Insight into interfaces and junction of polycrystalline silicon solar cells by kelvin probe force microscopy

Kelvin probe force microscopy (KPFM) is a powerful tool to measure surface potential with resolutions in the order of atomic/nanometer scales, and could also provide direct measurements of the surface potential on interfaces and junctions of solar cell devices. In this paper, the whole surface potential distribution along the cross-section of the polycrystalline silicon solar cell was illustrated by KPFM for the first time. Interestingly, the surface potential presents a two-stepwise downward profile from Al electrode to Ag electrode, and surface potential skip-steps occur at Al/p-Si interface and p-n junction, respectively. Notably, the p(+) layer due to the Al doping was firstly identified by KPFM. Devices of three different efficiencies are tested and showed that the skip-step value at Ag/Si interface is linearly correlated with the device efficiency. So the surface potential skip-step value at Ag/Si interface is proposed to be an important parameter to evaluate the quality of Ag/Si interface. By combination of SEM, TEM and KPFM characterizations with performance measurement of the solar cells, we get deep insight relationships of compositions and morphologies around metal/semiconductor interfaces and junction in the atomic and nanometer scales, and find correlations between these structures and electrical/photoelectrical properties of devices. These studies are helpful to understand the device physical properties and provide potential routes to improve device efficiency.