Measurement of CuInSe2 solar cell AC parameters

The AC parameters (cell capacitance and cell resistance) of Copper Indium Diselenide (CuInSe₂) solar cell are measured using time-domain technique. The cell capacitance is calculated from the open circuit voltage decay (OCVD) and cell resistance with solar cell I–V characteristics measured in dark. The solar cell exhibits high parallel resistance and low parallel capacitance. The doping concentration and built in voltage are derived from the 1/C_P² versus bias voltage graph. The built-in voltage of the solar cell shows good agreement with measurements published in the literature.     

Impedance spectroscopy coupled with voltammetry for quantitative evaluation of temperature and voltage dependent parameters of a silicon solar cell

The techniques of linear sweep voltammetry (LSV) and impedance spectroscopy (IS) are combined to study the detailed temperature and voltage dependencies of a range of performance-indicator parameters of a mono-crystalline n⁺–p Si solar cell. The complex nonlinear least square fitting procedure is employed for quantitative evaluation of the IS data. The underlying mechanisms of the observed temperature/voltage sensitive characteristics of the cell are examined using a collection of currently available theoretical models. The individual roles of minority carrier diffusion and defect-induced charge recombination are manifested in the voltage and temperature dependent signatures of the measured cell parameters. These parameters include the transition layer capacitance and built-in potential of the n⁺–p interface, the acceptor concentration in the base, the series, shunt and recombination resistances, and the effective diode ideality factor.     

Measurement of silicon and GaAs/Ge solar cells ac parameters

The ac parameters (cell capacitance and cell resistance) of Silicon (Si) and Gallium Arsenide (GaAs/Ge) solar cells are measured at different temperatures using time domain technique. The cell capacitance is calculated from the Open circuit voltage decay (OCVD) and the cell resistance from solar cell I–V characteristics measured under dark condition. It is observed that the solar cell capacitance increases whereas the cell resistance decreases with increase in temperature.     

Silicon (BSFR) solar cell AC parameters at different temperatures

The AC parameters of back surface field refiected (BSFR) silicon solar cell are measured at different cell temperatures (198–348 K) both in forward and reverse bias under dark condition using impedance spectroscopy technique. It is found that cell capacitance increases with temperature whereas cell resistance decreases, in forward bias voltage. Beyond maximum power point voltage, the cell inductance (0.28 μH) is measured, as the inductive reactance is comparable with cell series resistance. The measured cell parameters (cell capacitance, dynamic resistance, etc) are used to calculate the mean carrier lifetime and diode factor at different cell temperatures.     

GaAs/Ge solar cell AC parameters at different temperatures

The AC parameters of Gallium Arsenide (GaAs/Ge) solar cell were measured at different cell temperatures (198–348 K) by varying the cell bias voltage (forward and reverse) under dark condition using impedance spectroscopy technique. It was found that the cell capacitance increases with the cell temperature where as the cell resistance decreases, at any bias voltage. The measured cell parameters were used to calculate the intrinsic concentration of electron–hole pair, cell material relative permittivity and its band gap energy. The diode factor and the cell dynamic resistance at the corresponding maximum power point decrease with the cell temperature.     

Device simulation and modeling of microcrystalline silicon solar cells

Device modeling for p–i–n junction basis thin film microcrystalline Si solar cells has been examined with a simple model of columnar grain structure utilizing two-dimensional device simulator. The simulation results of solar cell characteristics show that open-circuit voltage (V_oc) and fill factor considerably depend on structural parameters such as grain size and acceptor doping in intrinsic layer, while short-circuit current density (J_sc) is comparatively stable by built-in electric field in the i-layer. It is also found that conversion efficiency of more than 16% could be expected with 1 μm grain size and well-passivated condition with 10 μm thick i-layer and optical confinement.     

Outdoor testing of single crystal silicon solar cells

The evaluation and assessment of the performance of photovoltaic (PV) cells requires the measurement of the current as a function of voltage, temperature, intensity, wind speed and radiation spectrum. Most noticeable of these parameters is the PV conversion efficiency η (defined as the maximum electrical power P_max produced by the PV cell divided by the incident photon power P_in) which is measured with respect to standard test conditions (STC). These conditions refer to the solar spectrum , solar radiation intensity , cell temperature and wind speed (2 mph). Tests under STC are carried out in laboratory-controlled environment.With an increase of ambient temperature, there is a deficiency in the electrical energy that the solar cell can supply. This situation is especially important in hot climates. Outdoor exposure tests of solar cells have been conducted in the Department of Physics, University of Brunei Darussalam. Preliminary results demonstrate that the efficiency of the single crystal silicon solar cell strongly depends on its operating temperature. It has been noted that at the operating temperature of 64 °C, there was a decrease of 69% in the efficiency of the solar cell compared with that measured at STC. Investigation of the effect of variation in intensities of sunlight on the solar cell performance showed that the efficiency of the cell is reduced as intensities of sunlight are reduced but at a rate different from the reduction in intensities.     

Thermal design influence on the performance of solar thermophotovoltaic devices

The present theory integrates all components of a thermophotovoltaic (TPV) device (i.e. the primary lens, the absorber, the photovoltaic (PV) cell and a photon recuperator system). Energy balances are derived for the absorber and PV cell. The TPV efficiency is maximized by using three optimization parameters, namely, absorber temperature, PV cell temperature and cell voltage. An ad-hoc numerical constraint optimization procedure is developed which allows computation of an optimum absorber temperature within 5–10?K error. In case of an improved thermal design, the ‘ideal’ bandgap for TPV solar-energy conversion is in the range 0.5–1?eV, in reasonable good agreement with present knowledge. Further improvement in thermal-design quality moves the optimum bandgap toward higher values. Improving thermal-design quality decreases the influence of the concentration ratio, on both optimum absorber temperature and optimum PV cell voltage. An improved thermal design allows almost all narrow-bandgap materials to operate at positive voltage. The results prove existence of an optimum concentration ratio.     

Optimization of arbitrarily doped MINP solar cell performance

A comprehensive model of four layered MINP⁺ (metal-tunnel insulator - NP⁺ semiconductor) structure incorporating an inhomogeneous impurity doping profile is numerically simulated. The influence of built-in electric field in the device on its performance is investigated. The drift fields are the result of the nonuniformaly doped semiconductor. The dependence of the minority carrier lifetime and mobility on the doping density are taken into account. Solution curves for the complete set of transport, continuity and Poisson equations for the device as solar cell are obtained. Significant device parameters are identified and model calculations are carried out over a range covering most physically realizable values for these parameters. The I-V characteristics are calculated as a function of insulator thickness, metal workfunction, substrate doping density and semiconductor thickness with present semiconductor fabrication technology.     

Investigation of non-aqueous electrodeposited CdS/Cd1−xZnxTe heterojunction solar cells

Polycrystalline Cd_1−xZn_xTe solar cells with efficiency of 8.3% were grown by cathodic electrodeposition on glass/ITO/CdS substrates using non-aqueous ethylene glycol bath. The deposit is characterised versus the process conditions by XRD and found to possess a preferred (1 1 1) orientation on Sb doping in the electroplating bath. The surface morphology of the deposit is studied using atomic force microscope. The average RMS roughness for the ternary film was higher than that for the binary CdTe. Optical properties of the films were carried out to study the band gap and calculation of molar concentration ‘x’. The effects of Sb doping in CdS/Cd_1−xZn_xTe heterojunctions have been studied. The short circuit current density (c) was found to improve and series resistance (R_s) reduced drastically upon Sb doping. This improvement in J_sc is attributed to an increase in quantum efficiency. The evaluation of solar cell parameters was also carried out using the current–voltage characteristics in dark and illumination. The best results were obtained when 2×10⁻³ M ZnCl₂ along with antimony were present in the deposition bath. Under AM 1.5 conditions the open circuit voltage, short circuit current density, and fill factor of our best cell were V_oc=600 mV, J_sc=26.66 mA/cm², FF=0.42 and efficiency, η=8.3%. The carrier concentration and built-in potential of Cd_1−xZn_xTe calculated from Mott–Schottky plot was 2.72×10¹⁷ cm⁻³ and 1.02 eV.     

Annual output estimation of concentrator photovoltaic systems using high-efficiency InGaP/InGaAs/Ge triple-junction solar cells based on experimental solar cell's characteristics and field-test meteorological data

The temperature dependences of the electrical characteristics of InGaP/InGaAs/Ge triple-junction solar cells under concentration were evaluated. For these solar cells, conversion efficiency (η) decreased with increasing temperature, and increased with increasing concentration ratio owing to an increase in open-circuit voltage. The decrease in η with increasing temperature decreases with increasing concentration ratio. Moreover, the annual output of a concentrator system with a high-efficiency triple-junction cell was estimated utilizing the experimental solar cell's characteristics obtained in this study and field-test meteorological data collected for 1 year at the Nara Institute of Science and Technology, and compared with that of a nonconcentration flat-plate system.     

Low temperature passivation of silicon surfaces by polymer films

A novel surface passivation method for silicon carrier lifetime measurements and solar cells using a polymer film is introduced. It is easy to apply, no special pre-treatment, e.g. no hydrofluoric acid (HF)-treatment, is necessary. The surfaces to be passivated are covered with the polymer solution, dried at 90°C and encapsulated. Surface recombination velocities (S) as low as S=30 cm/s for various doping concentrations have been observed, nearly independent of the bulk injection level. The passivation is stable for at least 6 h. For a polymer-passivated rear contact solar cell the same open circuit voltage is achieved as for a cell with thermally grown oxide.     

The circuit point of view of the temperature dependent open circuit voltage decay of the solar cell

The open circuit voltage decay (OCVD) technique has been used to determine the minority carrier lifetime. In this study, an experimental and analytical method is described for determination of minority carrier lifetime at porous Si based solar cell by photo induced OCVD technique. The cell is illuminated by a monochromatic light source (λ = 658 nm) in the open circuit configuration, and the decay of voltage is measured after abruptly terminating the excitation. For the analysis of the OCVD characteristic of solar cell device, equivalent electrical circuit has been proposed in which the diffusion capacitance is connected in series with the contribution of the solar cell interface. Exact minority carrier lifetimes at low (50-170 K) and high (190-330 K) temperature regions have been obtained as 28.9 and 2.65 μs from the temperature dependent OCVD measurements by using an alternative extraction technique.     

Concentrator silicon solar cells from silicon industry rejects

Two types of silicon (Si) substrates (40 n-type with uniform base doping and 40 n/n⁺ epitaxial wafers) from the silicon industry rejects were chosen as the starting material for low-cost concentrator solar cells. They were divided into four groups, each consisting of 20 substrates: 10 are n/n⁺ and 10 are n substrates, and the solar cells were prepared for different diffusion times (45, 60, 75 and 90 min). The fabricated solar cells on n/n⁺ substrates (prepared with a diffusion time of 75 min) showed better parameters. In order to improve their performances, particularly the fill factor, 20 new solar cells on n/n⁺ substrates were fabricated using the same procedure (the diffusion time was 75 min)—but with four new front contact patterns. Investigation of current–voltage (I–V) characteristics under AM 1.5 showed that the parameters of these 20 new solar cells have improved in comparison to previous solar cells' parameters, and were as follows: open-circuit voltage (V_OC=0.57 V); short circuit current (I_SC=910 mA), and efficiency (η=9.1%). Their fill factor has increased about 33%. The I–V characteristics of these solar cells were also investigated under different concentration ratios (X), and they exhibited the following parameters (under X=100 suns): V_OC=0.62 V and I_SC=36 A.     

Temperature-dependent quantum efficiency analysis of graded-gap Cu(In,Ga)Se2 solar cells

Despite the high solar cell efficiencies achieved with Cu(In,Ga)Se₂ (CIGS) absorbers, key parameters such as the carrier diffusion length and recombination lifetime are still under investigation. Here, we extract lifetime and diffusion length from temperature-dependent internal quantum efficiency (IQET) spectra of state of the art high efficiency CIGS solar cells. Two-parameter fits to the measured IQE curves using a model for double-graded gap solar cells show very good agreement in the studied temperature range T=146–293 K, allowing the extraction of the electron recombination lifetime in the absorber and the collection probability in the front region of the cell. The obtained results agree with current literature values obtained by other characterization techniques. Furthermore, the temperature dependence of the recombination lifetime is explained by Shockley–Read–Hall recombination through a single bulk defect level with an activation energy of 200 meV.     

Effect of temperature on dark current characteristics of silicon solar cells and diodes

The influence of temperature on the dark forward current–voltage characteristics of a single crystalline silicon solar cell and a small silicon diode within the range from 295–373 K has been analysed. It was shown that the forward voltage of the solar cell degrades 2 mV and in the case of diode 1 mV per 1 K temperature increases at constant forward current of 100 mA. Thermal resistance and heat transfer from the solar cell by using a thick copper plate as a heat sink have also been discussed. For the series resistance determination the current–voltage I–U characteristics of single crystalline silicon solar cells in different temperatures were measured in the dark. It was proved that series resistance of the silicon solar cells and diodes is temperature dependent and increases with temperature increase 0.65% K^?1. Therefore, protection of silicon solar cells as well as silicon diodes against overheating is essential during their exploitation. Copyright © 2005 John Wiley & Sons, Ltd.     

Carrier transport in high-efficiency ZnO/SiO2/Si solar cells

Carrier transport in ZnO/SiO₂/n-Si solar cell has been theoretically analyzed with a consideration that the photo-carrier transport from silicon to ZnO layer through the barrier is dominated by quantum mechanical tunneling process of minority carrier. It was found that the highest efficiency of the cell could be achieved at SiO₂ layer thickness of around 20 Å. The efficiency of the cells decreases as the surface states density Q_ss becomes higher. Moreover, the efficiency increases as the electron concentration of ZnO layer is increased due to the decrease of work function of ZnO. It was also found that the lower transmittance of the high carrier concentration ZnO due to the free-carrier absorption at infrared wavelength region does not give any significant effect to the cell performance. The efficiency of higher than 25% is achievable by optimizing the involved device parameters.     

2D-numerical analysis and optimum design of thin film silicon solar cells

Device modeling for p–i–n junction μc-Si basis thin film polycrystalline Si solar cells has been examined with a simple model of columnar grain structure and its boundary condition utilizing two-dimensional device simulator. As the simulation results of solar cell characteristics show, open-circuit voltage (V_oc) and curve fill factor (FF) considerably depend on those structural parameters, while short-circuit current density (J_sc) is comparatively stable by courtesy of homogeneous built-in electric field in the i layer. It has also been found that conversion efficiency over 12% could be expected with 1 μm grain size and well-passivated condition with 3 μm thick i-layer.     

Computer simulation of the effect of phosphorous doping of the i-layer in a thin-film a-Si:H p–i–n solar cell

The simulation RAUPV2 has been used to model a thin-film a-Si:H p–i–n solar cell, fabricated at the Rand Afrikaans University. For a physically acceptable set of input parameters, the simulated J–V curve agrees very well with the empirical J–V curve, under AM1.5 g illumination. The effect of boron- and phosphorous doping of the i-layer (B- and P-profiling) was studied. It was found that boron doping of the i-layer greatly reduced cell performance. On the other hand, there seemed to be an optimal phosphorous concentration in the i-layer, P_opt, for which cell performance, measured in terms of maximum power output, was a maximum. It was observed that as the P concentration in the i-layer was increased towards P_opt, the recombination rate in the front of the i-layer decreased, whilst that in the back part of the i-layer increased. The short-circuit current was seen to decrease under P-profiling. It was seen that as a consequence of P-profiling, the drift field in the back part of the i-layer was relatively insensitive to the effect of an applied voltage, for applied voltages up to about 0.55 V.     

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of J_sc, V_oc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the V_oc degradation while maximizing the short circuit current density (J_sc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in J_sc and V_oc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells.