Surface recombination can have a major impact both on the short-circuit current and on the open-circuit voltage. High recombination rates at the top surface have a particularly detrimental impact on the short-circuit current since the top surface also corresponds to the highest generation region of carriers in the solar cell. Lowering the high top surface recombination is typically accomplished by reducing the number of dangling silicon bonds at the top surface by using a "passivating" layer on the top surface. The majority of the electronics industry relies on the use of a thermally grown silicon dioxide layer to passivate the surface due to the low defect states at the interface1. For commercial solar cells, dielectric layers such as silicon nitride are commonly used.
Since the passivating layer for silicon solar cells is usually an insulator, any region which has an ohmic metal contact cannot be passivated using silicon dioxide. Instead, under the top contacts, the effect of the surface recombination can be minimized by increasing the doping. While typically such a high doping severely degrades the diffusion length, the contact regions do not participate in carrier generation, and hence the impact on carrier collection is unimportant. In addition, in cases where a high recombination surface is close to the junction, the lowest recombination option is to increase the doping as high as possible.
Back Surface Field (BSF)
A similar effect is employed at the rear surface to minimize the impact of rear surface recombination velocity on voltage and current if the rear surface is closer than a diffusion length to the junction. A "back surface field" (BSF) consists of a higher doped region at the rear surface of the solar cell. The interface between the high and low doped region behaves like a p-n junction. An electric field forms at the interface, which introduces a barrier to minority carrier flow to the rear surface. The minority carrier concentration is thus maintained at higher levels in the bulk of the device, and the BSF has a net effect of passivating the rear surface.2
- 1. , “Calculation of surface generation and recombination velocities at the Si-SiO2 interface”, Journal of Applied Physics, vol. 58, p. 4267, 1985.
- 2. , “Physical operation of back-surface-field silicon solar cells”, IEEE Transactions on Electron Devices, vol. 24, pp. 322 - 325, 1977.