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Current Losses Due to Recombination

In order for the p-n junction to be able to collect all of the light-generated carriers, both surface and bulk recombination must be minimised. In silicon solar cells, the two conditions commonly required for such current collection are:

  1. the carrier must be generated within a diffusion length of the junction, so that it will be able to diffuse to the junction before recombining; and
  2. in the case of a localized high recombination site (such as at an unpassivated surface or at a grain boundary in multicrystalline devices), the carrier must be generated closer to the junction than to the recombination site. For less severe localised recombination sites, (such as a passivated surface), carriers can be generated closer to the recombination site while still being able to diffuse to the junction and be collected without recombining.

The presence of localized recombination sites at both the front and the rear surfaces of a silicon solar cell means that photons of different energy will have different collection probabilities. Since blue light has a high absorption coefficient and is absorbed very close to the front surface, it is not likely to generate minority carriers that can be collected by the junction if the front surface is a site of high recombination. Similarly, a high rear surface recombination will primarily affect carriers generated by infrared light, which can generate carriers deep in the device. The quantum efficiency of a solar cell quantifies the effect of recombination on the light generation current. The quantum efficiency of a silicon solar cell is shown below.

quantum efficiency

Typical quantum efficiency in an ideal and actual solar cell, illustrating the impact of optical and recombination losses.