Impact of Titanium Dioxide Surface Defects on the Interfacial Composition and Energetics of Evaporated Perovskite Active Layers

Abstract

This investigation elucidates critical Brønsted and Lewis acid–base interactions at the titanium dioxide (TiO2) surface that control the interfacial composition and, thus, the energetics of vacuum-processed methylammonium lead iodide (MAPbI3) perovskite active layers (PALs). In situ photoelectron spectroscopy analysis shows that interfacial growth, chemical composition, and energetics of co-deposited methylammonium iodide (MAI)/PbI2 thin films are significantly different on bare and (3-aminopropyl)­triethoxysilane (APTES)-functionalized TiO2 surfaces. Mass spectroscopy analysis indicates that MAI dissociates into hydrogen iodide and methylamine in the gas phase and suggests that MAPbI3 nucleation is preceded by adsorption and coupling of these volatile MAI precursors. Prior to MAPbI3 nucleation on the bare TiO2 surface, we suggest that high coverages of methylamine adsorbed to surface defect sites (e.g., undercoordinated Ti atoms and hydroxyls) promote island-like growth of large, PbI2-rich nuclei that inhibit nucleation and lead to a thick substoichiometric interface layer that impedes charge transport and collection energetics. APTES functional groups passivate TiO2 surface defects and facilitate more conformal growth of small, PbI2-rich nuclei that enhance MAPbI3 nucleation and significantly improve interfacial energetics for charge transport and extraction. This work highlights the considerable influence of TiO2 surface chemistry on PAL composition and energetics, which are critical factors that impact the performance and long stability of these materials in emerging photovoltaic device technologies.

From Groundwork to Efficient Solar Cells: On the Importance of the Substrate Material in Co‐Evaporated Perovskite Solar Cells

“…This goes beyond previous studies, where detailed investigations showed that the substrate material can influence the composition and electronic structure of the perovskite thin film close to the underlying charge transport layer. [65][66][67][68] However, the present study supplements these findings by highlighting that, beyond the crystallization during the initial stage of thin-film formation, the incorporation of MAI into the perovskite framework, and thereby the composition of the perovskite, continuous to be influenced throughout the entire thin film.…”

“…However, compositional differences in the very first atomic layers (absorber thicknesses below 5 nm) as investigated in detail by Olthof et al and others may still be present. [65][66][67][68] The density of initial nuclei is relatively large, with several nuclei being present with areas of a few 100 × 100 nm 2 in this stage. In particular, for SnO 2 substrates that exhibit slightly larger grain sizes compared to the other substrate materials in the early phase of the perovskite thin-film formation, a Volmer-Weberlike island growth mode for co-evaporated thin films can be concluded from the individual crystallites forming at this stage.…”

“…Preliminary studies on the interdependence of substrate material and electronic structure were performed for the co-evaporation of PbI 2 and MAI for a limited number of substrate materials. [65][66][67][68] In addition, thin-film formation for ultrathin perovskite layers as well as for thin films based on PbCl 2 were studied. [69,70] Of particular note, metal oxide (e.g., titanium dioxide) substrate materials demonstrated only mediocre device performance but remarkable improvements were reported by adding a thin layer of an organic material between the metal oxide and the perovskite absorber.…”

Investigation of the Selectivity of Carrier Transport Layers in Wide‐Bandgap Perovskite Solar Cells

“…Indeed, as described by Boyd et al, this reaction, which is not specific to NiO x , can happen with other metal oxides and has been reported with TiO 2 and SnO 2 . [ 32–34 ]…”

Anchorable Perylene Diimides as Chemically Inert Electron Transport Layer for Efficient and Stable Perovskite Solar Cells with High Reproducibility

“…[ 8,9 ] Moreover, the unique surface chemistry of oxide semiconductors has recently been demonstrated to induce interfacial perovskite decomposition via chemical reactions (like redox reactions and acid–base reactions), thus unfavorable to interfacial charge extraction as well as long‐term stability. [ 10–14 ]…”