Absorption Induced by Mn Doping of ZnS for Improved Sensitized Quantum-Dot Solar Cells

ZnS quantum dots (QDs) have limited application potential in QD-sensitized solar cells because of their wide-band-gap, which does not allow absorption of sunlight in the visible and infrared regions. Introducing intermediate-energy levels in the QDs is one way to expand the absorption window into the visible region. We show that this effect is achieved in Mn-doped ZnS QDs. Mn-doped ZnS QDs are synthesized by laser ablation in water and solution-based methods. The structural, optical, and magnetic properties of the ZnS:Mn QDs are examined by x-ray diffraction (XRD), transmission electron microscope (TEM), photoluminescence (PL) emission, photoluminescence excitation (PLE), and magnetic susceptibility measurements. The average particle size of cubic phase ZnS:Mn estimated from the XRD and TEM is about 3 nm. The QDs show two PL peaks near 450 and 600 nm, which are attributed to the defect-related emission of ZnS and emission of Mn2? in a ZnS host, respectively. The PLE spectra exhibit near-band-edge absorption of ZnS at 350 nm and the absorption of Mn2? internal-energy levels around 468 nm. The latter absorption is due to the transitions of the 3d5 electronic states of Mn2? from the ground state 6A1 to excited states 4A1 and 4E and plays an important role in improving the absorption of the material in the visible region. ZnS:Mn QDs coated on Zn2SnO4 nanowires show greatly improved sensitization in the visible region as demonstrated by incident photon-to-electron conversion efficiency experiments. Our study also shows that the characteristics of solar-cell performance can be tuned with the Mn concentration.This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-10ER46728