Abstract:
Efficiency of earth abundant and pure sulfide kesterite Cu-Zn-Sn-S (CZTS) solar cell has been stagnant around 9.4% for years, while its
counterpart Cu-In-Ga-Se (CIGS) reports an efficiency of more than 22%. Low open circuit voltage (VOC) is the major challenging factor for
low efficiency due to severe nonradiative interface recombinations. The existence of higher defect states at the conventional CZTS-CdS interface due to undesirable energy level alignment and lattice misfit promotes trap-assisted recombinations and results in low VOC. In this work,
amorphous TiO2 (Eg ¼ 3:8 eV) is proposed as a promising substitute to the conventional and low bandgap CdS (Eg ¼ 2:4 eV) layer. The
surface and interface of the CZTS-TiO2 layer were investigated using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron
spectroscopy (UPS). The result reveals favorable “spike”-like conformations at the CZTS-TiO2 interface with a conduction band offset value
of 0.17 eV. The nanoscale probing of the interface by Kelvin probe force microscopy across CZTS-TiO2 layers shows a higher potential
barrier for interface recombination at CZTS-TiO2 in contrast to the conventional CZTS-CdS interface. Finally, the fast decay response and
lower persistent photoconductivity of photogenerated carriers for CZTS-TiO2 heterojunction based photodetectors further validate our
results. The energy level alignment and nanoscale interface studies signify TiO2 as a promising alternate buffer layer for earth abundant
CZTS solar cells.
