  The p-type amorphous/ n-type crystalline silicon (P+ a-Si/N+ c-Si)
heterojunction was simulated for developing the solar cells with high
conversion efficiency and low cost. The characteristic of such cells with
different work function of transparent conductive oxide (TCO) were calculated.
The energy band structure, quantum efficiency and electric field are analyzed
in detail to understand the mechanism of the heterojunction cell. Our results
show that the a-Si/c-Si heterojunction is hypersensitive to the TCO work
function, and the TCO work function should be large enough in order to achieve
high conversion efficiency for P+ a-Si:H/N+ c-Si solar cell. With the optimized
parameters set, the P+ a-Si:H/N+ c-Si solar cell reaches a high efficiency
({\eta}) up to 21.849% (FF: 0.866, VOC: 0.861 V, JSC: 29.32 mA/cm2).
