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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.
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.
Chalkopyrite gelten aufgrund ihrer direkten Bandlücke sowie der hohen Absorptions-koeffizienten als vielversprechende Materialien für Absorber in der Dünnschichtsolarzellentechnik. Speziell CuGaSe2 (CGS) mit einer für Kupfer-Chalkopyrite großen Bandlücke von 1.68 eV ist ein für die Photovoltaik interessantes Material in Hinblick auf Tandemsolarzellen mit CuInSe2. Das Grundverständnis der Eigenschaften des CGS-Materialsystems ist daher von entscheidender Bedeutung für effizienzsteigernde Optimierungen für Solarzellen auf Chalkopyrit-Basis.
Mit Hilfe der Molekularstrahlepitaxie wurden einkristalline CGS-Schichten unterschiedlicher Stöchiometrie auf GaAs-Substraten abgeschieden. Die Oberflächenzusammensetzung sowie die Valenzbandstruktur wurden mit Hilfe der Photoelektronen-Spektroskopie bestimmt. Zusätzlich erlaubten die einkristallinen Schichten eine Bestimmung der Periodizität der Oberfläche durch niederenergetische Elektronenstreuung. Dabei konnte eine (4x2) Rekonstruktion der Zinkblende-Ordnung der (001) Fläche für kupferreiche CGS-Schichten nachgewiesen werden. Für kupferarme bis nahstöchiometrische Präparationsbedingungen wurde eine (4x1) Rekonstruktion erhalten. Die (112) Fläche hingegen zeigte für kupferarme Stöchiometrien eine bisher noch nicht nachgewiesene (3x1) Rekonstruktion der Chalkopyrit-Ordnung mit zwei Rotationsdomänen, welche für höhere Cu/Ga Verhältnisse in eine (1x1) Struktur überging. Weiterhin gelang es die bereits für Kupfer-Chalkopyrite bekannte
Cu2-xSe Sekundärphase, welche unter kupferreichen Präparationsbedingungen auftritt, durch einen nachträglichen Heizschritt auf Wachstumstemperatur deutlich abzuschwächen bzw. zu entfernen.
Für die Effizienz der Solarzelle sind die Grenzflächen von Absorber zu Puffer- und Puffer- zu Fensterschicht von besonderer Bedeutung. Die Untersuchung des Heterokontakts von CGS zu ZnO stellt einen weiteren Schwerpunkt der vorliegenden Arbeit dar. Hierbei stand die Bandanpassung von Chalkopyrit zu ZnO im Vordergrund. Für das Wachstum von ZnO konnte mit Hilfe der Metall-Organischen-MBE auf CuGaSe2 das Auftreten einer dünnen
(1-2 nm) ZnSe-Zwischenschicht nachgewiesen werden. Im Gegensatz zu CuInSe2 wurde jedoch anstatt eines „Spikes“ in der Leitungsbandanpassung von CGS zu ZnO ein für die Solarzelleneffizienz ungünstiges „Cliff“ gefunden. Letzteres trat unabhängig von der CGS Substratorientierung auf. Mit Hilfe von niederenergetischer Elektronenstreuung gelang für das Wachstum von ZnO auf der (112) CGS Fläche trotz der großen Gitterfehlanpassung von ZnO zu CGS der Nachweis von geordnetem Wachstum. Für das Wachstum von ZnO auf CGS (001) wurde polykristallines Wachstum beobachtet.
Aufgrund der nicht optimalen Bandanpassung zum CGS scheint ZnO zusammen mit ZnSe als Pufferschicht kein geeignetes Fenstermaterial für dieses Kupfer-Chalkopyrit zu sein. Damit ist eine mögliche Erklärung für den, im Vergleich zu CuInSe2 basierten Zellen, geringen Wirkungsgrad von CGS Solarzellen gefunden.
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 chemical and electronic structure of chalcopyrite absorbers with different bulk band gap energies, Egbulk, [i.e., low-gap Cu(In,Ga)Se2 (CIGSe, Egbulk ~ 1.2 eV) and wide-gap CuInS2 (CIS, Egbulk ~ 1.5 eV)] and of buffer/absorber heterointerfaces based on these materials are studied with soft and hard x-ray spectroscopy techniques. Mechanisms that benefit (limit) the performance of low(wide)-gap chalcopyrite-based solar cells are identified. This knowledge is used to develop surface tailoring treatments to optimize buffer/absorber heterointerfaces based on wide-gap chalcopyrites and improve the performance of their solar cells.
Photoemission spectroscopy (PES) characterization of the two absorbers (i.e., CIGSe and CIS) reveal compositional-depth profiles. The changes detected in CIGSe include: a near surface Ga-depletion, a strongly Cu-poor surface and a strong presence of surface Na that (likely) occupies Cu vacancies. A similar Cu-deficiency is found in CIS. The depth-composition changes result in significant widening of the band gap at the surface, Egsurf, (i.e., CIGSe, Egsurf: 1.70 ± 0.2 eV and CIS, Egsurf: 1.88 ± 0.2 eV) as evident by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) measurements. Differences in the interaction of the CIGSe and CIS surfaces with deposited buffer materials are identified. PES and modified Auger parameter studies reveal strong intermixing at the CdS/CIGSe and ZnS/CIGSe heterointerfaces. S L2,3 x-ray emission spectroscopy (XES) measurements of CIGSe substrates submitted to CdS chemical bath deposition (CBD-CdS) treatments show the formation of In2S3 and defect-rich/nanostructured CdS at the interface, compounds with higher band gap values than the measured Egsurf for CIGSe. S L2,3 XES spectra of CIGSe substrates submitted to CBD-ZnS treatments reveal the formation of (Zn,In)(S,Se)2 chemical analogs at the interface. PES and XES measurement series show that the CdS/CIS heterointerface is more abrupt, with no detected interface chemical species. Direct measurement of the band alignment of these heterointerfaces reveals: an ideal conduction band offset (CBO) configuration for CdS/CIGSe (i.e., CBO: +0.11 ± 0.25 eV), a spike CBO configuration for ZnS/CIGSe (i.e., CBO: +1.06 ± 0.4 eV), and a highly unfavorable cliff CBO configuration for CdS/CIS (i.e., CBO: -0.42 ± 0.25 eV). The performance of solar cell devices based on these heterointerfaces is correlated to their CBO configuration.
Two surface tailoring approaches intended to correct the CBO configuration of the CdS/CIS heterointerface are presented. One method is based on rapid thermal processing (RTP) selenization treatments of CIS absorbers, aiming to exchange Se for S in treated samples. The idea behind this approach is to modify the surface of a wide-gap chalcopyrite so that it forms a more favorable heterointerface with CdS, such as heterointerfaces within low-gap chalcopyrite devices. X-ray fluorescence analysis and PES measurements of RTP-treated CIS samples show a greater treatment effect at the surface of the sample compared to the bulk (i.e., surface [Se]/[S+Se] range: 0.23 ± 0.05 to 0.83 ± 0.05, compared to bulk [Se]/[S+Se] range: 0.01 ± 0.03 to 0.24 ± 0.03). Tuning of the Cu:In:(S+Se) surface composition from a Cu-poor 1:3:5 to a 1:1:2 stoichiometry is observed in RTP-treated CIS samples with lower to higher surface Se contents, respectively. UPS measurements show a shift in valence band maximum toward the Fermi level in samples with higher surface Se content (i.e., -0.88 ± 0.1 to -0.51 ± 0.1 eV), as expected for a reduction in Egsurf due to exchange of Se for S. Ultraviolet-visible spectrophotometry reveals a reduction in the optical band gap of samples with greater Se incorporation (i.e., from 1.47 ± 0.05 to 1.08 ± 0.05 eV), allowing for a working window for optimization purposes.
The second tailoring method involves surface functionalization of CIS absorbers with dipole-charge-inducing self-assembled monolayers (SAM) of benzoic acid derivatives and thiol molecules. The introduction of dipole charges between a heterointerface can tune the relative alignment of the electronic bands composing its electronic structure; thus, use of a suitable dipole-inducing SAM could correct the CBO misalignment in the CdS/CIS heterointerface. UPS measurements of the secondary electron cut-off region of CIS samples treated with a selected set of SAMs show a work function modulation of CIS (i.e., 4.4 ± 0.2 eV - 5.2 ± 0.2 eV). Small gains in solar cell parameters of solar cells based on SAM-modified heterointerfaces are measured.
An overview of the performance of chalcopyrite(kesterite)-based solar cells in relation to the electronic properties of their corresponding buffer/absorber heterointerface suggests that optimization approaches extending beyond the buffer/absorber heterointerface may be needed for further performance gains in wide-gap chalcopyrite-based solar cell devices.
The aim of this work is to establish tools for optical characterization of defects in thin-film silicon solar cells. This is related to a challenging process of setup adjustments and careful interpretation of the measured raw data because of several artifacts and effects, which are typical for thin films. They are caused by the low layer/sample thickness and the related high impact of interfaces. Therefore, different thin-film samples were investigated to establish a process to correct/minimize these thin-film effects. The possibility of a knowledge transfer from mc-Si wafers with bulk thickness to thin Si films was checked. This would simplify a successful interpretation of the corrected data. Defects in mc-Si were investigated for many decades without the parasitic impact of thin films. Other Si phases, which are limited to thin-film samples, were investigated to learn details about their specific physical properties. These Si phases are amorphous and microcrystalline silicon. Additional to that electroluminescence investigations were performed on mc-Si solar cells. These investigation deals with topics, which are not even understood on bulk materials up to now. This could offer a basic for further knowledge transfers to thin-film Si.
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.
Ziel der Arbeit war es, erstmalig die Bandstruktur des in Dünnschichtsolarzellen verwendeten Absorbermaterials CuInS2 zu bestimmen. Zu diesem Zweck konnte ein neues GSMBE-Verfahren mit TBDS als Schwefel-Precursor etabliert werden, um die Verwendung von elementarem Schwefel in einem UHV-System zu ersetzen. Zusätzlich zu den abgeschiedenen Schichten wurde eine Spaltfläche präpariert. Die Charakterisierung der Proben erfolgte in situ mittels XPS/UPS und LEED. Für weitere ex situ Untersuchungen standen XRD und SEM zur Verfügung. Die ARUPS-Untersuchungen zur Bestimmung der Bandstruktur fanden unter Verwendung von Synchrotronstrahlung statt. Die aufgenommenen Bandstrukturen wurden mit Bandstrukturberechnungen Eyerts und Zungers verglichen. Dabei gestattete eine Zusammenarbeit mit Eyert eine exakte Verifizierung jenseits hochsymmetrischer Verläufe. CuInS2(001) und CuInS2(112) wurden auf Si und GaAs abgeschieden und zeigte Wachstumsraten, die im Vergleich mit MBE-Verfahren älterer Arbeiten um bis zu 85 % geringer waren. Dies führte auf Si zur Ausbildung von CuSi-Phasen, so dass sich Si als Substrat in diesem Verfahren als ungeeignet erwies. Die CuInS2-Abscheidung auf GaAs zeigte eine starke Abhängigkeit von der vorhandenen Oberflächenrekonstruktion. Eine 2x1-Rekonstruktion auf GaAs(100) führte zu einer terrassierten CuInS2-Schicht. Eine Abscheidung auf der 2x2-rekonstruierten GaAs(111)A-Oberfläche hatte eine facettierte CuInS2-Oberfläche zur Folge. Auf schwefelpassiviertem, rekonstruktionsfreien GaAs(111)B zeigte sich dagegen facettenfreies CuInS2(112)-Wachstum in vorherrschender Chalkopyrit-Struktur. An den Oberflächen von Cu-reichen CuInS2-Schichten bildeten sich CuS-Kristallite. Diese polykristallinen Ablagerungen führten bei ARUPS-Untersuchungen zu nicht-dispergierenden Zuständen, die die Bandstrukturen der CuInS2-Schichten überlagerten. Bei k-senkrecht-Messungen zeigte sich ein Zustand, dessen Dispersion stark von Berechnungen Eyerts abweicht. Sein Ursprung konnte nicht eindeutig abgeleitet werden konnte, da der Beobachtungsraum aufgrund von Endzustandseffekten und Satellitenstörungen stark eingeschränkt war. Generell zeigt ein Vergleich der Berechnungen nach Eyert bzw. Zunger mit entsprechenden Messungen, dass beide Modellrechnungen zu Bindungsenergien führen, die gegenüber den Messwerten zu niedrig sind. Die energetische Ausdehnung der charakteristischen Bereiche und deren Lage zueinander wird jedoch zutreffend beschrieben. Konsistent mit Beobachtungen in anderen Arbeiten an analogen Systemen erweist sich dabei die theoretische Behandlung der d-Zustände bei der Modellierung als kritisch. Die Bandlücke zwischen d-Charakter-dominierten und p-Charakter-dominierten Bereich wird von Eyert überschätzt, von Zunger dagegen unterschätzt. Dies lässt sich auf die verwendeten Näherungen (LDA (Zunger), GGA (Eyert)) des Austauschkorrelationspotentials zurückführen. Bei einer Anpassung der von Eyert berechneten Bandstrukturen durch einen geeigneten Energieoffset zeigten sich im Vergleich mit k-parallel-Messungen im Allgemeinen gute, nahezu perfekte Übereinstimmungen von beobachteten Dispersionsverläufen. Es konnten die effektiven Massen der Löcher für die k-parallel-Messungen bestimmt werden. Die Werte liegen in der Größenordnung des Literaturwertes, jedoch im Mittel bei doppelt so hohen Werten. Ein signifikanter, richtungsabhängiger Trend lässt sich aufgrund der starken Streuung nicht feststellen. Im Rahmen der vorliegenden Arbeit konnte ausserdem gezeigt werden, dass die Verwendung eines (111)-Substrates zu einer Domänenbildung aufwachsender CuInS2(112)-Schichten führt. ARUPS-Messungen an derartigen Schichten bestehen daher aus einer Überlagerung von Bandstrukturen entlang unterschiedlicher Richtungen. Für zukünftige Arbeiten bietet es sich daher an, die (111)-substratinduzierte Fehlorientierung von (112)-Schichten durch ein Step-Flow-Wachstum auf Substraten mit Miscut zu unterbinden.