Effect of Zn and Mg doping on CuInS thin films and solar cells
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F11.3.1
Effect of Zn and Mg doping on CuInS thin 2 films and solar cells Tobias Enzenhofer, Thomas Unold, Roland Scheer and Hans-Werner Schock Hahn-Meitner-Institut, Glienicker Strasse 100, 14109-Berlin, Germany ABSTRACT Solar cells based on CuInS2 still suffer from Voc limitation compared to the value expected from the bandgap. The open circuit voltage can be enhanced via controlled doping of small amounts of Zinc or Magnesium (< 1 at. %), however the underlying physical reason for this improvement is not understood so far. We have systematically studied the effect of diffusion of Zn and Mg into the absorber layers with photoluminescence and quantum efficiency measurements. ZnO (ZnMgO, MgF2) has been deposited onto the absorber layers and photoluminescence (PL) spectra at room temperature were recorded as a function of the annealing temperature. The modified emission spectra reveal a defect related transition with a strong luminescence signal. At the same time the open-circuit voltage of the Zn (Mg) doped CuInS2 cells are around 100 mV higher than the values found for the undoped cells from the same device process. We interpret these results as a reduced defect state density at the surface and an increased bulk defect concentration induced by the incorporation of Zn or Mg. Comparison of external quantum efficiency (EQE) and I/U measurements with the PL results support observed changes of the interface/bulk properties. INTRODUCTION The chalcopyrite System Cu-In-S is known as a very promising material for the development of high-efficiency thin-film solar cells [1]. CuInS2 solar cells with their bandgap of 1.5 eV offer the opportunity for high open circuit voltages above 1000 mV [2]. Braunger et al. have reported that the incorporation of small amounts of Zn (Zn/Cu=2%) via a CBD process leads to an increased open circuit voltage [3]. They assumed a change in the dominant recombination mechanism from interface to space charge recombination. We introduce Zn and Mg dopants in our sequential preparation process for investigation of the VOC limiting factors in CuInS2 thin film solar cells. From photoluminescence measurements we found that the enhancement of the open circuit voltage via doping is accompanied by a change in the PL spectrum and a strong increase in the Pl signal whereas no difference occurs for the dopants we used (Mg or Zn). From optical transmission and reflection measurements we assume for the significant increase of the open circuit voltage no bandgap widening. This result is in agreement with investigations of the Zn2-2xCuxInxS2 alloy system [4]. External quantum efficiency measurements show an overall reduction in spectral response which is in contrast to the enhanced VOC. Furthermore the rectangular shape of the EQE of CuInS2 solar cells exhibits lower red response and a not negligible response below the specified bandgap is found.
F11.3.2
In order to elucidate this contradictive behaviour we tried to find a correlation between doping of CuInS2 absorber layers, device properties and photoluminesce
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