Point-contacting by Localised Dielectric Breakdown: self-aligned metallisation for a solar cell rear surface

Abstract

The operating efficiency of commercial p-type silicon solar cells has been greatly improved in recent years by developments in the front surface design. This has been largely driven by new screen printed pastes and selective emitter technologies which have facilitated the use of high efficiency emitters. The rear surface offers next best way to increase cell efficiency by improving on the ubiquitous full area screen printed aluminium back surface field.

A new method is presented for creating contact to a doped region through a passivation dielectric. The technique is self aligned, fast, low tech and performed at room temperature. A voltage is applied between the silicon bulk and the gate metal of a metal insulator semiconductor device which contains a single doped point contact. Hard breakdown of the dielectric occurs over the doped region leading to stable low resistance ohmic contact. This work focuses on the potential for this method to be used in a rear point contact design similar to the PERL cell.

A detailed investigation is given of the mechanism of contact formation conclusively demonstrates its initiation by dielectric breakdown. Good fits to the Weibull function are observed for four commonly used surface passivation dielectrics, as supported by the percolation model for dielectric failure. After breakdown, a current path through the dielectric is been created where subsequent Joule heating occurs leading to low resistance ohmic contact. Structural analysis by Transmission Electron Microscope imaging show diffusion of the gate metal into the dielectric, confirming the localised nature of the resulting contact. Other competing mechanisms that may be involved in the contacting process, such as metal induced crystallisation, are investigated and ruled out.

Important figures of merit for performance in a solar cell are measured, including contact resistivity as low as 250 uOhm cm2 and effective surface recombination velocity down to 55 cm/s. This allows a comparison to other competing technologies and supports the potential for this technology to be used in high efficiency commercial p-type silicon solar cells.