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.