Exploration of Phosphorus Oxide in Heavily Phosphorus-Doped Polysilicon Films of Tunneling Oxide Passivation Contact Solar Cells

The investigation aims to reveal and confirm the phosphorus (P)–oxygen (O) bonds formed in the poly-Si (n+) film of the TOPCon device, which is beneficial for reducing the resistivity and the potential barriers of grain boundaries (GBs) due to further passivation of GBs within the film. To overcome the difficulty of gaining the chemical components in the interface and bulk of the n-Si/SiOx/poly-Si (n+) materials, we undertake X-ray photoelectron spectroscopy (XPS) with the depth profile by means of argon ion milling, in which the collision damage of ion with target surface has been neglected because of an effective detectable depth and longer mean escape depth of the optoelectron in the ionized state in the sampling point of the subsurface. High-resolution transmission electron microscopy (HR-TEM) has been employed to check the atomic lattice morphology of the multi-layer stacks for further distinguishing the crystallographic plane orientation of crystallites and ultrathin SiOx layer feature distribution in the interfacial region and bulk of SiOx/poly-Si (n+) films. The results of XPS-P 2p fitting peaks at 133.4 and 136.2 eV and −O 1s satellite peak at 535.3 eV, respectively, manifesting that the P–O bonds or derivatives (POx) may be present in the poly-Si (n+) film and localized in the GBs, through the heavily P dopants that induce tensile stress within crystallites and compress stress in GBs. The conclusion has been supported by the variation of thermodynamic functions, such as molar enthalpy, reaction entropy, Gibbs energy, and formation energy of silicon and phosphorus oxides, at the temperatures of 298 and 1173 K in terms of the minimum principle of energy.