Electronegativity-driven fluorotyrosine modulation of SnO_X ETLs via deposition control and oxygen-vacancy suppression for carbon-based perovskite solar cells exceeding 20%

In hole-transport-layer-free carbon-based perovskite solar cells (CPSCs), inefficient hole extraction at carbon electrodes and insufficient hole blocking by electron transport layers (ETLs) induce severe carrier recombination at the buried perovskite interface, thus limiting device performance. N-type SnO_X films prepared via the chemical bath deposition (CBD) method emerge as promising ETLs for CPSCs. Nevertheless, conventional CBD is dominated by rapid and uncontrolled Sn⁴⁺ hydrolysis, which induces premature nucleation and severe particle aggregation, resulting in non-uniform film growth. Meanwhile, the rapid precipitation of Sn species limits their complete oxidation, inherently inducing a high density of surface oxygen vacancies (V_O). Herein, we introduce a multifunctional chelating molecule, 3-fluorotyrosine (F-Tyr), to modulate the in-situ growth of SnO_X during CBD. The high electronegativity of fluorine and multidentate coordination synergistically regulate the hydrolysis of Sn⁴⁺, enabling controlled nucleation and suppressed particle aggregation. Furthermore, F atoms preferentially occupy V_O sites, substantially decreasing the surface defect density and thereby mitigating interfacial non-radiative recombination. Consequently, the F-Tyr-modified SnO_X ETLs enable HTL-free CPSCs to achieve a champion PCE of 20.15% with an enhanced V _OC of 1.12 V. Unencapsulated devices exhibit enhanced long-term stability, retaining 87.9% of initial efficiency after 180 days at 10 ± 5% RH.