Surface Recombination

Surface Recombination effects

As a part of the Solar Cell Design process, we need to consider Surface recombination. It can have a key influence on both the open-circuit voltage and short-circuit current. Since top surface resembles the highest carriers generation region in the solar cell, the top surface’s high recombination rates have an especially unfavorable effect on the short-circuit current. Using passivating layer on the top surface can be normally used to lower the high top surface recombination by the reduction of the number of silicon bonds dangling at the top surface. Most of the electronics industry rely on using a silicon dioxide layer -thermally grown- for passivating the surface due to the interface’s low defect states. Dielectric layers like silicon nitride are commonly used in commercial solar cells. The surface plays a significant role in recombination as well. Normally the surfaces complicate the bulk lifetime measurement. Surface lifetime (τ_s ) typically describes recombination at the surfaces, in which the fundamental decay mode is included but higher decay modes is ignored. Surface lifetime is a function of S₁ and S₂ (the surface recombination velocities), W (the cell width) and D (the minority carrier diffusivity). The exact solution is really complex but an estimated solution is available for special cases and is accurate enough for most purposes.  

Techniques for reducing the impact of surface recombination

Silicon dioxide can’t be used to passivate any area containing an ohmic metal contact because the passivating layer for silicon solar cells is usually an insulator. Alternately, the effect of the surface recombination can be diminished by increasing the doping under the top contacts. While diffusion length is normally severely degraded by the high doping, the regions of contact do not partake in carrier generation and therefore the impact on carrier collection is not important. Furthermore, when the high recombination surface is near the junction, increasing the doping to the highest possible level is the lowest recombination option.

Back Surface Field

For reducing the impact of rear surface recombination velocity on current and voltage,a similar effect is used at the rear surface if it is closer than the diffusion length to the junction. A back surface field (BSF) consists of a region that is higher doped at the solar cell’s rear surface. The interface between the low and high doped regions act similar to a p-n junction and an electric field is formed at the interface that introduces a barrier to flow of the minority carrier to the rear surface. Therefore the minority carrier concentration is maintained in the bulk of the device at the higher levels and the BSF has a passivating net effect on the rear surface. So by controlling it, we can reduce the effect Voc and Isc due to recombination in the process of designing efficient solar cells.