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 were synthesized by laser ablation in water and solution-based method. The structural, optical, and magnetic properties of the ZnS:Mn QDs have been examined by x-ray diffraction (XRD), transmission electron microscope (TEM), photoluminescence emission (PL) and 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 nm and 600 nm, which are attributed to the defect related emission of ZnS and emission of Mn 2+ in a ZnS host, respectively. The PLE spectra exhibit near band edge absorption of ZnS at 350 nm and the absorption of Mn 2+ internal energy levels around 468 nm. The latter is due to the transitions of the 3d 5 electronic states of 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. ZnS:Mn QDs coated on Zn 2 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 the solar cells performance can be tuned with the Mn concentration.

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Optimization of growth conditions of type-II Zn(Cd)Te/ZnCdSe submonolayer quantum dot superlattices for intermediate band solar cells J. Vac. Sci. Technol. B 31, 03C119 (2013); 10.1116/1.4797486 Effect of metal oxide morphology on electron injection from CdSe quantum dots to ZnO

ABSTRACT:Nanowires are a promising option for sensitized solar cells, sensors, and display technology. Most of the work thus far has focused on binary oxides for these nanowires, but ternary oxides have advantages in additional control of optical and electronic properties. Here we report on the diffuse reflectance, Low Temperature (LT) and Room Temperature (RT) Photoluminescence (PL), PL Excitation (PLE) spectrum, and Time Resolved PL (TRPL) of Zinc Tin Oxide (ZTO) nanowires grown by Chemical Vapor Deposition. The PL from the ZTO nanowires does not exhibit any band gap or near gap emission, and diffuse reflectance measurement confirms that these ZTO nanowires have a direct forbidden transition.The broad PL spectrum reveals two Gaussian peaks centered at 1.86 eV (red) and 2.81 eV (blue), representing two distinct defect states or complexes. The PL spectra was further studied by Time Resolved Emission Spectrum (TRES) and intensity dependent PL and TRPL. The time resolved measurements show complex non-exponential decays at all wavelengths, indicative of defect to defect transitions, and the red emissive states decay much slower than the blue emissive states. The effects of annealing in air and vacuum are studied to investigate the origin of the defect states in the nanowires showing that the blue 2 states are related to oxygen vacancies. We propose an energy band model for the nanowires containing defect states within the band gap and the associated transitions between these states that are consistent with our measurements.

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 cell technology.