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Optical Detection of Deep Defects in Cu(In,Ga)Se2

The aim of this thesis is to shed light on the deep defect structure in Cu(In,Ga)Se2 by photoluminescence measurements and to propose a possible conclusive defect model by attributing experimental findings to a literature review of defect calculations from first principles. Epitaxial films are grown on GaAs by metal organic vapor phase epitaxy and characterized by photoluminescence at room or low temperature. In CuGaSe2, deep defect bands at ca. 1.1 eV and 1.23 eV are resolved. A model for the power law behavior in excitation dependent measurements of the peak intensities is derived, which leads to the experimental finding of two deep donor-like defects as a result. In Cu(In,Ga)Se2, the deeper band at around 1.1 eV remains constant in energy when more and more gallium is replaced by indium in the solid solution. For decreasing Ga-contents, the band gap is mainly lowered by a decrease of the conduction band energy. From fitting models for electron-phonon coupling, the dominating deep donor-like defect is determined at 1.3 eV above the valence band maximum. This level is proposed to be crucial for high Ga-contents when it is deep inside the band gap and most likely acts as a recombination center. At low Ga-contents it is resonant with the conduction band. The larger open circuit voltage deficits for high Ga-contents are proposed to stem at least partly from this defect which is qualitatively supported by simulations. Additionally another defect band at around 0.7 eV is observed for high Ga-contents at low temperatures and at 0.8 eV for low Ga-contents. The intensity of the 0.8 eV band seems to disappear in a sample with Cu-deficiency. In general, deep luminescence is always observed with similar energies in all Cu-rich compositions, independent of the Ga-content. The deep defect involved could explain inferior efficiencies of Cu-rich devices which show increased non-radiative recombination in general. It is further discussed that the same deep defect could be the origin of a level at 0.8 eV which is observed in several photo-capacitance measurements in literature. Based on the literature review for intrinsic defect calculations by hybrid-functionals, a possible defect model for shallow and deep defects is derived with a focus on those results, where different authors using different methods agree. By comparing the experimental results in the scope of this thesis, the deep defect found at 1.3 eV above the valence band is attributed to the GaCu antisites. The single (0/-1) charge transition of CuIn and CuGa is proposed to be the main shallow acceptor in the near-band-edge luminescence of Cu-rich compositions at 60 - 100 meV, whereas the second (-1/-2) charge transition is attributed to the deep 0.8 eV defect band. The present findings could be useful for the improvement of Cu(In,Ga)Se2 solar cells with stochiometric absorber compositions (Cu-rich growth) or with high band gaps (high Ga-content). Furthermore, the results show a very good agreement of experiment and recent theoretical calculations of defects, which can be seen as a promising relation between photoluminescence spectroscopy and predictions from theory for other complex materials

Surface characterization of epitaxial Cu-rich CuInSe2 absorbers

We investigated the electrical properties of epitaxial Cu-rich CuInSe 2 by Kelvin probe force microscopy (KPFM) under ambient and ultra-high vacuum conditions. We first measured the sample under ambient conditions before and after potassium cyanide (KCN) etching. In both cases, we do not see any substantial contrast in the surface potential data; furthermore, after the KCN etching we observed outgrowths with a height around 2nm over the sample surface. On the other hand, the KPFM measurements under ultra-high vacuum conditions show a work function dependence according to the surface orientation of the Cu-rich CuInSe 2 crystal. Our results show the possibility to increase the efficiency of epitaxial Cu-rich CuInSe 2 by growing the materials in the appropriated surface orientation where the variations in work function are reduced

The impact of Kelvin probe force microscopy operation modes and environment on grain boundary band bending in perovskite and Cu(In,Ga)Se2 solar cells

An in-depth understanding of the electronic properties of grain boundaries (GB) in polycrystalline semiconductor absorbers is of high importance since their charge carrier recombination rates may be very high and hence limit the solar cell device performance. Kelvin Probe Force Microscopy (KPFM) is the method of choice to investigate GB band bending on the nanometer scale and thereby helps to develop passivation strategies. Here, it is shown that amplitude modulation AM-KPFM, which is by far the most common KPFM measurement mode, is not suitable to measure workfunction variations at GBs on rough samples, such as Cu(In,Ga)Se2 and CH3NH3PbI3. This is a direct consequence of a change in the cantilever-sample distance that varies on rough samples. Furthermore, we critically discuss the impact of different environments (air versus vacuum) and show that air exposure alters the GB and facet contrast, which leads to erroneous interpretations of the GB physics. Frequency modulation FM-KPFM measurements on non-air-exposed CIGSe and perovskite absorbers show that the amount of band bending measured at the GB is negligible and that the electronic landscape of the semiconductor surface is dominated by facet-related contrast due to the polycrystalline nature of the absorbers

Oxidation as Key Mechanism for Efficient Interface Passivation in Cu (In,Ga)Se2 Thin-Film Solar Cells

Copper-indium-gallium-diselenide (CIGS) thin-film solar cells suffer from high recombination losses at the back contact and parasitic absorption in the front-contact layers. Dielectric passivation layers overcome these limitations and enable an efficient control over interface recombination, which becomes increasingly relevant as thin-film solar cells increase in efficiency and become thinner to reduce the consumption of precious resources. We present the optoelectronic and chemical interface properties of oxide-based passivation layers deposited by atomic layer deposition on CIGS. A suitable postdeposition annealing removes detrimental interface defects and leads to restructuring and oxidation of the CIGS surface. The optoelectronic interface properties are very similar for different passivation approaches, demonstrating that an efficient suppression of interface states is possible independent of the metal used in the passivating oxide. If aluminum oxide (Al2O3) is used as the passivation layer we confirm an additional field-effect passivation due to interface charges, resulting in an efficient interface passivation superior to that of a state-of-the-art cadmium-sulfide (CdS) buffer layer. Based on this chemical interface model we present a full-area rear-interface passivation layer without any contact patterning, resulting in a 1% absolute efficiency gain compared to a standard molybdenum back contact. © 2020 authors. Published by the American Physical Society

Passivation of the CuInSe2 surface via cadmium pre-electrolyte treatment

Effective defect passivation of semiconductor surfaces and interfaces is indispensable for the development of high efficiency solar cells. In this study we systematically investigated the surface and grain boundary properties of CuInSe2 (CISe) with scanning tunneling microscopy (STM) and spectroscopy (STS) after different surface treatments such as potassium cyanide (KCN) etching, pre-electrolyte treatment with cadmium ions, and annealing in ultrahigh vacuum (UHV). We show that air exposed samples with a subsequent KCN etching step exhibits a highly defective surface. However, a Cd pre-electrolyte treatment passivates most of these defects, which manifests itself by a reduction of the high conductance in the STS measurements at positive sample biases. The origin of the improvement can be traced back to an increase in surface band bending, which leads to a type inversion, induced by a change in the concentration of Cu vacancies. We observe a defect passivation at the CISe surface and at the grain boundaries. Our results give a direct explanation of why the CdS buffer layer in CISe thin film solar cells is of utmost importance for high efficiency devices

Doping mechanism in pure CuInSe2

We investigate the dopant concentration and majority carrier mobility in epitaxial CuInSe2thin films for different copper-to-indium ratios and selenium excess during growth. We find that all copper-poor samples are n-type, and that hopping conduction in a shallow donor state plays a significant role for carrier transport. Annealing in sodium ambient enhances gallium in-diffusion from the substrate wafer and changes the net doping of the previously n-type samples to p-type. We suggest that sodium incorporation from the glass might be responsible for the observed p-type doping in polycrystalline Cu-poor CuInSe2solar cell absorbers