Effect of illumination intensity and temperature on the I–V characteristics of n-C/p-Si heterojunction

Krishna et al. (Sol. Energy Mater. Sol. Cells 65 (2001) 163) have recently developed an heterojunction n-C/p-Si in order to achieve low cost and high-efficiency carbon solar cell. It has been shown that for this structure, the maximum quantum efficiency (25%) appears at wavelength λ (600 nm). In this paper, the dependence of I–V characteristics of this heterojunction solar cell on illumination intensity and temperature has been systematically investigated. An estimation of the stability of the solar cell with temperature has been made in terms of the temperature coefficient of I_sc and V_oc. The intensity variation study has been used to estimate the series resistance R_s of the solar cell.The effect of illumination intensity on I–V of n-C/p-Si heterojunction is more complex because the carrier lifetime and the carrier mobility of amorphous carbon are small and also because drift of carriers by built-in electric field plays an important role in these cells. Therefore, the conventional analytical expression for I–V characteristic is not applicable to such solar cells. These structures will not obey the principle of superposition of illuminated and dark current. The experimental results have been analysed by developing empirical relation for I–V.The temperature sensitivity parameters α, the change in I_sc and β, the change in V_oc per degree centigrade have been computed and are found to be 0.087 mA/°C and 1 mV/°C, respectively. This suggests that the heterojunction n-C/p-Si has good temperature tolerance. The value of series resistance has been estimated from the family of I–V curves at various intensities. The R_s is found to be ≈12 Ω, which is on the higher side from the point of view of photovoltaic application.     

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.     

A method for the determination of dynamic resistance of photovoltaic modules under illumination

Dynamic resistance of solar cells and modules have been determined from a dark IV characteristic curve. In the determination, it is often assumed that series resistance R_s is small and the shunt resistance R_sh very large, their effects can be neglected. The resultant dynamic resistance can be called the external dynamic resistance R_D. R_D is normally taken to be the slope of the IV characteristic of a cell or a module. We present in this paper a new method to determine an internal dynamic resistance R_d of a photovoltaic module based on one illuminated IV curve, taken into account finite series and shunt resistance. In the experiment, we measure illuminated and dark characteristics of a 4.5 Wp commercial X-Si solar module (9 V, 500 mA) at 25–27 °C, and calculate R_d and R_D. We see that R_d and R_D can be markedly different. Comparison is also made on the series resistance R_s and shunt resistance R_sh determined from single IV curve and two IV curve methods.     

Method for analyzing series resistance and diode quality factors from field data Part II: Applications to crystalline silicon

The method described in a prior journal publication [1] is applied to the determination of module series resistance and diode quality factors for several crystalline silicon (c-Si) technology photovoltaic (PV) modules. This method makes use of the functional dependence of the slope of the current–voltage (I–V) characteristics at open circuit (R_oc) against the reciprocal of the short-circuit current density (J_sc), from multiple I–V curves taken under variable illumination. It is shown that calculations of the series resistance for six modules yield values in the range 1.0–1.6 Ω-cm², expressed in unit-cell area terms. The derived values for the series resistance (R_s) determined from the data are investigated for their effect on the module fill factor (FF) values and their dependence at higher light intensity levels. The diode quality factors also derived from the same data are shown to be somewhat larger than those obtained from the more canonical method — slope of the fit of the open-circuit voltage (V_oc) versus logarithm of J_sc. The differences between the two methods are explored within a two-diode model for c-Si. Deriving average values of diode quality factors for series-connected cells using either method is shown to exhibit problematic issues.     

Photoelectrochemical investigations on electrochemically deposited CdSe and Fe-doped CdSe thin films

Thin films of CdSe and Fe-doped CdSe (Fe:CdSe) were deposited onto stainless steel substrates by electrodeposition technique. The photoelectrochemical investigations have been carried out using the cell configurations CdSe/1 M (Na₂S–S–NaOH)/C and Fe:CdSe/1 M (Na₂S–S–NaOH)/C for studying the current–voltage (I–V) characteristics in dark and under illumination, photovoltaic output, spectral response, photovoltaic rise and decay characteristics. The studies reveal that films are n-type conductivity. The junction quality factor in light (n_l), series and shunt resistance (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. After Fe doping, efficiency and FF of PEC solar cell is found to be improved from 0.34% and 31.12 to 1.80% and 35.78, respectively.     

Solar cells parameters evaluation considering the series and shunt resistance

This paper presents a new technique for the evaluation of the parameters of illuminated solar cell with a single diode lumped circuit model and considering the series and shunt resistances. This method includes the presentation of the standard I=f(V) function as V=f(I) and the determination of the factors C₀, C₁, C₂ of this function that provide the calculation of the illuminated solar cell parameters. These parameters are usually the saturation current (I_s), the series resistance (R_s), the ideality factor (n), the shunt conductance (G_sh=1/R_sh) and the photocurrent (I_ph).Parameter values were extracted using the present method from experimental I-V characteristics of commercial solar cells and modules. The method proposed below appears to be accurate even in the presence of noise and/or random errors during measurement and it needs no a prior knowledge of the parameters compared to other methods.     

Effect of variation of I01/I02 on short-circuit current and fill factor of a real solar cell having resistive and current leakage losses

We analyze the effect of variation of I₀₁/I₀₂ on short-circuit current and the fill factor of a solar cell having resistive and current leakage losses. This analysis is particularly important for the polycrystalline solar cells, where higher values of I₀₂ and hence lower values of I₀₁/I₀₂ can be expected due to the space charge regions associated with the grain boundaries. Also in polycrystalline solar cells, we cannot ignore the effect of series and shunt resistances. It is observed that the value of fill factor depends on R_s, R_sh and I₀₁/I₀₂, while the value of I_sc depends only on R_s and I₀₁/I₀₂.     

Dependence of the I–V characteristics of amorphous silicon solar cells on illumination intensity and temperature

Current-voltage characteristics of amorphous silicon (a-Si) solar cells are systematically investigated as functions of the illumination intensity and ambient temperature. The principle of superposition of the short-circuit current and the dark current, which is usually assumed for crystalline silicon solar cells, is not applicable to a-Si solar cells. It is shown, that the output current of a-Si solar cells at a given illumination intensity E₂mW/cm²I_E2(V) is expressed by a relatively simple equation, I_E2(V) = I_d(V) + (_{E2/100) × (I100(V) — Id(V))}, when the series resistance of the solar cells is negligible. Here, I_d(V) is the dark current, I₁₀₀(V) is the output current at an illumination of 100 mW/cm², and V is the applied voltage. Empirical formula to describe the dependence of the current-voltage characteristics on the illumination intensity and the temperature are presented and discussed.     

Effect of temperature and electron irradiation on the I–V characteristics of Au/CdTe Schottky diodes

The results of the studies on the effect of temperature and 8 MeV electron irradiation on the current–voltage (I–V) characteristics of the Au/CdTe Schottky diodes are presented in this article. Schottky diodes were prepared by evaporating Au onto n-type CdTe films electrodeposited onto stainless steel substrates. The forward and reverse current–voltage characteristics of these diodes were studied as a function of temperature. The diodes were subjected to 8 MeV electron irradiation at various doses and their effect on the I–V characteristics was studied. Some intrinsic and contact properties such as barrier height, ideality factor, and series resistance were calculated from the I–V characteristics. Diode ideality factor of the junctions were greater than unity. The ideality factor and the series resistance R_s increase with decrease in temperature. The conduction seems to be predominantly due to thermionic emission–diffusion mechanism. The resistance was found to increase with increasing dose. The leakage current, ideality factor and barrier height were found to be unaffected by electron irradiation up to, a dose of about 40 kGy.     

Temperature dependence of I-V characteristics and performance parameters of silicon solar cell

The temperature dependence of open-circuit voltage (V_oc) and curve factor (CF) of a silicon solar cell has been investigated in temperature range 295-320 K. The rate of decrease of V_oc with temperature (T) is controlled by the values of the band gap energy (E_g), shunt resistance (R_sh) and their rates of change with T. We have found that R_sh decreases nearly linearly with T and its affect on dV_oc/dT is significant for cells having smaller R_sh values. Series resistance also changes nearly linearly with voltage. CF depends not only on the value of R_s and other parameters but also on the rate of change of R_s with voltage. The rate of decrease of R_s with voltage and T are important to estimate the value of CF and its decrease with temperature accurately.     

Study of silicon solar cell at different intensities of illumination and wavelengths using impedance spectroscopy

The electrical properties of an n⁺–p–p⁺ structure-based single-crystalline silicon solar cell were studied by impedance spectroscopy, I–V and spectral response. The impedance spectrum is measured in dark, under different intensities (14, 43, 57, 71, 86, 100 mW/cm²) of illumination and wavelengths (400–1050 nm) of light. Under dark and at low intensities of illumination (<50 mW/cm²) the impedance spectra show perfect semicircles but at high intensities the semicircles are distorted at low frequencies. It is found that illumination provides an additional virtual R₁C₁ network parallel to the initial bulk R_pC_p network observed under dark conditions. The value of virtual resistance R₁ depends on the illumination wavelength and shows an inverse relationship with the spectral response of the device.     

Determination of the diode parameters of a-Si and CdTe solar modules using variation of the intensity of illumination: An application

An attempt has been made for the determination of diode parameters viz. shunt resistance R_sh, series resistance R_s, diode ideality factor n and reverse saturation current density J₀ of three solar modules: a-Si 47-37, a-Si 51-13 and CdTe 14407. In this regard, two approaches namely (A) and (B) reported by Khan et al. (2010) have been used to determine all the four diode parameters R_sh, R_s, n and J₀. The data of slopes of J-V curve at open circuit conditions (m_oc) and open circuit voltage (V_oc) at different illumination intensities obtained by Del Cueto (1998) for two a-Si and one CdTe solar modules have been used to determine the above diode parameters. The determined values of diode parameters have been used to generate the theoretical J-V curves. The theoretical fill factor (FF) and V_oc have been calculated from the theoretical J-V curves and are plotted along with the experimental FF and V_oc values. The theoretical values of FF and V_oc obtained by the approach (B) of method of Khan et al. (2010) are in good agreement with the experimental values.     

Photovoltaic cells based on chemically deposited p-type SnS

The aim of this work is to attract the attention of the scientific workers in the field of PV conversion of solar energy to SnS polycrystalline thin film as a candidate for construction of cheap solar cells, since it posseses similar photoelectric properties as polycrystalline silicon, but it can be produced on any kind of substrate, by simple, economic and environmentally approved technique. By the use of the method of chemical deposition from two separate solutions, complete preparation of three types of cells was done. All of them use SnS as base absorbing layer, with a difference in the window layer electrode. The first one has CdO, the second one has Cd₂SnO₄ thin film window electrode, both prepared by the chemical deposition method. The third cell was purely Schottky barrier cell in which the window electrode was SnO₂:F, prepared by spray pyrolysis. The I–V, C–V and spectral characteristics were registered and the conclusion was drawn that the best performances has shown the cells with Cd₂SnO₄ film as a window electrode.     

Photoelectrochemical properties of spray deposited n-ZnIn2Se4 thin films

Zinc indium selenide (ZnIn₂Se₄) thin films have been prepared by spraying a mixture of an equimolar aqueous solution of zinc sulphate (ZnSO₄), indium trichloride (InCl₃), and selenourea (CH₄N₂Se), onto preheated fluorine-doped tin oxide (FTO)-coated glass substrates at optimized conditions of substrate temperature and a solution concentration. The photoelectrochemical (PEC) cell configuration of n-ZnIn₂Se₄/1 M (NaOH+Na₂S+S)/C has been used for studying the current voltage (I–V), spectral response, and capacitance voltage (C–V) characteristics of the films. The PEC study shows that the ZnIn₂Se₄ thin films exhibited n-type conductivity. The junction quality factor in dark (n_d) and light (n_l), series and shunt resistance (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. The measured (FF) and η of the cell are, respectively, found to be 0.435% and 1.47%.     

Identification of PV solar cells and modules parameters using the genetic algorithms: Application to maximum power extraction

In this paper, we propose to perform a numerical technique based on genetic algorithms (GAs) to identify the electrical parameters (I_s, I_ph, R_s, R_sh, and n) of photovoltaic (PV) solar cells and modules. These parameters were used to determine the corresponding maximum power point (MPP) from the illuminated current-voltage (I-V) characteristic. The one diode type approach is used to model the AM1.5 I-V characteristic of the solar cell. To extract electrical parameters, the approach is formulated as a non convex optimization problem. The GAs approach was used as a numerical technique in order to overcome problems involved in the local minima in the case of non convex optimization criteria. Compared to other methods, we find that the GAs is a very efficient technique to estimate the electrical parameters of PV solar cells and modules. Indeed, the race of the algorithm stopped after five generations in the case of PV solar cells and seven generations in the case of PV modules. The identified parameters are then used to extract the maximum power working points for both cell and module.     

Study of CdS/Cu(In,Ga)Se2 interface by using n values extracted analytically from experimental data

This paper presents that an analytical method based on Lambert W-function can be applied to estimate the value of the diode ideality factor n, of a ZnO/CdS/Cu(In,Ga)Se₂ (CIGS) solar cell by using its dark current-voltage characteristics. The method is tested at different temperatures in the dark and found that the resulting n(T) values are in good agreement with those estimated experimentally from the slopes of the straight-line regions of Log I-V plots. The suggested values of n(T) under illumination are also determined using the exact explicit analytic solutions for the current-voltage relation expressed in terms of Lambert W-functions and experimentally estimated parasitic series and shunt resistances (R_s, R_sh), diode saturation current (I_o), open circuit voltage (V_oc) and short circuit current (I_sc) values at various temperatures. Temperature dependence of the diode ideality factor revealed that after illumination still tunnelling enhanced interface recombination mechanism dominates the current transport with relatively low tunnelling energy as compared to the dark case.     

Determination of the interfacial dynamic velocity in silicon solar cells

A detailed theoretical method is presented for the determination of the interfacial dynamic velocity (IDV) S_d introduced at the edge of the space-charge region in the base of a solar cell. The method is based on a dynamic measurement at an arbitrary point on the I–V curve and exploits measurements carried out on a solar cell under illumination. A transient regime between two steady states around the operating point is investigated. The theory takes into account the carrier generation and recombination rates. The measured values of S_d are shown to depend on the cell operating conditions, and the error in the determination of S_d increases with the light intensity that is kept constant during measurements. The interfacial dynamic velocity characterizes the junction as an active interface related to the current flow through the device and appears to allow solar cell quality control since it also depends on the cell structure parameters.     

Clear separation of seasonal effects on the performance of amorphous silicon solar modules by outdoor I/V-measurements

The causes of seasonal variations on the performance of an amorphous silicon solar module were clearly separated using long-term outdoors I(V)-measurements. We normalized the data to a standard temperature, by using measured temperature coefficients of the characteristic parameters of the I(V)-curve, rather then extrapolating the curve itself. The resulting data were interpreted using a new model containing an effective μτ-product in the i-layer of the device (Merten et al. 1997). This μτ-product is accessed by variable illumination measurements (VIM) of the I(V) characteristic, which can be easily performed outdoors, making use of the natural variation in the illumination levels. The effective μτ-product of the module remains constant throughout its second year of outdoor exposure. We conclude that the enhanced efficiency in summer is, therefore, mainly a spectral effect, and operating temperatures exceeding the winter value of 60°C do not further increase the module's performance.     

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.     

Optical, electrical and photovoltaic characteristics of organic semiconductor based on oxazine/n-Si heterojunction

In this work, the construction and photoelectrical characterization of p-type organic semiconductor oxazine (OXZ) in junction with n-type silicon semiconductor are presented. The Stokes shift between absorption and emission of oxazine was analyzed. The analysis of the spectral behavior of the absorption coefficient (α) of OXZ, in the absorption region revealed a direct transition, and the energy gap was estimated as 1.82 eV. From the current–voltage, I–V, measurements of the Au/OXZ/n-Si/Al heterojunction in the temperature range 300–375 K, characteristic junction parameters and dominant conduction mechanisms were obtained. This heterojunction showed a photovoltaic behavior with a maximum open circuit voltage, V_oc, of 0.42 V, short-circuit current density, J_sc, of 3.25 mA/cm², fill factor, FF, of 0.35 and power conversion efficiency, η, of 3.2% under 15 mW/cm² white light illumination.