Three cascade solar cells with graded band-gap layer on the base of GaAs–AlGaAs heterosystem

A new structure of three cascade solar cells with graded band-gap layer on the base of GaAs–AlGaAs heterosystem is designed to decrease the thermal losses, arising as a result of absorption of short-wavelength radiation. These solar cells were created by combining liquid-phase epitaxy with gas-phase zinc diffusion technologies. In these structures multilayer cascade elements reduce losses, caused by generation of “hot” carriers and the upper graded band-gap layer improves the conversion of short-wavelength radiation. The cell structure exhibited high short-wavelength sensitivity and the following parameters (under 1-sun conditions): open-circuit voltage (V_oc=1.03 V); short-circuit current (I_sc=24.9 mA/cm²); fill factor (FF=0.74) and efficiency (η=22.3%). Therefore it can be successfully utilized as power supplies in high-located areas, and as sensors of ultra-violet radiation.     

Properties of an n-C:P/p-Si carbon-based photovoltaic cell grown by radio frequency plasma-enhanced chemical vapor deposition at room temperature

The phosphorus-doped amorphous carbon (n-C:P) films were grown by radiofrequency (RF) power-assisted plasma-enhanced chemical vapor deposition (PECVD) at room temperature using a solid phosphorus target. The influence of phosphorus doping on the material properties of n-C:P based on the results of simultaneous characterization are reported. Moreover, solar cell properties such as series resistance, short-circuit current density, open-circuit current voltage, fill factor and conversion efficiency along with the spectral response are reported for the fabricated carbon-based n-C:P/p-Si heterojunction solar cells by standard measurement technique. The cells’ performances have been given in the dark I–V rectifying curve and I–V working curve under illumination when exposed to AM 1.5 illumination condition (100 mW/cm², 25 °C). The maximum open-circuit voltage (V_oc) and short-circuit current density (J_sc) for the cells are observed to be approximately 236 V and 7.34 mA/cm², respectively, for the n-C:P/p-Si cell grown at a lower RF power of 100 W. The highest energy conversion efficiency (η) and fill factor (FF) were found to be approximately 0.84% and 49%, respectively. We have observed that the rectifying nature of the heterojunction structures is due to the nature of n-C:P films.     

Photovoltaic characteristics of phosphorus-doped amorphous carbon films grown by r.f. plasma-enhanced CVD

The phosphorus-doped amorphous carbon (n-C:P) films were grown by r.f. power-assisted plasma-enhanced chemical vapor deposition at room temperature using solid phosphorus target. The influence of phosphorus doping on material properties of n-C:P based on the results of simultaneous characterization are reported. Moreover, the solar cell properties such as series resistance, short circuit current density (J_sc), open circuit current voltage (V_oc), fill factor (FF) and conversion efficiency (η) along with the spectral response are reported for the fabricated carbon based n-C:P/p-Si heterojunction solar cell were measured by standard measurement technique. The cells performances have been given in the dark I–V rectifying curve and I–V working curve under illumination when exposed to AM 1.5 illumination condition (100 mW/cm², 25 °C). The maximum of V_oc and J_sc for the cells are observed to be approximately 236 V and 7.34 mA/cm², respectively for the n-C:P/p-Si cell grown at lower r.f. power of 100 W. The highest η and FF were found to be approximately 0.84% and 49%, respectively. We have observed the rectifying nature of the heterojunction structures is due to the nature of n-C:P films.     

Equivalent model of monocrystalline, polycrystalline and amorphous silicon solar cells

The current–voltage (I–V) characteristics of monocrystalline, polycrystalline and amorphous silicon solar cells are measured in the dark. A two diodes equivalent model is used to describe the electronic properties of solar cells. The non-linear curve fitting of the dark I–V curves obtain besides the diode ideality factors and the reverse saturation currents, the series and shunt resistance of the solar cells. These parameters determine the fill factor and the efficiency of the solar cells.     

Cu(In,Ga)Se2 based photovoltaic structure by electrodeposition and processing

CuIn_1−xGa_xSe₂ (CIGS) thin films were formed from an electrodeposited CuInSe₂ (CIS) precursor by thermal processing in vacuum in which the film stoichiometry was adjusted by adding In, Ga and Se. The structure, composition, morphology and opto-electronic properties of the as-deposited and selenized CIS precursors were characterized by various techniques. A 9.8% CIGS based thin film solar cell was developed using the electrodeposited and processed film. The cell structure consisted of Mo/CIGS/CdS/ZnO/MgF₂. The cell parameters such as J_sc, V_oc, FF and η were determined from I–V characterization of the cell.     

Concentrator silicon solar cells from silicon industry rejects

Two types of silicon (Si) substrates (40 n-type with uniform base doping and 40 n/n⁺ epitaxial wafers) from the silicon industry rejects were chosen as the starting material for low-cost concentrator solar cells. They were divided into four groups, each consisting of 20 substrates: 10 are n/n⁺ and 10 are n substrates, and the solar cells were prepared for different diffusion times (45, 60, 75 and 90 min). The fabricated solar cells on n/n⁺ substrates (prepared with a diffusion time of 75 min) showed better parameters. In order to improve their performances, particularly the fill factor, 20 new solar cells on n/n⁺ substrates were fabricated using the same procedure (the diffusion time was 75 min)—but with four new front contact patterns. Investigation of current–voltage (I–V) characteristics under AM 1.5 showed that the parameters of these 20 new solar cells have improved in comparison to previous solar cells' parameters, and were as follows: open-circuit voltage (V_OC=0.57 V); short circuit current (I_SC=910 mA), and efficiency (η=9.1%). Their fill factor has increased about 33%. The I–V characteristics of these solar cells were also investigated under different concentration ratios (X), and they exhibited the following parameters (under X=100 suns): V_OC=0.62 V and I_SC=36 A.     

Calculation of CIS and CdTe module efficiencies

An accurate and fast method to calculate the efficiency of Cu(In,Ga)Se₂ (CIGS) and CdTe thin-film solar modules is presented here. This comprises a new method to calculate the fill factor as a function of discrete and distributed series resistance, and of shunt conductance: a three-dimensional, third-order polynomial approximation is presented, and the expansion of the coefficients as a power series of 1/V_oc is given. Analytical expressions are presented which fit experimental data of the optical absorption in ZnO as a function of its thickness or sheet resistance. Together with a calculation outline of the series and shunt effects of the module integration, this constitutes a practical module design tool. This is illustrated with results of dependence of module efficiency on cell length, window and absorber sheet resistance, interconnect contact resistance, “softness” of the cell I–V curve, and absorber material (CIGS or CdTe). Optimal or critical values for these parameters are given.     

Al0.3Ga0.7As/GaAs concentrator solar cells

An Al_0.3Ga_0.7As/GaAs tandem solar cell was fabricated in a commercial reactor which has been specially modified for dual operation of both atomic layer epitaxy (ALE) and metalorganic chemical vapor deposition (MOCVD) growth modes. The p-type and n-type dopants were carbon and silicon, respectively, and the required doping concentrations were achieved by optimizing growth conditions such as V/III ratio (mole ratio of group V atoms to group III atoms), exposure times to reactant gases, and growth temperatures. The current-voltage (I–V) characteristics indicate that, up to 53 Suns, the tunnel junction does not seem to result in any appreciable deterioration in the multijunction solar cell's performance. The measured efficiency increases with increasing solar concentration up to a point where a region of negative resistance starts to appear in the I–V characteristics.     

Current transport in copper indium gallium diselenide solar cells comparing mesa diodes to the full cell

Mesa diodes were formed on CdS/CIGS/stainless steel solar cells to investigate current transport when edge leakage and spot defects are avoided. Current conduction mechanisms in the device were determined from current–voltage (I–V) and current–voltage–temperature (I–V–T) characteristics. Space charge limited (SCL) current in the mobility regime with an exponential distribution of traps was found in the voltage range of V>0.6 V based on I∝V^m where m>2. In the voltage region of 0.2V<V<0.6V, recombination was the dominant mechanism based on the ideality factor, n, in the equation I=Ae^(qV/nkT), close to 2. For −0.2V<V<0.2V, a combination of tunneling and SCL current in the ballistic regime was suggested because of the weak temperature dependency and approximation to I∝V^1.5. For the reverse bias region where V<−0.2 V, the device exhibited either SCL current in the velocity saturation regime or tunneling based on the unity I–V relation and the weak temperature dependency. A previous report on full size CIGS cells indicated a higher degree of tunneling for V<0.2 V. Thus, the mesa diodes show some difference in mechanism compared to “good” full cells and much difference compared to “poor” full cells.     

Estimation of global solar radiation on horizontal surfaces in Jeddah, Saudi Arabia

The measured data of global solar radiation on a horizontal surface, as well as the number of sunshine hours, mean daily ambient temperature, maximum and minimum ambient temperatures, relative humidity and amount of cloud cover, for Jeddah (latitude 21° 42′37″N, longitude 39° 11′12″E), Saudi Arabia for the period 1996–2006 are analyzed. The data are divided into two sets. The sub-data set 1 (1996–2004) are employed to develop empirical correlations between the monthly average of daily global solar radiation fraction (H/H₀) and various meteorological parameters. The nonlinear Angström type model developed by Sen and the trigonometric function model proposed by Bulut and Büyükalaca are also evaluated. New empirical constants for these two models have been obtained for Jeddah. The sub-data set 2 (2005, 2006) are then used to evaluate the derived correlations. Comparisons between measured and calculated values of H have been performed. It is indicated that, the Sen and Bulut and Büyükalaca models satisfactorily describe the horizontal global solar radiation for Jeddah. All the proposed correlations are found to be able to predict the annual average of daily global solar radiation with excellent accuracy. Therefore, the long term performance of solar energy devices can be estimated.     

A graphical method of measuring the performance characteristics of solar collectors

A graphical method to measure average and instantaneous efficiencies of a solar concentrator used for heating and boiling liquids and a flat plate collector is presented. The overall heat loss coefficient for the collectors and the optical loss factors: γ(τa)_b—the product for a concentrator and (τa)—the product for a flat plate collector, are also obtained. The method involves measuring the temperature of stagnated liquid in the absorber/collector as a function of time at noon. The efficiencies obtained are correct to within 5% of the efficiencies obtained from accurate measurements involving solar radiation data, the design parameters of collectors and the physical characteristics of the materials used in the fabrication of collectors.     

Dynamic evaluation of maximum power point tracking operation with PV array simulator

This paper presents an innovative method to measure the dynamic control ability of maximum power point tracking for PV inverters under the condition of irradiance fluctuation. The PV array I–V curve simulator is a kind of indoor testing facility and easy to be adopted by industries. Basic functions are given by a specially designed PV array I–V curve simulator composed of the active power load. Most of the parameters are controllable by sophisticated software with capability of treating a lot of 1-s data for a very long period of time. In this paper, detailed structure of the equipment is described and test examples are also given by using a commercial PV inverter.     

Effect of solar control glazings on human skin temperature

Human skin temperatures were measured under exposure to near-infrared radiation through solar control glazings containing SiWO_x films as solar control materials. It was found that the increase in skin temperature corresponds to the solar transmittance (T_s) value of the glazings. When the hand was exposed to radiation through a glazing containing solar films, the skin temperature after five minutes was less 0.9–1.8°C lower than when the corresponding glass without the films was used. The solar control glazings were effective in reducing the increase in skin temperature.     

Extraction and analysis of solar cell parameters from the illuminated current–voltage curve

In this work we describe, apply and analyze a procedure to extract the physical parameters of a solar cell from its I–V curve under illumination. We compare it with other procedures and assess the statistical significance of the parameters. Our method, called APTIV, uses separate fitting in two different zones in the I–V curve. In the first one, near short circuit, current fitting is used because the error in current dominates. In the second one, near open circuit, voltage fitting is used because this is the dominant error. The method overcomes some drawbacks of common procedures: voltage errors are properly managed and no accurate initial guesses for the parameters are needed. In addition, the numerical implementation is very simple.     

Hydrogenated nanocrystalline silicon p-layer in amorphous silicon n–i–p solar cells

This paper describes an investigation into the impacts of hydrogenated nanocrystalline silicon (nc-Si:H) p-layer on the photovoltaic parameters, especially on the open-circuit voltage (V_oc) of n–i–p type hydrogenated amorphous silicon (a-Si:H) solar cells. Raman spectroscopy and transmission electron microscopy (TEM) analyses indicate that this p-layer is a diphasic material that contains nanocrystalline grains with size around 3–5 nm embedded in an amorphous silicon matrix. Optical transmission measurements show that the nc-Si:H p-layer has a wide band gap of 1.9 eV. Using this nanocrystalline p-layer in n–i–p a-Si:H solar cells, the cell performances were improved with a V_oc of 1.042 V, whereas the solar cells deposited under similar conditions but incorporating a hydrogenated microcrystalline silicon (μc-Si:H) p-layer exhibit a V_oc of 0.526 V.     

Effects of illumination and Co γ-ray irradiation on the electrical characteristics of porous silicon solar cells

A new approach for hybrid metal–insulator–semiconductor (MIS) Si solar cells is adopted by Institute of Fundamental Problems for High Technology, Ukrainian Academy of Sciences. In order to interpret the effect of illumination and ⁶⁰Co γ-ray radiation dose on the electrical characteristics of solar cells are studied at room temperature. Before the solar cells are subjected to stressed irradiation six different illumination levels of forward and reverse bias I–V measurements are carried out at room temperature. The solar cells are irradiated with ⁶⁰Co γ-ray source irradiation, with a dose rate of 2.12 kGy/h and an over dose range from 0 to 500 kGy. Experimental results shows that both the values of capacitance and conductance increase with increasing illumination levels and give the peaks at high illumination levels. γ-ray irradiation induces an increase in the barrier heights Φ_b(C–V) which are obtained from reverse-bias C–V measurements, whereas barrier heights Φ_b(I–V) which are deducted from forward-bias I–V measurements remain essentially constant. This negligible change of Φ_b(I–V) is attributed to the low barrier height (BH) in regions associated with the surface termination of dislocations. Both the I–V and C–V characteristics indicate that the total-dose radiation hardness of the Si solar cells cannot be neglected according to illumination levels.     

The effects of temperature and light concentration on the GaInP/GaAs multijunction solar cell’s performance

Monolithic Ga_0.49In_0.51P/GaAs cascade solar cells with a p⁺/n⁺ GaAs tunnel junction were grown by MOCVD technique. The variation of the photovoltage, photocurrent, fill factor, efficiency, I–V characteristics and spectral response under different temperatures (25–75 °C), and light intensity values (1–40 sun), were investigated experimentally.The open-circuit voltage of the multijunction cell decreases with the temperature increase at a rate of 5.5 mV/°C. The efficiency of the cascade structure under investigation was increased with an increase in the light concentration up to a point where the series resistance and the tunnel junction effects become significant.     

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.     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

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).