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.     

Dye-sensitized solar cells: Scale up and current–voltage characterization

Large size, dye-sensitized solar cells (DSSC) have been prepared on silver grid embedded, transparent conductive oxide (TCO) glass substrate by screen printing method. Under one sun condition (AM 1.5, P_in 100 mW cm⁻²), achieved active area energy conversion efficiency of 5 cm×5 cm device approaches that of small-size DSSC prepared at similar condition. To improve the accuracy of efficiency measurement, current–voltage characterizations were carried out with shadow mask and different light reflective backgrounds. It was found that transparent and stripe-like non-active area, arising due to current collector and overcoat layer, of large size DSSC does not strongly influence the energy conversion efficiency.     

Effect of spatial variation of incident radiation on spectral response of a large area silicon solar cell and the cell parameters determined from it

Effect of spatial variation of incident monochromatic light on spectral response of an n⁺–p–p⁺ silicon solar cell and determination of diffusion length of minority carriers (L_b) in the base region and the thickness of the apparent dead layer (x_d) in the n⁺ emitter from the spectral response have been investigated. Spectral response of a few 10 cm diameter and 10×10 cm² pseudo-square silicon solar cells was measured with the help of a standard silicon solar cell of 2×2 cm² area in 400–1100 nm wavelength range. Different areas (4, 9, 16, 25 and total area 78.6 or 96 cm²) were exposed. The effect of the radial variation of incident radiation was determined quantitatively by defining a parameter f₁ as the ratio of the average intensity falling on the reference cell to that on the exposed area of the test cell. The value of f₁ varied between 1 and 1.15 (1.25) as the exposed area of the cell varied from 4 cm² to 78.6 (96) cm² indicating that the spatial inhomogeneity of intensity increased with the increase in the exposed cell area. Short-circuit current densities, J_sc, computed from spectral response data for AM1.5 spectrum were less compared to the directly measured values by a factor which was nearly equal to f₁. However, radial variation of intensity does not affect the determination of diffusion length of minority carriers in the base region (by the long wavelength spectral response, LWSR method using the measured spectral response data in 0.85<λ<1.05 μm range) and the thickness of the dead layer (by the method of Singh et al. using the data of 0.45<λ<0.65 μm range) significantly.     

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.     

Influence of the layer thickness and hydrogen dilution on electrical properties of large area amorphous silicon p–i–n solar cell

The aim of this work is to present data concerning the optimization of performances of a large area amorphous silicon p–i–n solar cell (30×40 cm²) deposited by plasma enhanced chemical vapour deposition (PECVD) at 27.12 MHz. In this work the solar cell was split into small areas of 0.126 cm², aiming to study the device performance uniformity, where emphasis was put on the role of the n-layer thickness. The solar cells were studied through the spectral response behaviour in the 400–750 nm range as well as by the behaviour of the AC impedance. Solar cells with fill factor of 0.58, open circuit voltage of 0.83 V, short circuit current density of 17.14 mA/cm² and an efficiency of 8% were obtained at growth rates higher than 0.3 nm/s.     

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.     

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.     

Relation between solar cell parameters and space degradation

Proton and electron irradiations induce the degradation of space solar cells. The rate of the degradation depends on the current regime in the junction; it is four times lower when the cell is in the diffusion current regime than in the recombination regime. The analysis of the current regime shows that the diffusion regime is predominant for voltages larger than a critical value, which reduces with large values of the surface recombination velocity, small-base doping levels and increasing concentration of recombination centres. We examine the conditions for which we can keep as minimum the degradation rate under irradiation.     

Measurement of solar cell parameters using impedance spectroscopy

Measurement of solar cell parameters is important for the design of satellite power systems. These parameters can be measured using impedance spectroscopy and an equivalent circuit model developed. In this study parameters of a Back Surface Reflector Field solar cell (BSFR) have been measured using impedance spectroscopy. The results show high diffusion capacitance of BSFR cells and their exponential relation to the operating voltage. Cell dynamic resistance, diode factor, transition capacitance, and series resistance could also be measured. The minority carrier life time also has been calculated.     

New method to extract the model parameters of solar cells from the explicit analytic solutions of their illuminated I–V characteristics

We present a new method to extract the intrinsic and extrinsic model parameters of illuminated solar cells containing parasitic series resistance and shunt conductance. The method is based on calculating the Co-content function (CC) from the exact explicit analytical solutions of the illuminated current–voltage (I–V) characteristics. The resulting CC is expressed as a purely algebraic function of current and voltage from whose coefficients the intrinsic and extrinsic model parameters are then readily determined by bidimensional fitting. The procedure is illustrated by applying it to experimental and synthetic I–V characteristics and an analysis of the errors is presented.     

Au–Cu/p–CdTe/n–CdO/glass-type solar cells

The heterostructure design proposed by us for the photovoltaic (PV) solar cell is: Au–Cu/p–CdTe:Sb/n–CdO:F/glass. The CdO:F films were grown by the sol–gel method, in conditions in order to get low resistivity 4.5×10⁻⁴ Ω-cm and an optical transmission higher than 85%. The CdTe:Sb films were prepared by means of the RF sputtering technique, in conditions to get resistivity value around 10⁶ Ω-cm, high crystalline quality and higher grain size. The Au–Cu contacts were thermally evaporated. For the study of PV-heterostructure a systematic variation of the preparation parameters were carried out. The parameters involved in the manufacture of the cell, in order to look for the highest efficiency were: (A) For the deposit of the p-CdTe:Sb films, a low argon pressure of 2.5 m Torr and high substrate temperature of 450 °C. The CdTe:Sb film thickness was varied in the interval 4.5–11 μm. (B) For the activation of the heterostructure: (i) The treatment temperature in vacuum, after the CdTe is deposited, was varied in the 350–550 °C range and (ii) the treatment temperature in Ar atmosphere, after the heterostructure is dipped in CdCl₂ solution, was studied in the 400–510 °C range. (C) Optimization of the Cu–Au contact with the adequate Cu-film thickness. The highest energy conversion efficiency (η) value was 5.48%. This work reports a systematic study of the parameters involved in the solar cell manufacture, for the search of a better value of η.     

Polycrystalline thin-film solar cells – A review

This paper reviews recent progress in polycrystalline thin-film solar cell research and development. Results from both small area cells and larger area modules/submodules are discussed. Emphasis is given to results from deposition techniques with potential for fabrication of large-area cells of the type CdS/CdTe and CdS/Cu(In,Ga)Se₂. Small area high-efficiency cell results are discussed in terms of manufacturability issues for large-area cells. A discussion of recent successes in electrodeposition of high-quality absorbing layers of Cu(In,Ga)Se₂ is given. Hybrid approaches involving final adjustment of local and overall stoichiometries using post-deposition vacuum evaporation of selected substituent elements and thermal anneals are also discussed.     

Self-calibration as a tool for high-accuracy determination of silicon solar cell internal quantum efficiency

Light nonuniformity, uncertainty in the illuminated photoactive area, and relative, but not absolute radiometric data for the reference detector, can be the reasons for the inaccuracy or impossibility of solar cell spectral response and quantum efficiency determination. The use of a self-calibration principle permits minimization of the errors caused by the above factors. This principle consists of quite precise calculation of the internal quantum efficiency Q(λ_m) of the test cell at λ_m≈0.8 μm, where the cell response is weakly dependent on emitter and base parameters. Experimentally determined short- and long-wavelength internal quantum efficiencies, Q(0.4) and Q(0.95), respectively, based on relative radiometric data for a reference detector, are used as starting data for the Q(λ_m) calculation. The ratio of the calculated to measured Q(λ_m) values gives the correction factor for shifting the experimental quantum efficiency curve. Computer modeling supports the assumption that uniform deviation of measured Q(λ) can be precisely corrected by calculation. Analysis of the accuracy of the self-calibration method demonstrates very small uncertainties in the corrections of quantum efficiency measurements, attainable for many practical situations. Confirmation of correctness of the proposed method is shown by analysis of the results of spectral response measurements of several solar cells.     

Space solar cells—tradeoff analysis

This paper summarizes the study that had the objective to tradeoff space solar cells and solar array designs to determine the best choice of solar cell and array technology that would be more beneficial in terms of mass, area and cost for different types of space missions. Space solar cells, which are commercially now available in the market and to be available in the near future, were considered for this trade study. Four solar array designs: rigid, flexible, thin film flexible and concentrator solar arrays were considered for assessment. Performance of the solar cells along with solar array designs were studied for two types of space missions: geo synchronous orbit (GEO) and low earth orbit (LEO) spacecraft. The Solar array designs assumed were to provide 15 kW power for 15 years mission life in GEO and 5 kW power for 5 years mission life in LEO altitudes. To perform tradeoff analysis a spread sheet model was developed that calculates the size, mass and estimates the cost of solar arrays based on different solar cell and array technologies for given set of mission requirements. Comparative performance metrics (W/kg, W/m², kg/m², and $/W) were calculated for all solar arrays studied and compared, at the solar array subsystem level and also at the spacecraft system level. The trade analysis results show that high-efficiency multijunction solar cells bring lot of cost advantages for both types of missions. The trade study also show that thin film solar cells with moderate efficiency with ultra lightweight flexible array design may become competitive with well-established single crystalline solar cell technologies in the future.     

Lateral current effects on the voltage distribution in the emitter of solar cells under concentrated sunlight

The design of the grid contact in silicon solar cells is one of the most important steps for the optimization and fabrication of these energy conversion devices. The voltage drop due to the lateral flow of current towards the grid fingers can be a limiting factor causing the reduction of conversion efficiency. For low current levels this voltage drop can be made small, for typical values of sheet resistance in the emitter, but for solar cells made to operate at high sun concentrations this efficiency loss can be important, unless there is a clear vision of the current and voltage distribution so that the emitter and grid design can be improved. Hence, it is important to establish and solve the current and voltage distribution equations for solar cells with a grid contact. In this work, first these equations are established and then they are solved in order to show the effects that the lateral current flow in the emitter cause on the voltage distribution, particularly at high illumination levels. In addition, it will be shown that the open circuit voltage is significantly reduced due to the lateral current flow as compared to the value predicted from a simple equivalent circuit with a lumped resistance model.     

Photovoltaic effects of a:C/C60/Si (p–i–n) solar cell structures

An attempt to improve the efficiency of the heterojunction p-a:C/n-Si has been made by introducing the highly insulating C₆₀ layer between the semiconductor layers of the cell structure. The conductivity of the implanted films is found to increase during the implantation process and it is attributed to the complete disintegration of the fullerene molecules. The efficiency of this structure is found to be 0.1% under AM 1.5 conditions which is ten times higher than the cell fabricated using the boron ion implanted fullerene without the insulating layer.     

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.     

Monitoring current–voltage characteristics and energy output of silicon photovoltaic modules

Photovoltaic (PV) system designers use performance data of PV modules to improve system design and make systems more cost effective. The collection of this valuable data is often not done due to the high costs associated with data acquisition systems. In this paper, we report on the design of a low-cost current–voltage (I–V) measuring system used to monitor the I–V characteristics of PV modules. Results obtained from monitoring seven crystalline silicon modules between October 2001 and November 2002 are presented and discussed. Results obtained also show the value of being able to continuously monitor the current–voltage characteristics of PV modules.     

On the sensitivity of open-circuit voltage and fill factor on dangling bond density and Fermi level position in amorphous silicon p–i–n solar cell

A simple method for calculation of current–voltage characteristics of an amorphous silicon solar cell is described in terms of excitation current, J_G, and excitation voltage, V_G, the latter being defined in terms of separation of quasi-Fermi levels. Contrary to the usual method of calculating the short-circuit current and dark current separately and assuming a linear superposition, in the present method the calculations are done first in the open circuit where the neutrality of space charge can be assumed and then the current has been calculated in terms of a gradient in the quasi-Fermi levels. We find that depending on other parameters, the open-circuit voltage is a weak function of dangling bond density except in cases of very large degradation. The sensitivity of open-circuit voltage, V_oc, to light-induced degradation can further be reduced by moving the thermal equilibrium Fermi level above the upper dangling bond level. Fill factor deterioration is found to be mainly due to conductivity modulation and is higher for the lower values of thermal equilibrium Fermi level.