Effect of Selenization Profile on Solar Cell Efficiency

The temperature and soaking times play two very important roles in determining the final efficiency of the CIGS solar cell. Many researches have revealed significant evidences for how the different sintering temperatures and soaking times have a direct impact on the efficiency outcome. The results of the experiment by Kadam et al demonstrate that a sintering temperature above 500 oC and a soaking period between 30 and 60 minutes can improve the efficiency of the cell significantly.In his experiments, when the samples were selenized at 400 °C for 10 minutes with a temperature ramp of 6°C per minute, it resulted in smaller, non-faceted grains. This is an indication that either the temperature or the soaking time is inadequate. When the soaking time was increased from 10 to 15 minutes, no significant improvements were noted in the results. Therefore, the ramp rates can be construed as having a larger impact on efficiency. In addition, Kadam et al studied the impact of various heating rates. Changes in the parameters were made: heating rate was increased from 6 °C per minute to 20 °C per minute and the temperature was increased to 425 °C with a soak period of 30 minutes. The resulting solar cell film appeared to be more homogeneous. Furthermore, the temperature was increased to see the effects on the grain size and cell efficiency, which indicated that at 475 °C, the grain size ranged from 0.5 um to 1.5 um with an efficiency of 5.56%. At the same time, they detected that increasing the peak temperatures above 500 °C could also help with improving efficiency. However, at higher temperatures the grains become well faceted and less thick. It is fair to conclude that high quality CIGS solar cells should be prepared at temperatures above 500 °C with an ideal soaking time between 30 and 60 minutes. Understanding that the heat rate, temperature, and soaking times are important factors in determining the ultimate cell efficiency, a belt furnace with a wide range of firing temperatures and a fast heat rate design are desired for constructing highly efficient solar cells.