Staebler Wronski effect in Amorphous Silicon.

In the early stage of technology, amorphous silicon was found to be an interesting material because with doping it can be made P-type or N-type and provided an opportunity for the formation of junction and leading to the formation of solar cells. But after introducing sunlight radiation on solar cell, efficiency decreases and this effect is called Staebler Wronski effect.

                    Large number of defects around 10²³ / cm³ is present in amorphous silicon due to large number of dangling bonds. There are several missing bonds and these missing bonds are called dangling bonds. Dangling bond reduces the performance because they act as traps and they absorb the carriers. This problem was solved by making hydrogenated amorphous silicon (a-Si: H). Hydrogen passivation during the manufacturing process creates hydrogen bond (with dangling bond) and reduces the defect densities ( to around 10¹⁶ / cm³ and improves the device performance. With this improvement, a lot of interest began in building in amorphous silicon and the manufacturing process started.

                     For solar applications hydrogenated amorphous silicon (a-Si: H) showed high absorption coefficient in the visible range of solar spectrum (AM 1.5G). As the absorption coefficient is inversely proportional to the absorption length, only a few micron layer thickness of hydrogenated amorphous silicon is required to absorb 90% of the visible spectrum. In 1976 first hydrogenated amorphous Silicon (a-Si: H) solar cell was developed by “Carlson and Wronski” with an efficiency of 2.4 %.

Figure 1 Energy Band Structure

                      
                    Figure 1 shows energy band of crystalline silicon and hydrogenated amorphous silicon (a-Si: H). It is observed that in case of crystalline silicon there is tetrahedral configuration with SP³ hybridization which indicates “no trap states”.  But in case of hydrogenated amorphous silicon (a-Si: H) instead of tetrahedral configuration, bonds with different length and angles are present which are less stable and are called as “weak bonds”.  When energy is given to material in the form of solar radiation, these week bonds break and causes degradation in inefficiency.  This effect is called Staebler Wronski effect. It is fortunate enough that efficiency does not drop continuously after light exposure.  After some time, it gets saturated to a value of 80 to 90% of the initial value.