Nucleation of Cu(In, Ga)Se 2 on Molybdenum Substrates
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Nucleation of Cu(In,Ga)Se2 on Molybdenum Substrates T. Schlenker, K. Orgassa, H. W. Schock, and J. H. Werner Institute of Physical Electronics, University of Stuttgart Pfaffenwaldring 47, 70569 Stuttgart, Germany
ABSTRACT We investigate growth mechanisms of Cu(In,Ga)Se2 on Mo films on glass, as the typical back contact for Cu(In,Ga)Se2 solar cells. A thermal evaporation process deposits Cu(In,Ga)Se2 of nominal 3 nm thickness at different rates R and substrate temperatures TSub. An ultrahigh resolution scanning electron microscope serves to investigate the nucleation behavior. The deposited Cu(In,Ga)Se2 forms three-dimensional isolated nuclei, known as Volmer-Weber growth mode. Deposition rate R and substrate temperature TSub control the areal density n of the Cu(In,Ga)Se2 nuclei. We observe a power law dependence between the island density n and the deposition rate R, and an exponential dependence of the island density n on substrate temperature TSub. The theory of homogeneous nucleation explains the Cu(In,Ga)Se2 cluster formation on polycrystalline Mo and the dependence of the island density on the growth conditions.
INTRODUCTION Solar cells based on Cu(In,Ga)Se2 (CIGS) have the potential to become the leading thin-film photovoltaic devices. High efficiency CIGS solar cells are fabricated using Mo coated soda lime glass [1]. Molybdenum complies well with the manifold requirements for a back contact of CIGS solar cells. So far, the achievements in optimizing the absorber growth have been mainly directed by intuition. The understanding of CIGS film growth seems to be limited. In order to grow 'better' CIGS absorbers, a profound understanding of growth mechanisms is needed. Pilot studies at our institute show, that the performance of solar cells with 0.5 µm thin single-layer absorbers are strongly influenced by the deposition parameters during the initial growth stage of CIGS on Mo. During this phase, we vary the substrate temperature and the deposition rate and find that a high substrate temperature TSub and low deposition rate R yield cells with the best performance and the highest reproducibility. Thus, it is of particular importance to obtain a picture of the initial growth behavior of CIGS.
EXPERIMENTAL DETAILS An ultrahigh vacuum chamber with a base pressure of 2×10-8 mbar and 1×10-7 mbar during deposition serves to deposit CIGS by thermal co-evaporation of Cu, In, Ga and Se. A mass spectrometer allows one to measure the elemental fluxes individually. During the evaporation process, we keep the deposition rate of each element constant. The elemental rate ratios correspond to the ones for a high efficiency absorber layer grown by a single-layer process with constant deposition rates [2], and lead to a composition of Ga/(Ga+In) = 0.30 and
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Cu/(In+Ga) = 0.81. Polycrystalli
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