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

We report on the electrical and physical properties of Cu2ZnSnSe4 (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 mu m thick and that the typical grain size is of the order of 1 mu m. Energy-dispersive X-ray spectroscopy measurements show Cu-poor and Zn-rich compositions with Cu/(Zn + Sn) similar to 0.8 and Zn/Sn similar to 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 x 1 cm(2) CZTSe solar cell shows an efficiency of 6.3% with a maximum short circuit current of 31.3 mA/cm(2), 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. (C) 2012 Elsevier B.V. All rights reserved.