ICAMS / Interdisciplinary Centre for Advanced Materials Simulation
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Role of the internal interfaces in solar cells

Date: 05.11.2013
Time: 09:40 a.m.
Place: Materials Day 2013, Ruhr-Universität Bochum, Bochum, Germany

Oana Cojocaru-Mirédin, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Torsten Schwarz, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Andreas Stoffers, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany
Daniel Abou-Ras, Helmholtz-Zentrum Berlin, Berlin, Germany
Roland Würz, Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg, Stuttgart, Germany
Christian-Herbert Fischer, Helmholtz-Zentrum Berlin, Berlin, Germany
Dierk Raabe, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany

Cu(In,Ga)Se2 (CIGS), Cu2ZnSnSe4 (CZTSe), and multicrystalline Si (mc-Si) solar cells possess a high efficiency, despite the polycrystalline structure of the absorber layer. One of the major factors controlling the cell efficiency is the diffusion of the impurities during the fabrication process into the absorber layer and to the p-n junction. However, the interaction between the defects and the impurities at the internal interfaces is not completely understood. This is due to a lack of information on the local chemical changes across the internal interfaces at the nanoscale.

In this work, the internal interfaces (p-n junction and grain boundaries) of different thin-film solar cells (CIGS and CZTSe), but also mc-Si solar cells will be presented. Electrical properties, structural properties, but also the chemical composition are the key parameters which need to be controlled in order to achieve high-efficiency solar cells, knowing that the internal interfaces are the one responsible for recombination processes in the material. The chemical composition at atomic scale was investigated by means of atom probe tomography (APT). The APT experimental findings for thin-film solar cells show that the absorber surface (first 2-3 atomic monolayers) is generally Cu-depleted and Cd-enriched. This observation is a strong indication for the existence of an inverted p-n junction within the first layers of the CIGS absorber layer [1-3]. This configuration is very stable (Fermi level pinning at the conduction band) and reduces the recombination rate at the p-n junction, leading thus to high-efficient solar cells.

Furthermore, the grain boundaries (GBs) are also responsible for the recombination processes in the solar cells. Indeed the alkali impurities decorate the GBs in CIGS and CZTSe solar cells [4-5], whereas in mc-Si solar cells the GBs are decorated by transition metal impurities [6]. The present results are compared with the existing “electronic” GBs models. The aim is to understand the correlation between impurities and point defects at the internal interfaces of CIGS, CZTSe, and mc-Si solar cells and possible consequences for the cell efficiency.

References :
[1] O. Cojocaru-Mirédin et al., Appl Phys. Lett. 98 (2011) 103504.
[2] O. Cojocaru-Mirédin et al., Appl Phys. Lett. 101 (2012) 181603.
[3] P. Choi et al., Surf. Interf. Anal. 44 (2012) 1386-1388.
[4] O. Cojocaru-Mirédin et al., J. of Photovoltaics. 1 (2011) 207-212.
[5] T. Schwarz et al., Appl. Phys. Lett. 102 (2013) 042101.
[6] A. Stoffers et al. “Grain Boundary segregation in m-Si solar cells: a correlative EBSD-APT-EBIC study” 2013, in preparation.

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