Disentangling Carrier‐Transport and Interfacial Carrier‐Recombination by Mitigating Na Interstitials for 11.9% Efficient Cd‐Free Cu₂ZnSnS₄ Solar Cells

High-performance photovoltaic (PV) devices require optimal carrier-transport properties and minimized carrier recombination. However, these two factors are found entangled in Cu₂ZnSnS₄ (CZTS) solar cells due to the unintentionally introduced Na interstitial defects. The Na interstitials behave as shallow donors which can charge-passivate the acceptor-like interfacial defects but reduce the hole density and electron mobility-suppressing interfacial recombination whilst hampering carrier transport in the bulk of CZTS. Herein, we demonstrate that Na interstitials can be reduced by a device annealing process at ≈220 °C for 5 min in open air based on the strong interaction between Na and O, thus successfully disentangling the carrier-transport and junction interface carrier-recombination, leading to increased hole density from 1.3 × 10¹⁵ cm^-3 to 9.8 × 10¹⁵ cm^-3 and electron diffusion length from 0.25 to 0.35 µm. This strategy not only yields a champion 11.9% efficiency of Cd-free CZTS solar cells but also advances the understanding of carrier transport in kesterite and other emerging PV materials.