Au Upconversion Nanocomposites with Broad-Band Excitation

Jan 24, 2014 - In the Yb2O3/Au composites, the extra photothermal effect induced by .... This work was supported by National Talent Youth Science ... ...
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Yb2O3/Au Upconversion Nanocomposites with Broad-Band Excitation for Solar Cells Tong Liu,†,‡ Xue Bai,† Chuang Miao,† Qilin Dai,† Wen Xu,† Yanhao Yu,† Qidai Chen,† and Hongwei Song*,† †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China ‡ College of Instrumentation & Electrical Engineering, Jilin University, 938 Ximinzhu Avenue, Changchun 130061, People’s Republic of China S Supporting Information *

ABSTRACT: Luminescent upconversion (UC) is a promising way to harvest nearinfrared (NIR) sunlight and improve the power conversion efficiency (PCE) of solar cells. However, most of efficient upconversion phosphors (UCPs) are based on 4f−4f transitions of rare earth (RE) ions, which have only a narrower excitation band matching with sun spectrum. To solve this significant problem, we designed and fabricated a novel kind of efficient UC nanocomposites, Yb2O3/Au, in which the upconversion luminescence (UCL) of Yb2O3 was a white broad band that originated from electron−hole recombination, and the excitation bands were expanded at least a range of 770−980 nm through the energy transfer (ET) from anisotropic gold nanoparticles (GNPs) to the Yb2O3 host. To our knowledge, the direct ET from the noble metal to the lanthanide phosphors has never been evidenced. Exploring Yb2O3/Au as the upconverter of a dyesensitized solar cell (DSSC), NIR photovoltaic response was successfully demonstrated as proof-of-concept.

1. INTRODUCTION Solar cells, which can convert sunlight into electric energy via a photovoltaic effect, represent promising routes to green and renewable energy generation and have attracted worldly scientific and industrial interests.1−3 In the solar spectrum, 55−60% of sunlight distributes within the near-infrared (NIR) region of 800−1700 nm. The inability of active materials to absorb the NIR photons (usually less than the band gap) limits the power conversion efficiency (PCE) of some new types of solar cell, such as organic (OPV) solar cells and dye-sensitized solar cells (DSSCs).4,5 The luminescent upconverter, which can convert two or more low-energy NIR excitation photons into a high-energy visible emission photon, is a promising route to resolve this problem.6−9 In theory, by the application of an ideal upconverter, the PCE of the solar cell can be increased from the limit of 30% by up to 43% under AM 1.5 filtered spectral illumination.10,11 However, to date, the PCEs of experimental solar cells based on the upconverter have been extremely low and merely serve as proof of principle.12,13 Currently, the most efficient upconversion phosphors (UCPs) are based on 4f−4f sharp line transitions of trivalent rare earth (RE) ions; for instance, in the well-known NaYF4:Yb3+,Er3+,14,15 Yb3+ ions act as the sensitizer agent, and Er3+ ions act as the activator agent. In view of upconverter applications of solar cells, these UCPs still have some common drawbacks to be overcome: (1) lower upconversion (UC) efficiency and (2) narrower excitation matching of UCPs with sunlight.16 Various methods have been explored to improve the © 2014 American Chemical Society

quantum efficiency and decrease the threshold of upconversion luminescence (UCL), such as choosing suitable host lattices and activators of UCPs, devices of core−shell structure and organic−inorganic composites, plasmon enhancement based on noble metal nanostructures and so forth, and some developments have been achieved.17−22 Yet, there is still a lack of effective strategy on extending the excitation band of UCPs. Very recently, Hummelen and his co-workers proposed a novel device using a set of organic molecules with overlapping absorption spectra acting as an extreme antenna, connected to suitable upconverters to expand the excitation matching of the upconverter with sunlight.22 Indeed, they obtained an expanded excitation band of NaYF4:Yb3+,Er3+ in the range of 750−850 nm and observed dramatically improved UCL efficiency through the antenna effect of infrared (IR) dye molecules. Schmidt and his co-workers made an integrated photovoltaic device, combining a DSSC and the organic molecules’ UC system.23 The photon UC in organic molecules is based on broad band triplet−triplet annihilation (TTA),24 and the integrated device displays enhanced current under sub-band gap illumination. It should be highlighted that IR dyes or organic molecules are unstable, difficult to prepare, and have small Stokes shifts; also, the band widths for one species of dye Received: August 25, 2013 Revised: January 18, 2014 Published: January 24, 2014 3258

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solution became clear. Then, 20 mL of the as-synthesized GNRs aqueous solution was added into the above Yb(NO3)3 and urea solution. After that, the obtained solution was heated to 90 °C and kept at that temperature for 2 h in the oil bath with refluxing. The resulting suspension was separated by centrifugation and collected after washing with deionized water three times. Then, the as-synthesized products [Au@Yb(OH)CO3] were dried at 60 °C for 6 h and calcined in air at selected temperatures (500, 600, and 700 °C) for 4 h to obtain the Yb2O3/Au nanocomposites. For further comparison, the Yb2O3 NP was also synthesized. Same amounts of Yb(NO3)3·6H2O and urea were dissolved in 30 mL of deionized water in a flask, and then, the solution was heated to 90 °C and kept at that temperature for 2 h. After washing, the suspension was dried at 60 °C and calcined in air at 700 °C for 4 h. 2.3. Fabrication of the Yb2O3/Au-DSSC. The TiO2 electrode (12 μm in thickness of the TiO2 layer and 0.5 cm × 0.5 cm in area) and the Pt counter electrode were bought from Dalian Heptachroma SolarTech Co., Ltd. (Dalian, China), which were fabricated on the FTO glass by screen-printing and magnetron sputtering, respectively. In the preparation of Yb2O3/Au-DSSCs, first, the Yb2O3/Au nanocomposites and polyethylene glycol (PEG) 600 with a mass ratio of 1:1 were added into ethanol in a beaker with stirring, and in the following 2 h, the suspension of Yb2O3/Au was further prepared by using ultrasonic dispersion. After that, Yb2O3/Au paste was obtained through evaporation. The UC layer of Yb2O3/Au paste was bladed on the incident light side of the TiO2 electrode through the doctor blading method (25 μm in thickness). Then, the TiO2 electrode coated with UC layer was annealed at 500 °C for 30 min. After cooling to room temperature, the TiO2 layer on one side of the FTO glass was immersed in a 0.5 mM N719 dye solution for 24 h at room temperature to complete the sensitizer uptake, while the UC layer on the other side of the FTO glass was kept out of the N719 dye solution during the immersion. Finally, the TiO2 anode and the Pt counter electrode were bonded together through a hot-melt spacer to form a sandwich structure, and a drop electrolyte consisting of 0.60 M BMII, 0.03 M I2, 0.10 M guanidine thiocyanate, and 0.05 M 4-tertbutylpyridine in a mixture of acetonitrile (85%) and valeronitrile (15%) was injected into the device through predrilled holes on the Pt counter electrode. The hole was sealed using a hot-melt ionomer film (35 μm thickness) and a cover glass (0.1 mm thickness). The PCE of the DSSC assembled with the bought electrodes (without the UC layer) under AM 1.5 filtered spectral illumination was 4.1% (Supporting Information, Figure S1). For comparison, the reference sample NaYF4:Yb3+,Er3+DSSC was prepared by a similar method. The β-phase NaYF4:Yb3+(15%),Er3+(3%) micropowders were purchased from Shanghai Gona Powder Technology Co., Ltd. 2.4. Measurements. The high-resolution transmission electron microscope (HR-TEM) images were recorded on a FEI Tecnai G2 S-Twin microscope operated with an accelerating voltage of 200 kV. The transmission electron microscope (TEM) images were recorded on a Hitachi H-800 TEM under a working voltage of 200 kV. The energy-dispersive X-ray (EDX) elemental mapping images were recorded on a FEI Tecnai G2 S-Twin microscope under a working voltage of 200 kV equipped with an EDX spectrometer. UV/vis−NIR absorption spectra were measured with a Shimadzu UV1800PC UV/vis−NIR scanning spectrophotometer in the

or organic molecule are not as broad as expected, which would limit their practical application unavoidably. In this work, we explore a novel approach of expanding the excitation bands of lanthanide UCPs, which is based on the antenna effect of a gold nanostructure and the effective energy transfer (ET) from the gold nanostructure to Yb2O3 UCPs. Gold nanoparticles (GNPs) have very strong surface plasmon absorption (SPA) bands, and in anisotropic nanoparticles (NPs) such as nanorods (NRs) and nanoshells, SPA bands can shift from the visible toward the NIR region due to the improved contribution of longitudinal vibration modes. For gold nanorods (GNRs), the location of SPA depends on the ratio of length to diameter.25 In principle, if the SPA of GNP is effectively transferred into an UCP, Yb2O3, broad-band excitation can be easily realized through the sensitization of GNPs. Actually, SPA-induced UC enhancement has been frequently observed in some core−shell composites based on GNPs and UCPs, such as NaYF4:Yb3+,Er3+/Tm3+@Au;21,26 however, the expansion of excitation bands through the direct ET from GNPs to UCPs has never been reported. Efficient UCL based on ET could be only realized on the following conditions: (1) the emission transition of donors and the excitation transition of acceptors are well-satisfied with spectral matching conditions based on Föster−Dexter theory; (2) they locate within an effective interaction distance ranging from one to several lattice constants; and (3) SPA of GNPs originates from vibration transitions with a decay rate of ∼100 ps,27 resulting in rapid thermal diffusion, thus requiring the emission transitions of UCPs to have better temperature stability. In most of the previous devices, these strict conditions could not be satisfied at the same time. Fortunately, in the present Yb2O3/Au composites, these conditions can be well-satisfied due to the unique structure of the composites and unusual UC mechanism of the Yb2O3 phosphor, which will be highlighted in the following text.

2. EXPERIMENTAL SECTION 2.1. Synthesis of GNRs. The GNRs were prepared according to the seed-mediated growth method by El-Sayed and co-workers.28 Briefly, a seed solution was prepared by mixing 7.5 mL of cetyltrimethyl ammonium bromide (CTAB) (0.1 M) and 2.5 mL of HAuCl4 (1 mM) with 0.6 mL of freshly prepared 10 mM NaBH4 solution under vigorous stirring at 28 °C. After 2 h, this seed solution was used for the synthesis of the GNRs. In a flask, 13.2 mL of 0.1 M CTAB was mixed with 12 mL of 1 mM HAuCl4, and then 240 μL of 10 mM silver nitrate aqueous solution and 220 μL of 2 M hydrochloric acid were added to the flask. After gently mixing the solution, 192 μL of 0.1 M ascorbic acid was added. With continuous stirring of this mixture, 120 μL of the as-synthesized seed solution was added finally to initiate the growth of the GNRs. These NRs were aged 5 h to ensure full growth at 28 °C. After preparation, excess CTAB was removed by centrifuging at 12 000 rpm for 10 min and then redispersed in 40 mL of deionized water. 2.2. Synthesis of the Yb2O3/Au Nanocomposite. The precursor of the basic carbonate composite was synthesized by the coprecipitation method. The Yb2O3 (99.999%) micropowders were purchased from Changchun Haipurui Rare Earth Material Technology Co., Ltd, and then, the Yb(NO3)3·6H2O was prepared by dissolving the corresponding Yb2O3 micropowders in nitric acid at elevated temperature. After that, 0.82 mmol of Yb(NO3)3·6H2O and 1.8 g of urea were dissolved in 10 mL of deionized water in a flask with stirring until the 3259

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amorphous phase into the crystalline phase at 600 °C, and the crystallinity of the oxide composite was improved considerably when the calcined temperature was elevated to 700 °C. Thus, the 700 °C annealed Yb2O3/Au and Yb2O3 samples were used for further spectral studies in the following experiments. In Figure 1c, most of the diffraction peaks agreed with the standard card 87-2374 of cubic Yb2O3, and the peak located at 2θ = 38.2° was assigned to the characteristic diffraction peak (111) of cubic gold (JCPDS No. 04-0784), implying the formation of Yb2O3/Au composites. In order to better understand the microstructure and formation mechanism of the final Yb2O3/Au products, the synthesis process of the Yb2O3/Au nanocomposites was illustrated, as shown in Figure 2. The aqueous solution was sampled (Yb(OH)CO3/Au) while the coprecipitation reaction was going on. The left part of Figure 2 shows the HR-TEM images of the intermediary products sampled at different reaction times, the 15th, 19th, 21st, and 40th min, respectively. It is interesting to observe that originally a GNR approached the Yb(OH)CO3 nanosphere with its end point, arising from electrostatic attraction.29 Actually, the GNR was positively charged because of residual CTAB on its surface, and the charges gathered on the end piont, while Yb(OH)CO3 was negatively charged in the alkaline solution,30 inducing the heterocoagulation process start from the end point of the GNR.29 In the fringe patterns of the Yb(OH)CO3/Au composites, an obvious boundary between GNR and Yb(OH)CO3 was obviously distinguished, and the fringe spacing of GNR was determined to be ∼0.24 nm, which corresponded closely with the spacing between the (111) plane of fcc gold (0.235 nm) (JCPDS No. 04-0784). We conducted the EDX mapping to determine the specific distribution of the Au element in the samples annealed at different temperatures. The results show that for the annealed samples, the Au element distributed continuously within the whole composite NP, similar to the distribution of the other two elements, O and Yb (Supporting Information, Figure S2). As the annealing temperature increased, the distribution of the Au element became more and more homogeneous, as shown in Figure 2. This means that in the annealed samples, the original GNRs probably became very small NPs owing to thermal diffusion. To further investigate the SPA properties of GNPs in the calcined samples, the three calcined products were dissolved in the dilute hydrochloric acid to avoid the scattering of light by Yb2O3 NPs. Then, the UV/vis−NIR absorptions of the dissolved solutions were measured, as shown on the right side of Figure 2. In all of the samples, two broad bands of SPA were identified, extending over 400−1000 nm. The band located at ∼550 nm corresponded to transverse SPA, and that located at the long wavelength side corresponded to the longitudinal SPA. It can be observed that the longitudinal SPA gradually shifted to the short wavelength side with elevated annealing temperature (shifted from 830 to 750 nm as the annealing temperature increased from 500 to 700 °C), implying the gradually decreasing anisotropy of the GNPs.25 In all of the samples, a relatively narrow band at 980 nm was identified, corresponding to absorption of Yb3+ ions dissolved in the solutions. Under the excitation of a 980 nm laser diode, Yb2O3 micropowders demonstrated bright white broad-band emissions, and the UC efficiency was estimated to be as high as 10% in vacuum, which was originally reported by Wang et al.31 We examined the UCL of Yb2O3 micropowders in air and

range from 400 to 1100 nm. Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max 2550 X-ray diffractometer using a monochromatized Cu target radiation resource (λ = 1.54 Å). In the measurements of powerdependent UCL spectra, a continuous 980 and 808 nm diode laser was used to pump the samples. A visible photomultiplier (350−850 nm) combined with a double-grating monochromator was used for spectral collection. In order to obtain the excitation spectrum of UCL, the integrated intensity over the 350−700 nm range of the emission was plotted versus the excitation wavelength, and that process was fulfilled by using a tunable continuous wave (CW) titanium:sapphire laser, pumped by a Coherent Verdi-V18. The photo current−voltage (I−V) curves were acquired by a source-measurement unit under the illumination of a continuous 980 nm diode laser or titanium:sapphire oscillator that worked in the CW mode (790−830 nm).

3. RESULTS AND DISCUSSION All of the GNRs used were synthesized according to the seedmediated growth method. Figure 1a shows the TEM image of

Figure 1. (a) TEM image of the prepared GNRs. (b) HR-TEM image of the 700 °C calcined Yb2O3/Au nanocomposites. (c) The XRD patterns of the precursor [Au@Yb(OH)CO3] and the calcined samples (500, 600, and 700 °C, respectively).

the GNRs, which indicated that the GNRs were uniform and well-dispersed, with an average diameter of ∼20 nm and length of ∼50 nm. There also existed a little bit of a small cube of gold, with a length of ∼20 nm. The precursor of the basic carbonate composite was synthesized by the coprecipitation method. According to the starting materials, the coprecipitation product should be Au@Yb(OH)CO3, and the precursors were readily converted into an oxide composite (Yb2O3/Au) via thermal decomposition, which was identified by the color change of the sample (from light blue to dark blue). Figure 1b shows the HRTEM image of the 700 °C calcined sample, which indicates that the products consisted of monodispersed nanospheres with a mean particle size of 120 nm. Figure 1c displays XRD patterns of the composites upon calcination. It implies that the sample annealed at 500 °C as well as the precursor remained in the amorphous phase. The composites began to transform from the 3260

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Figure 2. Synthesis mechanism of the Yb2O3/Au nanocomposite. The schematic diagram shows the HR-TEM images (presenting the growing process of the Yb(OH)CO3 onto the GNR), EDX mapping images, and the absorption spectra (presenting the distributions and the absorption properties of gold in the calcined samples).

compared it to the well-known NaYF4:Yb3+,Er3+ at the same experimental conditions, which showed that its integral UCL intensity was at least one order stronger than that of the corresponding NaYF4:Yb3+,Er3+ under the pumping of 1.8 W/ mm2 (Supporting Information, Figure S3). Furthermore, the integral UCL intensity of Yb2O3/Au was comparable to the NaYF4:Yb3+,Er3+ micropowders under 980 nm excitation of 1.18 W/mm2. In order to better understand the UCL mechanism of Yb2O3, which has not been clarified yet, the evolution of UCL with excitation power in Yb2O3 NPs and Yb2O3/Au nanocompsites was performed under 980 nm excitation in air. It is interesting to observe that under relatively low excitation power (0.04−0.19 W/mm2), the Yb2O3 sample exhibited several sharp emission lines (488, 501, and 512 nm) in the range of 450−550 nm, and the strongest peak was located at 501 nm (Figure 3a). This group of emission lines was well in accordance with the cooperative UCL of Yb3+−Yb3+ pairs.32 The overall intensity of these emission lines first increased with the increase of excitation power in the power range of 0.04−0.11 W/mm2 and then gradually decreased with the further increase of excitation power. As the excitation power was beyond 0.23 W/mm2, the line emissions that originated from Yb3+−Yb3+ pairs were completely quenched. The appearance of the broad band in Yb2O3 powders started from 0.36 W/mm2 (threshold power), and the original central wavelength of the broad band was located at around 650 nm (Figure 3b). As the excitation power increased further, the intensity of UCL gradually and rapidly increased with the increasing power, while the central wavelength gradually shifted to blue until 570 nm. It should be highlighted that the generation of the UC broad band was accompanied by the generation of thermal holes in the valence band. This was verified by the experiment of thermal conductivity, which was significant for understanding the UC mechanism of Yb2O3 and Yb2O3/Au (Supporting Information, Figure S4). The behavior of UCL in the Yb2O3/Au nanocomposites was similar to UCL of Yb2O3 under the same 980 nm excitation (Figure 3c). It should be highlighted that in contrast to Yb2O3, the threshold power of generating white broad bands in the

Figure 3. (a) The cooperative emission of Yb3+ in Yb2O3 under 980 nm laser excitation (0.04−0.19 W/mm2). (b) Power-dependent white light emission spectra of Yb2O3 (0.36−1.18 W/mm2) under 980 nm laser excitation. (c) Power-dependent white light emission spectra of Yb2O3/Au (0.25−1.18 W/mm2) under 980 nm laser excitation. (d) The white light emission spectra of Yb2O3/Au under different laser excitation wavelength (6.89 W/mm2). (e), The UC excitation spectra of Yb2O3 and Yb2O3/Au. (f) ln−ln plots of integrated white light emission intensities versus the 980 nm (Yb2O3/Au and Yb2O3 samples) and 808 nm (Yb2O3/Au sample) pumping power density.

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Yb2O3/Au nanocomposites degraded to 0.25 W/mm2. At relatively low excitation power, the UCL of Yb2O3/Au nanocomposites was much stronger than that of Yb2O3. As the excitation power further increased, the intensity difference of UCL between Yb2O3/Au and Yb2O3 gradually became smaller, and the UCL intensities of the two samples were comparable under high power excitation (Figure 3f). These facts will be explained further in the following text (refer to the discussions about the broad-band UCL mechanism of Yb2O3/ Au). It is exciting to observe that in the Yb2O3/Au nanocomposites, the UCL of the broad band was obtained not only under the excitation of 980 nm, corresponding to the excitation of Yb3+ ions, but also under the excitation of 770−880 nm laser light, corresponding to SPA of GNPs. Figure 3d shows typical UCL spectra of the Yb2O3/Au composites under the excitation of 770−810 nm laser light, which indicates that the UCL spectra are all the same and exactly in accordance with those under 980 nm excitation. This implies that corresponding to the surface plasmon excitation of GNPs, ET from GNPs to Yb2O3 happened effectively, leading to UCL of Yb2O3/Au composites. In order to prove this point further, the excitation spectra in a range of 770−900 nm in Yb2O3 nanocrystalline powders and Yb2O3/Au nanocomposites were measured and compared, as shown in Figure 3e. Within the excitation range of 770−880 nm, no UCL was distinguished in Yb2O3, while for Yb2O3/Au, obvious excitation bands were identified, centering around 790 nm, which was in accordance with the SPA of Yb2O3/Au very well. The power-dependent UCL behaviors of Yb2O3 and Yb2O3/ Au were also studied and compared, as shown in Figure 3f. It can be seen that in ln−ln plots, the UCL intensity (IUCL) as a function of excitation power (P) had a very strong power law (IUCLPn) for both Yb2O3 and Yb2O3/Au samples. In different power ranges, there existed two different slopes n for all of the UCL measurements. In detail, it was deduced that n = 12.6 ± 1.1 (0.36−0.58 W/mm2) and 4.0 ± 0.2 (0.58−1.18 W/mm2) for Yb2O3 under 980 nm excitation, n = 6.8 ± 0.3 (0.25−0.58 W/mm2) and 3.5 ± 0.1 (0.58−1.18 W/mm2) for Yb2O3/Au under 980 nm excitation, and n = 10.4 ± 2.3 (0.48−0.57 W/ mm2) and 4.1 ± 0.2 (0.57−0.76 W/mm2) for Yb2O3/Au under 808 nm excitation. In most of cases, the slope n represents the photon number of a two-photon or multiphoton process; however, it is not an absolute rule; for instance, when photon avalanche or pumping saturation effect happens, the value of n may either be much larger than the photon number or much smaller than the photon number.33,34 Here, we believe that the superstrong power law did not match the photon number of UCL in Yb2O3 NPs and Yb2O3/Au composites. Figure 4 demonstrates the UCL model for the broad-band emissions of Yb2O3 and Yb2O3/Au composites. As described in the above text, Yb3+−Yb3+ pairs could be easily pumped onto excited states under 980 nm excitation, and the exposure with high power density induced the temperature of Yb2O3 or Yb2O3/Au samples to increase considerably. In the valence band of Yb2O3 or Yb2O3/Au, a large number of holes were generated, which could be confirmed by the occurrence of thermostimulated conductivity (Supporting Information, Figure S4) and the fact that Yb2O3 and most of the other RE2O3 compounds were p-type semiconductors.35 The electrons on excited Yb3+−Yb3+ pairs recombined with holes in the valence band of Yb2O3, generating broad-band UCL. It should be pointed out that the radiative transition rate of electron−hole

Figure 4. Broad-band UCL mechanism of Yb2O3/Au. The schematic diagram shows the ET, populating, and electron−hole recombination processes for generating white light emission under the NIR laser excitation corresponding to SPA of GNPs.

recombination of the semiconductor was at least several orders larger than the 4f−4f transitions of Yb3+−Yb3+ pairs32,36,37 and thus could exceed the nonradiative relaxation rate of Yb3+−Yb3+ pairs, preventing the thermal quenching of UCL. On the contrary, due to the direct 4f−4f radiative transition rate of excited Yb3+−Yb3+ pairs being much smaller than the nonradiative relaxation rate at elevated temperature, this kind of UCL was unavoidably quenched. In the Yb2O3/Au composites, the extra photothermal effect induced by SPA of GNPs leads the sample temperature to surpass considerably that of the pure Yb2O3 naonpowders under the same power 980 nm excitation (Supporting Information, Figure S5). The increase of temperature in Yb2O3/Au was helpful for generating thermal holes in the valence band, resulting in the decrease of threshold power for generating white light. It is suggested that when the excitation power was lower, the extra increase of tempereature induced by the absorption of GNPs favored the generation of thermal holes on the valence band. On the contrary, when the excitation power was too strong, the extra absorption of GNPs would induce temperature quenching. It can explain that the UC intensity of Yb2O3/Au at low pump power is stronger than that of Yb2O3 but weaker when increasing the pump power, as shown in Figure 3f. In short, under the excitation corresponding to SPA of GNPs, phononassistant ET (release photon) happened from GNPs to the excited states of Yb3+−Yb3+ pairs, and the following electron− hole recombination induced the broad-band UCL. In the present work, we originally evidenced the ET from GNPs to Yb2O3 UCPs through the NIR excitation expansion of the UCL, which offered a novel thought to modify the matching of the upconverter with the sunlight spectrum. Actually, most of recent works doubted and even denied the possibility of ET from GNPs to lanthanide phosphors. Some literature considered that direct interaction of GNPs with nanophosphors would induce the quenching of photoluminescence due to the possible opposite ET from the nanophosphors to the GNPs. In order to obtain improved photoluminescence through the field enhancement effect of SPA, an intermediate layer (∼10 nm) was usually proposed between nanophosphors and GNPs to prevent this possible ET process.38,39 Herein, we should highlight that the present Yb2O3/Au composites possess the following distinguishing features in comparison to the UCPs in the other works, which is significant to realize efficient ET from GNPs to Yb2O3. First of all, the annealing-induced lattice diffusion of the Yb2O3/Au 3262

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heterostructure resulted in a broad distribution of GNPs/ clusters in Yb2O3 lattices, ensuring the effective interaction distance between GNPs/clusters and Yb2O3 NPs. At the same time, the anisotropy of the original GNR was retained basically after annealing. Second, despite the fact that the central wavelength of SPA of GNR did not match exactly the 2 F7/2−2F5/2 excitation transition of Yb3+, it was located on the higher-energy side of the 2F7/2−2F5/2 transition with a narrower energy gap, which was a benefit of the phonon-assisted ET process (releasing phonons). Third, the UCL mechanism of Yb2O3 differed largely from the traditional lanthanide UCPs, such as NaYF4:Yb3+,Er3+. The fast electron−hole recombination process promoted the realization of ET from GNPs to Yb2O3 lattices. Moreover, it also effectively suppressed the nonradiative transitions of excited Yb3+ ions and Yb3+−Yb3+ pairs, resulting in effective UCL, even at relatively high temperature. Finally, we should highlight that the realization of broad-band excitation of UCL in lanthanide phosphors is of vital significance for solar cell upconverters. It is also promised to enhance the performance of other optical and optoelectronic devices, such as light-emitting devices, biosensors, and highresolution fluorescence microscopes.40 Note that in the present Yb2O3/Au composites, the UCL efficiency induced by the ET of GNPs to Yb2O3 is not so high, owing to the essential phonon-assisted process. The extinction cross section of plasmatic NPs is much larger than that of the atoms or ions, organic fluorophores, or semiconductor quantum dots.40 Some lanthanide compounds such as Sm2O3, Er2O3, and Nd2O3 have sufficient intermediate energy levels, matching with SPA of noble metal NPs. We have reason to predict that the realization of resonant ET from noble metal NPs to UCPs could not only expand the excitation bands of UCL but also dramatically improve the UC efficiency. The Yb2O3/Au layer of ∼25 μm was further fabricated on the front side of the DSSC to build a NIR driving DSSC device. The NIR laser illuminated the Yb2O3/Au UCL film first, emitting white light, which was efficiently absorbed by the N719 sensitizer, generating photovoltaic responses. The schematic configuration of the DSSC consisting of an external front NIR UC layer is shown in Figure 5a. Figure 5b shows a comparison of photo I−V characteristics of the Yb2O3/AuDSSC and NaYF4:Yb3+,Er3+-DSSC under exposure of 980 nm light. Figure 5c shows photo I−V characteristics of the Yb2O3/ Au-DSSCs and the DSSC without the UC layer (the control group) under exposure of 790−830 nm light. It can be observed that photovoltaic responses of hundreds of microamperes could be obtained not only under the exposure of 980 nm light (1.18 W/mm2) but also under the irradiation of 790− 830 nm light (3.57 W/mm2), implying the successful expansion of NIR responses. Under the exposure of 980 nm light, the photovoltaic permanence for Yb2O3/Au-DSSC was comparable to that for NaYF4:Yb3+,Er3+-DSSC, in which commercial NaYF4:Yb3+,Er3+ micropowders were adapted for the UC layer. This shows that the present Yb2O3/Au nanocomposite upconverter is relatively effective (PCE = 0.0035%). When the wavelength of the NIR laser was tuned from 830 to 790 nm, the PCE of the DSSC with the Yb2O3/Au layer increased (Figure 5c inset), which was well in accordance with the excitation spectrum of the Yb2O3/Au nanocomposites. The PCE of the DSSC approached 0.01% under the irradiation of 790 nm light. It is worth highlighting that in all of the DSSCs based on luminescent upconverters, such as LaF3−TiO2:Yb3+,Er3+ and NaYF4:Yb3+,Er3+, NIR photovoltaic response could be obtained

Figure 5. (a) The schematic configuration of the cell consisting of an external front layer of Yb2O3/Au nanocomposites as the UC film. (b) The photo I−V characteristics of Yb2O3/Au-DSSC or NaYF4:Yb3+,Er3+-DSSC under 980 nm light illumination (1.18 W/mm2). (c) The photo I−V characteristics of the DSSC with and without the Yb2O3/Au layer under 790−830 nm illumination (3.57 W/mm2); the inset shows PCE (%) of Yb2O3/Au-DSSC versus the NIR excitation wavelength.

only under 980 nm irradiation, and the short-circuit current varied from several tens to hundreds of microamperes.5,13 It should be noted that presently, we could not provide expeimental results related to the AM 1.5 standard illumination because the threshold of excitation power density (∼0.25 W/ mm2 for 980 nm) of the NIR light is even much higher than the power density of the entire solar spectrum (1 mW/mm2). It is expected that the present device might be used in the concentrator solar cells.

4. CONCLUSIONS In conclusion, we designed and obtained a novel type of UC nanocomposites, Yb2O3/Au with broad IR excitation bands (770−980 nm), differing from any previous lanthanide compound UCPs. The broad excitation bands matched more widely with sunlight, and the integral UCL intensity of Yb2O3/ Au was comparable to that of the NaYF4:Yb3+,Er3+ micropowders under 980 nm excitation (1.18 W/mm2). It is suggested that the electron−hole recombination mechanism dominated the UCL of Yb2O3, where electrons came from excited Yb3+ ions pairs and holes aroused from the valence band. Therefore, the UCL had a much faster radiative rate and endured higher temperature. Because GNPs/clusters distributed randomly and maintained the anisotropy of the original GNRs after thermal diffusion, efficient ET occurred from GNPs/clusters to Yb2O3 phosphors with the assistance of phonons, which was evidenced from broad- band NIR 3263

dx.doi.org/10.1021/jp408501k | J. Phys. Chem. C 2014, 118, 3258−3265

The Journal of Physical Chemistry C

Article

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excitation bands in the range of 770−900 nm. Overall, this kind of composite UCP, which provides a novel thought for expanding spectral matching of the upconverter, might be of interest to use in concentrator solar cells.

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ASSOCIATED CONTENT

* Supporting Information S

Photovoltaic properties of the DSSC under stimulated AM 1.5 sunlight illumination (Figure S1), EDX elemental mapping of Yb2O3/Au (Figure S2), a comparison of the UCL strength between Yb2O3 (micropowders) and commercial NaYF4:Yb3+,Er3+ (Figure S3), thermal conductivity of Yb2O3 (Figure S4), and a comparison of the local thermal effect between Yb2O3 and Yb2O3/Au (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-431-85155129. Tel: +86-431-85155129. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Talent Youth Science Foundation of China (Grant No. 60925018) and the National Natural Science Foundation of China (Grant No. 61204015, 51002062, 11174111, 61177042, and 81201738). The China Postdoctoral Science Foundation Funded Project (2012M511337 and 2013T60327) and funding from the State Key Laboratory of Bioelectronics of Southeast University are also acknowledged.

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REFERENCES

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