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Enhancing efficiency and stability of perovskite solar cells through Nb-doping of TiO2 at low temperature Guannan Yin, Jiaxin Ma, Hong Jiang, Juan Li, Dong Yang, Fei Gao, Jing Hui Zeng, Zhike Liu, and Shengzhong (Frank) Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01063 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017

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ACS Applied Materials & Interfaces

Enhancing Efficiency and Stability of Perovskite Solar Cells Through Nb-doping of TiO2 at Low Temperature Guannan Yin,‡ Jiaxin Ma,‡ Hong Jiang, Juan Li, Dong Yang, Fei Gao, Jinghui Zeng, Zhike Liu* and Shengzhong Frank Liu*

Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China Keywords: perovskite solar cell, Nb doped TiO2, electron transport layer, air stability and thermal stability

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ABSTRACT The conduction band energy, conductivity, mobility and electronic trap states of electron transport layer (ETL) is very important to the efficiency and stability of a planar perovskite solar cell (PSC). However, as the most widely used ETL, TiO2 often needs to be prepared under high temperature and has unfavorable electrical properties such as low conductivity, high electronic trap states. Modifications such as elemental doping are effective methods for improving the electrical properties of TiO2 and the performance of PSCs. In this study, Nb doped TiO2 films are prepared by a facile one-port chemical bath process at low temperature (70oC), and applied as a high quality ETL for planar PSCs. Comparing with pure TiO2, the Nb doped TiO2 is more efficient for photogenerated electron injection and extraction, showing higher conductivity, higher mobility and lower trap-state density. A PSC with 1% Nb doped TiO2 yielded a power conversion efficiency of more than 19%, remains about 90% of its initial efficiency after storing for 1200 h in air or annealing at 80oC for 20 h in glove box.


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1. INTRODUCTION Planar PSC is a preferred structure for future applications due to its simple preparing process and low cost. Due to the favorable position of its conduction band (CB), a large band gap, long electron lifetimes and low fabrication costs, TiO2 is frequently used as electron transport layer in planar perovskite solar cells. Generally, to achieve high-performance PSCs, anatase- or rutile-phase TiO2 films are preferred, however high quality TiO2 films require sintering at over 450°C.1-2 Although the atomic layer deposited method (ALD),3 DC magnetron sputtering4 or chemical bath method (CBD)5 enable the fabrication of TiO2 at low temperature. However, ALD and DC magnetron sputtering are costly and not easily scalable, and there are many trap states in CBD deposited TiO2 films, the relatively high density of electronic trap states below CB of TiO2 have a large influence on charge transport by producing excessive charge accumulation at interface of TiO2 and perovskite,6-7 which in turn influence the efficiency and stability of perovskite solar cells, as well as generating light-soaking issues.8 In order to improve the quality of solution processed TiO2 films for high performance PSCs, a wide range of doping elements have been investigated for TiO2 doping in PSCs, such as Lanthanum (500 oC)9, Niobium (520 oC)10, Lithium (450 o 11-12

C

or 500 oC13), Indium (500 oC)14, Neodymium (600 oC)6, Aluminum (600 oC)15.

There are many advantages for doping elements in the TiO2 films: 1) improving the electronic properties and reducing the trap-state density of TiO2, for example, Indium doping can passivate the electronic defects or trap states caused by nonstoichiometric oxygen-induced defects within the TiO2 lattice;13-14 2) inducing a complex interplay of other effects that impact the device performance. For example, it can affect surface 3 ACS Paragon Plus Environment


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morphology of the TiO2 or change the crystallinity of perovskite and thereby influence the performance of PSCs;16 3) modifying the CB position of TiO2 by doping can either through a downward shift to increase electron injection from the perovskite film into the TiO24 or through an upward shift to increase the open-circuit voltage (Voc) of PSCs9. However, these doped TiO2 films are required to sinter at over 450 °C. Therefore, developing an easier method to dope the TiO2 layer at low temperature is very important. In this study, a simple and low temperature CBD method is applied to deposite large-scale TiO2 and Nb doped TiO2 as ETL for PSCs. The efficiency and stability of PSCs are improved by Nb doping of TiO2 films. The analyses indicate that Nb doping can increase the conductivity and mobility of TiO2 film, and reduce its trap-state density, leading to reduced electron recombination, increased electron transport, and increased stability in PSCs. A power output of more than 19% is achieved in planar PSCs with 1% Nb doped TiO2. PSCs with Nb doped TiO2 show great stability remaining about 90% of its initial value over 1200h in air, and are significantly more thermal stable than PSCs with pure TiO2. Therefore, this novel deposition method represents a breakthrough as it opens up a scalable and inexpensive way to dope TiO2 by one-port solution method at low temperature.

2. EXPERIMENTAL SECTION 2.1. Deposition of TiO2 and Nb doped TiO2 films. TiO2 and Nb doped TiO2 layers were grown by chemical bath deposition on fluorine-doped tin oxide (FTO) substrates. The deposition was made by putting the FTO/glass substrates in a glass container filled with the titanium chloride or niobium chloride/titanium chloride (molar ratios: 1%, 2%, and 3%) solution in a 70 oC lab oven for 1 h. The deposited 4 ACS Paragon Plus Environment

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substrates were rinsed by deionized water for 2 min to remove any loosely bound materials, dried in a stream of N2 and annealed for 30 min at 185oC on a hot plate. 2.2. Fabrication of PSCs. All TiO2/FTO/glass and Nb doped TiO2 FTO/glass substrates were treated by an UV-ozone treatment for 15 min before deposition of the perovskite films. The perovskite solution containing 1 mmol PbI2 (99.99%, Alfar Aesar), 1 mmol CH3NH3I (synthesized according to our earlier work4, 17-18), and 1 mmol dimethyl sulfoxide (DMSO, 99.9%, Aldrich) was dissolved in 600 mg of N, N-dimethylformamide (DMF, 99.8%, Alfa Aesar). The mixed solution was spin-coated on top of the ETLs (TiO2 and Nb doped TiO2) at 1000 rpm for 5 s and 4000 rpm for 45 s while dripping chlorobenzene onto the substrate during second spinning step19. All the samples were then heated at 100 °C for 10 min resulting in the formation of dark perovskite films. The spiro-OMeTAD (70 mM solution in chlorobenzene) doped with 4-tert-butylpyridine (TBP, Sigma Aldrich) and bis(trifluoromethylsulfonyl)imide lithium salt (Li-TFSI, Sigma Aldrich) is deposited by spin coating (4000 rpm for 20 s) as hole transport layer on top of pervoskite film20. Finally, a 100 nm-thick gold top electrode was deposited as top electrode by thermal evaporation using a shadow mask to form an active area of 9 mm2. 2.3. Characterization. The surface morphologies and cross-section images of TiO2 and Nb doped TiO2 films were characterized by field-emission scanning electron microscopy (SEM, HITACHI, SU-8020). The X-ray photoelectron spectroscopy (XPS) was carried out using a photoelectron spectrometer (ESCALAB250Xi, Thermo Fisher Scientific). Photoluminescence (PL) (excitation at 532 nm) was obtained with an Edinburgh Instruments Ltd FLS980 spectrometer. The current density versus voltage (J-V) characteristics of the PSCs were measured by using a Keithley 2400 source meter under the illumination of a AM 1.5 solar simulator with the light 5 ACS Paragon Plus Environment


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intensity of 100 mW/cm2 (AM 1.5G, SAN-EIELECTRIC XES-40S2-CE solar simulator), as calibrated by a NREL-traceable KG5 filtered silicon reference cell. The active areas of all solar cells were defined by a 9 mm2 metal mask. All devices scanned with reverse and forward under standard test procedure at a scan rate of 30 mV/s. Incident photon-to-current conversion efficiency (IPCE) spectra of the PSCs were measured by a Q Test Station 500TI system (Crowntech. Inc., USA). The monochromatic light intensity for IPCE was calibrated using a reference silicon detector. The cyclic voltammetry was measured by using an electrochemical workstation (CHI660D) in a standard three-electrode configuration with Ag/AgCl as the reference electrode and Pt mesh as the counter electrode in acetonitrile aqueous solution. The scanning speed is 0.3 V/s, whereas the amplitude potential was 10 mV.

3. RESULTS AND DISCUSSION Schematics of the simple one-port CBD deposition methods are shown in Figure 1a. TiCl4 or TiCl4/NbCl5 precursor solution is dissolved in deionized water. The CBD deposition is made by putting the FTO/glass substrates in a petri dish filled with the above solution, keeping in lab oven at 70oC for 1 h. The deposited substrates are rinsed by deionized water for 2 min to remove any loosely bound materials, and dried in a stream of dry air and annealed for 30 min at 185oC. Top-view scanning electron micrographs (SEM) of the undeposited and deposited FTO surface are shown in Figure 1b-d, FTO film has a clear feature, TiO2 deposited FTO shows many small particles on the surface, indicating a rougher surface (as same as the cross-section shown in Figure S1a). However, 1% Nb doped TiO2 (best niobiumin doping condition) shows a more uniform and dense surface. Cross-sectional SEM micrographs show that both films have a thickness of about 40 nm (Figure S1).


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Figure 1. (a) Schematic of chemical bath deposition method by heating NbCl5/TiCl4 mixed solution containing FTO/glass substrates in lab oven at 70 oC for 1 h followed by post-annealing at 185 oC for 30 min. Top-view scanning electron micrographs images of (b) bare FTO, (c) TiO2/FTO, (d) Nb doped TiO2/FTO. XPS measurements are conducted to elucidate the chemical compositions of TiO2 and Nb doped TiO2 deposited on FTO substrates. Figure 2a,b show the XPS spectra of the Ti2p and Nb3d peaks respectively for the two films. The Ti 2p1/2 and Ti 2p3/2 peaks of the TiO2 film have the binding energies of ≈461.6 and ≈457.8 eV, respectively, which are consistent with literature values21. For the Nb doped TiO2 film, the Ti 2p3/2 peak is shifted slightly higher to 459.2 eV. This shift can be explained by the Pauling electronegativity theory22, the electronegativity value of Ti is 1.5 and Nb is 1.6,23 which indicates negative charge transfer toward niobiumin in the Ti-O-Nb bond, thereby increasing the Ti 2p core level binding energy.24 The XPS spectra of the doped samples shows two peaks located at 206.2 eV and 209.0 eV corresponding to Nb3d5/2 and Nb3d3/2 transitions, respectively,25 demonstrating the presence of Nb5+ in the TiO2 crystal lattice. As expected, the undoped TiO2 does not 7 ACS Paragon Plus Environment


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have a corresponding peak. Figure 2c shows the transmittance spectra for TiO2 and Nb doped TiO2 coated on the FTO/glass substrates. The 1% Nb doped TiO2/FTO glass substrate shows almost the same transmittance compared to that of TiO2/FTO glass substrates. However, 2% and 3% Nb doped TiO2/FTO glass substrates show lower transmittance in the range from 300 nm to 750 nm and slight red-shift in the onset of transmittance compared to the TiO2/FTO glass substrates. This is attributed to the narrower band gap (2.70 eV) of the 3% Nb doped TiO2 than that of the pure TiO2 (3.07eV), as shown in (Table S1). The cyclic voltammetry is carried out to estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of pure TiO2 and 3% Nb doped TiO2 (as shown in Figure S2). Ag/AgCl is used as the reference electrode and Pt as counter electrode, acetonitrile solutions as the supporting electrolyte. The HOMO and LUMO levels of TiO2 and 3% Nb doped TiO2 are calculated from the equation26: ELUMO = -e(E'red+Vref); EHOMO = -e(E'ox+Vref)

(1)

where E'ox is the onset of the oxidation peak, E'red is the onset of the reduction peak, Vref is the potential of reference electrode versus vacuum. The calculated LUMO levels of TiO2 and 3% Nb doped TiO2 are -4.10 and -4.22 eV, respectively. The LUMO level of 3% Nb doped TiO2 is more negative than pure TiO2, the PSCs based on 3% Nb doped TiO2 with a deeper LUMO level is desired to reduce the VOC of PSCs. To probe the effect of the niobium doping on charge transfer at the interface of TiO2 and the perovskite layer, photoluminescence (PL) measurements are carried out 8 ACS Paragon Plus Environment

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to determine whether or not the photogenerated electrons were efficiently injected from the perovskite film into the ETL. Figure 2d shows that the spectral peaks at 766 nm related to the intrinsic fluorescence emission of CH3NH3PbI3 are quenched by contact with the undoped TiO2 and 1% Nb doped TiO227, indicating electron transfer from the perovskite to the ETLs. Compared to the perovskite sample, the PL intensities of the TiO2/perovskite and 1% Nb doped TiO2/perovskite samples are quenched about 93.2% and 96.2%, respectively, indicating that the charge extraction from the perovskite is more efficient for Nb doped TiO2.

Figure 2. The XPS spectra of (a) Ti 2p peaks and (b) Nb 3d peaks. (c) Transmittance spectra of TiO2/FTO, and Nb doped TiO2/FTO with different Nb contents. (d) Photoluminescence (PL) spectra of perovskite/glass and perovskite layers in contact with the TiO2/FTO or Nb doped TiO2/FTO, insert: enlarged PL spectra of 9 ACS Paragon Plus Environment


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TiO2/perovskite and Nb doped TiO2/perovskite. In order to determine the influence of Nb doping on the conductivity of TiO2 film, the devices composed of ITO/TiO2/Au and ITO/Nb doped TiO2/Au are fabricated and current-voltage characteristic is shown in Figure. S3, the conductivity is calculated by the flowing equation28: I = σ0AD-1V

(2)

where A is the area (9 mm2) and D is the thickness (40 nm) of the TiO2 or Nb doped TiO2; the calculated values of the conductivity are summarized in Table S2. It is found that the conductivity of the Nb doped TiO2 is increased about three times than that of undoped TiO2, demonstrating that the Nb doping is an effective way to increase the conductivity of TiO2 film. The mobility of pure TiO2 and 1% Nb doped TiO2 is obtained by fitting the current versus voltage (I-V) curves of electron-only devices (FTO/ TiO2 or 1% Nb doped TiO2/PCBM/Ag) (Figure. S4) by the Mott-Gurney equation29: J = qεoεrµV2/8L3

(3)

where J is the current density, εo is the vacuum permittivity, εr is the dielectric permittivity of the TiO2, µ is the electron mobility, V is the applied voltage of the device, L is the thickness of the TiO2 or doped TiO2 film, the calculated values of the electron mobility are 2.39 × 10-3 and 1.17 × 10-3 cm2V-1s-1 for pure TiO2 and 1% Nb doped TiO2, respectively, demonstrating that the Nb doping can effectively promote the electron transport in TiO2 film. To investigate the effects of Nb-doping on the electron trap-state density in the perovskite films, the electron-only devices (FTO/TiO2 or 1% Nb doped 10 ACS Paragon Plus Environment

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TiO2/CH3NH3PbI3/PCBM/Ag) are fabricated to measure the electron trap-state density in the perovskite absorber on different substrates. The I-V curves of the devices based on TiO2 and 1% Nb doped TiO2 are shown in Figure S5. The linear relation indicates an ohmic response of the electron-only device at low bias voltage, and the current quickly increases nonlinearly when the bias voltage exceeds the kink point, demonstrating that the trap-states are completely filled. The trap-state density can be determined by the trap-filled limit voltage (VTFL) using equation30:

VTFL =

entL2 2εε0

(4)

where e is the elementary charge of the electron (e =1.6×10-19 C), L is the perovskite film thickness (≈300 nm), ε is the relative dielectric constant of CH3NH3PbI3 (ε = 28.8), ε0 is the vacuum permittivity (εo = 8.854×10-12 F/m), and nt is the trap-state density of CH3NH3PbI3 film. The VTFL of the CH3NH3PbI3 films deposited on TiO2 and 1% Nb doped TiO2 substrates are 0.46V and 0.32 V, respectively. Therefore, the electron trap-state density of CH3NH3PbI3 coated on TiO2 is 1.63×1016 cm-3. When the CH3NH3PbI3 is deposited on 1% Nb doped TiO2, the electron trap-state density reduced to 1.13×1016 cm-3, indicating that the Nb doped TiO2 can improve the perovskite film quality and decrease its trap-state density. To investigate the effects of Nb doping on the electron trap-state density in the TiO2 films, the I-V curves of pure TiO2 or 1% Nb doped TiO2 film with electrodes are measured, as shown in Figure S6. The value of VTFL increases as electron traps increasing. From the I-V curves, it can estimate the values of VTFL. The VTFL of devices based on pure TiO2 (0.51 V) is higher than that based on Nb doped TiO2 (0.24 V). According to the relation between electron traps and VTFL (Equation 4), the 11 ACS Paragon Plus Environment


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density of electron traps in the pure TiO2 film (1.81×1016 cm-3) is more than two times of that in 1% Nb doped TiO2 film (0.85×1016 cm-3). It is clear that Nb-doping can passivate the electron traps in TiO2 film. The lower trap-state density in Nb doped TiO2 is likely to lead to an enhanced efficiency and stability in PSCs. A

normal

device

structure,

consisting

of

FTO/pure

or

doped

TiO2/CH3NH3PbI3/Spiro-OMeTAD/Au31, is prepared to verify the performance improvement provided by the Nb doped TiO2 compared to cells with a pure TiO2. Cross-sectional SEM image of a complete cell is shown in Figure S7. The thickness of the pure or doped TiO2 film is about 40 nm, capped by ～300 nm of CH3NH3PbI3 film. To optimize the performance of Nb doped TiO2 based cells, the cells on FTO substrates are prepared by using TiO2 with niobium doping concentrations varying from 0% to 4% (mol% to TiO2) as ETLs. The measured J-V under AM 1.5G irradiation at 100 mW/cm2 are presented in Figure 3a and the relevant photovoltaic parameters including open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF) and efficiency (η) are summarized in Table 1. As shown, the PSCs with undoped TiO2 shows a decent PCE of 18.14% (under reverse scan) with a VOC of 1.10 V, a JSC of 22.37 mA/cm2, and a FF of 73.7%. The PSCs with 1% Nb doped TiO2 can yield improved PCEs compared to the control device. The champion device shows a promising PCE of 19.23% with improved JSC (22.86 mA/cm2) and FF (76.5%). The enhanced photovoltaic parameters can be attributed to the more efficient charge separation/collection at Nb doped TiO2/perovskite interface due to the higher conductivity and electron mobility of Nb doped TiO2 film. It is important to note that the performance of the devices starts to decrease as the Nb doping ratio is exceeds 1%, which causes the VOC degradation due to the reduced LUMO level of Nb doped TiO2 (Table S1). The reduced JSC is may originate from increased charge 12 ACS Paragon Plus Environment

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recombination because the higher Nb content in the TiO2 resulted in more surface defects, which act as trap sites.32 The incident photon-to-current efficiency (IPCE) spectra of the best-performing devices with pure TiO2 or 1% Nb doped TiO2 as ETLs are shown in Figure S8. The device with 1% Nb doped TiO2 shows higher IPCE values than that of PSCs with pure TiO2 across the wavelength ranging from 350 to 700 nm, indicating enhanced charge separation and collection at 1% Nb doped TiO2/perovskite interface. Table1. Photovoltaic parameters of PSCs with pure TiO2 and Nb doped TiO2 as ETLs. Nb (%)

VOC (V)

JSC (mA/cm2)

FF (%)

η (%)

0

1.10

22.37

73.7

18.14

1

1.10

22.86

76.5

19.23

2

1.08

23.03

75.4

18.75

3

1.05

22.40

73.9

17.38

Up to now, all the reported PSCs with only planar TiO2 as HTLs generally encounter large hysteresis33. However, for PSCs with Nb doped TiO2, a reasonable PCE of 19.20% under reverse scan can be obtained and show small hysteresis, for which the PCE obtained under forward scan is 16.21% (Figure 3b). Whereas, the PSCs with undoped TiO2 showed a slight low PCE of 18.14% under reverse scan but exhibited a significant hysteresis, for which the PCE obtained under forward scan is only

12.72%

(Figure

3b).

The

hysteresis

ratio

defined

as

|PCEreverse

−PCEforward|/PCEreverse is calculated to be around 15.6% and 29.8% for the Nb doped PSCs and undoped devices, respectively. Previous studies suggest that electron accumulation, arising from in efficient charge transfer at perovskite interfaces, exaggerates current voltage hysteresis34-35. So the reduced hysteresis is attributed to efficient charge transport at Nb doped TiO2/perovskite interface. Meanwhile, the high conductivity of Nb doped TiO2 can decrease the charge accumulation at its interface, 13 ACS Paragon Plus Environment


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which it is another reason for the reduced hysteresis. As shown in Figure 3c,d, the statistics analysis confirmed that PSCs with 1% Nb doped TiO2 demonstrated a higher PCE than that with the undoped TiO2. The average PCE on a basis of 40 devices is 17.6% for PSCs with 1% Nb doped TiO2 and 16.3% for PSCs with undoped TiO2, respectively, unambiguously indicating that Nb doping appreciably improves the PSC performance.

Figure 3. (a) Current density-voltage curves of PSCs based on undoped TiO2 and Nb doped TiO2. (b) Hysteresis behavior of the PSCs based on TiO2 and 1% Nb doped TiO2 ETLs. Statistical PCE distribution of PSCs based on (c) TiO2 and (d) 1% Nb doped TiO2. To examine the impact of the Nb doping upon the device air stability, the unencapsulated perovskite devices with pure TiO2 and 1% Nb doped TiO2 are aged 14 ACS Paragon Plus Environment

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for 1200 h in air (humidity: ~20%). As showed in Figure 4a, the PCE of the PSCs is measured after a certain period of storage. The PCE of the devices with TiO2 is quickly dropped to 67.6% of the initial value, showing an obvious degradation of JSC and an unnormal J-V curve with a pseudo FF (≈84%) (Figure 4b) while the devices with 1% Nb doped TiO2 decrease very fast at the beginning and then tend to saturate after a certain period, retaining over 89.0% of their initial PCE after 1200 h. The enhanced stability of the doped devices is mainly due to faster charge transferring at doped TiO2 and perovskite interface, higher conductivity and electron mobility, and lower trap-state density of doped TiO2, which can facilitate the interfacial charge transfer between the perovskite and TiO2 so that charge accumulation and recombination within the devices can be avoided36. Finally, the thermal stability of devices is studied. As shown in Figure 4c, the efficiency of PSCs with 1% Nb doped TiO2 can be maintained more than 90% after annealing on a hotplate at 80oC for 20 h in glove box. However, the performance of PSCs with pure TiO2 decreased to 54% of its initial PCE under the same processing conditions. The degradation speed of PCE (η) can be defined as a function of time t by , where η (0) and η (t) are the initial PCE and the PCE at time t, respectively37. Therefore the devices with pure TiO2 show a degradation speed of α = 2.36%/h. However, the devices with 1% Nb doped TiO2 show α = 0.45%/h, indicating that 1% Nb doped TiO2 can slow down the degradation speed of PSCs under thermal annealing conditions. Importantly, no degradation of VOC in the aged devices with 1% Nb doped TiO2 occurred (Figure 4d), and the J-V curve is normal. However, the aged devices with pure TiO2 show an obvious degradation of VOC and JSC, and the J-V curve is also unnormal with a pseudo FF, which may due to the degradation of perovskite film caused by its high trap-state density, and/or trapped charges on pure 15 ACS Paragon Plus Environment


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TiO2 film for its low conductivity and mobility, and high trap-state density7, 38.

Figure 4. (a) The stability of PSCs with TiO2 and 1% Nb doped TiO2 as ETLs under air exposure (humidity: ~20%) for different time periods. (b) Current density-voltage curves of PSCs with TiO2 and 1% Nb doped TiO2 before and after storing in air for 1200 h. (c) The stability of PSCs with TiO2 and 1% Nb doped TiO2 as ETLs under thermal annealing at 80oC in glove box for different time periods. (d) Current density-voltage curves of PSCs with TiO2 and 1% Nb doped TiO2 before and after annealing at 80oC for 20 h.

4. CONCLUSIONS A low temperature and solution processed CBD method has been successfully used to dope Nb into TiO2 film, resulting in a highly efficient Nb doped TiO2 ETL with high quality, high electrical conductivity, electron mobility and low trap-state density. 16 ACS Paragon Plus Environment

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Such improvements lend crucial support to the enhanced injection and extraction of photogenerated electrons to avoid the accumulation of electron at the ETL/perovskite interface, which sequentially trigger remarkable increases in Jsc and FF, and stability of PSCs. Basing on the optimized 1% Nb doped TiO2 ETL, a highly efficient planar PSC with a best PCE of 19.2% is obtained. The PCE can be maintained about 90% of its initial efficiency after storing in air for 1200 h or annealing at 80oC for 20 h in glove box. In general, developed CBD method in this paper will pave the way for doping other elements into TiO2 in large area for high quality ETLs because it does not need a high temperature and can easily prepared in a solution process.

ASSOCIATED CONTENT

Supporting Information

Additional SEM and Cyclic voltammetry analysis of TiO2 and 1% doped TiO2 films. The electrical conductivity of TiO2 and 1% doped TiO2 films, the I-V curves of TiO2 and 1% doped TiO2 films fitting with the Mott-Gurney law, dark I-V curves of the electron-only devices (FTO/TiO2 or 1% Nb doped TiO2/CH3NH3PbI3/PCBM/Ag), the

I-V curves of FTO/TiO2 or Nb doped TiO2/Ag devices. Cross-sectional SEM micrograph of typical PSC based on 1% Nb doped TiO2. The IPCE of the PSCs based on the TiO2 and 1% doped TiO2 ETLs. AUTHOR INFORMATION Corresponding Author * Prof. Zhike Liu, [email protected]. * Prof. Shengzhong Frank Liu, [email protected]. 17 ACS Paragon Plus Environment


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ORCID Shengzhong Frank Liu: 0000-0002-6338-852X

Author Contributions ‡ Guannan Yin and Jiaxin Ma contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge all support from the National Key Research Program of China (2016YFA0202403)

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