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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43846−43854
Enhanced Efficiency and Long-Term Stability of Perovskite Solar Cells by Synergistic Effect of Nonhygroscopic Doping in Conjugated Polymer-Based Hole-Transporting Layer Chang Woo Koh,†,∥ Jin Hyuck Heo,‡,∥ Mohammad Afsar Uddin,† Young-Wan Kwon,§ Dong Hoon Choi,† Sang Hyuk Im,*,‡ and Han Young Woo*,† †
Department of Chemistry, Research Institute for Natural Sciences and ‡Department of Chemical & Biological Engineering, Korea University, Seoul 136-713, Republic of Korea § KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea S Supporting Information *
ABSTRACT: A face-on oriented and p-doped semicrystalline conjugated polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT), was studied as a holetransport layer (HTL) in methylammonium lead triiodidebased perovskite solar cells (PVSCs). PPDT2FBT exhibits a mid-band gap (1.7 eV), high vertical hole mobility (7.3 × 10−3 cm2/V·s), and well-aligned frontier energy levels with a perovskite layer for efficient charge transfer/transport, showing a maximum power conversion efficiency (PCE) of 16.8%. Upon doping the PPDT2FBT HTL with a nonhygroscopic Lewis acid, tris(pentafluorophenyl)borane (BCF, 2−6 wt %), the vertical conductivity was improved by a factor of approximately 2, and the resulting PCE was further improved up to 17.7%, which is higher than that of standard PVSCs with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiroOMeTAD) as an HTL. After BCF doping, the clearly enhanced carrier diffusion coefficient, diffusion length, and lifetime were measured using intensity-modulated photocurrent and photovoltage spectroscopy. Furthermore, compared to the standard PVSCs with spiro-OMeTAD, the temporal device stability was remarkably improved, preserving the ∼60% of the original PCE for 500 h without encapsulation under light-soaking condition (1 sun AM 1.5G) at 85 °C and 85% humidity, which is mainly due to the highly crystalline conjugated backbone of PPDT2FBT and nonhygroscopic nature of BCF. In addition, formamidinium lead iodide/bromide (FAPbI3−xBrx)-based PVSCs with the BCF-doped PPDT2FBT as an HTL was also prepared to show 18.8% PCE, suggesting a wide applicability of PPDT2FBT HTL for different types of PVSCs. KEYWORDS: perovskite solar cell, hole-transport material, semicrystalline polymer, p-type dopant, nonhygroscopic doping
1. INTRODUCTION Recently, organometal halide perovskite solar cells (PVSCs) have attracted significant attention as a next-generation solar cell technology because of their outstanding power conversion efficiency (PCE), low-cost fabrication, solution processability, flexibility, etc.1,2 During the past few years, PVSCs have been reported with certified PCEs over 22%; PVSCs show higher photovoltaic performances than polycrystalline and thin-film silicon solar cells.3 Among the various kinds of PVSC structures, including mesoscopic, planar, bilayer, and mesosuper structures,4−7 the most common and successful PVSC structure has a conventional device architecture of fluorinedoped tin oxide (FTO)/TiO2/perovskite/hole-transport layer (HTL)/Au.6,8 The choice of a hole-conductor requires suitable energy levels (highest occupied molecular orbital (HOMO) above −5.43 eV), a high hole mobility for rapid hole transport, and suppressed charge recombination. 2,2′,7,7′-Tetrakis(N,Ndi-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) has been widely utilized as the most efficient hole© 2017 American Chemical Society
transporting material (HTM) because of its favorable energetic configuration and suitable affinity toward the perovskite layer, reducing the potential losses.9−12 However, the low charge carrier mobility (∼10−5 cm2/V·s in pristine film) of spiroOMeTAD requires additional dopants or additives to improve its electrical properties.13−19 Thus, a redox-active p-type dopant, lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), has been widely adopted to improve the hole mobility and device performance. However, Li-TFSI can oxidize HTMs with oxygen in the presence of light or thermal excitation.20−22 Such oxidization processes in an atmospheric environment can negatively impact the device reproducibility because the uncontrollable oxidation degree deteriorates the device operational stability by the light-induced and moisture-induced gradual degradation processes.23,24 The pinhole channels Received: August 28, 2017 Accepted: November 29, 2017 Published: November 29, 2017 43846
DOI: 10.1021/acsami.7b12973 ACS Appl. Mater. Interfaces 2017, 9, 43846−43854
Research Article
ACS Applied Materials & Interfaces
based on Lewis acid, tris(pentafluorophenyl)borane (BCF),34 may suggest an efficient strategy to further improve the electrical properties without damaging the device stability against humidity. Although BCF was previously utilized as a dopant in PVSCs, no detailed study has been reported on the underlying mechanism (i.e., doping-induced conductivity change, carrier dynamics, etc.), morphological changes, and the device stability with BCF doping. Upon doping the PPDT2FBT HTL with a nonhygroscopic BCF, the vertical conductivity of the HTL was improved by ∼2 times, thereby exhibiting better efficiency (up to 17.7% PCE) than the standard MAPbI3 device (with Li-TFSI-doped spiro-OMeTAD). Furthermore, the PVSCs with the BCF-doped PPDT2FBT HTL exhibited excellent stability under continuous light soaking of 1 sun at 85 °C and 85% relative humidity, maintaining ∼60% of original PCE for 500 h without encapsulation. The molecular design and experimental results in this study may suggest a useful guideline for development of HTMs to optimize the performance and stability of PVSCs.
generated by the migration of Li-TFSI across the HTL can seriously degrade the performance of the devices.23 The hygroscopic nature of Li-TFSI seriously degrades the perovskite layer, thereby causing poor device stability. In addition, a polymeric HTM, polytriarylamine (PTAA), has been incorporated into PVSCs to show one of the highest efficiencies (>20%) with good film formability,25−27 but it still needs Li-salt dopant to enhance its intrinsic low hole mobility due to the amorphous nature of the polymeric structure. For these reasons, intense research efforts should be directed to develop solution-processable HTM alternatives to replace spiroOMeTAD (or PTAA). It is still challenging to develop new HTL and/or nonhygroscopic dopant better than spiroOMeTAD with Li-TFSI HTL to guarantee long-term stability and high efficiency. Semicrystalline p-type conjugated polymers can be good candidates as alternative HTMs to realize highly efficient and stable PVSCs because conjugated polymers with crystalline ordering have higher vertical hole mobilities than spiroOMeTAD, high thermal stabilities, tunable band alignment with perovskite, low-temperature solution-based film formation, etc. Recently, several attempts have been made to demonstrate the potential of conjugated polymers as HTMs. Ho et al. studied poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] as an HTL in dopant-free hybrid PVSCs with a PCE of 9.65%; it showed less hysteresis than the reference device with spiro-OMeTAD and a remarkably improved device stability against ambient moisture without encapsulation.28 Park et al. synthesized a benzodithiophene (BDT) and benzothiadiazole (BT)-based random copolymer HTM with a high hole mobility of ∼10−3 cm2/V·s and reported a 17.3% PCE with long-term stability.29 Marks et al. implemented a new low-band-gap polymeric HTM series based on semiconducting 4,8-dithien-2-yl-benzo[1,2-d;4,5-d′]bistriazole-alt-benzo[1,2b:4,5b′]dithiophenes, demonstrating high PCEs (14.5%) and environmentally stable perovskite cells.30 Recently, Jin et al. have introduced a face-on oriented and closely π−π stacked conjugated polymer based on the alkoxynaphthylthienylsubstituted BDT unit with fluorinated BT and measured a PCE of 17.28% with an excellent ambient stability up to 30 days.31 Inspired by these previous reports, we examined a face-onoriented semicrystalline polymer, poly[(2,5-bis(2hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT), as an HTL in planar-type methylammonium lead triiodide (MAPbI3)-based PVSCs. The polymer, PPDT2FBT, was designed to be planar by incorporating intra- and interchain noncovalent Coulombic interactions, such as hydrogen bonding and dipole−dipole interactions.32 This molecular design provides a highly crystalline morphology with a tight interchain packing in a face-on manner (π−π stacking distance of 3.72 Å) without losing a good solution processability. The crystalline morphology results in a remarkably high vertical hole mobility (∼10−3 cm2/V·s) in a wide range of film thickness up to ∼1 μm.32,33 The high carrier mobility with thickness tolerance makes it unique compared to other conjugated polymer HTMs. The thickness tolerance of PPDT2FBT can be an advantage for further improving the moisture barrier properties by adjusting the HTL thickness without deteriorating hole-transport properties. In addition to the great hole-transporting property, the crystalline PPDT2FBT improves the device stability against thermal stress and humidity. In addition, a hydrophobic dopant
2. EXPERIMENTAL SECTION 2.1. General. Tris(pentafluorophenyl)borane was purchased from TCI and used without further purification. UV−vis spectra were measured with a Jasco V-630 spectrophotometer. The number- and weight-average molecular weights were measured by gel permeation chromatography (GPC) with o-dichlorobenzene as an eluent at 80 °C on an Agilent GPC 1200 series, relative to polystyrene standards. Grazing incidence X-ray diffraction (GIXRD) measurements were performed at the PLS-II 9A U-SAXS beam line of Pohang Accelerator Laboratory, Pohang, Republic of Korea. The surface morphology of the films in the tapping mode was characterized by atomic force microscopy (AFM, XE-100, PSIA) using a silicon cantilever. Electron spin resonance (ESR) spectroscopy was performed at room temperature using a Jeol JES-FA200 ESR spectrometer (X-band; 8.75−9.65 GHz; Japan), and ESR signals were detected at 0.9980 mW incident microwave power. 2.2. Device Fabrication. To fabricate the perovskite solar cells, we first deposited an ∼50 nm thick dense TiO2 electron-conducting layer on a F-doped SnO2 (FTO, Pilkington) glass substrate by spray pyrolysis deposition method with 20 mM titanium diisopropoxide bis(acetylacetonate) (Aldrich) solution at 450 °C. For MAPbI3 perovskite film, 40 wt % of MAPbI3 perovskite solution was prepared by reacting 1:1 mole ratio of methylammonium iodide (DS Logics Co., Ltd.) and lead(II) iodide (PbI2, Aldrich) in 1 mL of N,Ndimethylformamide (DMF, Aldrich) at 60 °C for 30 min, and 100 μL of hydroiodic acid (Aldrich) was added at room temperature. The prepared MAPbI3 perovskite solution was spin-coated on the TiO2/ FTO substrate at 3000 rpm for 200 s and then dried on a hot plate at 100 °C for 2 min. For formamidinium lead iodide (FAPbI3−xBrx)based perovskite film, PbI2 (dimethyl sulfoxide (DMSO)) complex was prepared by reacting 50 g of PbI2 in 150 mL of DMSO (Aldrich) at 60 °C for 30 min; then, 350 mL of toluene (Aldrich) was slowly added to the mixture of PbI2 and DMSO. The white precipitate was filtered and dried in a vacuum oven at 60 °C for 5 h; 1 M PbI2(DMSO) complex was dissolved in DMF at 60 °C for 5 min. The PbI2(DMSO) complex solution was spin-coated on TiO2/FTO substrate at 3000 rpm for 30 s and sequentially the mixture (0.5 M) of formamidinium iodide (DS Logics Co., Ltd.) and methylammonium bromide (DS Logics Co., Ltd.) in isopropyl alcohol (Aldrich) was spin-coated at 5000 rpm for 30 s. Then, the film was dried on a hot plate at 150 °C for 20 min. After preparing the perovskite film, PPDT2FBT with 0, 2, 4, and 6 wt % of BCF was dissolved in odichlorobenzene (15 mg/1 mL) and then spin-cast on top of the MAPbI3 layer at 3000 rpm for 30 s. Finally, a 80 nm thick Au layer was deposited by thermal evaporation. Whole experiments were conducted in ambient atmosphere under a controlled relative humidity below ∼30%. The active area was fixed to 0.16 cm2. 43847
DOI: 10.1021/acsami.7b12973 ACS Appl. Mater. Interfaces 2017, 9, 43846−43854
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Figure 1. (a) Chemical structures of PPDT2FBT and dopant BCF, (b) cross-sectional SEM image of perovskite cell, (c) energy band diagram, and (d) J−V curves. 2.3. Space Charge Limited Current (SCLC) Measurements. To obtain the hole mobility of PPDT2FBT and BCF-doped PPDT2FBT, we fabricated hole-only devices, which are composed of ITO/PEDOT:PSS/PPDT2FBT or PPDT2FBT with BCF/Au and measured the current density−voltage (J−V) characteristics in the range of 0−8 V. The resulting J−V curves were fitted using the Mott− Gurney equation (eq 1)
JSCLC =
9 V2 εε0μ 3 8 L
2.5. Doping of Hole-Transport Layer. First, to prepare the stock solutions, PPDT2FBT (5 mg) and BCF (5 mg) were dissolved in 1 and 2 mL of o-dichlorobenzene, respectively. The BCF stock solution (0, 4.8, 9.6, and 14.4 μL) was added to 120 μL of the polymer stock solution to make PPDT2FBT solutions containing 0, 2, 4, and 6 wt % BCF, respectively; additional o-dichlorobenzene solvent was added to each solution to obtain a total solution volume of 150 μL. Subsequently, each polymer solution was spin-cast on top of the perovskite layer.
(1)
3. RESULTS AND DISCUSSION The polymer, PPDT2FBT, was synthesized in 80% yield by Stille cross-coupling of a dibrominated monomer, 1,4-dibromo2,5-bis(2-hexyldecyloxy)benzene, and bisstannylated 4,7-bis(5trimethylstannylthiophen-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole with Pd2(dba)3 as a catalyst in chlorobenzene using a microwave reactor according to our previously reported method.35 The number-average molecular weight and molecular weight distribution were measured to be 70 kDa and polydispersity index = 1.7, respectively. The HOMO energy level of PPDT2FBT (−5.35 eV) is higher than that of MAPbI3 (−5.43 eV) (Figure 1). The well-aligned energy levels provide facile hole transfer from the perovskite to the PPDT2FBT layer. In addition, the face-on orientation with tight π−π stacking and the high vertical hole mobility of 7.3 × 10−3 cm2/V·s (Figure S1) make PPDT2FBT a good candidate as an HTM for PVSCs. PVSCs with a planar-type device architecture of FTO (500 nm)/TiO2 (50 nm)/MAPbI3 (300 nm)/PPDT2FBT (50 nm)/ Au (80 nm) were fabricated to investigate the hole-transporting characteristics of PPDT2FBT, as shown in Figure 1b. Upon illumination, the MAPbI3 perovskite generates free electron− hole pairs or loosely bound electron−hole pairs at room temperature because of small exciton binding energy ( 320 nm) and longer carrier lifetime than the undoped device. Moreover, the small amount (2−4 wt %) of BCF dopants does not affect the crystalline packing structures of PPDT2FBT, showing the same π−π and lamella stacking distances with and without the dopants in the GIXRD measurement. Moreover, the devices with PPDT2FBT HTLs showed remarkably improved long-term stabilities to maintain ∼60% of original PCE without encapsulation under light-soaking and damp/heat conditions (1 sun AM 1.5G) at 85 °C and 85% humidity, compared to the reference device with a spiro-OMeTAD HTL. Importantly, the BCF-doped PVSCs
ORCID
Dong Hoon Choi: 0000-0002-3165-0597 Sang Hyuk Im: 0000-0001-7081-5959 Han Young Woo: 0000-0001-5650-7482 Author Contributions ∥
C.W.K. and J.H.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) of Korea (2016M1A2A2940911, 2012M3A6A7055540, and 20100020209). This research was also supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M1A2A2054991).
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DOI: 10.1021/acsami.7b12973 ACS Appl. Mater. Interfaces 2017, 9, 43846−43854
Research Article
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DOI: 10.1021/acsami.7b12973 ACS Appl. Mater. Interfaces 2017, 9, 43846−43854
