A new adaptive light beam focusing principle for scanning light stimulation systems

Abstract

In this article a novel principle to achieve optimal focusing conditions or rather the smallest possible beam diameter for scanning light stimulation systems is presented. It is based on the following methodology: First, a reference point on a camera sensor is introduced where optimal focusing conditions are adjusted and the distance between the light focusing optic and the reference point is determined using a laser displacement sensor. In a second step, this displacement sensor is used to map the topography of the sample under investigation. Finally, the actual measurement is conducted, using optimal focusing conditions in each measurement point at the sample surface, that are determined by the height difference between camera sensor and the sample topography. This principle is independent of the measurement values, the optical or electrical properties of the sample, the used light source, or the selected wavelength. Furthermore, the samples can be tilted, rough, bent, or of different surface materials. In the following the principle is implemented using an optical beam induced current system, but basically it can be applied to any other scanning light stimulation system. Measurements to demonstrate its operation are shown, using a polycrystalline silicon solar cell.

Scanning Light Stimulation System With Active Focus Correction at <inline-formula> <tex-math notation="LaTeX">$\mu \hbox{m}$</tex-math> </inline-formula> Resolution for PV Applications

“…We discussed a method for obtaining optimal focusing conditions on a sample surface based on an edge detection technique in combination with our previously introduced adaptive focusing principle [12]. Integrated into an OBIC/OBIV system, an image resolution down to ρ 1/e 2 ≈ 4.3 μm is achieved.…”

“…The original focusing principle, as previously described in detail, is integrated into an optical beam induced current (OBIC) system [12]. It enables an adaptive focusing in each measurement point on a sample surface, and is based on three basic steps: first, the smallest possible beam diameter is adjusted at a reference point (x ref , y ref ) on a CMOS sensor surface, and the corresponding z-position is defined as Fig.…”

“…Recently, we presented a focusing principle which does not have these disadvantages, and operates independent of the optical or electrical sample properties, the light source and the selected wavelength [12]. Furthermore, samples can be tilted, rough, bent, of different surface materials or covered by an encapsulating optical medium [13].…”

Optical Characterization and Photovoltaic Performance Evaluation of GaAs p-i-n Solar Cells with Various Metal Grid Spacings

“…LBIC mapping is a convenient tool for analyzation of the carrier collection efficiency at different positions on the surface and to find the low efficiency areas. Various groups have used the LBIC technique to investigate grain boundary distribution and to explain the minority carrier diffusion length and surface recombination velocity at a grain boundary or surface [14,15].…”

Injection in light beam induced current systems: An analytical model

“…The issue of estimating injection maxima in both cases arises also for LBIC systems using lamp and monochromator as light source as well as solar LBIC systems , because of focusing the light sharply. In these cases, focusing the light beam creates a Gaussian distribution of light intensity that might hinder the requirement of spatial uniformity of the light source.…”