AMPS (Analysis of microelectronic and photonic structures) mode,which was developed by Pennsylvania State University,has been used to module the light J_V c haracteristics of a_Si solar cells with a structure of TCO/p_a_SiC:H/i_a_Si:H/n_ a_Si:H/metal.The effects of valence band offset and contact barriers at p/i and TOC/p,n/metal interfaces on the light J_V characteristics have been examined .The modeling has qualitatively categorized and explained the non_ideal J_V behaviors (rollover,crossover,Voc shift,and rollunder) observed in a_Si based so lar cells.