Improvement of a-Si1−xGex:H single-junction thin-film solar cell performance by bandgap profiling techniques

In this work, the effect of hydrogen dilution on the Ge content of the film and the effect of bandgap grading in the a-Si1−xGex:H absorber near the p/i and the i/n interfaces on cell performance were discussed. The a-Si1−xGex:H single-junction solar cells were improved by employing both p/i grading and i/n grading. The i/n grading increased the VOC and the FF while it also reduced the JSC as compared to the cell without grading. Presumably the potential gradient established by the i/n grading facilitates the hole transport hereby improved by the FF. On the contrary, the potential barrier established by the p/i grading seemed to limit the cell performance and constrain the p/i grading width below 20 nm due to the drop in FF and JSC. Combining the effects of bandgap grading on the VOC, JSC and FF, the suitable thicknesses of the p/i and the i/n grading were 20 nm and 45 nm, respectively. Finally, the grading structures accompanied with further optimization in doped layers were integrated to achieve a cell efficiency of 8.59%.

► We improve the a-SiGe:H single-junction solar cell performance by bandgap grading. ► Effect of bandgap grading width on the solar cell performance was demonstrated. ► Effect of hydrogen dilution on film Ge content was also showed. ► The increase in hydrogen dilution increased the film Ge content significantly. ► Appropriate bandgap grading width improved VOC and FF while slightly reducing JSC.