High efficiency a-Si:H/a-SiGe:H tandem solar cells fabricated with the combination of V- and U-shaped band gap profiling techniques

Hydrogenated amorphous silicon germanium (a-SiGe:H) films prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a mixture of SiH4, H-2, and GeH4 were investigated for their use as the bottom cell of amorphous silicon/amorphous silicon germanium (a-Si:H/a-SiGe:H) tandem solar cell structures. Narrow optical band gaps (E-opt) in the range of 1.5 to 1.6 eV were obtained by varying the GeH4/(SiH4 + GeH4) gas flow rate ratio in low-temperature deposition. The a-SiGe:H films deposited with various GeH4/(SiH4 + GeH4) gas flow rate ratios were used as intrinsic layers for the a-Si:H/a-SiGe:H tandem solar cells with different graded band gaps:V-, VU-, and U-shapes. It was found that using the VU-shape improves the solar cell efficiency owing to a higher J(sc) when compared with using V-shape. The VU-shape's V-oc and FF are also improved when compared with the U-shape's V-oc and FF. As a result, a high efficiency of 11.0% (V-oc = 1.74 V, J(sc) = 9.07 mA/cm(2), and FF = 0.70) was successfully achieved with the solar cells fabricated using the VU-shape graded band gap technique. (C) 2015 The Japan Society of Applied Physics