Defect distributions in Cu(In,Ga)(S,Se)2 solar cells with three different buffer layers

Defect levels and interface states in ZnO:B/buffer/Cu(In,Ga)(S,Se)2/Mo/glass solar cells with a Zn(S,O)/CdS, CdS, or Zn(S,O) buffer layer were investigated by capacitance–voltage (C–V) and admittance spectroscopy. Two acceptor levels, A1 (with Ea,A1 = 0.09±0.01 eV activation energy) and A2 (Ea,A2 = 0.26±0.02 eV) were observed from the temperature dependence of the C–V profiles. Spatial distributions of these acceptors were determined by the bias dependence of the profiles. In addition, two kinds of defect signals, S1 (Ea,S1=0.06 eV and 0.07 eV) and S2 (Ea,S2=0.25 eV), were detected from admittance spectroscopy. Each solar cell exhibited only one signal: S1 in Zn(S,O)/CdS- or Zn(S,O)-buffered solar cells, and S2 in solar cell with a CdS buffer layer. These results demonstrate that buffer layers influence the spatial distributions of intrinsic defect states in the region of the Cu(In,Ga) (S,Se)2 absorber close to the hetero-interface.