A 5.5% efficient co-electrodeposited ZnO/CdS/Cu2ZnSnS4/Mo thin film solar cell

•Cu–Zn–Sn–S mixed layer is co-electrodeposited in single step.•Sn poor and bi-layered Cu2ZnSnS4 absorber is formed after sulfurization.•Lattice vibrational modes of kesterite Cu2ZnSnS4 are indicated via polarized Raman and FTIR spectra.•Best efficiency reaches 5.5% for CdS/Cu2ZnSnS4 hetero-junction solar cells.•Low electrical barrier (44 meV) across the cell is demonstrated by dark J–V.

Thin film solar cells with a structure of ZnO/CdS/Cu2ZnSnS4 (CZTS)/Mo were fabricated successfully by sulfurization of co-electrodeposited Cu–Zn–Sn–S precursors at 590 °C for 15 min. The best solar cell performance was achieved with an efficiency of 5.5% (VOC=673.8 mV, JSC=18.7 mA cm−2, FF=44%). The vibrational properties and phase identification of the absorber were studied by the polarised Raman and IR spectra. The CZTS absorber layer shows a bi-layered structure comprising of a well-crystallized photovoltaic layer with metallic ratios of Cu/Zn=1.95 and Zn/Sn=1.56 and a particulate-like bottom layer with a heavily Sn poor content. These particulates in the bottom CZTS layer feature with a high density of stacking faults along with ZnS phases and contribute to the low minority lifetime, the high recombination losses, and the increased series resistances in the device. In the well-crystallized CZTS absorber layer the trace of a twin defect on {1 1 2} plane is discovered. Dark electrical analysis indicates there is a relatively small energy barrier height of 44 meV across the cell.

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