Light Current-voltage Characteristics Research Articles

Metastable Defect Distributions in CIGS Solar Cells and Their Impact on Device Efficiency

AbstractMetastabilities in the electrical characteristics of CIGS devices are commonly observed phenomena originating from persistent changes of shallow and deep levels distributions within the absorber. We examine characteristic changes induced by voltage bias and light together with their relaxation behavior and interpret them as the consequences of a negative-U type of centers predicted by theoretical calculations of Lany and Zunger. It is shown how the properties of these centers justify a model of p+ layer explaining specific features of light and dark current-voltage characteristics. The discussion showing the impact of various charge distributions on carrier transport is presented. The arguments are provided, that centers responsible for metastable effects are also to blame for majority of photovoltaic losses exhibited in various devices.

Investigation of electrodeposited p-Si/Cd 1 − xZn xS 1 − ySe y heterojunction solar cells

Polycrystalline films of n-Cd 1 − x Zn x S 1 − y Se y with different x and y values (0.1 ≤ x ≤ 0.8 and 0 ≤ y ≤ 0.2) were prepared by electrodeposition from solution of CdCl 2 + ZnCl 2 + Na 2S 2O 3 + SeO 2 (or Na 2Se 2O 3) on p-Si substrates. The dependence of the electric and photoelectric parameters in these heterojunctions on the x, y and heat treatment condition were investigated. It was found that annealing the heterojunctions at T a = 400 °C for τ a = 5 ÷ 6 min in air leads to an improving of the electric and photoelectric parameters. The evaluation of solar cell parameters was carried out using the dark and light current–voltage characteristics. Under AM 1.5 condition the open circuit photovoltage and short circuit photocurrent density of cells p-Si/n-Cd 03Zn 0.7S 0.8Se 02 were V oc = 0.6 V, J sc = 22.8 mA/cm 2, respectively.

Scaling quantum-dot light-emitting diodes to submicrometer sizes

We introduce a device structure and a fabrication technique that allow the realization of efficient light-emitting diodes (LEDs) with dimensions of the active area in the ≈100 nm range. Using optical lithography, selective oxidation, and an active region consisting of InAs quantum dots (QDs), we fabricated LEDs with light–current–voltage characteristics which scale well with nominal device area down to 600 nm diam at room temperature. The scaling behavior provides evidence for strong carrier confinement in the QDs and shows the potential for the realization of high-efficiency single-photon LEDs operating at room temperature.

Reduction of Dark Current Under Reverse Bias in a-Si:H p-i-n Photodetectors

AbstractThis paper presents the development of low dark current amorphous silicon (a-Si:H) based heterojunction photodiodes. A series of p-i-n and n-i-p structures have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Junction properties and carrier transport are investigated in terms of dark and light current-voltage characteristics, time dependence of the dark current, and spectral photoresponse measurements. It is demonstrated that a thin (∼4 nm) undoped a-SiC:H buffer layer introduced between the p and i layers reduces the leakage current and improves the diode ideality factor. A dark current density of ∼10 pA/cm2 at reverse bias of 1 V was achieved for the n-i-p structure. Optimization of device design for further improvement of dark current and photoresponse is discussed.

Protocrystalline Si:H p-type Layers for Maximization of the Open Circuit Voltage in a-Si:H n-i-p Solar Cells

AbstractWe have revisited the issue of p-layer optimization for amorphous silicon (a-Si:H) solar cells, correlating spectroscopic ellipsometry (SE) measurements of the p-layer in the device configuration with light current-voltage (J-V) characteristics of the completed solar cell. Working with p-layer gas mixtures of H2/SiH4/BF3 in rf plasma-enhanced chemical vapor deposition (PECVD), we have found that the maximum open circuit voltage (Voc) for n-i-p solar cells is obtained using p-layers prepared with the maximum possible hydrogen-dilution gas-flow ratio R=[H2]/[SiH4], but without crossing the thickness-dependent transition from the a-Si:H growth regime into the mixed-phase amorphous + microcrystalline [(a+μc)-Si:H] regime for the ∼200 Å p-layers. As a result, optimum single-step p-layers are obtained under conditions similar to those applied for optimum i-layers, i.e., by operating in the so-called “protocrystalline” Si:H film growth regime. The remarkable dependence of the p-layer phase (amorphous vs. microcrystalline) and n-i-p solar cell Voc on the nature of the underlying i-layer surface also supports this conclusion.

Microcrystalline n- and p-layers at the tunnel junction of a-Si:H/a-Si:H tandem cells

Completely microcrystalline layers have been incorporated at the tunnel junction of a tandem cell in the configuration SnO 2:F/p-a-SiC:H/i-a-Si:H/n- μc-Si:H/p- μc-Si:H/i-a-Si:H/n-a-Si:H/Ag and an efficiency of 9.89% has been achieved. We present here our experimental findings and our modelling predictions using the computer programme AMPS. A wide band gap a-Si:H buffer layer between p- μc-Si:H and i-layer a-Si:H was necessary for obtaining good performance of the bottom cell. The spectral response in the blue region is reduced with the increase of buffer layer thickness and thickness as small as 1.0 nm was necessary to achieve the best efficiency. Computer prediction shows that recombination in the p- μc-Si:H could be very high when no buffer is used because of the combination of low band gap and high (>10 17 cm −3) defect density. Buffer layer prevents electron back diffusion into the μc-Si:H p-layer. Computed light current voltage characteristics did not change significantly when tunnelling across the thin buffer layer was incorporated in the model. To achieve larger efficiencies in the tandem configuration, insertion of oxide layers between a-Si:H layer and n- μc-Si:H layer of the top cell and between n- μc-Si:H layer and p- μc-Si:H layer was necessary. Our simulations show that, (a) the recombination rate is increased at the junction due to lower mobility gap of the microcrystalline layers, (b) the density of trapped carriers is reduced with microcrystalline junction, not only in the p- and n-layers but also in the i-layers, (c) the recombination rate at the microcrystalline junction is increased with an oxide layer at the p- μc-Si:H and n- μc-Si:H interface showing an improvement of both FF as well as V oc.

Light-emitting diodes based on polythiophene: influence of the metal workfunction on rectification properties

Light emission and current-voltage characteristics of conjugated polymer poly(3-octylthiophene) (P3OT) light-emitting diodes (LEDs) are discussed for two types of top electrodes. On changing the workfunction of the top contact by 0.9 eV from 4.3 (Al) to 5.2 eV (Au) we find that the main direction of rectification changes sign: when Al forms the top contact rectification ratios up to 10 4 were obtained, while in the case of Au this ratio is 10 2 in the opposite direction. As a consequence, the rectification ratio changes effectively by almost six orders of magnitude. In the case of Au as a top electrode no electroluminescence is observed, which suggests that a one-carrier type (‘hole-only’) device is made. The observation of light emission for indium-tin-oxide (ITO)/P3OT/Al devices in both modes of operation is discussed in terms of two injection mechanisms (thermionic emission over a Schottky-like contact versus tunneling into the transport bands). For these devices the onset of light emission in forward and reverse modes of operation occurs at the same current densities (6.25×10 −4 A/cm 2), but at much higher electric fields in the reverse mode.

Investigations on hydrogenated amorphous silicon films grown at high rate in a UHV plasma CVD system

In this paper we analyze the results of an extensive characterisation study involving scanning electron microscopy, spectroscopic ellipsometry, Laser Raman spectroscopy, optical band gap, dark and photoconductivity measurements as a function of temperature and light intensity, current-voltage characteristics and capacitance versus frequency measurements on Schottky diodes of a-Si:H films, prepared in a UHV plasma CVD system at varying deposition rates upto ≈ 10.0 Å/s. We find that the values of imaginary part of the dielectric constant ( ϵ 2max), dark and photoconductivity, activation energy, conductivity pre-exponential factor, exponent Λ of photoconductivity decrease, whereas the values of diode quality factor n and density of defect states increase and the values of the width of TO-peak in Raman spectra, photosensitivity and optical band gap do not change significantly with the increasing rate of deposition. The less severe deterioration of opto-electronic properties observed in these films, even when grown at high rates in our plasma CVD reactors, is attributed to the effective control of secondary plasma reactions in the interelectrode space when kept very low (10–12 mm). Implications of obtaining high growth rate for i-layer, in a p-i-n solar cell structure, is discussed with the available results of modelling analysis.

An empirical theory of isc and voc in the ZnzCd1−zS/Cu2S solar cell

A simple empirical theory has been given for the short-circuit current (isc ) and the open-circuit voltage (voc ) of the ZnzCd1−zS/CU2S solar cell. The heterojunction band lineup at the interface is assumed to be determined by the pinning of the Fermi level at the collector surface by the copper impurity/cation vacancy level. Since the impurity level remains fixed with respect to the collector valence-band edge and the collector band gap goes up with z, such a pinning produces a potential barrier (‘‘spike’’) at the junction. The height of the spike increases with z. An estimate is made of the Cd impurity level in the emitter. This level is believed to act as the recombination center for electrons diffusing back to the emitter from the collector conduction band. These electrons would normally face a strong opposing field but because of an expected incipient Zener breadkdown of the potential barrier in the narrow highest field region they would face a substantially smaller opposing field. Zener breakdown field for CdS is known and for the mixed sulfide we have considered this field as a parameter obtained by fitting the voc values. The parameter, thus obtained, varies with z smoothly in a logarithmic scale and is within an order of magnitude of the Zener breakdown field for CdS. We have given some physical arguments to show that the Zener breakdown and consequent reduction of the field does not necessarily mean a widening of the depletion layer. While for CdS the known Zener breakdown field gives a value of voc in good agreement with the experiment, it is a measure of the expected correctness of the fitted values of this field that, in conjunction with the heights of the conduction-band spike, they give reasonable isc values. The generally consistent values of the parameters of the cell measured in different specimens are ascribed to the self-doping and self-compensating properties of the collector material. An explanation of the crossover of dark and light current-voltage characteristics for different types of cells has been given. Since it is futile to aim at any numerical accuracy in view of the uncertainties in some of the material parameters, the main emphasis has been on the explanation of the trends of the variation of the parameters with z. This explanation has been given in terms of a few physically reasonable assumptions. Although at least one experimental result tends to confirm the position of the recombination level, a few specific experiments have been suggested which would hopefully sustain the assumptions of the theory.

Measurement of the nonlinearity of light diode current-voltage characteristics

Measurement of the nonlinearity of light diode current-voltage characteristics

Theoretical modeling of amorphous silicon-based alloy p-i-n solar cells

We have developed a computer model of amorphous silicon based alloy p-i-n solar cells based on the solution of the electron and hole continuity equations, together with Poisson’s equation. This model takes into account the spatial and energy variations of the localized state spectrum, nonuniform doping profiles, and nonuniform optical excitation. Heavily doped p+ and n+ regions are used as contacts, and these enable us to realistically set up the boundary conditions. Appropriate boron profiles have been added to the intrinsic layer, with the density of states being dependent on the doping concentration. The computed dark and light current–voltage characteristics are in good agreement with experimental data. In particular, our results indicate the importance of residual boron doping in the intrinsic layer for cells illuminated through the n+ layer, and show that for optimized p-i-n devices carrier back diffusion is not a significant effect and that the open circuit voltage is determined by the recombination current.

Computer simulation of amorphous silicon based alloy p-i-n solar cells

We report on the results of a computer simulation of amorphous silicon-based alloy p-i-n solar cells based on the complete set of transport equations. Our model takes into account the spatial and energy variations of the localized state spectrum, nonuniform doping profiles, and nonuniform optical excitation. The computed dark and light current-voltage characteristics are in good agreement with experimental data. Our results suggest that carrier back diffusion is not a significant effect in optimized p-in devices and that the open circuit voltage is determined by the recombination current. We also show the importance of residual boron doping in the intrinsic layer for cells illuminated through the n+ layer, and that hole transport limits device performance.

Cold pressed cadmium selenide photoanodes for electrochemical solar cells

Cold pressing of cadmium selenide powder was investigated as a technique for producing photoanodes for electrochemical solar cells. Physical properties such as density, resistivity and surface morphology were determined and related to solar cell performance via wavelength response, quantum efficiency and white light current-voltage characteristics. The spectral response indicated a bandgap of 1.7 eV. Pellets pressed at higher pressure showed an improved quantum efficiency. Pressures above 69 MPa produced fractures in the pellets. Conversion efficiencies under white light (tungsten halide lamp) at 100 mWcm −2 were on the order of 1.5%.

Impurity gettering of polycrystalline solar cells fabricated from refined metallurgical-grade silicon

A damage-gettering technique is described which reduces the impurity content in grown crystals and enhances cell performance of diffused solar cells. Crystalline ingots were Czochralski-grown from an acid-leached metallurgical-grade source. Damage gettering was performed by preparing a mechanically damaged layer on the wafer back surface and subsequent annealing. Optimum annealing conditions were investigated as a function of ambient gas species, temperature, and time. In an O 2 ambient, the fill factor of the cells degraded to 0.25, while cell performance was greatly improved by annealing in N 2 . Conversion efficiency tends to increase with annealing time at higher temperatures. Maximum conversion efficiencies attained for mono- and polycrystalline solar cells fabricated from MG-Si are 9.8 and 7.7 percent, respectively. Light current-voltage characteristics and the leakage-current variations with depth were analyzed. It was found that impurity gettering begins at the wafer surfaces and proceeds gradually into the bulk regions.

Detailed light-current-voltage analysis of GaP electroluminescent diodes

The light-current-voltage (LIV) characteristics of both red- and green-emitting GaP p-n junctions have been analyzed in detail at room temperature. Device current has been quantitively and accurately decomposed into components corresponding to the Sah-Noyce-Shockley space-charge recombination and to diffusion current. Data analysis is performed rapidly using an automatic iterative procedure developed for this purpose. In the course of calculation, diode resistance in series with the junction is measured and corrected for, thus considerably extending the upper limit of bias current which may be usefully studied. This study has also determined the current dependence of injection efficiency, electroluminescent quantum efficiency, and the effective luminescent recombination efficiency of the bulk semiconductor region adjacent to the junction. Although this method is applicable to broad classes of junction devices, its use involves certain limitations and ambiguities, which are explicitly discussed. This analysis has been applied to the LIV characteristics of representative examples of both red- and green-light-emitting GaP p-n junctions, grown by double liquid-phase epitaxy. For both these cases, it is demonstrated that the simple current models employed (SNS space charge and diffusion) are in excellent agreement with the observed I-V characteristics over more than nine decades of device current. It seems apparent that these techniques will prove relevant to studies of the physics of light-emitting diodes.

On a model of photo-field-emission from p-type semiconductors

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