Experimental investigation of Cu(In 1− x,Ga x)Se 2/Zn(O 1− z,S z) solar cell performance

Abstract

In this study we investigate the performance of Cu(In 1− x ,Ga x )Se 2/Zn(O 1− z ,S z ) solar cells by changing the gallium content of the absorber layer in steps from CuInSe 2 to CuGaSe 2 and at each step vary the sulfur content of the Zn(O,S) buffer layer. By incorporating more or less sulfur into the Zn(O,S) buffer layer it is possible to change its morphology and band gap energy. Surprisingly, the best solar cells with Zn(O,S) buffer layers in this study are found for close to or the same Zn(O,S) buffer layer composition for all absorber Ga compositions. In comparison to their CdS references the best solar cells with Zn(O,S) buffer layers have slightly lower open circuit voltage, V oc, lower fill factor, FF, and higher short circuit current density, J sc, which result in comparable or slightly lower conversion efficiencies. The exception to this trend is the CuGaSe 2 solar cells, where the best devices with Zn(O,S) have substantially lowered efficiency compared with the CdS reference, because of lower V oc, FF and J sc. X-ray photon spectroscopy and X-ray diffraction measurements show that the best Zn(O,S) buffer layers have similar properties independent of the Ga content. In addition, energy dispersive spectroscopy scans in a transmission electron microscope show evidence of lateral variations in the Zn(O,S) buffer layer composition at the absorber/buffer layer interface. Finally, a hypothesis based on the results of the buffer layer analysis is suggested in order to explain the solar cell parameters.