Analysis of High-Voc Single-Crystal CZTSe Solar Cells via Admittance Spectroscopy

Solar cells fabricated on single-crystalline Cu
₂
ZnSnSe
₄
exhibiting V
_OC
's as high as 428 mV are presented. Two devices with varied Cu and Zn content are characterized to determine the factors contributing to the improved V
_OC
in the more Cu-depleted specimen. A trade-off between increased voltages and decreased FF is observed, likely due to a change in carrier density and thus series resistance. External quantum efficiency shows a dramatic improvement in the absorption edge and a 20 meV increase in extracted bandgap for the higher V
_OC
device Finally, admittance spectroscopy is used to probe differences in the defect landscape between the devices. Two metastable states are induced via light and voltage biasing prior to admittance measurements. The improved V
_OC
is accompanied by an admittance activation energy near 200 meV likely due to interface defects and another near 160 meV with unclear origins. An additional lower energy artifact is observed in both devices and attributed to a Cu
_Zn
antisite defect.