Theory of beam induced current characterization of grain boundaries in polycrystalline solar cells

Abstract

A theoretical analysis is given of the induced current profiles at grain boundaries in polycrystalline solar cells, as obtained by light or electron beam excitation. The area A and the variance σ2 of the contrast profile of a grain boundary are calculated for realistic generations as functions of the interface recombination velocity vs and the minority carrier diffusion length L. A graphical new procedure is proposed which allows the simultaneous determination of vs and L from the measured values of A and σ. The evaluation of an experimental electron beam induced current profile illustrates the applicability of the theory.

A contactless method for measuring the recombination velocity of an individual grain boundary in thin-film photovoltaics

“…Furthermore, complementary SEM techniques such as energy/wavelength dispersive x-ray and electron backscattered diffraction can be used to relate the recombination velocity of a grain boundary to its composition and misorientation. Donolato 9 proposed using the area and variance of the electron beam induced current ͑EBIC͒ profile across a grain boundary to determine the minority carrier diffusion length and recombination velocity. However, the EBIC method is restricted to grain boundaries a͒ Electronic mail: b.g.mendis@durham.ac.uk.…”

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope

“…This effect is caused by the locally increased carrier recombination. From the contrast profile of the EBIC signal across the LAGB (cf., Figure 5H), a diffusion length of (60 ± 12) µm and a recombination velocity of (4.1 ± 0.4) x 10 4 cm sec -1 were determined for the minority charge carriers in the framework of the model by Donolato 14 . The single dark points in the EBIC image, spread over the entire sample surface and concentrated especially in the vicinity of the LAGB, indicate the positions of threading dislocations.…”

“…The extended defects usually show an increased carrier recombination so that they appear darker in an EBIC image than defect free regions. In the framework of physically based models of defects 14 , a quantitative evaluation of the spatial dependence of the EBIC signal, which is called contrast profile, enables the determination of the minority carrier diffusion length and lifetime as well as the surface recombination velocity. Because these parameters are dependent on temperature, EBIC investigations should also be performed at low temperature (cryo-EBIC) to obtain an enhanced signal to noise ratio.…”

Investigations on the recombination activity of grain boundaries in MC silicon

“…These normalized linescans, the so called contrast profiles, allow the determination of the surface recombination velocity at the grain boundary as well as the diffusion length of the minority charge carriers in each grain. The evaluation of these parameters is carried out making use of a theoretical model based on the solution of the carrier flow continuity equations [2] with suitable boundaries conditions. We used here the direct fitting procedure approach which allows better accuracy of the extracted values since none of the information contained in the experimental contrast data is discarded [3].…”

“…For cells with a hydrogen-rich SiNx antireflective coating hydrogenation takes place during the firing step where the Al BSF is formed. A MgF2 evaporation of a second layer antireflection coating (DARC) is optional and was only applied to the best 2x2 cm 2 cell from each 5x5 cm 2 wafer. In order to investigate the influence of the grain size on the solar cell parameters, mc FZ material with 0.2 mm and 1 mm average grain size was processed.…”