Designing New Atomic Layer Deposition Precursors for Third-Generation Photovoltaics

Atomic layer deposition (ALD) holds a critical position in nanofabrication because of its excellent uniformity, conformality to nanostructured surfaces, subnanometer thickness control, and ability to synthesize high-quality materials and interfaces. Its abilities arise from its chemical mechanism: precursor molecules saturate the surface with a selflimiting monolayer and prevent continuous growth of material, then react with another co-reactant to deposit about one atomic layer of material at a time. Consequently, ALD first requires precursors with suitable volatility, chemical reactivity, and thermal stability. Motivated by the prospects of using ALD to fabricate next-generation solar cells and the unusual quantum mechanical behaviour of Pb-containing materials at the nanoscale, we set out to design, develop, and understand new Pb-containing ALD precursors. We began by attempting to prepare Pb(II) analogues of -heterocyclic carbenes, eventually finding the stable lead(II) rac- , -di-tert-butylbutane-2,3-diamide (1Pb) that displayed the best volatility of any Pb(II) complex reported in the literature (1 Torr vapour pressure at 94 C). With 1Pb and the well-known lead(II) bis[bis(trimethylsilylamide)] (0Pb), we then developed an ultralow-temperature ALD process for PbS using H 2 S as the S-containing co-reactant. Pure and crystalline PbS films were deposited with ideal ALD growth behaviour at temperatures as low as 45 C with 1Pb and 65 C with 0Pb, enabling deposition on thermally sensitive substrates like CH 3 NH 3 PbI 3 (MAPI) perovskites. Not only did the process effectively encapsulate MAPI thin films, they also had tunable -type conductivity, hole mobility, and carrier concentrations. An unusual reduction to elemental Pb(0) of 1Pb by H 2 S was found to begin at 95 C and was studied experimentally and theoretically by density functional theory (DFT) to understand its mechanism. Despite their success at low temperatures, 0Pb and 1Pb were limited by their thermal stability (<155 C), so we set out to design a more stable Pb-containing precursor. The surprising volatility of the dimeric bis[lead(II) N,N -di-tert-butyl-1,1-dimethylsilanediamide] [(4Pb) 2 ] prompted an in-depth theoretical study of its bonding, thermodynamics, and molecular electrostatic properties. By studying (4Pb) 2 and some of its derivatives, we gained a more nuanced view of the chemical nature of volatility with implications for future precursor design. The success of (4Pb) 2 led us to adapt its ligand for Co, yielding the unprecedented spirocyclic low-spin Co(IV) bis[ , -di-tert-butyl-1,1-dimethyldiamidosilane] (4Co). Finally, we explored Sn(II) and Sn(IV) trifluoroacetates as green potential ALD precursors to the transparent conductor F-doped SnO x . We analyzed their structure and bonding by X-ray diffraction (XRD) in the solid state, and gas-phase electron diffraction (GED) and DFT in the vapour phase, showing that their polymeric structures actually increased their volatility, and may represent a new precursor design strategy altogether.

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