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Annealing-induced solid phase crystallization of In₂O₃:H leads to a significantly improved electron mobility, which is confirmed by Hall measurements. Indium hydroxide dehydroxylation occurs in In₂O₃:H during annealing, which is well responsible for the structural transformation and a high electron mobility with a decreased carrier concentration in crystallized In₂O₃:H. A significant decrease in the intensity of occupied gap states is observed in crystallized In₂O₃:H, possibly due to a decrease in carrier concentration. Doped In₂O₃ variants have been found to have a quite deeper allowed transition level below the valence-band edge than undoped In₂O₃, which in particular applies to crystallized In₂O₃:H, but most likely attributed to a change of the crystal structure upon annealing and/or a different O 2p-In 4d coupling near the VBM compared to amorphous In₂O₃:H.
To well understand the interface properties of Ag/In₂O₃:H upon annealing, a thin Ag film was grown on the In₂O₃:H substrate and annealed in vacuum up to 300 °C. During annealing, the potential Ag diffusion into the bulk In₂O₃:H and/or a change of an annealing-induced Ag topography (i.e., cluster formation) occurs, with a small Ag oxidation (i.e., Ag₂O and AgO). With Ag deposition, an initial downward band bending of (0.11±0.05) eV was present in In₂O₃:H, attributed to a Schottky contact formed at the Ag/In₂O₃:H interface. Upon annealing, the downward band bending reduces gradually, and the Schottky-barrier height at the Ag/In₂O₃:H interface also decreases.
A thickness series of the individual materials on the respective “substrate” (i.e., MnS/Si, GaN/MnS, and ZnO/GaN) was epitaxially grown on Si (100) wafer, and the interfacial chemistry and energy-level alignment at the respective interfaces are examined using photoelectron spectroscopy. At the MnS/Si interface, an interface-induced band bending (IIBB) appears in Si, which of values are found to be (0.15±0.07) and (0.23±0.07) eV for 4 and 15 nm MnS/Si stacks, respectively. The MnS/Si heterointerface shows a type-II (staggered) band lineup with a VBO of (-0.37±0.10) eV and the corresponding CBO of (2.27±0.10) eV. For the GaN/MnS interface, a significant diffusion of Mn into the GaN layer takes place during GaN deposition. In addition, an interface-induced band bending (IIBB) by ~0.30 eV is observed in MnS. The GaN/MnS interface shows a type-II (staggered) band lineup with a VBO of (1.46±0.10) eV and the corresponding CBO of (-1.09±0.10) eV. At the ZnO/GaN interface, a significant N diffusion from GaN into ZsnO takes place, i.e., Zn-N bonds, when ZnO is grown on the GaN layer. Also, an interfacial oxide (GaOx) layer was formed during ZnO deposited on GaN films. The ZnO/GaN heterointerface shows a type-II (staggered) band lineup with a VBO of (2.48±0.10) eV and the corresponding CBO of (-2.50±0.10) eV, respectively.
The chemical and electronic structure of the interfaces of kesterite-based and related absorber materials in thin-film solar cell device-relevant layer stacks have been investigated with x-ray based spectroscopic techniques (XPS, HAXPES, XAES, XES, and XAS). The analyzed materials are CZTS absorbers with their interfaces to the Mo back contact and to different Zn(O,S)-buffer layers, SnS absorbers with partly N-doped ZnO buffer layers, and 2 absorbers with (Cd,Zn)S buffer layers.
We find an unchanged CZTS surface in this series which has a profound effect on the ZnOS growth. Instead of a constant composition throughout the buffer thickness, we find S-rich material growing directly on the CZTS in the ALD process. The conduction band offset is found to be around -0.21(+-0.15) eV. The measured cliff is reduced by chemical interface modification towards a flat band alignment.
The interface effect during annealing at the CZTS back contact where MoS2 forms occurs even at low annealing temperatures below 500°C. Sn spectra of liftoff CZTS back sides, annealed on the Mo contact, resemble the precursor at all temperatures while Sn spectra of CZTS front sides are almost single phase after standard annealing. A TiN interlayer was effectively introduced to prevent reactions at the CZTS/Mo interface.
We find a strong impact of ALD deposited ZnO on oxidized SnS absorber surfaces, resulting in metallic Sn formation. In contrast, doped ZnO:N reduces the SnS much less agressively. The strong downwards bending for both ZnO and SnS at the ZnO/SnS junction that we find for undoped ZnO contact layers is drastically reduced if ZnO:N contact layers are used. The smaller cliff in the conduction band of 0.2 eV for ZnO:N/SnS makes this junction much more interesting for solar cell applications than the ZnO/SnS with a large cliff of 0.55 eV.
The chemical analysis of the (Cd,Zn)S:Ga/CuSbS2 reveals chemical interaction at the junction. We find an additional species on the high energy side of the Cu peak for all samples. The interface species can be explained with the changed buffer composition at the interface, particularly low S and Cd content and increased Ga concentration, and could also be affected by the presence of O and hydroxides. We find an upwards surface band bending of -0.15 eV for the bare absorber. At the junction, we see downwards band bending of up to 0.2(+-0.09) eV in the absorber and strong upwards band bending of -0.75(+-0.28) eV in the buffer. The junction is not an abrupt transition from one homogenious material to another. The buffer growth is strongly affected by the absorber. Also the electronic structure on both sides is significantly affected. We find a conduction band alignment with a large cliff of -1.3(+-0.3) eV.
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.
The properties of Si thin films for solar cells, the interaction with different substrates and the influence of dopants are examined with synchrotron based x-ray spectroscopy – primarily x-ray emission spectroscopy (XES) and hard x-ray photoelectron spectroscopy (HAXPES). The films are studied as-deposited (i.e., amorphous, a-Si) and after conversion into polycrystalline (poly-Si) employing solid phase crystallization (SPC). Si L2,3 XES spectra of thin-film Si samples can be described by a superposition of a-Si and monocrystalline Si-wafer (c-Si) reference spectra. According to a quantification based on that superposition principle, none of the investigated samples are completely crystallized – a measurable a-Si component always remains (5-20 %) regardless of deposition and treatment conditions. Based on additional results from electron back scattering diffraction different models are developed which may explain this finding. According to these models, the remnant a-Si component can be attributed to amorphous/disordered material at the grain boundaries. Using one of these models, the thickness of this grain-surrounding material s could be approximated to be (1.5 ± 0.5) nm. Further investigations of the SPC process reveal a faster crystallization for boron-doped samples, and a slower crystallization for phosphorous-doped samples, when compared to the crystallization of undoped a Si:H thin films. The peculiarities of B K XES spectra (and observed changes upon SPC) indicate that boron could act as a nucleation center promoting crystallization. Si L2,3 XES spectra of a-Si:H and P-doped poly-Si exhibit spectral features above the valence band maximum at 100 eV that could be attributed to a-Si defect states and n+-dopant states, respectively. The SPC crystallization velocity of Si thin films on ZnO:Al/glass is found to be faster than that on SiNx/glass substrate. Multiple indications for oxidization at the poly-Si/ZnO:Al interface are found based on our Si L2,3 XES analysis. Spatially resolved x-ray photoelectron spectroscopy data support this and even suggest the formation of sub-oxides or zinc silicate as an interface species. The electronic structure of the buried a-SiOx:H(B)/ZnO:Al and µc-Si:H(B)/ZnO:Al interfaces are unraveled with “depth resolved” hard x-ray photoelectron spectroscopy. A surface band bending limited to the very surface of the silicon layers is found. The valence band maxima for the Si cover layers and the ZnO:Al TCO are determined and interface induced band bending for both interfaces are derived. At the a-SiOx:H(B)/ZnO:Al interface a tunnel barrier of (0.22 ± 0.31) eV and at µc-Si:H(B)/ZnO:Al interface a tunnel barrier of (-0.08 ± 0.31) eV is determined. This explains a previously empirically found solar cell efficiency increase produced by introducing a µc-Si:H(B) buffer layer between an a-Si p-i-n cell and the ZnO:Al/glass substrate.