Solar cells produce electricity by generating current from incident light. Solar cells are electrical modules that directly convert sunlight into electric energy. Current is generated by incident light in the photovoltaic cells. Excess Carriers are generated when the semiconductor absorbs the light due to the energy of the photon, which if is large enough, will break the bond between the semiconductor’s two atoms and cause the electron hole to be formed
Light generating Current process
The process of generating current in a solar cell – which is the light generated current, includes two main processes:
The absorption of incident photons for creating electron-hole pairs is the first process. Pairs of electron-hole would be created in the solar cell given that the incident photons possess an energy bigger than the energy of the band gap. Though, electrons in p type material, and holes in the n type material are in a state of meta stability and, on average, would only exist for a time length which is equal to the lifetime of minority carrier before recombining. After carriers recombining, no current or power can be generated as the light-generated electron-hole pair is lost.
The second process is the carriers collection by the pn junction, as recombination is prevented using a pn junction to separate the electron and hole spatially. The action of the electric field present at the pn junction separate the carriers. When the light generated minority carrier touches the pn junction, the electric field at the junction sweeps it across the junction, where it now becomes a majority carrier. If the base and emitter of the solar cell are together connected (as in when short circuiting the solar cell), the light generated carriers flow across the exterior circuit.
The "collection probability" defines the likelihood that the light generated carrier that is absorbed in a specific region of the semiconductor device will be captured by the pn junction and consequently will contribute to the light generated current. This likelihood depends on the distance which a light generated carrier have to travel in comparison to the diffusion length and also on the surface properties of the device.
Collection probability of carriers created in the depletion region is always unity because all the electron hole pairs are rapidly swept apart by the electric field and are then collected. Far from the junction, the collection probability drops down when:
The light generated carrier is further than a diffusion length away from the junction, then the collection probability of this carrier is for sure quite low.
Also, when the carrier is generated near to a region as in the case of a surface with higher recombination than the junction, and as a result the carrier will recombine.