Bandgap Tunable Ternary Cd Sb_2- S_3-δ Nanocrystals for Solar Cell Applications

We report the synthesis and photovoltaic performance of a new nonstoichiometric ternary metal sulfide alloyed semiconductor-Cd Sb_2- S_3-δ nanocrystals prepared by the two-stage sequential ionic layer adsorption reaction technique. The synthesized Cd Sb_2- S_3-δ nanocrystals retain the orthorhombic structure of the host Sb₂S₃ with Cd substituting a fraction (x = 0-0.15) of the cationic element Sb. The Cd Sb_2- S_3-δ lattice expands relative to the host, Sb₂S_3, with its lattice constant a increasing linearly with Cd content x. Optical and external quantum efficiency (EQE) spectra revealed that the bandgap E _g of Cd Sb_2- S_3-δ decreased from 1.99 to 1.69 eV (i.e., 625-737 nm) as x increased from 0 to 0.15. Liquid-junction Cd Sb_2- S_3-δ quantum dot-sensitized solar cells were fabricated using the polyiodide electrolyte. The best cell yielded a power conversion efficiency (PCE) of 3.72% with the photovoltaic parameters of J _sc = 15.97 mA/cm², V _oc = 0.50 V, and FF = 46.6% under 1 sun. The PCE further increased to 4.86%, a respectable value for a new solar material, under a reduced light intensity of 10% sun. The PCE (4.86%) and J _sc (15.97 mA/cm²) are significantly larger than that (PCE = 1.8%, J _sc = 8.55 mA/cm²) of the Sb₂S₃ host. Electrochemical impedance spectroscopy showed that the ZnSe passivation coating increased the electron lifetime by three times. The EQE spectrum of Cd Sb_2- S_3-δ has a maximal EQE of 82% at λ = 350 nm and covers the spectral range of 300-750 nm, which is significantly broader than that (300-625 nm) of the Sb₂S₃ host. The EQE-integrated current density yields a J _ph of 11.76 mA/cm². The tunable bandgap and a respectable PCE near 5% suggest that Cd Sb_2- S_3-δ could be a potential candidate for a solar material.