Performance improvement of microcrystalline thin film silicon solar cells by back reflector with high resistivity and low absorption

In this paper, a series of microcrystalline silicon (μc-Si:H) solar cells were fabricated on different back reflectors by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). The results indicated that the performance of μc-Si:H solar cells strongly depended on their back reflector structures. First of all, the various Al:ZnO films with different optical and electrical properties were fabricated, and the effects on the performance of μc-Si:H solar cells as the back reflector materials were investigated. Unlike the previous studies for a-Si:H solar cells, all the μc-Si:H cells with various Al:ZnO back reflectors are showing similar I-V characteristics. However, it was interesting result that the back reflector with highest resistivity, fabricated by oxygen reactive sputtering, showed the best fill factor. As the next step, the n-μc-SiO layer with high resistivity was introduced as the new back reflector materials substituting for the conventional Al:ZnO. The optimal deposition condition for the n-μc-SiO layer was selected considering the low refractive index under 1.85, the reasonable electrical resistivity around 1E+3 Ω·cm and low absorption spectra near IR region. For the new back reflector structures, all the cell parameters were increased drastically at n-μc-SiO thicker than 300 nm, and a conversion efficiency of as high as 9.3 % (V_oc: 0.501 V, J_sc: 27.4 mA/cm², F.F: 0.68) was obtained. The performance gain for V_oc and F.F was more obvious in the thicker back reflectors, suggesting that the high-resistivity n-μc-SiO layer could reduce the shunt current at the back contacts of μc-Si:H cells.