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

in Nano Energy (2021), 88

An in-depth understanding of the electronic properties of grain boundaries (GBs) in polycrystalline semiconductor absorbers is of high importance since their charge carrier recombination rates may be very ... [more ▼]

An in-depth understanding of the electronic properties of grain boundaries (GBs) 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 the workfunction, measured with amplitude modulation (AM)-KPFM, which is by far the most common KPFM measurement mode, is prone to exhibit measurement artifacts at grain boundaries on typical solar cell absorbers 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. [less ▲]

Surface characterization of epitaxial Cu-rich CuInSe2 absorbers

in IEEE (2020)

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

in Nature Communications (2020)

Surface characterization of epitaxial Cu-rich CuInSe2 absorbers

in IEEE Photovoltaic Specialists Conference. Conference Record (2019, July)

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 ... [more ▼]

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. [less ▲]

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

Doctoral thesis (2018)

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 ... [more ▼]

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. [less ▲]

Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers

in Nature Communications (2018)

Defects in Cu(In,Ga)Se2: Photoluminescence vs Theory

Poster (2018)

Luminescence detection of the 0.8 eV defect

Poster (2017)

Revisiting radiative deep-level transitions in CuGaSe2 by photoluminescence

in Applied Physics Letters (2016), 109

Excitation dependence of photoluminescence transitions

Poster (2016)