Light Beam Induced Current Technique Research Articles

EBIC and LBIC studies of the properties of extended defects in plastically deformed silicon

The results of comparative experimental studies of one- and two-dimensional defects in plastically deformed silicon by the electron-beam-induced current (EBIC) and light-beam-induced current (LBIC) techniques are reported. It is shown that the contrast of two-dimensional defects (dislocation trails) in the LBIC method can by much more pronounced than that in the EBIC technique, which is in good agreement with the results of calculations. The higher sensitivity of the LBIC technique is mainly due to deeper penetration of the optical beam into the material in comparison to the penetration of the electron beam of a scanning electron microscope.

Printed metal back electrodes for R2R fabricated polymer solar cells studied using the LBIC technique

The performance of printable metal back electrodes for polymer solar cells were investigated using light beam induced current (LBIC) mapping of the final solar cell device after preparation to identify the causes of poor performance. Three different types of silver based printable metal inks were employed. Organic solvent based, UV-curable and water based silver inks were tested. Both grid electrodes and full electrodes were employed and it was shown via the grid electrode that the organic solvent based ink adversely affects the device performance under the printed metal whereas both the UV-curable and the water based inks were neutral to improving device performance. Complete roll-to-roll (R2R) processed modules were also tested and some limitations of the LBIC technique was identified for serially connected modules.

Interaction between process technology and material quality during the processing of multicrystalline silicon solar cells

Multicrystalline silicon is the most used material for the production of silicon solar cells. The quality of the as grown material depends on the quality of the feedstock and the crystallization process. Bulk impurities, crystal defects like dislocations and of course the grain boundaries determine the material quality and thus the solar cell conversion efficiency. Therefore minority carrier lifetime measurements are often done to characterize the material quality. But the measured values are from limited use because it is known that the solar cell process itself can dramatically change the minority carrier lifetime and the solar cell efficiency. In order to obtain more detailed information of the behaviour of different defect types additionally high-resolution LBIC (light beam induced current)-measurements have been done. Since LBIC needs a pn-junction for photocurrent generation the LBIC technique has been combined with the a-Si/c-Si heterojunction cell process, which makes it possible to manufacture solar cells even from as cut wafers without changing the material quality. With this combination of measurement and preparation techniques it was possible to analyze the influence of the diffusion process and the firing process on the behaviour of the three different defect types: grain boundaries, dislocation networks and bulk impurities.

Light beam induced current and infrared thermography studies of multicrystalline siliconsolar cells

In this paper we demonstrate the parallel application of light beam induced current (LBIC)and lock-in infrared thermography for the investigation of strong shunting regions inmulticrystalline silicon solar cells. Usually both mappings are not correlated, but in thiscase the shunts could be imaged by both techniques. If for a locally generated photocurrentthe conductance through a shunt lying nearby is comparable to that across the emitter intothe current amplifier, local shunts become visible in the LBIC as dark regions. After therear contact of the cell was removed, the LBIC technique was performed fromthe rear side of the cell. The images point to the existence of inversion layersalong grain boundaries crossing the bulk of the cell. Obviously, these inversionlayers represent the dominant material-induced shunt type in multicrystallinesilicon solar cells. Moreover, it has been shown that cracks may lead to shunts.

CELLO: an advanced LBIC measurement technique for solar cell local characterization

A solar cell is a large area device, and thus its global IV-characteristic and efficiency depend strongly upon its local properties. Local defects, such as a locally reduced diffusion length, a strong local shunt resistance or a high local series resistance will adversely influence the cell’s global properties. Experimental techniques suitable for mapping the spatial distribution of such local parameters can provide valuable information, and thus help to improve the technology for the production of efficient and reproducible solar cells. The LBIC (Light Beam Induced Current) technique [1] allows calculation of the local diffusion length of the solar cell material from local photocurrent data obtained under short-circuit conditions. As well as allowing LBIC measurements, the PVScan 5000 mapping analyzer by NREL [2] can be used to map defects and grain boundaries using reflectivity data and special surface etching. Localized shunts can be mapped by CCD cameras [3,4] or nematic liquid crystals [5]. The electron-beam-induced current (EBIC) is an alternative technique for the investigation of defects in solar cells [6]. Destructive techniques, MASC [7] for local IV-characterisation and RAMP [8] using a scanning (by scratching) tungsten electrode, have also recently been developed.

Fast LBIC in-line characterization for process quality control in the photovoltaic industry

The photovoltaic industry asks for fast, non-destructive techniques for in-line characterization tools in solar cells production. We shall show in this paper that the use of the light beam induced current technique (LBIC) is capable to get in a few seconds time photocurrent maps of large area solar cells and to correlate these data with the cell efficiency. The samples analysed in this study are industrial 10×10 cm 2 multicrystalline silicon solar cells. The LBIC setup works with three laser sources at 633, 780 and 830 nm, taking the laser beam power below 1 mW. The laser beams are moved on the wafer surfaces using a galvanometer x–y scanner system and the beam size on the focus has a diameter of about 65 μm. We demonstrated the possibility to obtain quantitative information about the cell quality in <3 s from the photocurrent maps with a pretty good correlation with the efficiency data.

Impurity Removing at Dislocations in Float Zone Silicon by Aluminium-Silicon Alloying

ABSTRACTAluminium-silicon alloying is applied to dislocation-containing FZ silicon samples in order to verify if this external gettering technique is able to remove impurities trapped by these defects. Samples were scratched (front surface), bent and annealed at 750°C for 6h in order to create dislocation arrays. Standard samples and also nickel contaminated samples were investigated by light beam induced current (LBIC) mapping technique before and after alloying at 900°C for 4h with a 1 μm thick aluminium layer deposited on the backside. The LBIC technique detects the features of the dislocation array in agreement with X ray topographs. In the standard samples, the contrast is relatedto an inadvertent contamination of the samples and disappears after gettering. In the contaminated samples, dislocation contrast is higher, the aluminium treatment attenuates strongly the dislocations electrical activity but not so neatly as for the standard samples. This result is explained by the possible microprecipitation of nickel at dislocations.

Detection of junction failures and other defects in silicon and III–v devices using the LBIC technique in lateral configuration

A scanning light microscope was used to obtain light beam induced current (LBIC) profiles in samples containing a p-n junction either parallel or perpendicular to the surface scanned by the beam. Using this technique we studied the quality of the junction of InP(n)-InP(p) diodes, which are one of the intermediate structures obtained during the processing of InGaAsP/InP lasers. The same technique was successfully employed also for the quality control of silicon power diodes. It was thus demonstrated that the LBIC technique, operated in the standard and in the lateral configuration at a resolution better than 10 μm, is a powerful, nondestructive tool which can be efficiently used for the quantitative measure of the damage present at any kind of diode junctions.

LBIC investigation of impurity-dislocation interaction in FZ silicon wafers

In the present work, dislocation arrays are investigated in float zone (FZ) grown silicon wafers by the light beam induced current (LBIC) mapping technique at different wavelengths and by deep level transient spectroscopy (DLTS). The LBIC technique appears to be able to recognize and to detect these arrays and to evaluate their recombination strength. In FZ dislocated wafers, a phosphorus diffusion attenuates strongly the LBIC contrast of dislocations, depending on the duration and temperature of the treatment. Electrical activity at room temperature of the defects, still physically present, seems to disappear. Simultaneously, the peak intensity of DLTS spectra related to dislocations is reduced and this evolution depends on the phosphorus diffusion temperature and duration.

Influence of phosphorus diffusion on the recombination strength of dislocations in float zone silicon wafers

Dislocation arrays are investigated in float zone (FZ) grown silicon wafers by the light beam induced current (LBIC) mapping technique at various wavelengths and by deep level transient spectroscopy (DLTS). The LBIC technique allows us to recognize and detect these arrays and to evaluate their recombination strength. Dislocations are found to be less recombining in (100)-oriented FZ samples than in (111) oriented ones. In FZ dislocated wafers, a phosphorus diffusion strongly attenuates the LBIC contrast of dislocations, depending on the duration and temperature of the treatment. Electrical activity of the defects, which are still physically present, as verified by x-ray topography, seems to disappear. Simultaneously, the peak intensities of DLTS spectra related to dislocations are reduced and this reduction depends on the phosphorus diffusion temperature and duration.

A novel method for the determination of the radiation damage effects in silicon detectors

The result of a study about the effects of radiation damage on silicon detectors, carried out using the LBIC (light beam induced current) technique are thoroughly reported. As this technique is capable to detect any change in the minority carriers diffusion lenght associated to radiation damage, it was possible to show that the main degradation effect, associated to fast neutron irradiation up to a fluence of 10 13 n cm 2, is a reduction of the lifetime of the minority carriers, which reduces the collection efficiency. A secondary effect which has been also clearly detected is a drift of the Fermi level, presumably associated to damage-induced compensation effects.