Current Losses Due to Recombination
Recombination losses has an effect on both the current collection (thus the short circuit current) and also the forward bias injection current (thus the open circuit voltage). Recombination is typically classified based on the region of the cell in which recombination occurs. Normally, surface recombination (recombination at the surface) or bulk recombination (in the bulk of the solar cell) are the key recombination areas. Another area where recombination can occur is the depletion region (depletion region recombination.
Surface and bulk recombination should be both minimized in order for the p n junction to be able to collect all of the light generated carriers. In the silicon solar cells, there are two conditions typically needed for a similar current collection, as detailed below:
- The carrier should be generated within the diffusion length of the junction, so that the carrier can diffuse to the junction before any recombination
- The carrier should be generated closer to the junction than to the recombination site in the case of a localized site of high recombination(as an example at the unpassivated surface or at the grain boundary of the multicrystalline devices). For localised recombination sites which are less severe, (as an example a passivated surface), carriers can still be generated closer to the recombination site than to the junction, yet still be able to diffuse to the junction and be collected without recombining.
Presence of Localized recombination sites
The existence of the localized recombination sites at both the front surface and the rear surface of a silicon solar cell explains the different collection probabilities based on the different energy the photons have. Because blue light poses a higher absorption coefficient and is absorbed really close to the front surface, it is not probable that it will generate minority carriers that can be collected by the junction if the front surface is a site that has high recombination. Likewise, carriers generated by infrared light will be mainly affected by the high rear surface recombination, which can generate the carriers deeper in the device. The quantum efficiency of a solar cell can be used to quantify the recombination effect on the light generation current.