Grain boundaries in solar cells


CALL: 2017

DOMAIN: MS - New Functional and Intelligent Materials and Surfaces


LAST NAME: Redinger



HOST INSTITUTION: University of Luxembourg

KEYWORDS: thin film solar cells; grain boundaries; Scanning Probe Microscopy; Hall effect; Atom Probe Tomography, Kelvin Probe Force Microscopy

START: 2018-02-01



Submitted Abstract

Within the GRISC project (GRain boundaries In Solar Cells) scanning probe microscopy (SPM), Hall effect and Atom Probe tomography (APT) measurements will be used to study the grain boundaries (GBs) in Cu(In,Ga)Se2 (CIGSe).This material is used to manufacture thin film solar cells. In industry, the growth of the absorber layer is carried out on low cost glass substrates. This results in polycrystalline films with a lot of grain boundaries which need to be passivated and understood as good as possible in order to further increase the power conversion efficiency of the solar cells.In contrast to most other reports in literature, we will grow CIGSe absorbers on GaAs wafers with one defined GB (i.e. on a bicrystal). This allows combining the results from the Hall measurements (which is a macroscopic technique) with SPM and APT measurements, which are local techniques. We will use scanning tunneling microscopy and spectroscopy to study the local density of states at the grain boundaries and we will use Kelvin Probe force microscopy to study work function changes at GBs. Moreover conductive atomic force microscopy will be applied to study current transport along the GBs. All measurements will be carried out in ultra-high vacuum and the samples will be transferred into the SPM chamber without prior air exposure. Hall effect measurements will be carried out to measure the temperature dependence of the conductivity and of the mobility across the GBs.The CIGSe samples will be grown on GaAs wafers in a metal organic vapor phase epitaxy system. We will grow pure CuInSe2 and Cu(In,Ga)Se2 absorbers and the growth will be carried out on ¿3 and random high angle GBs. We will study the GB properties prior and after a post deposition treatment (PDT) with alkali metals such as Na and F. The PDT treatment will be carried out in a molecular beam epitaxy system where NaF or KF will be thermally evaporated. In addition metallic Na or F will be evaporated in the SPM machine for which alkali metal dispensers will be used. The PDT treatments will be done without air exposure to increase reproducibility and to assure that SPM measurements are not falsified by surface contaminations. Finally, we will supplement the measurements with APT measurements carried out at the RWTH. This allows us to link the GB properties as measured with SPM and Hall to the grain boundary composition.The GRISC project will substantially improve the understanding of the GBs in CIGSe since we combine local measurements carried with SPM, transport measurements via Hall and compositional measurements via APT on a well defined GB. This combination makes the project unique and the outcomes will be of high importance for CIGSe solar cells and beyond.

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