FIGURE 3  The interface images between c-Si(n)/SiO_x/SrF₂/Al were observed using STEM-HAADF  (A) A high-resolution STEM HAADF microscopy picture of the n-Si/SrF₂/Al/ stack. (B) EDX mapping was performed with a 10 nm resolution for the signals of Al, Sr, F, and Si. (C) An EDX line scan was performed across the interfaces.

To demonstrate the effect of the SrF₂/Al electron-selective on silicon solar cells’ performance, The authors successfully produced a series of proof-of-concept c-Si solar cells on the n-type substrates with the partial area rear SrF₂/Al contact architecture. Figure 4A illustrates a schematic diagram of an n-Si solar cell.  To accomplish passivation and improve optical performance, the emitter of the fabricated solar cells was coated with an Al₂O₃/SiN_x stack fabricated by PECVD. In Figure 4B, V_oc increases from 622 mV with Al contact directly to 645 after inserting a 2 nm SrF₂ layer. Then, the V_oc reaches its peak at 654 mV for the 4 nm SrF₂ layer and remains at a slight drop but still above 650 mV. The observed improvement in the performance of the solar cell can be explained by the decrease in recombination, which is due to the Fermi-level depinning effect occurring at the Al/n-Si interface. ³⁸ The V_oc of the solar cell with illumination intensity has also been characterized to further demonstrate the removing Schottky barrier phenomenon (Figure S1). The fill factor (FF) and short-circuit current (J_sc)values depict a similar trend, as shown in figures 4C and D. The values fall as increasing the SrF₂ thickness beyond 4 nm, which is identified with the trend with contact resistance. Figure 4E depicts the PCE of the fabricated devices with various SrF₂ layer thicknesses.

Figure 4F compares the J-V characteristics obtained under 1 sun illumination of both cells with and without the optimal inserting layer SrF₂ (4 nm). The PCE of the fabricated solar cell was increased from 18.06% to 21.56%, indicating that it has the potential to be a new potential representative DF-ECSC for dopant-free silicon solar cells. This device has a V_oc of 654 mV, J_sc of 41.91 mA·cm^-2, and FF of 78.68%. Due to the low contact resistance, the FF increased by 7.88% in comparison to the reference cell with Al contact directly. Both V_oc and J_sc saw a small performance improvement (from 622 mV, 40.98 mA·cm^-2 to 654 mV, 41.91 mA·cm^-2).