Investigation of electrical shading effects in back-contacted back-junction silicon solar cells using the two-dimensional charge collection probability and the reciprocity theorem

Abstract

In this study, short-circuit current losses in high-efficiency n -type back-contacted back-junction silicon solar cells caused by the electrical shading effect have been investigated by two-dimensional simulations of the charge carrier collection probability. Based on the reciprocity theorem, the homogenous partial differential equation describing the probability of charge carriers being collected by the p-n junction on the rear side of the solar cell has been solved numerically using the finite element method implemented in the partial differential equation solver COMSOL Multiphysics. The method has been applied to study the impact of geometrical parameters of the solar cell, such as the pitch distance, as well as the emitter and back surface field width, on the local and global internal quantum efficiency and on the short-circuit current density. The influence of the rear surface recombination velocity of an undiffused gap and the effective rear surface recombination velocity of the back surface field region on the short-circuit current density is also presented. It has been found that the width and the surface recombination velocity of the undiffused gap on the rear side of the solar cell have a strong impact on the charge collection probability in the base. Thus, the surface recombination velocity of the undiffused gap has to be minimized or the undiffused gap has to be reduced or omitted completely in order to increase the short-circuit current density significantly. Furthermore, it has been found that low base doping concentrations are essential for minimizing the effective rear surface recombination velocity of the back surface field. It has also been shown that in the case of low base doping concentrations, the short-circuit current density reaches its maximum value and that in this case it is nearly independent of the back surface field doping concentration

Inorganic photovoltaics – Planar and nanostructured devices

“…The same effect is also demonstrated in a back-contacted diffused 2421 junction solar cell [359]. Fill factor increases along with emitter coverage up to a certain 2422 value and then decreases because of the increased series resistance related to the longer travel Fig.…”

Back-Contacted Silicon Heterojunction Solar Cells With Efficiency >21%

“…Conversely, the IBC-SHJ device FF is 73.0%, which is significantly lower than any typical high-performance Std-SHJ device. Based on spectral response and light-beam-induced current measurements, we link the still relatively modest J sc gain in our IBC-SHJ devices (versus our best Std-SHJ device) to not fully minimized parasitic absorption losses (both in the short-and long-wavelength parts of the spectrum), rather than to electrical shading effects [30], [31]. Further details on this topic will be the subject of future research.…”

Back-Contacted Silicon Heterojunction Solar Cells: Optical-Loss Analysis and Mitigation

“…In our case, it seems reasonable to consider 0.5-mm-wide electron-collecting stripes as a practical limit for our process flow. Alternatively, as the electrical shading is also dependent on the surface-passivation level of the electron-collecting regions [33], it can be reduced by enhancing the passivation quality in our IBC-SHJ devices. To which quantitative extent these two levers will allow mitigating J medium still needs to be investigated.…”