A new method to determine the optimum load of a real solar cell using the Lambert W-function

An exact explicit solution based on the Lambert W-function is presented to express the optimum load of an illuminated solar cell containing a parasitic series resistance and a shunt resistance. The W-function expressions are derived using Matlab software. The method is validated by comparing the model-predicted results to the experimental data for three real solar cells. The impacts of the series resistance and shunt resistance on the optimum load are also studied and the results show a good consistency with the data reported in the literature.     

Unusual photocapacitance properties of a mono-crystalline silicon solar cell for optoelectronic applications

The current-voltage characteristics of mono-crystalline solar cell device under dark and illumination of 100 mW/m² (AM1.5) were measured. The efficiency of the studied device under AM1.5 was found to be 14.22%±0.2 compared with the company standards. The capacitance properties of mono-crystalline silicon solar cell device were investigated under dark and illumination conditions. The studied mono-crystalline silicon solar cell exhibits an unusual photocapacitance ranging from 50.4 to 4585 nF under dark and 100 mW/m² (AM1.5) of white light, respectively. The drastic increase in the capacitance of the solar cell is due to the space charge polarization induced by the increasing number of photogenerated carriers. The photocapacitance mechanism of the solar cell was interpreted by modified Goswami and Goswami (MGG) model. The relative capacitance C_ph/C_d (the ratio between the capacitance under illumination to the capacitance under dark) and the relative resistance R_ph/R_d (the ratio between the resistance under illumination to the resistance under dark) as a function of the applied frequency at different illuminations were interpreted. The values of the interface state density N_ss and interface capacitance C_ss are increased with the increasing illumination intensities. The prepared mono-silicon solar cell device is a good candidate for photocapacitive and photoresistive sensors in modern electronic and optoelectronic devices.     

Evaluating the optimum load range for box-type solar cookers

This paper introduces a new concept of Optimum Load Range (OLR) for solar cookers. OLR gives the load values for which cooker preferably shows good thermal as well as good cooking performance; it may be considered a crucial parameter for solar cookers. This OLR concept is based on the dependence of rate of rise of load temperature on different heat transfer processes between load and cooker interior. This concept illustrates solar cooking in two simple steps. The total time required to complete these steps puts an essential constraint for cooking of any load amount. The maximum value of load (upper limit of OLR) till which cooker shows satisfactory cooking may be determined from this constraint. This constraint requires determination of two OLR parameters which are t_{step I} and t_{step II}. The load for which cooker remain almost 30% efficient, may be referred as lower limit (minimum value) of OLR. For the verification of OLR, experimental studies have been conducted with a solar cooker named SFSC. The OLR parameters along with different thermal performance parameters (TPPs) (second figure of merit (F₂), utilization efficiency (η_u) etc.) suggested by different researches for solar cookers in water load condition have been computed from the measured thermal profiles of different loads (0.8–3.0 kg). From the curve analysis of different TPPs with load, the existence of upper limit of OLR is observed. The values of rate of rise of load temperature at water temperatures 80, 85 and 90 °C for different loads also confirm the same. The OLR of SFSC is found to be 1.2–1.6 kg.     

Photovoltaic solar cell array used for supplemental power generation

A study of the performance of a silicon cell non-tracking photovoltaic array has been made over a three year period. The array provided power in parallel with commercial utility power in a shared mode which makes use of all the solar energy generated. Tests of degradation, dirt accumulation, compatability with telephone plant, and day by day performance were performed. A method is developed for predicting the energy output of a non-tracking array based on standard global insolation measurements.     

A very simple solar tracker for space and terrestrial applications

A new very simple, reliable solar tracker is described. The solar tracker is based on a new arrangement of auxiliary bifacial solar cell connected directly to D.C. motor. Auxiliary solar cells (panels) can both sense and provide energy for tracking.     

Wet-chemical passivation and characterization of silicon interfaces for solar cell applications

The interface properties of silicon solar cell structures were characterized by the two non-destructive and highly surface-sensitive spectroscopic techniques: surface photovoltage and spectroscopic ellipsometry. The resulting charge and density of interface states as well as the microscopic surface roughness and oxide coverage were investigated during silicon wafer preparation and during sample storage in air. The surface state density of hydrogen-terminated silicon surfaces as well as the long-time stability of the hydrogen termination were found to primarily depend on the surface morphology resulting from the wet-chemical oxidation procedures applied before. The smallest interface state densities were obtained by NH₄F treatment subsequent to oxidation in ultra-pure water at 80°C. Surfaces prepared using this procedure are found to be much more stable upon exposition to clean-room air than those prepared by conventionally prepared H-terminated surfaces.The successful application of the new passivation procedures in photovoltaics is shown for selected examples of different solar cell concepts.     

A method for estimating monthly global solar radiation

In this paper a method is followed for estimating monthly totals of global solar radiation from a combination of calculations of monthly cloudless global solar radiation, surface meteorological observations, and empirical formulae relating sunshine duration to global solar radiation. The percent deviation of calculated from observed values is not negligible, but is much less than errors obtained by using extraterrestrial solar radiation totals. In case of values of monthly global solar radiation which are estimated for other regions, the resulted possible errors should be determined again. Some techniques leading to adaptation of the regression equations for other areas are argued.     

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.     

The present potential of CdS solar cells as a future contender for photovoltaic space and terrestrial power applications

A brief assessment is given of the current position on the relative merits of CdS and silicon solar cells for both space and terrestrial applications. After drawing attention to the absence of any known major stability problem with CdS cells, two other criteria are adopted for comparing the two cells. The first of these is the expected cost of medium-scale production of solar modules for remote and portable terrestrial applications. Details are given of an integrated approach to the CdS module construction which uses current technology but results in significant cost savings. The second criterion is the power-to-weight ratio for space arrays. In both of these cases, CdS cells are shown to possess significant potential advantages.     

A simplified method for estimating direct normal solar irradiation from global horizontal irradiation useful for CPV applications

The design and analysis of CPV systems require knowledge of direct normal solar irradiation but ground-based measurements of these data are only available for very few locations. Nowadays, meteorological databases that estimate direct normal irradiation from satellite images and other data sources are used. However, values provided by the different existing databases show large dispersion due to different estimation methods, input data and base years. In this paper, a simplified method for calculating direct normal irradiation is presented. It has been obtained from previous models proposed by several authors. One of its advantages is that it only requires latitude and global horizontal irradiation as input data. As global irradiation is easy to find or measure, the procedure becomes a useful tool in renewable energy applications. The accuracy of this method is similar to that of the existing databases and it is able to easily generate a mass of direct normal irradiation data for different areas worldwide.     

Recent progress of amorphous silicon solar cell applications and systems

Fifteen years have passed since the first industrial use of amorphous silicon (a-Si) solar cells for consumer products. At present, a-Si solar cells are entering a new age of use in power generating systems at private residences and other outdoor applications. This paper reviews recent advances in amorphous silicon (a-Si) solar cells and their applications. Technological developments in the field of a-Si solar cells are discussed. Various applications and systems that take advantage of the a-Si solar cell are then introduced. Finally, future prospects are discussed, including a new concept of GENESIS system for worldwide energy generation and transmission.     

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

One of the most promising ways to reduce the cost of photovoltaics is thin-film crystalline silicon solar cells. This paper, together with part 1, reviews the current state of research in thin-film crystalline silicon solar cells. Deposition on silicon, novel techniques which use a high-quality, reusable silicon substrate and light trapping have been described in part 1 of this paper. This paper describes deposition on glass and ceramics and discusses cell designs for thin-film crystalline silicon solar cells.     

A review of thin-film crystalline silicon for solar cell applications. Part 1: Native substrates

Approximately half the cost of a finished crystalline silicon solar module is due to the silicon itself. Combining this fact with a high-efficiency potential makes thin-film crystalline silicon solar cells a growing research area. This paper, written in two parts, aims to outline world-wide research on this topic. The subject has been divided into techniques which use native substrates and techniques which use foreign substrates. Light trapping, vapour- and liquid-phase deposition techniques, cell fabrication and some general considerations are also discussed with reference to thin-film cells.     

Modeling and optimization of solar thermoelectric generators for terrestrial applications

In this paper we introduce a model and an optimization methodology for terrestrial solar thermoelectric generators (STEGs). We describe, discuss, and justify the necessary constraints on the STEG geometry that make the STEG optimization independent of individual dimensions. A simplified model shows that the thermoelectric elements in STEGs can be scaled in size without affecting the overall performance of the device, even when the properties of the thermoelectric material and the solar absorber are temperature-dependent. Consequently, the amount of thermoelectric material can be minimized to be only a negligible fraction of the total system cost. As an example, a Bi₂Te₃-based STEG is optimized for rooftop power generation. Peak efficiency is predicted to be 5% at the standard spectrum AM1.5G, with the thermoelectric material cost below 0.05 $/W_p. Integrating STEGs into solar hot water systems for cogeneration adds electricity at minimal extra cost. In such cogeneration systems the electric current can be adjusted throughout the day to favor either electricity or hot water production.     

An optimum design of antireflection coating for spherical silicon solar cells

Antireflection (AR) coatings for spherical crystalline silicon solar cells are theoretically optimized from the viewpoint of achieving the largest photon densities in the spherical crystalline silicon solar cells. Because the AR film thickness is optimized with regard to the photon densities in the spherical crystalline silicon solar cells, tolerance in the film thickness can be evaluated. Also, the optimized AR film thicknesses for the spherical crystalline silicon solar cells and planar crystalline silicon solar cells are compared, and analytic expressions for the optimized AR film thicknesses are derived as a function of a quarter-wavelength film thickness.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

Operating temperatures of linear solar concentrators for optimum power output

In this paper, operating temperatures of linear solar concentrators for optimum power output have been calculated considering the operating temperature, emissivity of absorber and wind velocity dependence of the heat loss factor. Results are presented graphically and discussed.     

Monitoring of silicon solar cell technology via the surface photovoltage method

The surface photovoltage (SPV) technique adapted to thin samples was used to monitor solar cell technology. The relatively short minority carrier diffusion length from 70 to 80 μm found in p-bulk of the cells results from the presence of a layer with structural defects near the surface. The measurement of successively etched samples reveals that freshly cut off silicon wafers are already strongly destroyed to a depth of at least 35 μm. A diffusion length of about 300 μm was evaluated in the samples after removing the disturbed layer.     

Fabrication of amorphous silicon carbide films using VHF-PECVD for triple-junction thin-film solar cell applications

Preparation of intrinsic hydrogenated amorphous silicon carbide (i-a-SiC:H) thin films for use as a top cell of triple junction solar cells is presented. These films were deposited using very-high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) technique with monomethyl silane (MMS) gas as the carbon source. Deposition conditions were explored to obtain films with a wide gap and low defect density. It was confirmed that the hydrogen dilution ratio plays an important role in enhancing the film properties. Employing a-SiC:H film as an intrinsic layer of single junction cell, open-circuit voltage as high as 0.99 V has been achieved.