FIGURE 1 The thermally evaporated SrF2 film was characterized using XPS and UPS measurements, as shown in the following statistics: (A) F 1s core level spectrum. (B) Sr 3d core spectrum. (C) Extracted WF of SrF2 from the secondary electron cut-off spectrum. (D) Valence band spectrum of the SrF2 film.
2.2. Contact Resistivity and Stability.
The contact resistivity (ρc) of the c-Si(n)/SrF2/Al stack is a key factor in evaluating its electron selectivity ability. It was evaluated by the transfer length method (TLM) in Figure 2, with a schematic of the test structure inset.35 The Schottky barrier height ( > 0.7 eV) is caused by the potent Fermi-level pinning effects when an aluminum electrode is in straight contact with the lightly doped n-Si and behaves in a rectifying fashion. By contrast, Figure 2A exhibits that the interposition of a nanoscale SrF2 thin film between the c-Si(n) substrate and aluminum electrode considerably enhances the contact presence and permits an Ohmic contact. The value of ρc is calculated as 2 mΩ∙cm2 from the 4 nm SrF2/Al stack, which is lower than the great majority of other dopant-free structures in c-Si solar cells. DF-ECSCs works in kinds of literature report that the minimum ρc can be achieved by EuFx (15.6 mΩ·cm2), YF3 (17.8 mΩ·cm2), MgOx (17.5 mΩ·cm2), CeF3 (10.96 mΩ·cm2), MgAcac (7.6 mΩ·cm2), MgFx (35 mΩ·cm2), TiO2(20 mΩ·cm2), TaNx (42 mΩ·cm2) prepared by thermal evaporation or by atomic layer deposition (ALD).11,18-20,22,26,36,37 And the value of ρc resembles the most typical material LiFx (1 mΩ·cm2).7 However, lithium fluoride is toxic and harmful to human health and the environment, and Li resources and its compounds are in short supply on Earth, which is not conducive to large-scale applications. In contrast, SrF2 is commonly used as an additive for toothpaste in humans’ daily lives, which is non-toxic to physical health and the environment. Hence, it has more prospects for large-scale application. The low ρc formed by n-Si/SrF2/Al may be attributed to the lower WF of SrF2. The energy band of c-Si(n) is forced to bend downwards further as Fermi depinning, which makes it easier for an electron aggregation area to form, thereby realizing the selective transmission of electrons.22,38
It can be seen in Figure 2B that ρc remains in Ohmic contact (< 42 mΩ·cm2) as the thickness of SrF2 variates from 1.5, 4, 6, to 9 nm, indicating its high tolerance for thickness, thus providing a wide processing window. By contrast, the most researched electron-selective structures, n-Si/LiFx/Al and n-Si/MgOx/Al have a narrow manufacturing window (1.5 ± 0.5 nm). The contact resistance stability of these c-Si/SrF2/Al stack samples under air storage was investigated. Despite being exposed for 168 hours, the ρc is still less than 45 mΩ·cm2 for all thicknesses investigated. Moreover, the ρc for SrF2 within 9 nm remains below 140 mΩ·cm2 over one and a half years. The results indicate that SrF2 has considerable potential as a material for DF-ECSCs in c-Si solar cells, making it a promising candidate for large-scale manufacturing.