Michal Raska

Noise Diagnostics of Solar Cells

This paper deals with the use of micro‐plasma noise for solar cells diagnostic purposes. When a high electric field is applied to a PN junction containing some technological imperfections, enhanced impact ionization arises in micro‐sized regions, thus producing so‐called micro‐plasmas, which in turn can lead to the deterioration in quality or destruction of the PN junction. It is therefore advisable to use methods which can indicate the presence of micro‐plasma in the junction and make the quality assessment and quantitative description of the tested cells possible.

Diagnostics of Forward Biased Silicon Solar Cells Using Noise Spectroscopy

Our research is above all focused on non‐destructive testing of the solar cells. We study a single‐crystal silicon solar cells n+p and we don’t have serious information about features of a pn junction and impurities distribution. The main point of our study is characterization of the local defects in samples. These defects lead to live‐time reduction and degradation of reliability. Flicker noise in forward biased solar cells is subject of this paper. We will discuss our measurement with Kleinpenning approaches for inhomogeneous semiconductors and we suggest the physical nature of the samples behaviour.

Noise sources of p-type CdTe single crystal

We have carried out noise measurements of CdTe single crystals, prepared by Institute of Physics, Charles University in Prague. These crystals are used as radiation detectors. Three types of significant noise were observed: low frequency noise, generation-recombination noise and thermal noise. The sample had very high value of 1/f noise, much higher than theoretical estimation. The depleted layers nearby the contacts are the dominating sources of noise. The contact whereof the hole concentration reduces with an applied electric field exerts prevailing effect on the noise. The resulting noise voltage is much higher than it must have been with the calculated quantity of charge carriers in the sample as a result of the depleted layers effect on the noise. Consequently the significant noise voltage appeared in the area nearby the contacts sums with the much lower value of the noise voltage in the homogenous area of the sample.

Usage of microplasma signal noise for solar cells diagnostic

This work deals with the usage of micro-plasmas signal noise for solar cells diagnostic. When high electric field is applied to PN junction with some technological imperfections it produces in tiny areas of enhanced impact ionization called micro-plasmas which could lead to deterioration in quality or destruction of PN junction. On this account it is possible to use methods which indicate presence of micro-plasma in junction and enable quality and quantitative description of tested cells.

Bulk resistance decay in CdTe

Bulk resistance decay of cadmium telluride (CdTe) single crystals was investigated. The bulk resistance of each CdTe single crystal was measured during long time interval. The samples were placed into a cryostat. That allowed us to hold the temperature constant during the measurements and eliminate the illumination influence. The measurements started and were continued at 300 K and after some period of time it was sharply raised up to 390 K. We observed the resistance slow decreasing with time with temperature T = 300 K and T = 390 K. All the samples have very high value of relaxation time. The presented samples must have not one but four acceptor or donor levels, some of them are deep levels. We have discovered that the interactions between the valence band and deep acceptor levels or between the conductivity band and deep donor levels cause this long value of relaxation time.

Noise of Reverse Biased Solar Cells

The non‐destructive testing and analysis of single crystal silicon solar cell is the focal point of our research. The noise spectroscopy and I–V curve measurement of reverse biased pn junction provide information that is connected with solar cell reliability and that provide for not only local defect characterization. We propose a new electric solar cell model, as a base for an enhanced noise model, which is in accordance with the experimentally obtained I–V curves. We suggest the physical nature of an unconventional behavior in reverse I–V characteristics, which is typical for solar cells without apparent local avalanche breakdowns.

Experimental Analysis of Noise in CdTe Radiation Detectors

Noise characteristics of CdTe gamma and X‐ray detectors have been carried out. Samples were prepared at Physical Institute of Charles University in Prague by traveling heater method (THM). Measurements of high‐ohmic detectors with two golden contacts and low‐ohmic detectors with four contacts were carried out. Two voltage contacts were used to distinguish between metal‐semiconductor junction area with depleted region and homogeneous part of the sample. The resistance of high‐ohmic samples is in the range from hundreds of M□ up to several G□. The noise characteristics of the samples were measured in dark and with the illumination in the range of radiation from ultraviolet to infrared.

Simulation of Microplasma Noise in PN Junctions

Local avalanche breakdowns take place in the neighbourhood of PN junction local defects at sufficiently high reverse voltages. These breakdowns give rise to microplasma noise. Mathematical description of the noise type was suggested on the basis of its analysis. Verification of results was carried out by means of microplasma noise simulation.

New mixed rules on composite materials

The paper studies dielectric properties of elastomer filled with Ni spherical particles of different sizes within the range 35 - 115 mum. The experimental analysis is followed by the examination permittivity/conductivity mixing rules. A new mixing formula for the composite system consisting of spherical metallic particles and polymer matrix is presented. This formula includes both Ni particle content and Ni particle size as two independent variables.

New mixed rules

The paper studies dielectric properties of elastomer filled with Ni spherical particles of different sizes within the range 35-115 mum. The experimental analysis is followed by the examination permittivity/conductivity mixing rules. A new mixing formula for the composite system consisting of spherical metallic particles and polymer matrix is presented. This formula includes both Ni particle content and Ni particle size as two independent variables.