Access to finely tuned thin films that can act as electron transport layer (ETL) and adapt to the absorber composition and whole cell fabrication process is key to achieve efficient perovskite-based solar cells. In this study, the growth of mixed niobium-titanium oxide (Nb-TiO2) thin films by atomic layer deposition and its use to extract photogenerated electrons is reported. Films were obtained at 200 °C from titanium (IV) i-propoxide, (t-butylimido)tris(diethylamido)niobium(V), and water by introducing Nb2O5 growth cycle in a TiO2 matrix. Process parameters (order of precursor introduction, cycle ratio) were optimized; the growth mechanism and the effective Nb incorporation were investigated by an in situ quartz crystal microbalance and x-ray photoelectron spectroscopy. The composition, morphology, structural, and optoelectronic properties of the as-deposited films were determined using a variety of characterization techniques. As a result, a fine control of the film properties (between TiO2 and Nb2O5 ones) could be achieved by tuning Nb content. To allow a successful implementation in solar devices, a comprehensive annealing study under several conditions (temperatures, various atmospheres) was conducted leading to an evolution of the optical properties due to a morphological change. Ultimately, the incorporation of these 15 nm-thick films in mesoscopic perovskite solar cells as ETL shows an improvement of the cell performances and of their stability with increasing Nb content, in comparison of both TiO2 and Nb2O5 pure compounds, reaching power conversion efficiency up to 18.3% and a stability above 80% of its nominal value after 138 h under illumination.

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See supplementary material online for additional data on substrate preparation, in situ QCM measurements (experimental details, mass variations during Nb-TiO2 film growth (precursor sequence A, various n1 values), characterizations of as-deposited (GPC measured by XRR and SE, surface homogeneity assessment by XPS, XPS depth profiling, angle-resolved XPS measurements, transmission-reflection measured by spectrophotometry, light absorption spectra, SE data and evolution of the optical bandgap, cyclovoltammetry), and annealed (GIXRD diffractograms, cyclovoltammetry, SEM) thin films, and solar cells (description of all fabrication steps, structural characterization of the perovskite absorber by SEM, XRD, UV-Vis absorption spectra of the stacks).

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