Oxygen-defect characterization for improving R&D relevance and Cz-Si solar cell efficiency

摘要

Most high-efficiency solar cells are fabricated from monocrystalline Czochralski (Cz) silicon (Si) wafers because of the high quality of the material. Despite the considerable heritage from microelectronics, the Cz-Si substrate quality can still limit cell performance. A key issue regarding wafer quality relates to the presence of oxygen in the bulk. Cz wafers indeed feature high oxygen concentrations owing to the silica crucible dissolution during crystal growth. Although not harmful as such, oxygen is a very reactive impurity that can form a plurality of recombination-active defects before, during, or after cell processing. It is now well known that the formation of these defects can lead to reductions in PV conversion efficiency of several per cent absolute. Furthermore, the formation/annihilation of these defects can interfere with cell process optimizations; it can also impact R&D activity or the integration/ramping up of new process tools in a production line. As a consequence, it is of paramount importance to both wafer and cell manufacturers that the Cz substrate quality be controlled. This paper introduces a characterization technique called OxyMap, which aims to provide a detailed analysis of oxygen and related defects in Cz-Si wafers. The capabilities of the OxyMap technique are presented and illustrated with the help of statistical data obtained for wafers and cells from the main PV players in the industry. Large variations in material quality are revealed among the tested wafers, highlighting the need for quality control in order to optimize the ingot yield/quality and to increase the cell efficiency/reliability.