Analysis of electrical parameters in heterojunctions based on poly 3-octylthiophene and cadmium sulfide thin films

Planar hybrid heterojunctions were built with poly 3-octylthiophene (P3OT) and chemical bath-deposited cadmium sulfide (CdS) thin films on a conductive glass substrate. The organic material, P3OT, acts as a light absorber and the inorganic one, CdS, as the electron acceptor. Two types of CdS films had been used: one is as-deposited and the other doped with HgCl₂. Heterojunctions were formed by casting a chemically synthesized P3OT solution onto CdS films. The P3OT film thickness was also varied for heterojunction studies. Current vs. potential (I–V) characterizations under dark and illumination conditions were performed for the P3OT/CdS heterojunctions under 88 mW/cm² irradiance level, which show photovoltaic effect with different open circuit voltage (V_OC) levels, being as high as 1 V for some devices. A parametric analysis of I–V curves details the effect of CdS resistivity and P3OT film thickness on series and shunt resistance of the heterojunctions.     

Quantitative field performance analysis of a photovoltaic array

A quantitative analysis of the long-term field performance of a photovoltaic (PV) array has been made considering such field variables as solar insolation, temperature and long-term degradation of a solar cell. It deals with (i) the numerical analysis of two degradation factors of a single crystal solar cell, namely, increased lumped series resistance R_s and decreased short circuit current I_sc, (ii) the field condition V_m — I_m relation, and (iii) the ratio of I_m and I_sc for a PV array. The value of I_m has been computed from the solution of a transcendental equation using Newton–Raphson approximation technique. This approach has been verified by field data obtained in Delhi, India, over a period of four years for the single crystal silicon PV modules (manufactured by CEL, India). It can be used for the study of the long-term field performance of a polycrystalline and amorphous silicon PV module.     

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.     

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%.     

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.     

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.     

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.     

A new method of determination of series and shunt resistances of silicon solar cells

A new method of measurement of series resistance R_s and shunt resistance R_sh of a silicon solar cell is presented. The method is based on the single exponential model and utilizes the steady state illuminated I–V characteristics in third and fourth quadrants and the V_oc–I_sc characteristics of the cell. It enables determination of values of R_sh and R_s with the intensity of illumination. For determination of R_s it does not require R_sh to be assumed infinite and realistic values of R_sh can be used. The method is very convenient to use and in the present study it has been applied to silicon solar cells having finite values of R_sh. We have found that R_sh is independent of intensity but the R_s decreases with both the intensity of illumination and the junction voltage.     

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.     

Copper metallization for crystalline Si solar cells

Cu metallization for crystalline Si solar cells was investigated using either Ti or Ti/TiN diffusion barriers. The resistivity and the specific contact resistance change were measured for both Ti(30 nm)/Cu(100 nm) and Ti(30 nm)/TiN(30 nm)/Cu(100 nm) contact structures under various annealing conditions. As the annealing temperature increased, the efficiency of the cells increased mainly due to the increase in fill-factor and I_SC, which was correlated with the series resistance (R_S) of the metal layer. The solar cells with Ti/TiN/Cu contacts generally showed the higher efficiencies than those with Ti/Cu, because in Ti/Cu contacts Cu diffused through Ti and increased R_S.     

Density of interface states, excess capacitance and series resistance in the metal–insulator–semiconductor (MIS) solar cells

Dark and illuminatied current–voltage (I–V) characteristics of Al/SiO_x/p-Si metal–insulator–semiconductor (MIS) solar cells were measured at room temperature. In addition to capacitance–voltage (C–V) and conductance–voltage (G–V), characteristics are studied at a wide frequency range of 1 kHz–10 MHz. The dark I–V characteristics showed non-ideal behavior with an ideal factor of 3.2. The density of interface states distribution profiles as a function of (E_ss−E_v) deduced from the I–V measurements at room temperature for the MIS solar cells on the order of 10¹³ cm⁻² eV⁻¹. These interface states were responsible for the non-ideal behavior of I–V, C–V and G–V characteristics. Frequency dispersion in capacitance for MIS solar cells can be interpreted only in terms of interface states. The interface states can follow the a.c. signal and yield an excess capacitance, which depends on the relaxation time of interface states and the frequency of the a.c. signal. It was observed that the excess capacitance C_o caused by an interface state decreases with an increase of frequency. The capacitances characteristics of MIS solar cells are affected not only in interface states but also series resistance. Analysis of this data indicated that the high interface states and series resistance leads to lower values of open-circuit voltage, short-circuit current density, and fill factor. Experimental results show that the location of interface states and series resistance have a significant effect on I–V, C–V and G–V characteristics.     

Influence of 1-methyl-3-propylimidazolium iodide on I3/I redox behavior and photovoltaic performance of dye-sensitized solar cells

In this paper, we investigated redox behavior of I⁻ and I₃⁻ in 3-methoxypropionitrile (MePN) with different concentrations of 1-methyl-3-propylimidazolium iodide (MPII) and iodine by cyclic voltammetry and electrochemical impedance spectroscopy. It was found that the apparent diffusion coefficient (D) values of triiodide and iodide ions, the serial resistance (R_s) and the charge-transfer resistance (R_ct) decreased slightly with increase of the concentration of I₃⁻ in MePN containing 1.4 M MPII. Moreover, the R_ct and D values of triiodide and iodide ions affection on dye-sensitized solar cells (DSCs) should be considered as a whole. The DSCs with the electrolyte (1.4 M MPII, 0.1 M LiI, 0.1 M I₂, 0.5 M TBP, in MePN) gave short circuit photocurrent density (J_sc) of 14.44 mA/cm², open circuit voltage (V_oc) of 0.72 V, and fill factor (FF) of 0.69, corresponding to the photoelectric conversion efficiency (η) of 7.17% under one Sun (AM1.5).     

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.     

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.     

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.     

Correlating beam radiation with sunshine duration

For seven locations in north and central India, monthly averaged data of daily beam irradiation on a horizontal surface, I, have been correlated with bright sunshine duration, s, using the relation (I/I₀) = b₀(s/S′) where I₀ is the monthly mean value of extraterrestrial irradiation over a day, S′ is the corresponding duration over which Campbell-Stokes type sunshine recorders remain sensitive. It is found that b₀ is a constant that is period and region dependent. Monthly estimates of beam radiation using the relation above show that for the seven stations rms errors are within 3–6%. Tests made for other stations in India also gave good estimates. For the computation of I, global radiation is not required unlike most estimation techniques and therefore the method should find a wide application.It may be observed that b₀ values turn out to be roughly equal to exp(-md T ) where m is the relative airmass, d is the Raleigh optical thickness and T is the Linke turbidity factor.     

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.     

Studies on the interface between novel type ZnO and p-a-SiC:H in 1.5 eV a-SiGe:H pin diodes in comparison to SnOx and ITO

1.5 eV a-SiGe:H pin diodes with p-a-SiC:H window layers (E_g = 1.95 eV) have been deposited on conventional ITO, SnO_x and novel Al-doped ZnO (ZnO:Al) transparent conductive oxides (TCO) in order to compare the influence of TCO/p-a-SiC:H interfaces on diode parameters. Characterization by transmission, sheet resistance and SEM photographs reveals comparable features for the three types of TCOs, ZnO:Al additionally shows a very high chemical stability in hydrogen containing plasmas. Transmission and adsorption experiments also were performed on the layer-system TCO + thin p-a-SiC:H indicating different growth rates of p-a-SiC:H on ZnO:Al and SnO_x. SCLC experiments on sandwich structures consisting of TCO/n⁺ in⁺/Cr demonstrate an increase in the midgap defect density of states N_t(E) for a-SiGe:H layers on ITO but no dependence of N_t(E) on the crystallite size of the substrates. In the case of ZnO:Al the analysis of I–V characteristics underAM1 illumination (100 mW/cm²) displays a clear improvement of open-circuit voltage V_oc and short-circuit current density I_sc. This is due to a higher work function of ZnO:Al in comparison to SnO_x and, therefore, increased built-in potential V_bi. Diffusion of Zn and In into the p-a-SiC:H-layer, however, causes a rise in series resistance R_S and lowering of the fillfactor FF for ZnO:Al and ITO as front contacts. With respect to extended optimization, in particular avoidance of the high series resistance, the very stable, high temperature deposited ZnO:Al seems to be a promising material for a-Si:H based solar cells.     

Technical note An approximation to solar cell equation for determination of solar cell parameters

An approximation was applied to a solar cell equation to eliminate reverse saturation current. I–U curves obtained from the approximated equation were fitted to experimentally measured data by using the iteration technique. All instrumentation errors were taken into account in the fitting procedure. The Filling Factor (FF) and the series resistance (R_s) appeared to be important for fitting accuracy. The fitting procedure was repeated for different illuminations and solar cell temperatures.     

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.