Energy level alignment and nanoscale investigation of a-TiO₂/Cu-Zn-Sn-S interface for alternative electron transport layer in earth abundant Cu-Zn-Sn-S solar cells

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 (V_OC) 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 V_OC. In this work, amorphous TiO₂ (E g = 3.8 eV) is proposed as a promising substitute to the conventional and low bandgap CdS (E g = 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.