SrFx-based electron-selective contact with high tolerance to thickness for crystalline silicon solar cells enabling efficiency over 21%

Wide-bandgap metal compound-based dopant-free passivating contacts have been explored to fabricate crystalline silicon (Si) solar cells to mitigate the high carrier recombination rate of metal-Si contact directly. Here, an over 4-nm-thick single-layer strontium fluoride (SrF_x) and a double-layer SrF_x/lithium fluoride (LiF) films deposited by a facile vacuum thermal evaporation are developed to act as high-performance electron-selective contacts. SrF_x with ultra-low work function (2.8 eV) induces a strong downward band bending at the n-type Si (n-Si)/SrF_x interface, and a dipole active layer exists at the SrF_x/aluminum (Al) interface, enabling a low contact resistivity (ρ_c) of 34.1 mΩ cm² and thus yielding an impressive fill factor (FF) of 82.8%. Eventually, a power conversion efficiency (PCE) of 20.1% is achieved in the SrF_x-based solar cell. Moreover, in the n-Si/SrF_x/LiF/Al contact, the diffusion of Li in the SrF_x film favors facilitating electron transport as well as relaxing its thickness restriction, inhibiting carrier recombination. And an impressive FF of 83.7% with a low ρ_c of 25.9 mΩ cm², an improved open-circuit voltage of 631 mV, and a short-circuit current density of 39.9 mA/cm² are attained, resulting in a champion PCE of 21.1%. Double-layer SrF_x/LiF deposited by a simple process provides a grand opportunity to fabricate low-cost and high-PCE photovoltaic devices.