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shell nanoparticles as spectral converters
Electrochimica Acta 282 (2018) 743e749
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhancement of dye sensitized solar cell efficiency through introducing concurrent upconversion/downconversion core/shell nanoparticles as spectral converters Tong Chen a, Yunfei Shang a, Shuwei Hao a, b, *, Li Tian a, Yuedan Hou a, Chunhui Yang a, b, c, ** a
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China Key Laboratory of Micro-systems and Micro-structures Manufacturing of Ministry of Education, Harbin Institute of Technology, China c State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2018 Received in revised form 29 May 2018 Accepted 15 June 2018 Available online 18 June 2018
Extending the spectral absorption of dye-sensitized solar cells from the visible into near-infrared and ultra-violet range enables the minimization of non-absorption loss of solar photons. Here, we report a viable strategy to implement simultaneously near-infrared upconversion and ultra-violet downconversion for dye-sensitized solar cells through constructing a type of upconversion-core/ downconversion-shell-structured nanoparticles. For the first time, NaYF4:20%Yb,2%Er@NaYF4:7%Eu core/shell nanoparticles are applied to TiO2 photoanode for fabricating near-infrared/ultra-violet-enabled dye-sensitized solar cell devices. The incorporation of designed nanoparticles into TiO2 photoanode of dye-sensitized solar cells achieves high efficiency of 7.664% under one sun illumination, increasing the power conversion efficiency by about 13.95%. We confirm that the enhancement of overall efficiency includes 4.82% upconversion contribution, 7.58% downconversion function and 1.55% scattering effect. Our strategy opens the path for further broadening the solar spectral use to improve the performance of photovoltaic devices. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Concurrent upconversion/downconversion Core/shell structured nanoparticles Extend NIR and UV spectrum respond Dye-sensitized solar cells
1. Introduction In recent decades, extensive efforts have been devoted to the development of new photovoltaic devices or enhancement of the photoelectric conversion efficiency of existing solar cells in order to alleviate energy pressures arising from the excessive consumption of fossil fuels. Among these various photovoltaic technologies, Dyesensitized solar cells (DSSCs) have been regarded as highly promising next-generation devices, which have the potential advantages of their low cost, ease of processing and relatively high power conversion efficiencies (PCEs) [1,2]. The maximum DSSCs
* Corresponding author. MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. ** Corresponding author. MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail addresses: [email protected] (S. Hao), [email protected] (C. Yang). https://doi.org/10.1016/j.electacta.2018.06.111 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
conversion efficiency can theoretically reach ~30% [3]. However, the current conversion efficiency record is still limited to ~12% [4e7]. This problem is principally arose from the inability to absorb and utilize extra photons involving near-infrared (NIR) or ultra violet (UV) lights that constitute almost half of the radiant energy from the sun [8e13]. One strategy that increases the conversion efficiency of DSSC devices is to convert photons with energies below the bandgap of photovoltaic devices into visible photons (<750 nm) that lie in the absorption region of the N719 dye [14e18]. To date, various host materials doped with Yb3þ/Er3þ ion pairs (e.g.,Y2O3:Yb3þ/Er3þ [19], LaF3:Yb3þ/Er3þ [20] and NaYF4:Yb3þ/Er3þ [21]) have been applied as spectral converters in DSSC in an effort to preferably harvest the NIR solar photons. In particular, uniform b-NaYF4:Yb3þ/Er3þ have been demonstrated to possess higher upconversion (UC) efficiency due to the low phonon energy of NaYF4 matrix [21]. Despite the unique NIR harvesting and converting properties, the inevitable surface defects and ligands of such nanomaterials is a major drawback as it leads to severe electron recombination.
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Consequently, the efficiency enhancement from UC contribution is merely about 1% over the overall DSSC efficiency, which is lower than that induced by scattering effect of UC particles. In order to address this issue, the core/shell-structured UC materials have been developed with the aim of decreasing the surface defects. The introduced core/shell nanostructures has been proven as effective route for improving NIR photons absorption, while maintaining good electron transport through TiO2 photoanode film [20,22e24]. For instance, Zhang et al. incorporated a novel NaYF4:Yb3þ/Er3þ/ TiO2 core/shell nanoparticle into TiO2 photoanode, which can effectively overcome the electron trapping by suppressing the surface defects [20]. Liang et al. showed that the insulating SiO2 layer in designed b-NaYF4:Yb3þ/Er3þ@SiO2@TiO2 submicroprisms can minimize the electron trapping caused by NaYF4:Yb3þ/Er3þ nanocrystals, increasing the UC contribution to 2.17% in enhancement efficiency [22]. The NaxGdFyOz:Yb/Er@TiO2 UC/semiconductor core/shell nanoparticles were also utilized as NIR photons converter for DSSCs, achieving efficiently light trapping through harvesting of NIR solar photons and light scattering [23]. Note that, the inert NaYF4 shell was also used as shield layer onto NaYF4:Yb3þ/Er3þ core for directly mitigating the electron-trapping problem [24]. Subsequently, our group has introduced a new dyesensitized NaYF4:Yb3þ/Er3þ@NaYF4:Nd core/shell nanoparticles for DSSCs with improved NIR harvesting performance [25]. These promising studies highlight the value of employing well-preformed core/shell-structured UC converters to efficiently harvest NIR photons for DSSC devices. However, all of the mentioned converters only harvest NIR light, it is lack of expanding the UV harvesting for the use in DSSCs. Recently, an improvement of solar cell UVrespond has also been reported through using rare-earth doped fluorides materials to modify tin oxide compact layer [26e28]. It has been demonstrated that the rare-earth modified tin oxide strategy not only facilitates enhancement of infrared response, but also efficiently mitigates dye degradation and photoelectron recombination. Along this line, the Al2O3:Eu3þ-TiO2:Er3þ,Yb3þ composite further confirms its excellent capacity to convert UV and NIR radiation for solar cells [29]. Yet, concurrent and efficient upconversion and downconversion (DC) processes in the same nanoparticles incorporated in the photoanode for DSSCs remain unexplored. Herein, we report the application of a novel NaYF4:Yb3þ/ 3þ Er @NaYF4:Eu3þ core/shell nanoparticle for the improvement of NIR and UV lights harvesting in DSSC devices. This architecture of NaYF4:Yb3þ/Er3þ core and NaYF4:Eu3þ shell domain are able to absorb simultaneously NIR and UV photons and efficiently reemit visible lights, which matches the main absorption range of the commonly used N719 dye. DSSCs treated by the optimized core/ shell nanoparticles reach a PCE of 7.664% under AM 1.5G, higher
than that of regular DSSC (6.726%), leading to a 13.95% improvement over the overall DSSC efficiency.
2. Results and discussion Fig. 1 illustrates a schematic diagram for the application of NaYF4:Yb3þ/Er3þ@NaYF4:Eu3þ core/shell nanoparticles enabled the NIR/UV DSSC device by converting simultaneously low-energy NIR and high-energy UV photons into absorbable visible photons. The UC/DC core/shell nanoparticles are deposited onto the mesoporous TiO2 layer for constructing the improved photoanode, and the configuration of a DSSC structure is shown in left part of Fig. 1. The light-absorbing N719 dye in regular DSSC devices possesses strong absorption with typically defined range of 450e700 nm (the right part of Fig. 1). The presence of Yb3þ/Er3þ ion pairs in the core can absorb 980 nm NIR photons beyond 700 nm. The embedment of Eu3þ ions in shell layer extends the spectrum respond of DSSC devices to the underutilized UV photons below 400 nm. Moreover, the absorbed solar photons can be converted to visible photons that match the absorption spectrum of the N719 dye. Along this line, the strategy will activate a synergistic effect in UC core and DC shell layers for improving DSSC efficiency [30,31]. Hexagonal-phase NaYF4 is chosen as the host matrix for lanthanide ions because it is known to be one of the most efficient materials for UC and DC processes [32,33]. We firstly optimized the optical matching phenomenon between UC/DC emitting spectrum of various lanthanide ions and absorption of N719 dye. Yb3þ/Er3þ ion pair, effectively upconverting NIR photons to green ones (peaks at 540 nm), was demonstrated to be excellent UC emitter for the maximized absorption of N719 dye (Fig. S1, ESI). The DC process in Eu3þ ion has been verified to effectively extend N719 dye respond to 395 nm (Fig. S2, ESI) [34,35]. In order to enable concurrent and high-efficiency UC and DC processes within the same nanoparticles, a core/shell structure was constructed to separately bear UC and DC lanthanide ions in its different layers. We subsequently discuss two doping strategies involving the UC process in core and shell layer. Core/shell nanocrystals of NaYF4:20%Yb,2%Er@NaYF4:5% Eu and NaYF4:5%Eu@NaYF4:20%Yb,2%Er nanocrystals were synthesized through an adapted procedure from the literature [36]. Details of synthesis steps were described in the ESI. The transmission electron microscopy (TEM) results (Fig. 2aed) show that the NaYF4:20%Yb,2%Er (UC core), NaYF4:20%Yb,2%Er@NaYF4:5%Eu (UC@DC), NaYF4:5%Eu (DC core) and NaYF4: 5%Eu@NaYF4:20% Yb,2%Er (DC@UC) are hexagonal and uniform, with a mean size of 37.4 ± 0.3, 44.5 ± 0.2, 38.1 ± 0.3 and 44.9 ± 0.2 nm, respectively. This implies that the shell layer has a thickness of ~3.5 nm for the core/ shell nanoparticles. The X-ray diffraction (XRD) patterns confirm that the all core and core/shell nanoparticles are of hexagonal
Fig. 1. Near-infrared and ultraviolet sunlight harvesting and then spectral conversion into visible range to activate N719 dye for the enhancement of a DSSC device.
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Fig. 2. Transmission electron microscopy (TEM) images of (a) NaYF4:20%Yb,2%Er, (b) NaYF4:5%Eu, (c) NaYF4:20%Yb, 2%Er@NaYF4: 5%Eu and (d) NaYF4: 5%Eu@ NaYF4:20%Yb, 2%Er. PL spectra of NaYF4:20%Yb, 2%Er@NaYF4: 5%Eu and NaYF4: 5%Eu@ NaYF4:20%Yb, 2%Er excited at (e) 980 nm (~2 W cm2) and (f) 395 nm (~2 W cm2).
phase with good crystallinity (Fig. S3, ESI). The above analysis of structural properties verifies that these UC@DC and DC@UC samples have no difference in crystallographic phases, shapes and sizes. To allow an accurate comparison of the luminescence efficiency, exactly the same concentration of nanoparticles and the same measurement conditions are applied. By placing UC ions pair in the core, interestingly, we find their all emissions at 408, 525/540 and 660 nm, corresponding to the transitions Er3þ: 2H9/2 / 4I15/2, 2H11/ 4 4 4 4 4 2 / I15/2, S3/2 / I15/2 and F9/2 / I15/2 are greatly enhanced at 980 nm excitation (See Fig. 2e and Fig. S4a). Note that, comparing with the DC@UC structure, UC@DC nanoparticles with 395 nm excitation also illustrate the enhanced. DC emissions at 585, 610 and 698 nm, corresponding to the
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D0/7F1、5D0/7F2 and 5D0/7F4 transitions of the Eu3þ ions (Fig. 2f and Fig. S4b). This observation also suggests that this design of UC in the core domain and DC in the shell domain benefits the highest-efficiency and concurrent UC/DC emissions. This is because this UC@DC structure can make the UV excitable DC process close to the nanoparticle surface, evading the disadvantages of high scattering and weak penetration for UV light. Meanwhile, the UC process in the core refrains from surface quenching effect, and have low scattering in this arrangement, allowing the combination of efficient UV-excitable DC and NIR-excitable UC processes within the core/shell nanostructure [37]. To convert efficiently NIR and UV radiation to visible emission within this core/shell nanomatrix to the maximum extent, we
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exploited the precise doping effect of activator Eu in the outermost layer of core/shell nanoparticles. The size of NaYF4:20%Yb,2% Er@NaYF4:x%Eu core/shell nanoparticles with varying Eu3þ content (x ¼ 0, 1, 3, 5, 7 and 10 mol%) in shell layer is determined to be 42 ± 0.4, 43 ± 0.2, 42 ± 0.2, 43 ± 0.3, 42 ± 0.3, and 44 ± 0.6 nm, showing an identical size to the core/shell particles in Fig. 2c and d (Fig. S5, ESI). Moreover, the X-ray diffraction (XRD) patterns, as shown in Fig. S6 (in ESI), confirm that the all core/shell nanoparticles with varying Eu3þ concentration have same crystallographic structures of hexagonal phase. These results thus imply the successful incorporation of Eu3þ ions in shell layer without significant effect on the morphology and size of particles. However, subtle variation in the Eu3þ concentration leads to prominent influence on the UC and DC luminescence (Fig. 3a and b). As can be seen in Fig. 3a, all DC emitting bands in the range of 550e750 nm is increased gradually when elevating the Eu3þ concentration in shell layer, and enhances by factors of 2.9 and 3.0 times for Eu3þ of 7 mol % and 10 mol%, respectively. This is mainly because, for given excitation energy, the excitation received by overall Eu3þ ion increased with the increase of Eu3þ ions concentration. On the contrary, with the increase of Eu3þ content in shell layer, the UC emission intensity shows a gradual downward trend. This is attributed to deleterious cross relaxation quenching processes between Er3þ and Eu3þ ions shown in Fig. 3d. The higher doping concentration of Eu3þ unavoidably provides an increased interaction between Eu3þ and Er3þ ions, thus leading to enhanced cross relaxation quenching effect. This conclusion is supported by the photoluminescence decay recorded at 540 nm for NaYF4:20%Yb,2%
Er@NaYF4:x%Eu (x ¼ 0, 1, 3, 5, 7, 10) core/shell nanoparticles. The average lifetime of 4S3/2 states of Er3þ decreased from 619s to 383s with the increase of Eu3þ ions doping concentrations from 0% to 10% (Fig. S7, ESI). It is worth to note that the nanoparticles of NaYF4:20%Yb,2%Er@NaYF4:7%Eu display the most intense luminescence by integrating the UC and DC emitting intensity as shown in Fig. 3c, suggesting that the 7% Eu3þ ions was determined to the optimized doping strategy. In order to demonstrate unequivocally that the introduction of NaYF4:20%Yb,2%Er@NaYF4:7%Eu core/shell nanoparticles can lead to photocurrent and efficiency enhancement via concurrent UC/DC functions of NIR and UV lights, devices of regular DSSC, DSSC with NaYF4@NaYF4, DSSC with NaYF4:20%Yb,2%Er@NaYF4 and DSSCs incorporated with NaYF4:20%Yb,2%Er@NaYF4:X%Eu (X ¼ 0,1,3,5,7,10) were fabricated. The DSSCs based on UC/DC nanoparticles preparation process is schematically shown in Fig. S8a. The energy converter of the same size had been successfully deposited onto the surface of TiO2 photoanode (Fig. S9, ESI), as indicated by the morphology change shown in FESEM images (Figs. S8b and c, ESI). The thicknesses of the TiO2 electrode and UC/ DC nanoparticles layer are determined to be 17.6 and 5.7 mm, respectively (Fig. S8d, ESI). We first optimized the photovoltaic performances of the DSSCs assembled with NaYF4:20%Yb,2% Er@NaYF4:X%Eu (X ¼ 0,1,3,5,7,10) core/shell nanoparticles under direct AM1.5 G simulated sunlight irradiation (100 mW cm2), and the results are shown in Fig. S10 and Table S1. The NaYF4:20%Yb,2% Er@NaYF4 doped 7% Eu3þ sample was found to play supreme role on the enhancement of DSSC efficiency, which agrees well with the
Fig. 3. PL spectra of NaYF4:20%Yb, 2%Er@NaYF4: x%Eu (x ¼ 0, 1, 3, 5, 7 and 10) excited at (a) 980 nm (~2 W cm2) and (b) 395 nm (~2 W cm2). (c) Contrasted luminescence intensity of UC (green line) and DC (red line) processes in the core-shell UC@DC NPs. (d) Proposed energy transfer mechanisms between Yb3þ/Er3þ ions pair and Eu3þ ion. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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luminescence result shown in Fig. 3c, implying that deed the designed core/shell structured-nanoparticles is beneficial for harvesting extra solar photons and operates to DSSCs. To further demonstrate the NIR and UV harvest functions by UC/DC processes in the same nanoparticles, we investigated the photocurrent density-voltage curves (J-V) of regular DSSC, DSSC with NaYF4:20% Yb,2%Er@NaYF4 and DSSC with NaYF4:20%Yb,2%Er@NaYF4:7%Eu under direct AM1.5 G simulated sunlight irradiation (100 mW cm2). The tested results are shown in Fig. 4a with photovoltaic parameters summarized in its inset. The regular DSSC (control cell) shows a short-circuit current density (Jsc) of 11.912 mA cm2, open-circuit voltage (Voc) of 0.768 V, fill factor (FF) of 0.730, and conversion efficiency (h) of 6.726%. After separately deposited NaYF4:20%Yb,2%Er@NaYF4 and NaYF4:20%Yb,2% Er@NaYF4:7%Eu core/shell nanoparticles of the same size onto TiO2 photoanode, the Jsc values was increased to 12.887 mA cm2 and 14.30 mA cm2, respectively. Consequently, the h of improved cells is added up to 7.154% and 7.664%, yielding 6.36% and 13.95% efficiency enhancement over the regular DSSC, respectively. These prominent enhancements in photocurrent and efficiency can be attributed to the scattering effect and contributions from harvesting NIR and UV lights, which can be also verified by the IPCE data as shown in Fig. 4b and Fig. S11. Interestingly, the NaYF4:20%Yb,2% Er@NaYF4 and NaYF4:20%Yb,2%Er@NaYF4:7%Eu treated DSSC devices showed higher IPCE values at ~980 nm than the regular DSSC cell under illumination of 980 nm laser with an output of 1 W power, further confirming the UC contribution. As a control experiment, the NaYF4@NaYF4 nanoparticles without any dopants were applied to the TiO2 photoanode, achieving a Jsc of 12.613 mA cm2 and h of 6.83% (Fig. S12, ESI), which confirmed the 1.55% efficiency
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enhancement was induced by scattering effect. The addition of NaYF4:20%Yb,2%Er@NaYF4 nanoparticles with only UC ions into TiO2 photoanode make the DSSC reach an enhanced Jsc of 12.887 mA cm2 and h of 7.154%, confirming a 4.82% enhanced efficiency from UC function compared with the DSSC treated with NaYF4@NaYF4 nanoparticles. The DC contribution to efficiency enhancement of DSSC can be demonstrated to be 7.58% through comparing the DSSC coupling with NaYF4@NaYF4:7%Eu nanoparticles with the DSSC treated with NaYF4@NaYF4 nanoparticles, as shown in Fig. S12. The NIR and UV harvest function by the designed core/shell nanoparticles was further verified by measuring these three cells under 980 nm NIR and 395 nm UV laser irradiations with a power of 1 W. As can be seen in Fig. 4c, the significant enhancement of photovoltaic performances was observed for DSSC treated by NaYF4:20%Yb,2%Er@NaYF4 nanoparticles (h of 0.252%) compared with the pristine DSSC (h of 0.003%), verifying that the improved DSSC device was endowed with NIR light harvesting ability. Furthermore, the core/shell nanoparticles with concurrent UC and DC processes make the DSSC device produce a higher h of 0.0375%, demonstrating that the NIR and UV lights can be simultaneously utilized. Along this line, we further demonstrated that the embedded converters enable the DSSC device responding to extra light under periodic concurrent NIR/UV lasers ON/OFF irradiation as shown in Fig. 4d. The photocurrent value and corresponding h of the DSSC with NaYF4:20% Yb,2%Er@NaYF4:7%Eu nanoparticles were tested as a function of time at a constant forward bias of 0.8 V under AM1.5G with the simultaneous 980 nm NIR and 395 nm UV lasers switched on and off every 20 s. The stable photocurrent density of 14.303 mA cm2 and h of 7.664% were generated under the standard sunlight
Fig. 4. (a) The current density-voltage (JeV) characteristics and (b) the IPCE spectrum of DSSC, DSSC with UC NPs and UC/DC NPs under AM1.5 G simulated sunlight irradiation (100 mW cm2) and illumination of 980 nm laser with an output of 1 W power. (c) J-V curves measured under 980/395 nm laser irradiation (1 W cm2) for DSSC, DSSC with UC NPs and UC/DC NPs. (d) The photocurrent density measured at 0.8 V and the corresponding h of DSSC with UC/DC NPs under AM 1.5G with 980/395 nm laser switched on and off every 20 s.
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irradiation. However, the photocurrent density was increase to 14.856 mA cm2 together with an enhanced h of 7.888% aroused from the switching of 980 nm NIR and 395 nm UV lasers. This result strongly validates that the designed core/shell nanoparticles are able to absorb simultaneously NIR and UV lights for generation of additional photocurrent. 3. Conclusion In conclusion, for the first time, the novel core/shell-structured NaYF4:Yb,Er@NaYF4:Eu nanoparticles with concurrent UC and DC functions are successfully prepared and employed for proof-ofconcept study in DSSCs as an energy converter on top of the transparent TiO2 layer. The DSSC devices coupling with designed core/shell nanoparticle possess an excellent capacity to extend light absorption through enabling simultaneously harvesting and utilization of NIR and UV light by the UC core and DC shell. The optimal Jsc of 14.30 mA cm2 and h of 7.664% are achieved in the improved DSSC system, which is a noticeable improvement of 13.95% compared to the regular DSSC (6.726%). This great improvement is most likely due to the combined effects of enhanced UC/DC processes and light scattering. Our work provides a feasible strategy to broaden the solar spectrum use to UV and NIR sunlight for DSSCs or other types of solar cells.
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Conflicts of interest There are no conflicts to declare. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51402071), the National Science Fund for Distinguished Young Scholars (Grant No. 51325201).
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhancement of dye sensitized solar cell efficiency through introducing concurrent upconversion/downconversion core/shell nanoparticles as spectral converters Tong Chen a, Yunfei Shang a, Shuwei Hao a, b, *, Li Tian a, Yuedan Hou a, Chunhui Yang a, b, c, ** a
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China Key Laboratory of Micro-systems and Micro-structures Manufacturing of Ministry of Education, Harbin Institute of Technology, China c State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2018 Received in revised form 29 May 2018 Accepted 15 June 2018 Available online 18 June 2018
Extending the spectral absorption of dye-sensitized solar cells from the visible into near-infrared and ultra-violet range enables the minimization of non-absorption loss of solar photons. Here, we report a viable strategy to implement simultaneously near-infrared upconversion and ultra-violet downconversion for dye-sensitized solar cells through constructing a type of upconversion-core/ downconversion-shell-structured nanoparticles. For the first time, NaYF4:20%Yb,2%Er@NaYF4:7%Eu core/shell nanoparticles are applied to TiO2 photoanode for fabricating near-infrared/ultra-violet-enabled dye-sensitized solar cell devices. The incorporation of designed nanoparticles into TiO2 photoanode of dye-sensitized solar cells achieves high efficiency of 7.664% under one sun illumination, increasing the power conversion efficiency by about 13.95%. We confirm that the enhancement of overall efficiency includes 4.82% upconversion contribution, 7.58% downconversion function and 1.55% scattering effect. Our strategy opens the path for further broadening the solar spectral use to improve the performance of photovoltaic devices. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Concurrent upconversion/downconversion Core/shell structured nanoparticles Extend NIR and UV spectrum respond Dye-sensitized solar cells
1. Introduction In recent decades, extensive efforts have been devoted to the development of new photovoltaic devices or enhancement of the photoelectric conversion efficiency of existing solar cells in order to alleviate energy pressures arising from the excessive consumption of fossil fuels. Among these various photovoltaic technologies, Dyesensitized solar cells (DSSCs) have been regarded as highly promising next-generation devices, which have the potential advantages of their low cost, ease of processing and relatively high power conversion efficiencies (PCEs) [1,2]. The maximum DSSCs
* Corresponding author. MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. ** Corresponding author. MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China. E-mail addresses: [email protected] (S. Hao), [email protected] (C. Yang). https://doi.org/10.1016/j.electacta.2018.06.111 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
conversion efficiency can theoretically reach ~30% [3]. However, the current conversion efficiency record is still limited to ~12% [4e7]. This problem is principally arose from the inability to absorb and utilize extra photons involving near-infrared (NIR) or ultra violet (UV) lights that constitute almost half of the radiant energy from the sun [8e13]. One strategy that increases the conversion efficiency of DSSC devices is to convert photons with energies below the bandgap of photovoltaic devices into visible photons (<750 nm) that lie in the absorption region of the N719 dye [14e18]. To date, various host materials doped with Yb3þ/Er3þ ion pairs (e.g.,Y2O3:Yb3þ/Er3þ [19], LaF3:Yb3þ/Er3þ [20] and NaYF4:Yb3þ/Er3þ [21]) have been applied as spectral converters in DSSC in an effort to preferably harvest the NIR solar photons. In particular, uniform b-NaYF4:Yb3þ/Er3þ have been demonstrated to possess higher upconversion (UC) efficiency due to the low phonon energy of NaYF4 matrix [21]. Despite the unique NIR harvesting and converting properties, the inevitable surface defects and ligands of such nanomaterials is a major drawback as it leads to severe electron recombination.
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Consequently, the efficiency enhancement from UC contribution is merely about 1% over the overall DSSC efficiency, which is lower than that induced by scattering effect of UC particles. In order to address this issue, the core/shell-structured UC materials have been developed with the aim of decreasing the surface defects. The introduced core/shell nanostructures has been proven as effective route for improving NIR photons absorption, while maintaining good electron transport through TiO2 photoanode film [20,22e24]. For instance, Zhang et al. incorporated a novel NaYF4:Yb3þ/Er3þ/ TiO2 core/shell nanoparticle into TiO2 photoanode, which can effectively overcome the electron trapping by suppressing the surface defects [20]. Liang et al. showed that the insulating SiO2 layer in designed b-NaYF4:Yb3þ/Er3þ@SiO2@TiO2 submicroprisms can minimize the electron trapping caused by NaYF4:Yb3þ/Er3þ nanocrystals, increasing the UC contribution to 2.17% in enhancement efficiency [22]. The NaxGdFyOz:Yb/Er@TiO2 UC/semiconductor core/shell nanoparticles were also utilized as NIR photons converter for DSSCs, achieving efficiently light trapping through harvesting of NIR solar photons and light scattering [23]. Note that, the inert NaYF4 shell was also used as shield layer onto NaYF4:Yb3þ/Er3þ core for directly mitigating the electron-trapping problem [24]. Subsequently, our group has introduced a new dyesensitized NaYF4:Yb3þ/Er3þ@NaYF4:Nd core/shell nanoparticles for DSSCs with improved NIR harvesting performance [25]. These promising studies highlight the value of employing well-preformed core/shell-structured UC converters to efficiently harvest NIR photons for DSSC devices. However, all of the mentioned converters only harvest NIR light, it is lack of expanding the UV harvesting for the use in DSSCs. Recently, an improvement of solar cell UVrespond has also been reported through using rare-earth doped fluorides materials to modify tin oxide compact layer [26e28]. It has been demonstrated that the rare-earth modified tin oxide strategy not only facilitates enhancement of infrared response, but also efficiently mitigates dye degradation and photoelectron recombination. Along this line, the Al2O3:Eu3þ-TiO2:Er3þ,Yb3þ composite further confirms its excellent capacity to convert UV and NIR radiation for solar cells [29]. Yet, concurrent and efficient upconversion and downconversion (DC) processes in the same nanoparticles incorporated in the photoanode for DSSCs remain unexplored. Herein, we report the application of a novel NaYF4:Yb3þ/ 3þ Er @NaYF4:Eu3þ core/shell nanoparticle for the improvement of NIR and UV lights harvesting in DSSC devices. This architecture of NaYF4:Yb3þ/Er3þ core and NaYF4:Eu3þ shell domain are able to absorb simultaneously NIR and UV photons and efficiently reemit visible lights, which matches the main absorption range of the commonly used N719 dye. DSSCs treated by the optimized core/ shell nanoparticles reach a PCE of 7.664% under AM 1.5G, higher
than that of regular DSSC (6.726%), leading to a 13.95% improvement over the overall DSSC efficiency.
2. Results and discussion Fig. 1 illustrates a schematic diagram for the application of NaYF4:Yb3þ/Er3þ@NaYF4:Eu3þ core/shell nanoparticles enabled the NIR/UV DSSC device by converting simultaneously low-energy NIR and high-energy UV photons into absorbable visible photons. The UC/DC core/shell nanoparticles are deposited onto the mesoporous TiO2 layer for constructing the improved photoanode, and the configuration of a DSSC structure is shown in left part of Fig. 1. The light-absorbing N719 dye in regular DSSC devices possesses strong absorption with typically defined range of 450e700 nm (the right part of Fig. 1). The presence of Yb3þ/Er3þ ion pairs in the core can absorb 980 nm NIR photons beyond 700 nm. The embedment of Eu3þ ions in shell layer extends the spectrum respond of DSSC devices to the underutilized UV photons below 400 nm. Moreover, the absorbed solar photons can be converted to visible photons that match the absorption spectrum of the N719 dye. Along this line, the strategy will activate a synergistic effect in UC core and DC shell layers for improving DSSC efficiency [30,31]. Hexagonal-phase NaYF4 is chosen as the host matrix for lanthanide ions because it is known to be one of the most efficient materials for UC and DC processes [32,33]. We firstly optimized the optical matching phenomenon between UC/DC emitting spectrum of various lanthanide ions and absorption of N719 dye. Yb3þ/Er3þ ion pair, effectively upconverting NIR photons to green ones (peaks at 540 nm), was demonstrated to be excellent UC emitter for the maximized absorption of N719 dye (Fig. S1, ESI). The DC process in Eu3þ ion has been verified to effectively extend N719 dye respond to 395 nm (Fig. S2, ESI) [34,35]. In order to enable concurrent and high-efficiency UC and DC processes within the same nanoparticles, a core/shell structure was constructed to separately bear UC and DC lanthanide ions in its different layers. We subsequently discuss two doping strategies involving the UC process in core and shell layer. Core/shell nanocrystals of NaYF4:20%Yb,2%Er@NaYF4:5% Eu and NaYF4:5%Eu@NaYF4:20%Yb,2%Er nanocrystals were synthesized through an adapted procedure from the literature [36]. Details of synthesis steps were described in the ESI. The transmission electron microscopy (TEM) results (Fig. 2aed) show that the NaYF4:20%Yb,2%Er (UC core), NaYF4:20%Yb,2%Er@NaYF4:5%Eu (UC@DC), NaYF4:5%Eu (DC core) and NaYF4: 5%Eu@NaYF4:20% Yb,2%Er (DC@UC) are hexagonal and uniform, with a mean size of 37.4 ± 0.3, 44.5 ± 0.2, 38.1 ± 0.3 and 44.9 ± 0.2 nm, respectively. This implies that the shell layer has a thickness of ~3.5 nm for the core/ shell nanoparticles. The X-ray diffraction (XRD) patterns confirm that the all core and core/shell nanoparticles are of hexagonal
Fig. 1. Near-infrared and ultraviolet sunlight harvesting and then spectral conversion into visible range to activate N719 dye for the enhancement of a DSSC device.
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Fig. 2. Transmission electron microscopy (TEM) images of (a) NaYF4:20%Yb,2%Er, (b) NaYF4:5%Eu, (c) NaYF4:20%Yb, 2%Er@NaYF4: 5%Eu and (d) NaYF4: 5%Eu@ NaYF4:20%Yb, 2%Er. PL spectra of NaYF4:20%Yb, 2%Er@NaYF4: 5%Eu and NaYF4: 5%Eu@ NaYF4:20%Yb, 2%Er excited at (e) 980 nm (~2 W cm2) and (f) 395 nm (~2 W cm2).
phase with good crystallinity (Fig. S3, ESI). The above analysis of structural properties verifies that these UC@DC and DC@UC samples have no difference in crystallographic phases, shapes and sizes. To allow an accurate comparison of the luminescence efficiency, exactly the same concentration of nanoparticles and the same measurement conditions are applied. By placing UC ions pair in the core, interestingly, we find their all emissions at 408, 525/540 and 660 nm, corresponding to the transitions Er3þ: 2H9/2 / 4I15/2, 2H11/ 4 4 4 4 4 2 / I15/2, S3/2 / I15/2 and F9/2 / I15/2 are greatly enhanced at 980 nm excitation (See Fig. 2e and Fig. S4a). Note that, comparing with the DC@UC structure, UC@DC nanoparticles with 395 nm excitation also illustrate the enhanced. DC emissions at 585, 610 and 698 nm, corresponding to the
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D0/7F1、5D0/7F2 and 5D0/7F4 transitions of the Eu3þ ions (Fig. 2f and Fig. S4b). This observation also suggests that this design of UC in the core domain and DC in the shell domain benefits the highest-efficiency and concurrent UC/DC emissions. This is because this UC@DC structure can make the UV excitable DC process close to the nanoparticle surface, evading the disadvantages of high scattering and weak penetration for UV light. Meanwhile, the UC process in the core refrains from surface quenching effect, and have low scattering in this arrangement, allowing the combination of efficient UV-excitable DC and NIR-excitable UC processes within the core/shell nanostructure [37]. To convert efficiently NIR and UV radiation to visible emission within this core/shell nanomatrix to the maximum extent, we
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exploited the precise doping effect of activator Eu in the outermost layer of core/shell nanoparticles. The size of NaYF4:20%Yb,2% Er@NaYF4:x%Eu core/shell nanoparticles with varying Eu3þ content (x ¼ 0, 1, 3, 5, 7 and 10 mol%) in shell layer is determined to be 42 ± 0.4, 43 ± 0.2, 42 ± 0.2, 43 ± 0.3, 42 ± 0.3, and 44 ± 0.6 nm, showing an identical size to the core/shell particles in Fig. 2c and d (Fig. S5, ESI). Moreover, the X-ray diffraction (XRD) patterns, as shown in Fig. S6 (in ESI), confirm that the all core/shell nanoparticles with varying Eu3þ concentration have same crystallographic structures of hexagonal phase. These results thus imply the successful incorporation of Eu3þ ions in shell layer without significant effect on the morphology and size of particles. However, subtle variation in the Eu3þ concentration leads to prominent influence on the UC and DC luminescence (Fig. 3a and b). As can be seen in Fig. 3a, all DC emitting bands in the range of 550e750 nm is increased gradually when elevating the Eu3þ concentration in shell layer, and enhances by factors of 2.9 and 3.0 times for Eu3þ of 7 mol % and 10 mol%, respectively. This is mainly because, for given excitation energy, the excitation received by overall Eu3þ ion increased with the increase of Eu3þ ions concentration. On the contrary, with the increase of Eu3þ content in shell layer, the UC emission intensity shows a gradual downward trend. This is attributed to deleterious cross relaxation quenching processes between Er3þ and Eu3þ ions shown in Fig. 3d. The higher doping concentration of Eu3þ unavoidably provides an increased interaction between Eu3þ and Er3þ ions, thus leading to enhanced cross relaxation quenching effect. This conclusion is supported by the photoluminescence decay recorded at 540 nm for NaYF4:20%Yb,2%
Er@NaYF4:x%Eu (x ¼ 0, 1, 3, 5, 7, 10) core/shell nanoparticles. The average lifetime of 4S3/2 states of Er3þ decreased from 619s to 383s with the increase of Eu3þ ions doping concentrations from 0% to 10% (Fig. S7, ESI). It is worth to note that the nanoparticles of NaYF4:20%Yb,2%Er@NaYF4:7%Eu display the most intense luminescence by integrating the UC and DC emitting intensity as shown in Fig. 3c, suggesting that the 7% Eu3þ ions was determined to the optimized doping strategy. In order to demonstrate unequivocally that the introduction of NaYF4:20%Yb,2%Er@NaYF4:7%Eu core/shell nanoparticles can lead to photocurrent and efficiency enhancement via concurrent UC/DC functions of NIR and UV lights, devices of regular DSSC, DSSC with NaYF4@NaYF4, DSSC with NaYF4:20%Yb,2%Er@NaYF4 and DSSCs incorporated with NaYF4:20%Yb,2%Er@NaYF4:X%Eu (X ¼ 0,1,3,5,7,10) were fabricated. The DSSCs based on UC/DC nanoparticles preparation process is schematically shown in Fig. S8a. The energy converter of the same size had been successfully deposited onto the surface of TiO2 photoanode (Fig. S9, ESI), as indicated by the morphology change shown in FESEM images (Figs. S8b and c, ESI). The thicknesses of the TiO2 electrode and UC/ DC nanoparticles layer are determined to be 17.6 and 5.7 mm, respectively (Fig. S8d, ESI). We first optimized the photovoltaic performances of the DSSCs assembled with NaYF4:20%Yb,2% Er@NaYF4:X%Eu (X ¼ 0,1,3,5,7,10) core/shell nanoparticles under direct AM1.5 G simulated sunlight irradiation (100 mW cm2), and the results are shown in Fig. S10 and Table S1. The NaYF4:20%Yb,2% Er@NaYF4 doped 7% Eu3þ sample was found to play supreme role on the enhancement of DSSC efficiency, which agrees well with the
Fig. 3. PL spectra of NaYF4:20%Yb, 2%Er@NaYF4: x%Eu (x ¼ 0, 1, 3, 5, 7 and 10) excited at (a) 980 nm (~2 W cm2) and (b) 395 nm (~2 W cm2). (c) Contrasted luminescence intensity of UC (green line) and DC (red line) processes in the core-shell UC@DC NPs. (d) Proposed energy transfer mechanisms between Yb3þ/Er3þ ions pair and Eu3þ ion. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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luminescence result shown in Fig. 3c, implying that deed the designed core/shell structured-nanoparticles is beneficial for harvesting extra solar photons and operates to DSSCs. To further demonstrate the NIR and UV harvest functions by UC/DC processes in the same nanoparticles, we investigated the photocurrent density-voltage curves (J-V) of regular DSSC, DSSC with NaYF4:20% Yb,2%Er@NaYF4 and DSSC with NaYF4:20%Yb,2%Er@NaYF4:7%Eu under direct AM1.5 G simulated sunlight irradiation (100 mW cm2). The tested results are shown in Fig. 4a with photovoltaic parameters summarized in its inset. The regular DSSC (control cell) shows a short-circuit current density (Jsc) of 11.912 mA cm2, open-circuit voltage (Voc) of 0.768 V, fill factor (FF) of 0.730, and conversion efficiency (h) of 6.726%. After separately deposited NaYF4:20%Yb,2%Er@NaYF4 and NaYF4:20%Yb,2% Er@NaYF4:7%Eu core/shell nanoparticles of the same size onto TiO2 photoanode, the Jsc values was increased to 12.887 mA cm2 and 14.30 mA cm2, respectively. Consequently, the h of improved cells is added up to 7.154% and 7.664%, yielding 6.36% and 13.95% efficiency enhancement over the regular DSSC, respectively. These prominent enhancements in photocurrent and efficiency can be attributed to the scattering effect and contributions from harvesting NIR and UV lights, which can be also verified by the IPCE data as shown in Fig. 4b and Fig. S11. Interestingly, the NaYF4:20%Yb,2% Er@NaYF4 and NaYF4:20%Yb,2%Er@NaYF4:7%Eu treated DSSC devices showed higher IPCE values at ~980 nm than the regular DSSC cell under illumination of 980 nm laser with an output of 1 W power, further confirming the UC contribution. As a control experiment, the NaYF4@NaYF4 nanoparticles without any dopants were applied to the TiO2 photoanode, achieving a Jsc of 12.613 mA cm2 and h of 6.83% (Fig. S12, ESI), which confirmed the 1.55% efficiency
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enhancement was induced by scattering effect. The addition of NaYF4:20%Yb,2%Er@NaYF4 nanoparticles with only UC ions into TiO2 photoanode make the DSSC reach an enhanced Jsc of 12.887 mA cm2 and h of 7.154%, confirming a 4.82% enhanced efficiency from UC function compared with the DSSC treated with NaYF4@NaYF4 nanoparticles. The DC contribution to efficiency enhancement of DSSC can be demonstrated to be 7.58% through comparing the DSSC coupling with NaYF4@NaYF4:7%Eu nanoparticles with the DSSC treated with NaYF4@NaYF4 nanoparticles, as shown in Fig. S12. The NIR and UV harvest function by the designed core/shell nanoparticles was further verified by measuring these three cells under 980 nm NIR and 395 nm UV laser irradiations with a power of 1 W. As can be seen in Fig. 4c, the significant enhancement of photovoltaic performances was observed for DSSC treated by NaYF4:20%Yb,2%Er@NaYF4 nanoparticles (h of 0.252%) compared with the pristine DSSC (h of 0.003%), verifying that the improved DSSC device was endowed with NIR light harvesting ability. Furthermore, the core/shell nanoparticles with concurrent UC and DC processes make the DSSC device produce a higher h of 0.0375%, demonstrating that the NIR and UV lights can be simultaneously utilized. Along this line, we further demonstrated that the embedded converters enable the DSSC device responding to extra light under periodic concurrent NIR/UV lasers ON/OFF irradiation as shown in Fig. 4d. The photocurrent value and corresponding h of the DSSC with NaYF4:20% Yb,2%Er@NaYF4:7%Eu nanoparticles were tested as a function of time at a constant forward bias of 0.8 V under AM1.5G with the simultaneous 980 nm NIR and 395 nm UV lasers switched on and off every 20 s. The stable photocurrent density of 14.303 mA cm2 and h of 7.664% were generated under the standard sunlight
Fig. 4. (a) The current density-voltage (JeV) characteristics and (b) the IPCE spectrum of DSSC, DSSC with UC NPs and UC/DC NPs under AM1.5 G simulated sunlight irradiation (100 mW cm2) and illumination of 980 nm laser with an output of 1 W power. (c) J-V curves measured under 980/395 nm laser irradiation (1 W cm2) for DSSC, DSSC with UC NPs and UC/DC NPs. (d) The photocurrent density measured at 0.8 V and the corresponding h of DSSC with UC/DC NPs under AM 1.5G with 980/395 nm laser switched on and off every 20 s.
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irradiation. However, the photocurrent density was increase to 14.856 mA cm2 together with an enhanced h of 7.888% aroused from the switching of 980 nm NIR and 395 nm UV lasers. This result strongly validates that the designed core/shell nanoparticles are able to absorb simultaneously NIR and UV lights for generation of additional photocurrent. 3. Conclusion In conclusion, for the first time, the novel core/shell-structured NaYF4:Yb,Er@NaYF4:Eu nanoparticles with concurrent UC and DC functions are successfully prepared and employed for proof-ofconcept study in DSSCs as an energy converter on top of the transparent TiO2 layer. The DSSC devices coupling with designed core/shell nanoparticle possess an excellent capacity to extend light absorption through enabling simultaneously harvesting and utilization of NIR and UV light by the UC core and DC shell. The optimal Jsc of 14.30 mA cm2 and h of 7.664% are achieved in the improved DSSC system, which is a noticeable improvement of 13.95% compared to the regular DSSC (6.726%). This great improvement is most likely due to the combined effects of enhanced UC/DC processes and light scattering. Our work provides a feasible strategy to broaden the solar spectrum use to UV and NIR sunlight for DSSCs or other types of solar cells.
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Conflicts of interest There are no conflicts to declare. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51402071), the National Science Fund for Distinguished Young Scholars (Grant No. 51325201).
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