ORIGIN OF THE BENEFICIAL EFFECT OF CAESIUM IN HIGHLY EFFICIENT CIGS SOLAR CELLS
Record efficiency Cu(In,Ga)Se2 solar cells are today obtained upon incorporation of caesium. The beneficial effect of caesium was revealed using a combinatory approach of synchrotron-based and electron microscopy techniques, as well as ab-initio calculations. Caesium accumulates at specific grain boundaries, where it passivates defects, thus reducing the recombination of carriers.
Solar cells based on Cu(In,Ga)Se2 (CIGS) absorbers have recently reached record conversion efficiencies of over 23 % . Such record performance of CIGS solar cells is currently only attainable by the incorporation of heavy alkali metals, like Cs, into the absorber through an alkali fluoride post-deposition treatment (PDT). In order to reveal how the incorporated alkali metals cause beneficial effects, their spatial distribution must be determined with an ultimate resolution, and the concurrent impact on the electrical performance on the CIGS absorber has to be evaluated.
Solar cells without and with a CsF-PDT were produced within the same CIGS deposition run, and the beneficial effect of the treatment was clearly confirmed by electrical measurements. Subsequently, thin cross-sectional lamellas were prepared out of the solar cells using a focused ion beam. These lamellas were analysed via high-resolution X-ray fluorescence analysis (nano-XRF) at the ID16B nano-analysis beamline with a spot size of approximately
50 x 50 nm2. Different electron microscopy techniques were performed on the same lamella, correlating the distribution of caesium and the CIGS elements with the microstructure.
A scanning transmission electron microscopy (STEM) image in partial high-angle annular dark-field (pHAADF) geometry acquired from the lamella extracted from the CIGS solar cell, which was exposed to the CsF-PDT, is presented in Figure 71a. It clearly reveals the polycrystalline CIGS absorber including vertical as well as horizontal grain boundaries. The nature of the grain boundaries was investigated by means of electron backscatter diffraction. Figure 71b shows the resulting inverse pole figure map in which the orientation of each grain is given by a certain colour. Comparing the orientation of neighbouring grains yields the type of grain boundary formed by these adjacent grains. Highly symmetrical and electrically benign S3 twin boundaries are highlighted. All other grain boundaries are random grain boundaries, which are known to be detrimental to the solar cell performance.
Fig. 71: Cu(In,Ga)Se2 microstructure, Cs distribution measured with nano-XRF and respective ab-initio calculations. a) Electron microscopy image of the Cu(In,Ga)Se2 absorber revealing different micrometre-sized grains. b) Colour-coded map of the grain orientation. S3 twin boundaries are marked by white lines. c) Cs intensity map, showing accumulation of Cs at random grain boundaries (arrows) rather than at S3 twin boundaries and at the surface of the absorber. d) Atomic structure of a Cs-decorated S3(114) grain boundary together with the respective density of states.