FG Experimentalphysik und funktionale Materialien
Refine
Document Type
- Doctoral thesis (3)
Has Fulltext
- yes (3)
Is part of the Bibliography
- no (3)
Language
- English (3)
Keywords
- Solarzelle (3) (remove)
Institute
This thesis focuses on the investigation and characterization of the surfaces and interfaces of chalcopyrite-based Cu(In,Ga)Se2 (CIGSe) thin film solar cells using various x-ray and electron spectroscopies. In particular, the impact of alkali post deposition treatments (PDT) on the chemical and electronic surface and interface structure of CdS/CIGSe absorbers is studied.
The structure of “real world” CdS/CIGSe interfaces and how they are impacted by different alkali PDTs was investigated by a combination of different x-ray spectroscopies. The interface formation is characterized by studying sample sets with different CdS thicknesses. The chemical environment for indium and cadmium is revealed by deriving the modified Auger parameter α'(In) and α'(Cd) using the kinetic energy of most prominent Auger line together with the binding energy of the chosen core level. A more complex situation is found for CdS/CIGSe samples that underwent NaF+KF PDT, where a K-In-Se compound is initially present on top of the chalcopyrite absorber. The conversion of the K-In-Se type species into a Cd-In-(O,OH,S,Se) interface compound is recorded at short CBD-CdS deposition times. It appears the majority of K that is present at the surface of the NaF+KF PDT CIGSe absorber is dissolved in the CBD and partially re-deposited as K-O type species. The Cd/S ratio clearly deviates from the stoichiometry expected for CdS, and a Cd(O,OH,S)-like compound is likely formed. The electronic structure of CdS/CIGSe interface is similarly more complex for the NaF+KF PDT compared to the NaF PDT case, where only Cd(O,OH,S) buffer was formed.
In an attempt to shed more light into this complex situation, the impact of evaporated alkali metals (K, Rb, Cs) on the surface structure of CIGSe was studied in-system by synchrotron-based hard x- ray photoelectron spectroscopy (HAXPES), aiming at understanding the underlying mechanism of the interfacial effect of alkalis on the performance of CIGSe devices. In the case of K deposition, two K species are observed by x-ray absorption near-edge structure (XANES) and HAXPES, one of which species disappears at high annealing temperature. Furthermore, three new In contributions (In-O and K-In-Se, metallic In species) can be observed after K evaporation. The evolution of chemical contribution supports the formation of a K-In-Se and Cu-poor CIGSe (1:3:5) bilayer structure that is similar to what was reported for “real world” NaF+KF PDTs. Deposition of heavy alkali metals (Rb, Cs) induced the formation of alkali selenide phases after alkali evaporation and during low temperature annealing. Similar chemical changes as seen for the K composition (i.e. presence of metallic In, In-O, and alkali-O) are observed. However, detailed analysis of the Alk/Se ratio and composition provide direct evidence for the formation of a Alk-(In)-Se and (Cu,Alk)(In, Ga)Se2 bilayer.
The insights from these studies promise to provide crucial aid to fully exploit alkali pre-treatments in scientific and industrial CIGSe production, and will deliberate use of this means of surface/interface tailoring to push efficiencies even further.
The aim of this work is to study the optical properties of crystal defects in multicrystalline solar silicon and poly-/microcrystalline silicon thin films on glass substrate.
First a setup for photoluminescence imaging on multicrystalline silicon solar wafers was developed. This system is suitable for detecting band-to-band luminescence as well as defect-related luminescence at room temperature on large-scale wafers at different stages of their processing.
Spectroscopic photoluminescence investigations of multicrystalline silicon solar wafers indicated a new intense luminescence line at ≈ 0.91 eV at room temperature. The origin of this line is probably found in a specific grain boundary. Furthermore, luminescence in the region of 0.8 eV was investigated in detail, and it was found that probably oxygen is responsible for a peak at 0.77 eV at 80 K.
Electroluminescence investigations at room temperature at both materials exhibit extended defect structures such as grain boundaries. Furthermore, it can be concluded that electroluminescence imaging in reverse bias mode indicate on serious breakdown points in solar cells, which can lead to destruction of solar cells and modules. By comparing defect-related and reverse bias electroluminescence images, a difference in the spatial distribution of defects emitting D1 radiation and defects emitting light under reverse bias beyond -12 V is detectable.
In addition, there seems to be a correlation in the distribution of non-doping impurities and photoluminescence. Concerning this, vertical slabs of two silicon blocks were examined by means of Fourier-transform infrared spectroscopy and photoluminescence. A correlation of the distributions of interstitial oxygen and the band-to-band luminescence profiles could be found. Additionally, a correlation between D3/D4 luminescence profile and nitrogen distribution in the blocks was observed.
Finally, the growth process, particularly the transition from amorphous to microcrystalline silicon by PECVD, was studied by combined photoluminescence and Raman investigations. Formation of silicon nano-grains was detected by means of photoluminescence and Raman spectroscopy.
The aim of this work is to describe and explain the properties of defects in multicrystalline (mc) and thin-film solar silicon (Si). For this reason, investigations with scanning electron microscope methods were performed, namely cathodoluminescence (CL), electron beam induced current (EBIC), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Additionally, photoluminescence (PL) and reverse-biased electro luminescence (ReBEL) measurements were also conducted. Through correlation of PL, ReBEL and EBIC, it was possible to localize breakdown sites at mc-Si solar cells. Problems that occurred during the thin-film EBIC investigations could be demonstrated and explained. For the first time cross sectional EBIC investigations could be performed on thin-film silicon tandem cells. At mc-Si, it was possible to observe the oxygen related P-line next to the common D1-line luminescence at 10 K clearly distinguishable from each other at once. Furthermore, a hitherto not comprehensively discussed intense luminescence line at 0.93 eV could be described in detail. Through correlation of PL, CL, EBIC, EBSD, and TEM measurements, the origin of the now named Di luminescence at 0.93 eV is postulated to be in connection with Frank partial dislocations, with two energetic levels inside the band gap, one at 112±9 meV below the conduction band and the other at 93±10 meV above the valence band. Finally, it was attempted to explain the behavior of twin boundaries at temperatures below 30 K, where these show an enhanced collection efficiency in comparison to the surrounding grains. An alteration of the local “freeze out” temperature, possibly by a local band gap narrowing, is suggested as a reason. Another conceivable explanation is a breakdown of the diode potential at the grains.