Effect of oxide based graded buffer and bottom n-layer on the performance of the single junction amorphous silicon solar cells

Abstract

We have developed oxide based window layer, graded buffer layer and n-layer by radio frequency plasma enhanced chemical vapour deposition (rf-PECVD) method for use in the fabrication of single junction amorphous silicon (a-Si) small area (1 cm2) solar cell. From our earlier study we had observed that higher stabilized efficiency can be obtained using oxide based window and buffer layer. In this study, we have replaced conventional buffer layer by graded buffer (graded with four different band gaps) and conventional n-a-Si:H by wide band gap n-a-SiO:H to improve p/i interface and current density further of a single junction solar cell. The use of these two materials in a single junction a-Si structure enhances the open circuit voltage (Voc) and short circuit current of the cell and the conversion efficiency as well. Graded buffer layer can minimize the p/i interface defects to a large extent causing an improvement of Voc. The bandgap of n-a-SiO:H film is much higher and optical absorption is less than that of n-a-Si:H film having the dark conductivity almost same. Due to low absorption and wide optical gap of n-oxide layer allows more fraction of light enter into the absorber layer after reflection from back reflector resulting in increase in short circuit current. 9.53% of initial efficiency is achieved by the use of graded buffer layer and wide band gap n-layer in single junction small area (1 cm2) solar cell.