Effect of edge junction isolation on the performance of laser doped selective emitter solar cells

The effect of laser and chemical edge junction isolation on electrical performance of industrially manufactured laser doped selective emitter solar cells with light induced plated n-type contacts is investigated in this work. Directly after the formation of the aluminium back surface field, photoluminescence images indicates that laser edge junction isolation causes substantial damage around the perimeter of the cell, extending several millimeters from the laser edge isolation groove. On finished devices, regions of high series resistance are evident around the perimeter, caused by parasitic plating nucleating in the damaged laser grooved region which induce shunting and inhibits further plating taking place in the surrounding regions. The use of chemical edge junction isolation eliminates both of these issues and can result in efficiency gains of more than 2% absolute compared to that fabricated using laser edge isolation, suggesting a far superior method of edge junction isolation for the industrial manufacture of laser doped selective emitter solar cells with light induced plated contacts.     

Classification of shunting mechanisms in crystalline silicon solar cells

The efficiency of a solar cell is given by its average electrical parameters. On inhomogeneous materials and especially on large-area solar cells the inhomogeneity of the short circuit current, the open circuit voltage and the fill factor are important factors to reach high and stable efficiencies and may limit the overall performance of the device.A locally increased dark forward current (shunt) reduces the fill factor and the open circuit voltage of the whole cell. The inhomogeneity of the forward current in a solar cell can be measured using lock-in thermography. The quantitative and voltage-dependent evaluation of these thermographic investigations of various solar cell types on mono- or multi-crystalline silicon enables the classification of the different shunting mechanisms found. By further microscopic investigations the physical reasons for the increased dark forward currents can be determined.It turns out that a high density of crystallographic defects like dislocation tangles or microdefects can be responsible for an increased dark forward current. Unexpectedly, grain boundaries in solar cells on multicrystalline silicon do not show any measurable influence on the local dark forward current. In most cases shunts caused by process-induced defects are dominating the current–voltage characteristic at the maximum power point of the solar cell. In commercial solar cells shunts at the edges are most important, followed by shunts beyond the grid lines.     

Absorption of solar energy in a room

The purpose of this study is to develop an analysis that allows an examination of the spatial distribution of the absorbed solar energy for a room. The room is composed of several areas that consist of an exterior window where solar beam and diffuse energies originate, an interior window, a door, and six opaque areas. Each area is subdivided into a number of discrete surfaces and an analysis based on the radiosity-irradiation method is used to determine the absorbed solar energy for each surface. A geometrically-based method is used to describe the location and magnitude of the directly incident solar beam energy for each surface. The influence of the number of discrete surfaces on the findings is examined. The results show that a grid pattern that produces discrete surfaces having dimensions of less than 0.3 m yields accurate results. Results are reported for different orientations of the room. As expected, the absorbed solar energy is high in regions where the solar beam energy irradiates a surface. The spatial distribution of the absorbed solar energy does vary significantly over the areas of the room. Measured solar energy values are used to compute the absorbed solar energy as a function of time of day. The movement of the energy across the surfaces of the room creates a time-varying value for the absorbed solar energy. Numerical values for the ratio of the absorbed solar energy for each area to the available solar energy from the exterior window are supplied.     

Temperature distribution in a solar irradiated liquid film flowing over a solid wall

A mathematical model is developed to predict the thermal behavior of a thin liquid film flowing along an inclined surface and exposed to an intense solar radiation flux. The pertinent equations are solved assuming a uniform velocity across the film. Radiation losses are combined with convection through a heat transfer coefficient at the free surface of the film. Numerical data indicate that temperature differences on the order of 100°C can exist across the fluid layer although its thickness may not exceed 0.5 mm. An important conclusion of this study is that a moderately blackened heat transfer medium is sufficient to accomplish the required task in an optical cavity.     

An InP solar cell with a light-trapping front surface

Low-angle (311) V-grooves can be produced on a (100) InP surface by a maskless anisotropic etch. This can reduce the surface reflectivity of a glass-covered InP solar cell. Light reflected from grooved surface is trapped by total internal reflection at the glass/air interface and directed back to the solar cell. Results from ITO/InP solar cells on low-angle V-grooved substrates are presented, showing a 5.8% increase in short-circuit current, and a nearly equal rise in efficiency.     

An assessment of a revised Olmo et al. model to predict solar global radiation on a tilted surface at Beer Sheva,Israel

A model to convert horizontal solar global radiation to that on a tilted surface is presented. It is based upon a relatively simple model proposed by [Olmo FJ, Vida J, Foyo I, Castro-Diez Y, Alados-Arboledas L. Prediction of global irradiance on inclined surfaces from horizontal global irradiance. Energy 24 (1999) 689–704]., which requires only measurements of horizontal solar radiation but was found to produce significant errors when tested with data from another site. The present model assumes the availability of databases for at least two of the three solar radiation types, viz., global, beam and diffuse. The horizontal global radiation is converted to that on a tilted surface by applying the Olmo model to the diffuse component, whereas the beam component is converted by using the geometrical relationship between the two surfaces. The original Olmo anisotropic radiation correction factor is now assumed to be a function of sky conditions. The solar radiation databases were converted to subsets corresponding to clear, partially cloudy and cloudy sky based upon clearness index values. The three anisotropic correction factors were determined by fitting to a 12-months database. The present model was then tested by applying it to a second database consisting of 24-months not involved in the model development. It was found to give better results than three highly regarded more complex models.     

Enhanced performance of solar cells with anti-reflection layer fabricated by nano-imprint lithography

Because of increase in demand for clean energy sources, photovoltaic device is becoming more important as a new power plant. To replace fossil fuels with photovoltaics, generating electricity with solar cells should meet cost effectiveness and high efficiency. An anti-reflection technique is one of the effective methods to achieve high efficiency.To reduce the reflection at the surface of concentrated photovoltaic device, a nanometer scale dot-pattern array was formed on the surface of GaInP/Ga(In)As/Ge solar cells by nano-imprint lithography. Since this nano-pattern is smaller than the wavelength of visible light, the effective refractive index near the surface changes gradually. Thus, the reflection of the light at the surface with moth-eye structure can be effectively reduced for overall spectral region. As a result, a solar cell with the moth-eye pattern as an anti-reflection layer showed lower reflectance and enhanced total conversion efficiency, compared to a solar cell without a moth-eye patterned layer.The patterned solar cell was characterized using UV-vis spectrophotometer, atomic force microscope (AFM) and scanning electron microscope (SEM), and its total conversion efficiency was measured by solar simulator.     

The analysis of light trapping and internal quantum efficiency of a solar cell with DBR back reflector

A theoretical analysis of the total internal quantum efficiency (IQE) of a flat-band p-n homo-junction silicon solar cell with back reflector using distributed Bragg reflectors to improve the light trapping is presented and contributions of different regions of the structure to IQEs are simulated. An optical model for the determination of generation profile of the cell is adopted and multiple light passes are considered and compared to previous single light pass approach. It is found that the spatial widths of the cell, the surface recombination velocities, the front surface transmittance and the back reflector have significant impacts on the IQEs. With two light passes and normal incident light, the simulation result shows the IQEs can be increased over the one pass value by 6.34% and with a 60° light reflection angle, the IQEs can be further increased by 9.01% while assuming the reflectance at back structure closed to 100%. The effect on IQEs by back reflectance is more significant than that by front transmittance. Under multiple light passes simulation, up to 51 light trapping passes have been considered at wavelength range 900-1100 nm, the cell IQEs can be enhanced by about 26.98%.     

An 11.4% efficient polycrystalline thin film solar cell based on CuInS2 with a Cd-free buffer layer

The fabrication of a 11.4% efficient thin film solar cell based on CuInS₂ with an In_x(OH,S)_y buffer layer is described. The device parameters and performance are compared to heterojunctions with a standard Us buffer layer. A junction breakdown at negative bias under illumination is related to the buffer layer. A simple model implying photoconductive shunting paths is presented.     

Empirical correlations as a means for estimating monthly average daily global radiation: A critical overview

In regions where solar energy is abundant, solar energy can play a vital role in attaining energy sustainability. Sizing solar energy systems requires the availability of solar radiation data on horizontal surface which can then be used to calculate solar radiation intensity on any tilted surface using appropriate conversion factors or formula. In many parts of the world, especially in developing countries, such data is not readily available. Many researchers have found that monthly average daily value of global solar radiation on horizontal surface can be estimated when meteorological parameters such as duration of sunshine, number of rainy days, relative humidity, etc. are available. Many empirical correlations have been developed based on this approach. The development of such a correlation has been made possible through the availability of solar and other meteorological data required for their validation. This paper presents a review on the existing empirical correlations and critically looks at the practicality of such correlations. This raises the question on the appropriateness of the past and present approaches adopted by researchers in this field. The paper also discusses various related aspects and proposes new directions for future research.     

Characterization of high open-circuit voltage double sided buried contact (DSBC) silicon solar cells

The double sided buried contact (DSBC) silicon solar cells have consistently shown high open-circuit voltages (V_oc) than its single sided buried contact counterpart because of better rear surface passivation. The rear surface passivation which is provided by the rear floating junction is effective only when there is no leakage in the rear floating junction. However, the partial shunting of the rear floating junction can cause a drop in the fill factor of the cell which has been the only parameter limiting the realization of the structure's potentials. In this paper, LBIC (light beam induce current), spectral response, dark I-V and J_sc-V_oc measurements for DSBC cells have been carried out to help explain some of the experimentally observed attributes of this structure. The partly shunted rear floating junction has been identified by LBIC measurement as low current regions near the rear metal contacts.     

Resistivity topography: a grain boundary characterisation method

Solar cells made from multicrystalline silicon are the ideal basis for photovoltaic systems. The solar cell efficiencies are still limited by the crystal defects (dislocations, grain boundaries) and their electrical activity. As a fast and efficient assessment of the electrical activity of specific grain boundaries high resolution resistivity maps will be used, measured on a set of wafers as cut coming from the same block. Resistivity maps proved to be a precise and yet simple method to characterise grain boundary activity. While measurements in the bulk of the crystal are symmetric because of the isotropy of the material, measurements crossing rain boundaries are depending on the relative orientation of the boundary with respect to the measurement geometry. The large number of measurement points of a high resolution resistivity map allows a statistical treatment of the data to evaluate one quantitative value for the grain boundary electrical activity.Solar cells from those have been processed. In the regions with a higher electrical activity at grain boundaries, the open circuit voltage of the solar cell decreased. A specific evaluation of the electrical activity of grain boundaries on the basis of the thermionic model allowed the fine tuning of production parameters leading to a homogeneous quality of cast mc-Silicon wafers bearing the potential of approaching highest efficiencies in industrial production processes.     

Low-cost rear side floating junction solar-cell issues on mc-Si

Rear side floating junction solar cells with localised contacts often show parasitic shunting losses. For p-type material, this is due to tunnelling that takes places between the passivating n⁺-type rear junction and the p⁺-type back surface field region underneath localised rear contacts. To avoid this, the rear metallisation and the passivation layer should electronically be separated. Alternatively, the (induced) doping concentration of these layers could be optimised. This paper discusses the constraints met upon incorporation of such lowly recombinative structures into a low-cost solar cell process on multi-crystalline Si (mc-Si) material.     

Effect of short-wavelength radiation on the photoresponse of MIS-IL solar cells (experimental study)

MIS-IL solar cells are potentially low-cost devices, due to their simplicity and avoidance of the high-temperatures in their fabrication processing. In addition, inversion-layer (IL) solar cells are demonstrated to have higher collection efficiencies at short wavelengths. The spectral response of MIS-IL solar cells has been measured in the wavelength range 340–1100 nm. From these measurements, the external and internal quantum efficiencies have been determined. Also, a comparative study of the quantum efficiencies of inversion layer and diffused junction cells has been carried out. The IL cell has a higher collection-efficiency for carriers generated by ultra-violet light than the conventional p–n junction solar cell. These results reveal one of the important advantages of IL solar cells. Also, it can be seen that IL solar cells can be improved by surface texturing in a similar way to a conventional diffused solar cells.     

Effect of the front surface field on crystalline silicon solar cell efficiency

The present paper reports on a simulation study carried out to determine and optimize the effect of the high–low junction emitter (n⁺-n) on thin silicon solar cell performance. The optimum conditions for the thickness and doping level of the front surface layer with a Gaussian profile were optimized using analytical solutions for a one dimensional model that takes on the theory relevant for highly doped regions into account. The photovoltaic parameters of silicon solar cells with front surface field layer (n⁺-n-p structure) and those of the conventional one (n-p structure) are compared. The results indicate that the most important role played by the front surface field layer is to enhance the collection of light-generated free carriers, which improves the efficiency of the short wavelength quantum. This is achieved by a drastic reduction in the effective recombination at the emitter upper boundary, a property primarily responsible for the decrease in the emitter dark current density. The findings also indicate that the solar cell maximum efficiency increase by about 2.38% when the surface doping level of the n⁺-region and its thickness are equal to 2.10²⁰ cm^?3 and 0.07 μm, respectively.     

Optimization of random diffraction gratings in thin-film solar cells using genetic algorithms

Optical coupling and light trapping in thin-film solar cells are studied numerically using rigorous solutions of Maxwell's equations. The solar cell investigated consists of a ZnO/a-Si/ZnO/Ag structure, though results may be generalized to any thin-film solar cell technology. Varying diffraction gratings were studied, including periodic rectangular gratings, a four-level rectangular grating, and an arbitrary grating resembling a randomly textured surface. A genetic algorithm was used to optimize multi-level rectangular and arbitrary gratings. Solar cells with optimized multi-level rectangular gratings exhibit a 23% improvement over planar cells and 3.8% improvement over the optimal cell with periodic gratings. Solar cells with optimized arbitrarily shaped gratings exhibit a 29% improvement over planar cells and 9.0% improvement over the optimal cell with periodic gratings. The enhanced solar cell efficiencies for multi-level rectangular and arbitrary gratings are attributed to improved optical coupling and light trapping across the solar spectrum.     

Large-area metallisation wrap through solar cells using electroless plating

In this publication we present a realisation of a large-area metallisation wrap through (MWT) back-contact-solar cell with electroless-plated contacts. The MWT cell is a very promising back-contact solar-cell concept since the additional effort required to process MWT instead of conventional cells is limited to the formation of a small number of holes. In addition, the concept can easily be applied to very large area cells.We introduce a 9.8 cm×9.8 cm MWT solar cell, which reaches an efficiency of 16.1%. Up to this point cell performance is limited by local shunting, which results from non-optimal nickel sintering.     

Experimental investigation of natural convection heat exchange within a physical model of the manifold chamber of a thermosyphon heat-pipe evacuated tube solar water heater

The high capital costs associated with heat-pipe evacuated tube solar water heating systems can be reduced by replacing forced circulation with thermosyphon circulation. Currently research on thermosyphon heat-pipe evacuated tube solar water heaters is limited. An experimental investigation of the natural convective heat exchange regime that exists within the manifold chamber of a proprietary heat-pipe evacuated tube solar water was undertaken. This paper presents experimental data from a heat-pipe Evacuated Tube Solar Water Heater (ETSWH) subjected to the Northern Maritime Climate at the University of Ulster’s outdoor solar testing facility located at the Jordanstown campus. The thermal performance of this across solar noon (±30 min) was experimentally determined to be comparable to two physical laboratory 10 pin-fin model manifolds constructed to the same dimensions and geometry as the manifold chamber of the heat-pipe ETSWH when operated under steady laboratory conditions. When the surface temperatures of the pin-fins (simulated condensers) in the model manifold were normalised with respect to the lowest most pin-fin in the array the influence of buoyant flow was observed. Similarly to related studies in this field it was found that normalised surface temperatures on downstream pin-fins do not increase monotonically as would be expected if no interactions occur. It was found that at the pin-fin diameter to pitch used in the model manifold that normalised surface temperatures decrease at certain points in the array due to the action of buoyant flow generated from upstream pin-fins which increased heat transfer. Two-dimensional Particle Imaging Velocimetry (2D-PIV) was used to visualise the thermosyphon fluid flow regime. It was observed that the fluid flow regime varied across the model due to interactions between the fluid, chamber walls and pin-fins.     

A numerical model of p–n junctions bordering on surfaces

Many solar cell structures contain regions where the emitter p–n junction borders on the surface. If the surface is not well passivated, a large amount of recombination occurs in such regions. This type of recombination is influenced by the electrostatics of both the p–n junction and the surface, and hence it is different from the commonly described recombination phenomena occurring in the p–n junction within the bulk. We developed a two-dimensional model for the recombination mechanisms occurring in emitter p–n junctions bordering on surfaces. The model is validated by reproducing the experimental I–V curves of specially designed silicon solar cells. It is shown under which circumstances a poor surface passivation, near where the p–n junction borders on the surface, reduces the fill factor and the open-circuit voltage. The model can be applied to many other types of solar cells.     

Solar cell system using a polished concave Si-crystal mirror

We demonstrated a solar cell system comprising a concave-Si-mirror solar cell and a conventional small Si solar cell set at the focal point of the concave-mirror solar cell. The concave-Si-crystal mirror was prepared by mechanically polishing a single-crystal Si wafer. It was used both as a solar cell and mirror. This system can make effective use of the photons reflected from the concave-mirror solar cell because the number of total photons is the sum of photons from both the mirror solar cell and the small solar cell set at the focal point. When the conventional small solar cell with an efficiency of 13.0% was used, the total conversion efficiency of the present system increased to 12.2% compared with the conversion efficiency of 11.5% for only the concave-Si-mirror solar cell. These results show that the present solar cell system has the capability to achieve high efficiency through the effective use of the photons reflected from conventional solar cells.