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Iron Detection in Crystalline Silicon by Different Lifetime Measurement Techniques - ScienceDirect

In areas of increased concentration of free charge carriers due to the small cross sections, there is a large current density which can lead to strong local heating and then to the local diffusion and heat breakdown. To set a suitable reverse voltage, which leads to emission of radiation from defects, computer-controlled voltage source VS , filter capacitor C , and parallel load resistance were used Figure 1. The circuit was connected to the reverse state monocrystalline solar cell wafer SC with area of.

The electrical signal was detected at the load resistor. The obtained signal was amplified by preamplifier PA and amplifier A. To measure noise voltage characteristics and photoelectric signals in the sample, two noise detectors NDS selective Nanovoltmeters Unipan have been used.

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The voltage was measured by digital voltmeter DV and values were stored on PC. With this setup, the effective values of noise current versus reverse-bias voltage of electric and optical signals were measured.

Figure 2 represents the relation between defect noise current and reverse-bias voltage over whole solar cell wafer. By repeating this measurement, the noise signals appeared for the same values of bias voltages. The corresponding optical application in upper arm provides localization of defects or imperfections. When the reverse-bias voltage was applied, any noise signal from defect site has not been observed up to first important current peak at peak 1 in Figure 2.

With further increasing of the voltage, other near-surface defects appeared in different sites on the sample. When bias voltage reached a value of , several very intensive spots, originated mainly in ill-cutting edges of solar cell, defects in p-n junction, or imperfections of structure, have been clearly localized Figure 3 , and a corresponding current signal was quite strong peak 5 in Figure 2.

For other values, the location of these sites could vary in function of defects nature.

Advances in Optical Technologies

In the second method near-field NOBIC experiment [ 18 , 19 ], a very small area of silicon solar cell surface approx. During the scan the tip-sample distance is kept constant at using an optical shear force feedback control. Thanks to this setup, the xy current distribution map of solar cell has been obtained.

The photo-induced current signal has been detected by a lock-in nanovoltmeter while the solar cell was reverse-biased or unbiased [ 20 ]. The accuracy of this method depends primarily on the light spot size and on the scanning step of the piezo driver, which are inversely proportional. By long step the accuracy of the method is low, but whole measurement process accelerates because of reduced number of measured points. The topography of the sample with a pyramidal texture is shown in Figure 5 a.

The electrical response signals, corresponding reflectance, and topography are demonstrated by dependence on spatial coordinate in Figure 5 b. Black curve corresponds to the profile of the sample surface. Blue one corresponds to the electrical response signal and purple one represents a local surface reflectance in one scanning line. Relative electrical response mapped by color scale onto original topography of the sample is shown in Figure 6.

The pyramidal structure form Figure 5 has been etched so to decrease the electrical response on the tops of texture, which allow obtaining higher electric efficiency of the cell.

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This new mesa structure of the samples is presently the object of intensive study. The novel combination of methods for samples local electric detection and optical localization with micro- and nano-scale resolution for the study of monocrystalline silicon solar cell wafer is presented. At short circuit condition, the NOBIC photocurrent of this cell dominates over the variation of the reflectivity.

Based on correlations with aperture-SNOM, the sites corresponding to largest and smallest reflections have been assigned. The photocurrent is the smallest on top of protruding peaks which have a greater local reflectivity, and is the largest in the valleys with the smallest reflectivity. Using this correlation we have found, that the photocurrent for applying a forward voltage decreases inhomogeneously at different locations Figures 5 and 6. The measurement has shown smaller relative fall of the photocurrent for the illumination of valleys in comparison with the peaks in the structure.

Proposed characterization method based on scanning probe microscopy technique SNOM allows nondestructive and noncontact sample study defects in p-n junction, structure imperfections, and local photoelectric measurements. A maximum of optically excited photocurrent is indicator of local conversion efficiency due to local light constant energy excitation, and number of imperfections is a quality indicator for solar cell lifetime and only precise testing can help to determine a nature of defects.

At present time, it is quite difficult find a correlation between defect nature and its appearance. Therefore, for further improvement of monocrystalline silicon solar cells efficiency, more intensive mapping and nonmapping measurements of optical and electric properties are challenged. Advances in Optical Technologies. Abstract Near-surface defects in solar cell wafer have undesirable influence upon device properties, as its efficiency and lifetime.

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Detection and Localization of Defects in Monocrystalline Silicon Solar Cell.
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Iron Detection in Crystalline Silicon by Different Lifetime Measurement Techniques - ScienceDirect

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