The surfaces of solar cells are an important multifunctional interface, critical to solar device operation. At the surface of a semiconductor, the periodicity of the atomic lattice ends, and atoms at the surface lack sufficient neighbours to bond with. This creates localised sites where the atomic outer shell consists of vacant ‘dangling’ bonds, which are highly recombination active. Dangling bonds introduce discrete energy levels across the forbidden energy gap, which electrons and holes use as sites to facilitate a transition from the conduction to the valence band in recombination processes.
Any process which seeks to reduce the surface recombination rate is known as surface passivation. Recombination is a complementary process which requires both an electron and hole to be present, and at the surface, additional interface states are also a participant in the process as sites which facilitate capture. Surface passivation methods can be categorised into two broad strategies:
- Reduce the number of interface sites at the surface.
- Reduce the population of either electrons or holes at the surface.
Point one above usually involves the formation of hydrogen and silicon bonds and is commonly referred to as ‘chemical passivation. Field or charge-effect passivation can be achieved by doping, or by the introduction of electrostatic charge at the surface interface, which repels minority carriers from the surface. In the instance of thin silicon nitride (SiNx) layer, positive charge is introduced to the surface, which is beneficial for n-type surfaces as this reduces the minority (hole) carrier population, however on p-type surfaces this will increase the minority carrier (electron) population increasing recombination. In the instances of a p-type substrate, aluminium oxide (AlOx) can be used—as is the case in the rear passivation of PERC solar cells—as this dielectric introduces net negative fixed charge to the surface which, in the case of a p-type surface, will attract majority carriers (holes) and repel minority carriers (electrons).
Furthermore, both SiNx and AlOx films typically require an anneal to ‘activate’ the surface passivation. In the instance of SiNx, this is performed during a firing step, whereby hydrogen is released from the film, which then passivates dangling bonds at the surface of the silicon. Surface passivation of solar cells is increasingly important as the wafers become thinner since a greater proportion of the overall recombination occurs at the surface regions.