last, calculated from 1-year field data with the simulation model, the time-series battery state-of-charge (SOC) has been statistically analyzed considering the monthly and hourly variations as well as the probability distributions. The results have shown the battery working states in the real hybrid solar–wind power generation system.     

Comparative study of different approaches of multicrystalline silicon texturing for solar cell fabrication

Alkali etchant cannot produce uniformly textured surface to generate satisfactory open circuit voltage as well as the efficiency of the multi-crystalline silicon (mc-Si) solar cell due to the unavoidable grain boundary delineation with higher steps formed between successive grains of different orientations during alkali etching of mc-Si. Acid textured surface formed by using chemicals with HNO₃–HF–CH₃COOH combination generally helps to improve the open circuit voltage but always gives lower short circuit current due to high reflectivity. Texturing mc-Si surface without grain boundary delineation is the present key issue of mc-Si research. We report the isotropic texturing with HF–HNO₃–H₂O solution as an easy and reliable process for mc-Si texturing. Isotropic etching with acidic solution includes the formation of meso- and macro-porous structures on mc-Si that helps to minimize the grain-boundary delineation and also lowers the reflectivity of etched surface. The study of surface morphology and reflectivity of different mc-Si etched surfaces has been discussed in this paper. Using our best chemical recipe, we are able to fabricate mc-Si solar cell of 14% conversion efficiency with PECVD AR coating of silicon nitride film. The isotropic texturing approach can be instrumental to achieve high efficiency in mass production using relatively low-cost silicon wafers as starting material with the proper optimization of the fabrication steps.     

Oxygen in Czochralski silicon used for solar cells

Compared to the Czochralski (CZ) silicon used in microelectronic industry (M-CZ Si), the annealing behavior of oxygen in the CZ silicon used for solar cells (S-CZ Si) was investigated by means of FTIR and SEM. It was found that the oxygen concentration in S-CZ Si crystal was lower than in the M-CZ Si crystal. During single-step annealing in the temperature range of 800–1100°C, the oxygen in S-CZ Si was hard to precipitate, even if the material contained higher carbon concentrations. After pre-annealing at 750°C, many more oxygen precipitates were formed. The amount and density of the oxygen precipitates were almost the same as in M-CZ Si annealed in single step. It is considered that oxygen has no significant influence on the efficiency of solar cells made from Cz silicon if it is annealed only by a single step in the range of 800–1100°C.