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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.