Control of Defect-Mediated Charge Recombination in Kesterite Absorbers through Oxygen-Sodium Interplay

Area of Science:

• Materials Science
• Solid State Physics
• Photovoltaics

Background:

• Kesterite Cu₂ZnSnS₄ (CZTS) solar cells exhibit limited performance due to open-circuit voltage losses.
• Defect-mediated nonradiative recombination is a primary cause of these voltage losses.

Purpose of the Study:

• To elucidate the atomistic mechanisms by which oxygen and sodium influence sulfur-vacancy-induced charge recombination in CZTS.
• To establish principles for rational defect passivation in kesterite photovoltaics.

Main Methods:

• Utilized *ab initio* nonadiabatic molecular dynamics simulations.
• Investigated the role of sulfur vacancies, atomic oxygen, molecular oxygen, and sodium on defect states and carrier lifetimes.

Main Results:

• Doubly positively charged sulfur vacancies create deep donor-like trap states via Sn-5s/S-3p hybridization, reducing carrier lifetime.
• Atomic oxygen passivates sulfur vacancies in oxygen-poor conditions, extending carrier lifetimes threefold.
• Interstitial oxygen, formed from molecular oxygen in oxygen-rich conditions, introduces mid-gap states and accelerates recombination.
• Sodium stabilizes oxygen configurations, suppressing recombination and restoring carrier lifetimes.

Conclusions:

• Oxygen and sodium play critical, coupled roles in regulating defect-mediated recombination in CZTS.
• Understanding these interactions enables rational design strategies for defect passivation in kesterite solar cells.