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The chemical and electronic structure of hybrid organometallic (CH₃NH₃PbI₍₃₋ₓ₎Clₓ) and inorganic (CsSnBr₃) perovskite materials on compact TiO₂ (c-TiO₂) is studied using x-ray and electron based spectroscopic techniques. The morphology and local elemental composition of CH₃NH₃PbI₍₃₋ₓ₎Clₓ, used as absorbers in PV devices, defining the film quality and influencing the performance of respective solar cells is studied in detail by using photoemission electron microscopy (PEEM). An incomplete coverage, with holes reaching down to the c-TiO₂ was revealed; three different topological regions with different degrees of coverage and chemical composition were identified. Depending on the degree of coverage a variation in I oxidation and the formation of Pb⁰ in the vicinity of the c-TiO₂ is found. The valence band maxima (VBM) derived from experimental data for the perovskite and c-TiO₂, combined with information from literature on spiro-MeOTAD suggests an energy level alignment resulting in an excellent charge selectivity at the absorber/spiro-MeOTAD and absorber/c-TiO₂ interfaces respectively. Further, the derived energy level alignment indicates a large recombination barrier (~2 eV), preventing shunts due to direct contact between c-TiO₂ and spiro-MeOTAD in the pin-holes.
In-situ ambient pressure hard x-ray photoelectron spectroscopy (AP-HAXPES) studies of 60 and 300 nm CH₃NH₃PbI₍₃₋ₓ₎Clₓ have been performed under varies conditions (i.e. vacuum/water and dark/UV light) to gain insight into the degradation mechanism responsible for the short lifetime of the absorber. The 60 nm perovskite forms Pb⁰ in water vapor (non-defined illumination) in presence of x-rays. The 300 nm perovskite sample shows a complex behavior under illumination/dark. In water vapor/dark the perovskite dissolves into its organic (MAI) and inorganic (PbI₂) components. Under illumination PbI₂ further decomposes to Pb⁰ induced by UV light and x-rays.
For alternative inorganic CsSnBr₃ perovskites, the impact of SnF₂ on the chemical and electronic structure is studied to identify its role for the improved performance of the solar cell. HAXPES and lab-XPS measurements performed on CsSnBr₃ with and without SnF₂ indicate two Sn, Cs, and Br species in all samples, where the second Sn species is attributed to oxidized Sn (Sn⁴⁺). When adding SnF₂ to the precursor solution, the coverage is improved and less Sn⁴⁺ and Cs and Br secondary species can be observed, revealing an oxidation inhibiting effect of SnF₂. Additionally, SnF₂ impacts the electronic structure, enhancing the density of states close to the VBM.
This thesis focuses on the investigation and characterization of the surfaces and interfaces of chalcopyrite-based Cu(In,Ga)Se₂ (CIGSe) and organo-metal mixed halide perovskites, specifically CH₃NH₃PbI(₃-ᵪ)Clᵪ thin film solar cell absorbers using various x-ray and electron spectroscopies. In particular, the impact of alkali treatments on the chemical and electronic surface and near-surface structure of CIGSe absorbers is studied. For CH₃NH₃PbI(₃-ᵪ)Clᵪ the compound formation is monitored and the peculiarities of the interface formation of CH₃NH₃PbI(₃-ᵪ)Clᵪ on compact and mesoporous TiO₂ are examined.
Laboratory and synchrotron-based photoelectron spectroscopy are used to gain a depth-dependent picture of the chemical and electronic structure in the surface and near-surface region of CIGSe absorbers, focusing on the influence of NaF and NaF/KF post-deposition treatments (PDT) when compared to alkali-free CIGSe absorbers. The alkali-free and NaF-PDT absorbers show similar chemical properties, having a Cu and Ga poor surface region compared to the nominal bulk and the same chemical environment for indium and selenium. For the NaF/KF-PDT samples a K-In-Se compound is present on top of a Cu-In-Ga-Se compound, with a nanopatterned surface that is almost devoid of Cu and Ga.
Further, for the NaF-PDT sample a near-surface electronic band gap of 1.61[+0.14/-0.51] eV is derived. In contrast, a large and more gradual change towards the surface is obtained for the NaF/KF-PDT absorber exhibiting a significant band gap widening of the surface, which is in agreement with a Cu- and Ga-devoid surface region and the formation of a K-In-Se surface compound.
To in-situ monitor the compound formation of CH₃NH₃PbI(₃-ᵪ)Clᵪ on compact TiO₂ (c-TiO₂) hard x-ray photoelectron spectroscopy is used. During in-situ annealing a drop-casted layer of precursor solution on c-TiO₂ shifts in the perovskite related core levels can be observed upon reach the “transition temperature” of 80-100°C. Further the Cl signal decrease at the onset of annealing and shows a depletion in the surface region for annealing temperatures above 50°C. In addition, the chemical and electronic structure of the interface between perovskite and mesoporous TiO₂ (mp-TiO₂) or c-TiO₂ using different perovskite layer thicknesses are studied.