Metastability of Zn1−xMgxO/Cu(In,Ga)Se2 solar cells with different conduction band offset values

The metastable changes due to the white light exposure on the current-density-voltage (J-V) characteristics of (Zn,Mg)O·(ZMO)/Cu(In,Ga)Se2·(CIGS) solar cells with the controlled conduction band offset (CBO) between window and CIGS layers were measured. This phenomenon, so called as the light soaking (LS) effect, was not observed when the CBO values were −0.36 and +0.03 eV; however, the LS effect was observed when the CBO value was +0.35 eV. Here, plus and minus signs of CBO indicate the conduction band minimums of the window layers above and below that of the CIGS layers, respectively. When the CBO value is positive, a notch is formed at the window/CIGS interface, which can act as a barrier for photo-generated electrons in the CIGS layer. The photo and dark J-V characteristics of the CIGS solar cell with the CBO value of +0.35 eV showed that the photo current mainly increased due to the exposure to white light. External quantum efficiency measurements showed that the effective barrier height reduced due to the exposure to white light. These results indicate that the CBO relates to the LS effect and one of the key factors of the LS effect is the barrier for the photo-generated electrons.