Open-circuit voltage loss and instability from surface Sn(II) oxidation and high-density Sn vacancies pose great hurdles for developing highperformance Sn-based perovskite solar cells (PSCs). Turning attention from the bulk microstructure to surface reconstruction is promising to push the performance enhancement of Sn-based PSCs. Herein, a surface-modulation strategy based on 6-maleimidohexanehydrazide trifluoroacetate is rationally designed to reconstruct the surface structure of FASnI 3 films to manage the Fermi level and passivate defects. The electronic state evolution results in an n-type Fermi level shift of the shallow surface, thereby forming an extra back-surface field for electron extraction. Meanwhile, the ion-pairing agent affords passivating cationic and anionic defects, thereby nullifying the charged-defect-rich surface. In particular, the reductive hydrazide group and carboxyl groups alleviate superficial Sn(IV) and inhibit Sn(IV) formation, homogenizing surface potential and prolonging carrier lifetime. Accordingly, devices deliver a champion power conversion efficiency (PCE) of 13.64% and an elongated lifespan, with over 75% of the original PCE after 1000 h of illumination (O 2 < 50 ppm). This work presents a new insight on the surface reconstruction strategy for developing high-performance Sn-based PSCs.
Although binary Sn–Pb perovskites possess optimal band gap approaching to the Shockley–Queisser limit efficiency, the enhancement on power conversion efficiency (PCE) of Sn–Pb perovskite solar cells (PSCs) is impeded by the detrimental oxidation of Sn2+. Herein, a novel and effective strategy is developed to introduce pseudohalide anion thiocyanate (SCN–) with similar ionic radius to iodide to occupy the X-site of the perovskite lattice, thus restraining the rapid oxidation of Sn2+ to Sn4+. The incorporation of SCN– into perovskite stabilizes the perovskite crystal structure thermodynamically and increases the adsorption-energy-barrier of oxygen molecules. The coordination between Sn2+ and SCN– can reduce the defect density by healing the undercoordinated Sn2+ and suppressing the Sn and I vacancies. With the incorporation of SCN–, the ion migration behavior and lattice strain associated with the defects are remarkably relaxed. The study on carrier dynamics based on steady-state and time-resolved photoluminescence suggests that the carrier lifetime and non-radiative recombination rate of SCN– PSCs can be remarkably prolonged and depressed, respectively. As a result, FASn0.5Pb0.5I3-based PSCs achieve a 14.5% increase in PCE, reaching 13.74% under AM 1.5G illumination. This strategy takes a noteworthy step toward high efficiency and high stability FA-based Sn–Pb PSCs.
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