Investigation of Patterned Plasma Etching Processes for HJT-IBC Solar Cells: Keys to Maintaining a High Electronic Quality Surface

We examine a cell fabrication process involving a novel patterned plasma etching step to define the interdigitated back-contact (IBC) structure for heterojunction (HJT) crystalline silicon (c-Si) solar cells. In this process, specific plasma surface treatments are necessary to achieve good device performance. X-ray Photoelectron Spectroscopy (XPS) and High-Resolution Transmission Electron Microscopy (HRTEM) are used to investigate the underlying reasons for the effectiveness of these treatments. Two experimental conditions are explored: (1) etching the hydrogenated nanocrystalline silicon (nc-Si:H) and amorphous silicon (a-Si:H) layers down to c-Si surface before depositing the final doped layer, and (2) leaving a >5 nm intrinsic a-Si:H (i-a-Si:H) after etching. In the first case, a gentle NF₃ etching step suffices to recover diode-like behavior without S-shape, but results in cells with very low open-circuit voltage (V_OC). In the second case, an additional H₂ plasma cleaning step is required to recover both diode-like behavior without S-shape and good minority carrier lifetime (and high V_OC). XPS analysis reveals that both NF₃ etching and H₂ plasma can remove N, F, and O from the interface seen by the patterning plasma, although with different effectiveness for c-Si and a-Si:H surfaces. Critically, avoiding an air break between NF₃ etching and H₂ plasma reduces oxidation at the interface between i-a-Si:H and the final doped layer to background levels, thereby achieving the best device performance. HRTEM provides supporting insights that explain the necessity of the etching steps and the importance of stopping the etching at the i-a-Si:H layer before reaching the c-Si surface.