Improved Up-Conversion Luminescence from Er3+:LaF3 Nanocrystals

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Improved Up-Conversion Luminescence from Er :LaF Nanocrystals Embedded in Oxyfluoride Glass Ceramics via Simultaneous Tri-Wavelength Excitation Zhi Chen, Guobo Wu, Hong Jia, Kaniyarakkal Sharafudeen, Wubin Dai, Xiaowen Zhang, Shengfeng Zeng, Jianmin Liu, Rongfei Wei, Shichao Lv, Guoping Dong, and Jianrong Qiu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 6, 2015

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The Journal of Physical Chemistry

Improved Up-Conversion Luminescence from Er3+:LaF3 Nanocrystals Embedded in Oxyfluoride Glass Ceramics via Simultaneous Tri-wavelength Excitation Zhi Chen,a Guobo Wu,a Hong Jia,b Kaniyarakkal Sharafudeen,c Wubin Dai,a Xiaowen Zhang,a Shengfeng Zeng,a Jianmin Liu,a Rongfei Wei,a Shichao Lv,a Guoping Donga* and Jianrong Qiua*

a

State Key Laboratory of Luminescent Materials and Devices, and Guangdong

Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510641, China.

b

College of Physical and Electronic Information, Luoyang Normal College, 471022, Luoyang, Henan, China.

c

Escola de Engenharia de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, SP,

Brazil.

* To whom correspondence should be addressed. E-mail: [email protected], [email protected] Fax: +86 020-87113646 Tel: +86 13588003708

1

ACS Paragon Plus Environment


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Abstract Up-conversion (UC), harvesting near-infrared (NIR) sunlight, is highly desirable for photovoltaic (PV) cells. In regard to this concept, most of the reported experiments on UC materials and their applications, however, were conventionally studied upon monochromatic laser with narrower excitation band, which is difficult to meet the requirement of solar spectrum conversion. Given the practical applications in PV cells, investigations for UC materials upon simultaneous multi-wavelength even broadband NIR sunlight excitation are much more meaningful. Herein, we studied the UC luminescence properties of germanate oxyfluoride glass ceramics (GCs) containing LaF3:Er3+ nanocrystals with lower phonon energy upon simultaneous tri-wavelength excitation. The UC emission intensities upon simultaneous tri-wavelength excitation were drastically enhanced in comparison with the case that by monochromatic excitation. The UC luminescence mechanisms were interpreted in depth in terms of synergetic UC effect owing to the perturbation in the excited states established by different excitation wavelengths. We demonstrated the application of the simultaneous tri-wavelength excited GCs by adding it to the rear face of the thin-film hydrogenated amorphous silicon (a-Si:H) solar cells. The photoactive current generated by the reflected UC light upon simultaneous tri-wavelength excitation was dramatically enhanced in contrast to the case that upon monochromatic excitation. This Er3+-doped germanate oxyfluoride GCs, harvesting broader NIR sunlight photons via simultaneous multi-wavelength excitation, has colossal potential to improve the power conversion efficiency in PV cells in the near future. 2


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1. Introduction With the depletion of fossil fuels and the associated serious environmental pollution, it is particularly important to develop novel, green and clean energies. Solar power is regarded as one of the most sustainable energies due to its abundance and renewability. Solar cells, directly converting sunlight to electricity via photovoltaic (PV) effect, have made it possible for utilization of solar energy. However, owing to the discrete band structure of semiconductors solar cells, only photons with energies equal to or greater than the band gap energy of the semiconductors can be absorbed and utilized. Most near-infrared (NIR) sunlight photons are transmitted through solar cells, and therefore do not contribute to the photocurrent generation. Fortunately, this drawback can be overcome through solar spectrum conversion.1-2 It has been promulgated that NIR sunlight photons can be effectively utilized by adding a layer of rare-earth (RE) ions activated up-conversion (UC) materials emitting intense visible light at the rear face of PV cells,3-5 which has immense potential to further enhance the power conversion efficiency (PCE) of solar cells. For a long time, in most of the proof-of-concept experiments, UC materials and their application in PV cells have been commonly investigated using monochromatic laser.6-10 Nevertheless, the nature of the sunlight might be overlooked, that it exhibits a continuous and polychromatic spectrum. Sunlight pumping can be considered as simultaneous multi-wavelength excitation. More importantly, investigation for UC materials under simultaneous multi-wavelength NIR excitation, that too with broadband NIR sunlight pumping are much more meaningful for the practical system. 3



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Regretfully, few works have been undertaken to our knowledge.4, 11-16 Herein, we believe that UC upon simultaneous multi-wavelength NIR excitation can broaden the utilization of NIR sunlight photons, which has great potential for the enhancement of the PCE in the next-generation PV cells. In this work, we focused on the UC luminescence properties, mechanisms and applications of germanate oxyfluoride glass ceramics (GCs) containing LaF3:Er3+ nanocrystals upon simultaneous tri-wavelength NIR excitation. In most RE3+, Er3+ is regarded as the most efficient ion for UC, which is attributed to its abundant energy cascades of 4f-4f transitions and larger NIR absorption cross-section.17-18 Compared to other UC materials, the GCs containing LaF3:Er3+ nanocrystals possess the advantages: lower phonon energy, higher chemical durability and mechanical strength, more intense UC luminescence, weaker light scattering as well as better-matched refractive index.19-22 Moreover, UC luminescence properties, mechanisms and applications of the GCs upon simultaneous tri-wavelength excitation have rarely been reported to our knowledge. Here, we studied the UC luminescence properties of the germanate oxyfluoride GCs containing LaF3:Er3+ nanocrystals upon simultaneous tri-wavelength NIR excitation. The mechanisms of the UC luminescence upon simultaneous tri-wavelength NIR excitation were discussed in depth. We also demonstrated the application by applying the germanate oxyfluoride GCs containing LaF3:Er3+ nanocrystals to the thin-film hydrogenated amorphous silicon (a-Si:H) solar cells of which the spectrum response is ranging from 380 to 750 nm.23 Upon simultaneous tri-wavelength excitation, the photocurrent from the solar cell coupled 4


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with the GCs was remarkably enhanced comparing the case that upon monochromatic excitation. Germanate oxyfluoride GCs containing LaF3:Er3+ nanocrystals, harvesting broader NIR sunlight photons via simultaneous multi-wavelength excitation, has vast potential to shorten the distance for the improvement of the PCE in the next-generation PV cells.

2. Experimental Section 2.1 Fabrication of samples. The preparation of Er3+:LaF3 nanocrystals embedded germanate oxyfluoride GCs is similar with our previous works.24-25 The precursor glasses with a composition of 49GeO2-22Al2O3-13LaF3-15LiF-3ErF3 (in mol %) were prepared at 1300 oC for 1h by conventional melt-quenching technique. The precursor glasses were cut into blocks with a size of 10×10 mm2 and heat-treated at 700 oC for 4h to achieve GCs through crystallization. For further optical measurements, the samples were optically polished with a thickness of 1mm. 2.2 Measurements and Characterization. The crystalline phase of the samples were identified by X-ray diffraction (XRD) on an X’Pert PRO X-ray diffractometer (PANalytical, Netherland) using Cu Kα (λ = 1.5418 Å) radiation. The absorption spectra were recorded using a Lambda 900 spectrophotometer (Perkin Elmer, USA) with a spectral range from 200 to 2500 nm. High-resolution transmission electron microscope (HR-TEM, 2100F, JEOL, Japan) was employed to analyze the microstructure of the samples. UC emission spectra were measured with a spectrofluorometer (iHR 320, Jobin-Yvon, France) equipped with 808, 980 and 1530 nm laser diodes (LDs) and a 500 W solar simulator (OSR500, NBeT, China) as 5



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excitation sources, respectively. The UC luminescence lifetimes were collected with a spectrometer (iHR 320, Jobin-Yvon, France) with a digital oscilloscope (TDS 3012B, Tektronix, USA) and pulse LDs as excitation source (LE-LS-808-5000TFC, LE-LS-980-5000TFCA and LE-LS-1550-3000TFCC, LEO Photoelectric, China). Photocurrent responses of the PV-UC device were measured using an IM6ex electrochemical workstation (Zahner, Germany). All the measurements were carried out at room temperature.

3. Results and Discussion In order to characterize glassy phase and GCs, an exhaustive and systematic study was conducted, using XRD and TEM analysis. As shown in Fig. 1(a), the crystallization peaks correspond to the precipitation of LaF3 (JCPDS file no. 32-0483) nanocrystals, indicating the formation of LaF3 in the glass matrix. The mean nanocrystals size in GCs can be estimated through Scherrer’s equation:25-26 𝐾𝜆

𝐷 = 𝛽 cos 𝜃

(1)

where D is the nanocrystals size, K is a dimensionless shape factor, i.e., K=0.89, λ is the x-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) of diffraction peak, and θ is the Bragg angle. The size of the fine nanoparticle calculated is about 17 nm for the sample heat treated at 700

o

C. TEM

characterizations of these composites show that heat treatment at 700 oC induced the precipitation of nanocrystals with an average of 19 nm in diameter (Fig. 1(c) and 1(d)), corresponding to the result calculated by Scherrer’s equation. Importantly, the obtained composites still maintain translucent (the inset photographs in Figure 1b) 6


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despite the in situ precipitation of nanocrystals. The intense NIR absorptions peaked at about 808, 980 and 1530 nm in Fig. 1(b) enlighten us to investigate the UC luminescence behaviors of the obtained composites upon simultaneous tri-wavelength excitation, which will provide a new and simple method for harvesting much broader NIR photons for the enhancement of PCE of the next-generation solar cells.

Fig. 1 (a) XRD patterns and (b) Absorption spectra of 3% Er3+-doped precursor glass and GCs heat-treated at 700 oC for 4h. Insets: the images of 3% Er3+-doped precursor glass and GCs. (c) TEM images of 3% Er3+-doped GC heat-treated at 700 oC for 4h. The inset shows the corresponding SAED pattern. (d) The HRTEM image of the corresponding nanocrystals in figure 1(c). To confirm the improvement of the UC luminescence intensities with simultaneous tri-wavelength excitation, the 3% Er3+-doped GCs were excited by 808 & 980 & 1530 7



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nm laser simultaneously. As shown in Fig. 2, upon monochromatic excitation, the color presents pure red or green to the naked eye. However, it is striking that the green and red emission bands intensify abruptly upon simultaneous tri-wavelength excitation, which appears bright yellow-green color to the naked eye. This is because, upon simultaneous tri-wavelength excitation, the fluorescence branching ratios of green/red vary from the case that upon monochromatic excitation, leading to the bright yellow-green light. The integrated intensity of green or red emission is enhanced by more than one order of magnitude upon simultaneous tri-wavelength excitation in contrast to the case that upon monochromatic laser. We propose that the synergistic UC effect of simultaneous tri-wavelength excitation has a higher contribution to the enhancement of the UC emission intensity. Compared to the total UC emission intensities excited by three monochromatic lasers, the integrated intensity of green and red emission is increased by a factor of 2.28 and 3.13 upon simultaneous tri-wavelength excitation, respectively. The results, somehow, suggest different microscopic mechanisms for the enhanced green and red UC emissions upon simultaneous tri-wavelength excitation.

Fig. 2 UC emission spectra of 3% Er3+-doped GCs upon simultaneous tri-wavelength 8


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excitation. The insets show the optical photographs under different excitation conditions of (a) simultaneous tri-wavelength, i.e., 808 (1.4×103 W/cm2) & 980 (1.2×103 W/cm2) & 1530 nm (0.8×103 W/cm2), (b) 808 nm (1.4×103 W/cm2), (c) 980 nm (1.2×103 W/cm2), and (d) 1530 nm (0.8×103 W/cm2). To shed more light on the microscopic mechanisms of enhanced UC luminescence phenomenon upon simultaneous tri-wavelength excitation, we performed power dependence analysis on the UC luminescence intensity versus monochromatic or simultaneous tri-wavelength irradiation power. The number of photons required to populate the excited state can be obtained from the formula: (2) Where Iem is the emission intensity, P is the pump laser power, and n is the number of laser photons.27-28 As depicted in Fig. 3(d)-(f), using least-squares fitting, the n-values for green and red emissions are all approaching to 2 when excited by monochromatic laser, indicating a two-photon mechanism. Under tri-wavelength excitation simultaneously, as shown in Fig. 3(a)-(c), the slopes n of the dependences of the green and red band on 800, 980 and 1530 nm lasers are all smaller than 1, which are much different from the case that upon monochromatic excitation. Upon traditional monochromatic excitation, owing to its nonlinear optical characteristic, two photons or more are needed to populate the UC emission levels.29 On the contrary, due to the synergistic UC effect of the simultaneous tri-wavelength excitation, the UC luminescence is caused by simultaneous absorbing one photon of the adjusted laser and one photon of the fixed laser, leading to high efficient utilization of photons.30-31 9



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This feature inspires us to further discuss the microscopic mechanisms of UC luminescence processes upon tri-wavelength excitation.

Fig. 3 Dependences of the green and red emission band integrated intensities on (a) 808 nm laser intensity, while 980 and 1530nm laser power remains at a fixed value; (b) 980 nm laser intensity, while 808 and 1530 nm laser power remains at a fixed value; (c) 1530 nm laser intensity, while 808 and 980 nm laser power remains at a fixed value; (d) 808 nm laser intensity; (e) 980 nm laser intensity and (f) 1530 nm laser intensity. In an attempt to analyze the UC dynamic processes related to the 3% Er3+-doped GCs, we have measured the UC luminescence decay kinetics at 542 and 667 nm upon excitation of 808 nm pulse LDs. As depicted in Fig.4(a) and 4(b), the decay profiles can be well fitted into a double exponential function as:32-34 (

=

)

(

)

(3)

Where I and I0 are the luminescence intensity at time t and 0, A1 and A2 are constants, t is the time, and τ1 and τ2 represent the rapid and slow lifetimes for the exponents, respectively. The average lifetimes (τ) can be calculated as follows: =(

) ( 10


)

(4)

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Fig. 4 The fluorescence decay curves of Er3+: 4S3/2 level (542 nm emission) and Er3+: 4F9/2 level (667 nm emission) in GCs upon LDs of 808 nm, respectively. The UC luminescence lifetimes at 542 and 667 nm upon various excitation wavelengths were analyzed and summarized in Table 1. The decay time of the red emission is longer than the green emission, as observed in previous reports.35-36 It is worth noting, however, that the UC luminescence lifetimes are influenced by various excitation wavelengths; as a result, leading to different decay times for an identical UC emission level. As previous reports,19 the quantum efficiency (η) of the UC fluorescence can be expressed by =

=

(5)

Where A is the spontaneous emission probability, WNR is the non-radiative decay rate and τf is the lifetime of the UC emission level. Hence, various excitation wavelengths may have different contribution to the UC emission efficiency. We propose that different excitation wavelengths might have disparate electrons population at a specific emitting level. Photons of longer-wavelength are more likely to be populated at the lower level originating from the ground state absorption (GSA). When the material is interacted by another excitation of a certain wavelength with the primeval 11



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excitation simultaneously, the electrons at the first excited state are effortlessly pumped to the green and red luminescence levels with lower energy losses. As a result, the utilizing efficiency of the NIR photons was effectively improved, leading to the enormous enhancement of the overall UC emission.

Table 1 Summary of the fluorescence decay curves of Er3+: 4S3/2 level (542 nm emission) and Er3+: 4F9/2 level (667 nm emission) in the GCs upon pulse LDs of 808, 980 and 1550 nm, respectively. Samples

GCs

Pulse LDs

Lifetimes (τ/ms)

(nm)

542 nm (4S3/2 → 4I15/2)

667 nm (4F9/2 → 4I15/2)

808

1.51

3.86

980

1.97

9.57

1550

8.93

20.01

For a closer insight to the UC luminescence microscopic mechanisms, the plausible energy level transitions are illustrated in Fig. 5. Owing to the nature of the simultaneous tri-wavelength excitation, it is reasonable to assume that the probability of UC processes could be enhanced as photons with resonant energy for each transition are available, e.g., excited state absorption (ESA) has a higher contribution owing to the presence of a wider range of resonant photons.12 For , 4I9/2 state of Er3+ is first populated through GSA process with an 808 nm photon, following the ESA process by further absorbing a 1530 nm photon to give 2H11/2 state. Then the 12


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populated state 2H11/2 would be depopulated to ground state through the green emission (2H11/2, 4S3/2 → 4I15/2) or relaxed to the 4F9/2 state by multi-phonon relaxation process to give the red emission by 4F9/2 → 4I15/2 transition.31 For and , the first populated state 4I9/2 via GSA process with an 808 nm photon would be relaxed to the 4F9/2 state by multi-phonon relaxation process. By an ESA process, the 4F7/2 and 4

F9/2 state is populated by absorbing a 980 and 1530 nm photon, respectively; which

finally contribute to the green and red emission.30, 37-38 For , 4I11/2 state is first populated through GSA process with a 980 nm photon, following the ESA process by further absorbing a 1530 nm photon to give 4F9/2 state. Then the populated state 4F9/2 would be radiative transition to ground state through the red emission (4F9/2 → 4

I15/2).39 For , 4I11/2 state is first populated through GSA process with a 1530 nm

photon, following the ESA process by further absorbing an 808 nm photon to give 2

H11/2 state of Er3+. The following processes are very similar to .30 In principle, in

comparison with monochromatic excitation, the synergistic UC effect of simultaneous tri-wavelength excitation has a higher contribution to the abrupt enhancement of the UC emission intensity. This is mainly because, under simultaneous tri-wavelength excitation, the ESA steps have maximum absorption cross sections at different NIR excitation wavelengths. Compared to the case that excited by monochromatic laser, the efficiencies of the ESA were optimized independently and the energy losses rooting from the multi-phonon relaxation process were reduced, leading to a substantial gain in overall UC performance.40 The advantage of multiple resonances obviously overwhelms in total the disadvantage of lower irradiance at a specific 13



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monochrome,41 which is expected to shorten the distance for the remarkable enhancement of the PCE in the next-generation PV-UC cells.

Fig. 5 Schematic energy level diagrams of Er3+ ion in GCs related to possible UC processes with the excitation of tri-wavelength simultaneously. To further validate the feasibility of the novel UC strategy that we proposed for harvesting broader NIR photons via 3% Er3+-doped GCs for using in solar cells, the demonstration for photocurrent “ON–OFF” response has been simply performed on a thin-film a-Si:H solar cell with Er3+-doped GCs attached to the rear face of the cell. The schematic representation is briefly sketched in Fig. 6(a). As depicted in Fig. 6(b), weak short-circuit current densities (Jsc) of about 0.87 mA/cm2, 0.06 mA/cm2 and 0.03 mA/cm2 are observed when the modified PV-UC cell upon monochromatic laser of 808, 980, and 1530 nm, respectively. Nevertheless, upon simultaneous tri-wavelength excitation, the Jsc of the modified PV-UC cell strikingly jumps to about 1.70 mA/cm2, which is increased about 1.77 times comparing to the total photocurrent generated by monochromatic laser of 808, 980, and 1530 nm, respectively. The spectrum response of the thin-film a-Si:H solar cell is ranging from 380 to 750 nm as shown in Fig. S1. Hence, the photocurrent is originated from the reflected UC luminescence produced 14


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by the Er3+-doped GCs. Owing to the synergistic UC effect of simultaneous tri-wavelength excitation, the UC luminescence intensities are markedly enhanced in comparison with the case that upon monochromatic excitation. The enhanced UC emissions are reflected back to the thin-film a-Si:H solar cell and again absorbed, resulting in the remarkably increased photocurrent generation. To give a deep insight and practical applicability of simultaneous multi-wavelength excited UC for PV cells, the UC emission spectra of the GCs modified with and without the thin-film a-Si:H solar cell were recorded under irradiation of concentrated incoherent NIR sunlight with wavelength λ > 800 nm (Here, an 800 nm long-pass filter shown in Fig. S2 was used to block the short wavelength lights of the simulated sunlight source). As shown in Fig. S3, the UC luminescence spectrum of the modified system cannot be observed, which can further validate that the UC emission can be absorbed by the thin-film a-Si:H solar cell. This result proves the possibility of the novel UC strategy we proposed via simultaneous multi-wavelength excitation for harvesting incoherent broadband NIR sunlight photons in virtue of Er3+-doped GCs, which has tremendous potential for the enhancement of the PCE of next-generation solar cells.

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Fig.6 (a) Schematic representation of the integrating Er3+-doped GCs material with solar cells for the demonstration of NIR photoresponse upon simultaneous tri-wavelength excitation; (b) Photocurrent responses of thin-film a-Si:H solar cell coupled with 3% Er3+-doped GCs on the rear face of the cell at an applied potential of 0V under different excitation conditions with 30 s laser on/off cycles.

4. Conclusions In conclusion, we investigated the UC luminescence properties of the germanate oxyfluoride GCs containing LaF3:Er3+ nanocrystals upon simultaneous tri-wavelength excitation. The UC emission intensities upon simultaneous tri-wavelength excitation are drastically enhanced in comparison with the case that by monochromatic excitation. We discussed the enhanced mechanisms of the UC luminescence in depth, 16


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which can be put down to the synergetic UC effect rooting from ESA based on various excitation wavelengths. More importantly, we successfully demonstrated that the improved photoactive current upon simultaneous tri-wavelength excitation can be detected on a thin-film a-Si:H solar cell attached with the GCs. This germanate oxyfluoride GCs containing LaF3:Er3+ nanocrystals, broadening utilization of NIR sunlight photons via simultaneous multi-wavelength excitation, has vast potential to shorten the distance for the improvement of the PCE in PV cells in the near future.

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grants no. 51132004, 61475047), Guangdong Natural Science Foundation (Grants no. S2011030001349, 2014A030306045, 1045106410104887). The authors also thank X. W. Niu and J. Y. Zheng (Zhejiang Astronergy Technology Co. Ltd., Hangzhou, China) for the thin-film a-Si:H solar cells fabrication.

Supporting Information External quantum efficiency (EQE) of thin-film a-Si:H solar cell (Figure S1), Transmission spectrum and photograph of 800 nm long-pass filter (Figure S2), UC luminescence spectra upon irradiation of concentrated incoherent NIR sunlight (Figure S3). This information is available free of charge via the Internet at http://pubs.acs.org.

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Improved Up-Conversion Luminescence from Er3+:LaF3 Nanocrystals

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