Enhanced Efficiency and Stability of Perovskite Solar Cells via Anti

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Enhanced Efficiency and Stability of Perovskite Solar Cells via AntiSolvent Treatment in Two-Step Deposition Method Minghua Li,† Xiaoqin Yan,*,† Zhuo Kang,† Xinqin Liao,† Yong Li,† Xin Zheng,† Pei Lin,† Jingjing Meng,† and Yue Zhang*,†,‡ †

State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China ‡ Beijing Municipal Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing, 100083, China S Supporting Information *

ABSTRACT: The low-cost inorganic−organic lead halide perovskite materials become particularly promising for solar cells with high photovoltaic conversion efficiency. The uniform and pinholefree perovskite films play an important role for high-performance solar cells. We demonstrate an antisolvent treatment by controlling the PbI2 morphology to enhance the perovskite conversion and photophysical properties, including high absorption, crystallinity, and rapid carrier transfer. The fabricated perovskite solar cells show tremendous PCE improvement to about 16.1% from 12% with less hysteresis, and retain over 90% initial PCE after 30 days in ambient and dark atmosphere. In prospect, this antisolvent treatment will be a feasible route to prepare high-quality perovskite films including favorite photophysical properties. KEYWORDS: antisolvent treatment, PbI2 morphology, stability, TiO2/perovskite heterojunction, perovskite solar cell

1. INTRODUCTION In recent decades, solar cells have been rapidly developed for their high-efficiency and low-cost in harvesting solar energy, especially the third generation solar cells, such as copper indium gallium selenide solar cells (CIGS), dye-sensitized solar cells (DSSC), and organic solar cells (OPV).1−6 Recently, the lead halide hybrid−perovskite solar cells based on AMX3 (where A is an organic ammonium cation, such as methylammonium (MA) or formamidinium (FA), M is Pb or Sn, and X is a halide anion) composition become one of the most promisingly shining stars with the efficiency jumping from 3.8% to over 20%.7−21 The magic perovskite materials show outstanding optoelectronic properties such as high absorption throughout the visible spectrum, ability to transfer electrons and holes, and a tunable band gap from 1.1 to 2.3 eV by modulating the element composition.12,22,23 As a result, these wonderful perovskite materials with superb optoelectronic properties have been applied widely in bilayer mesoporous metal oxides and planar heterojunction solar cells serving as the light absorption layer.18,20,24,25 To realize high-quality perovskite films, a variety of approaches have been developed, such as a one-step method, a two-step method, and a vapor-assisted method.9,11,26 In a typical two-step deposition of MAPbI3, PbI2 is first deposited on the substrate and then transformed to perovskite by exposing to methylammonium halide solution. Burschka and © 2017 American Chemical Society

co-workers applied the mesoporous TiO2 to facilitate the perovskite conversion.11 Recently, it has been reported that a thick uniform and smooth perovskite capping layer on the TiO2 nanostructure can enhance the film absorption and boost the device performance.18,24,27 Hence, the deposition of a PbI2 capping layer is the precondition for forming compact and smooth perovskite films on the TiO2 nanostructure. However, the methylammonium halide penetration becomes a serious issue, due to volume expansion when the MAI inserts into the PbI2 crystal lattice. This will lead to incomplete transformation of PbI2 to perovskite on account of the fact that the formed compact layer blocks the MAI diffusion and insertion into the deeper PbI2 layer.20,28 Moreover, the residual and uncontrolled amount of PbI2 has a detrimental effect on the carrier separation and transport and consequent device performance, such as reproducibility and stalibity.28,29 To address this problem, many methods have been developed. Han’s group and Seok’s group utilized the PbI2(DMSO) complex to increase the transformation process, leading to high-performance solar cells.20,30 Huang and co-workers developed a thermal-driven interdiffusion method to produce a compact MAPbI3 layer.31 Zhao and co-workers added some MAI into PbI2 precursor to Received: January 23, 2017 Accepted: February 14, 2017 Published: February 14, 2017 7224

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Figure 1. (a) Schematic illustration of device structure, (b) energy level diagram, and (c) the schematic fabrication process: the topside is the conventional method with reaction from topside, the downside is the antisolvent treatment with reaction in the whole film.

produce enough intervoids and interspaces which can serve as a solution penetrating channel in the PbI2 layer. To produce plenty of pores and voids in the PbI2 layer, the chlorobenzene (CB) solution is chosen to load onto the PbI2 wet film to extract DMF solution before annealing. The typical images of the PbI 2 films on the mesoporous TiO 2 nanostructure with or without CB treatment are shown in Figure 2. The pristine PbI2 film displays as compact and flat on the nanocrystalline TiO2. After CB dropping, plentiful pores and voids generate on the surface (in Figure 2b), and penetrate through the whole film (in the cross-sectional image Figure S1

generate adjustable volume expansion before the MAI contact to produce efficient and reproducible solar cells.28 In this work, we develop a new method to produce compact and uniform perovskite films in the two-step deposition process, by producing enough voids and spaces in the capping PbI2 layer. To prepare the mesoporous structure, the antisolvent (Chlorobenzene) was employed to treat the ascast PbI2 film before annealing. Due to the different solubility of the solute and solvent, the DMF can be extracted quickly, leading to PbI2 rapid crystallization and film volume reduction. The mesoporous PbI2 structure can provide enough space for the methylammonium halide solution infiltration into the whole film instead of staying on the film surface, resulting in rapid and complete perovskite conversion. The obtained perovskite films exhibit favorite light-absorption, crystallinity, and photophysical properties. The prepared TiO2/perovskite heterojunction shows rapid and effective electron separation and transfer process, which is beneficial for enhancing the device power output and suppressing hysteresis behavior. Upon this method, the fabricated solar cells show an enhanced PCE from 12.05% to 16.09% with less hysteresis and long stability.

2. RESULTS AND DISCUSSION We fabricate an FTO/cp-TiO2/mp-TiO2/perovskite/SpiroOMeTAD/Au structure depicted in Figure 1a. In the typical two-step method, the perovskite is formed once the PbI2 and the methylammounium halide solution contact. According to the reports, the CB and VB for the MAPbI3 is −3.93 and −5.43 eV, respectively. The CB and VB for the PbI2 is −3.45 eV and −5.75 eV, respectively.26 The residual PbI2 in the TiO2/ perovskite interface can block photoinduced carrier separation and transfer due to the wide band gap (in Figure 1b). To improve the conversion process, we adopt antisolvent treatment to change the perovskite conversion process from interface interdiffusion to intercalation reaction in the whole PbI2 film (illustrated in Figure 1c). It is very important to

Figure 2. SEM images of films with or without CB treatment: (a) and (c) are top-view images of the pristine PbI2 film and corresponding perovskite film without CB treatment, (b) and (d) are top-view images of the PbI2 film and corresponding perovskite film with CB treatment. 7225

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similar appearance, except for the difference in grain size and film roughness. The perovskite crystal with CB treatment grows larger to about 170 nm from about 140 nm (in Figure S4), which can reduce the recombination sites due to the decreased grain boundaries. This grain growth derives from the MAI intercalating into the large crystal lattice of PbI2 with CB treatment. And the grown perovskite grain generates higher film RMS of 11.1 nm in contrast to 9.65 nm of the pristine perovskite film. But the RMS difference of the two perovskite film is very little compared with those reported by others. By comparing the cross-sectional images of the two perovskite films (Figure S5), there exists a dense and uniform perovskite capping layer with CB treatment without any voids (dashed circles in the pristine perovskite film), suggesting a beneficial infiltration into the TiO2 scaffold to form high-quality perovskite/TiO2 heterojunction. In brief, this antisolvent treatment provides the good premise for excellent optoelectric properties on account of the reduced the recombination sites and favorite interface contact. In the perovskite conversion process, the color of the PbI2 with CB treatment changed from yellowish to dark-brown immediately once the methylammonium halide solution dropped. While the color of the pristine PbI2 films maintained light brown even after annealing. This striking comparison indicates the PbI2 film with antisolvent treatment can transfer to perovskite more rapidly and effectively, because the methylammonium halide solution penetrates into the whole film instead of staying on the top of compact PbI2 layer. The absorption spectra of the two perovskite films were conducted in Figure 3a. The perovskite film with CB treatment shows stronger absorption throughout the whole spectra. The peak at about 500 nm in the PbI2 film is disappeared in the perovskite film with CB treatment, confirming the effective phase conversion without PbI2 phase residue. To further verify the film phase composition, the XRD patterns are depicted in Figure 3b. A series of new diffraction peaks, which can be identified for the tetragonal MAPbI3 phase, emerge after the contact with methylammonium halide solution. The perovskite film from pristine compact PbI2 layer still presents 12.6° diffraction peak, deriving from the characteristic (001) plane of the PbI2 phase. In contrast, the PbI2 peaks almost disappear in the perovskite film with the CB treatment. The results of absorption spectra and XRD patterns suggest that the CB treatment can enhance the efficiency of the PbI2 transforming into perovskite. The steady-state and time-resolved photoluminescence spectra were wildly investigated to get insight into perovskite photophysical properties.34−37 When the perovskite film is deposited on the insulating layer, such as glass or quartz, the PL intensity is mainly dependent on the recombination of the excitons in the film. While with charge transfer layer, the PL intensity depends greatly on the carrier transfer process. In here, the two perovskite films are deposited on the TiO2 ETL to characterize the carrier transfer kinetics. Figure 3c shows the steady-state photoluminescence of the perovskite films with or

of the Supporting Information). Comparing the UV−vis absorption spectra (Figure 3a), the treated PbI2 film holds

Figure 3. Film characteristics with or without CB treatment. (a) UV− vis absorption spectra, (b) the XRD diffraction patterns, (c) the steady-state PL spectra, and (d) the time-resolved photoluminescence, respectively.

higher absorption due to stronger light scattering in the pores and voids. Regarding the XRD patterns (in Figure 3b), the two PbI2 films present similar appearance. The Scherrer formula is employed to detect the antisolvent effect on the grain size evolution. The full width at half-maximum (FWHM) at 12.6° are 0.433° and 0.361° for the PbI2 film without and with antisolvent treatment, and the corresponding calculated grain sizes are 19.33 and 23.19 nm, respectively. It is noteworthy that identifying the crystal size change from the SEM images is difficult due to the aggregated grains forming large domains. Interestingly, the PbI2 layer presents grown crystal grains and viewed pores distributed in the whole film after the antisolvent treatment. In order to study the origin of the pores and holes in the PbI2 film, the Fourier Transform Infrared Spectroscopy (FTIR) measurement was conducted in Figure S2. As already reported, the characteristic CO bonding of the DMF solvent is at about 1650 cm−1.32,33 After CB dipping, the characteristic mode of the DMF weakens distinctly, indicating that the DMF solution is almost extracted. This rapid DMF volume reduction will lead to plentiful space in the deposited layer and fast crystallization of the supersaturated PbI2. On the basis of the PbI2 films with or without CB treatment, the perovskite films are prepared by loading a drop of methylammonium halide solution subsequently. The SEM images and topographies of the two perovskite films are shown in Figure 2 and Figure S3. The two perovskite films show

Table 1. Fitted Parameters of the TRPL Curves of TiO2/Perovskite Films with or without Treatmenta

a

sample

τ1 (ns)

A1

τ2 (ns)

A2

τ−average (ns)

pristine perovskite perovskite with treatment

0.446 0.291

22.727 61.410

13.929 14.077

77.273 38.590

10.87 5.61

τ−average = A1*τ1 + A2*τ2. 7226

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ACS Applied Materials & Interfaces without CB treatment. The PL intensity of the perovskite film with treatment drops significantly in contrast to that of the pristine perovskite film, suggesting the excitons can separate and transfer rapidly from perovskite film into TiO2 layer. To clarify the transient photoinduced carrier lifetime, the timeresolved photoluminescence (TRPL) spectra are conducted in Figure 3d. The TRPL spectra were fitted by a biexponential function: PLintensity = A1 exp(−t/τ1) + A2 exp(−t/τ2); where A1 and A2 are time independent coefficients of amplitude fraction for each decay component and τ1 and τ2 are decay time of fast and slow component, respectively. The fitted parameters are depicted in Table 1. It can be observed clearly that the decay time decreases from 10.87 to 5.61 ns, which suggests a fast charge transfer and collection process occurs in the perovskite/ TiO2 heterojunction with the treatment. This improvement in photoinduced carrier transfer and injection process might reduce the J−V hysteresis by avoiding the formation of interface capacitance or accumulated charge.38 To further characterize the photophysical property of perovskite films with or without CB treatment, kelvin probe measurements in the dark and illumination were performed. The surface of the perovskite film can accumulate carriers, due to located trap state, to result in band bending at the surface as shown in Figure 4a.39−41 The magnitude of band bending

acceptor type accumulating electrons. The SPVs of the pristine perovskite film without and with treatment shift down 120 mV from 720 mV to 600 mV and 70 mV from 610 mV to 540 mV, respectively. The lower magnitude of SPV obtained in the perovskite film with treatment manifests the CB treatment can lower the density of trap state, leading to less nonradiative recombination which is responding for better photophysical properties of the perovskite film and favorite photovoltaic performance in assembled device. The photovoltaic performance of the perovskite solar cells with or without treatment is shown in Figure 5. The perovskite

Figure 5. Photovoltaic performance of the perovskite solar cells with or without treatment: (a) the J−V curves under reverse scan of the best solar cells, (b) the EQE spectra, (c) the steady current and corresponding PCE measured at the bias near the maximum output point of the two perovskite solar cells without (at 0.8 V) and with (at 0.86 V) treatment, and (d) the durability of the unencapsulated solar cells stored in dark and ambient condition.

Figure 4. KPFM measurement. (a) Schematic band diagram for the surface photovoltage (SPV) measurement at the perovskite surface. Evac, Ec, Ef, and Ev represent the vacuum level, the conduction band minimum, the Fermi level, and the valence band maximum. Wdark and Willu represent the Ws in the dark or under illumination. The green solid lines and the black dashed lines indicate the condition in the dark and illumination, respectively. (b) Representative VCPD potential histograms change in the dark and under illumination, the top is the perovskite film with the treatment, the bottom is the perovskite film without treatment. The tip potential is calibrated by the fresh pyrolytic graphite surface (in Figure S6). The KPFM images of the two perovskite films are shown in Figure S7.

solar cells with treatment have improved PCE from 12.05% to 16.09% (from the statistic parameters in Table 2). The PCE improvement arises from increased Voc from 1.004 to 1.063 V, Jsc from 18.61 mA cm−2 to 21.26 mA cm−2, and FF from 0.645 to 0.712. The increased Jsc, which is confirmed by the EQE spectra in Figure 5b, originates from the enhanced light absorption and favorite crystallinity. The Voc and FF improvement might own to the good property of the TiO2/perovskite heterojunction, which has favorite photoinduced carrier separation, extraction and transfer. To obtain accurate power output, the steady-state current density and PCE in Figure 5c were conducted. The stabilized power output of the perovskite solar cell is about 1.75 times higher than that in the pristine perovskite solar cell, and can exhibit over 16.2% after 60 s light soaking. The J−V curves of the two perovskite solar cells were measured at reverse and forward scan, and shown in Figure S8. The perovskite solar cell with CB treatment behaves less hysteresis of 6.7% difference factor in contrast to the pristine perovskite solar cell with 38.9% difference factor. This decreased hysteresis might depend on effective carrier transfer and weak capacitance response at low-frequency region. With the purpose of detecting the perovskite film stability, the XRD patterns of the two perovskite films stored for 4 weeks

depends on the density of carrier accumulation, and the direction of band bending is determined by the type of accepted carrier. There exists an upward band bending when the surface state is acceptor type accumulating electrons; while the bending is downward if the surface state is of donor type accumulating holes. Under illumination, the degree of band bending reduces on account of the screen effect of photoinduced carriers. The difference of the surface work function in the dark and under illumination is referred to as surface photovoltage (SPV), indicating the band bending shift and the density of surface trap. The contact potential difference (CPD) is the relative potential between the tip and the sample surface, and the value of VCPD is determined by the equation: ϕtip − ϕsample

VCPD = . Thus, the VCPD evolution is equivalent to −e the perovskite surface potential change. In Figure 4b, the VCPD shifts down, indicating that the surface trap state is of the 7227

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ACS Applied Materials & Interfaces Table 2. Performance Statistic Parameters of the Perovskite Solar Cells with or without Treatment sample

Voc (V)

Jsc (mA cm−2)

FF

PCE (%)

pristine perovskite champion-pristine perovskite with treatment champion-treatment

1.00 ± 0.02 1.03 1.06 ± 0.02 1.08

18.61 ± 0.64 19.32 21.26 ± 0.59 21.96

0.65 ± 0.02 0.66 0.71 ± 0.01 0.72

12.05 ± 0.63 13.14 16.09 ± 0.56 17.11

were conducted in the Figure S9. The perovskite film with antisolvent treatment maintains high and stable crystallinity for a long time without PbI2 phase emergency. In contrast, the pristine perovskite film presents distinct phase decomposition with decreased perovskite peak intensity and increased PbI2 peak intensity along with storage time. After four week storage, there exists only trace amounts of perovskite. Obviously, the residual PbI2 has a detrimental effect on the perovskite longterm stability, which is consistent with some reported results.29,42,43 Moreover, the device stability is also tested in Figure 5d. After 30 days aging, the PCE of the perovskite solar cell with CB treatment remains over 90% of the initial PCE, while the PCE of the pristine perovskite solar cell drops over 50% from 12.79% to 6.2%. The evolution of detailed performance parameters is presented in Figure S10. The performance degradation is mainly attributed to the Jsc decline due to perovskite film decomposition. There are two reasons for the antisolvent treatment to enhance the device performance stability. The first aspect is the antisolvent treatment can develop highly and purely crystalline perovskite without unconsumed PbI2 phase, which can accelerate the perovskite decomposition. Besides, the voids (labeled in Figure S5a), where the moisture in the air can penetrate into, were eliminated by the perovskite well infiltrating into the TiO2 nanostructure. In short, the perovskite solar cell with CB treatment presents improved PCE, less hysteresis and longterm stability, on account of the elevated optoelectronic properties of the TiO2/perovskite coupling heterojunction. In addition, electronic impedance spectroscopy characterization was measured to understand the electron transfer and recombination property in the perovskite solar cells. The Nyquist plots in Figure 6a demonstrate two irregular semicircles. Accordingly to the previous reports, the series resistance (Rs) is obtained at the intercept point of the Nyquist plot on the real axis. The high-frequency semicircle is related to the electronic transport resistance (R s ) and chemical capacitance. The low-frequency component is usually assigned to recombination resistance (Rrec) and the capacitance between the perovskite film and the electron transport layer. The Nyquist plots were fitted according to the equivalent circuit in Figure S11. The recombination resistance (Rrec) in Figure 6b was obtained under different applied voltage. The perovskite solar cell with treatment has higher Rrec values in the whole range, implying a slower recombination process and less recombination loss. And this is the reason for the enhanced Voc. To get insight into the electrical defect characteristics in the perovskite solar cells, the capacitance−frequency (C−f) curves in Figure 6c were measured at short-circuit condition without bias voltage.44−47 The frequency-dependent capacitance is related to the ion defects or defects dipole in the perovskite and can be influenced by illumination, applied bias, or temperature.46−51 In order to investigate the effect of antisolvent treatment on the capacitance response, the frequency-dependent capacitance was measured in the dark to rule out illumination or temperature influence. At the low-

Figure 6. Impedance spectroscopy characteristics of the perovskite solar cells with or without antisolvent treatment in the form of (a) the Nyquist plots, (b) the evolution of the recombination resistance (Rrec) at different applied bias voltages, (c) the frequency-dependent capacitance curves, and (d) the Mott−Schottky plots.

frequency region, the capacitance in the perovskite solar cell with treatment is lower than that in the control device, which might be attributed to the enhanced crystallinity or favorite interface contact. This decreased capacitance response at the low−frequency region manifests the less native defects and weaker electrode polarization, and will result in less hysteresis behavior. Figure 6d shows the Mott−Schottky plots of the two perovskite solar cells with or without treatment. The result is analyzed by fitting the curves according to the equation: 1 C2

=

2 qεε0N

(V − V

bi

−

kT q

)where the C represents the space

charge capacitance of per unit, and q, ε, and ε0 are the elementary electron, the dielectric constant of the semiconductor, and the vacuum permittivity, respectively. N, V, Vbi, k, and T are the charge density, the applied voltage, the build-in potential, the Boltzmann constant, and the absolute temperature, respectively. In the M-S equation, the interfacial charge density (N) is inversely proportional to the line slope. The perovskite solar cell with treatment holds higher slope of −2.37 × 1014 in contrast to that of −1.09 × 1014 in the pristine solar cell, showing the low charge density at the interface. The build-in potential for the perovskite solar cell with CB treatment is about 0.1 V larger than that in the pristine perovskite solar cell, resulting in enhanced driving force for effective carrier transfer and recombination suppression under extended depletion region. Meanwhile, the lower carrier accumulation explains the lower hysteresis and the higher Voc for the perovskite solar cell with the treatment.52,53 These impedance measurements explain the reason that the treatment 7228

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with Cu Kα X-ray radiation source. Steady state photoluminescence was measured using Horiba JY-HR800 with excitation at 514 nm. In the time-resolved photoluminescence spectra, the measurement was conducted at 765 nm with Edinburgh Instruments (F900) with a pulsed diode laser (PDL 800-B) at 485 nm (with an intensity of 0.12 mW cm−2) at a pulse frequency of 1 MHz. The photovoltaic curves were measured under simulated AM 1.5G irradiation (100 mW cm−2) using a Xenon-lamp solar simulator (Oriel, 911A). A Si-reference cell citified by NIST was used to calibrate the lamp light. The J−V curves, electrochemical impedance spectroscopy and Mott−Schottky were performed by an electrochemical workstation (Solartron, SI1287/ SI1260) in the atmosphere. The electrochemical impedance spectroscopy were carried out under different applied bias at 20 mV perturbation in the range of 1 MHz to 1 Hz in dark. The frequency-dependent capacitances were detected in the parallel equivalent circuit mode at zero bias voltage in dark. The Mott− Schottky plots were extracted from the capacitance spectra at the frequency of 10k under the dark condition. The EQE spectrum was measured by Stanford Research Systems model SR830 DSP lock-in amplifier coupled with WDG3 monochromator and 500 W xenon lamp.

improve the performance, resulting from lower recombination loss and effective charge extraction and transfer.

3. CONCLUSIONS In summary, we develop a facile antisolvent treatment to prepare compact and uniform perovskite films. The perovskite film with this treatment exhibits favorable properties with enhanced light absorption and high crystallinity. The wellstructured TiO2/perovskite heterojunction presents excellent carrier separation and transfer property, resulting in less electron accumulation at the interface. By fabricating the solar cell using this antisolvent treatment, the device performance increased about 33% from 12.05% to 16.09% (the best PCE of 17.11%) with less hysteresis. And the PCE of the perovskite solar cell with treatment remains over 90% of the initial PCE after 30 days in ambient air stored in the dark. Utilizing this method, higher PCE can be feasibly obtained with higher Jsc after further modulating the fabrication process. This strategy adopting antisolvent treatment is expected to be useful and meaningful for other perovskite materials and devices to realize outstanding photovoltaic performance

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01136. SEM images of PbI2 and perovskite films, AFM topography and KPFM measurement of perovskite films, FTIR spectra, grain size distribution, J−V curves, photovoltaic parameters under long time storage, XRD patterns, and schematic equivalent circuit (PDF)

4. EXPERIMENTAL SECTION 4.1. Materials Preparation. All chemicals and regents were purchased from Sigma-Aldrich and used as-received, except as otherwise noted. The TiO2 precursor solution was prepared according to the acidic titanium solution: 0.23 M TTIP and 0.013 M HCl in IPA solution. Then, the synthetic TiO2 precursor solution was stored in a desiccative vessel for long-term use. The PbI2 precursor in DMF was kept at the concentration of 461 mg/mL at 70 °C. Then, 15 mg CH3NH3I and 1 mg CH3NH3Cl powder was dissolved in 1 mL IPA solution at room temperature. The HTM formulation was SpiroOMeTAD/chlorobenzene (72.3 mg/mL) containing 17.5 μL LiTFSI/acetonitrile (520 mg/mL) and 28.8 μL 4-tert-butylpyridine. 4.2. Device Fabrication. The perovskite thin film and solar cells were fabricated on precleaned FTO-coated glass with a sheet resistance of 10 Ω sq−1. The compact TiO2 layer was deposited on the patterned FTO glass using the TiO2 precursor solution at 2000 rpm for 30s followed by a ramped sintering at 500 °C for 30 min. The mesoporous TiO2 layer was deposited by spin-coating a commercial TiO2 paste (Dyesol 18NRT, diluted in ethanol at 1:6 weight ratio) at 5000 rpm for 30s. After drying at 125 °C for 5 min, the TiO2 films were gradually heated to 500 °C, followed by annealing for 30 min. The prepared substrates were again sintered at 500 °C for 30 min before use. To reduce the atmosphere influence, the substrates were transferred into N2-filled glovebox.54,55 Then, the PbI2 precursor was spin-coated on the mesoporous layer at 4000 rpm for 30s. Subsequently, the as-cast PbI 2 film was loaded a drop of chlorobenzene and spun at 4000 rpm for 30s quickly, followed by annealing at 70 °C for 30 min. After cooling down to room temperature, the film was coated by the CH3NH3I/CH3NH3Cl solution for 30 s, followed by spin-coating at 4000 rpm for 20s, and drying at 100 °C for 30 min. It is also worth pointing out the samples were stored overnight in dark under dry condition. Then, the asprepared Spiro-OMeTAD solution was coated at 3000 rpm for 30 s. Finally, a ∼70 nm thick gold layer was deposited on the top of the device using thermal evaporation. 4.3. Film and Device Characterization. The film morphology investigation was performed using the Field Emission Scanning Electron Microscope (FEI, Quanta 3D) for SEM. The topography and KPFM were carried out with Bruker Dimension ICON system. The work function of the kelvin probe was calibrated by a freshly cleaved highly ordered pyrolytic graphite surface, which has known work function of 4.65 eV. The optical absorption spectra was measured by UV/vis spectrophotometer (Varian Cary 5000). The FTIR spectra was performed by NICOLET 6700 (Thermo scientific). The XRD measurements were characterized by the Rigaku DMAX-RB equipped

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Y.). *E-mail: [email protected] (Y.Z.). ORCID

Pei Lin: 0000-0002-3300-5241 Yue Zhang: 0000-0001-7772-3280 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 51372023, 51527802, and 51232001), the National Major Research Program of China (No. 2013CB932602), the Program of Introducing Talents of Discipline to Universities (B14003), Beijing Municipal Science & Technology Commission, and the Fundamental Research Funds for Central Universities.

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REFERENCES

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DOI: 10.1021/acsami.7b01136 ACS Appl. Mater. Interfaces 2017, 9, 7224−7231

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DOI: 10.1021/acsami.7b01136 ACS Appl. Mater. Interfaces 2017, 9, 7224−7231