FORMATION OF CHARGE PUMP IN PHOTOELECTRIC CELL
The concept of creating “charge pumps” within the structure of Photo-electric cell was introduced by the Russian Academy of Sciences. The Charge Pumps are formed due to formation of spatial defect-dopant complexes, which stimulus a qualitative change in transport mechanism of light generated charges in the volume of the P-type solar cell. The initial results demonstrated that it is possible to increase short circuit current and maximum power output of a P-type c-Si photoelectric cell with the use of Local Photon Annealing (LPA) technique. Conceptually, this can be done by reducing the lifetime of charge carriers by creating n-regions in P-type c-Si to promote the separation of generated electron-hole pairs and reducing their recombination rate.
Experiments were carried out on p-type Cz-grown silicon which was chosen from different manufacturers. The concentration of boron was 1015 cm-3 (ρv =8-10Ω cm), oxygen (0.8-1.2)1018 cm-3, the surface orientation was (100). The conventional annealing was carried out on the whole surface after polishing wafers. The annealing method is accomplished through non-thermal or “cold” photonic annealing with the help of standard halogen lamps. The luminous radiation power was 45 W/cm2 and the temperature range 1000 o C. This method is known as Local Photon Annealing (LPA). A photo-mask is placed over the surface of wafer to control the temperature so that it shouldn’t exceed more than 45-55 o C. The photo-mask serves as a kind of thermal screen which prevents excessive heating of the wafer during LPA. During LPA photo-mask provides numerous spatially distant areas which fabricate tensile strain in wafer. As a result, LPA forms clusters of impurities, especially boron and oxygen in selected areas. The formation of defect containing areas in wafer shows experimentally the possibility of creating latent n-areas in the volume of wafers. The hidden n-areas were proposed as charge pumps in volume of p-type solar cells. These regions will lead to the construction of floating potential which restrain the recombination of electron-hole pairs and compels their movement towards contacts. The figure 1 shows the creation and work of charge pumps in photoelectric cell.
Due to effectiveness and simplicity of the process, it does not require large industrial asset and investments.