Enhanced dual-wavelength sensitive red upconversion luminescence in Bi2O3:Yb3+/Er3+ phosphors via optical-inert ions doping
Graphical abstract
Enhance the UC efficiency by simultaneous dual-wavelength (980 and 1550 nm) excitation and single-, co- and tri-doping Na+, Ca2+ and/or Gd3+ ions.
Introduction
Lanthanide (Ln) doped UC materials have aroused extensive attention owing to their potential applications in fields such as displays [[1], [2], [3]], biological imaging [[4], [5], [6]], biodetection [[7], [8], [9]] and efficiency enhancement of solar cells [[10], [11], [12]], etc. In generally acknowledgment, dye-sensitized solar cells have a high absorption in the visible range (400–700 nm), and their wide bandgap (1.7–1.8 eV) implies that near infrared (NIR) solar radiation would be lost because of transmission [10,13]. Therefore, enlarging the solar spectrum response is a potential strategy to increase the photovoltaic efficiency of solar cells. It is worthwhile pointing out that UC materials for enhancing the photovoltaic efficiency of solar cells, are currently attracting extensive interest [[14], [15], [16], [17], [18], [19]].
Actually, Er3+ is regarded as one of the most efficient ion for UCL, owing to its abundant energy cascades of 4f−4f transitions and larger NIR absorption cross section. Besides being excited under common wavelength such as 980 nm, the Er3+ ion can also be excited at 1550 nm, because the metastable 4I13/2 level of Er3+ has a long lifetime [[20], [21], [22], [23]]. Therefore, it is essential to study the excellent UCL performance of Er3+ ions induced by excitation under 1550 nm in order to enlarge the solar spectrum response. Recently, some works on the optical characteristics of Er3+ ions and Er3+–Yb3+ ion pairs under 1550 nm excitation have sought to improve the efficiency of solar cells [[24], [25], [26], [27], [28], [29]]. Lu et al. [27] reported red and green bi-colour UC emissions in hexagonal Y2O2S:Yb3+/Er3+ nanocrystals under 1550 nm excitation. Fu's group [28] have investigated that Ba5Zn4Y8O21:Er3+/Yb3+ phosphor exhibits extremely weak green emission and a considerably strong red emission with good color purity and stability under 1550 nm excitation. Arnaoutakis et al. [29] reported that by addition of NaYF4:Er3+ UC phosphors, the peak external quantum efficiency was 2.5% in a bifacial silicon solar cell under excitation at 1523 nm. Nevertheless, the nature of sunlight might be overlooked, that it exhibits a continuous and polychromatic spectrum. Sunlight pumping can be considered as simultaneous multiwavelength excitation. More importantly, investigations for UC materials under simultaneous multiwavelength NIR excitation, that too with broadband NIR sunlight pumping, are much more meaningful for practical systems. Regretfully, as far as we know only a few works on UC with a broadband spectral response were undertaken until now [[30], [31], [32], [33], [34], [35], [36], [37], [38]]. For example, Qiu's group [30,32,33,36], Jia's group [31], Zhang's group [34] and Chen's group [37] have proposed and demonstrated an approach to improve the UC efficiency by multi-wavelength excitation in Er3+, Tm3+ or Ho3+ single-doped fluoride and halide systems (LaF3, NaYF4, BaCl2 and so on). Afterwards, this research was expanded to Yb3+/Er3+ co-doped CaF2 and Yb3+/Tm3+ co-doped NaYF4 materials by Zhang et al. [35] and Jia's group [38]. They illustrated that the multiwavelength simultaneously excited up-conversion process allowed better and broader harvesting of near-infrared solar energy, which was expected to open the possibilities of the remarkable improvement of the power conversion efficiency of next-generation solar cells. However, simultaneous multiwavelength excitation UCL systems are not only limited to fluoride and a few halide systems but also are still scarce. Therefore, it is essential to carry out novel work about multiwavelength response materials for possible potential application in solar cells.
In general, the UC efficiency could be enhanced by reducing the possibility of non-radiative transition process using a host lattice material with low phonon energy, such as fluoride and sulfide matrices [[39], [40], [41]]. However, in view of poor physical and chemical stabilities, high cost, the applications of rare earth ions doped fluoride and sulfide materials are restricted even though they exhibit high UC efficiencies. On the contrary, highly efficient oxide and complex oxide UC materials have been extensively studied by reason of good chemical durability and thermal stability [42,43]. While most of the oxides host materials are the rare earth oxide [[44], [45], [46]], much higher costs are required to obtain highly pure rare earth materials compared to the main group elements like bismuth, which could be easily purified in large quantities by techniques like zone refining. Especially, Bismuth is an ecofriendly strategic metal because of nontoxic and harmless properties, which is called “Green bismuth”. Importantly, bismuth is low cost, which makes it hold great appeal in many areas, besides, bismuth is the only heavy metal that is nontoxic and thermodynamic stable at high temperature [47,48]. Thus, Bi2O3 may also be a more suitable UC host material. Zhang et al. have researched Bi2O3:Yb,Er/Ho/Tm UC nanoparticles which exhibit excellent UCL and high contrast image in biological imaging under 980 nm excitation [49,50]. To further improve the UC emission intensity, optical-inert ions that can be used to enhance the UC efficiency by altering electronic transition probabilities were also introduced to this system in many ways [[51], [52], [53], [54], [55], [56], [57], [58]]. However, there is rare research about the effect of combination optical inert metal ions doping and lack of knowledge on the intercommunication between codopants on the UCL properties to the best of our knowledge.
Based on the above consideration, in this work, the Bi2O3:Er3+ and Bi2O3:Yb3+, Er3+ phosphors induced by simultaneous dual-wavelength excitation under 980 and 1550 nm were studied in order to enlarge the solar spectrum response. Meanwhile, we achieve this improvement by substituting or co-substituting Na+, Ca2+ and Gd3+ for Bi3+ in the host lattice. Largescale structure of the host remains unchanged, but the nearest neighbor environment of any given Yb3+ or Er3+ ion becomes distorted. In this fashion, we are able to increase transition dipole moments without deleteriously affecting the global lattice.
Section snippets
Materials
Bi(NO3)3·5H2O (AR grade), Yb2O3 (99.99%), Er2O3 (99.99%) and Gd2O3 (99.99%) were purchased from Aladdin Chemical Reagent Factory (China). NH4HCO3 (AR grade), NaCl (AR grade), CaCl2 (AR grade) and ethanol were supplied by Tianjin Kemiou Chemical Reagent Co., Ltd. China. They were used as starting materials without further purification. Deionized water was used as solvent throughout the experiment.
All the rare earth nitrates were obtained by dissolving the corresponding rare earth oxides in
UCL properties under 980 nm, 1550 nm or simultaneous dual wavelength (980 and 1550 nm) excitation
The crystal phase of as-prepared Bi2O3:15% Yb3+, 2% Er3+ and Bi2O3:2% Er3+ samples were investigated by XRD, as shown in Fig. 1. The phase of the Bi2O3:2% Er3+ sample matched well with the standard tetragonal Bi2O3 (JCPDS card no. 27-0050). However, the XRD pattern of the Bi2O3:15% Yb3+, 2% Er3+ sample was in good agreement with the standard pattern of cubic Bi2O3 (JCPDS card no. 52-1007), and no significant impurity peaks were observed, indicating that Yb3+ and Er3+ ions have embedded into the
Conclusion
Higher doping concentration of Yb3+/Er3+ is beneficial to form cubic phase of Bi2O3 with nanometer or micron blocky particles with uneven size. By contrast, lower doping concentration of Yb3+/Er3+ is beneficial to form tetragonal phase of Bi2O3. Compared to the sum of emission intensities excited by two single-wavelength (980 nm and 1550 nm) separately, the red emission from Bi2O3:Yb3+/Er3+ sample excited by two-wavelength simultaneously is enhanced by a factor of 2.41. Furthermore, compared to
Acknowledgements
The authors gratefully acknowledge the China Postdoctoral Science Foundation (2016M592308), Chinese Academy of Sciences (Grants XDA09030203).
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