Upconversion Luminescence from Ln3+(Ho3+

Apr 16, 2018 - Ho3+ ions in 5F4, 5S2 states first relax to 5F5 state and then to the ground state 5I8, giving the red emissions. Another process is th...
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Article Cite This: J. Phys. Chem. C 2018, 122, 9606−9610

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Upconversion Luminescence from Ln3+(Ho3+,Pr3+) Ion-Doped BaCl2 Particles via NIR Light of Sun Excitation Hong Jia,*,†,‡ Zhongli Liu,† Lamei Liao,† Yanhong Gu,† Chaoliang Ding,† Jianguo Zhao,† Weiying Zhang,† Xiaoke Hu,† Xun Feng,† Zhi Chen,*,§ Xiaofeng Liu,‡ and Jianrong Qiu*,‡,§ †

College of Physics and Electronic Information, Henan Key Laboratory of Electromagnetic Transformation and Detection, Luoyang Normal University, Luoyang 471934, China ‡ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China § State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510641, China S Supporting Information *

ABSTRACT: Generation of visible photons by upconverting materials under excitation by the near-infrared part of sunlight is very attractive for complete utilization of solar energy. Visible upconversion luminescence is efficiently generated in BaCl2:Ln3+ (Ho3+ or Pr3+) particles via excitation with broad band near-infrared part of sunlight. We observed greatly enhanced photocurrent response from the thin-film hydrogenated amorphous silicon solar cell covered with the upconversion layer of BaCl2:Ho3+ phosphor, which demonstrates a prospective application for near-infrared photoresponse devices.

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INTRODUCTION Upconversion (UC) materials doped with lanthanide ions are expected to find potential applications in area of biomarkers, color display, and photovoltaics (PV).1−12 Attaching an UC layer at the rear face of a PV device is very promising for enhancement of power conversion efficiency.13−17 However, the reported efficiencies of these UC-enhanced solar cell demonstrated only small improvement and these early experiments merely served as proof-of-principle demonstration. This problem is due to the fact that the efficiency of UC is very low, and the excitation spectral region is very narrow due to the 4f−4f transition nature of rare earth (RE) ions compared to the solar spectrum.18−21 This problem could be solved by the use of broad band UC materials, which however are rare. Recently, Zou et al.19 showed that using cyanine dye molecule enabled a broad band excitation of NaYF4 nanoparticles codoped with Yb3+ sensitizers and Er3+ activator ions by energy transfer between dye and Yb3+ ions. However, indirect excitation through heterogeneous energy transfer suffers competitive energy loss; what is more, the poor photostability of dyes hinders their application for long-term solar irradiation. Alternatively, broad band near-infrared (NIR) photons excited visible−NIR UC by combination of the absorption bands of different RE ions through core−shell nanofabrication.22 However, it is hard to fully harvest the solar energy with continuous energy distribution due to the discrete characteristic © 2018 American Chemical Society

absorptions of RE ions. In addition, heterogeneous doping will inevitably suffer detrimental energy transfer among RE ions. To improve the upconverted emission efficiency required by practical applications, another approach is to use multiwavelength excitation. Blue-green UC laser has been achieved by two-color (two NIR lights) pumping,23 and especially, Ho3+doped UC materials have promised for enhancement of UC luminescence under multiwavelength excitation.24−26 It is therefore feasible to use this new strategy to realize broad band-excited UC, aiming at the complete utilization of solar energy. However, up to now, in most of the proof-of-concept experiments, UC materials and their application are commonly investigated using monochromatic lasers with high power densities. The use of noncoherent light source to generate UC emission from Ho3+ or Pr3+ ion-doped materials and the simulation of a hybrid cell combining such UC materials in a close-to-reality working conditions remains a tremendous challenge. Herein, UC luminescence of BaCl2:Ho3+/BaCl2:Pr3+ phosphor powders, which give bright UC luminescence under excitation by concentrated incoherent broad band NIR sunlight, is systematically investigated. We further demonstrate Received: March 12, 2018 Revised: April 10, 2018 Published: April 16, 2018 9606

DOI: 10.1021/acs.jpcc.8b02434 J. Phys. Chem. C 2018, 122, 9606−9610

Article

The Journal of Physical Chemistry C the use of BaCl2:Ho3+ powder as a rear face of solar cell application for NIR light harvest (Figure 1). The rational design

diffractometer (PANalytical, Netherlands) using Cu Kα (λ = 1.5418 Å) radiation. Diffuse reflectance spectra were recorded using a Cary 5000 UV−vis−NIR spectrophotometer (Varian) equipped with a double out-of-plane Littrow monochromatic using BaSO4 as a standard reference. The luminescence property of the samples was investigated by a high-resolution spectrofluorometer (Edinburgh Instruments FLS920) equipped with a 500 W solar simulator (NBeT, OSR500, China) as excitation source. Long-pass filters (800 and 1000 nm) were used to cut off the short wavelength lights of the simulated sunlight source, and short-pass filters 990 nm were used to cut off the long wavelength lights of the simulated sunlight source, which were purchased from Andover Co. The sunlight was focused to a cross section of about 0.25 cm2 using an optical lens. Photocurrent responses of the PV-UC device were measured using an IM6ex electrochemical workstation (Zahner, Germany).

Figure 1. Schematic representation of the PV-UC cells for measurement of NIR photoresponse.

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RESULTS AND DISCUSSION Figure 2a shows the XRD patterns of the BaCl2:15 mol % Ho3+ and BaCl2:5 mol % Pr3+ samples. The positions and intensities of diffraction peaks match well with the orthorhombic phase of BaCl2 (JCPDS no. 01-72-1388), and no other phase can be identified. It means that there is no structural change of the orthorhombic phase after doping with Ho3+ or Pr3+ in BaCl2. The scanning electron microscopy (SEM) images of BaCl2:15% Ho3+ and BaCl2:5% Pr3+ show similar morphologies with micron-scale particle size (Figure S7). The sharp absorption bands around 900, 1157, and 1610 nm are observed in Figure 2b, which are due to the intraconfigurational f−f transitions of Ho3+ from the 5I8 ground state to the 5I5, 5I6, and 5I7 excited states, respectively. The a-Si:H solar cells do not directly respond to NIR photons above 800 nm. Because Ho3+ ion has remarkable absorption bands at around 900, 1157, and 1610 nm, the corresponding NIR photons might be efficiently converted to visible-light emissions, which can be absorbed by the a-Si:H solar cells for the generation of photocurrent. Figure 3 shows the UC emission spectra of BaCl2:Ho3+ under excitation by concentrated incoherent NIR light of sunlight (wavelength λ > 800 nm). The UC emissions around 658, 546, and 489 nm are attributed to the 5F5 → 5I8, 5F4 → 5I8, and 5F2 → 5I8 transitions of Ho3+, respectively. Clearly, the UC luminescence of the BaCl2:Ho3+ particles strongly depends on the doping concentration of Ho3+ ions. The UC emission

is briefly sketched in Figure 1. The BaCl2:Ho3+ particles as a upconverter layer absorb and convert the NIR sunlight to visible luminescence by UC processes; then, visible light was reflected and efficiently reabsorbed by hydrogenated amorphous silicon (a-Si:H) solar cells and produced photoelectrons. The a-Si:H solar cells respond to the spectral range from 380 to 750 nm.27 The back contact is a transparent conducting material, which serves as the electrode and permits the incoherent broad band NIR sunlight to excite the UC layer. This new simple and effective methodology may facilitate the application of UC materials in NIR photodetectors and solar cells.

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EXPERIMENTAL METHODS Stoichiometric amounts of 99.99% purity PrCl3, 99.99% purity BaCl2, 99.99% purity HoCl3, and 99.5% NH4Cl (molar ratio HoCl3/NH4Cl = 1:24) were uniformly mixed in high-purity Ar gas and then transferred into a covered cylinder glassy crucible and heated in a tubular furnace under the protection of Ar. The furnace was heated to 950 °C and kept at this temperature for 1.5 h; then, the furnace was cooled down slowly to room temperature. BaCl2:Ho3+ phosphor was fabricated by this solidstate reaction method.28 The crystal structures of the as-synthesized powders were studied by X-ray diffraction (XRD) on a X’Pert Pro X-ray

Figure 2. (a) XRD patterns for samples of BaCl2:15 mol % Ho3+ and BaCl2:5 mol % Pr3+. (b) Absorption spectrum of BaCl2:Ho3+ phosphor. 9607

DOI: 10.1021/acs.jpcc.8b02434 J. Phys. Chem. C 2018, 122, 9606−9610

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The Journal of Physical Chemistry C

Figure 5. Energy level diagrams of Ho3+ ion involving possible UC processes.

Figure 3. UC emission spectra of BaCl2:Ho3+ under excitation by NIR part (wavelength λ > 800 nm) of sunlight.

state before finally decayed to the ground states (5F5 → 5I8). In the third process, the Ho3+ ions absorb a 900 nm photon and are excited to the 5I5 state and then to the 5F5 state by absorbing a 1610 nm photon, afterward relaxed to the5I8 state. The green emission comes from the transition of Ho3+ ions between the 5F4, 5S2 states to the ground state (5I8), and the blue luminescence is associated with the F2 → 5I8 transitions. Both the green and blue emissions are the result of the sequential absorption of a 900 nm photon (to the 5I5 state) and an 1157 nm photon (to the 5F2 state), whereas the cross relaxation between the 5I7 → 5I6 and 5F4, 5S2 → 5F5 of Ho3+ ions contributes to the red UC luminescence. To confirm the process of energy transfer, we investigate the UC emission intensity of BaCl2:Ho3+ under excitation by incoherent NIR light (from sunlight) for wavelengths 990 nm > λ > 800 nm and λ > 990 nm (Figures S1 and S3, respectively). It is strange that the UC emission around 546 nm is absent under excitation at wavelengths from 800 to 990 nm compared with the spectrum recorded at an excitation wavelength of λ > 800 nm. Meanwhile, the UC emission around 486 nm disappeared and the emissions around 765 nm arise under excitation at wavelength λ > 990 nm. The energy level diagrams of Ho3+ ion involving possible UC processes are shown in Figures S2 and S4. These results are regarded as the evidences for the energy transfer process (Figure 5). To prove that Ln3+ ion-doped BaCl2 can generate UC emissions under excitation by broad NIR light from the sun, we optimize the UC emission properties of BaCl2:Pr3+ phosphor and found that BaCl2:5% Pr3+ exhibits strongest red UC emission upon excitation by concentrated incoherent NIR sunlight with wavelength λ > 800 nm (Figure S5). The absorption spectrum of the BaCl2:5% Pr3+ shows six major peaks located at 850, 980, 1460, 1596, 2000, and 2190 nm, which are attributed to the 1G4 → 3P1 + 1I6, 3H4 → 1G4, 3H4 → 3 F4, 3H4 → 3F3, 3H4 → 3F2, and 3H4 → 3H6 transitions of Pr3+ ions, respectively. Possible energy level diagrams of Pr3+ ion involving UC process excitation by incoherent NIR (wavelength λ > 800 nm) from sunlight are depicted in Figure S8. The Pr3+ ions are first excited to the 3P1 + 1I6 states through multiple ways under excitation by broad NIR light and then produced UC emissions around 470, 537, 610, and 636 nm by transitions of 3P1 + 1I6 → 3H4, 3P0 → 3H4, 3P1 + 1I6 → 3H6, and 3 P1 + 1I6 → 3F2 of Ho3+, respectively. On the other hand, Pr3+ ions can be also excited to the 1D2 states and then relaxed to the 3H4 level by phonon-assisted process to give the red emission around 610 nm. The presence of these absorption

3+

intensity reaches a maximum value when Ho ion concentration is 15%. When the Ho3+ ion concentration increases from 0.3 to 28%, more Ho3+ ions become available to furnish the excitation light and result in the higher UC emission intensity by ion−ion energy transfer. If the concentration of Ho3+ ions further increased, the emission intensity decreases gradually, this may be ascribed to the concentration-dependent quenching. The dependences of the luminescence intensity of BaCl2:15 mol % Ho3+ phosphor under NIR light of sunlight (wavelength λ > 800 nm) are presented in Figure 4. The slopes

Figure 4. Power dependence of UC intensity of BaCl2:15% Ho3+ measured at 658, 546, and 489 nm.

for red (658 nm), green (546 nm), and blue (489 nm) emissions are 0.77, 0.71, and 0.59, respectively. Because of the use of broad band excitation, the slope values are much smaller than the results obtained by single-wavelength excitation,26 in which the number of photons involved in the UC process is close to observed slopes. The energy transfer mechanism under NIR light of sunlight (wavelength λ > 800 nm) excitation in the BaCl2:15 mol % Ho3+ phosphor is depicted in Figure 5. The main emissions around 658 nm may be explained by three main process. The Ho3+ ions after absorption at a 1157 nm photon are promoted first to its excited state (5I6) and then to 5F4, 5S2 states through absorption of another 1157 nm photon. The Ho3+ ions in 5F4, 5S2 states first relax to 5F5 state and then to the ground state 5I8, giving the red emissions. Another process is that the Ho3+ ions in 5I6 relax to 5I7 first and then go to 5F5 9608

DOI: 10.1021/acs.jpcc.8b02434 J. Phys. Chem. C 2018, 122, 9606−9610

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UC emission spectra of BaCl2:Ho3+ under excitation by NIR part (990 nm > wavelength λ > 800 nm) of sunlight (Figure S1); energy level diagrams of Ho3+ ion involving possible UC process under excitation by incoherent NIR (990 nm > wavelength λ > 800 nm) from sunlight (Figure S2); UC emission spectra of BaCl2:Ho3+ under excitation by incoherent NIR (wavelength λ > 990 nm) from sunlight (Figure S3); energy level diagrams of Ho3+ ion involving possible UC process excitation by incoherent NIR (wavelength λ > 990 nm) from sunlight (Figure S4); UC emission intensity of BaCl2:Pr3+ under excitation by incoherent NIR (wavelength λ > 800 nm) from sunlight (Figure S5); absorption spectrum of BaCl2:Pr3+ phosphor (Figure S6); energy level diagrams of Pr3+ ion involving possible UC process excitation by incoherent NIR (wavelength λ > 800 nm) from sunlight (Figure S7); SEM images of BaCl2:15% Ho3+ and BaCl2:5% Pr3+ (Figure S8) (PDF)

bands results in the absorption of NIR part of sunlight, and the results also suggest that UC emission can be also generated by sunlight excitation in other Ln3+-doped UC materials as long as NIR absorption lines are present. As an immediate application of the sunlight-generated UC, the BaCl2:Ln3+ phosphor powders are attached to the rear surface of an a-Si:H thin-film solar cell for the examination of NIR photoresponse. As depicted in Figure 6, a weak short-

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected] (H.J.). *E-mail: [email protected] (Z.C.). *E-mail: [email protected], [email protected]. Fax: +86 02087113646 (J.Q.).

Figure 6. Photocurrent response of a-Si:H solar cell coupled with and without BaCl2:Ho3+ (15 mol %) and BaCl2:Pr3+ (5 mol %) phosphors on the rear face of the cell at an applied potential of 0 V under irradiation of concentrated incoherent NIR sunlight with wavelength λ > 800 nm with 60 s sunlight on/off cycles.

ORCID

Hong Jia: 0000-0003-1075-5717 Xiaofeng Liu: 0000-0003-2932-022X Jianrong Qiu: 0000-0003-3148-2500 Notes

The authors declare no competing financial interest.

circuit current density (Jsc) of 0.1 mA/cm2 is observed when the thin-film a-Si:H solar cell is covered with BaCl2 powder, whereas bare thin-film a-Si:H solar cell only shows feeble photoresponse tailing toward 800 nm. Nevertheless, the Jsc values of the thin-film a-Si:H solar cell coupled with the BaCl2:Ho3+ (15 mol %) and BaCl2:Pr3+ (5 mol %) phosphors strikingly jump to 0.25 and 0.16 mA/cm2, respectively. This result demonstrates that the spectral response of the a-Si:H solar cell to incoherent broad band NIR sunlight can be greatly enhanced using the BaCl2:Ln3+ phosphor due to its highly efficient UC emission. This study is predicted to be of general interest for potential applications in NIR photoresponse devices.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 61404071, 61575091, 61675094, 51132004, 51072054, and 51102209), the National Basic Research Program of China (2011CB808100), the Natural Science Foundation of Henan Province of China (162300410201), the Henan Provincial Department of Science and Technology Research Project (162102210303), the Fundamental and Cutting-Edge Technology Research Program of Henan Province (152300410218), the Key Scientific Research Projects of Henan Province (Grants 16A140014 and 16A140016), and the Program for Young Teachers of Higher School in Henan Province (2013GGJS-153), and sponsored by Program for Science and Technology Innovation talents in University of Henan Province (16HASTIT044).

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CONCLUSIONS In summary, we have showed that UC emissions of BaCl2:Ho3+ phosphor can be efficiently generated by excitation with incoherent NIR sunlight at wavelength λ > 800 nm and explained the related mechanisms of upconversion luminescence. In addition, we demonstrated the notable photoactive current generation from an industrial a-Si:H solar cell attached with the BaCl2:Ln3+ (Ho3+ or Pr3+) and phosphor layer excited by NIR part of sunlight (λ > 800 nm). This new strategy of excitation under incoherent broad band NIR sunlight will open a new avenue toward the applications of upconversion materials in NIR light-harvesting devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02434. 9609

DOI: 10.1021/acs.jpcc.8b02434 J. Phys. Chem. C 2018, 122, 9606−9610

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