Article Cite This: Macromolecules 2019, 52, 4757−4764
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Dopant-Free Hole-Transporting Polycarbazoles with Tailored Backbones for Efficient Inverted Perovskite Solar Cells Yuanyuan Xie,†,§,∥ Xuxian Wang,†,∥ Qing Chen,† Sizhou Liu,† Yikai Yun,† You Liu,† Cheng Chen,† Jungan Wang,† Yezhou Cao,† Fangfang Wang,*,† Tianshi Qin,*,†,‡ and Wei Huang‡,§ †
Institute of Advanced Materials (IAM), Nanjing Tech University (NJ Tech), 5 Xinmofan Road, Nanjing 210009, P. R. China Key Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an 710072, P. R. China § College of Chemistry and Chemical Engineering, Inner Mongolia University, 235 West Daxue Street, Hohhot 010021, P. R. China Downloaded via IDAHO STATE UNIV on July 18, 2019 at 01:33:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Three conjugated polymers based on different linkage sites of carbazole repeat units, 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7-PCzTPA, were obtained through judicious molecular engineering. We observed that structure differences between 2,7- and 3,6-carbazole linkage sites could significantly influence intra- and intermolecular architectures and electronic states of materials. Herein, 3,6-PCzTPA and 3,6-2,7-PCzTPA with 3,6-carbazole units exhibited higher hole mobilities owing to the formation of radical cations, compared to 2,7-PCzTPA with 2,7-carbazole units. As a result, by using 3,6-2,7-PCzTPA as the hole-transporting material, perovskite solar cells with the p−i−n structure demonstrated the highest power conversion efficiency up to 18.4%. The outstanding device performance originated from compositive values of open-circuit voltage and fill factor, which were attributed to the suitable energy level as well as a high hole mobility of 3,6-2,7-PCzTPA. Moreover, its straightforward synthesis strategy, fine film-formation ability, and nondopant requirement indicated 3,6-2,7-PCzTPA as an ideal hole-transporting material for perovskite solar cells.
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homogeneous thin film via a straightforward solution processing. These properties are beneficial to scale-up printing techniques such as inkjet or slot-die coating.4 To date, the PCE of PSCs based on commonly used poly-[bis(4-phenyl)(2,4,6trimethylphenyl)amine] (PTAA) has reached up to 22.1%.5 However, the high cost and dopant requirement of PTAA limit the scale-up fabricability and long-term stability of PSCs.6 Therefore, it is important to develop dopant-free polymeric HTMs for achieving more affordable and more stable PSCs.7 Nevertheless, mostly reported PCEs based on pristine polymer HTMs were around 10−16%,8−12 only very few examples up to 18%,13−16 due to the limitation on hole mobilities of dopant-free HTMs. Therefore, it is still a challenge to design conjugated polymers with a matching highest occupied
INTRODUCTION Perovskite solar cells (PSCs) have been regarded as the nextgeneration solution-processable photovoltaics with many advantages such as high efficiency, low cost, and large-scale realizability. In recent few years, the power conversion efficiencies (PCEs) have been increased from 3.8 to 24.2%.1 However, most current materials used in PSCs have not reached all requirements toward future large-scale manufacturing.2 Among all functional layers in PSCs, the hole-transporting layer (HTL) plays an important role in extracting and transporting positive charges from the photoactive layer to the electrode, which is essential for achieving efficient charge separation and high PCE. Although tremendous efforts have been devoted on developing hole-transporting materials (HTMs), most research studies focused on small molecules in contrast to conjugated polymers.2,3 It is well known that conjugated polymers with several advantages of good solubility, processability, and fusibility can readily form a © 2019 American Chemical Society
Received: February 22, 2019 Revised: May 30, 2019 Published: June 12, 2019 4757
DOI: 10.1021/acs.macromol.9b00372 Macromolecules 2019, 52, 4757−4764
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Figure 1. Chemical structures and calculated molecular conformations of 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7-PCzTPA. The molecular conformations were calculated at the B3LYP/6-31+G* level in vacuum by using WaveFunction Spartan’14 software.
Scheme 1. Synthetic Routes of 3,6-PCzTPA, 3,6-2,7-PCzTPA, and 2,7-PCzTPAa
(a) 1-Fluoro-4-iodobenzene, Cs2CO3, DMF, 150 °C, 24 h. (b) Bis(4-methoxyphenyl)amine, t-BuOK, Pd2(dba)3, P(t-Bu)3, o-xylene, 105 °C, 20 h, N2. (c) Bis(pinacolato)diboron, KOAc, Pd(dppf)Cl2, DMF, 90 °C, 20 h, N2. (d) Aliquat 336, Pd(PPh3)4, 2 M aqueous K2CO3, toluene, 85 °C, 20 h, N2. a
molecular orbital (HOMO) energy level, a good film-forming property, and appropriate hole mobility precisely and simultaneously. More importantly, a fundamental understanding on the relationship between intra- or intermolecular structures of conjugated polymers and their optoelectronic properties is essential for researchers to design novel polymeric HTMs with high performances. Carbazole derivatives possess many advantages such as straightforward starting reagents, easy functionalization, good hole-transporting capability, as well as high chemical and environmental stability,17 which have widespread usage as small-molecule HTMs18−27 and polymeric HTMs12,15,28,29 in
PSCs. Furthermore, carbazole-containing polymers are also used in organic light-emitting diodes,30,31 organic field-effect transistors,32,33 and organic solar cells. For example, poly(Nvinylcarbazole) is a typical photoconductive polymer employed as organic photorefractive and photovoltaic materials.17,34 Poly-[[9-(1-oc-tylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] is desired as the polymeric hole conductor in heterojunction solar cells.35 In these carbazole-containing polymers, the architectures and electronic states can be precisely modulated by coupling different positions of repeat units. In 2,7-linked polycarbazoles, the nitrogen atoms located at the meta position 4758
DOI: 10.1021/acs.macromol.9b00372 Macromolecules 2019, 52, 4757−4764
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Table 1. Detailed Molecular and Material Parameters on the Optical, Electrochemical, and Photoelectrical Properties of the Synthesized HTMs HTMs
Tg (°C)
Td (°C)
Mn (g mol−1)
Mw (g mol−1)
PDI
λabsa (nm)
λabsb (nm)
Egc (eV)
EHOMOd (eV)
EHOMOe (eV)
ELUMOf (eV)
μh (cm2V−1 s−1)
3,6-PCzTPA 2,7-PCzTPA 3,6-2,7-PCzTPA
289 261 303
436 422 434
4231 6638 4886
5816 12656 7558
1.50 2.45 2.53
322 380 345
324 383 352
3.18 2.86 3.00
−5.18 −5.23 −5.22
−5.22 −5.37 −5.34
−2.04 −2.51 −2.34
1.46 × 10−5 1.49 × 10−6 1.74 × 10−5
Maximum absorption measured in solution. bMaximum absorption measured on the film. cOptical band gap (Eg) was calculated from the equation, Eg = (1240/λonset), where λonset is the absorption onset wavelength of the UV−vis spectra for three polycarbazoles. dDetermined first oxidation potential from solution-based CV. eDetermined by photoelectron spectroscopy. fLUMO calculated by LUMO = HOMO + Eg. a
Thermal properties of three polycarbazoles were measured via thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). As shown in Figure 2a, three polymers
induce the entire conjugation along the polymer backbone. In contrast, the nitrogen atoms at the para position in 3,6-linked polycarbazoles can be easily oxidized.36 Moreover, the nitrogen atoms can be easily functionalized with a large variety of substituents to modulate the properties of the carbazole without increasing steric hindrance in polymerization. Thus, it is an ideal strategy of molecular design to introduce triphenylamine derivatives with excellent electron-donating properties on the nitrogen positions of polycarbazoles.37,38 In this work, we synthesized three types of polycarbazole analogues 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7-PCzTPA by coupling carbazole repeat units at different sites (Figure 1). Then, we investigated the relationship between polycarbazole backbone conformation and their properties, including energy level, hole extraction, charge mobility, and film morphology. Finally, we used abovementioned three polycarbazoles as dopant-free HTMs in PSCs and discussed their device performances.
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RESULTS AND DISCUSSION The simulated molecular conformations in Figure 1 exhibited diverse backbone curvatures of 3,6-PCzTPA, 2,7-PCzTPA and 3,6-2,7-PCzTPA based on different linkages of carbazole units. The synthetic routes were depicted in Scheme 1. First, intermediates 1 and 2 were obtained by coupling commercial reagents 1-fluoro-4-iodobenzene with 3,6-dibromocarbazole or 2,7-dibromocarbazole. Then, 1 and 2 reacted with 4,4′dimethoxydiphenylamine via Buchwald−Hartwig coupling, respectively, providing the key monomers 3 and 4. Partial compounds 3 and 4 were further boronated by bis(pinacolato)diboron to obtain other monomers 5 and 6. Eventually, a series of polycarbazoles with different linkage sites, named as 3,6-PCzTPA, 3,6-2,7-PCzTPA, and 2,7PCzTPA, were obtained by the Suzuki−Miyaura crosscoupling polymerization according to monomers 3 + 5, 4 + 5, and 4 + 6, respectively. Detailed synthetic procedures and structural characterizations of these polycarbazoles were described in the Experimental Section. The chemical structures of monomers were characterized by 1H NMR, 13C NMR, and MALDI-TOF mass spectra. The target polymeric HTMs were readily soluble in common organic solvents such as chloroform, toluene, and chlorobenzene (CB). The average molecular weights (Mn and Mw) and polydispersity index (PDI) of three polymers were measured via gel permeation chromatography (GPC) (Table 1). The weight-average molecular weights (Mw) were 5800, 12 700, and 7600 g mol−1 for 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7-PCzTPA, respectively. The difference of molecular weights was probably due to low steric hinderances of 2,7-carbazole linkage sites, facilitating the accessibility of the reactive chain for further polymerization.
Figure 2. (a) TGA and (b) DSC curves of 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7-PCzTPA. Normalized UV−vis absorption spectra of three polycarbazoles (c) in toluene solution and (d) thin films, respectively.
exhibited outstanding thermal stabilities with decomposition temperatures (Td, 5% weight loss temperature) of 436 °C for 3,6-PCzTPA, 422 °C for 2,7-PCzTPA, and 434 °C for 3,6-2,7PCzTPA. In Figure 2b, glass phase transition temperatures (Tg) of 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7-PCzTPA polymers were 289, 261, and 303 °C, respectively. Both TGA and DSC results indicated that three polycarbazoles kept in amorphous states during the thermal annealing process were essential for device fabrication. Normalized UV−vis absorption spectra of toluene solutions and thin films for three polycarbazoles were shown in Figure 2c,d, respectively. Because different connection sites of carbazole units led to the distinction of polymeric backbone conformations, 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7PCzTPA exhibited different absorption spectral features with absorption peaks at 322, 380, and 345 nm, respectively. Based on the absorption edges at 390, 434, and 414 nm, the calculated optical band gaps (Eg) were 3.18 eV for 3,6PCzTPA, 2.86 eV for 2,7-PCzTPA, and 3.00 eV for 3,6-2,7PCzTPA, respectively. Compared to absorption spectra in solutions, three polymeric films showed slightly bathochromic shifts with absorption peaks at 324, 383, and 352 nm, perhaps ascribing to the similar intra- and intermolecular conformations in both solution and the solid state.39,40 It was noted that 2,7-PCzTPA exhibited red-shifted maximum absorption wavelength compared to 3,6-PCzTPA and 3,6-2,7-PCzTPA in both 4759
DOI: 10.1021/acs.macromol.9b00372 Macromolecules 2019, 52, 4757−4764
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Macromolecules solution and film, suggesting its longer effective conjugation length owing to the planar molecular structure.41 HOMO energy levels of three polycarbazoles were measured by photoelectron spectroscopy in air (PESA), as shown in Figure 3a. 3,6-PCzTPA, 2,7-PCzTPA, 3,6-2,7-PCzTPA, and
optical, electrochemical, and photoelectrical properties of the synthesized HTMs were summarized in Table 1. Time-resolved photoluminescence (TRPL) measurements were carried out to determine hole extraction at the perovskite/HTM interface. Figure 3c showed TRPL traces acquired from a perovskite film deposited on glass or HTL substrates. The pristine perovskite film on a glass substrate exhibited a long photoluminescence (PL) lifetime of 163.7 ns. When HTLs were applied, perovskites exhibited faster PL decays due to the interfacial hole extractions.42 The PL lifetimes of perovskites with 3,6-PCzTPA and 3,6-2,7-PCzTPA were 36.3 and 34.4 ns, comparable with that of PTAA reference (38.2 ns), suggesting the similar hole extraction abilities. Nevertheless, the perovskite film showed a comparably long PL lifetime of 48.5 ns by using 2,7-PCzTPA. This result demonstrated a poor hole extraction from perovskite to 2,7-PCzTPA, perhaps due to the relatively small gap between their HOMO energy levels. The difference of hole extraction could be further confirmed by steady-state PL measurement as shown in Figure S1. Hole mobilities of these pristine polycarbazoles were determined by using space-charge-limited current (SCLC) measurement. The hole-only devices were prepared as the following device structure: indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/HTM/MoO3/Ag, showing the recorded curves in Figure 3d. 3,6-PCzTPA and 3,6-2,7-PCzTPA manifested comparable hole mobilities of 1.46 × 10−5 and 1.74 × 10−5 cm2 V−1 s−1, respectively, whereas 2,7-PCzTPA exhibited 1 order of magnitude lower hole mobility of 1.49 × 10−6 cm2 V−1 s−1 in contrast to two other counterparts. It is well known that charge transport in conjugated polymers is mainly determined by three processes,43 (i) intramolecular conducting along the polymer main chain, (ii) intermolecular charge hoping across chains owing to π−π stacking, and (iii) charge tunneling between conducting units separated by less conducting segments, usually observed in doped polymers. As depicted in Scheme 2b, 2,7-PCzTPA could form the quinoidal structure under the oxidation state, which allowed charges to delocalize and migrate effectively along the polymer backbones. The main charge-transporting processes were intra- and intermolecular
Figure 3. (a) Data of PESA of three polymers. (b) CV curves of three polymers in acetonitrile with the 0.1 M TBAPF6 supporting electrolyte. (c) TRPL traces of perovskite films on glass or HTL substrates. (d) SCLC curves of three polycarbazoles.
PTAA exhibited HOMO levels at −5.22, −5.37, −5.34, and −5.22 eV, respectively, which were suitable with a valence band of −5.55 eV for the perovskite film (Figure 5b). We have also estimated the HOMO energy levels of three polymers via cyclic voltammetry (CV) of the solid-state films coated on platinum carbon electrodes, as shown in Figure 3b. The CV data were consistent with the results from the PESA measurement with a small difference of ∼0.1 eV, probably due to variation between two different measurements. Lowest unoccupied molecular orbital (LUMO) levels of the polymers were estimated according to optical band gaps from absorption onsets. Detailed molecular and material parameters on the
Scheme 2. Possibilities of Quinoid States for Three Polycarbazoles and Their Main Charge Transport Processes
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Figure 4. AFM images of (a) 3,6-PCzTPA, (b) 2,7-PCzTPA, (c) 3,6-2,7-PCzTPA, and (d) PTAA films.
charge transfers. Nevertheless, the quinoidal structures of 3,6PCzTPA and 3,6-2,7-PCzTPA, containing 3,6-linked carbazole units might produce the discontinuity of the conjugation at the nitrogen atoms and yield the conjugation break in the polymer backbone, as shown in Scheme 2a,c.36 The positive charges localized in the nitrogen atoms were thus formed, which were confirmed by the aforementioned CV measurement (Figure 3b). Polymers with 3,6-linked carbazole repeat units showed reversible oxidation waves, indicating the formation of electrogenerated radical cations, while 2,7-PCzTPA displayed poor chemical reversibility. In this case, high hole mobilities of 3,6-PCzTPA and 3,6-2,7-PCzTPA with poor conjugation were probably dominated by the charge-tunneling process due to the formation of radical cations, resulting in improved interchain charge transfers. Therefore, it was concluded that charge mobilities could be significantly improved by the formation of oxidation states on the nitrogen atoms in 3,6linked carbazole structures, even though 2,7-linked carbazole structures possessed better charge delocalization.36,41 Film morphologies of HTLs were also very important for achieving efficient inverted PSCs, which employed a p−i−n structure: transparent conducting oxide substrate/hole-transport layer (p)/perovskite (i)/electron-transport layer (n)/ electrode. We investigated the uniformity and roughness of three polycarbazole films by atomic force microscopy (AFM). Figure 4 exhibited the AFM images of our polycarbazole and PTAA reference films, which were prepared by using solutions with the concentration of only 2 mg mL−1 polymers in CB and spin-coating with a speed of 6000 rpm on ITO substrates. All three pristine 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7-PCzTPA films showed smooth surfaces with root-mean-square (RMS) roughnesses of 2.45, 1.99, and 3.51 nm, respectively, which were better than or at least similar to the referenced PTAA film (RMS = 3.30 nm). Furthermore, perovskite films on different polycarbazoles showed smooth surface morphologies with a low RMS roughness of ∼25 nm owing to good wettability of perovskite onto polycarbazoles, which were similar to the control film on PTAA (Figure S2). We further fabricated inverted PSCs with the following architecture: ITO/HTLs/(FAPbI3)0.85(MAPbBr3)0.15/[6,6]phenyl-C61-butyric acid methyl ester (PC61BM)/C60/bathocuproine (BCP)/Ag, as shown in Figure 5a. 3,6-PCzTPA,
Figure 5. PSC architecture and characterization based on 3,6PCzTPA, 2,7-PCzTPA, 3,6-2,7-PCzTPA, and PTAA film. (a) Inverted PSC architecture. (b) Schematic energy diagram of the corresponding devices. (c) J−V curves of the champion PSCs based on three polycarbazoles and PTAA reference. (d) IPCE spectra and integrated short-circuit current densities as a function of wavelength.
2,7-PCzTPA, 3,6-2,7-PCzTPA, and referenced PTAA were used as HTLs, respectively. The energy-level diagram was shown in Figure 5b, wherein the values of the perovskite energy level were obtained from the previous reports.44 Details of whole device fabrication were listed in the Supporting Information. Figure 5c demonstrated the current density− voltage (J−V) characteristics under AM 1.5G simulated solar illumination of 100 mW cm−2 . Detailed photovoltaic parameters were listed in Table 2. The PSC based on 3,6Table 2. Photovoltaic Parameters of the Devices Based on 3,6-PCzTPA, 2,7-PCzTPA, 3,6-2,7-PCzTPA, and PTAA under AM 1.5G Simulated Sunlight of 100 mW cm−2
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HTMs
Voc (V)
Jsc (mA cm−2)
FF (%)
PCE (%)
3,6-PCzTPA 2,7-PCzTPA 3,6-2,7-PCzTPA PTAA
0.96 1.07 1.04 1.04
21.93 21.78 22.66 22.47
77.9 73.2 77.9 77.6
16.4 17.1 18.4 18.1
DOI: 10.1021/acs.macromol.9b00372 Macromolecules 2019, 52, 4757−4764
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the B3LYP/6-31+G* level in vacuum by using WaveFunction Spartan’14 software. 1H and 13C NMR were acquired at 400 MHz by a Bruker spectrometer using tetramethylsilane as the internal standard. MALDI-TOF MS spectra were measured on Waters Q-Tof Premier mass spectrometry. The number- and weight-average molecular weights of the polymers were determined by Agilent PLGPC220 series GPC with dichlorobenzene as the eluant and polystyrene as the standard. Absorption spectra of the materials were recorded on a Shimadzu UV-1750 spectrophotometer using toluene solution and spin-coating films. TGA and DSC were performed on Shimadzu DSC-60A and Shimadzu DTG-60H at a heating rate of 10 °C min−1 under the nitrogen atmosphere, respectively. CV studies were carried out by using a CHI660E system in a typical three-electrode cell with a glass carbon working electrode, a platinum wire counter electrode, and a silver/silver chloride (Ag/ AgCl) reference electrode. All electrochemical experiments were carried out under a nitrogen atmosphere at room temperature in an electrolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile at a sweeping rate of 0.1 V s−1. The electrochemical potential was calibrated using the ferrocene/ ferrocenium (Fc/Fc+) couple as an external standard. The conversion E(Fc/Fc+) = 0.63 V versus the normal hydrogen electrode was used. The oxidation potential of Fc/Fc+ was 0.35 V versus the Ag/AgCl electrode. According to the onset oxidation potential of the CV measurements, the HOMO was estimated based on the vacuum energy level of ferrocene (5.1 eV): HOMO = −(Eonset − 0.35 V) − 5.1 eV. Atomic force microscope images were observed by the Park XE-70 instrument in the tapping mode. The thickness of the organic films was measured using KLA Tencor-P-7. A JSM-7800F field emission scanning electron microscope (SEM) was used to obtain the SEM images. TRPL Measurements. The TRPL traces were recorded by a fluorescence spectrometer (FLS 980, Edinburgh Instruments Ltd.) with an excitation light source at 520 nm. The model used to fit the TRPL lifetime is the two-exponential decay function as R(t) = B1 e−t/τ1 + B2 e−t/τ2, where R is the photon number, B1 and B2 are decay factors, t is time, and τ1 and τ2 are decay lifetimes. Mobility Measurements. The hole-only device for hole mobility measurements consisted of ITO/PEDOT:PSS/HTM/MoO3/Ag. The HTMs were dissolved in anhydrous CB (20 mg mL−1 for all polymers) and stirred at 70 °C for 30 min. A pre-etched ITO substrate was treated with UV−ozone for 30 min, and then a 40 nmthick PEDOT:PSS layer was spin-coated onto the ITO substrate from an aqueous solution, followed by baking in an oven at 150 °C for 10 min. Then, the HTM films were spin-coated on the PEDOT:PSS layer at 3000 rpm, respectively. A bilayer cathode structure of MoO3 (20 nm)/Ag (100 nm) was thermally evaporated on top of the HTM layers. By using the following equation, the hole mobilities of the devices were determined with the SCLC method
PCzTPA showed a relatively low efficiency of 16.4% with a low open-circuit voltage (Voc) of 0.96 V. The devices made from 2,7-PCzTPA and 3,6-2,7-PCzTPA exhibited higher Voc values of 1.07 and 1.04 V, respectively. We speculated that the difference of Voc was probably determined by the HOMO levels of three polycarbazoles because of a similar variation trend between the open-circuit voltages and the HOMO levels. PSCs of 2,7-PCzTPA demonstrated a PCE of 17.1% with a low fill factor (FF) of 73.2%, which was mainly due to the relatively low hole extraction and transport ability of 2,7-PCzTPA. It was worth noting that 3,6-2,7-PCzTPA-based PSC achieved a champion PCE of 18.4% because 3,6-2,7-PCzTPA possessed appropriate energy-level alignment with the perovskite, good hole extraction and transport ability, as well as excellent film morphology. As a reference, the PSC using PTAA as HTM showed a PCE of 18.1% under equivalent conditions. Incident photon-to-electron conversion efficiency (IPCE) measurements were carried out to validate the current densities of devices. As displayed in Figure 5d, all IPCE spectra showed an onset of around 800 nm (∼1% IPCE) and exhibited quantum efficiency values of over 80% from 400 to 750 nm. The integrated short-circuit current densities (Jsc) calculated from the IPCE spectra were 20.56 mA cm−2 for 3,6-PCzTPA, 20.89 mA cm−2 for 2,7-PCzTPA, 21.80 mA cm−2 for 3,6-2,7PCzTPA, and 21.41 mA cm−2 for PTAA, which matched well (∼5%) with the Jsc measured under AM 1.5G illumination. The forward and reverse J−V scans of PSCs based on three polycarbazoles and referenced PTAA showed negligible hysteresis in Figure S3. To evaluate the repeatability of device performance, we prepared 30 cells independently for each polycarbazoles under the same experimental conditions. The histograms of average PCEs (Figure S4) presented a good repeatability, and the average efficiencies were ∼14.7, ∼15.5, and ∼16.5% for 3,6-PCzTPA, 2,7-PCzTPA, and 3,6-2,7PCzTPA devices, respectively.
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CONCLUSIONS In summary, by judicious molecular engineering on three dopant-free polycarbazole HTMs with different connection sites of carbazole monomers, the influences of polycarbazole conformation on the performance of HTLs and PSCs were investigated. Polycarbazoles with 3,6-linked carbazole units showed higher hole mobilities than 2,7-PCzTPA, stemming from charge tunneling that occurred in the formation of radical cations on the nitrogen atoms of carbazole units. However, 3,6PCzTPA displayed relatively low Voc due to its unmatched HOMO level to the perovskite. Therefore, upon a compositive optimization of electrical properties such as efficient hole extraction, high charge-carrier mobility, and matchable HOMO levels, 3,6-2,7-PCzTPA was presented as an ideal dopant-free HTM and achieved a superior performance of 18.4% PCE in inverted PSCs. Our results revealed the relationship between the molecular structure of polycarbazoles and performance of materials and devices, providing a guidance to design novel and dopant-free polycarbazole HTMs for high-performance PSCs.
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J=
9 V2 εrε0μ h 3 8 L
where ε0 is the permittivity of free space, εr is the dielectric constant of the HTM, μh is the hole mobility, V is the voltage drop across the device, and L is the HTM thickness; In V = Vappl − Vr − Vbi, Vappl is the applied voltage to the device, Vr is the voltage drop due to constant resistance and series resistance across the electrodes, and Vbi is the built-in voltage due to the difference in work function of the two electrodes. The dielectric constant is assumed to be 3 in our analysis, which is a typical value for organic molecules. The current density versus voltage characteristics were recorded on a Keithley 2400 source meter. Device Fabrication and Characterization. CH 3 NH 3 Br (MABr), NH2CHNH2I (FAI), PbI2, PbBr2, BCP, and PTAA (Mn = 3200 g mol−1, Mw = 4900 g mol−1) were purchased from Xi’an Polymer Light Technology Corp., China. PC61BM was purchased from Nano-C Tech., USA. CB, N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich. Buckminsterfullerene (C60) was purchased from Xiamen Funano New
EXPERIMENTAL SECTION
General Materials and Methods. 3,6-Dibromocarbazole, 2,7dibromocarbazole, 4,4′-dimethoxydiphenylamine, and other reagents were used as received from commercial sources and used without further purification. All the solvents were treated according to the standard procedures. The molecular conformations were calculated at 4762
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Material Technology Corp., China. 3,6-PCzTPA, 2,7-PCzTPA, 3,62,7-PCzTPA, and PTAA solutions were prepared by dissolving 2 mg mL−1 polymers in CB and stirring at 25 °C for 2 h, respectively. The mixed perovskite precursors consisted of 1.1 M FAI, 0.2 M MABr, 1.19 M PbI2, and 0.21 M PbBr2 in a co-solvent of DMSO/DMF (4:1, by volume). PC61BM solution was prepared by dissolving 10 mg of PC61BM into CB (1 mL) and stirring at 25 °C for 12 h. Glass/ITO substrates were cleaned with diluted detergent, deionized water, acetone, and ethanol in sequence in ultrasonic baths for 30 min and then dried by nitrogen flow. After treatment by UV−ozone for 30 min, the ITO substrates were transferred to a N2-filled glovebox with H2O and O2 concentrations of
