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Voltage Losses due to Recombination

Recombination can have many consequences and voltage losses is one of them. As previously discussed, 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. As we study Design of Solar cells this effect should be explored. The open-circuit voltage (Voc) is an important performance characteristic of solar cells, and properly the limiting effect on Voc is critical for device optimization. First we can define Voc. The open circuit voltage is the max voltage the solar panel outputs with no load on it or the voltage at which the current of forward bias diffusion is exactly the same as to the short circuit current. The current of forward bias diffusion is dependent on the recombination amount in a pn junction and when recombination increases the forward bias current also increases. As a result, high recombination leads to increase in the forward bias diffusion current, which then reduces the open circuit voltage so effectively reducing maximum output voltage with no load.  

Factors affecting Recombination

Diode saturation current is the physical factor which defines recombination in forward bias. Recombination is controlled by many factors like:
  • Number of minority carriers at the edge of the junction
  • The speed with which the carriers move away from the junction
  • The speed of recombination

Factors affecting Voc

Those factors can be further analyzed to explain effect on open voltage current. The following parameters affect the dark forward bias current, and also the open circuit voltage:
  1. The minority carriers number at the edge of the junction. The minority carriers number injected from the other side is merely the minority carriers number in equilibrium times an exponential factor that depends on both voltage and temperature. Thus, by reducing the concentration of equilibrium minority carrier, recombination is reduced. The equilibrium carrier concentration is reduced by increasing the doping .
  2. The material’s diffusion length. A short diffusion length leads to quick disappearance of the minority carriers from the edge of the junction due to recombination, therefore allowing additional carriers to cross and the forward bias current increases. Subsequently, to reduce recombination and obtain a high voltage, a high diffusion length is needed. The diffusion length relies on the material type, the doping and processing history of them. High doping leads to reduction in the diffusion length, thus introducing a the tradeoff between keeping a high diffusion length (that affects both the current and voltage) and having a high voltage.
  3. The existence of localised recombination sources inside a junction’s diffusion length of the junction. A high recombination source near the junction (typically a surface or a grain boundary) will permit carriers to quickly move to this recombination source and recombine, thus significantly increasing the recombination current. The surface recombination impact can be reduced by passivating the surfaces.
  So by controlling the above factors we can reduce Voltage losses due to recombination in the process of designing efficient solar cells.    
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