ABSTRACT
The CaTiO3:Er3+/Yb3+ upconversion phosphor was synthesized using a simplified co-precipitation method and the effect of Mg2+ ion co-doping was investigated on the structural and optical properties focusing on the near-infrared (NIR)-to-visible upconversion. The introduction of Mg2+ ions into the host lattice produced substantial changes in the crystal structure, grain size, and absorption, thus leading to the enhancement in upconversion emission intensities. X-ray diffraction (XRD) analysis indicated the formation of polycrystalline CaTiO3-Ca4Ti3O10 composite crystals and an increase in the crystallite size was observed upon increasing the Mg2+ ion concentration in the samples. Elemental analysis by energy dispersive spectroscopy (EDS) suggested the substitution of Ca2+ ions by Mg2+ ions in the CaTiO3 host lattice. Moreover, a change in the Yb3+/Er3+ ratio from 0.25 to 1.1 indicated the redistribution of the Er3+ or Yb3+ ions caused by the Mg2+ ions. These lattice deformations further resulted in an improved absorption of Er3+ ions, exhibiting a â¼3-fold enhancement in the upconversion emission intensity (at the excitation intensity of â¼1 W cm-2).
ABSTRACT
High purity Zn2SiO4:Mn crystals were synthesized by impregnating a precursor solution into mesoporous silica followed by sintering process. The effects of doping alkali metal ions (Li+, Na+, K+) on the structural, morphological and photoluminescence properties were investigated. Formation of single phase α-Zn2SiO4:Mn crystals was confirmed from X-ray diffraction. The crystal size was significantly decreased from 54 nm to 35 nm with increasing molar concentration of alkali metal ion dopants in Zn2SiO4:Mn. Zn2SiO4:Mn crystals co-doped with alkali metal ions showed stronger emission and faster decay times compared to the un-doped Zn2SiO4:Mn phosphor. The highest emission quantum yields (EQEs) of 68.3% at λ exc 254 and 3.8% at λ exc 425 nm were obtained for the K+ ion doped samples with Mn2+ : K+ ratio of â¼1 : 1. With alkali metal ions (Li+, Na+, K+) co-doping, the decay time of Zn2SiO4:Mn crystals was shortened to â¼4 ms, whereas the emission intensity was elevated, with respect to un-doped Zn2SiO4:Mn crystals. Zn2SiO4:Mn crystal growth in silica pores together with selective doping with alkali metal ions paves a way forward to shorten the phosphor response time, without compromising emission efficiency.
ABSTRACT
The effect of nickel concentration has been investigated in ZnO/NiO nanocomposites synthesized using the co-precipitation method. The X-ray diffraction and TEM measurements confirm the distinct phase of NiO in the ZnO/NiO samples. Furthermore, the Raman study shows the sharp modes at 99 cm-1 and 438 cm-1 corresponding to E(low) 2, E(high) 2 of hexagonal wurtzite ZnO structure and, 1080 cm-1 associated to the two-phonon (2P) mode of NiO, respectively. We also compared the effect of Ni concentration on the formation of ZnO/NiO by analyzing Ehigh 2 Raman mode of ZnO with the help of spatial correlation model. The correlation lengths, broadening and asymmetry ratio obtained from the fitting showed good agreement with the experimental results.
ABSTRACT
Ion migration has been suspected as the origin of various irreproducible and unstable properties, most notably the hysteresis, of lead halide perovskite photovoltaic (PV) cells since the early stage of the research. Although many evidence of ionic movement have been presented both numerically and experimentally, a coherent and quantitative picture that accounts for the observed irreproducible phenomena is still lacking. At the same time, however, it has been noticed that in certain types of PV cells, the hysteresis is absent or at least within the measurement reproducibility. We have previously shown that the electronic properties of hysteresis-free cells are well represented in terms of the conventional inorganic semiconductors. The reproducibility of these measurements was confirmed typically within tens of minutes under the biasing field of -1 V to +1.5 V. In order to probe the effect of ionic motion in the hysteresis-free cells, we extended the time scale and the biasing rage in the electronic measurements, from which we conclude the following: (1) From various evidence, it appears that ion migration is inevitable. However, it does not cause detrimental effects to the PV operation. (2) We propose, based on the quantitative characterization, that the degradation is more likely due to the chemical change at the interfaces between the carrier selective layers and perovskite rather than the compositional change of the lead iodide perovskite bulk. Together, they give much hope in the use of the lead iodide perovskite in the use of actual application.
ABSTRACT
Low-temperature solution-processed perovskite solar cells are attracting immense interest due to their ease of fabrication and potential for mass production on flexible substrates. However, the unfavorable surface properties of planar substrates often lead to large variations in perovskite crystal size and weak charge extractions at interfaces, resulting in inferior performance. Here, we report the improved performance, reproducibility, and high stability of "p-i-n" planar heterojunction perovskite solar cells. The key fabrication process is the addition of the amine-polymer poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-P1) to a simple spin-coating process. The PFN-P1 works as a surfactant and helps promote uniform crystallization. As a result, perovskite films with PFN-P1 have a uniform distribution of grain sizes and improved open circuit voltage. Devices with PFN-P1 showed the best efficiency (13.2%), with a small standard deviation (0.40), out of 60 cells. Moreover, â¼90% of the initial efficiency was retained over more than 6 months. Additionally, devices fabricated from PFN-P1 mixed perovskite films showed higher stability under continuous operation at maximum power point over 150 h. Our results show that this approach is simple and effective for improving device performance, reproducibility, and stability by modifying perovskite properties with PFN-P1. Because of the simplicity of the fabrication process and reliable performance increase, this approach marks important progress in low-temperature solution-processed perovskite solar cells.
ABSTRACT
The lead halide perovskite photovoltaic cells, especially the iodide compound CH3NH3PbI3 family, exhibited enormous progress in the energy conversion efficiency in the past few years. Although the first attempt to use the perovskite was as a sensitizer in a dye-sensitized solar cell, it has been recognized at the early stage of the development that the working of the perovskite photovoltaics is akin to that of the inorganic thin film solar cells. In fact, theoretically perovskite is always treated as an ordinary direct band gap semiconductor and hence the perovskite photovoltaics as a p-i-n diode. Despite this recognition, research effort along this line of thought is still in pieces and incomplete. Different measurements have been applied to different types of devices (different not only in the materials but also in the cell structures), making it difficult to have a coherent picture. To make the situation worse, the perovskite photovoltaics have been plagued by the irreproducible optoelectronic properties, most notably the sweep direction dependent current-voltage relationship, the hysteresis problem. Under such circumstances, it is naturally very difficult to analyze the data. Therefore, we set out to make hysteresis-free samples and apply time-tested models and numerical tools developed in the field of inorganic semiconductors. A series of electrical measurements have been performed on one type of CH3NH3PbI3 photovoltaic cells, in which a special attention was paid to ensure that their electronic reproducibility was better than the fitting error in the numerical analysis. The data can be quantitatively explained in terms of the established models of inorganic semiconductors: current/voltage relationship can be very well described by a two-diode model, while impedance spectroscopy revealed the presence of a thick intrinsic layer with the help of a numerical solver, SCAPS, developed for thin film solar cell analysis. These results point to that CH3NH3PbI3 is an ideal intrinsic semiconductor, which happens to be very robust against accidental doping, and that the perovskite photovoltaic cell is in fact a model p-i-n diode. The analytical methods and diagnostic tools available in the inorganic semiconductor PV cells are useful and should be fully exploited in the effort of improving the efficiency. One outstanding question is why the perovskite stays intrinsic. Considering the defects and impurities that must abound in the perovskite layers formed by the spin-coating process, for example, there must be physicochemical mechanism keeping it from being doped. This may be related to the special band structure making up the band gap in this ionic solid. Understanding the mechanism may open a door for the wider utility of this class of solid.
ABSTRACT
The controlled growth of oriented ZnO nanostructures is desirable for their varied applications. In this study, an electrochemical technique is used for deposition of ZnO. The large area morphology shows sheet-like structures oriented perpendicular to the substrate. X-ray diffraction studies show that the crystallinity and purity of electrodeposited ZnO improved by using a moderate thermal annealing. TEM shows randomly oriented rod-like structures. The Raman spectra further confirm the crystallinity of the films. The photoluminescence spectra show emission peaks in the blue-green visible region. Furthermore, we also investigate the potential of the electrochemically-formed ZnO nanostructures as a suitable template for electrochemical deposition of polypyrrole (Ppy).
