Interface Chemistry and the Role of an LiF Interlayer in TiO _x /Al Electron-Selective Passivating Contacts for Crystalline Si Solar Cells: Insights from Hard X-ray Photoelectron Spectroscopy

Titanium oxide (TiO x ) has emerged as a promising dopant-free electron-selective contact material for crystalline silicon (Si) solar cells owing to its excellent surface passivation capability. However, direct metallization with low work function metals such as aluminum (Al) severely degrades this passivation, underscoring the critical importance of interfacial control for practical applications. In this study, hard X-ray photoelectron spectroscopy (HAXPES) is employed to directly probe the buried n -Si/TiO x /Al interfaces, enabling chemical and electronic analysis even beneath metallic overlayers─an ability that is crucial for realistic device architectures. The measurements reveal that Al deposition induces a pronounced chemical reduction of TiO x, which is the primary cause of passivation loss. Remarkably, introducing an ∼2-nm-thick LiF interlayer between TiO x and Al not only suppresses this interfacial reduction but also tunes the band alignment by lowering the electron energy barrier at the n -Si/TiO x interface by ∼0.1 eV, thereby promoting Ohmic carrier transport while preserving passivation. As a result, open-circuit voltages of 661 and 683 mV are demonstrated in full-area and locally metallized device designs, respectively. These insights highlight how interfacial engineering governs both chemical stability and carrier selectivity, offering a versatile design strategy that extends to a broad spectrum of Si-based devices.