Simulation of I–V characteristics of a PV module with shaded PV cells

The authors are studying a diagnostic method of a PV power generating system. We consider that the change of I–V characteristics can be utilized for the diagnosis. However, the report on the change of I–V characteristics is very little. In this paper, we investigate the relation between the output lowering due to shaded PV cells and the change of I–V characteristics, utilizing the computer simulation. It was proven from the simulation that I–V characteristics are changed by the condition of the shadow, which covered the module. The change of I–V characteristics of a PV module with shaded PV cells is discussed by the shift of the avalanche breakdown voltage of shaded PV cells.     

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.     

Capacitive load based on IGBTs for on-site characterization of PV arrays

This paper describes the practical design of a portable capacitive load based on insulated gate bipolar transistors (IGBTs), which is used to measure the I–V characteristics of PV arrays with short-circuit currents up to 80 A and open circuit voltages up to 800 V. Such measurement allows on-site characterization of PV arrays under real operating conditions and also provides information for the detection of potential array anomalies, such as broken cells or defective connections. The presented I–V load is easy to reproduce and low-cost, characteristics that are within the reach of small-scale organizations involved in PV electrification projects.     

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.     

Studies on the temperature dependence of I–V and C–V characteristics of electron irradiated silicon photo-detectors

The current transport mechanisms of n⁺–p silicon (Si) photo-detectors in different temperature and bias regions before and after irradiation with a dose of 350 kGy has been investigated and presented in this article. Temperature-dependent dark current–voltage (I–V) studies under forward and reverse bias were carried out for this purpose. In the temperature range studied, the dark current contribution in the low bias range is believed to be due to the generation-recombination of minority carriers in the space-charge region. Electron irradiation does not seem to have altered the dark current conduction mechanism. Capacitance–voltage (C–V) at various temperatures was measured to identify the presence of deep levels in the device.     

I–V curve simulation by multi-module simulator using I–V magnifier circuit

This paper presents an I–V curve simulation of PV array/modules using multi I–V magnifier circuits. The circuit magnifies an I–V output of a pn photo-sensor, which is regarded as a small solar cell, by analog technology. About 30 W I–V curve simulator circuits were made and their characteristics were evaluated. LED light irradiated into the photo-sensor works like irradiation of sun light on real PV modules. It has been confirmed that each voltage gain and current gain of the circuit is independently adjustable and the circuit magnifies an I–V output of the photo-sensor successfully. FF of the circuit can be modified by shunt register connected to the photo-sensor in parallel. The circuit showed enough response ability to apply the maximum power point tracker evaluation of PV inverters. Temperature dependence of module output can be simulated by temperature control of the photo-sensor. The result of output characteristics of series connection of the I–V magnifier circuits suggests that the simulator system composed of multi I–V magnifier circuit could simulate partial shading effect of PV array output.     

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.     

Photovoltaic and electronic properties of quercetin/p-InP solar cells

In this work, quercetin/p-InP heterojunction solar cell has been fabricated via solution-processing method and characterized by current–voltage and capacitance–voltage measurements at room temperature. A barrier height and an ideality factor value of 0.86 eV and 3.20 for this structure in dark have been obtained from the forward bias current–voltage characteristics. From the capacitance–voltage measurement, the barrier height and free carrier concentration values for the quercetin/p-InP device have been calculated as 1.63 eV and 3.8×10¹⁷ cm⁻³, respectively. Also, series resistance calculation has been performed by using Cheung theory. The device exhibits a strong photovoltaic behavior with a maximum open circuit voltage V_oc of 0.36 V and short-circuit current I_sc of 35.3 nA under 120 lx light intensity only.     

Transient analysis of a PV power generator charging a capacitor for measurement of the I–V characteristics

Measuring the I–V characteristics is of high importance since it can be considered as a quality and performance certificate for each PV generator. The most precise and inexpensive measuring method is represented in capacitor charging by the PV generator. Using the equivalent circuit of the PV generator with a capacitor as load and applying transient analysis on the circuit, we obtain the capacitor charging voltage and current as a function of time, as well as their differentials as a function of short circuit current and capacitor size. The derived equations facilitate the calculation of proper capacitance size for measuring the I–V characteristics, and considers the acquisition speed of the measuring system as demonstrated through two measurement samples in this paper. The capacitor size is directly and indirectly proportional to the short circuit current and open circuit voltage of the PV generator, respectively. Accordingly, the paper presents a capacitance calculation chart, which enables selecting the correct capacitance for measuring the I–V characteristics by a computerized data acquisition system.     

Experimental results of controlled PV module for building integrated PV systems

Last issues about Building Integrated Photovoltaic Systems (BIPV) still show average Performance Ratio (PR) values in the range of 0.75–0.80. The main causes well known: partial shadows, temperature effects, PV inverter losses, thermal losses, etc. and mismatching losses. Ideally, all the modules work in the same conditions, but differences between modules really exist due to differences in the working temperature, the inclination or orientation angles, differences in the I–V characteristic coming from the manufacturing process, etc. The effect is that the output power of the complete PV system is lower than the addition of the power of each PV module.These mismatching losses can be decreased by means of suitable electronics. This paper presents the experimental results obtained over PV systems equipped with controlled PV modules, PV modules with low cost and high efficiency DC–DC converters, including MPPT algorithm and other functions, such as power control and Power Line Communications (PLC).Tests have been divided into two great categories: tests on the electronic performance of the DC–DC converter and tests on grid-connected PV systems with multiple DC–DC converters. Many of these tests have been carried out taking advantage of the PV System Test Platform, a powerful tool especially designed by Robotiker to evaluate all kind of PV systems, especially systems with differences between modules. Aspects of the DC–DC converter performance have been detailed and among the most important experiments, the paper analyses different situations such as partial shadows, different inclined planes, PV systems with different PV modules, and finally a comparison between a conventional system and a system composed by controlled PV modules have been described. To sum up, the importance of a good system dimensioning is analysed, with very interesting results.     

A study of a two stage maximum power point tracking control of a photovoltaic system under partially shaded insolation conditions

A photovoltaic (PV) array shows relatively low output power density, and has a greatly drooping current–voltage (I–V) characteristic. Therefore, maximum power point tracking (MPPT) control is used to maximize the output power of the PV array. Many papers have been reported in relation to MPPT. However, the current–power (I–P) curve sometimes shows multi-local maximum point mode under non-uniform insolation conditions. The operating point of the PV system tends to converge to a local maximum output point which is not the real maximal output point on the I–P curve. Some papers have been also reported, trying to avoid this difficulty. However, most of those control systems become rather complicated. Then, the two stage MPPT control method is proposed in this paper to realize a relatively simple control system which can track the real maximum power point even under non-uniform insolation conditions. The feasibility of this control concept is confirmed for steady insolation as well as for rapidly changing insolation by simulation study using software PSIM and LabVIEW.     

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.     

Experimental and theoretical investigations of a new potential barrier due to sharp a-Si/c-Si heterointerfaces buried in the solar cell emitter

Several new concepts to increase the conversion efficiency of solar cells have been presented over the last few years. One possibility is the multi-interface novel device solar cell with a highly doped amorphised substructure inserted in the emitter. This active nanostructure is created by P ion implantation followed by an adequate thermal treatment necessary to form two sharp a-Si/c-Si heterointerfaces. After an incomplete initial thermal treatment at 500°C, dark current–voltage (I–V) characteristics were measured after each of several complementary thermal treatments. In this paper, we show that the classical two-diode model has to include a voltage reduction resulting from the two low–high-type interfaces in order to correctly fit the experimental curves.     

The effect of temperature on the charge transport and transient absorption properties of K27 sensitized DSSC

The charge transport and transient absorption properties of K27 dye-sensitized solar cell have been investigated. The current–voltage (I–V) characteristics of the solar cell were analyzed by the thermionic emission theory. The ideality factor, barrier height and series resistance values of the solar cell were determined. The ideality factor higher than unity indicated the presence of non-ideal behavior in current–voltage characteristics at lower voltages. At the higher voltages, the charge transport mechanism for the solar cell is controlled by a space-charge limited current (SCLC) with an exponential distribution of traps. The built potential values are determined from capacitance–voltage plot and were found to be 0.14 and 0.58 V, respectively. The transient absorption data of K27 DSSC device suggest that the fast and slow phases are taking place. While the fast phase corresponds to regeneration of the dye cation by the iodide redox couple, the slow phase corresponds to the decay of long-lived I₂⁻/ TiO₂ electron absorption. The best conversion efficiency for K27 DSSC was found to be 0.317% under 100 mW/cm² (FF=0.584, V_oc=480 mV, I_sc=1.131 mA). The photocurrent results indicate that the photogeneration of charge carriers is a monophotonic process.     

Analysis of anomalous current–voltage characteristics of silicon solar cells

The current–voltage characteristics of solar cells, under illumination and in the dark, represent a very important tool for characterizing the performance of the solar cell.The PC-1D computer program has been used to analyze the deviation of the dark current–voltage characteristics of p–n junction silicon solar cells from the ideal two-diode model behavior of the cell, namely the appearance of “humps” in the I–V characteristics. The effects of the surface recombination velocity, the minority-carrier lifetimes in the base — and emitter regions of the solar cell, as well as the temperature dependence of the I–V characteristics have been modeled using PC-1D.It is shown that the “humps” in the I–V characteristics arise as a result of recombination within the space-charge region of the solar cell, occurring when conditions for recombination are different from the simple assumptions of the Sah–Noyce–Shockley theory.     

Constant voltage I–V curve flash tester for solar cells

Flash testers are commonly used for measuring solar cells and modules but in their usual implementation are complex, expensive, and susceptible to transient errors. This work presents a new tester design that is simple, low cost, and reduces transient errors by use of a constant-voltage cell-bias circuit. A novel feature of the system is that it extracts a family of I–V curves over a decade range of light intensity, which provides comprehensive information on cell performance. The new design has been tested and used extensively.     

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.     

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.     

An analytical study of the porosity effect on dye-sensitized solar cell performance

The porosity in a dye-sensitized solar cell (DSSC) can affect light absorption and electron diffusion that govern the overall electrical current–voltage (I–V) characteristics. In this research, two methods, namely, constant overlap and variable overlap, were developed to determine the connectivity of dye-sensitized TiO₂ particles in high and low porosity levels, respectively. In turn, the light absorption coefficient α and the electron diffusion coefficient D were analytically derived in terms of the porosity P. Subsequently, the electron diffusion differential equation involving α and D was solved for the I–V output as a function of P. A parametric analysis showed that the optimal porosity was equal to 0.41 for maximum I–V output. The analytical results agree well with experimental data reported in the literature. Besides DSSC, the analytical model can be applied to predict the performance of solid-state DSSC as well as dye-sensitized photoelectrochemical cells applied to hydrogen production and water purification.     

Temperature dependence and the oscillatory behavior of the opto-electronic properties of a dye-sensitized nanocrystalline TiO2 solar cell

A dye-sensitized TiO₂ solar cell was developed and characterized. The I–V (current–voltage) characteristics were studied at different temperatures from −40°C to 80°C. The opto-electronic properties of the cell depend on factors like ambient temperature and the time constants of the redox processes at the cell interfaces. The temperature dependence of V_oc and I_sc were clearly demonstrated. I_sc increased with increasing temperature above room temperature, where as V_oc increased with decreasing temperature below room temperature. The opto-electronic properties showed oscillatory behavior especially at low temperatures, which may be attributed to the different velocities of the redox processes occurring at the TiO₂/dye, dye/electrolyte and the electrolyte/counter electrode interfaces.