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 3d(5) electronic states of Mn2+ from the ground state (6)A(1) to excited states (4)A(1) and E-4 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.