Development of bandgap graded a-SiOx:H solar cells for two-terminal and four-terminal applications

Abstract

In this thesis, graded bandgap energy a-SiO_x:H solar cells are developed to attain a high JSC without compromising their high V_OC × FF product. First, a grading method which takes into account the interdependence of bandgap energy, deposition rate and the desired grading profile for a-SiO_x:H materials is developed. Then, graded bandgap energy a-SiO_x:H solar cells are fabricated to investigate the optimum grading width. The bandgap energy is linearly graded from 2.1 eV to 1.96 eV. It was observed that, when fabricating graded bandgap energy a-SiO_x:H solar cells, the grading width at both the ends of the absorber layer should be as small as possible. A 10-nm thick grading at the front end and 20-nm thick grading in the rear of the absorber layer showed the best result among all the graded bandgap cells developed during the optimization, with an efficiency of 8.2 % (V_OC : 0.96 V, J_SC : 13.36 mA/cm2, FF: 0.64).

The effects of grading the bandgap energy on the performance of a-SiO_x:H solar cells were also investigated. For instance the bandgap variation within the graded region was investigated. We found that increasing the range over which the bandgap energy is varied within a fixed graded region was detrimental to the J_SC and FF. When increasing the thickness of the graded bandgap energy absorber layer for a-SiOx:H, J_SC as high as 14.4 mA/cm2 was achieved. Also, the increase in the thickness of these graded bandgap energy absorber layers did not result in a drastic drop in efficiency as seen for a-SiOx:H solar cells without bandgap energy grading.

Finally, optical simulations of graded bandgap energy a-SiO_x:H solar cells were attempted. Graded bandgap energy i-a-SiO_x:H layers were simulated. The matching of the simulated absorbance to the measured absorbance of the graded bandgap absorber layers were found to be satisfactory. However, the simulations of the graded bandgap energy a-SiO_x:H solar cells were not satisfactory and requires more attention.

Further research into improving the FF of graded bandgap energy solar cells can help increase the VOC × FF product and efficiency of such devices. Moreover, the increase in J_SC obtained by grading the bandgap energy makes the device a suitable candidate for a top cell in four-terminal (4T) applications.