Surface potential imaging of PV cells with a Kelvin probe

This paper describes a Kelvin probe based inspection technique which measures surface potentials (SP) at various illumination conditions and derives the delta surface potential (ΔVSP) and surface band bending (ϕ) of photovoltaic (PV) cells. The ΔVSP is the difference of surface potential measured in light and dark conditions respectively. The delta surface potential (ΔVSP) is related to charge injection and carrier lifetime, while the surface band bending (ϕ) is associated with the surface condition. A white light and short wavelength light illuminations are used to distinguish the bulk effects from that of the surface.A scanning vibrating Kelvin probe system was built to image PV cells with and without illumination. Experiments were performed on both single crystalline silicon (sc-Si) and multi-crystalline silicon (mc-Si) cells. It is found that the sc-Si cells possess a higher ΔVSP but a lower ϕ than that of mc-Si cells. The average ΔVSP and ϕ are 350 mV and 50 mV for the sc-Si cells, and 280 mV and 110 mV for the mc-Si cells. Process defects on a surface could affect both parameters, and it is found that the ΔVSP is not uniform and could be reduced to 170 mV at a defect on the mc-Si cells. The high ΔVSP and low ϕs are expected to contribute to the high conversion efficiency on a cell.

► A Kelvin probe system with various illuminations was built for PV inspection.
► Delta surface potential and surface band bending were derived.
► Macroscopic defects and process variation were mapped on full-sized cells.
► A great variability was observed on both metal firing and cell processing.
► High delta surface potential and low band bending contribute to high cell efficiency.