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.     

Dark I–U–T measurements of single crystalline silicon solar cells

The effect of parasitic resistances on silicon solar cell performance was discussed. The current–voltage I–U characteristics of single crystalline silicon solar cells at different temperatures were measured in the dark. A one and two diodes equivalent model was used to describe the electronic properties of the solar cells. The diode ideality factors, the series and shunt resistance, that determine the fill factor and the efficiency of the solar cell, have been estimated. It was proved that the performance of the tested silicon solar cell can be described with enough accuracy by the one diode equivalent model with series resistance r_s equal to 0.1 Ω and an empirical ideality factor m_id equal to 1.4.     

Photoconductive Schottky diode based on Al/p-Si/SnS2/Ag for optical sensor applications

The Schottky barrier junctions of tin disulfide (SnS₂) on p-silicon were fabricated using sol–gel spin technique. The photoresponse and junction properties of the diode were investigated. The ideality factor and barrier height of the Al/p-Si/SnS₂/Ag diode were obtained to be 1.54 and 0.53 eV, respectively. The photocurrent properties of the device under various illuminations were also explored. The photocurrent in the reverse bias voltage is increased by increasing photo-illumination intensity. The transient photocurrent results indicate that photocurrent under illumination is higher that the dark current. The capacitance–voltage characteristics of diode were also investigated at different frequencies. The capacitance decreases with increasing frequency due to a continuous distribution of the interface states. These results suggest that the fabricated diode can be used for optical sensor applications.     

Analysis of illumination-intensity-dependent j–V characteristics of ZnO/CdS/CuGaSe2 single crystal solar cells

Current–voltage characteristics of ZnO/CdS/CuGaSe₂ single crystal solar cells measured at room temperature are investigated depending on illumination intensity. The characteristics can be described using the two-diode model, indicating two current transport mechanisms acting in the cells. The first and dominant mechanism is recombination of carriers at the interface between CdS and CuGaSe₂. The second one is recombination in the depletion region, which has been found to have a small effect on the solar cell photovoltaic performance. Both the diode ideality factor and the saturation current density of the dominant diode increase under illumination. A model based on interface recombination can explain these results. This model allows the estimation of diffusion voltage, capture cross-section of holes at the interface and mobility of electrons in the CdS layer.     

A new method to determine the diode ideality factor of real solar cell using Lambert W-function

A new method using Lambert W-function is presented to determine the diode ideality factor of real solar cell.     

Organic and inorganic solar cells parameters evaluation from single I–V plot

This paper presents a new method to determine the five solar cell parameters of the single diode lumped circuit model. These parameters are usually the saturation current, the series resistance, the ideality factor, the shunt conductance and the photocurrent. This method is based on the measured current–voltage data. The method has been successfully applied to a commercial silicon solar cell, a module and an organic solar cell.     

Parameter identification for solar cell models using harmony search-based algorithms

Recently, accurate modeling of current vs. voltage (I–V) characteristics of solar cells has attracted the main focus of various researches. The main drawback in accurate modeling is the lack of information about the precise values of the models parameters, namely, photo-generated current, diode saturation current, series resistance, shunt resistance and diode ideality factor. In order to make a good agreement between experimental data and the models results, parameter identification with the help of an optimization technique is necessary. Because I–V curve of solar cells is extremely non-linear, an excellent optimization technique is required. In this paper, harmony search (HS)-based parameter identification methods are proposed to identify the unknown parameters of the solar cell single and double diode models. Simple concept, easy implementation and high performance are the main reasons of HS popularity to solve complex optimization problems. For this aim, three state-of-the-art HS variants are used to determine the unknown parameters of the models. The effectiveness of the HS variants is investigated with comparative study among different techniques. Simulation results manifest the superiority of the HS-based algorithms over the other studied algorithms in modeling solar cell systems.     

Fill factor losses in ZnO/CdS/CuGaSe2 single-crystal solar cells

Current–voltage characteristics of ZnO/CdS/CuGaSe₂ single-crystal solar cells with solar conversion efficiency values of η=3.5%, 6.0%, 6.7% and 9.7% were analyzed using the single diode equation. The effect of each of the achieved parameters on the fill factor was calculated. The calculations revealed that the fill factor reduction due to the series resistance remained below Δff=4.4% under illumination, while this effect would have been much higher if the illumination had not reduced the series resistance markedly. The calculation furthermore revealed that the fill factor reduction due to the shunt resistance remained below Δff=3.6% under illumination. This effect would have been negligible if the illumination had not also reduced the shunt resistance in all studied cells. The increase of the saturation current density under illumination has brought about considerably high fill factor losses (at least Δff=8.3%) in all studied cells. Already the dark saturation current density and the diode ideality factor in such cells have been found to be much higher than the ones in the cells based on CuInSe₂. This seems to be the most substantial restriction to the fill factor, and thus to the performance, of solar cells based on CuGaSe₂. An explanation for this different behavior seems to lie in the different band structures of these cells.     

太阳电池模型参数对并联组件输出特性的影响

根据太阳电池I-V方程和基本电路联接理论,推导出选择具有相同最大功率点电压V_m的单体太阳电池组成并联组件可以获得最大输出功率;分别计算出二极管理想因子A,短路电流I_(sc),反向饱和电流I_o,电池串联内阻R_s对最大功率点电压V_m和组件失配损失的影响,确定出对它们影响最大的模型参数是二极管理想因子A。     

Determination of solar cell parameters from its current–voltage and spectral characteristics

An algorithm for the calculation of solar cell parameters (series and parallel resistance, diode coefficient, reverse current density) calculation from its current–voltage characteristics at fixed illumination intensity is proposed. The possibility of determining the p–n junction depth on the basis of spectral dependencies of diode photocurrent at different values of the applied bias voltage is shown.     

Lift-off process reducing crack formation and its Cu(In,Ga)Se2 thin film solar cell applications

To reduce cracks caused by the lift-off process in a Cu(In,Ga)Se₂ (CIGS) layer, we focused on increasing the transferred layer thickness. We investigated the relationship between crack formation and the transferred layer thickness which is controlled by a Mo back electrode thickness. We found that the cracks were reduced by increasing the back electrode thickness. We suggest that the dominant factor of the crack reduction is attributed to the increase of the film hardness by increasing the Mo back electrode thickness. Next, we applied this crack reduction method to the solar cell fabrication. From the comparison of the 0.2-μm-thick Au single and 0.2-μm-thick Au/1.6-μm-thick Mo stacked back electrode lift-off CIGS solar cells, we investigated advantages of our crack reduction method. The crack formation was reduced only for the stacked back electrode lift-off solar cell. From the spatial distribution evaluation of an external quantum efficiency (EQE), we found that the crack reduction leads to not only the increase of an average EQE but also the decrease of EQE dispersion. In the solar cell parameters, the stacked back electrode lift-off solar cell without cracks showed the short-circuit current density and fill factor higher than those of the single back electrode lift-off solar cell with cracks. As a result, the conversion efficiency improvement as high as approximately 1% (an absolute value) was obtained. Moreover, the stacked back electrode lift-off solar cell showed the diode parameters (the diode ideality factor, the saturation current density, and series resistance) better than those of the single back electrode lift-off solar cell in the dark current density-voltage characteristics. We concluded that this high fill factor was attributed to the better diode performance. We therefore found that the stacked back electrode structure was very effective for improving the solar cell performance using the lift-off process.     

Effect of interface recombination on solar cell parameters

A model is presented for p–n hetero-junction solar cells in which interface recombination is the dominant diode current transport mechanism. The model explains the large diode ideality factor (n>2) and the increased saturation current density in terms of increased density of interface states N_ir. Furthermore, the model allows us to explain the non-translation between illuminated and dark J–V characteristics. The explanation is based on the assumption that, for high interface state density values, both the depletion layer width and the diffusion voltage in the p- and n-side of the junction are functions of N_ir. The interface recombination leads to lower values of the open-circuit voltage, short-circuit current density, and fill factor. These results are illustrated by numerical calculations of solar cell parameters and compared with experimental data achieved for ZnO/CdS/CuGaSe₂ single-crystal solar cells.     

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.     

Separation of solar cell current into its constituent parallel currents under illumination

A method is presented which suggests the sharing out of total cell short circuit current among the constituent parallel branches of the equivalent circuit, allowing for the identification of their contributions to the total current–voltage (I–V) characteristics under illumination in solar cells described through two parallel diodes. The method is based on the fact that all parallel branches are always under the same voltage; in particular, the open circuit voltage is the same for the cell as a whole as well as for each of the two diodes. With the help of the parameters of each diode and its share in the cell short circuit current, its light I–V characteristic can be drawn, and its individual fill factor calculated. Furthermore, the method suggests a way to estimate the cell fill factor with the help of the individual fill factors. The application of the method is carried out on experimental data of a ZnO/CdS/CuGaSe₂ single crystal solar cell.     

8 MeV electron irradiation studies on electrical characteristics of Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se₂ (CIGS) solar cells are gaining considerable interest due to their high optical absorption coefficient and adjustable band gap, which enables them to achieve high conversion efficiency and also present many promising applications in space power systems. In this paper we report the results of the effect of temperature and 8 MeV electron irradiation on the electrical characteristics of ZnO/CdS/Cu(In,Ga)Se₂/Mo polycrystalline thin-film solar cells under forward and reverse bias studied in the temperature range 270-315 K. The solar cells were subjected to 8 MeV electron irradiation from the Microtron accelerator and were exposed to graded doses of electrons up to 75 kGy. I-V characteristics of the cells under dark and AM 1.5 illumination condition were studied before and after the irradiation. Capacitance measurements were also carried out at various frequencies before and after irradiation. In the measured temperature range, the dark current contribution is due to the generation-recombination of the minority carriers in the depletion region. The ideality factor is found to decrease with increase in temperature. It seems that electron irradiation has not altered the dark current conduction mechanism significantly. The effect of electron irradiation on the solar cell parameters such as fill factor (FF), conversion efficiency (η), saturation current (I_o), short circuit current (I_sc), open circuit voltage (V_oc), and ideality factor (n) was studied. They were found to be stable up to 75 kGy of electron dose as only small changes were observed in the solar cell parameters.     

Temperature-dependent properties of organic-on-inorganic Ag/p-CuPc/n-GaAs/Ag photoelectric cell

A thin organic film of copper phthalocynanine (CuPc) as p-type semiconductor was deposited by vacuum evaporation on n-type GaAs single-crystal semiconductor substrate. Electrical, photoelectrical and frequency response of the cells were investigated at a temperature interval of 23–74 °C. Photoelectric characteristics were measured under semiconductor laser beam injection illumination (), while frequency response was investigated by laser beam modulated with a frequency range of 10 Hz–100 kHz.It was observed that cell parameters such as rectification ratio; threshold voltage; nonlinearity coefficient; junction, shunt and series resistances; diode ideality factor and power conversion efficiency were temperature-dependent. Moreover, experimental data showed that open-circuit voltage decreases with an increase in frequency whereas short-circuit AC current falls with frequency but remains constant as a function of temperature. It was further observed that the short-circuit DC current remained constant with an increase in frequency as well as temperature. Based on the experimental data an equivalent circuit of photoelectric cell was proposed to explain the observed behavior.     

Degradation of solar cells made of upgraded metallurgical grade silicon

The aging effect of solar cells made of upgraded metallurgical grade silicon under continued exposure to ultraviolet light has been studied. It has been observed that in 3 years the cell degrades from 6.6% to 4.7% in efficiency mainly because of an increase in reflectivity and a decrease in the shunt resistance. Measurements of diode ideality factors and reverse saturation current densities revealed that the overall junction and the material quality suffered significantly large damages.     

Temperature dependence of Cu2ZnSn(SexS1−x)4 monograin solar cells

The temperature dependence of open-circuit voltage (V_oc), short-circuit current (I_sc), fill factor (FF), and relative efficiency of monograin Cu₂ZnSn(Se_xS_1−x)₄ solar cell was measured. The light intensity was varied from 2.2 to 100 mW/cm² and temperatures were in the range of T = 175-300 K. With a light intensity of 100 mW/cm²dV_oc/dT was determined to be −1.91 mV/K and the dominating recombination process at temperatures close to room temperature was found to be related to the recombination in the space-charge region. The solar cell relative efficiency decreases with temperature by 0.013%/K. Our results show that the diode ideality factor n does not show remarkable temperature dependence and slightly increases from n = 1.85 to n = 2.05 in the temperature range between 175 and 300 K.     

Design and Optimization of Amorphous Based on Highly Efficient HIT Solar Cell

The objective of this paper is to improve the power conversion efficiency of HIT solar cell using amorphous materials. A high efficiency amorphous material based on two dimensional heterojunction solar cell with thin intrinsic layer is designed and simulated at the research level using Synopsys/RSOFT-Solar Cell utility. The HIT structure composed of TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n⁺)/Ag is created by using of RSOFT CAD. Optical characterization of the cell is performed by Diffract MOD model based on RCWA (rigorous coupled wave analysis) algorithm. Electrical characterization of the cell is done by Solar cell utility using based on Ideal diode method. In addition, optimization of the different layer thickness in the HIT structure is executed to improve the absorption and thereby the photocurrent density. The proposed HIT solar cell structure resulted in an open circuit voltage of 0.751 V, a short circuit current density of 36.37 mA/cm² and fill factor of 85.37% contributing to the total power conversion efficiency of 25.91% under AM1.5G. Simulation results showed that the power conversion efficiency is improved by 1.21% as compared to the reference HIT solar cell. This improvement in high efficiency is due to reduction of resistive losses, recombination losses at the hetero junction interface between intrinsic a-Si and c-Si, and optimization of the thicknesses in a-Si and c-Si layers.