Enhanced Performance for Planar Perovskite Solar Cells with

Article pubs.acs.org/JPCC

Enhanced Performance for Planar Perovskite Solar Cells with Samarium-Doped TiO2 Compact Electron Transport Layers Yan Xiang,† Zhu Ma,† Jia Zhuang,*,† Honglin Lu,† Chunyang Jia,‡ Junsheng Luo,‡ Haimin Li,† and Xiaowei Cheng† †

School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, 610500, People’s Republic of China School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu, 610054, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: The tactics of ion doping in metal oxide is normally used to improve the film quality, achieve an appropriate energy band, and enhance carrier mobility. Here, a rare earth element (samarium) was doped into TiO2 compact electron transport layers (ETLs) by adding samarium trinitrate into the titanium precursor solution. The results show that perovskite solar cells (PSCs) with Sm-doped TiO2 exhibit 10.3% enhancement with a power conversion efficiency (PCE) of 14.10%, compared to nondoped devices. It is found that Sm doping can upward shift the Fermi energy level of the ETLs, increase the carrier transport ability, and inhibit the carrier recombination. The results indicate that rare earth ion doping could be a promising method for producing effective ETLs and high performance PSCs.

1. INTRODUCTION Organic−inorganic hybrid perovskite has become the most competitive photoelectric material because of its excellent properties including high carrier mobility, an adjustable spectral absorption range, long diffusion lengths, and the simplicity and affordability of fabrication, and it has been used in photovoltaics, light emitting diodes (LEDs), photodetectors, lasers, and more.1−4 The power conversion efficiency (PCE) of organic− inorganic hybrid perovskite solar cells (PSCs) was increased from 3.81 to 22.1% via material synthesis, film crystal growth control, and interface and device engineering.5−10 Among these strategies, materials which were employed as the charge carrier selective contacts play an important role in PSCs. Moreover, the materials for electron selective contact remain one of the most challenging scientific issues.11−13 In general, inorganic metal oxide semiconductors, such as TiO2,12 SnO2,13,14 ZnO,15,16 Nb2O5,17−19 and SrTiO3,20 can be applied as electron transport materials for solar cells. Among these n-type metal oxide materials, TiO2 is a promising candidate for planar perovskite solar cells due to its high transparency, excellent carrier separation ability, environmental stability, and easy fabricating process.21 In a planar heterojunction PSC, TiO2 film with a thickness of tens of nanometers is demanded to cover the conductive substrate. However, it simultaneously suffers from low conductivity and carrier accumulation due to the numerous trap states. In order to circumvent these problems, a kind of tactic was developed. Graphene and graphene quantum dots were introduced as additives to increase the conductivity and carrier extraction.22 Organic silane self-assembled monolayer © XXXX American Chemical Society

was used to tune the interface electronic structure and passivate the recombination process.23 Moreover, ion doping is another effective method to control the trap states and carrier transport. For example, Nb-doped TiO2 prepared by a facile chemical bath process at low temperature (70 °C) is more efficient for photogenerated electron injection and extraction, showing higher conductivity, higher mobility, and lower trap-state density.24 Al-doped TiO2 exhibited a striking impact upon the density of subgap states and enhanced the conductivity by orders of magnitude, which dramatically improved the device efficiencies.25 Moreover, lithium, cesium, magnesium, etc. were doped into TiO2 to control the carrier dynamics in PSCs;26−28 nevertheless, rare earth element doping has been rarely reported. Rare earth elements have been widely used in various fields, such as high-quality phosphors, upconversion materials, catalyst materials, superconductors, magnetic materials, solar materials, etc.29−33 Most of the useful functions of rare earth elements originate from the electron transitions within the 4f shell, and are highly sensitive to the composition and structures of the rare earth compounds, especially to the complexation state and the crystal field of the matrix in which rare earth ions are trapped.32 Many rare earth compounds nanostructures have been successfully synthesized by the sol−gel method, hydrothermal synthetic pathway, and chemical coprecipitation method.30,33,34 Moreover, it can be seen that the sol−gel method can be used to Received: June 15, 2017 Revised: August 28, 2017

A

DOI: 10.1021/acs.jpcc.7b05880 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. SEM images of (a) FTO, (b) FTO/TiO2, and (c) FTO/0.3% Sm-doped TiO2.

Figure 2. EDX elemental mapping of (a) FTO, (b) FTO/TiO2, and (c) FTO/0.3% Sm-doped TiO2.

except that Sm(NO3)3·6H2O was dissolved into HCl−ethanol solution ranging from 0.1 to 1.0% (Sm/Ti, molar ratio). 2.3. Preparation for Perovskite Precursor and Hole Transport Material. The perovskite precursor solution was prepared by dissolving 0.5532 g of PbI2 and 0.1908 g of CH3NH3I in 1 mL of N,N-dimethylformamide (DMF) with stirring at 60 °C overnight. Hole transport material spin-coating solution was prepared by dissolving 72.3 mg of spiro-OMeTAD, 28.8 μL of tBP, and 17.5 μL of Li-TFSI solution (520 mg of LiTFSI in 1 mL of acetonitrile) in 1 mL of chlorobenzene. 2.4. Device Fabrication. Fluorine-doped tin oxide (FTO) coated glasses (15 Ω/sq, Yingkou,China, OPV Tech New Energy Co.) used were ultrasonically cleaned with acetone, ethanol, and deionized water, and then dried with a nitrogen stream. The compact TiO2 layers were coated on the FTO glasses by spincoating the titanium precursor solution at 2000 rpm for 50 s, followed by heating at 150 °C for 15 min, and then were annealed at 500 °C in a muffle furnace for 30 min. After cooling to room temperature, the compact TiO2 film was treated using TiCl4 solution at 70 °C for 30 min, and then sintered at 500 °C in a muffle furnace for 30 min. The perovskite layer was fabricated using a spin-coating method on the compact TiO2/FTO substrates at 3000 rpm for 55 s. During the spin-coating process, 80 μL of chlorobenzene was dropped in the center of the substrate. After the spin-coating process, the film was heated at 100 °C for 20 min. The spiro-OMeTAD solution was spincoated on MAPbI3 film at 3000 rpm for 30 s. Ag top electrodes were deposited through a mask using magnetron sputtering technique under a vacuum of 1 × 10−3 Pa and a sputtering power of 40 W. 2.5. Material and Device Characterization. The film morphology was investigated using a scanning electron

easily realize rare earth ion doping in TiO2 nanostructures by preparation of mix precursor solution. In this study, samarium-doped TiO2 electron transport layers (ETLs) are prepared by adding samarium trinitrate into the TiO2 precursor solution for planar perovskite solar cells. The devices show a 14.10% PCE with improved short circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), compared to nondoped devices. It is found that the Sm doping can upward shift the Fermi energy level of the ETL, increase the free electron density, and decrease the density of deep trap states.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. 2,2′,7,7′-Tetrakis(N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD, 99.5%), CH3NH3I (99.5%), PbI2 (99.99%), lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI; 99.9%) and 4-tertbutylpyridine (tBP; 96%) were purchased from Xi’an Polymer Light Technology Corp. Titanium isopropoxide (99.999%), chlorobenzene (99.9%), N,N-dimethylformamide (DMF, 99.9%), and acetonitrile (99.9%) were achieved from SigmaAldrich. Sm(NO3)3·6H2O (99.9%) was obtained from Aladdin. All other solvents and chemicals were obtained from commercial sources and used as received without further purification. 2.2. Preparation for TiO2 and Sm-Doped TiO2 Precursor Solution. The compact TiO2 layer solution was prepared as previously reported.35 A 369 μL volume of titanium isopropoxide was added into 2.53 mL of ethanol; at the same time 35 μL of 2 M HCl solution was added into 2.53 mL of ethanol in another vial. The HCl−ethanol solution was added dropwise to the titanium isopropoxide solution under stirring for 2 h; subsequently the mixture solution was filtered with a 0.22 μm filter. Sm-doped TiO2 precursor solutions were obtained with a similar approach B


Article


Figure 3. XPS analysis of TiO2 and Sm-doped TiO2 compact ETLs. (a) Full survey spectrum, (b) enlarged XPS spectrum with binding energy range from 1100 to 1060 eV, (c) O 1s core-level spectrum, and (d) Ti 2p core-level spectrum.

film of FTO shows coarse grain with sharp edges and corners (Figure 1a). Homogeneous, flat and thin TiO2 and Sm-doped TiO2 films without pinhole were obtained upon FTO, where TiO2 particles are uniformly distributed on the surface of Fdoped SnO2 film (Figure 1b,c). Compared to the TiO2 film, Smdoped TiO2 film does not show any significant changes. The elemental mapping, which was taken with energy dispersive Xray spectroscopy (EDX),36,37 was used to analyze the chemical composition of the FTO film, TiO2 and 0.3% Sm-doped TiO2 compact ETLs, and the presence of Sm (Figure 2). The tin and fluorine elements are derived from the FTO substrate, and titanium element is derived from the generated TiO2. The existence of samarium element shows the Sm doping for TiO2 by adding samarium nitrate into titanium precursor solution is feasible, which further has an influence on the performance of the PSCs. X-ray photoelectron spectra (XPS) were recorded to investigate the chemical composition of the compact ETLs and the chemical states of constituent elements. The full survey spectrum shown in Figure 3a affirms the coexistence of Ti, O, and C in TiO2 and Sm-doped TiO2 compact films. The existence of C 1s is attributed to adsorbed hydrocarbon (Figure S1 in the Supporting Information). The peak of Sm 3d is loaded in the high binding energy area, and it is superimposed with the Ti LMM Auger electron spectrum. As a result, Sm is not observed in the full spectrum because of low Sm-doping concentration in this study. Although Sm is not observed directly in the full spectrum, the enlarged XPS spectra show the notable chemical shift (about

microscopy (SEM; ZEISS EV0MA15), where the system was connected to an energy dispersive X-ray spectroscopy (EDX) detector. The crystal structure of the samples was characterized by X-ray diffraction (XRD; DX-2700, Dandong) with Cu Kα radiation (λ = 0.154 06 nm) at a scanning rate of 4 deg/min. XPS measurements were obtained using a X-ray photoelectron spectrometer (KRATOS, AXIS Ultra DLD) with the monochromated Al Kα X-ray source (hν = 1486.6 eV, 200 W). The UV−vis absorption spectra of films were measured using a UV− vis−NIR spectrophotometer (UV-2600, SHIMADZU). The time-resolved photoluminescent (TRPL) spectra were measured on a Fluorolog-3 spectrofluorometer (Horiba JobinYvon) with a PMT (H10330-75, Hamamatsu) as the detector (λem = 760 nm). The current−voltage characteristic was recorded using an electrochemical workstation under a simulated solar spectrum (AM1.5) provided by a solar simulator (CEL-S500, Beijing, China). The monochromatic incident photon-to-current efficiency (IPCE) was measured using an IPCE system (PVE 300, Bentham, Inc.) from 300 to 800 nm. The active area of solar cells was controlled to 0.12 cm2 using a mask.

3. RESULTS AND DISCUSSION 3.1. Structure and Properties of Compact ETLs. The compact TiO2 and Sm-doped TiO2 films interfaced to FTO glasses (of ≈350 nm thickness of F-doped SnO2) were investigated. Figure 1 shows the SEM images of the FTO film, TiO2, and 0.3% Sm-doped TiO2 compact ETLs. F-doped SnO2 C


Article


Figure 4. (a and b) Top-view SEM images of CH3NH3PbI3 film based on TiO2 and Sm-doped TiO2 ETLs, (c) cross section of PSCs (FTO/compact Sm-doped TiO2/CH3NH3PbI3/spiro-OMeTAD/Ag), (d) device model diagram, (e) XRD patterns of CH3NH3PbI3 films (▲ represents FTO), and (f) UV−vis absorption spectra of CH3NH3PbI3 based on TiO2 and Sm-doped TiO2 ETLs.

2 eV) of Ti LMM when the doping concentration of Sm3+ reached 1%. The peak located at 1086.2 eV of 1% Sm-doped TiO2 film basically matches with Sm 3d5/2, and that is not abundantly clear in pure TiO2 and 0.3% Sm-doped TiO2 films. In addition, samarium doping has an influence on the Ti 2p and O 1s core-level spectra, which verified the existence of Sm indirectly. Figure 3d shows the Ti 2p core-level spectra of TiO2, 0.3% Sm-doped TiO2, and 1% Sm-doped TiO2compact films. The peaks of pure TiO2 compact film located at 458.6 and 464.3 eV indicate the spin−orbit splitting of the Ti 2p components (2p3/2 and 2p1/2, respectively). The Ti 2p3/2 peaks of 0.3 and 1% Sm doped TiO2 compact film are located at 458.4 and 458.5 eV, which shows a chemical shift of 0.2−0.3 eV. In addition, the O 1s peaks of TiO2 compact film, 0.3% Sm-doped TiO2, and 1% Sm-doped TiO2 compact films are located at 529.8, 530, and 530.1 eV (Figure 3c), which shows a chemical shift of 0.2−0.3 eV. The chemical shifts of O 1s and Ti 2p are attributed to the difference in the chemical and coordination environments of the Sm3+ ions caused O−Ti−O → Sm3+ chemical adsorption. Generally, most of the useful functions of rare earth elements originate from the electron transitions within the 4f shell. The increases in binding energy of O 1s and Ti 2p are attributed to the decreased electron densities of O and Ti atoms due to the charge

transfer from the 2p orbit of O atom and the 2p orbit of Ti atom to the 4 f orbit of Sm. 3.2. Performance of PSCs. Figure 4a,b shows the top-view SEM images of CH3NH3PbI3 film based on TiO2 and Sm-doped TiO 2 ETLs via a one-step spin-coating process with CH3NH3PbI3/DMF solution and chlorobenzene as an antisolvent. Full coverage of perovskite layer onto TiO2 and Smdoped TiO2 ETL substrates with no pinholes contribute to high Jsc, Voc, and fill factor (FF), which is crucial for achieving highefficiency perovskite solar cells. Figure 4c,d shows the SEM image of the cross section of the solar cells and the device model diagram. The functional layer is closely connected with adjacent layers, which is beneficial to the transfer of the carrier. The thicknesses of the ETL and the CH3NH3PbI3 layer are about 80 and 600 nm, respectively. Figure 4e shows the XRD measurement of CH3NH3PbI3 thin film loaded on TiO2 and Sm-doped TiO2 ETLs. It demonstrates pure perovskite structure was formed without PbI2 and the addition of samarium nitrate did not affect the crystal structure of perovskite. Figure 4f shows the UV−vis absorption spectra of CH3NH3PbI3 films based on TiO2 and Sm-doped TiO2 ETLs. It displays two almost coincident curves and the band gap of 1.55 eV (inset image). It further proves Sm doping did not affect the structure and performance of perovskite films. D


Article


Figure 5. Photovoltaic performance of PSCs based on TiO2 and Sm-TiO2 ETLs. (a) J−V curves, (b) IPCE spectra, and (c) J−V curves for hysteresis of PSCs, and (d) J−V curves for stability of PSCs.

Table 1. Photovoltaic Parameters of PSCs Derived from J−V Measurements max −2

average −2

ETLs

Voc (V)

Jsc (mA cm )

FF

PCE (%)

Voc (V)

Jsc (mA cm )

FF

PCE (%)

TiO2 0.1% Sm-doped TiO2 0.3% Sm-doped TiO2 0.5% Sm-doped TiO2 0.7% Sm-doped TiO2 1.0% Sm-doped TiO2

1.032 1.036 1.044 1.049 1.055 1.053

18.11 18.31 19.13 18.93 17.57 17.15

0.683 0.708 0.706 0.705 0.710 0.705

12.78 13.43 14.10 14.00 13.16 12.73

1.027 1.034 1.040 1.038 1.027 1.039

17.18 17.71 18.07 17.53 17.05 16.83

0.673 0.694 0.697 0.702 0.696 0.699

11.86 12.73 13.10 12.76 12.31 12.17

Figure 6. (a) UV−vis absorption spectra of TiO2 and Sm-doped TiO2 compact ETLs, and (b) indirect band gap model.

Figure 5a shows the current−voltage (J−V) characteristics of PSCs based on TiO 2 and Sm-doped TiO2 ETLs, and

corresponding parameters are summarized in Table 1. The cell based on TiO2 ETL exhibits an open-circuit voltage (Voc) of E


Article


Figure 7. (a) Mott−Schottky analyses of TiO2 and Sm-doped TiO2 compact ETLs. (b) Density of electronic states in TiO2 including the density of conduction band states (DCB), valence band states (DVB), and trap states (DT), the shift of quasi-Fermi level, and the reduced surface trap density via Smdoping. (c) Energy diagram of the cell.

shows histogram of the device PCE measured for 30 devices based on TiO2 and Sm−TiO2 electron transport layers. 3.3. Effect of the Charge Transport Property. Figure 6a shows the UV−vis absorption spectra of the TiO2 and Sm-doped TiO2 compact ETLs. The spectra of Sm3+-doped TiO2 show a blue shift in the band gap transition (inset image in Figure 6a). The blue shift of UV−vis absorption spectra can be attributed to the charge-transfer transition between rare earth ion 4f orbit electrons and the TiO2 conduction or valence band.38 The absorption edge shift to a shorter wavelength is dependent on the amount of Sm3+. The transitions of 4f orbit electrons of rare earth elements led to the enforcement of the separation of photogenerated electron−hole pairs. Voc strongly depends on the energy level offset at the electrode interfaces and can be increased linearly by using energy-leveltailored ETLs and hole transport layers (HTLs) at the interface of perovskite solar cells. The maximum Voc is achieved when the energy levels of HTLs and ETLs are pinned to the valence band maximum (VBM) and conduction band minimum (CBM) of perovskites.39 The Kubelka−Munk function is used to determine the optical band gap, i.e., (αhν)1/2 versus the photon energy of the exciting light (hν), where α is the absorption coefficient, h is Planck’s constant, and ν is the light frequency.40 Figure 6b shows the band gap of TiO2 increased from 3.28 to 3.33 eV when 0.3% Sm(NO3)3 was added to TiO2 precursor solution. It is beneficial for increasing the Voc in perovskite solar cells. Mott−Schottky analyses of the TiO2/electrolyte interface were carried out to investigate the changes of electrical properties caused by the introduction of samarium in a three-electrode cell, with the compact layer coated on FTO substrate used as the working electrode, a platinum sheet used as the counter electrode, and a standard Ag/AgCl in saturated KCl solution used as the reference electrode. The flat band potential (Efb) of the semiconductor is related to the space charge capacitance, and the relationship can be expressed using eq 1:41

Figure 8. Time-resolved photoluminescent (TRPL) decay curves of FTO/TiO2/MAPbI3 film and FTO/0.3% Sm-doped TiO2/MAPbI3 film.

1.032 V, a short circuit current density (Jsc) of 18.11 mA cm−2, a fill factor (FF) of 0.683, and a PCE of 12.78%. When Sm3+ ion was doped into TiO2 ETLs, the performances of PSCs were improved. As seen from Table 1, the Voc and FF of PSCs were increased when Sm3+ ion was doped into TiO2. The Jsc was increased when TiO2 ETLs were doped with 0.1−0.5% Sm. When the doping concentration exceeded 0.5%, the Jsc values dropped rapidly. When 0.3% Sm was doped into TiO2, the cell achieved the most remarkable improvement of PCE (14.10%). Obviously, Sm3+ ion was added to increase the short-circuit current, which led to the improvement of conversion efficiency. Figure 5b shows the incident photon-to- current efficiency (IPCE) spectra of the corresponding devices, which is in good agreement with the J−V characteristics. J−V measurements were performed with different scan rates and directions to analyze the hysteresis of the PSCs (Table S1 in the Supporting Information). Figure 5c shows that proper Sm doping is beneficial to reduce the hysteresis of the device. As shown from Figure 5d, the device without encapsulation at about 40% humidity in air retains over 80% of its initial efficiency (storage time of 100 h). Figure S2

csc−2 =

2(E − Efb − kBT /e) Ndεε0eA2

(1)

where E is the electrode potential, ε is the relative dielectric constant, A is the active surface, and T and kB are the absolute temperature and Boltzmann constant, respectively. The Efb of the

Table 2. TRPL Data for Perovskite Films on Different Substrates sample

τave (ns)

τ1 (ns)

amplitude of τ1 (%)

τ2 (ns)


τ3 (ns)


TiO2 0.3% Sm-doped TiO2

672.2 640.5

346.2 293.5

15.79 12.26

662.4 649.0

81.82 86.85

1384.8 1174.0

2.39 0.89

F



■

TiO2 electrode was obtained from the intercept of the linear portion in the Mott−Schottky plots. Figure 7a shows the Mott− Schottky analyses of TiO2 and Sm-doped TiO2 compact films. The calculated Efb values are about −0.18 V for the pure TiO2 film, −0.28 V for the 0.3% Sm-doped TiO2 film, and −0.32 V for 0.7% Sm-doped TiO2 film, respectively. This illustrates a negative shift of the flat band potential with a small amount of Sm-doped TiO2. Correspondingly, the shifted Vfb leads to a higher quasiFermi level (Figure 7b).42 Thus, the Sm doping slightly increased the Fermi level and reduced the surface state density (DT), which is beneficial for a higher Voc in the planar perovskite solar cell.43 It is also beneficial for obtaining an enhanced Jsc due to the decreased energy barrier between the CH3NH3PbI3 light harvesting layer and compact TiO2 layer which contribute to the charge injection process (Figure 7c). To further discuss the charge transfer process, time-resolved PL spectroscopy was used.44,45 As seen from Figure 8, the timeresolved PL spectra were best fitted by a triexponential function as eq 2:19 y = y0 +

⎛ t⎞ ⎟ ⎝ τi ⎠

∑ Ai exp⎜−

∑ Aiτi 2/∑ Aiτi

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05880. XPS of C 1s core-level spectrum, histogram of PCE measured for 30 cells, photovoltaic parameters of PSCs (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 13550396098. Fax: +86 02883033286. E-mail: [email protected]. ORCID

Jia Zhuang: 0000-0001-5304-2720 Chunyang Jia: 0000-0001-6326-1679 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Young Scholars Development Fund of SWPU (Grant 201699010017) and the scientific research starting project of SWPU (Grant 2017QHZ021).

(2)

■

where Ai and τi are the amplitude and decay time of the process, and the corresponding data is shown in Table 2. The PL decay times of the MAPbI3/TiO2 film are τ1 = 346.2 ns, τ2 = 662.4 ns, and τ3 = 1384.8 ns; the corresponding amplitudes are 15.79, 81.82, and 2.39%, respectively. For the MAPbI3 deposited on Sm-TiO2, the τ1, τ2, and τ3 drop to 293.5, 649.0, and 1174.0 ns, and the corresponding intensities are 12.26, 86.85, and 0.89%. The average recombination lifetime (τave) is estimated with the τi and Ai values from the fitted curve data using eq 3:19 τave =

Article

REFERENCES

(1) Chen, Q.; De Marco, N.; Yang, Y.; Song, T. B.; Chen, C. C.; Zhao, H.; Hong, Z.; Zhou, H.; Yang, Y. Under the spotlight: The organic− inorganic hybrid halide perovskite for optoelectronic applications. Nano Today 2015, 10, 355−396. (2) Zhang, W.; Eperon, G. E.; Snaith, H. J. Metal halide perovskites for energy applications. Nature Energy 2016, 1, 16048. (3) Kim, H. S.; Im, S. H.; Park, N. G. Organolead Halide Perovskite: New Horizons in Solar Cell Research. J. Phys. Chem. C 2014, 118, 5615− 5625. (4) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat. Photonics 2014, 8, 506−514. (5) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (6) 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.; et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2012, 2, 591. (7) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.; Duan, H.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542−546. (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) Yi, C.; Li, X.; Luo, J.; Zakeeruddin, S. M.; Gratzel, M. Perovskite photovoltaics with outstanding performance produced by chemical conversion of bilayer mesostructured lead halide/TiO2 films. Adv. Mater. 2016, 28, 2964−2970. (10) Yang, W. S.; Park, B. W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; et al. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017, 356, 1376−1379. (11) Yang, G.; Tao, H.; Qin, P.; Ke, W.; Fang, G. Recent progress in electron transport layers for efficient perovskite solar cells. J. Mater. Chem. A 2016, 4, 3970−3990. (12) Etgar, L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Grätzel, M. Mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells. J. Am. Chem. Soc. 2012, 134, 17396−17399. (13) Ke, W.; Fang, G.; Liu, Q.; Xiong, L.; Qin, P.; Tao, H.; Wang, J.; Lei, H.; Li, B.; Wan, J.; et al. Low-temperature solution-processed tin

(3)

The τave of MAPbI3/0.3% Sm-doped TiO2 film (640.5 ns) is shorter than that of the MAPbI3/TiO2 film (672.2 ns). This shows that the electrons could be more efficiently transported from the perovskite absorber layer to Sm-doped TiO2 than that of TiO2. The more efficient electron transport from perovskite to the Sm-doped TiO2 ETL indicates the suppressed electron recombination due to the perfect match in energy levels between the conduction band level of the perovskite and the Sm-doped TiO2 layer.19,46

4. CONCLUSIONS The rare earth Sm3+ ion has been introduced as an effective ion dopant to enhance the performance of planar perovskite solar cells. The Jsc and Voc are increased due to the upward shift of the Fermi energy level and increased carrier transport ability of the compact Sm-doped TiO2 ETL. The device performance depends on the concentration of Sm doping. As a result, the PCE of planar perovskite solar cells based on Sm-doped TiO2 with the Sm doping amounts of 0.3% (Sm/Ti, atomic ratio) has achieved 14.10%, which was 10.3% higher than that of undoped TiO2based planar perovskite solar cells. This indicates that rare earth ion doping with low doping concentrations for TiO2 ETLs is an effective method to improve the performance of perovskite solar cells. G


Article

The Journal of Physical Chemistry C oxide as an alternative electron transporting layer for efficient perovskite solar cells. J. Am. Chem. Soc. 2015, 137, 6730−6733. (14) Song, J.; Zheng, E.; Bian, J.; Wang, X. F.; Tian, W.; Sanehira, Y.; Miyasaka, T. Low-temperature SnO2-based electron selective contact for efficient and stable perovskite solar cells. J. Mater. Chem. A 2015, 3, 10837−10844. (15) Jung, K.; Lee, J.; Kim, J.; Chae, W.; Lee, M. Solution-processed flexible planar perovskite solar cells: A strategy to enhance efficiency by controlling the ZnO electron transfer layer, PbI2 phase, and CH3NH3PbI3 morphologies. J. Power Sources 2016, 324, 142−149. (16) Ma, Z.; Lu, H. L.; Zhao, F. G.; Xiang, Y.; Zhuang, J.; Li, H. M. Lowtemperature dynamic vacuum annealing of ZnO thin film for improved inverted polymer solar cells. RSC Adv. 2017, 7, 29357−29363. (17) Fernandes, S. L.; Véron, A. C.; Neto, N. F. A.; Nüesch, F. A.; Dias da Silva, J. H.; Zaghete, M. A.; Graeff, C. F. d. O. Nb2O5 hole blocking layer for hysteresis-free perovskite solar cells. Mater. Lett. 2016, 181, 103−107. (18) Ling, X.; Yuan, J.; Liu, D.; Wang, Y.; Zhang, Y.; Chen, S.; Wu, H.; Jin, F.; Wu, F.; Shi, G.; et al. Room-Temperature Processed Nb2O5 as the Electron-Transporting Layer for Efficient Planar Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 23181−23188. (19) Feng, J.; Yang, Z.; Yang, D.; Ren, X.; Zhu, X.; Jin, Z.; Zi, W.; Wei, Q.; Liu, S. E-beam evaporated Nb2O5 as an effective electron transport layer for large flexible perovskite solar cells. Nano Energy 2017, 36, 1−8. (20) Okamoto, Y.; Fukui, R.; Fukazawa, M.; Suzuki, Y. SrTiO3/TiO2 composite electron transport layer for perovskite solar cells. Mater. Lett. 2017, 187, 111−113. (21) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107, 2891−2959. (22) Tavakoli, M. M.; Tavakoli, R.; Hasanzadeh, S.; Mirfasih, M. H. Interface Engineering of Perovskite Solar Cell Using a ReducedGraphene Scaffold. J. Phys. Chem. C 2016, 120, 19531−19536. (23) Liu, L.; Mei, A.; Liu, T.; Jiang, P.; Sheng, Y.; Zhang, L.; Han, H. Fully printable mesoscopic perovskite solar cells with organic silane selfassembled monolayer. J. Am. Chem. Soc. 2015, 137, 1790−1793. (24) Yin, G.; Ma, J.; Jiang, H.; Li, J.; Yang, D.; Gao, F.; Zeng, J.; Liu, Z.; Liu, S. Enhancing Efficiency and Stability of Perovskite Solar Cells through Nb-Doping of TiO2 at Low Temperature. ACS Appl. Mater. Interfaces 2017, 9, 10752−10758. (25) Pathak, S. K.; Abate, A.; Ruckdeschel, P.; Roose, B.; Gödel, K. C.; Vaynzof, Y.; Santhala, A.; Watanabe, S. I.; Hollman, D. J.; Noel, N.; et al. Performance and stability enhancement of dye-sensitized and Perovskite solar cells by Al doping of TiO2. Adv. Funct. Mater. 2014, 24, 6046− 6055. (26) Heo, J. H.; You, M. S.; Chang, M. H.; Yin, W.; Ahn, T. K.; Lee, S. J.; Sung, S. J.; Kim, D. H.; Im, S. H. Hysteresis-less mesoscopic CH3NH3PbI3 perovskite hybrid solar cells by introduction of Li-treated TiO2 electrode. Nano Energy 2015, 15, 530−539. (27) Wang, W.; Zheng, H.; Liu, Y.; Sun, J.; Gao, L. Enhanced Perovskite Solar Cells with Cesium-Doped TiO2 Compact Layer. J. Nanosci. Nanotechnol. 2016, 16, 12768−12772. (28) Zhang, H.; Shi, J.; Xu, X.; Zhu, L.; Luo, Y.; Li, D.; Meng, Q. Mgdoped TiO2 boosts the efficiency of planar perovskite solar cells to exceed 19%. J. Mater. Chem. A 2016, 4, 15383−15389. (29) Hoppe, H. A. Recent developments in the field of inorganic phosphors. Angew. Chem., Int. Ed. 2009, 48, 3572−3582. (30) Yuan, S.; Sheng, Q.; Zhang, J.; Chen, F.; Anpo, M.; Zhang, Q. Synthesis of La3+ doped mesoporous titania with highly crystallized walls. Microporous Mesoporous Mater. 2005, 79, 93−99. (31) de la Mora, M. B.; Amelines-Sarria, O.; Monroy, B. M.; Hernández-Pérez, C. D.; Lugo, J. E. Materials for downconversion in solar cells: Perspectives and challenges. Sol. Energy Mater. Sol. Cells 2017, 165, 59−71. (32) Wang, X.; Li, Y. Rare-Earth-compound nanowires, nanotubes, and fullerene-like nanoparticles: synthesis, characterization, and properties. Chem. - Eur. J. 2003, 9, 5627−5635.

(33) Park, D. J.; Sekino, T.; Tsukuda, S.; Hayashi, A.; Kusunose, T.; Tanaka, S. I. Photoluminescence of samarium-doped TiO2 nanotubes. J. Solid State Chem. 2011, 184, 2695−2700. (34) Jayachandraiah, C.; Krishnaiah, G. Influence of cerium dopant on magnetic and dielectric properties of ZnO nanoparticles. J. Mater. Sci. 2017, 52, 7058−7066. (35) Chen, B.; Zheng, X.; Yang, M.; Zhou, Y.; Kundu, S.; Shi, J.; Zhu, K.; Priya, S. Interface band structure engineering by ferroelectric polarization in perovskite solar cells. Nano Energy 2015, 13, 582−591. (36) Moreno-Tovar, R. M.; Terrés, E.; Rangel-Mendez, J. R. R. Oxidation and EDX elemental mapping characterization of an ordered mesoporous carbon: Pb (II) and Cd (II) removal. Appl. Surf. Sci. 2014, 303, 373−380. (37) Flak, D.; Coy, E.; Nowaczyk, G.; Yate, L.; Jurga, S. Tuning the photodynamic efficiency of TiO2 nanotubes against HeLa cancer cells by Fe-doping. RSC Adv. 2015, 5, 85139−85152. (38) Štengl, V.; Bakardjieva, S.; Murafa, N. Preparation and photocatalytic activity of rare earth doped TiO2 nanoparticles. Mater. Chem. Phys. 2009, 114, 217−226. (39) Sha, W. E. I.; Ren, X.; Chen, L.; Choy, a. W. C. H. The efficiency limit of CH3NH3PbI3 perovskite solar cells. Appl. Phys. Lett. 2015, 106, 221104. (40) Yang, Z.; Wang, B.; Cui, H.; An, H.; Pan, Y.; Zhai, J. Synthesis of Crystal-Controlled TiO2 Nanorods by a Hydrothermal Method: Rutile and Brookite as Highly Active Photocatalysts. J. Phys. Chem. C 2015, 119, 16905−16912. (41) Gelderman, K.; Lee, L.; Donne, S. W. Flat-band potential of a semiconductor: using the Mott−Schottky equation. J. Chem. Educ. 2007, 84, 685−688. (42) Wang, W.; Zheng, H.; Liu, Y.; Sun, J.; Gao, L. Enhanced Perovskite Solar Cells with Cesium-Doped TiO2 Compact Layer. J. Nanosci. Nanotechnol. 2016, 16, 12768−12772. (43) Xiang, Y.; Yu, J.; Zhuang, J.; Ma, Z.; Li, H. TiO2 photoanode surface modification via combined action of samarium and titanium salt in dye-sensitized solar cells. Sol. Energy Mater. Sol. Cells 2017, 165, 45− 51. (44) Luo, J.; Jia, C.; Wan, Z.; Han, F.; Zhao, B.; Wang, R. The novel dopant for hole-transporting material opens a new processing route to efficiently reduce hysteresis and improve stability of planar perovskite solar cells. J. Power Sources 2017, 342, 886−895. (45) Han, F.; Luo, J.; Wan, Z.; Liu, X.; Jia, C. Dissolutionrecrystallization method for high efficiency perovskite solar cells. Appl. Surf. Sci. 2017, 408, 34−37. (46) Heo, J. H.; You, M. S.; Chang, M. H.; Yin, W.; Ahn, T. K.; Lee, S. J.; Sung, S. J.; Kim, D. H.; Im, S. H. Hysteresis-less mesoscopic CH3NH3PbI3 perovskite hybrid solar cells by introduction of Li-treated TiO2 electrode. Nano Energy 2015, 15, 530−539.

H


Enhanced Performance for Planar Perovskite Solar Cells with

Recommend Documents