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 $\mathrm{ZnS}:\mathrm{Mn}$ 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 $\mathrm{ZnS}:\mathrm{Mn}$ 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 ${\mathrm{Mn}}^{2+}$ in a ZnS host, respectively. The PLE spectra exhibit near-band-edge absorption of ZnS at 350 nm and the absorption of ${\mathrm{Mn}}^{2+}$ internal-energy levels around 468 nm. The latter absorption is due to the transitions of the $3d^5$ electronic states of ${\mathrm{Mn}}^{2+}$ from the ground state ${}^6{A}_1$ to excited states ${}^4{A}_1$ and ${}^{4}E$ and plays an important role in improving the absorption of the material in the visible region. $\mathrm{ZnS}:\mathrm{Mn}$ QDs coated on ${\mathrm{Zn}}_2{\mathrm{SnO}}_4$ 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.

• Received 10 July 2014

DOI:https://doi.org/10.1103/PhysRevApplied.3.024011