Shunting Problems Due to Sub-Micron Pinholes in Evaporated Solid-Phase Crystallised Poly-Si Thin-Film Solar Cells on Glass

TLDR: In this paper, two distinctly different approaches are shown to reduce the shunting problem to a negligible level: (i) to contact only a small fraction of the rear Si surface via a point contacting scheme, whereby the metal layer needs to be thin and the fractional area coverage small (<5%), and (ii) to deposit line contacts in a bifacial inter-digitated scheme.

Abstract: Recent progress in the metallisation of poly-silicon thin-film solar cells on glass, created by solid phase crystallisation (SPC) of evaporated amorphous silicon (EVA), revealed that shunting through sub-micron holes (density 100–200 mm−2) in the films causes severe shunting problems when the air-side metal contact is deposited onto these diodes, by creating effective shunting paths between the two highly doped layers of EVA cells. We present evidence of these pinholes by optical transmission and focussed ion beam (FIB) microscopic images and confirm the point-like pinhole shunts using lock-in thermographic images. The latter revealed that the Al rear electrode induces strong ohmic shunts below the grid lines and a high density of weak non-linear shunts away from the grid lines. Two distinctly different approaches are shown to reduce the shunting problem to a negligible level: (i) to contact only a small fraction of the rear Si surface via a point contacting scheme, whereby the metal layer needs to be thin (<1 µm) and the fractional area coverage small (<5%), and (ii) to deposit line contacts in a bifacial interdigitated scheme, whereby a thick layer of metal is deposited followed by a wet-chemical etching step that effectively reduces shunting by preferentially etching away the shunting paths. Test devices with an area of 1 cm2 achieve pseudo fill factors (pFF) of above 75% and diode ideality factors of below 1·3, demonstrating that the proposed methods are well suited for the metallisation of the rear surface of EVA solar cells. Copyright © 2008 John Wiley & Sons, Ltd.