Analysis of the types of recombination mechanisms of solar cells

The conversion efficiency of solar cells is also reduced because electron-hole pairs recombine before they can be effectively used. Light of appropriate wavelength shines on a semiconductor to generate electron-hole pairs. Therefore, the carrier concentration of the material will exceed the value when there is no light. If the light source is cut off, the carrier concentration decays to the value when they are in equilibrium. This decay process is generally called recombination. Several different recombination mechanisms are introduced below.

(1) Radiative recombination

Radiative recombination is the reverse process of light absorption, where electrons return from a high energy state to a lower energy state, releasing light energy. This type of recombination is common in semiconductor lasers and light-emitting diodes, but is not significant for silicon solar cells.

(2) Auger recombination

Auger recombination is the reverse process of the impact ionization effect. The electron and hole recombination releases excess energy, which is absorbed by another electron. Subsequently, the electron that absorbed the excess energy relaxes back to the original energy state and releases phonons. Auger recombination is particularly significant in heavily doped materials. When the impurity concentration exceeds 1017cm-3, Auger recombination becomes the most important recombination process.

(3) Recombination through traps

Impurities and defects in semiconductors create allowed energy levels in the forbidden band. These defect levels induce a very efficient two-level recombination process. In this process, electrons relax from the conduction band energy level to the defect energy level, and then relax to the valence band, resulting in recombination with a hole.

(4) Surface composite

The surface can be said to be a place with serious defects in the crystal structure. There are many allowed energy states with energy in the forbidden band at the surface. Therefore, according to the mechanism described above, recombination is easy to occur at the surface.

In actual batteries, the combined effect of the above compound loss factors results in a spectrum response. The task of battery designers is to overcome these losses and improve battery performance. The characteristics of battery design reflect the characteristics of the battery, and the different design characteristics of the battery also distinguish the various commercial components on the market.