From Event: SPIE Organic Photonics + Electronics, 2017
We demonstrate how amino-terminated silane monolayers alter the chemical and energetic composition of the TiO2 surface, which controls the interfacial nucleation, growth and energetics of device-relevant, hybrid perovskite (PVSK) thin films. The surface chemistry and energetics of compact TiO2 thin films are modified with a 3-aminopropyltriethoxysilane (APTES) monolayer that can either weakly coordinate Pb2+ ions (–NH2/free base form) or act as a surrogate organic cation (–NH3+/acid form) at the TiO2/PVSK interface, providing for significant differences in the nucleation free energy for the PVSK active layer as a function of NH3+/NH2 ratio. XPS spectra of amine-modified TiO2 surfaces (N 1s core level) demonstrate that we can achieve NH3+/NH2 ratios of between 3:1 and 1:3 depending upon subsequent acid and base treatment, respectively. Methylammonium lead triiodide (MAPbI3) films are incrementally co-evaporated on TiO2, TiO2/APTES-NH3+ and TiO2/APTES-NH2 interfaces, and the chemical composition, growth dynamics and energetics are systematically investigated using in situ X-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS). The XPS and UPS results reveal that initial nucleation and subsequent growth of the MAPbI3 PVSK film strongly depends on the chemical functionality of the TiO2 surface. The evaporated films display island-like growth on the bare TiO2 surface, which hinders nucleation of the PVSK phase until ca. 15 nm of precursor material is deposited. Conversely, film growth is more layer-by-layer on the amine-modified TiO2 interfaces, which promote nucleation of the PVSK phase within the first ca. 5 nm of deposition. In addition to vacuum evaporated thin films, we show how these TiO2 surface modifications control the morphology and crystallinity of solution-processed PVSK films based on formamidinium and methylammonium organic cations. These studies elucidate the role of TiO2 surface chemistry on the formation mechanism of hybrid PVSK active layers and the interfacial and bulk energetics, which have significant consequences related to the processing and operation of next-generation optoelectronic device platforms.
R. Clayton Shallcross, Selina Olthof, Klaus Meerholz, and Neal R. Armstrong, "Understanding the role of titanium dioxide (TiO2) surface chemistry on the nucleation and energetics of hybrid perovskite films (Conference Presentation)," Proc. SPIE 10363, Organic, Hybrid, and Perovskite Photovoltaics XVIII, 103631I (Presented at SPIE Organic Photonics + Electronics: August 10, 2017; Published: 19 September 2017); https://doi.org/10.1117/12.2274524.5581122032001.
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