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    Absorption Induced by Mn Doping of ZnS for Improved Sensitized Quantum-Dot Solar Cells
    (2015-02-23) Horoz, Sabit; Dai, Qilin; Maloney, Scott; Yakami, Baichhabi; Pikal, Jon; Zhang, X.; Wang, J.; Wang, Wenyong; Tang, Jinke
    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.
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    CdSe quantum dots synthesized by laser ablation in water and their photovoltaic applications
    (Applied Physics Letters, 2012-11-27) Horoz, Sabit; Dai, Qilin; Chen, Jiajun; Yakami, Baichhabi; Pikal, Jon; Wang, Wenyong; Tang, Jinke
    CdSe quantum dots (QDs) have been prepared by a facile and clean synthesis method––laser ablation in water. The structural and luminescent properties of the CdSe QDs have been investigated. The CdSe QDs of wurtzite crystal structure have an average particle size of about 5 nm. The QDs can be attached to ZnO nanowires making them ideal for applications in QD-sensitized nanowire solar cells. A uniqueness of the QDs attached to the ZnO nanowires by this laser ablation method is that they do not contain ligands, and the preparation avoids the complicated process of ligand exchange.
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    Controlled synthesis of Eu2+ and Eu3+ doped ZnS quantum dots and their photovoltaic and magnetic properties
    (2016-04) Horoz, Sabit; Yakami, Baichhabi; Poudyal, Uma; Pikal, Jon; Wang, Wenyong; Tang, Jinke
    Eu-doped ZnS quantum dots (QDs) have been synthesized by wet-chemical method and found to form in zinc blende (cubic) structure. Both Eu2+ and Eu3+doped ZnS can be controllably synthesized. The Eu2+ doped ZnS QDs show broad photoluminescence emission peak around 512 nm, which is from the Eu2+ intra-ion transition of 4f6d1 – 4f7, while the Eu3+ doped samples exhibit narrow emission lines characteristic of transitions between the 4f levels. The investigation of the magnetic properties shows that the Eu3+ doped samples exhibit signs of ferromagnetism, on the other hand, Eu2+ doped samples are paramagnetic of Curie-Weiss type. The incident photon to electron conversion efficiency is increased with the Eu doping, which suggests the QD solar cell efficiency can be enhanced by Eu doping due to widened absorption windows. This is an attractive approach to utilize benign and environmentally friendly wide band gap ZnS QDs in solar cells.