Effect of Zn and Mg doping on CuInS thin films and solar cells

Abstract

Solar cells based on CuInS₂ still suffer from V_oc 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.