Hexagonal Î²-NaYF4:Yb3+, Er3+ Nanoprism-Incorporated

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Hexagonal #-NaYF4:Yb3+, Er3+ Nanoprism-incorporated Upconverting Layer in Perovskite Solar Cells for Near-infrared Sunlight Harvesting Jongmin Roh, Haejun Yu, and Jyongsik Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04760 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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ACS Applied Materials & Interfaces

Hexagonal β-NaYF4:Yb3+, Er3+ Nanoprismincorporated Upconverting Layer in Perovskite Solar Cells for Near-infrared Sunlight Harvesting

Jongmin Roh, Haejun Yu, and Jyongsik Jang*

School of Chemical and Biological Engineering, College of Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-742, Korea

*Corresponding author E-mail: [email protected] Tel.: (+82) 2-880-7069 Fax: (+82) 2-888-1604

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ABSTRACT Hexagonal β-NaYF4:Yb3+, Er3+ nanoprisms, successfully prepared using a hydrothermal method, were incorporated into CH3NH3PbI3 perovskite solar cells (PSCs) as an upconverting mesoporous layer. Due to their near-infrared (NIR) sunlight harvesting, the PSCs based on the upconverting mesoporous layer exhibited a power conversion efficiency of 16.0%, an increase of 13.7% compared with conventional TiO2 nanoparticle-based PSCs (14.1%). This result suggests that the hexagonal β-NaYF4:Yb3+, Er3+ nanoprisms expand the absorption range of the PSC via upconversion photoluminescence, leading to an enhancement of the photocurrent.

KEYWORDS: NaYF4:Yb3+, Er3+, upconversion, nanoprism, perovskite solar cell


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Since organolead trihalide perovskite (CH3NH3PbX3, X = Cl, Br, I) was rediscovered as the sensitizer in solid-state solar cells,1 perovskite solar cells (PSCs) have increasingly improved their power conversion efficiency, rising from 9.7%1 to 20.1%.2 Organolead trihalide perovskite is considered to be a particularly promising material, due to its unique properties that include a large absorption coefficient,3 ambipolar charge transport,4 long electron–hole diffusion lengths,5,6 and facile solution processability.7-9 To date, there have been numerous attempts to improve the photovoltaic performance and stability of PSC devices, such as by controlling the processing conditions of perovskite films,10,11 engineering the chemical composition of perovskite,12,13 introducing additives to perovskite,14 and designing interface energy alignments.15,16 However, the perovskite sensitizer has a bandgap of 1.55 eV and absorbs only a small portion of incident light in the visible spectrum (up to 800 nm),1,17 thus resulting in energy loss due to the non-absorption of near-infrared (NIR) light. Light harvesting of low-energy photons below the absorption threshold of perovskite is required for high-performance PSC devices. One interesting approach to solve this NIR energy loss problem is to broaden the light absorption spectrum using upconversion materials. In the photon upconversion process, two or more incident photons are converted into one higher energy photon,18-20 generating additional photocurrent in solar cells.21 Among many upconversion materials, ytterbium and erbium codoped beta-phase sodium yttrium fluoride (β-NaYF4:Yb3+, Er3+) is well known as the most efficient upconversion phosphor for bright green photoluminescence (PL) and is widely used in lasers,22 solar cells,23, 24 and bioimaging.25 In NaYF4:Yb3+, Er3+ phosphor, Yb3+ ions act as a NIR sensitizer and Er3+ ions act as a visible photon emitter.26 NaYF4:Er3+ upconversion phosphors, applied to the bottom layer of silicon solar cells, theoretically increase the power conversion efficiency of the cells from 20% to 25%.23 In dye-sensitized solar cells (DSSCs), large NaYF4:Yb3+, Er3+ nanoplates and nanoprisms have been utilized as 3 ACS Paragon Plus Environment


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upconverting and light-scattering materials to increase the optical pathlength of the incident light and to obtain better NIR response.24, 27 It is expected that upconversion nanomaterials can be used to improve the photovoltaic performance of PSCs, as well as silicon solar cells and DSSCs. In this work, we report a novel upconverting mesoporous layer based on TiO2 nanoparticles and hexagonal β-NaYF4:Yb3+, Er3+ nanoprisms for PSCs. NaYF4:Yb3+, Er3+ nanoprisms were introduced to the TiO2 mesoporous layer in a PSC device as upconverting centers (Figure 1a). Low-energy NIR light absorbed by NaYF4:Yb3+, Er3+ nanoprism cores can be converted into high-energy visible light that can be utilized in PSCs. Thus, the NaYF4:Yb3+, Er3+ nanoprisms expand the absorption range of the PSC via upconversion PL, resulting in an enhancement of the photocurrent.

Figure 1. a) Schematic configuration and b) cross-sectional scanning electron microscopy (SEM) image of the perovskite solar cells (PSCs) device with a hexagonal NaYF4:Yb3+, Er3+ nanoprism upconverting layer. Hexagonal NaYF4:Yb3+, Er3+ nanoprisms were fabricated via a simple hydrothermal process using an aqueous solution containing trivalent lanthanide ions (Y3+:Yb3+:Er3+ = 78:20:2), sodium citrate, and ammonium fluoride.28 TiO2 nanoparticle- and NaYF4:Yb3+, Er3+ nanoprism-based pastes were prepared by mixing with ethanol, terpineol, ethyl cellulose, and lauric acid. To introduce an upconverting mesoporous layer into the PSC, diluted TiO24 ACS Paragon Plus Environment

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NaYF4:Yb3+, Er3+ mixed paste was spin-coated onto the compact TiO2 layer and annealed at 500°C for 30 min. Finally, the upconverting mesoporous film was post-treated with aqueous TiCl4 solution and heated at 500°C for 30 min to reduce charge recombination at the perovskite/TiO2 and perovskite/NaYF4:Yb3+, Er3+ interface.29 The surface morphology of the upconverting mesoporous layer and the cross-section of the PSC with the upconverting mesoporous film are shown in Figure S1 (Supporting Information) and Figure 1b, respectively; in Figure 1b, the thickness of the TiO2 mesoporous layer is ca. 200 nm and large hexagonal NaYF4:Yb3+, Er3+ nanoprisms are exposed over the TiO2 mesoporous layer. The amount of NaYF4:Yb3+, Er3+ nanoprisms on the surface of the upconverting mesoporous layer increases with the concentration of NaYF4:Yb3+, Er3+ nanoprisms in the mixed paste. The morphology of the NaYF4:Yb3+, Er3+ nanoprisms was investigated by scanning electron microscopy (SEM). Figure 2a shows a representative SEM image of the NaYF4:Yb3+, Er3+ nanoprisms. The fabricated NaYF4:Yb3+, Er3+ nanoprisms have a uniform hexagonal prism structure with good monodispersity and well-defined facets. The nanoprisms had an average size of 550 nm in diameter and 600 nm in height. The crystallinity of the NaYF4:Yb3+, Er3+ nanoprisms was confirmed by X-ray diffraction (XRD), as shown in Figure 2b. The diffraction peaks of the NaYF4:Yb3+, Er3+ nanoprisms corresponded exactly to the pure crystalline hexagonal β-NaYF4 phase (JCPDS No. 16-0334). Compared with the cubic α-NaYF4 phase, hexagonal β-NaYF4 is a more efficient host lattice of various lanthanide ions for upconversion PL, due to its high upconversion quantum yield.28. Figure 2c shows the upconversion PL spectrum of NaYF4:Yb3+, Er3+ nanoprisms under 980 nm excitation. In the upconversion process of the NaYF4:Yb3+, Er3+ system, the energy in Yb3+ excited by NIR light is transferred to Er3+ ions and released as high-energy visible light. Four Er3+ emission peaks at 408, 523, 543, and 655 nm were observed; these peaks were assigned to 2H9/2 → 4I15/2 (408 nm), 2H11/2 → 4I15/2 (523 nm), 4S3/2 → 4I15/2 (543 nm), and 4F9/2 5 ACS Paragon Plus Environment


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→ 4I15/2 (655 nm) transitions (Figure 2e).28 Notably, the 4S3/2 → 4I15/2 transition, a dominant peak in the emission spectrum of NaYF4:Yb3+, Er3+ nanoprisms, corresponds to bright green fluorescence under 980 nm excitation (Figure 2d). This result suggests that the NaYF4:Yb3+, Er3+ nanoprisms facilitate NIR light absorption by the perovskite sensitizer as upconversion PL, broadening the absorption spectrum range of the PSC device. Additionally, the Er3+ ions can be directly excited in broadband NIR by the 4I13/2 state of Er3+, followed by excited-state absorption from 4I13/2 to 4F9/2 emits visible light with 650 nm. Furthermore, the three-photon absorption around 1540 nm also results in same emission wavelength of light from Er3+. These additional upconversion paths can increase the emission of NaYF4:Yb3+, Er3+ phosphor, and contribute the additional photocurrent in the PSCs.26


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Figure 2. a) SEM image and b) X-ray diffraction (XRD) pattern of hexagonal NaYF4:Yb3+, Er3+ nanoprisms. c) Upconversion photoluminescence (PL) spectrum and d) digital photograph of hexagonal NaYF4:Yb3+, Er3+ nanoprisms under 980 nm near-infrared (NIR) laser excitation. e) Detailed energy-level diagram and corresponding energy transitions in the NaYF4:Yb3+, Er3+ system. The upconverting NaYF4:Yb3+, Er3+ nanoprisms are incorporated into the mesoporous layer in CH3NH3PbI3 PSCs, as shown in Figure 1b. To optimize the concentration of NaYF4:Yb3+, Er3+ nanoprisms in the mesoporous layer, the photocurrent density–voltage (J–V) characteristics of the PSCs with different weight ratios of NaYF4:Yb3+, Er3+ nanoprisms in the TiO2 mesoporous layer were investigated (Figure 3a); the corresponding photovoltaic properties of the various samples are summarized in Table S1. The PSC with the TiO2 control mesoporous film exhibited a short-circuit current density (Jsc) of 18.85 mA cm−2 and a power conversion efficiency (η) of 14.05%. All PSCs with a NaYF4:Yb3+, Er3+ nanoprism-added mesoporous layer exhibited enhanced Jsc and η, compared with the TiO2 reference PSC. Jsc and η values increased with the amount of NaYF4:Yb3+, Er3+ nanoprisms, up to 75 wt% (Figure S2). The best performance was obtained using a TiO2 mesoporous layer with 75 wt% NaYF4:Yb3+, Er3+ nanoprisms, demonstrating Jsc and η values of 20.23 mA cm−2 and 15.98%, respectively. These results represent a 13.74% enhancement in efficiency compared with the TiO2 reference device. This noticeable enhancement in Jsc and η was attributed to the upconversion luminescence properties of the NaYF4:Yb3+, Er3+ nanoprisms from NIR to visible light via the CH3NH3PbI3 perovskite layer. The performance improvement by upconversion effect was also identified from J–V characteristics obtained under 980 nm NIR laser condition. (Figure S3, Table S2). The Jsc of 0.027 mA cm-2 and Voc of 0.374 V were



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estimated from the measurement respectively. From this result, it is underpined that NaYF4:Yb3+, Er3+ nanoprisms function as upconverting materials in PSCs well. To investigate the charge transfer characteristics in PSC devices, electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 1 Hz to 1 MHz at −0.8 V bias under dark conditions. Figure 3b shows the Nyquist plots of the PSCs for various weight ratios of NaYF4:Yb3+, Er3+ nanoprisms in the TiO2 mesoporous layer. Typically, two semicircles appear in the Nyquist plots in the higher- and lowerfrequency regions. The small semicircles at higher frequencies show the charge transfer at the interfaces of the hole transport layer (HTL)/Au electrode; in this case, the semicircles were too small to be distinguished in the Nyquist plots. The large semicircles at lower frequencies are related to the charge recombination of electrons at the mesoporous film/perovskite/HTL interfaces.30 The recombination resistance values of the fabricated PSCs at the mesoporous film/perovskite/HTL interfaces (Rct) were similar with respect to the various weight ratios of NaYF4:Yb3+, Er3+ nanoprisms (Figure 3b); this implies that there is little charge transfer difference in the TiO2 layer and TiO2 layer with NaYF4:Yb3+, Er3+ nanoprisms. Thus, the TiCl4 post-treatment that passivates the recombination sites on the upconverting mesoporous layer was believed to be responsible for reducing charge recombination between the mesoporous film/perovskite interface.29 To meticulously figure out the charge transport resistance at the interfaces, EIS measurement was also conducted under illumination conditions. (Figure S4) Actually, there is no significant difference on the charge transport resistance, however, the slightly improved charge transport behavior was observed by replacing pure TiO2 based mesoporous layer with NaYF4:Yb3+, Er3+ nanoprisms incorporated mesoporous layer. It is ascribed to the enhanced pore-filling of perovskite in cramped TiO2 mesoporous layer, since thickness of mesoporous TiO2 layer was slightly lowered by decreasing concentration of TiO2 in stock paste. Furthermore, when some sections of TiO2 8 ACS Paragon Plus Environment

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based mesoporous layer are occupied by large NaYF4:Yb3+, Er3+ nanoprisms, the electrons excited in perovskite have a high chance to be transferred through high crystalline perovskite phase rather than TiO2 layer. So the charge migration efficiency would be enhanced.

Figure 3. a) Photocurrent density–voltage (J–V) characteristics and b) Nyquist plots of PSCs using an upconverting mesoporous layer with varied ratios of NaYF4:Yb3+, Er3+ nanoprisms.



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Electrochemical impedance spectroscopy (EIS) measurements were performed under dark conditions. To elucidate the NIR photovoltaic capability of NaYF4:Yb3+, Er3+ nanoprisms, the optimal photocurrent density–voltage (J–V) curves of the PSCs using TiO2 nanoparticles, TiO2 nanoparticles with NaYF4 nanoprisms, and TiO2 nanoparticles with NaYF4:Yb3+, Er3+ nanoprisms are presented in Figure 4a; the associated photovoltaic parameters are listed in Table 1. The PSC with NaYF4 nanoprisms in the mesoporous layer exhibited a Jsc of 18.52 mA cm−2 and a η of 14.26%, similar to the TiO2 reference device (Jsc of 18.85 mA cm−2, η of 14.05%). However, by replacing the NaYF4 nanoprism film with a NaYF4:Yb3+, Er3+ nanoprism upconverting layer, Jsc increased from 18.52 mA cm−2 to 20.23 mA cm−2. Consequentially, the best device performance of 15.98% was achieved, which is a 12.06% enhancements in efficiency compared with the NaYF4 nanoprism film. Figure 4b displays incident photon-to-current conversion efficiency (IPCE) spectra of the PSCs using TiO2 nanoparticles, TiO2 nanoparticles with NaYF4 nanoprisms, and TiO2 nanoparticles with NaYF4:Yb3+, Er3+ nanoprisms. A little change was observed in IPCE result when NaYF4 nanoprisms were added in the mesoporous layer. However, The IPCE value over the entire region of 400-750 nm was considerably enhanced with incorporating NaYF4:Yb3+, Er3+ nanoprisms in the mesoporous layer. The PSCs using NaYF4:Yb3+, Er3+ nanoprisms exhibits a maximum IPCE value of 85.4% at 525 nm owing to upconversion PL of NaYF4:Yb3+, Er3+ nanoprisms. This result is consistent with the photovoltaic performance shown in Figure 4b. From this perspective, application of NaYF4:Yb3+, Er3+ nanoprisms to PSCs represents a way to supplement the energy loss caused by non-absorption of NIR light via photon upconversion. When incident light is projected to the PSCs, the visible photons which energies are higher than the bandgap of perovsksite sensitizer is converted into a photocurrent 10 ACS Paragon Plus Environment

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directly. While NIR photons with energies lower than the bandgap of perovskite are effectively converted into visible photons by 980 nm NIR sensitization of Yb3+ followed by energy transfer to Er3+ and direct excitation of Er3+ by broadband NIR illumination ranged from 1000 to 1600 nm.26



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Figure 4. a) Photocurrent density–voltage (J–V) characteristics of PSCs using TiO2 nanoparticles, NaYF4 nanoprisms, and NaYF4:Yb3+, Er3+ nanoprisms and b) their corresponding incident photon-to-current conversion efficiency (IPCE) spectra. c) The schematic diagrams of the energy transfer process in PSCs using an upconverting mesoporous layer. Table 1. Photovoltaic parameters of the perovskite solar cells (PSCs) using TiO2 nanoparticles, NaYF4 nanoprisms, and NaYF4:Yb3+, Er3+ nanoprisms. Measurements were performed under AM 1.5G sunlight intensity of 100 W cm−2. Average values of all parameters were obtained for 15 devices.

Devicesa

JSCb (mA cm-2)

VOCc (V)

FFd

η e (%)

TiO2 (Reference)

18.85 ± 0.69

1.09 ± 0.01

0.68 ± 0.01

14.05 ± 0.59

TiO2 with 75% NaYF4

18.52 ± 0.41

1.10 ± 0.01

0.70 ± 0.01

14.26 ± 0.61

TiO2 with 75% NaYF4:Yb3+, Er3+

20.23 ± 0.94

1.10 ± 0.01

0.72 ± 0.02

15.98 ± 0.93

a

Active area of the fabricated PSC devices is 0.09 cm2; b short-circuit current; c open-circuit

voltage; d fill factor; and e power conversion efficiency. In conclusion, hexagonal β-NaYF4:Yb3+, Er3+ nanoprisms, successfully fabricated using a simple hydrothermal method, were incorporated into a PSC mesoporous layer as upconverting cores. By optimizing the amount of NaYF4:Yb3+, Er3+ nanoprisms in the TiO2 mesoporous layer, the PSC with a NaYF4:Yb3+, Er3+ nanoprisms exhibited the best power 12 ACS Paragon Plus Environment

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conversion efficiency of 15.98, an overall enhancement of 13.74% compared with that of a TiO2-based PSC device. This enhanced PSC performance was attributed to the additional photocurrent generation via NIR photon upconversion of NaYF4:Yb3+, Er3+ nanoprisms. This work provides new possibilities for the introduction of NIR harvesting nanomaterials in PSCs, further promoting broad light absorption for high-performance PSCs.



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ASSOCIATED CONTENT Supporting Information. Experimental details, SEM images of upconverting mesoporous layer and photovoltaic parameters of all PSC devices. EIS measurements under illumination condition. Photocurrent density-voltage characteristics and photovoltaic parameters evaluated under 980 nm NIR laser. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program on the Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea (2011-0031573). This work was supported by Development Fund of Seoul National University funded by Dongjin Semichem Co., Korea (045820130066).

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Chen,

D.;

Huang,

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Highly

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Luminescence

in

Yb/Er:NaGdF4@NaYF4 Core-shell Nanocrystals with Complete Shell Enclosure of the Core. Dalton Trans. 2014, 43, 11299-11304. (21) Auzel, F., Upconversion and Anti-Stokes Processes with f and d Ions in Solids. Chem. Rev. 2004, 104, 139-174. (22) Scheps, R., Upconversion Laser Processes. Prog. Quantum Electron. 1996, 20, 271358. (23) Shalav, A.; Richards, B. S.; Trupke, T.; Krämer, K. W.; Güdel, H. U., Application of NaYF4:Er3+ Up-Converting Phosphors for Enhanced Near-Infrared Silicon Solar Cell Response. Appl. Phys. Lett. 2005, 86, 013505. 17 ACS Paragon Plus Environment


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(24) Liang, L.; Liu, Y.; Zhao, X.-Z., Double-shell β-NaYF4:Yb3+, Er3+/SiO2/TiO2 Submicroplates as a Scattering and Upconverting Layer for Efficient Dye-Sensitized Solar Cells. Chem. Commun. 2013, 49, 3958-3960. (25) Zhou, L.; He, B.; Huang, J.; Cheng, Z.; Xu, X.; Wei, C., Multihydroxy Dendritic Upconversion Nanoparticles with Enhanced Water Dispersibility and Surface Functionality for Bioimaging. ACS Appl. Mater. Interfaces 2014, 6, 7719-7727. (26) Goldschmidt, J. C.; Fischer, S., Upconversion for Photovoltaics – a Review of Materials, Devices and Concepts for Performance Enhancement. Adv. Opt. Mater. 2015, 3, 510-535. (27) Shan, G.-B.; Assaaoudi, H.; Demopoulos, G. P., Enhanced Performance of DyeSensitized Solar Cells by Utilization of an External, Bifunctional Layer Consisting of Uniform β-NaYF4:Er3+/Yb3+ Nanoplatelets. ACS Appl. Mater. Interfaces 2011, 3, 3239-3243. (28) Li, C.; Quan, Z.; Yang, J.; Yang, P.; Lin, J., Highly Uniform and Monodisperse βNaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, and Yb/Tm) Hexagonal Microprism Crystals: Hydrothermal Synthesis and Luminescent Properties. Inorg. Chem. 2007, 46, 6329-6337. (29) Law, M.; Greene, L. E.; Radenovic, A.; Kuykendall, T.; Liphardt, J.; Yang, P., ZnO−Al2O3 and ZnO−TiO2 Core−Shell Nanowire Dye-Sensitized Solar Cells. J. Phys. Chem. B 2006, 110, 22652-22663. (30) Zhang, J.; Hu, Z.; Huang, L.; Yue, G.; Liu, J.; Lu, X.; Hu, Z.; Shang, M.; Han, L.; Zhu, Y., Bifunctional Alkyl Chain Barriers for Efficient Perovskite Solar Cells. Chem. Commun. 2015, 51, 7047-7050.


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Hexagonal Î²-NaYF4:Yb3+, Er3+ Nanoprism-Incorporated

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