Atomic Layer Deposition of Electron Selective SnOx and ZnO Films on Mixed Halide Perovskite: Compatibility and Performance

The compatibility of atomic layer deposition directly onto the mixed halide perovskite formamidinium lead iodide:methylammonium lead bromide (CH(NH₂)₂, CH₃NH₃)Pb(I,Br)₃ (FAPbI₃:MAPbBr₃) perovskite films is investigated by exposing the perovskite films to the full or partial atomic layer deposition processes for the electron selective layer candidates ZnO and SnO_x. Exposing the samples to the heat, the vacuum, and even the counter reactant of H₂O of the atomic layer deposition processes does not appear to alter the perovskite films in terms of crystallinity, but the choice of metal precursor is found to be critical. The Zn precursor Zn(C₂H₅)₂ either by itself or in combination with H₂O during the ZnO atomic layer deposition (ALD) process is found to enhance the decomposition of the bulk of the perovskite film into PbI₂ without even forming ZnO. In contrast, the Sn precursor Sn(N(CH₃)₂)₄ does not seem to degrade the bulk of the perovskite film, and conformal SnO_x films can successfully be grown on top of it using atomic layer deposition. Using this SnO_x film as the electron selective layer in inverted perovskite solar cells results in a lower power conversion efficiency of 3.4% than the 8.4% for the reference devices using phenyl-C₇₀-butyric acid methyl ester. However, the devices with SnO_x show strong hysteresis and can be pushed to an efficiency of 7.8% after biasing treatments. Still, these cells lacks both open circuit voltage and fill factor compared to the references, especially when thicker SnO_x films are used. Upon further investigation, a possible cause of these losses could be that the perovskite/SnO_x interface is not ideal and more specifically found to be rich in Sn, O, and halides, which is probably a result of the nucleation during the SnO_x growth and which might introduce barriers or alter the band alignment for the transport of charge carriers.

High-Performance Zinc Tin Oxide Semiconductor Grown by Atmospheric-Pressure Mist-CVD and the Associated Thin-Film Transistor Properties

Zinc tin oxide (Zn-Sn-O, or ZTO) semiconductor layers were synthesized based on solution processes, of which one type involves the conventional spin coating method and the other is grown by mist chemical vapor deposition (mist-CVD). Liquid precursor solutions are used in each case, with tin chloride and zinc chloride (1:1) as solutes in solvent mixtures of acetone and deionized water. Mist-CVD ZTO films are mostly polycrystalline, while those synthesized by spin-coating are amorphous. Thin-film transistors based on mist-CVD ZTO active layers exhibit excellent electron transport properties with a saturation mobility of 14.6 cm²/(V s), which is superior to that of their spin-coated counterparts (6.88 cm²/(V s)). X-ray photoelectron spectroscopy (XPS) analyses suggest that the mist-CVD ZTO films contain relatively small amounts of oxygen vacancies and, hence, lower free-carrier concentrations. The enhanced electron mobility of mist-CVD ZTO is therefore anticipated to be associated with the electronic band structure, which is examined by X-ray absorption near-edge structure (XANES) analyses, rather than the density of electron carriers.

Probing the highly transparent and conducting SnOx/Au/SnOx structure for futuristic TCO applications

A SnO_x/Au/SnO_x transparent conductive oxide (TCO) multilayered film was fabricated with a total thickness of 75 nm using both e-beam and thermal evaporation techniques.

Applications of atomic layer deposition in solar cells

Atomic layer deposition (ALD) provides a unique tool for the growth of thin films with excellent conformity and thickness control down to atomic levels. The application of ALD in energy research has received increasing attention in recent years. In this review, the versatility of ALD in solar cells will be discussed. This is specifically focused on the fabrication of nanostructured photoelectrodes, surface passivation, surface sensitization, and band-structure engineering of solar cell materials. Challenges and future directions of ALD in the applications of solar cells are also discussed.