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James Stanfill1 Kara Saunders1 R. Shallcross1 Neal Armstrong1

1, University of Arizona, Tucson, Arizona, United States

Herein we demonstrate that the surface composition of TiO2 electron-selective interlayers strongly influence the electrical properties of the interlayer and the nucleation/growth of MA-doped FAPbI3 perovskite active layers for use in PVs. Simultaneous control of active layer nucleation/growth and interfacial electrical properties with the underlying contact is predicted to significantly influence the energy conversion efficiencies and stabilities of perovskite PVs. Variable incident-angle Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) was used to monitor depth-dependent changes in the perovskite film structure on TiO2 surfaces that were compositionally modified with O2-plasma and/or UV-light pretreatment as measured by X-ray photoemission. Conducting tip AFM (cAFM) was used to map how these oxide pretreatments also affect the electrical properties of the oxide and the combined TiO2/perovskite sample. TiO2 thin films deposited by chemical vapor deposition are extremely reactive toward ambient small molecules and to perovskite precursor ions as a result of 5-6 different proposed surface defects that can act as adsorption/redox sites. Controlling these native or reacted surface defects by O2-plasma or UV-light pretreatment impacts TiO2 charge harvesting ability and interactions with a nucleating film. For the first time, angle-resolved GIWAXS shows that such oxide pretreatments influence the structure of the perovskite thin films first at the TiO2/perovskite interface which then propagates into the remaining bulk film. By advancing the incident X-ray angle from slightly below to slightly above the critical angle of the perovskite film, thus changing the X-ray probe depth between tens and hundreds of nanometers, GIWAXS reveals a preference for <101> parallel perovskite crystal orientation closer to the buried TiO2/perovskite interface, versus the top of the film, in all sample types. Furthermore, O2-plasma treated oxide films, followed by UV-light exposure, show the highest perovskite film preferred orientation and crystallinity at this buried interface amongst all oxide pretreatments. cAFM also demonstrates that UV-O2-samples contain the most electrically active TiO2 interlayer when surveyed across the surface, which translates, along with the highest perovskite film orientation/crystallinity, to the most conductive TiO2/perovskite heterojunction for optimal electron harvesting. We believe these results are explained by UV-O2 treatment removing chemi- and physi-sorbed carbon-based species at or proximal to Ti3+ defects which can then act as nucleation sites for perovskite precursor ions and as electron injection points. The fact that interface chemistry at nanometer length scales at the TiO2/perovskite heterojunction controls perovskite structure tens to hundreds of nanometers away from that interface, as well as overall sample electrical activity, highlights the the the importance of this interface in the optimization of perovskite PVs.

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