Chemical insights into electrophilic fluorination of SnO<inf>2</inf>for photoelectrochemical applications

Recently, there has been substantial interest in the fluorination of nanomaterials-based thin films used in various optoelectronic devices for optimum charge transport across semiconducting layers. The discovery of electrophilic fluorinating agents such as Selectfluor® (F-TEDA) has led to the development of novel methods for fluorination of metal oxides such as tin oxide (SnO2) in this work. Herein, we elucidate the fluorination of SnO2thin films using X-ray photoelectron spectroscopy (XPS) depth profiling. The interaction of the F-TEDA molecule with the SnO2surface occursviaN-F bonds. Fluorine is found to occupy interstices and substitutional sites in the SnO2lattice. The interstitial fluorine (1.21 at%) decays off by a depth of 61 nm in the SnO2film. The substitutional fluorine (1.28 at%) in SnO2results in remarkable changes in its electronic structure due to the lowering of oxygen defects by ∼80%. The electrical properties of the F-SnO2film is examined by impedance spectroscopy analysis. F-SnO2exhibits an increase in electrical conductivity by ∼1-2 orders of magnitude and an increase in electron density by ∼65%, making it suitable as a charge transport layer in photoelectrochemical cells (PECs). The PEC in aqueous medium at neutral pH with F-SnO2as the charge transport layer shows ∼81% increase in the photocurrent density (at 1.6 VversusRHE) and decrease in charge transfer resistance by ∼36%. Thus, the efficient transport of photogenerated charge carriers is observed in PECs with minimal recombination losses for the fluorinated SnO2films. This study helps in understanding the role of defect passivationviasingle-step fluorination of metal-oxide for charge transport layers which can be extended to perovskite solar cells in the future.