Electrical characterization of Cu2ZnSnSe4 solar cells from selenization of sputtered metal layers

We report on the electrical and physical properties of Cu ₂ZnSnSe₄ (CZTSe) solar cells consisting of an absorber layer fabricated by selenization of sputtered Cu, Zn, Sn multilayers. Cross-section scanning electron microscopy images show that the polycrystalline absorber layers are approximately 1 μm thick and that the typical grain size is of the order of 1 μm. Energy-dispersive X-ray spectroscopy measurements show Cu-poor and Zn-rich compositions with Cu/(Zn + Sn) ~ 0.8 and Zn/Sn ~ 1.2. Solar cells are fabricated out of this absorber material using a standard process flow for chalcogenide solar cells. Under AM1.5 G illumination, the best 1 × 1 cm² CZTSe solar cell shows an efficiency of 6.3% with a maximum short circuit current of 31.3 mA/cm², an open circuit voltage of 0.39 V and a fill factor of 52%. Doping density of the absorber layers is derived using the drivel level capacitance profiling (DLCP) technique, showing low p-type doping density which seems to increase exponentially with the Zn/Sn ratio. Comparing the values obtained from DLCP to the ones derived from Mott-Schottky plots of the same devices, it is shown that for CZTSe care has to be taken when deriving the doping density. Similar to copper indium gallium selenide junctions, Mott-Schottky plots overestimate the amount of free carriers in the buffer due to the presence of fast defect states inside the bandgap.