Eugene Smirnov

Investigation of the solar cell emitter quality by LBIC-like image techniques

Abstract The distributed properties of solar cell emitters and p–n junctions such as sheet resistance, local potential and shunt resistance were studied by original methods based on the laser scanning technique. To study the shunt resistance defects the dynamical optical reflectance thermography technique has been developed thanks to new possibilities of signal treatment and 2D scanning. The experimental factors that influence on PV LBIC-like signal are analyzed. It is shown that the information about the local potential and sheet resistance distribution is contained in the LBIC-like signal measured in the galvanostatic circuit and extracted by comparison of the signal distributions obtained under different applied voltage and laser beam parameters. The samples of the mapping of the resistive properties are presented for multicrystalline silicon solar cells without and with selective porous silicon antireflection coating.

Dual-beam dual-frequency scanning laser radar for investigation of ablation profiles

High-quality eye treatment needs sophisticated and accurate instrumentation for cornea profile measurement before and after laser ablation. Plastic PMMA specimens are in ordinary use for pre-operational adjustment of laser intensity distribution, supposed that the results of the cornea surface ablation and the ablation of the plastic surface are identical. We investigated dual-beam technique for path- difference measurement of two laser beams, reflected from two neighboring points of the surface to be measured. The beams are frequency shifted relative to each other. Interference in the plane of receiver gives differential frequency, its phase being dependent on path difference. Acousto-optical scanning results in 2D distribution of path difference. To get surface profiles, along-line videosignals undergo the mathematical operation of integration. The sensitivity of several nanometers was got to flat specimens. For convenience of image processing and visualization, TV type scanning is applied to laser beams.

Three-beam scanning laser radar microprofilometer

In many scientific and technologic applications, for example, in ophthalmology, microelectronics, etc, the problem exists of profile investigation with nanometer accuracy. Double-frequency phase-difference interferometric technique is known, that uses phase difference measurement between two beams propagation to the investigated surface and back at two different carrier frequencies. These measurements give profiles derivative along the line of scanning, that can be reconstructed into the profile using mathematical operation of integration. To get high accuracy, integration error must be excluded or compensated. It can be easily done, if special specimen plate is used with reference plane. Live objects, like human cornea, have no reference planes or points. We report here on the technique using measurement of two partial derivatives, thus enabling correct reconstruction of a 3D surface. Three laser beams are configured in acousto-optical system of modulators and scanners, in which partial derivatives are measured in each point for orthogonal X and Y directions. For this purpose, two modulators are positioned in series, producing alternatively neighboring beam pairs oriented along X or Y axis and having the same frequency difference for both directions. For image format 512 X 512, corresponding to investigated area 7 X 7 mm, with chosen geometry and frequency difference (f1 - f2) equals 1 MHz, time of measurement in each point is about 1 microsecond, resulting in total time less than 0.3 sec. per frame. The instrument is sensitive to profile variations less than 4 nm.

High-precision double-frequency interferometric measurement of the cornea shape

To measure the shape of the cornea and its declinations from the necessary values before and after PRK operation, s well as the shape of other spherical objects like artificial pupil, a technique was used of double-frequency dual-beam interferometry. The technique is based on determination of the optical path difference between two neighboring laser beams, reflected from the cornea or other surface under investigation. Knowing the distance between the beams on the investigated shape. The shape itself is reconstructed by along-line integration. To adjust the wavefront orientation of the laser beam to the spherical shape of the cornea or artificial pupil in the course of scanning, additional lens is involved. Signal-to-noise ratio is ameliorated excluding losses in the acousto-optic deflectors. Polarization selection is realized for choosing the signal needed for measurement. 2D image presentation is accompanied by convenient PC accessories, permitting precise cross-section measurements along selected directions. Sensitivity of the order of 10-2 micrometers is achieved.

Detection of a subsurface flaw with the total Internal Reflection Ultrasonic Sensor

A novel ultrasonic sensor aimed specifically at detecting subsurface flaws is developed and investigated experimentally. The sensor exploits the phenomenon of frustrated total internal reflection and operates as a two-port device whose insertion loss is affected by a flaw beneath the surface of a tested object. It is designed as a prism of a TeO2 single crystal, which in the experiment was put in contact with tested specimens using an optical contact technique. An artificial “defect” in the defected specimen of fused quartz was an epoxy layer of 28 μm in thickness. Testing the flawless and defected specimens has shown a distinctive difference between the sensor responses that proves the sensor capability to detect a subsurface flaw.

New technique for investigation of solar cell sheet resistance distribution by laser beam scanning

Laser beam scanning was applied for evaluating the distribution of sheet resistance of solar cell emitter. It was shown that the voltage drop around the illuminated spot has an information about the local sheet resistance since the most part of the voltage drop occurs near the illuminated area. The current under local illumination in reverse direction depends on the local quantum efficiency while in forward direction it depends on the same local properties and on the local sheet resistance. The processing of laser beam induced current images at different bias voltage gives a map of local sheet resistance complementing other techniques for investigation the electron devices. We investigated one and dual-beam technique for amplitude and phase LBIC measurement by means of universal laser scanning microscope worked in amplitude and differential-phase regimes. Acousto-optical scanning results in 2D distribution of amplitude or phase LBIC. For convenience of image processing and visualization, TV type scanning is applied to laser beams.

Plane-wave expansion based modelling of laser beam propagation in anisotropic medium

An efficient method of numerical calculation of optical field distribution during propagation through generally anisotropic medium is proposed. The method is based on plane-wave representation of the vector field and is valid for nonparaxial case of highly focused laser beams and fully describe polarization effects. It allows to specify arbitrary distribution of intensity, phase and polarization at source plane and include both evanescent field and propagating modes into consideration. The computation algorithm is fast and easily parallelizable because conventional two-dimensional fast Fourier transform is used as a core.

Optical visualization of ultrasonic pulses in the Total Internal Reflection Ultrasonic Sensor

An acousto-optic technique exploiting spatial filtration of light diffraction orders is used to visualize ultrasonic pulses in a TeO2 crystal, which is an ultrasonic sensor body. In the experiments, images of a sought shear and a spurious longitudinal waves emitted by a shear wave transducer were recorded. An unexpected occurrence of a satellite ultrasonic pulse due to a mismatched size of the radiating transducer was observed. No conversion of a SH mode at its reflection from the oblique free surface of the crystal was observed. At the same time, an inexplicable displacement of an ultrasonic beam was discovered that might be a manifestation of the acoustic Goos-Hänchen effect.

Laser scanning for sensing and study the operation of semiconductor devices

Advanced scanning technique on the base of laser microscope and evaluation of the induced photovoltaic (PV) signal is discussed. It is able to obtain non-destructively the 2D distribution of the several important parameters of semiconductor PV devices such as surface potential, sheet resistance, efficiency and losses of energy conversion, nanosize deviations of their layers.

Series of laser scanning techniques as a nondestructive tool for testing solar cells and batteries

We have designed a series of original techniques able to obtain non-destructively the 2D distribution of the several important parameters. Laser scanning of dynamically loaded solar cell has been shown to give a distribution of the dark current density since this value determines the local heat power and the reflectivity of the cell surface. An additional positive voltage bias at local laser irradiation and typical working conditions applies to the solar cell due to the concentration of current lines within the illuminated spot and due to the voltage drop depending of the local emitter resistivity. Quantitative comparison of the resistance sensitive photocurrent distribution and such distribution in the resistance independent conditions gives an information about local resistivity. The analysis of LBIC signal obtained from the whole battery at the different bias voltage and illumination conditions has shown to investigate the contribution of each cell to the total efficiency and the energy losses, switching to the output contacts of the battery only. This approach makes it possible to monitor the cell characteristics in encapsulated modules directly in loading conditions and without their destruction.