High-Performance Perovskite Solar Cells with a Weak Covalent TiO2

Subscriber access provided by University of Florida | Smathers Libraries

Article

High-Performance Perovskite Solar Cells with a Structure of Weak Covalent TiO2:Eu3+ Mesoporous Ling Jiang, Jia-Wei Zheng, Wangchao Chen, Yang Huang, Linhua Hu, Tasawar Hayat, Ahmed Alsaedi, Changneng Zhang, and Songyuan Dai ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00008 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Energy Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

High-performance Perovskite Solar Cells with a Structure of Weak covalent TiO2:Eu3+ Mesoporous Ling Jiang1,2, Jiawei Zheng2, Wangchao Chen2, Yang Huang2, Linhua Hu2, Tasawar Hayat3, Ahmed Alsaedi3, Changneng Zhang2*, and Songyuan Dai4*

1

College of Engineering and Technology, Jilin Agricultural University, Changchun, Jilin,

130118, P. R. China 2

Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of

Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui, 230088, P. R. China 3

NAAM Research Group, Department of Mathematics, Faculty of Science, King

Abdulaziz University, Jeddah 21589, Saudi Arabia 4

Beijing Key Laboratory of Novel Thin Film Solar Cells, North China Electric Power

University, Beijing, 102206, P. R. China

ABSTRACT: The mesoporous TiO2 nanocrystal attached with Eu3+ ions in amorphous state are employed as structure for high efficient perovskite solar cells. The down-conversion

TiO2:Eu3+

nanocrystal

shows

good

photoluminescence

characteristic to convert the incident high-energy photon into lower-energy photon, which improves the UV light utilization capability of perovskite solar cells. The device within 1.0 wt% Eu3+ ions shows an improvement of 29.2% from 12.22% to 15.79% in PCE compared with the typical devices without Eu3+ in TiO2, which is attributed to the increased utilization of incident UV light and the dramatically suppressed electron recombination process. To summarize, the combination of high UV light-harvesting capability and slow electron recombination process with TiO2:Eu3+ mesoporous structure could successfully develop a more efficient and stable PSCs. Keywords: Photoluminescence, down-conversion, interface recombination, UV

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

stability, perovskite solar cells.

1. Introduction Lead halide perovskite solar cells (PSCs) have achieved great success in the field of power conversion with ideal direct bandgap and high absorption coefficient in visible region. [1-10] The typical sandwiched structure of PSC includes TiO2 electron transporting layer, light-harvesting active layer (CH3NH3PbI3 crystal) and hole transport materials (HTM).[11] Under the irradiation of visible light, CH3NH3PbI3 can absorb the incident light, generating electron and hole, and the created electrons are injected into the conduction band of TiO2, while the extracted holes are simultaneously scavenged by HTMs which are finally collected by electrodes through the external load forming perovskite solar cells. As an outstanding photovoltaic material, TiO2 has shown the excellent performance of the electronic collection and transmission of semiconductor oxides. [12-15] But TiO2 can generate a redox ability of electron-hole pairs after absorbing UV light, which will destroy the perovskite crystal and decrease the PSC efficiency. [16,17] Common method for solving this problem is to coat on outer surface of PSCs with polymer ultraviolet light filter to avoid photodegradation of PSCs. Whereas the sealed UV light filter makes PSCs difficult to use ultraviolet light. [18] In order to extend the spectral response range of PSC to UV region, transparent rare earth complex luminescent down-converting layer and fluorinated polymer film were applied on the back side of PSC to convert UV light into useful visible light, which helps to improve the PSC efficiency. [18,19] To further enhance the photovoltaic performance of PSCs, researchers have made great progress in the TiO2 mesoporous electron transport materials, such as


Page 2 of 27

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SrTiO3, Nb doped TiO2, graphite-TiO2, graphite-SrTiO3 and other stable metal oxide SnO2 and Al2O3 as the electron transport layer. [20-23] However, their structures have low UV light utilization and the perovskite materials will gradually degrade, leading to a decline in cell performance. Rare-earth ions have a charge-transfer absorption band in the UV region and their emission spectra are mainly located in the visible region, so they are commonly used as the luminescence center to enhance PSC performance. But rare-earth ions only possess weak light absorption due to the specific 4f electronic structure. Recently, TiO2 as an extremely stable oxide has attracted extensive attention due to their unique optical properties and prospective applications in optoelectronic devices. Zeng et al have found that Eu3+ ions have good luminescent properties in TiO2:Eu3+ nanomaterial. [24] Chen et al used the induced luminescence of Eu3+ ion as spectroscopic probe to prove that Eu3+ ion reduced the symmetry of local structure by multigrid occupying TiO2. [25] Subsequently, they doped Sm3+and Nd3+ into TiO2 nanocrystals to obtain efficient energy transfer from TiO2 to Sm3+ and Nd3+ ions. [26] It is suggested that TiO2 induced luminescence of rare-earth ions could achieve better broad-spectrum ultraviolet energy and transfer to multi photons with long-wavelength visible light. Therefore, down-conversion luminescence of rare-earth ions induced by TiO2 has opened a new way to achieve high performance of PSCs in efficient lighting management. In this work, a modified sol-gel method is adopted to obtain the semiconductor light-emitting materials for synthesis of novel down-conversion TiO2:Eu3+ crystal for PSCs. Eu3+ ion as the luminescence center was incorporated to form a weaker force between Eu3+ ions and TiO2, and TiO2 nanocrystals posses with a better absorption in



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

the UV region to induce Eu3+ ions and realize the effective transfer to Eu3+ ions. The effective use of UV light with TiO2:Eu3+ luminescent for PSCs could help them to achieve higher photocurrent and efficiency. And UV stability of PSCs based TiO2:Eu3+ is improved without any additional UV filtration. 2. Materials and Methods Materials Europium (III) nitrate hexahydrate was purchased from J&K. Lead (II) iodide, dimethyl sulfoxide, lithium bis(trifluoromethylsulphonyl) imide (Li-TFSI), and 4-tert-butylpyridine

(TBP)

were

purchased

from

Aldrich.

Acetylacetone,

N,N-dimethylformamide, ethanol and chlorobenzene were obtained from Sinopharm. Methylammonium Iodide and Spiro-MeOTAD were from Xi’an p-OLED. And TiO2 paste (18 NRT) was purchased from Dyesol. All chemicals were commercially available and used without further purification. Device fabrication FTO glass (Pilkington TEC 15 15 Ω/□) was patterned by etching using zinc powder and hydrochloric acid (1 M). The substrates were ultrasonically cleaned in ethanol and deionized water for 20 min, respectively, then treated at 510 °C for 30 min to remove the organic residue. The dense blocking layer of TiO2 was deposited on patterned FTO substrates by spray pyrolysis of 7 mL of anhydrous isopropanol containing 0.6 mL of titanium diisopropoxide and 0.4 mL of bis (acetylacetonate), using dry air as carrier gas, at 460 °C to avoid direct contact between the FTO and hole transport layer (HTL). The mesoporous TiO2 layer was prepared by spin-coating of diluted TiO2 pastes (Dyesol 18NRT, 1:5.5 mass ratio with ethanol) with different amount of Eu(NO3)3·6H2O (0.0, 0.5, 1.0 and 1.5 wt% with undiluted TiO2 paste) on


Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the dense blocking layer of TiO2 at 4000 rpm for 20 s. The substrates were dried at 80°C and then annealed at 500 °C for 30 min. The CH3NH3PbI3 layer was formed by two-step spin-coating process. The mixed solution of PbI2 and CH3NH3I was prepared by dissolving PbI2 (530 mg) and CH3NH3I (180 mg) into a mixed solution of DMSO and DMF (1 mL, volume ratio 85:15) under stirring at 70 °C. Then, 50 μL of the solution was put on the mesoporous TiO2 layer at 1000 rpm for 20 s and 4000 rpm for 30 s. During the second spin-coating step, the substrates were treated with chlorobenzene at the last 10 s, which helps to form the perovskite crystal. After that, the substrates were annealed at 105 °C for 1 h. Subsequently, 30 μL of solution containing spiro-OMeTAD

(72.3

mg),

4-tert-butylpyridine

(28.8

μL),

lithium

bis

(trifluoromethylsulphonyl) imide (17.5 μL), Co (III)-complex (8 μL) and chlorobenzene (1 mL) was coated on the CH3NH3PbI3 layer by spin-coating at 3000 rpm for 20 s to form the HTM layer. Finally, a gold layer with a thickness of 60 nm was thermally evaporated on the HTM layer to form the back contact. Characterization The surface and cross-sectional morphology of the TiO2 and TiO2:Eu3+ layers were investigated by scanning electron microscopy (FEI XL-30 SFEG coupled to a TLD). Transmission electron microscopy (TEM) micrographs of the nanoparticles were obtained from the JEM-2010 transmission electron microscope (JEOL, Japan). The XRD patterns were measured using a Bruker-AXS Microdiffractometer (model D5005) with Cu Kα radiation (λ=1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo-VG Scientific ESCALAB 250 instruments, and the binding energies were referred to the C 1s level of saturated hydrocarbons at 284.6 eV. UV-vis absorption spectroscopy was recorded using the U-3900H UV-vis



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

spectrophotometer (Hitachi, Japan). Photoluminescence (PL) spectra were examined using a fluorescence spectrophotometer (QM400-TM, America). Transient absorption (TA) was measured by LKS.80 and LP.920 to analyze the charge recombination in PSCs with TiO2 and TiO2:Eu3+. The IPCE values were confirmed as a function of wavelength from 300 to 900 nm (PV Measurements, Inc.). The J-V curves were carried out on a solar simulator (solar AAA simulator, Oriel) with a source meter (Keithley Instruments, Inc.) at 100 mW/cm2, AM 1.5G illumination. The irradiance was calibrated using a Si reference cell certified by NREL. The applied bias voltage for the reverse scan was from 1.2 to -0.1 V, which was contrary to that of forward scan. The scan rate is 50 mV/s. The photovoltaic performance of our devices is not confirmed from independent certification laboratories. The forward bias for steady-state output of the PSC with TiO2:Eu3+ was held close to 0.805 V. The active area was 0.09 cm2. The UV-light-soaking test was under a light source (UV−Hg-2000, Beijing Lighting Research Institute). 3. Results and Discussion


Page 6 of 27

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. SEM images of TiO2 and 1.0 wt%-TiO2:Eu3+ layer (a and b); SEM images of TiO2/CH3NH3PbI3 and 1.0 wt%-TiO2:Eu3+/CH3NH3PbI3 layer (c and d); cross-sectional SEM images of PSCs based TiO2 and 1.0 wt%-TiO2:Eu3+ (e and f).

The surface morphologies of down-conversion 1.0 wt%-TiO2:Eu3+ and pure TiO2 nanoparticles are investigated by SEM as shown in Figure 1a and 1b. Average crystal size of TiO2 and Eu3+ ions doped TiO2 calculated have uniform spherical nanoparticle shape with sizes of about 20 nm. Obviously, the distribution of TiO2 particles become more uniform and the gap between particles is reduced after incorporating Eu3+ ions, which is beneficial to the electron transfer in the mesoporous layer. The morphologies of CH3NH3PbI3 perovskite deposited on pure TiO2 or 1.0 wt% TiO2:Eu3+ films are shown in Figure 1c and 1d. The particle size of CH3NH3PbI3 deposited on pure TiO2 film is differed, while, the CH3NH3PbI3 deposited on 1.0 wt% TiO2:Eu3+ film shows relatively uniform in particle size. It is possible that Eu3+ ions decrease the gap



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

between TiO2 particles, which provides a smooth substrate for the homogeneous growth of CH3NH3PbI3. From the cross-sectional SEM images of PSCs as shown in Figure 1e and 1f, we can learn that the substrate of TiO2 doped Eu3+ ions is relatively smooth and the growth of CH3NH3PbI3 thin film is also flatter.

0.350 nm (101)

0.347 nm

(101)

Figure 2. TEM images of TiO2 and 1.0 wt%-TiO2:Eu3+ nanoparticles (a and c); HRTEM images of TiO2 and 1.0 wt%-TiO2:Eu3+ nanoparticles (b and d).

The TEM images display that TiO2 particle is almost unchanged after doping with Eu3+ ions in Figure 2a and 2c. The average size of these nanocrystalline was observed to be in the range of 20-30 nm, which were in coincidence with those estimated by SEM. HRTEM images reveal that the lattice spaces of pure TiO2 and 1.0 wt% TiO2:Eu3+ are 3.50 Å (Figure 2b) and 3.47 Å (Figure 2d) respectively, corresponding to (101) crystal plane of TiO2. However, the smooth edge and unclear crystal morphology of 1.0 wt% TiO2:Eu3+ particles are shown in Figure 2d, indicating a coating formed to be amorphous substance on the surface of TiO2 particles. Those analyses show that Eu3+


Page 9 of 27

ions cannot enter TiO2 lattice, but possibly adhere to TiO2 surface in amorphous state. a

b O1s

TiO2

Eu3d

1000

800 600 400 Binding Energy (eV)

200

0

468

466

1140

1138

1136

1134

1132

d

O1s

0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

Intensity (a.u.)

Ti2p

464 462 460 Binding Energy (eV)

1130

Binding Energy (eV)

c 0.0 wt% 0.5wt% 1.0wt% 1.5wt%

0.5 wt% 1.0 wt% 1.5 wt%

Intensity (a.u.)

Intensity (a.u.)

3+

Ti2p

1200

Eu(NO3)3

Eu3d

TiO2 : Eu

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


458

456

531

530 Binding Energy (eV)

529

Figure 3. XPS spectra of mesoporous TiO2 with Eu3+ ions powder: (a) Survey (pure TiO2 and 1.0 wt%-TiO2:Eu3+), (b) Eu3d, (c) Ti2p, and (d) O1s. Table 1 Binding energy values of Eu3d, Ti2p and O1s for TiO2 with Eu3+ ions. Eu3d Ti2p O1s Eu(NO3)3 1135.30 ― ― TiO2 ― 459.20 530.40 0.5 wt% 1135.48 459.17 530.38 1.0 wt% 1135.50 459.15 530.37 1.5 wt% 1135.47 459.16 530.38 Table 2 The atomic content of Eu3+ ions dopant in TiO2. Sample (Wt%) 0.0 0.5 Content (At. %) 0.46 1.04

1.0 1.75

1.5 1.83

To investigate the chemical surrounding of Eu3+ ions in TiO2 nanoparticles, we characterized XPS signals of down-conversion TiO2:Eu3+(0.0, 0.5, 1.0, 1,5 wt%) as shown in Figure 3 and Table 1. Table 2 shows the atomic content of Eu3+ ions in TiO2. There occurs the Eu3d binding energy near 1135 eV in XPS spectra of 1.0 wt% TiO2:Eu3+, demonstrating the existence of Eu (Figure 3a). The Eu3d binding energy of



Eu(NO3)3 is observed at 1135.30 eV, while, the corresponding binding energies in TiO2:Eu3+ are observed at high energies and the intensity of Eu3d peak becomes larger with increase of Eu3+ ions doping content (Figure 3b). The Ti2p binding energy decreases slightly after doping Eu3+ ions and the intensity of Ti2p peak decreases with the increase of Eu3+ ions doping content as shown in Figure 3c. Along with the introduction of Eu3+ ions, the O1s peak intensity decreased, but the binding energy displays no change (Figure 3d). Those results indicate that Eu3+ ions are not incorporated into the TiO2 nanocrystal, but attached on surface of the TiO2 nanocrystal by weak force to affect its chemical environment. [27-30] Furthermore, the variation intensity of each element peak shows that the weaker force between Eu3+ ions and TiO2 increases with the increase of Eu doping content.

(101)

0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

(200) (004)

(105) (211) (224)

20

30

40 2θ (degree)

50

60

Figure 4. XRD patterns of TiO2:Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%).

The phase structure of TiO2:Eu3+ nanophosphor is determined by X-ray diffraction analysis. Figure 4 shows the XRD patterns of pure TiO2 and TiO2:Eu3+ nanophosphor sintered at 510 ºC for 30 min. The diffraction peaks of TiO2:Eu3+ nanophosphor at 2θ = 25.5°, 38.7°, 48.1°, 54.2°, 55.3° and 62.8° correspond to (101), (004), (200), (105), (211) and (224) planes of pure anatase form (JCPDS 21-1272).


Page 11 of 27

There appears no trace of characteristic peaks in the pattern of TiO2:Eu3+ nanophosphor, indicating the crystallization of TiO2 is almost not affect by Eu3+ as illustrated in Figure 1 and 2. However, the diffraction peaks of (101) crystal plane of TiO2:Eu3+ nanophosphor decrease gradually with the increase of Eu3+ ions doping concent. This may be due to the larger ionic radius of Eu3+ ion, making it difficult for Eu3+ ions to enter the nanocrystalline phase of TiO2 but embedded in the surface sites adjacent to the nanocrystal, resulting in weakening the intensity of diffraction peaks. In addition, the (101) crystal plane of TiO2:Eu3+ nanophosphor have a panning in contrast to (101) crystal plane of pure TiO2, which could be attributed to the influence of Eu3+ ion on the composition of TiO2 valence bonds.

a

b λex=270 nm Intensity (a.u.)

Intensity (a.u.)

λem=415 nm

260

280

300

320

340

360

380

300

400

500

c

600

700

Wavelength (nm)

Wavelength (nm)

d

396 nm

5

7

D0→ F2

0.5 wt% 1.0 wt% 1.5 wt%

λex= 396 nm Intensity (a.u.)

λem= 614 nm

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


360

380

5

415 nm

384 nm

5

400 Wavelength (nm)

420

440

520

7

D0→ F1 7 D0→ F0

560

5

5

600 640 Wavelength (nm)

7

D0→ F4

7

D0→ F3

680

720

Figure 5. Excitation spectra (a) and emission spectra (b) of TiO2; excitation spectra (c) and emission spectra (d) of TiO2:Eu3+ (0.5, 1.0 and 1.5 wt%).

The excitation and emission spectra of pure TiO2 down-conversion TiO2:Eu3+ are



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

measured at room temperature to illustrate the down-conversion process in Figure 5. The light under 280 nm contribute to the emission of TiO2 (Figure 5a). The typical intra 4f transitions direct excitation peak at around 396 nm in the excitation spectrum of TiO2:Eu3+ due to the 7F0–5LJ transition of Eu3+. [31] And the weaker peak at around 415 nm overlaps with the emission bands of TiO2, revealing that TiO2 could induce luminescence of Eu3+ ions. Moreover, a little shoulder peak at around 384 nm corresponding to the energy gap of TiO2 (3.22 eV) is observed in the excitation spectrum (Figure 5b and 5c). On the basis of the excitation spectrum, the Eu3+ ions can be excited with UV light not only through the TiO2 host lattice but also through the absorption by themselves. [32] Figure 5d shows the characteristic emission bands of TiO2:Eu3+ nanophosphor under 396 nm excitation located at 578, 591, 614, 652, and 700 nm, which are connected to the 5D0→7F0, 5D0→7F1, 5D0→7F2, 5D0→7F3 and 5D0→7F4 forbidden transitions, respectively and with the increase of Eu3+ ions doping content, the fluorescence intensity is getting stronger. It should be noticed that the 5D0→7F0 transition is symmetry forbidden, the observation of 5D0→7F0 forbidden transition indicates that Eu3+ ions occupy low symmetry and no inversion center sites. A highly polarizable chemical environment around Eu3+ ions contributes to the bright red emission of the compound. [33] The site symmetry and coordination state of the Eu3+ ion is measured by equation as follows: R= I(5D0→7F2)/I(5D0→7F1)

(1)

Where R is the asymmetry ratio, I is the excitation peak intensity. According to this formula, the R values of different Eu3+ ions doping content (0.5, 1.0 and 1.5 wt%) are 6.5, 7.5 and 8, respectively, showing that Eu3+ ions occupy low symmetry sites


Page 13 of 27

and the more the content of Eu3+ ions doping in TiO2 nanocrystal, the lower the symmetry, and the higher the degree of covalent. Since the R value disagrees with the higher symmetry of Ti4+ ions in the anatase structure, which means that Eu3+ ion cannot take the place of Ti4+ but form a weak-covalent bond on surface of TiO2 nanocrystal, which is consistent with XPS analysis. b

a

80

4

0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

70 60 IPCE (%)

3 Abs (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

50 40

0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

30 20

1

10 0 300

400

500

600

700

800

0 300

400

Wavelength (nm)

500 600 Wavelength (nm)

700

800

Figure 6. (a) UV-vis absorption spectra of TiO2:Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%)/CH3NH3PbI3 and (b) IPCE curves of PSCs with the sructure of TiO2:Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%)/CH3NH3PbI3.

UV-Vis

absorption

behaviors of TiO2:Eu3+

(0.0,

0.5,

1.0

and

1.5

wt%)/CH3NH3PbI3 are displayed in Figure 6a. Despite the same CH3NH3PbI3 deposition method, the TiO2:Eu3+/CH3NH3PbI3 thin films exhibit stronger absorption than TiO2/CH3NH3PbI3 from 370 to 400 nm in the ultraviolet region. Moreover, with the increase of Eu3+ in content the ultraviolet absorption becomes more intense. This stronger absorption indicates that more light in the ultraviolet range is utilized by TiO2:Eu3+ nanophosphor. In accord with this interpretation is the result that the light harvesting efficiency (LHE) of these films employing TiO2:Eu3+ nanophosphor is greatly higher in the wavelength range between 370 and 400 nm than that for the undoped TiO2 particles. The incident photon-to-current conversion efficiency (IPCE) spectra exhibit a current response from 300 to 850 nm, with a maximum of more than 80% in the



wavelength range of 370-400 nm for down-conversion TiO2:Eu3+, and more than 70% for TiO2 (Figure 6b). This difference originates from the TiO2:Eu3+ absorbing UV light and re-emitting red light (the quantum yield is around 9%), which helps to improve the ultraviolet light harvesting capability of the device. Furthermore, the efficiency of carrier injection and/or recombination could be increased by Eu3+. Interestingly, when the doping content of Eu3+ ions reaches more than 1.5 wt%, the UV spectral response of PSC is lower than that of 1.0 wt%-TiO2:Eu3+. The possible reason is that the excessive Eu3+ ions increases the boundary defect of TiO2, influencing the electron transport. As TiO2:Eu3+ has almost no response in the visible wavelength region from 400 to 850 nm (shown in the excitation spectrum), thus, the thin perovskite films based on TiO2:Eu3+ mesoporous structures have almost the same light harvesting (shown in the absorption spectra) and conversion efficiency (the similar IPCE curves). From this phenomenon, it can be confirmed that TiO2: Eu3+ could enhance the light response of PSC in the ultraviolet region (370-400 nm), broaden the range of spectral response, and improve the photovoltaic performance.

λex= 473 nm

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

650

700

750 Wavelength (nm)

800

850

Figure 7. Emission spectra of FTO/TiO2:Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%)/CH3NH3PbI3.

To get information of Eu3+ doped TiO2 on electron transport in mesoporous TiO2


Page 15 of 27

structures, the photoluminescence spectroscopy measurements were made on TiO2:Eu3+/CH3NH3PbI3 for PSCs with TiO2 and Eu3+-doped TiO2 samples. Figure 7 showed the characteristic photo luminescence spectra of FTO/TiO2:Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%)/CH3NH3PbI3 samples at the excitation wavelength of 473 nm. Clear reduction of the photoluminescence intensity is observed in the TiO2:Eu3+ samples due to the PL quenching caused by Eu3+ ion. When the Eu3+ doping content reaches 1 wt%, the quenching efficiency is up to ~50%. It means that Eu3+ doped in mesoporous TiO2 structures is beneficial for efficient electron transport. However, excessive doping content (1.5 wt%) will hinder the electron transport in mesoporous TiO2 structures. a

b 0.5 wt% fitting ∆ OD(a.u.)

∆OD (a.u.)

0.0 wt% fitting

τ = 17.0 ns

40

60

80 Time (ns)

100

120

τ = 22 ns

40

60

80 Time (ns)

100

120

d

c

1.5 wt%

1.0 wt% fitting

fitting ∆OD (a.u.)

∆ OD (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


τ = 24.8 ns

40

60

80

100

120

τ = 23 ns

40

60

Time (ns)

80 Time (ns)

100

120

Figure 8. Transient absorption (TA) spectra of TiO2:Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%)/CH3NH3PbI3.

To further investigate electron recombination in PSCs, the transient absorption spectroscopy was undertaken. It has been confirmed that the transient absorption



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

spectroscopy is a useful tool to characterize the electron recombination dynamics in the films. [34] The excited CH3NH3PbI3 depositing on both TiO2 and TiO2:Eu3+ with a pump wavelength of 473 nm was detected the recombination dynamics in CH3NH3PbI3/TiO2 without and with Eu3+, i.e., interface recombination between electrons in mesoporous TiO2 and holes in CH3NH3PbI3, by measuring the TA responses with a probe wavelength of 658 nm, according to the theory of Yoshihara. [35,36] Figure 8 show the normalized TA response of CH3NH3PbI3/TiO2 without and with Eu3+. A bleaching signal with a slow decay is observed clearly. There is one process in the TA decay, and the TA responses can be fitted very well to a monoexponential decay equation as follows: y = A exp (-x/τ)

(2)

Where τ is the time constant, A is the contribution from the relevant component and the fitting lifetimes are shown in Figure 8. In the case of CH3NH3PbI3 perovskite deposited on TiO2, a time constant of 17 ns was obtained. However, the TA decays of CH3NH3PbI3/TiO2:Eu3+ become slower and the time constant is obviously increased from 17 to 22 (0.5 wt%), 24.8 (1 wt%) and 23 ns (1.5 wt%) respectively. These fitting results indicate that interface recombination between electrons in TiO2 and the holes in CH3NH3PbI3 can be suppressed greatly by the incorporation of Eu3+ ions into TiO2, which is consistent with the photoluminescence observation (Figure 7).


Page 17 of 27

Figure 9. Schematic illustration of the mechanism explaining the enhanced photoelectric performance of PSC with TiO2:Eu3+ ions.

The scheme in Figure 9 intuitively illustrates the incident light enters into PSCs from the back side of FTO, most photons are directly absorbed by perovskite layer, while part of photons with high energy are converted into low energy photons by TiO2:Eu3+ for better absorption of perovskite layer, which can improve the efficient utilization of ultraviolet light and reduce the charge loss in PSCs. [37] Such a phenomenon is observed from UV-vis spectra of TiO2:Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%)/CH3NH3PbI3. Furthermore, Eu3+ ions embedded in the surface sites adjacent to the TiO2 nanocrystal reducing the gap clearance between TiO2 particles, which is beneficial to the electron transfer in the mesoporous layer and obviously retard the recombination between electrons in TiO2 and the holes in CH3NH3PbI3.

b 24

24

2

Current Intensity (mA/cm 2)

20 16 12 8

Forward

Reverse

0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

4

Jsc

20

20

16

16

PCE 12

12

8

8

4

4

0 0 0.0

0 0.2

0.4

0.6 Voltage (V)

0.8

50

1.0

100

150

200

250

PCE (%)

a Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 300

Time (s)

Figure 10. (a) J-V curves of reverse and forward scans (1 sun, AM 1.5G) for PSCs with Eu3+ (0.0,



0.5, 1.0 and 1.5 wt%) in mesoporous TiO2 structure. (b) The steady-state output of photocurrent density and PCE for the PSC with 1.0 wt% Eu in TiO2 held close to 0.805 V forward bias. Table 3. Photovoltaic parameters of reverse and forward scans for PSCs with Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%) in mesoporous TiO2 structure. wt% 0.0 0.5 1.0 1.5

Jsc(mA/cm2) Reverse Forward 17.01 17.00 18.35 18.16 19.34 19.26 18.23 18.15

Voc(V) Reverse Forward 1.00 0.98 1.04 1.01 1.08 1.06 1.05 1.03

a

FF(%) Reverse Forward 71.34 71.28 74.55 74.35 75.67 75.59 73.95 73.84

PCE(%) Reverse Forward 12.22 11.96 14.22 13.63 15.79 15.42 14.26 13.91

b

16

19

15 18

PCE (%)

2

Jsc (mA/cm )

14 13 12

17

16

11 10

15

0.0 wt%

0.5 wt%

1.0 wt%

1.5 wt%

c

0.0 wt%

0.5 wt%

1.0 wt%

1.5 wt%

0.0 wt%

0.5 wt%

1.0 wt%

1.5 wt%

d

1.1

FF (%)

75

Voc (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

1.0

70

65

0.9

0.0 wt%

0.5 wt%

1.0 wt%

1.5 wt%

Figure 11. Performance parameter distributions of 20 PSCs with Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%) in mesoporous TiO2 layer: (a) PCE, (b) Jsc, (c) Voc and (d) FF (reverse scan from 1.2 to −0.1 V) .

J-V characteristics of the PSCs employing mesoporous TiO2:Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%) structures were recorded in Figure 10a, and the detailed parameters are shown in Table 3. The reverse and forward scans for PSCs before and after doping with Eu3+ are almost overlap, indicating that there is nearly no hysterisis. The reference PSCs with a pure mesoporous TiO2 layer displayed an open circuit voltage


Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Voc), short-circuit current density (Jsc) and fill factor (FF), respectively, of 1.00 V, 17.01 mA/cm2, and 71.34%, leading to a PCE of 12.22%. The efficiency is gradually increased with the addition of Eu3+ and the device fabricated with 1.0 wt%-TiO2:Eu3+ as mesoporous layer resulted in a higher Jsc of 19.34 mA/cm2, Voc of 1.08 V, gaining a PCE of 15.79%, which increased by 29.2% compared with that of referenced device. The steady-state output of photocurrent density and PCE for the PSC with 1.0 wt% Eu in TiO2 is shown in Figure 10b. The photocurrent density and PCE of the PSC remained stable within 300 s, suggesting that Eu doped TiO2 is conducive to an efficient and stable PSC. The performance parameter distributions of 20 PSCs with Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%) in mesoporous TiO2 structures are summarized in Figure 11. Clearly, the Jsc is significantly enhanced after incorporating Eu3+ ions into TiO2 and when the doping content of Eu3+ ions reaches 1.0 wt%, the Jsc is more than 19 mA/cm2. We attribute this significant improvement to the increased light absorption. Moreover, the Eu3+ ions reduce the gap clearance between TiO2 particles by embeding in the surface sites adjacent to the TiO2. This helps to suppress the recombination between electrons and the holes in TiO2 and CH3NH3PbI3, resulting in the improved Voc. [38] However, when the content of Eu3+ is further increased (1.5 wt%), the device performance deteriorates, which could be ascribed to the increase of the TiO2 boundary defect by incorporating excessive particle with higher ionic radius and because the gaps between TiO2 particles are filled, the insufficient filling of perovskite in mesoporous TiO2 structures lead to the decreased FF.


b 1.0

0.8

0.8 Normalized Jsc

a 1.0

0.6 0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

0.4 0.2

Page 20 of 27

0.6 0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

0.4 0.2 0.0

0.0 0

2

4 6 Time (hours)

8

0

10

d

1.0

1.0

0.8

0.8 Normalized FF

c

Normalized Voc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Normalized PCE


0.6 0.4

0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

0.2

4 6 Time (hours)

8

10

4 6 Time (hours)

8

10

0.6 0.0 wt% 0.5 wt% 1.0 wt% 1.5 wt%

0.4 0.2

0.0

2

0.0 0

2

4 6 Time (hours)

8

10

0

2

Figure 12. Normalized stability with error lines of the PSCs with different content of Eu3+ (0, 0.5, 1.0 and 1.5 wt%) in mesoporous TiO2 layer.

To confirm the effect of TiO2:Eu3+ on the stability of PSCs, a UV light-soaking measurement on the PSCs with Eu3+ (0.0, 0.5, 1.0 and 1.5 wt%) in mesoporous TiO2 structure is conducted. The variations of the performance parameters with time of the four types of PSCs are plotted in Figure 12. The over temperature and relative humidity levels are not controlled during the measurements and the PSCs are not encapsulated. The stability of PSC without Eu3+ in mesoporous TiO2 structure is poor, and the efficiency decreases to less than 50% after UV irradiation for 10 h, as shown in Figure 12a. Since CH3NH3PbI3 is sensitive to light and heat, long time exposure to UV light, the thermal lattice vibration will be enhanced, which could destroy the lattice structure, reduce the charge collection efficiency and cause a decline of short circuit current density (Figure 12b). The photocurrent behavior of PSC employed


Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mesoporous TiO2:Eu3+ structure is improved obviously. For the PSC with 1 wt% Eu3+ in mesoporous TiO2 structure, the Jsc value only decrease by 2% after 2 h of UV irradiation. However, the Jsc value of PSC without Eu3+ reduces by 10%. This indicates that doping Eu3+ ions can effectively reduce the UV degradation and improve the stability of PSCs. The Voc of the four kinds of cells do not display a heavy degradation (Figure 12c). Due to the better smoothness of perovskite caused by Eu3+ ions, the FF of the PSC with Eu3+ ions show better stability (Figure 12 d). Moreover, the narrow distribution of the four kinds of PSC confirms that the errors are small. In conclusion, Eu3+ ions doped mesoporous TiO2 plays an important role in improving the utilization of UV light and reducing the thermal lattice vibration. 4. Conclusion In conclusion, we have studied the effect of TiO2:Eu3+ nanocrystal on the enhanced performance in the hybrid lead halide perovskite CH3NH3PbI3 solar cells. Eu3+ ion has a larger ionic radius than Ti4+ and it is hard to substitute for Ti4+ but could embed in weaker force between Eu3+ ions and TiO2. By means of the one-step spin-coating process, the devices employing 1.0 wt%-TiO2:Eu3+ as the mesoporous layer and spiro-OMeTAD as the hole transport layer have achieved a PCE of 15.79% under one sun illumination. Experimental results showed that the remarkable increase in Jsc from 17.01 to 19.34 mA/cm2 in the presence of Eu3+ ions is mainly attributed to the enhanced response of light harvesting of PSC in the ultraviolet region through the light down-conversion offered by TiO2:Eu3+. And the interface recombination between electrons in TiO2 and holes in CH3NH3PbI3 is found to be obviously suppressed by the induced luminescence of Eu3+ ions in mesoporous TiO2 structure. Our results indicated that the introduction of down-conversion TiO2:Eu3+ is an



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

effective way for further improvement of photovoltaic performance and UV stability of PSCs without UV treatment.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Basic Research Program of China (Grant No. 2015CB932200); the External Cooperation Program of BIC, Chinese Academy of Science (Grant No. GJHZ1607) and the National Natural Science Foundation of China (Grants 21173227, U1205112).

Reference 1. Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of the American Chemical Society, 2009, 131, 6050-6051. 2. Im J H, Lee C R, Lee J W, Park S W, Park N G. 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale, 2011, 3, 4088-4093. 3. Kim H S, Lee C R, Im J H, Lee K B, Moehl T, Marchioro A, Moon S J, Humphry-Baker R, Yum J H, Moser J E, Grätzel M, Park N G. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Scientific Reports, 2012, 2, 591(1-7). 4. Lee M M, Teuscher J, Miyasaka T, Murakami T N, Snaith H J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science, 2012, 338, 643-647. 5. Burschka J, Pellet N, Moon S J, Humphry-Baker R, Gao P, Nazeeruddin M K,


Page 22 of 27

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Grätzel

M.

Sequential

deposition

as

a

route

to

high-performance

perovskite-sensitized solar cells. Nature, 2013, 499, 316-319. 6. Liu M, Johnston M B, Snaith H J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature, 2013, 501, 395-402. 7. Stranks S D, Eperon G E, Grancini G, Menelaou C, Alcocer M J, Leijtens T, Herz L M, Petrozza A, Snaith H J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science, 2013, 342, 341-344. 8. Yang W S, Noh J H, Jeon N J, Kim Y C, Ryu S, Seo J, Seok S I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science, 2015, 348, 1234-1237. 9. National Renewable Energy Laboratory. Best Research-Cell Efficiencies; www.nrel.gov/pv/assets/images/efficiency_chart.jpg. 10. Green M A. Solar cell efficiency tables. Progress in Photovoltaics Research & Applications, 2017, 25, 3-13. 11. Snaith H. J. Perovskites: the emergence of a new era for low-cost, high- efficiency solar cells. Journal of Physical Chemistry Letters, 2013, 4, 3623-3630. 12. Yang Y, Ri K, Mei A Y, Liu L F, Hu M, Liu T F, Li X, Han H W. The size effect of TiO2 nanoparticles on a printable mesoscopic perovskite solar cell. Journal of Materials Chemistry A, 2015, 3, 9103-9107. 13. Sung S D, Ojha D P, You J S, Lee J, Kim J, Lee W I. 50 nm sized spherical TiO2 nanocrystals for highly efficient mesoscopic perovskite solar cells. Nanoscale, 2015, 7, 8898-8906. 14. Lee J W, Lee T Y, Yoo P J,Grätzel M, Mhaisalkar S, Park N G. Rutile TiO2-based perovskite solar cells. Journal of Materials Chemistry A, 2014, 2, 9251-9259. 15. Huang Y, Zhu J, Ding Y, Chen S, Zhang C, Dai S. TiO2 sub-microsphere film as scaffold layer for efficient perovskite solar cells. ACS Applied Materials & Interfaces, 2016, 8, 8162-8167. 16. Leijtens T, Eperon G E, Pathak S, Abate A, Lee M M, Snaith H J. Overcoming ultraviolet light instability of sensitized TiO2 with meso-superstructured organometal tri-halide perovskite solar cells. Nature Communications, 2013, 4, 2885(1-8).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

17. Philippe B, Park B W, Lindblad R, Oscarsson J, Ahmadi S, Johansson E M J, Rensmo H. Chemical and electronic structure characterization of lead halide perovskites and stability behavior under different exposures-A photoelectron spectroscopy investigation. Chemistry of Materials, 2015, 27, 1720-1731. 18. Bella F, Griffini G, Correa-Baena J P, Saracco G, Grätzel M, Haqfeldt A, Turri S, Gerbaldi C. Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers. Science, 2016, 354, 203-206. 19. Jiang L, Chen W, Zheng J, Zhu L, Mo L, Li Z, Hu L, Hayat T, Alsaedi A, Zhang C, Dai S. Enhancing the photovoltaic performance of perovskite solar cells with a down-conversion Eu-complex. ACS Applied Materials Interfaces, 2017, 9, 26958-26964. 20. Bera A, Wu K, Sheikh A, Alarousu E, Mohammed O F, Wu T. Perovskite oxide SrTiO3 as an efficient electron transporter for hybrid perovskite solar cells. Journal of Physical Chemistry C, 2014, 118: 28494-28501. 21. Kim D H, Han G S, Seong W M, Lee J W, Kim B J, Park N G, Hong K S, Lee S, Jung H S. Niobium doping effects on TiO2 mesoscopic electron transport layer-based perovskite solar cells. ChemSusChem, 2015, 8, 2392-2398. 22. Wang J T, Ball J M, Barea E M, Abate A, Alexander-Webber J A, Huang J, Saliba M, Mora-Sero I, Bisquert J, Snaith H J, Nicholas R J. Low-temperature processed electron collection layers of graphene/TiO2 nanocomposites in thin film perovskite solar cells. Nano Letters, 2014, 14, 724-730. 23. Wang C, Tang Y, Hu Y, Yang W. Graphene/SrTiO3 nanocomposites used as an effective electron-transporting layer for high-performance perovskite solar cells. Rsc Advances, 2015, 5, 52041-52047. 24. Zeng Q G, Ding Z J, Zhang Z M. Synthesis, structure and optical properties of Eu3+/TiO2 nanocrystals at room temperature. Journal of Luminescence, 2006, 118, 301-307. 25. Luo W, Li R, Liu G, Antonio M R, Chen X. Evidence of trivalent europium incorporated in anatase TiO2 nanocrystals with multiple sites. Journal of Physical Chemistry C, 2009, 113, 5340-5340. 26. Luo W, Li R, Chen X. Host-sensitized luminescence of Nd3+ and Sm3+ ions incorporated in anatase titania nanocrystals. Journal of Physical Chemistry C,


Page 24 of 27

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2009, 113, 8772-8777. 27. Hafez H, Saif M, Abdel-Mottaleb M S A. Down-converting lanthanide doped TiO2 photoelectrodes for efficiencyenhancement of dye-sensitized solar cells. Journal of Power Sources, 2011, 196, 5792-5796. 28. Jeon N J, Lee J, Noh J H, Nazeeruddin M K, Grätzel M, Seok S. Efficient inorganic-organic hybrid perovskite solar cells based on pyrene arylamine derivatives as hole-transporting materials. Journal of the American Chemical Society, 2013, 135, 19087-19090. 29. Krishna A, Sabba D, Li H, Yin J, Boix P P, Soci C, Mhaisalkar S G, Grimsdale A C. Novel hole transporting materials based on triptycene core for high efficiency mesoscopic perovskite solar cells. Chemical Science, 2014, 5, 2702-2709. 30. Krishna A, Sabba D, Yin J, Bruno A, Boix P P, Gao Y, Dewi H A, Gurzadyan G G, Soci C, Mhaisalkar S G, Grimsdale A C. Facile synthesis of a furan–arylamine hole-transporting material for high-efficiency, mesoscopic perovskite solar cells. Chemistry, 2015, 21, 15113-15117. 31. Park D H, Sang H C, Ji S K, Sohn K S. Combinatorial screening of Eu3+ doped TiO2–Y2O3–Gd2O3 ternary system at soft UV excitation. Journal of Alloys & Compounds, 2008, 449, 196-201. 32. Li J G, Wang X, Watanabe A K, Ishigaki T. Phase structure and luminescence properties of Eu3+-doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists. Journal of Physical Chemistry B, 2006, 110, 1121-1127. 33. Gaponenko N V, Molchan I S, Thompson G E, Skeldon P, Pakes A, Kudrawiec R, Bryja L, Misiewicz J. Photoluminescence of Eu-doped titania xerogel spin-on deposited on porous anodic alumina. Sensors & Actuators A Physical, 2002, 99, 71-73. 34. Manser J S, Kamat P V. Band filling with free charge carriers in organometal halide perovskites. Nature Photonics, 2014, 8, 737-743. 35. Yoshihara T, Katoh R, Furube A, Tamaki Y, Murai M, Hara K, Murata S, Arakawa H, Tachiya M. Identification of reactive species in photoexcited nanocrystalline TiO2 films

by

wide-wavelength-range

(400-2500

nm)

transient

spectroscopy. Journal of Physical Chemistry B, 2004, 108, 3817-3823.


absorption


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

36. Shen Q, Ogomi Y, Chang J, Tsukamoto S, Kukihara K, Oshima T, Osada N, Yoshino K, Katayama K, Toyoda T, Hayase S. Charge transfer and recombination at the metal oxide/CH3NH3PbClI2/spiro-OMeTAD interfaces: uncovering the detailed mechanism behind high efficiency solar cells. Physical Chemistry Chemical Physics, 2014, 16, 19984-19992. 37. Huang X, Han S, Huang W, Liu X. Enhancing solar cell efficiency: the search for luminescent materials as spectral converters. Chemical Society Reviews, 2013, 44, 173-201. 38. Falcomer D, Daldosso M, Cannas C, Musinu A, Lasio B, Enzo S, Speghini A, Bettinelli M. A one-step solvothermal route for the synthesis of nanocrystalline anatase TiO2 doped with lanthanide ions. Journal of Solid State Chemistry, 2006, 179, 2452-2457.


Page 26 of 27

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC


High-Performance Perovskite Solar Cells with a Weak Covalent TiO2

Recommend Documents