Diode Saturation Current Density Research Articles

ACTIVATION METHOD OF CADMIUM TELLURIDE THIN FILMS FOR CREATION EFFECTIVE SOLAR CELLS

The influence of annealing in freon on the cadmium telluride films crystal structure obtained by closed space sublimation method and the output parameters and light diode characteristics of solar cells based on these films were studied. Annealing of structures was performed at a temperature of 400°С for different times. The studies of the crystal structure were performed by the X-ray diffraction analysis. These studies have shown that the annealing in freon leads to a recrystallization of cadmium telluride films, which reduces microstress. It is evidenced by the lattice period decrease after annealing from а = 6.491 Å to а = 6.488 Å. The study of the influence of annealing in freon on the output parameters and light diode characteristics of solar cells was performed. For this purpose the light current-voltage characteristics of ITO/SnO2/CdS/CdTe structures, annealed for 20‑40 minutes, were studied. It was found that the optimal annealing time in freon is 30 minutes. It is shown that the solar cells based on such films have the highest efficiency in 11.7% after annealing in freon for 30 minutes. A simulation of light diode characteristics influence on the efficiency showed that the solar cells efficiency increase due to the diode saturation current density decrease when the annealing time in freon increases to 30 minutes. When the annealing time increases to 40 minutes the efficiency decrease is associated with a shunt resistance decrease. It is also shown that the pre-irradiation of solar cells in the AM1.5 mode for 20 minutes leads to an additional efficiency increase to 12.2%. According to the simulation results, such efficiency increase is also due to the diode saturation current density decrease.

Improving the efficiency of industrial samples of silicon solar elements

Possibilities of increasing the efficiency by more than 20 % for silicon photoelectric converters made in China have been investigated. It has been established by the method of computer simulation that the life-times of nonequilibrium charge carriers, which are 520 μs, realized in such photoelectric converters, do not limit the possibility of increasing their efficiency by more than 20 %. It is shown that an increase in the photocurrent density to 43.1 mA/cm2 leads to an increase in efficiency to 20.1 %, and a decrease in the diode saturation current density to 3.1∙10-14 A/cm2 leads to an increase in efficiency to 20.4 %. Simultaneous change of these diode characteristics leads to an increase in efficiency to 23.1 %. The paper proposes physical and technological approaches to increase the photocurrent density and reduce the diode saturation current density in ready-made photovoltaic converters. The study of the influence of operating temperature on the efficiency of crystalline silicon photoelectric converters is carried out in the article. It is shown that with increasing operating temperature the relative decrease in the efficiency of single-crystal devices is –0.7 relative %/C, which is significantly higher than in the instrument structures of European production and due to non-traditional decrease in short-circuit current density. Mathematical modeling of the influence of light-emitting diode characteristics on the efficiency of crystalline silicon solar cells showed that the decrease in the efficiency of instrument structures with increasing operating temperature is due not only to an increase in diode saturation current density from 10-13 A to 3·10-13 A, which is 300 %, but also by reducing the shunt resistance from 2.5 kOhm to 1.5 kOhm. A study of the effect of operating temperature on the diode saturation current showed that the height of the potential barrier in the studied silicon photovoltaic converters is 0.87 eV, due to the insufficient level of doping of the base material. The limited height of the potential barrier leads to an unconventional decrease in the shunt resistance with increasing operating temperature.

Modeling and optimization of organic tandem photovoltaic solar cells using silver nanowires as electrode and interconnecting layer

Much research has been conducted to improve the performance of photovoltaic solar cells. Transparent conductive film and interconnection layers have a significant impact on the performance of photovoltaic cells. In this work, we analyze the experimental results obtained on tandem organic photovoltaic solar cells with simple inverted structures using silver nanowires AgNW as transparent conductive electrode (TE) and as interconnection layer (ICL) between PEDOT: PSS and ZnO. This type of contact leads to a strong ohmic contact in both sub-cells having P3HT: ICBA as the lower active layer and having PTB7: PC71BM (1: 1.5) as the upper active layer with a good complement of the absorption spectrum. To study the advantages of using AgNWs as an interconnection layer (PEDOT: PSS/AgNWs/ZnO) in tandem photovoltaic solar cells and as an anode and its impact on the performance of these organic cells, we have simulated the electrical characteristics obtained by these tandem organic photovoltaic cells using an equivalent circuit model. This model is based on a single diode model with five photovoltaic parameters. We therefore extracted all the physical parameters of the illuminated photovoltaic cell from its experimental characteristics (J–V), such as the diode saturation current density (J0), the series and shunt resistors (RS, RSh), the ideality factor (n) and the photogenerated current density (JPh). For this we have solved the analytical equations of the current density using Newton Raphson's method. The equations are derived from the single diode equivalent circuit proposed to simulate the measured current density as a function of the voltage of the manufactured tandem type organic solar cells. A good agreement was obtained between the theoretical model and the experimental electrical characteristics. This confirms that the use of AgNWs between PEDOT: PSS and ZnO as an interconnection layer in reverse geometry of these tandem devices, has improved the efficiency (PCE = 9.24%) and is proving to be an efficient recombination layer for tandem organic photovoltaic solar cells.

Efficient Type-II Heterojunction Nanorod Sensitized Solar Cells Realized by Controlled Synthesis of Core/Patchy-Shell Structure and CdS Cosensitization.

Here, we report the successful application of core/patchy-shell CdSe/CdSe xTe1- x type-II heterojunction nanorods (HNRs) to realize efficient sensitized solar cells. The core/patchy-shell structure designed to have a large type-II heterointerface without completely shielding the CdSe core significantly improves photovoltaic performance compared to other HNRs with minimal or full-coverage shells. In addition, cosensitization with CdS grown by successive ionic layer adsorption and reaction further improves the power conversion efficiency. One-diode model analysis reveals that the HNRs having exposed CdSe cores and suitably grown CdS result in significant reduction of series resistance. Investigation of the intercorrelation between diode quality parameters, diode saturation current density ( J0) and recombination order (β = (ideality factor)-1) reveals that HNRs with open CdSe cores exhibit reduced recombination. These results confirm that the superior performance of core/patchy-shell HNRs results from their fine-tuned structure: photocurrent is increased by the large type-II heterointerface and recombination is effectively suppressed due to the open CdSe core enabling facile electron extraction. An optimized power conversion efficiency of 5.47% (5.89% with modified electrode configuration) is reported, which is unmatched among photovoltaics utilizing anisotropic colloidal heterostructures as light-harvesting materials.

Effect of nanoscale tin-dioxide layers on the efficiency of CdS/CdTe-based film solar elements

Comparative investigations of the output parameters and optical diode characteristics of ITO/CdS/CdTe/Cu/Au and SnO2: F/CdS/CdTe/Cu/Au film solar cells are carried out with the aim of optimizing the design of the front electrodes. It is established that the high voltage and large filling factor of the solar elements with SnO2: F films are caused by a lower diode saturation current density and series resistance due to the stability of the crystal structure and electrical properties of these films against chloride treatment of the base layer during device fabrication. At the same time, solar elements with an ITO front electrode exhibit a higher short-circuit current density due to the larger average light transmittance of the ITO layers. The use of nanoscale SnO2 layers in the ITO front contacts allows the efficiency of the CdS/CdTe-based solar elements to be enhanced to 11.4% on account of stabilization of the crystal structure and electrical properties of the ITO films and a possible reduction in the cadmium-sulphide-layer thickness without shunting the device structure.

A Fill Factor Loss Analysis Method for Silicon Wafer Solar Cells

The fill factor of silicon wafer solar cells is strongly influenced by recombination currents and ohmic resistances. A practical upper limit for the fill factor of crystalline silicon solar cells operating under low-level injection is set by recombination in the quasi-neutral bulk and at the two cell surfaces. Series resistance, shunt resistance, and additional recombination currents further lower the fill factor. For process optimization or loss analysis of solar cells, it is important to determine the influence of both ohmic and recombination loss mechanisms on the fill factor. In this paper, a method is described to quantify the loss in fill factor due to series resistance, shunt resistance, and additional recombination currents. Only the 1-Sun J-V curve, series resistance at the maximum power point, and shunt resistance need to be determined to apply the method. Application of the method is demonstrated on an 18.4% efficient inline-diffused p-type silicon wafer solar cell and a 21.1% efficient heterojunction n-type silicon wafer solar cell. Our analysis does not require J-V curve fitting to extract diode saturation current densities or ideality factor; however, the results are shown to be consistent with curve fitting results if the cell's two-diode model parameters can be unambiguously determined by curve fitting.

Silicon solar cell electrical parameter measurements through quantitative lock‐in carrierographic (photoluminescence) and thermographic imaging

We demonstrate theoretically and experimentally quantitative long‐pass filtered lock‐in carrierography (LIC) or lock‐in photoluminescence (LIP) imaging and perform comparisons with lock‐in thermography (LIT) imaging of industrial Si solar cells. Optoelectronic LIP and thermoelectronic LIT imaging modalities are used for non‐destructive, non‐contact measurements of the electrical parameters of solar cells from images obtained at various external load resistances. It is shown that quantitative surface‐averaged pixel brightness statistical distributions derived from near‐infrared (NIR) LIP and mid‐infrared LIT images can be used to measure, among other parameters, photogeneration current density, diode saturation current density, ideality factor, and maximum power photovoltage of a multi‐crystalline Si solar cell. LIP images were found to have superior contrast and spatial resolution to the respective LIT images. The electrical parameter values measured from the images were found to be in very good agreement with each other and with electrical measurements (EM). The theory further allows for the generation of derivative diode saturation current density and ideality factor images.

Benefit of Selective Emitters for p-Type Silicon Solar Cells With Passivated Surfaces

We compare homogeneous and selective emitters on monocrystalline silicon solar cells with passivated surfaces and present an analysis of the saturation current densities influencing the open-circuit voltage <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> and the fill factor FF . The cells' surfaces are passivated by a thin thermal oxide. Selective emitters are fabricated by laser doping from phosphosilicate glass. On both Czochralski-grown silicon (Cz-Si) as well as float zone silicon (FZ-Si), we find higher conversion efficiencies for the cells featuring a selective emitter. An efficiency up to 20.0% is reported on FZ-Si with an area of 148.4 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . For the selective emitter cells, 8 mV higher open-circuit voltages are found compared with the baseline. A saturation current analysis reveals that these cells exhibit a lower diode saturation current density of ideality 2 ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> ), as well as improved shielding of the minorities in the emitter from the front contact. The selective emitter cells show a minor loss in short-circuit current density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> of 0.5 % <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rel</sub> due to the presence of highly doped, illuminated areas. Front contact quality of the cells featuring a selective emitter is found to be superior compared with the cells with a homogeneously doped emitter.

Modeling solar cells with the dopant‐diffused layers treated as conductive boundaries

ABSTRACTModeling of transport and recombination of charge carriers in solar cells is useful for understanding and improving the device performance. We implement the fully coupled transport equations for electrons and holes into the finite‐element partial differential equation solver COMSOL. The dopant‐diffused surface regions such as junctions, floating junctions, or back surface field layers are treated as conductive boundaries of the volume in which the semiconductor equations are solved. This so‐called conductive boundary (CoBo) model characterizes diffused layers by their sheet resistances and diode saturation current densities. Both are directly experimentally accessible. The CoBo model exhibits excellent numerical stability and enables two‐dimensional simulations on a laptop. We find agreement when testing the two‐dimensional COMSOL implementation of the CoBo model for one‐dimensional devices against simulations using the code PC1D. We apply the CoBo model to elucidate how the sheet resistance of diffused vias impacts the power conversion efficiency of emitter wrap through solar cells. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Loss analysis of back-contact back-junction thin-film monocrystalline silicon solar cells

We investigate power losses in back-contact back-junction monocrystalline thin-film silicon solar cells fabricated using the porous silicon layer transfer process. Our loss analysis combines two-dimensional finite element modeling and resistance network simulations. The input parameters of the finite element modeling are determined experimentally by measuring saturation current densities and sheet resistances on test samples prepared identically to the solar cells. Characteristic solar cell parameters such as short circuit current, open circuit voltage, fill factor, and efficiency of measured and network simulated current voltage characteristics investigated in this study match within an uncertainty of 5%. Free energy loss analysis serves as comparison of all losses in units of power per area at the maximum power point. The largest loss is bulk recombination due to a carrier lifetime of 2 μs in the epitaxial Si layer. Further significant losses result from recombination at the base contacts characterized by a diode saturation current density of 50 000 fA cm−2 as well as resistive losses due to lateral majority carrier current flows within the solar cell base and contact resistance losses.

Effect of copper-deficiency on multi-stage co-evaporated Cu(In,Ga)S 2 absorber layers and solar cells

Solar cell absorber films of Cu(In,Ga)S 2 have been fabricated by multi-stage co-evaporation resulting in compositional ratios [Cu]/([In] + [Ga]) = 0.93–0.99 and [Ga]/([In] + [Ga]) = 0.15. Intentional doping is provided by sodium supplied from NaF precursor layers of different thicknesses. Phases, structure and morphology of the resulting films are investigated by X-ray diffraction (XRD) and scanning electron microscopy. The XRD patterns show CuIn 5S 8 thiospinel formation predominantly at the surface in order to accommodate decreasing Cu content. Correlated with the CuIn 5S 8 formation, a Ga-enrichment of the chalcopyrite phase is seen at the surface. Since no CuS layer is present on the as-deposited films, functioning solar cells with CdS buffer and ZnO window layers were fabricated without KCN etch. The open-circuit voltage of solar cells correlates with the copper content and with the amount of sodium supplied. The highest efficiency cell (open-circuit voltage 738 mV, short-circuit current 19.3 mA/cm 2, fill factor 65%, efficiency 9.3%) is based on the absorber with the least Cu deficiency, [Cu]/([In] + [Ga]) = 0.99. The activation energy of the diode saturation current density of such a cell is extracted from temperature- and illumination-dependent current-voltage measurements. A value of 1.04 eV, less than the band gap, suggests the heterojunction interface as the dominant recombination zone, just as in cells based on Cu-rich grown Cu(In,Ga)S 2.

Formation of the charge selective contact in solar cells with extremely thin absorber based on ZnO-nanorod/In2S3/CuSCN

Annealed and not-annealed solar cells with extremely thin absorber based on ZnO-nanorod /In2S3/CuSCN structures have been compared. Significantly higher external quantum efficiencies have been recorded on annealed devices. The temperature dependent current-voltage characteristics in the dark and under illumination were analyzed in detail. The short-circuit current density increased with the temperature and depended on the light intensity by a power law with a power coefficient of 0.85 that was independent of the annealing or measurement temperature. The temperature dependence of the ideality factor dominated the temperature dependencies of the diode saturation current density and of the open circuit voltage. The activation energies increased strongly after annealing. We propose that the limiting charge selective contact is driven away from the highly defective In2S3/CuSCN interface into the In2S3 layer due to stimulated by the annealing Cu diffusion.

New Two-Diode Model for Detailed Analysis of Multicrystalline Silicon Solar Cells

In order to extract electrical properties of multicrystalline silicon (Mx-Si) solar cells precisely, current–voltage (I–V) measurement in the dark condition was carried out. A new two-diode model, which includes a diffusion-current dominant area (DCA) and a recombination-current dominant area (RCA), with nonequivalent series resistances, was proposed as a new equivalent circuit. Electrical properties as fitting parameters were successfully extracted using a successive approximation method. This characterization derived J01 and J02, which are the diode saturation current densities for n=1 and 2 diodes, respectively. To ensure this diode model, temperature characteristics were measured, and the validity of this model was shown through calculation of the energy band gap from J01. J01 and J02 indicate a power factor and a loss factor of solar cells, respectively, which are newly proposed to classify solar cell performance.

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Diode characteristics in state-of-the-art ZnO/CdS/Cu(In1?xGax)Se2 solar cells

We report a new state of the art in thin-film polycrystalline Cu(In,Ga)Se2-based solar cells with the attainment of energy conversion efficiencies of 19·5%. An analysis of the performance of Cu(In,Ga)Se2 solar cells in terms of some absorber properties and other derived diode parameters is presented. The analysis reveals that the highest-performance cells can be associated with absorber bandgap values of ∼1·14 eV, resulting in devices with the lowest values of diode saturation current density (∼3×10−8 mA/cm2) and diode quality factors in the range 1·30 1;20% in thin-film polycrystalline Cu(In,Ga)Se2 solar cells. Published in 2005 John Wiley & Sons, Ltd.

Investigations of ammonium sulfide surface treatments on GaAs

X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED) results are presented for ammonium sulfide treated (100)GaAs surfaces. XPS shows that the sulfur coverage is independent of the ammonium sulfide concentration, although the relative amount of arsenic decreases as the sulfide concentration increases. RHEED patterns show that higher temperatures are required for the surface to restructure following treatment with higher sulfide concentration. In addition to the order of magnitude changes in the diode saturation current densities following ammonium sulfide treatment, we observe that the characteristics of gold and aluminum Schottky barriers on sulfide-treated GaAs surfaces also vary with the substrate temperature during metal deposition.

Minority carrier injection and charge storage in epitaxial Schottky barrier diodes

Minority carrier injection by so-called noninjecting metal-semiconductor contacts is considered under conditions of moderate to heavy forward bias. The injection ratio, γ (ratio of minority carrier current to total current), rises linearly with forward current for sufficiently large applied bias. The reason for this rise in γ is that the minority carrier current is enhanced by a drift-field component much larger than the diffusion current which dominates at low bias. In this range the injection ratio is given by γ = n 2 ij bN D 2j ns where n i and N D are the intrinsic and doping concentrations, b the mobility ratio, j ns the Schottky diode saturation current density, and j the diode forward current density. As an example a 5 Ω-cm n-type silicon-gold diode will obtain an injection ratio of 5% at a current density of 350 A/cm 2. The minority carrier stored charge per unit area ( Q), for Schottky diodes made on thin epitaxial layers, depends upon the characteristics of the epitaxi-substrate interface and can become very significant when this interface is highly reflecting (i.e., has a low value of surface recombination velocity). For large applied bias and negligible bulk recombination the stored charge is given by Q = qn i 2D pj N Dj nsσ where q is the electronic charge, D p the diffusion constant, and σ the surface recombination velocity. In measurements on experimental epitaxial diodes the interface is not found to be highly reflecting but is characterised by a recombination velocity of about 2000 cm/sec. This value applied to the 5 Ω-cm silicon-gold diode yields a storage time ( Q j ) of about 1 3 nsec. Normalised curves of γ and Q j vs. forward current are presented with parameters chosen in the range expected for silicon epitaxial Schottky barrier diodes. The theory has been verified by experiments on diodes made by evaporating gold contacts on silicon.