- Areas of defect, such as at the surface of solar cells where the lattice is disrupted, recombination is very high.
- Surface recombination is high in solar cells, but can be limited.
- Understanding the impacts and the ways to limit surface recombination leads to better and more robust solar cell designs.
Any defects or impurities within or at the surface of the semiconductor promote recombination. Since the surface of the solar cell represents a severe disruption of the crystal lattice, the surfaces of the solar cell are a site of particularly high recombination. The high recombination rate in the vicinity of a surface depletes this region of minority carriers. As explained in the Diffusion Page, a localized region of low carrier concentration causes carriers to flow into this region from the surrounding, higher concentration regions. Therefore, the surface recombination rate is limited by the rate at which minority carriers move towards the surface. A parameter called the "surface recombination velocity", in units of cm/sec, is used to specify the recombination at a surface. In a surface with no recombination, the movement of carriers towards the surface is zero, and hence the surface recombination velocity is zero. In a surface with infinitely fast recombination, the movement of carriers towards this surface is limited by the maximum velocity they can attain, and for most semiconductors is on the order of 1 x 107 cm/sec.
The dangling bonds (unpaired electrons) at the surface of a semiconductor cause a high local recombination rate.
The defects at a semiconductor surface are caused by the interruption to the periodicity of the crystal lattice, which causes dangling bonds at the semiconductor surface. The reduction of the number of dangling bonds, and hence surface recombination, is achieved by growing a layer on top of the semiconductor surface which ties up some of these dangling bonds. This reduction of dangling bonds is known as surface passivation.
The answer to a question "Why does limiting surface recombination lead to longer minority-carrier lifetimes in a cell?" is the following:
The minority-carrier lifetime of the material is contingent upon both bulk recombination and surface recombination. Limiting surfacing recombination can lessen the rate at which minority carriers are depleted. If the rate of minority carrier depletion can be limited, the observed minority-carrier lifetime of the material can be extended.