Showing papers by "Stefan W. Glunz published in 2007"

Diffusion lengths of silicon solar cells from luminescence images

Abstract: A method for spatially resolved measurement of the minority carrier diffusion length in silicon wafers and in silicon solar cells is introduced. The method, which is based on measuring the ratio of two luminescence images taken with two different spectral filters, is applicable, in principle, to both photoluminescence and electroluminescence measurements and is demonstrated experimentally by electroluminescence measurements on a multicrystalline silicon solar cell. Good agreement is observed with the diffusion length distribution obtained from a spectrally resolved light beam induced current map. In contrast to the determination of diffusion lengths from one single luminescence image, the method proposed here gives absolute values of the diffusion length and, in comparison, it is much less sensitive to lateral voltage variations across the cell area as caused by local variations of the series resistance. It is also shown that measuring the ratio of two luminescence images allows distinguishing shunts or surface defects from bulk defects.

Metal aerosol jet printing for solar cell metallization

Abstract: Metal aerosol jet printing is a new non-contact direct-write technique for the front side metallization of highly efficient industrial silicon solar cells. With this technique the first layer of a two-layer contact structure is created. It features a low contact resistance and good mechanical adhesion to the silicon surface. The second layer is formed by light-induced silver plating (LIP) to increase the line conductivity. To form the first layer a metal-containing aerosol is created in the printer and focused via a second surrounding gas stream through a nozzle and deposited onto the substrate. The focussing gas avoids the contact between the aerosol and the nozzle tip. In addition, line widths significantly smaller than the outlet diameter of the nozzle tip can be reached. Fine and continuous lines with a width of 14 μm were printed using a metal organic ink. As the adhesion of these layers was not sufficient, a commercially available screen-printing paste for solar cell metallization was modified and tested. Monocrystalline silicon solar cells of 12·5cm X 12·5cm with an aluminum back surface field were processed, achieving energy conversion efficiencies up to 17·8%.

High-Efficiency Crystalline Silicon Solar Cells

Abstract: The current cost distribution of a crystalline silicon PV module is clearly dominated by material costs, especially by the costs of the silicon wafer. Therefore cell designs that allow the use of thinner wafers and the increase of energy conversion efficiency are of special interest to the PV industry. This article gives an overview of the most critical issues to achieve this aim and of the recent activities at Fraunhofer ISE and other institutes.

Analyzing Back Contacts of Silicon Solar Cells by Suns-Voc-Measurement at High Illumination Densities

Abstract: This work demonstrates the feasibility and usefulness of a new method to analyse the quality of the rear contact of silicon solar cells separated from other ohmic loss channels as e.g. the resistive loss in the front contact grid. The measurement is based on SunsVoc data at high illumination densities between 1 and 1000 suns. Generally the rear contacts can be described as a Schottky diode with a shunt resistor in parallel. At 1 sun operation conditions the back contact is fully dominated by the shunt showing an ohmic behaviour. However, at high illumination densities the Schottky diode can not be shunted completely anymore resulting in an increasing voltage which is opposed to the pn junction voltage. Finally a reversal point in the SunsVoc characteristics can be observed, i.e. the voltage decreases with increasing illumination density. The evaluation of this characteristic behaviour is used to extract physical parameters like the barrier height of the contact. Additionally the contact quality is assessed for different contact types and base doping concentrations. The predicted contact quality is in good correlation with the measured fill factors of the cells.

Series resistance characterization of industrial silicon solar cells with screen-printed contacts using hotmelt paste

Abstract: This work presents the results of a detailed series resistance characterization of silicon solar cells with screen-printed front contacts using hotmelt silver paste. Applying the hotmelt technology energy conversion efficiencies up to 18·0% on monocrystalline wafers with a size of 12·5 cm × 12·5 cm have been achieved, an increase of 0·3% absolute compared to cells with conventional screen-printed contacts. This is mainly due to the reduction in the finger resistance to values as low as 14 Ω/m, which reduces the series resistance of the solar cell significantly. To retrieve the lumped series resistance as accurately as possible under the operating condition, different determination methods have been analyzed. Methods under consideration were fitting of the two-diode equation function to a dark IV-curve, integration of the area A under an IV-curve, comparison of a jsc–Voc with a one-sun IV-curve, comparison of the jsc and Voc points of a shaded curve with the one-sun IV-curve as well as comparison of a dark IV-curve with a one-sun IV-curve, and comparison of IV-curves measured at different light intensities. The performed investigations have shown that the latter four methods all resulted in reliable series resistance values. Copyright © 2007 John Wiley & Sons, Ltd.

Determining the defect parameters of the deep aluminum-related defect center in silicon

Abstract: Through a combined application of two characterization methods, deep-level transient spectroscopy and lifetime spectroscopy, the lifetime-limiting defect level in intentionally aluminum-contaminated Czochralski silicon has been analyzed and a complete set of defect parameters could be obtained. This aluminum-related defect center is found to be located at an energy level of Et−EV=0.44±0.02eV and exhibits an asymmetric capture cross section, with σp=3.6×10−13cm2 and σn=3.1×10−10cm2 being the hole and electron capture cross sections, respectively. The investigated defect center is attributed to the aluminum-oxygen complex (Al–O).

Progress with luminescence imaging for the characterisation of Silicon wafers and solar cells

Abstract: Electroluminescence (EL) and photoluminescence (PL) imaging have recently been demonstrated to be fast experimental techniques that allow measurement of the spatial distribution of the diffusion length in silicon solar cells and of the minority carrier lifetime in large area silicon wafers (1, 2). A practical advantage of these techniques is that data acquisition times for high resolution luminescence images are typically on the order of only one second. This paper reviews previous work related to process monitoring by luminescence imaging techniques and discusses some recent progress in the areas of several experimental and theoretical aspects of luminescence imaging. It is shown that luminescence imaging is an exceptionally versatile tool that provides spatially resolved information about a variety of material and solar cell parameters with data acquisition times that are compatible with in-line process monitoring.

Cobalt related defect levels in silicon analyzed by temperature- and injection-dependent lifetime spectroscopy

Abstract: Temperature- and injection-dependent lifetime spectroscopy (TIDLS) as a method to characterize point defects in silicon with several energy levels is demonstrated. An intentionally cobalt-contaminated p-type wafer was investigated by means of lifetime measurements performed at different temperatures up to 151°C. Two defect energy levels were required to model the lifetime curves on basis of the Shockley-Read-Hall statistics. The detailed analysis is based on the determination of the recently introduced defect parameter solution surface (DPSS) in order to extract the underlying defect parameters. A unique solution has been found for a deep defect level located in the upper band gap half with an energy depth of EC−Et=0.38±0.01eV, with a corresponding ratio of capture cross sections k=σn∕σp=0.16 within the interval of uncertainty of 0.06–0.69. Additionally, a deep donor level in the lower band gap half known from the literature could be assigned to a second energy level within the DPSS analysis at Et−EV=0.41...

Electronic properties and dopant pairing behavior of manganese in boron-doped silicon

Abstract: One of the authors T.R. gratefully acknowledges a scholarship of the German Federal Environmental Foundation Deutsche Bundesstiftung Umwelt. Another D.M. is supported by an Australian Research Council QEII Fellowship.

Solarzelle und solarzellenmodul mit verbesserten rückseiten-elektroden sowie verfahren zur herstellung

Abstract: Vorliegende Erfindung betrifft Ruckseitenkontaktsolarzellen sowie daraus gefertigte Solarzellenmodule, die eine spezielle Elektrodenstruktur aufweisen sowie ein Verfahren zu deren Herstellung. Dabei sind die Elektroden, uber die der Strom der Ruckseitenkontaktzelle abgegriffen wird, durch eine isolierende Schicht von den Fingerkontakten, die mit dem n- bzw. p-Halbleiterelement der Solarzelle in Verbindung stehen, durch eine isolierende Schicht getrennt. Das Verfahren ermoglicht eine wesentliche Vereinfachung zur aus dem Stand der Technik beschriebenen Herstellungsmethoden durch raumliche Abkoppelung der Strukturierung der Elektroden von der Solarzelle.

Method for producing a metal contact structure of a solar cell

Abstract: The invention relates to a method for producing a metal contact structure of a solar cell. Said method comprises the following steps: applying a metal contact structure to a surface of the solar cell, reinforcing the metal contact structure in an electrolytic bath. The invention is characterized in that the metal contact structure is applied by applying a metal-containing ink to the surface of the solar cell by means of at least one pressure nozzle.

Method for applying electrical contacts on semiconducting substrates, semiconducting substrate and use of the method

Abstract: An electrical contact is applied on a semiconducting substrate, such as a solar cell. A layer of metallic powder is applied on the substrate. A laser beam is the guided over the substrate for local sintering and/or melting of the metallic powder. The non-sintered or non-melted metallic powder is then removed from the substrate.

Method for determination of recombination activity of cylindric conduction channels for back-contacted solar cells

Abstract: A method for the determination of the surface recombination velocity of cylindric vias for the application in back-contacted silicon solar cells is proposed. By applying a theoretical framework for the lifetime dependence on dislocation density, the measured minority carrier lifetime as a function of via density can be described accurately; thus the value for the surface recombination velocity at the cylinder barrel Svia is extracted. Accordingly, Svia after SiNx deposition from the front and the rear of the wafers systematically decreases if material surrounding the laser drilled via is removed prior to passivation by etching from 106to100cm∕s.

Efficient upconversion systems for silicon solar cells

Abstract: Frequency upconversion of sub-bandgap photons is one approach to push the efficiency limit of solar cells with one bandgap. A highly efficient upconversion system is Erbium doped NaYF 4. In this paper we present photoluminescence studies on NaYF 4 with 10%, 20% and 30% Erbium doping. We show experimental results of an upconversion system consisting of an upconverting powder on a bifacial silicon solar cell. The system has a quantum efficiency of 2.5 � 10 -3 % at 1520nm wavelength. We model the efficiency enhancement of an upconversion system by spectral concentration with fluorescent dyes. To enhance the upconversion efficiency, fluorescent dyes can be used for spectral concentration. Our theoretical calculation show that an eightfold increase in upconversion efficiency is within reach.

Method for applying electric contacts to semi-conductor substrates, semi-conductor substrate and use of said method

Abstract: The invention relates to a method for applying at least one electric contact to a semi-conductor substrate, in particular solar cells, by means of a laser sintering method. The invention also relates to a semi-conductor substrate produced according to said method, in particular a solar cell, and to the use of said method.

Solar cell and solar cell module with improved rear-side electrodes, and production method

Abstract: The present invention relates to rear-side contact solar cells and solar cell modules produced therefrom which have a special electrode structure, and to a method for producing them. In this case, the electrodes via which the current of the rear-side contact cell is tapped off are isolated by an insulating layer from the finger contacts connected to the n-type and p-type semiconductor element of the solar cell. The method enables a significant simplification with respect to production methods described from the prior art, by spatially decoupling the structuring of the electrodes from the solar cell.