Electropolymerization Porous Aromatic Framework Film As a Hole

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Electropolymerization Porous Aromatic Framework Film As a HoleTransport Layer for Inverted Perovskite Solar Cells with Superior Stability Yudi Wang,† Shuhao Zhang,† Jionghua Wu,§ Kuan Liu,∥ Dongmei Li,§ Qingbo Meng,§ and Guangshan Zhu*,†,‡ †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China § Key Laboratory for Renewable Energy (CAS), Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ∥ Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China ‡

S Supporting Information *

ABSTRACT: PAF-86 film is electropolymerized (EP) by targeted monomer M1 tethered bifunctional carbozolyl moieties which not only serve in electron donation but also provide effective electrochemical (EC) active sites. The resulting PAF-86 film possesses a fairly compact surface, remarkable stability, efficient hole extraction capacity, and hole-transporting materials (HTMs) for inverted heterojunction perovskite solar cells (PSCs). Likewise, our investigation shows that PAF-86 film based perovskite solar cells (PSCs) retained about 80% power conversion efficiency (PCE) without encapsulation in air, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) based PSCs devices reduce to 4% under the same conditions. More impressively, the electropolymerization approach is convenient, controlled, and operated at ambient conditions which elude post heat-treatments and are appropriate for industrial application. KEYWORDS: porous aromatic framework, electropolymerization, perovskite solar cells, hole transport, long-term stability

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INTRODUCTION Perovskite solar cells (PSCs) are captivated from various photovoltaic devices with the power conversion efficiency (PCE) increasing from 3.8%1 to over 20%2 in the last seven years. Organometallic trihalide perovskite exhibits high lightabsorption coefficient, broad light absorption range, fast charge carrier mobility, and long diffusion length compared to thirdgeneration solar cells entitled dye-sensitized solar cells (DSSCs) with liquid electrolyte. Practically, solid-state PSCs have acquired more fame due to their ability to evade major technical difficulties of encapsulation on large-scale application. In general, a PSC is comprised of an electron selective layer (ESL), a mesoscopic scaffold, a perovskite layer, a holetransport material (HTM), and a metal electrode. Commonly, a mesoscopic scaffold was made by TiO2 with sintering up to 500 °C and used as a support for perovskite material,3,4 which was an energy-intense process. Afterward, planar heterojunction architectures of PSCs were developed without the mesoscopic scaffold. Meanwhile, some planar heterojunction structure holds TiO2 as the ESL, which also required sintering with the temperature range from 150 to 450 °C in order to form TiO2 © XXXX American Chemical Society

crystallites for the sake of effective electrical conductivity. Although Grätzel et al.5 employed chemical bath deposition of TiCl4 at 70 °C/1 h for the TiO2 crystallite formation, heat treatment for a period of time was still required, which was intricate and energy intense for flexible and large-scale application. Another planar heterojunction structure was inverted as ITO/HTM/perovskite/PCBM/metal electrode. Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) was used as an effective HTM.6,7 Unfortunately, the acidic nature may lead to corrosion toward substrates especially for flexible devices. Furthermore, the hygroscopicity of PEDOT causes poor chemical stability for PEDOT-based devices, which to a great extent limits the lifetime of solar cells for practical application. In order to enhance the stability, numerous inorganic compounds were applied as HTMs for PSCs such as NiOx,8−10 Cu2O,11,12 CuSCN,13 CuI,14 and graphene oxide15 etc. However, these inorganic compounds Received: September 16, 2017 Accepted: November 28, 2017 Published: November 28, 2017 A

DOI: 10.1021/acsami.7b14073 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Structural formula of monomer M1. (b) Cyclic voltammetry curve of the electropolymerization process for M1.

Scheme 1. Synthesis Route to M1

face the severe challenge of solution process to form a thin film, which should overcome the poor solubility due to a stiff network by cross-linked frameworks. Unlike traditional organic synthesis processes which form powder materials,23−32 some methods of fabricating films have been introduced such as surface-initiated synthesis and hyper-branched soluble CMPs.33,34 However, the complexity of the film surface and lack of generality curtail their further application. Thus, there is an urgent need to produce a controllable, economical, reproducible, and easy to operate film fabrication approach for porous framework materials. It is well-known that certain organic functionalities such as phenylamine, carbazole, alkenyl, thiophene, pyrrole, as well as their derivatives were polymerized and deposited on electrodes through the electrochemical oxidization approach.35−38 Scherf et al. synthesized micro-

also entail post heat-treatment to obtain a dense layer. To the best of our knowledge, there is no report on hole-transporting materials substitutes for PEDOT with superior stability, a convenient fabrication approach, and dispensible post heattreatment so far. Herein, we introduced porous aromatic frameworks (PAFs) into the PSC system. PAF materials, with excellent properties of porosity, large specific surface area, and high chemical, thermal, and mechanical stabilities, have been proven to have superiorities in terms of gas storage and separation, heterogeneous catalysis, electricity, and sensors.16−22 As concerned with environmental degeneration and traditional energy consumption, a lot of materials were developed for the application of super capacitors, light catalysis, and electronic devices. Porous framework materials applied to these arenas B


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109.6, 55.2. MS (micrOTOF) m/z: calculated for C56H42N4O2: 802.3, found: 826.0 (+Na+). Electropolymerization of PAF86 Film. The PAF-86 film was polymerized on ITO through the potentiostatic method from a solution consisting of monomer M1 (0.06 mg/mL) and TBAPF6 (0.1 mol/L) in dichloromethane at a potential of 1.7 V, with Pt as counter electrode and Ag/AgCl (saturated KCl, 0.2 V vs NHE) as reference electrode. The film thickness was controlled by scanning time. The PAF-86 film used as HTM for PSCs was potentiostatic deposited for 12 s, and a potential of 0 V was applied for 10 s to depolarize (dedoped) the deposits. Fabrication of Perovskite Solar Cells. After rinsing with water, acetone, and isopropanol, respectively, ITO was first covered by a HTM layer. The PAF-86 film was electropolymerized on ITO without any post heat processing other than simply rinsed by ethanol, and the obtained thickness of the film was around 30 nm. PEDOT:PSS acting as a HTM was also prepared for comparison. PEDOT:PSS solution (Baytron PVP AI 4083, Germany) was spin-coated on cleaned ITO at 3000 rpm for 30 s, followed by 120 °C sintering for 15 min. The perovskite layer was deposited by a sequential deposition method according to the literature.45 PbI2 of 1.25 M was dissolved in a mixture solvent of N,N-dimethylformamide and dimethyl sulfoxide at a volume ratio of 9:1 and stirred at 60 °C overnight, which was spin coated on HTM-covered ITO at 3000 rpm for 30 s in a glovebox and vacuumdried for 20 min. Then, the solution of CH3NH3I and CH3NH3Cl (w/ w = 9:1) in isopropanol (30 mg/mL) was dropped onto the PbI2 layer while spinning at 4000 rpm in the air to form the perovskite layer of CH3NH3PbIxCl3−x and subsequently annealed at 85 °C for 1 h. PCBM (20 mg/mL) in chlorobenzene and PDIN46 (1.5 g/mL) in methanol were then spin-coated on a perovskite layer in order at 1000 rpm for 60 s and 5000 rpm for 20 s, with a 60 °C treatment for 5 min before the PDIN layer was formed. Finally, Ag was vacuum evaporated on top of the device to form back contact with thickness of 80 nm. The active area of the devices was 0.1 cm2. Characterizations. 1H and 13C NMR spectra were obtained using a Varian instrument (300 MHz for 1H NMR, 75 MHz for 13C NMR). High-resolution mass spectra (HRMS) were obtained using a Bruker microTOF II by the means of the ESI technique. FT-IR spectra were measured using a Bruker IFS 66v/S Fourier transform infrared spectrometer. Powder X-ray diffraction (PXRD) patterns were obtained on a Riguku D/MAX2550 diffractometer using Cu−Kα radiation, 40 kV, and 200 mA with scanning rate of 1°/min (2θ). Thermogravimetric analysis (TGA) was performed on a Netzch Sta 449c thermal analyzer system at a heating rate of 10 °C/min in air atmosphere. The SEM analysis was carried out on a JEOL model JSM6700F scanning electron microscope and iridium (IXRF Systems) software with an accelerating voltage of 5 kV. Field emission transmission electron microscopy (TEM) was performed on an FEI Tecnai G2 F20 S-TWIN transmission electron microscope. Ultraviolet−visible light (UV−vis) absorption spectra were recorded on a UV−vis spectrophotometer (UV3600, Shimadzu). AFM images were obtained with a commercial AFM unit (SPA400-SPI4000, Seiko Instruments Inc., Japan). UPS was measured on a Prevac XPS/UPS System with the He (I) (21.2 eV) line using a negative bias voltage applied to the samples in order to shift the spectra from the spectrometer threshold. The photocurrent−voltage (J−V) curves of PSCs were measured on a Keithley 2602 SourceMeter under AM 1.5 illumination (100 mW/cm2) from an Oriel Solar Simulator 91192. J− V characteristics were obtained at a scan rate of 100 mV/s starting from short circuit to forward bias. Incident-photon-to-current conversion efficiency (IPCE) was measured by the direct current (DC) method using a lab-made IPCE setup under 0.3−0.9 mW/cm2 monochromic light illumination without bias illumination. Potentiostatic and cyclic voltammetry (CV) measurements were carried out in a three-electrode system using the electrochemical workstation mentioned above. The thickness of PAF film was measured on an Ambios Technology XP-2 profilometer. Time-resolved photoluminiscence (PL) measurements were acquired using a PL spectrometer, FLS 900, Edinburgh Instruments, excited with a picosecond pulsed diode laser (EPL-445) and measured at 640 nm after excitation at 445

porous polymer networks (MPNs) containing a carbazolyl and thienyl group through electrochemical oxidization.39−43 Y. Ma et al. and D. Jiang et al. electrodeposited conjugated microporous polymer (CMPs) films containing a carbazolyl group.35,44 In the case of in situ polymerization, particularly the electropolymerization (EP) approach is controllable and convenient, and the films obtained can be adjusted through various anodic potential and scan rate which opens a new avenue for porous framework materials. In this work, we are presenting an electropolymerization (EP) process for fabrication of porous aromatic framework (PAF-86) film that acts as a versatile stable HTM applied to inverted perovskite solar cells for the first time. Considering the amorphous nature of the PAF, it is highly possible to form film. Compared with other existing HTMs and their preparations, electropolymerization of PAF-86 film has advantages of easy operational process, large fabrication area, high chemical and mechanical stability, little monomer dosage, controllable start and end of reaction, and mild reaction conditions absent from heat treatment so as to reduce energy and cost consumption, which is the most promising way to realize flexible, lightweight, and portable devices. By keeping the aforementioned captivating salient feature, we intentionally synthesized an EP monomer M1 with carbazolyl functionality serving as an electrochemically active site for electropolymerization as well as electrodonation, as shown in Figure 1a, that was then processed to EP films (PAF-86) on ITO glass, acting as a HTM layer for PSCs. Worthy performance with excellent stability was obtained, which will both broaden the application prospect for PAFs and expand the options of HTM for PSCs. This method will also be favorable to the fields of catalysis, solar cells, fuel cells, hydrogen generation, and organic matter decomposition and so on.

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EXPERIMENTAL SECTION

Materials. Unless noted otherwise, all starting materials were purchased from commercial suppliers and used as received. Synthesis of Monomer (M1). The synthesis route of the monomer of M1 is shown in Scheme 1. A mixture of tris(4bromophenyl)amine (4.82 g, 10 mmol), carbazole (3.34 g, 20 mmol), CuI (0.38 g, 2 mmol), 18-crown-6 (0.18 g, 0.67 mmol), potassium carbonate (5.53 g, 40 mmol), and 1,3-dimethyl-3,4,5,6-tetrahydro2(1H)-pyrimidinone (DMPU) (5 mL) was stirred at 170 °C for 12 h under nitrogen. After cooled to room temperature, the mixture was quenched by 1 M HCl and then washed with NH4OH and water. The resulting residue was purified by column chromatography with hexane/methylene chloride as eluant, and white solid compound 1 with 56% yield was obtained. 1H NMR (300 MHz, DMSO): δ/ppm 8.25 (d, 4 H, J = 7.7 Hz), 7.74 (d, 1 H, J = 8.7 Hz), 7.64−7.55 (m, 5 H), 7.48−7.43 (m, 8 H), 7.40 (d, 4 H, J = 8.6 Hz), 7.33−7.26 (m, 4 H), 7.24 (d, 1 H, J = 8.7 Hz), 7.10 (d, 1 H, J = 8.7 Hz). For the synthesis of monomer M1, the mixture of compound 1 (6.54 g, 10 mmol), 4,4′-dimethoxydiphenylamine (3.44 g, 15 mmol), tris(dibenzylideneacetone) dipalladium (0.91 g, 1 mmol), tri-tertbutylphosphine (0.76 g, 3.75 mmol), sodium tert-butoxide (22.2 g, 160 mol), and methylbenzene (350 mL) was reacted at 120 °C for 24 h under argon. After cooling to room temperature, methylbenzene was removed by vacuum distillation. After rinsing by water, the resulting residue was purified with column chromatography using hexane/ methylene chloride as eluant, yield 68%. 1H NMR (300 MHz, DMSO): δ/ppm 8.24 (d, 4 H, J = 7.7 Hz), 7.56 (d, 4 H, J = 8.7 Hz), 7.50−7.39 (m, 8 H), 7.36 (d, 4 H, J = 8.7 Hz), 7.32−7.24 (m, 4 H), 7.21 (d, 2 H, J = 8.8 Hz), 7.09 (d, 4 H, J = 8.9 Hz), 6.93 (d, 4 H, J = 8.9 Hz), 6.88 (d, 2 H, J = 8.8 Hz), 3.74 (s, 6 H). 13C NMR (75 MHz, DMSO-d6): δ/ppm 140.3, 140.1, 130.6, 127.8, 127.6, 127.4, 126.6, 126.4, 126.0, 125.9, 123.3, 122.5, 120.6, 120.3, 119.8, 119.3, 114.9, C


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cm−1) is also observed for PAF-86 film. The IR data confirm that the electropolymerization reactions have taken place effectively. The result is consistent with XRD, TGA, SEM, TEM, and AFM observations. Thermogravimetric analysis (TGA) was used to measure the thermal stability of PAF-86 film which was shown in Figure S2 of the Supporting Information. It can be seen from the TGA curve that PAF86 has a slow weight loss before 500 °C, and then the decomposition of the framework accelerates, indicating a good thermal stability for the electropolymerized PAF-86 film. Besides, the chemical stability of the PAF86 film was tested by additional experiments. The tested solvents include acid (concentrated sulfuric acid and hydrochloric acid), base (potassium hydroxide, pH14), and organic solvents of high solubility (dimethylformamide and dimethyl sulfoxide). The obtained PAF86 film was insoluble in any tested solvents, which is another direct reflection of its cross-linked structure in contrast with the high solubility of the precursor in dichloromethane. These results reveal that PAF-86 film possesses splendid chemical and thermal stability, which is crucial for its application to photovoltaic devices. The morphology of the as-synthesized PAF-86 film was visualized by scanning electron microscopy (SEM) and shown in Figure S3a. The SEM image displayed a compact film layer, which is imperative in applying to perovskite solar cells. Transmission electron microscopy (TEM) was also measured to study the detailed structure of the PAF-86 film (see Figure S3b of the Supporting Information). X-ray diffraction measurements (Figure S4) demonstrated the amorphous nature of the EP PAF-86 film, and no clear diffractions were observed. Moreover, the atomic force microscopy (AFM) image shown in Figure 3a exhibits a fairly smooth surface of the film with the root-mean-square (RMS) roughness of 5.39 nm, which is equally important for the application in photovoltaic devices. To deliberate the suitability of PAF-86 film to be a holetransport layer for perovskite solar cells, ultraviolet photoelectron spectrometer (UPS) measurements were carried out and shown in Figure S5a. The UPS spectrum of PAF-86 film (He 1α, hν = 21.2 eV) was referred to the Fermi level of the spectrometer at zero binding energy. The work function (WF) value of PAF86 film is calculated as −4.92 eV, which is energy level matching with the valence band of MAPbI xCl 3−x perovskite (−5.3 eV) indicating a capacity of hole extraction. The UPS result manifests the well-matched energy level that can facilitate the transfer of holes from perovskite to the PAF86 film, which is an essential prerequisite for highly efficient HTM. Meanwhile, an ultraviolet−visible light (UV−vis) absorption spectrum was conducted to measure the band gap. As shown in Figure S5b, the band gap of PAF-86 film is estimated to be at 2.61 eV, thus the corresponding LUMO energy level of the film is calculated as −2.72 eV. The much higher LUMO energy level of the PAF-86 film than that of MAPbIxCl3−x perovskite (−3.8 eV) demonstrates the effective electron blocking property for the PAF-86 film, which reduces the recombination chance. The UV−vis measurement reveals that the PAF-86 film acts as a HTM and as an electron blocking layer. Furthermore, space charge limited current (SCLC) measurement was tested to determine the hole mobility of the PAF-86 film as shown in Figure 3b. Hole mobility was extracted by fitting the current density−voltage curve with the modified Mott−Gurney equation below

nm. A hole-only diode was fabricated using the architecture of ITO/ PAF film/MoO3/Au. Hole mobility was extracted through space charge limited current (SCLC) by fitting the current density−voltage curves using the Mott−Gurney relationship. Contact angle was measured by OCA25 DataPhysics using water with dosing volume of 2 μL and dosing rate of 0.5 μL s−1.

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RESULTS AND DISCUSSION The electropolymerization (EP) process of monomer M1 was studied by cyclic voltammetry (CV) measurements. Four reversible redox peaks appeared as shown in Figure 1b. The obtained results depict that the polymeric PAF-86 film grew only when the anodic potential reaches 1.48 V, which is supposed to be derived from the cationic radical of M1 gaining sn electron from another monomer M1 forming a cationic radical of a unit increment, namely, a dimeric carbazole cation.47 The relatively low anodic potentials of 0.56 and 0.98 V should correspond to the generation of two kinds of cationic radicals on account of 1,4-N-substituted phenylene and 1-N-4O-substituted phenylene units,48 while the redox peaks at around 1.25 V appearing from the second scan cycle should be ascribed to the reversible conversion between the oxidation and reduction state of a carbazole dimer.35 The gradual increase in current density of redox peaks with repeated scan indicates the gradual deposition of PAF-86 polymeric film on the working electrode. The Fourier transform infrared (FT-IR) spectra of monomer M1 and PAF-86 film (EP film) were recorded in Figure 2 (for

Figure 2. FT-IR spectra of the monomer M1 and PAF86 film.

the full spectra see Figure S1 of the Supporting Information). The band in the 910−730 cm−1 region that belongs to C−H deformation vibration of the benzene ring varies obviously after EP reaction. In this region, the C−H absorption peaks at 770− 735 cm−1 are involved in an adjacent four hydrogen vibrations, 860−800 cm−1, for two hydrogen atoms, and the bands appeared at 910−860 cm−1 for an isolated hydrogen atom. Compared to the monomer, the band of the PAF-86 film centered at 750 cm−1 becomes weaker in intensity, and this is ascribed to the reduction of four adjacent ring hydrogen atoms stemming from 1,2-disubstituted benzene of carbazole. An additional new band at 876 cm−1 for PAF-86 film corresponds to C−H deformation vibration of ring-isolated hydrogen derived from the dimeric carbazole, indicating the occurrence of polymerization reaction. Furthermore, the absorption peaks of characteristic functional groups for monomer stemmed from C−N stretching vibration of an aromatic tertiary amine (1309 and 1263 cm−1), and Ar−O−C stretching vibration (1022 D


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Figure 3. (a) Atomic force microscopy image of the EP PAF-86 film; (b) J−V curve for the hole-only device; and (c) time-resolved PL decay curves of the MAPbIxCl3−x layer deposited on ITO and PAF-86 film.

Figure 4. (a) Cross-section SEM image of the planar inverted solar cells; (b) J−V curve of the PAF film based PSC; and (c) IPCE spectra and integral photocurrent of the PAF film based PSC.

JSCLC =

⎞ V 2 ⎛ 0.89β 9 ε0εr μ in3 e⎜ Vin ⎟ ⎝ ⎠ 8 L L

rational capacity of hole transportation, which is essential for photovoltaic devices. Time-resolved PL decay measurement was used to determine the minority carrier lifetime of MAPbIxCl3−x deposited on different substrates of ITO and PAF-86 film, and the minority carrier lifetime data were fitted by an exponential diffusion model. As Figure 3c depicts, the minority carrier lifetime of MAPbIxCl3−x deposited on ITO is 307.4 ns. When deposited

(1)

where ε0 and εr are defined as electrical permittivity; μ is the carrier mobility; β is the field activation factor; V is the applied voltage; and L is the film thickness. The hole mobility of PAF86 film is estimated at 2.33 × 10−5 cm2 V−1 S−1, exhibiting a E


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ACS Applied Materials & Interfaces on PAF-86 film, the intensity of PL was quenched sharply, and the minority carrier lifetime shortened significantly to 5.79 ns. The minority carrier lifetime of MAPbIxCl3−x interfaced with PAF-86 film is remarkably reduced by almost 50 times that with ITO, indicating the more efficient extraction of photoinduced holes across the interface between MAPbIxCl3−x and PAF-86 film. From the above analyses it is revealed that the obtained EP PAF-86 film is a quite decent HTM and adapts to perovskite solar cells. The inverted PSC based on MAPbIxCl3−x perovskite using PAF-86 film as an HTM was constructed, and the SEM image of cross-section was demonstrated in Figure 4a. The thickness of PAF-86 film was around 30 nm, which was measured from the SEM image in Figure 4a. The photocurrent−voltage (J−V) characterization was measured under illumination (100 mW cm−2). The freshly prepared PSC processed an average power conversion efficiency (PCE) of 5.04% with the short-circuit current density (Jsc) of 13.1 mA cm−2, open-circuit voltage (Voc) of 0.98 V, and fill factor (FF) of 0.39. However, a more interesting thing was that the performance of the prepared PSC gradually improved to a relative steady state after 5 days without encapsulation, which was almost twice the initial performance. Figure 4b demonstrates the J−V curve of the PSC with stabilized performance. A decent PCE of 9.84% was obtained, with Jsc of 18.23 mA cm−2, Voc of 0.91 V, and FF of 0.59. The great increase in both Jsc and FF resulting in the improvement of PCE is considered to be a better surface contact between perovskite and PAF-86 film after a time, in view of the amorphous cross-linked framework nature of PAF86 film leading to a much better hole extraction and transportation. The corresponding IPCE spectrum (Figure 4c) shows that the IPCE value can reach around 70% over the range from 390 to 760 nm of wavelength and yielding an integrated current density of 18.23 mA cm−2, which is in harmony to the J−V result. Keeping practical application in consideration, the long-term stability of PSC devices is a key factor, which directly determines whether the PSC devices can be industrially produced and applied to practice. Thus, the long-term stability tests of PAF-86 based PSCs as well as PSCs based on traditional HTM of PEDOT:PSS for comparison were implemented. Figure 5 plots the normalized photovoltaic parameters for the duration over 500 h (the absolute values of that are shown in Figure S7 of the Supporting Information). The devices were stored similarly without encapsulation in air at ambient conditions with the room temperature at 20−30 °C and the relative humidity of 30−35%. It can been seen that the PAF-86 based PSC can retain almost 80% of the maximum PCE after 500 h, whereas the PEDOT:PSS based PSC degraded sharply to 4% of its initial performance. This result reveals that the PAF-86 based PSCs are much more stable than the PEDTO:PSS based PSCs. The PAF-86 and PEDOT:PSS based PSCs were both fabricated and stored under the same conditions with variation solely in HTM, demonstrating entirely distinct performance, indicating the enormous influence of the HTM on stability for PSCs. To further illustrate the difference in stability of PAF-86 and PEDOT:PSS, the contact angle (CA) between HTM and water was measured and shown in Figure 6. The PEDOT:PSS exhibits a CA of 11.5° due to its hygroscopic nature, which is prone to absorb the moisture in humid air leading to the degradation of PSCs. As for the PAF-86 film, the CA is significantly increased to approximately 77.7° on account of the

Figure 5. Photovoltaic parameters of PAF film-based (sphere colored olive) and PEDOT:PSS-based (cubic colored blue) device variation during the duration without encapsulation in air at ambient conditions. (a) Normalized PCE; (b) normalized Jsc; (c) normalized Voc; (d) normalized FF.

Figure 6. Contact angle between water and (a) PAF film or (b) PEDOT:PSS.

insoluble nature in water, which reflects a hydrophobic surface of PAF-86 and the stability in humidity. Furthermore, the IR spectrum of the PAF-86 film was recorded after storing for 500 h under ambient conditions in air which is shown in Figure 7 (for the full spectra see Figure S6 of the Supporting Information). The absorption peaks of characteristic functional groups for EP film stemming from C−N stretching vibration of the aromatic tertiary amine (1309 and 1263 cm−1) and Ar−O−C stretching vibration (1022 F


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ACS Applied Materials & Interfaces Author Contributions

Y.W., S.Z., and G.Z. contributed to the conception and design of the experiments. Y.W. and S.Z. synthesized the materials. Y.W., J.W., and K.L. carried out the experiments. Y.W., D.L., Q.M., and G.Z. analyzed the data. Y.W. wrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support from National Basic Research Program of China (973 Program, grant no. 2014CB931804) and NSFC (grant no. 21501064&21531003). The authors also thank Muhammad Faheem for his linguistic assistance.

Figure 7. Infrared spectra of the brand new EP PAF-86 film and that after 500 h storage.

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cm−1) remained as well as the absorption peak at 876 cm−1 derived from C−H deformation vibration of ring-isolated hydrogen derived from the dimeric carbazole. Hence, the IR results confirm the stability of the EP PAF-86 film even after 500 h.

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CONCLUSION In summary, porous aromatic framework film (PAF-86) with compact and smooth surface, reasonable hole extraction, and electron-blocking capacity and superior stability were intentionally synthesized through an in situ electropolymerization method, which is a simple and controllable process also suitable for large-scale preparation. The EP PAF-86 film was successfully applied in inverted perovskite solar cell as an HTM, receiving worthy performance of 10% with splendid stability. The PSC fabricated with PAF-86 film as an HTM can retain 80% of the maximum PCE after 500 h storage in air without encapsulation under ambient conditions with the temperature of 20−30 °C and the relative humidity of 30−35%. In comparison, the performance of PEDOT:PSS based PSC decreased sharply to only 4% of its initial one during the same storing conditions. This work will broaden the application prospect for PAFs and expand the options of HTMs with simply controllable operation process and remarkable stability as well, which will be an impetus to industrial production of PSCs for commercial and civilian use. This will also be beneficial to the fields of catalysts, fuel cells, hydrogen generation, and organic matter decomposition, and so on.

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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.7b14073. Full spectrum of FTIR, thermogravimetric analysis, SEM and TEM images, powder X-ray diffraction patterns, and UPS and UV−vis absorption spectra (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 431-8516-8331. Tel.: +86431-85168887. ORCID

Qingbo Meng: 0000-0003-4531-4700 Guangshan Zhu: 0000-0001-6841-737X G


Research Article

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Electropolymerization Porous Aromatic Framework Film As a Hole

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