The basic idea of balancing the carrier drift velocity in the absorption layer was proposed to improve the conversion efficiency of Si-based thin film solar cells. Using the graded microcrystalline i-SiGe absorption layer to modulate the energy band, the driven electric field of holes was increased from 5.92 kV/cm to 7.26 kV/cm, while the driven electric field of electrons was kept at 5.92 kV/cm. Compared with the step i-SiGe absorption layer, the drift velocity ratio of electrons and holes was more balanced. The improvement mechanism of the p-Si/graded-i-SiGe/n-Si solar cells was further analyzed using the measurement of the biased quantum efficiency. Consequently, the short-circuit current density and the associated conversion efficiency of the p-Si/graded-i-SiGe/n-Si solar cells were improved from 21.40 +/- 0.47 mA/cm(2) to 26.36 +/- 0.56 mA/cm(2) and from 7.43 +/- 0.23% to 9.15 +/- 0.25%, respectively compared with the p-Si/step-i-SiGe/n-Si solar cells. (C) 2015 Elsevier Ltd. All rights reserved.