AVS 71 Session TF2-MoA: Thin Films for Energy II

Antimony chalcogenide (Sb₂X₃, X=S, Se, Te) thin film solar cells have garnered significant interest due to their optimal bandgap, high absorption coefficient, and earth-abundant constituent elements. Interface engineering plays a pivotal role in optimizing the performance of these solar cells, as the interfaces between different layers critically influence charge carrier dynamics. This talk focuses on systematically investigating and engineering interfaces in Sb₂X₃-based thin film solar cells to enhance their efficiency and stability. Precise control over interface properties was achieved by employing techniques such as sputtering and chemical bath deposition (CBD). The introduction of buffer layers, passivation treatments, and surface modifications were explored to mitigate recombination losses and improve charge extraction. Advanced characterization methods including X-ray photoelectron spectroscopy (XPS), were utilized to analyze the interfacial properties and their impact on device performance. The findings highlight the critical importance of interface quality and the potential of tailored interface engineering to unlock higher efficiencies in antimony chalcogenide thin film solar cells. This work provides a pathway for further optimization and commercial viability of Sb2X3 solar cells, contributing to the advancement of sustainable photovoltaic and photoelectrochemical technologies.

The oxygen evolution reaction (OER) is the rate limiting step inhibiting the production of fuel from water.^1,2 Barium nickelate (BNO) is a promising catalyst, due to a tunable oxygen stoichiometry without significant structural changes to the crystalline phases. Tuning oxygen stoichiometry in transition metal oxides can modulate the band structure and increase catalytic activity.³ The R32 phase of BNO is proposed to exhibit increased OER catalytic activity due to a theoretically predicted e_g orbital filling of Ni near the optimal value of 1.2 for OER.¹ However, the range of stoichiometries, and therefore Ni oxidation states, that the R32 phase of BNO can exhibit are not yet known.

Here, we stabilized the R32 phase of BNO in thin films using a BaNi_0.93O_2.68 target by pulsed laser deposition (PLD) on (0001) Al₂O₃ substrates. The films were deposited at a substrate setpoint temperature of 400 °C, oxygen partial pressures ranging from 0.012-0.091 mbar, and a laser fluence of 2.5 J/cm². X-ray diffraction shows that the films are solely (100) oriented and only the R32 phase is observed over this range of partial pressures. Energy dispersive X-ray spectroscopy reveals that the film stoichiometries range from BaNi_1.10O_2.26 to BaNi_1.03O_2.14. The range of stable oxygen stoichiometries is further increased by post annealing in an oxygen atmosphere beyond those available by PLD. X-ray photoelectron spectroscopy indicates the presence of Ni²⁺, Ni³⁺, and Ni⁴⁺. Four-point probe measurements show an increase in resistivity with increasing oxygen stoichiometry; decreasing oxygen stoichiometry to BaNi_1.03O_2.14 resulted in a conductivity of 0.161 S/m. Then films were deposited on conductive (111) Nb:SrTiO₃ substrates for impedance spectroscopy, dielectric, and piezoelectric measurements. The piezoelectric response was measured using laser doppler vibrometry in an e_11,f wafer flexure system. Impedance spectroscopy results further show the role of oxygen vacancies migration on the electronic and ionic conductivity.

References

1) Lee et al., J. Am. Chem. Soc. 2016, 138 (10), 3541–3547.

2) Plevová et al., Journal of Power Sources 2021, 507, 230072.

3) Wang et al., ACS Catal. 2018, 8 (1), 364–371.

Ultrahigh purity hydrogen is essential in many fields including semiconductor processing, fuel cells, and fusion energy.Palladium-based foils offer high flux and perfect selectivity for hydrogen purification, but are prohibitively expensive for many applications. BCC metals like vanadium (V) and niobium (Nb) are ~600X less expensive and offer an order of magnitude greater hydrogen permeability, but require the application of a catalyst to efficiently dissociate and recombine molecular hydrogen and protect V and Nb from uptake of nonmetal impurities. In this work sputter deposition is used to fabricate composite membranes to unlock the full potential of BCC metal foils. Pd is a great catalyst, but will rapidly interdiffuse with BCC metals. In the first strategy we deposit a TiN interdiffusion barrier between Pd and the underlying V foil.TiN was deposited via reactive sputtering, and films with the desired (200) orientation were obtained in the metallic regime at 400 °C with the application of a 200 V bias. The permeability of thin-film TiN was determined with palladium-based sandwich structures. TiN layers up to 10 nm resulted in a minimal decrease in flux (~20%) relative to a freestanding PdCu foil, which was attributed to the interfacial resistance. Composite Pd|TiN|V|TiN|Pd membranes exhibited permeability values up to three times greater than pure palladium, exhibiting stability at 450 °C for over 100 h. However, the membranes were unstable at 500 °C, which was attributed to the instability of the thin Pd layer and loss of catalytic activity. The second strategy involved using sputter–deposited TiO₂ as a catalyst layer. It was found that anatase TiO₂ was an excellent catalyst for H₂ activation, producing composite membranes that achieved theoretical permeability at 500 ºC.After 20 hours of operation these membranes begin to display a slow decline in performance. Extensive materials characterization techniques are employed to understand the underlying reasons driving performance and to further improve the stability of these composite membranes.

Atomic layer deposition (ALD) enables angstrom level control over material synthesis, making it a powerful platform for engineering catalytic thin films. In this study, we report the deposition of ultrathin Ruthenium (Ru) films on TiO2 substrates with (100),(101) and (110) orientations via thermal ALD using organometallic precursor (Ru(ETCP)₂) and oxygen as co-reactant. The process yield conformal, uniform and highly crystalline as confirmed by X ray photoelectron spectroscopy (XPS) and X ray diffraction (XRD). XRD showed hexagonal closed packed structure and XPS confirmed the binding energy of Ru3d_5/2 is associated with Ru⁽⁰⁾. Electrochemical evaluation demonstrates hydrogen and oxygen evolution reactions in 0.1 M H₂SO₄, achieving enhanced catalytic activity for Ru on TiO2(110) which also exhibited lower charge transfer resistance in EIS plot. Tafel slopes also highlighted different kinetics for different Ru films. To understand surface chemistry and guide future refinement, first principles density functional theory (DFT) calculations will be employed to explore reaction mechanisms to unravel the energetics and rate limiting steps during growth of Ru films. DFT will help in understanding precursor-ligand decomposition, oxygen interaction and surface termination dynamics during the ALD cycle. This integrated experimental-computational approach illustrates how nanoengineered Ru films can play pivotal role in enabling next generation clean energy infrastructure to achieve terawatt scale hydrogen production.