We proposed an in situ postdeposition ultraviolet treatment in an Ar ambient (UTA) to improve the p/i interface of amorphous silicon based solar cell. We have increased the conversion efficiency by similar to16% by improving the built-in potential and reducing recombination at the p/i interface. Through spectroscopic ellipsometry and Fourier-transform infrared measurements, it is concluded that the UTA process induces structural modification of the p-type hydrogenated amorphous silicon-carbide (p-a-SiC:H) window layer. An ultrathin p-a-SiC:H contamination layer formed during the UTA process acts as a buffer layer at the interface. (C) 2004 American Institute of Physics.