Enhanced Efficiency and Long-Term Stability of Perovskite Solar Cells

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Enhanced efficiency and long-term stability of perovskite solar cells by synergistic effect of non-hygroscopic doping in conjugated polymer-based hole transporting layer Chang Woo Koh, Jin Hyuck Heo, Mohammad Afsar Uddin, YoungWan Kwon, Dong Hoon Choi, Sang Hyuk Im, and Han Young Woo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12973 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Enhanced efficiency and long-term stability of perovskite solar cells by synergistic effect of non-hygroscopic 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, Korea University, Seoul

136-713, Republic of Korea. ‡

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.

1 ACS Paragon Plus Environment

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ABSTRACT: A face-on oriented and p-doped semi-crystalline conjugated polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT) was studied as a hole-transport layer (HTL) in methylammonium lead triiodide (MAPbI3)-based 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 wellaligned 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 non-hygroscopic Lewis acid, tris(pentafluorophenyl)borane (BCF, 2–6 wt%), the vertical conductivity was improved by approximately a factor of 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 spiroOMeTAD, 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 non-hygroscopic 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, Semi-crystalline polymer, Ptype dopant, Non-hygroscopic doping


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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, bi-layer, and meso-super structures,4-7 the most common and successful PVSC structure has a conventional device architecture of fluorine-doped 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,N-di-p-methoxyphenylamine)9,9’-spirobifluorene (spiro-OMeTAD) has been widely utilized as the most efficient holetransporting 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 spiro-OMeTAD 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 lightinduced and moisture-induced gradual degradation processes.23,24 The pinhole channels generated by the migration of Li-TFSI across the HTL can seriously degrade the performance 3 ACS Paragon Plus Environment


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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 efficiency (>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 spiro-OMeTAD (or PTAA). It is still challenging to develop new HTL and/or nonhygroscopic dopant better than spiro-OMeTAD with Li-TFSI HTL in order to guarantee long-term stability and high efficiency. Semi-crystalline 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 spiro-OMeTAD, high thermal stabilities, a tunable band alignment with perovskite, and 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)-1,4phenylenevinylene] 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[1,2-

b:4,5b′]dithiophenes, demonstrating high PCEs (14.5%) and environmentally stable perovskite cells.30 Recently, Jin et al. introduced a face-on oriented and closely π-π stacked 4 ACS Paragon Plus Environment

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conjugated polymer based on the alkoxynaphthylthienyl-substituted 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-on oriented semi-crystalline 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) 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 processibility. 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 based on Lewis acid, tris(pentafluorophenyl)borane (BCF)34 may suggest an efficient strategy to further improve electrical properties without damage in 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., dopinginduced conductivity change, carrier dynamics, etc.), morphological changes and the device stability with BCF doping. Upon doping the PPDT2FBT HTL with a non-hygroscopic BCF, the vertical conductivity of the HTL was improved by ~2 times, thereby exhibiting better 5 ACS Paragon Plus Environment


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efficiency (up to 17.7% PCE) than the standard MAPbI3 device (with Li-TFSI doped spiroOMeTAD). 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.

2. EXPERIMENTAL 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 oC 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. Device fabrication To fabricate the perovskite solar cells, we firstly deposited ~50 nm-thick dense TiO2 electron conducting layer on a F-doped SnO2 (FTO, Pilkington) glass substrate by spray pyrolysis

deposition

(SPD)

method

with

20

mM

of

titanium

diisopropoxide 6

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bis(acetylacetonate) (Aldrich) solution at 450 oC. For MAPbI3 perovskite film, 40 wt% of MAPbI3 perovskite solution was prepared by reacting 1:1 mole ratio of methyl ammonium iodide (MAI, DS LOGICS CO., LTD.) and lead (II) iodide (PbI2, Aldrich) in 1 mL N,Ndimethylformamide (DMF, Aldrich) at 60 oC for 30 min, and 100 µL of hydroiodic acid (HI, Aldrich) was added at room temperature. The prepared MAPbI3 perovskite solution was spincoated on the TiO2/FTO substrate at 3000 rpm for 200 s, and then was dried on the hot plate at 100 oC for 2 min. For formamidinium lead iodide (FAPbI3-xBrx)-based perovskite film, PbI2(dimethylsulfoxide, DMSO) complex was prepared by reacting 50 g of PbI2 in 150 mL DMSO (Aldrich) at 60 oC for 30 min, and 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 oC for 5 h. 1 M of PbI2(DMSO) complex was dissolved in DMF at 60 oC 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 (FAI, DS LOGICS CO., LTD.) and methyl ammonium bromide (MABr, DS LOGICS CO., LTD.) in isopropyl alcohol (IPA, Aldrich) was spin-coated at 5000 rpm for 30 s. Then the film was dried on the hot plate at 150 oC for 20 min. After preparing the perovskite film, PPDT2FBT with 0 wt%, 2 wt%, 4 wt%, and 6 wt% of BCF was dissolved in o-dichlorobenzene (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 controlled relative humidity below ~30%. The active area was fixed to 0.16 cm2. 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 7 ACS Paragon Plus Environment


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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 (1)

=

(1)

where is the permittivity of free space (8.85×1014 F/cm). is the relative dielectric constant of PPDT2FBT (3.2), is the hole mobility, V is the potential across the device ( = − − ), and L is the active layer thickness. By preparing the blank device (ITO/PEDOT:PSS/Au), the potential loss due to the series resistance (Vr) of ITO and the built-in potential (Vbi) were carefully corrected in order to ensure accuracy in the measurements. Device characteristics measurement The J-V characteristics were measured by a solar simulator (Peccell, PEC-L01) with potentiostat (IVIUM, IviumStat) under 1 sun illumination (100 mW/cm2 AM 1.5G), which was calibrated with a Si-reference cell (Peccell, PEC-SI02 certified by Japanese Industrial Standards). The light source of solar simulator is 150 W Xenon lamp (USHIO, UXL-151DO). J-V curves of all devices were measured at a scan rate of 50 mV/s with 10 mV interval by masking the active area with a metal mask of 0.096 cm2. Intensity-modulated photocurrent and photovoltage were measured by potentiostat (IVIUM, IviumStat) with light emitting diode (IVIUM, IM1225). The external quantum efficiency (EQE) was measured using 150W Xenon lamp (ABET, 13014) with a monochromator (DONGWOO OPTRON Co., Ltd., MonoRa-500i) and potentiostat (IVIUM, IviumStat). The EQE was calibrated by Si-reference cell (Peccell, PEC-SI02 certified by Japanese Industrial Standards). The measurement range is 300-900 nm at 5 nm interval without white bias. Device stability test was measured by solar simulator without UV cut filter, which is ABA grade (spectral coincidence: class A/JIS C8912, uniformity: class B/JIS C8912 and temporal fluctuation: class A/JIS C8912). 8 ACS Paragon Plus Environment

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Doping of hole-transport layer First, to prepare the stock solutions, PPDT2FBT (5 mg) and BCF (5 mg) were dissolved in 1 mL 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 total solution volume of 150 µL. Subsequently, each polymer solution was spin-cast on top of the perovskite layer.

3. RESULTS AND DISCUSSION

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.



6

4 2 0

1.10

14

15 16 17 Ave. PCE (%)

6

2

21

(e)

2 wt% 15.62 ± 0.80 %

4

0

18

(b)

14

15 16 17 Ave. PCE (%)

18

4 wt%

14

15 16 17 Ave. PCE (%)

6 wt%

4 2 0

18

18

(g)

1.08

14

15 16 17 Ave. PCE (%)

18

(h)

17

1.02

F.F. (%)

1.04

PCE (%)

2

)

0.75

20

JSC(mA/cm

1.06

1.00

(d)

15.46 ± 0.97 %

2 0

6

16.30 ± 0.75 %

4

0.80

(f)

(c)

Counts

pristine 15.51 ± 0.75 %

Counts

(a)

Counts

Counts

6

V oc (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.70

19

2% 4% 6% [BCF] (wt %)

15 14

0.65

0%

16

0% Ave. PCE (%)

2% 4% [BCF] (wt %)

6%

0%

2% 4% 6% [BCF] (wt %)

13

0%

2% 4% 6% [BCF] (wt %)

Figure 2. (a-d) Average efficiency and (e-h) deviations of photovoltaic parameters for 20 samples with changing [BCF]. The polymer, PPDT2FBT, was synthesized in 80% yield by a Stille cross-coupling of a dibrominated monomer, 1,4-dibromo-2,5-bis(2-hexyldecyloxy)benzene, and bisstannylated 4,7-bis(5-trimethylstannylthiophen-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 (polydispersity index = 1.7). 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 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 holetransporting characteristics of PPDT2FBT, as shown in Figure 1b. Upon illumination, the 10 ACS Paragon Plus Environment

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MAPbI3 perovskite generates free electron-hole pairs or loosely bound electron-hole pairs at room temperature because of small exciton binding energy (< 2kT = 50 meV, where k is the Boltzmann constant and T is the absolute temperature).36,37 The generated charge carriers are then transported through the perovskite layer (self-transporting mode); the electrons are transferred into TiO2 and the holes are transferred into the PPDT2FBT HTL because of the proper energy band alignment, as shown in Figure 1c.38 The optimized thickness of PPDT2FBT is ~50 nm, which is thinner than the typical spiro-OMeTAD layer (~200 nm) or other small molecule HTLs.39-41 Because of the higher solution viscosity of polymeric PPDT2FBT, thinner PPDT2FBT films can form homogeneous and high-quality films which effectively cover the rough perovskite surface than small molecular spiro-OMeTAD. The shorter hole path length between the perovskite layer and the electrode also improves the charge extraction with minimized charge recombination.42 The J-V characteristics of the PVSCs were measured under AM 1.5G with simulated solar irradiation at an intensity of 100 mW/cm2. The J-V curves of planar-type MAPbI3 PVSCs are shown in Figure 1d, and the photovoltaic properties are summarized in Table 1. The PVSC with a PPDT2FBT HTL exhibits a short-circuit current density (JSC) of 20.4 mA/cm2, open-circuit voltage (VOC) of 1.06 V, fill factor (FF) of 74.9%, and resulting PCE of 16.2% in the forward scan. In the reverse scan, a slightly improved PCE of 16.8% was obtained with a JSC of 20.4 mA/cm2, VOC of 1.06 V, and FF of 77.6%. The device shows almost identical J-V curves in the forward and reverse scans with negligible hysteresis. PPDT2FBT shows excellent compatibility with MAPbI3 and demonstrates a great potential as a dopant-free HTL. It shows an almost identical performance to that of the standard perovskite device with a spiro-OMeTAD HTL and Li-TFSI as a dopant, as shown in Figure S2 (best PCE of 17.3% with JSC of 20.5 mA/cm2, VOC of 1.1 V, and FF of 76.6% in the 11 ACS Paragon Plus Environment


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reverse scan). The PCEs of the PPDT2FBT-based PVSCs are similarly reproducible with the standard spiro-OMeTAD based device, as shown in the histogram in Figure 2, which confirms the efficient hole transport owing to the face-on oriented crystalline morphology of the HTM.

Table 1 Summary of photovoltaic parameters of PVSCs containing PPDT2FBT HTL with different weight percent of BCF dopant. Dopant

Scan

Conc.

direction Forward

VOC(V) 1.06

JSC Calculated Jsc FF(%) PCE(%) PCEavg(%) (mA/cm2) (mA/cm2) 20.4

74.9

16.2

77.6

16.8

76.6

16.7

78.7

17.2

77.9

17.4

79.1

17.7

76.1

16.6

77.1

17.0

20.2

0 wt% Reverse

1.06

20.4

Forward

1.06

20.5

2 wt%

15.51 ± 0.75

20.4 Reverse

1.06

20.6

Forward

1.08

20.7

4 wt%

15.62 ± 0.80

20.5 Reverse

1.08

20.7

Forward

1.05

20.8

6 wt%

16.30 ± 0.75

20.7 Reverse

1.06

20.8

15.46 ± 0.97

To further improve the device performance, we also tried to dope PPDT2FBT using a Lewis acid p-type dopant. The choice of dopant is critical because the well-known Li-TFSI dopant in PVSCs shows several stability-related problems. Considering this point, we introduced BCF, which is a moisture-stable and hydrophobic p-type dopant. BCF acts as a Lewis acid and forms a Lewis acid-base complex with p-type PPDT2FBT, which increases the number of holes in the conjugated backbone of PPDT2FBT. The hydrophobic nature of the triphenyl rings in BCF (even in its salt form) and the conjugated backbone in PPDT2FBT 12 ACS Paragon Plus Environment

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provides further protection against humidity. The statistical photovoltaic characteristics of the planar-type MAPbI3 PVSCs with different BCF contents in the PPDT2FBT HTL are summarized in Figure 2 and Table 1. As the concentration of the BCF dopants in the PPDT2FBT HTM increases, the JSC and FF values gradually improve, which is consistent with the EQE spectra (see inset in Figure 3b). The device performance also gradually improves, showing average PCEs (based on 20 separate devices) from 15.5% to 16.3% with increasing [BCF] up to 4 wt%. The maximum PCE of 17.7% was obtained for the device with 4 wt% BCF (VOC of 1.08 V, JSC of 20.7 mA/cm2, and FF of 79.1%). However, the PCE decreases when the [BCF] is further increased to 6 wt%. The decreased PCE with 6 wt% BCF may be related to the film morphology. We examined the morphologies of the PPDT2FBT HTMs with different BCF contents on the MAPbI3 perovskite layer using atomic force microscopy (AFM), as shown in Figure S3. The AFM images clearly show that the HTL films with 0 to 4 wt% BCF have even and smooth surfaces, but the film with 6 wt% BCF shows a rougher surface caused by segregation of the HTM polymer and BCF.

(a)

pristine 2 wt% 4 wt% 6 wt%

0.15

0.10 0.004

0.05

0.000 1000

0.00

400

600

800

(b) 100 80 EQE (%)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

60

85

40

80

20

75

0 300

400

Wavelength (nm)

600

500

700

600

700

800

900

Wavelength (nm)

Figure 3 (a) UV-vis absorption spectra of PPDT2FBT with changing [BCF] in film. (b) EQE spectra of MAPbI3 perovskite solar cells with PPDT2FBT HTL with changing [BCF].



600

(a)

Current (mA)

400 200 0

pristine 2 wt% 4 wt% 6 wt%

-200 -400 -600

4.0x10

-6

3.0x10

-6

2.0x10

-6

1.0x10

-6

-4

-2

0 Voltage (V)

2

4

(b)

2

D n (cm /s )

0.50 0.75

0.75 1.00 1.25 2 Current density (mA/cm )

1.50

0.75 1.00 1.25 2 Current density (mA/cm )

1.50

(c)

τ n (ms)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.50

0.25 0.50

Figure 4 (a) I-V curves of ITO/PPDT2FBT:BCF/Au, (b) diffusion coefficient measured by IMPS and (c) charge carrier lifetime measured by IMVS for planar type MAPbI3 solar cells with different BCF content. The doping effect of BCF on PPDT2FBT was first studied using UV−vis absorption spectroscopy (Figure 3a). As the doping concentration of BCF increases from 2 to 6 wt%, the 14 ACS Paragon Plus Environment

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main absorption at ~650 nm decreases, and additional absorption is gradually generated in the long wavelength region (800–1100 nm). This indicates that positive polarons are generated by the formation of Lewis acid-base BCF-PPDT2FBT charge transfer complexes.43 We also studied radical generation by BCF doping by ESR spectroscopy. We prepared the solid thin films of pristine and BCF-doped PPDT2FBT. As shown in Figure S4, upon doping with BCF, a clear ESR signal was detected, confirming an efficient charge transfer reaction with radical generation. The EQE spectra of planar-type MAPbI3 PVSCs with different BCF contents are shown in Figure 3b. The EQE spectra are not significantly changed, but the EQE values gradually increase in the range of 500–700 nm with increasing [BCF]. For the PVSCs with PPDT2FBT HTL, the absorptivities of the MAPbI3 perovskite layers must be similar because they were prepared under the same condition, but the ~300 nm thick MAPbI3 layer cannot fully absorb the light because of its weaker absorptivity in the long wavelength region. Therefore, the transmitted light is partially absorbed by the PPDT2FBT HTM with the BCF additive, and the reflected light from Au is re-absorbed by MAPbI3 and HTM. As shown in Figure 3a, the main absorption peak of PPDT2FBT at ~650 nm decreases gradually with increasing [BCF], and the resulting transmitted light can be re-absorbed by the MAPbI3 layer, which contributes to the enhancement of EQE in the range of ~500–700 nm. In addition, to compare the EQE spectra of PVSCs with spiro-OMeTAD and BCF-doped PPDT2FBT, we plotted the ∆EQE = EQE(with spiro-OMeTAD) - EQE(with BCF-doped PPDT2FBT). As shown in Figure S5, the EQE values of PPDT2FBT based PVSCs are higher in the range of 300-550 nm but lower in the 550-700 nm range. The ∆EQE becomes small in the 550-700 nm wavelength range with increasing [BCF] = 2~6 wt% in the PPDT2FBT HTL. Therefore, the resulting JSC values of PPDT2FBT and spiro-OMeTAD based devices were measured to be similar. 15 ACS Paragon Plus Environment


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To determine why BCF doping in HTL improves the photovoltaic performance, we also examined the conductivity of the PPDT2FBT HTL with changing [BCF] by fabricating a sandwich-type device of Au/PPDT2FBT:BCF/indium tin oxide, as shown in Figure 4a. The conductivity (σ) was determined by σ = Id/VA (where I, d, V, and A are the current, film thickness (200 nm), voltage, and active area (0.16 cm2), respectively); the conductivity increases with increasing BCF contents: 7 × 10-6, 1.2 × 10-5, 1.6 × 10-5, and 1.7 × 10-5 S/cm for undoped, and 2, 4, and 6 wt% BCF-doped PPDT2FBT films, respectively. We also performed intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) measurements for PVSCs containing PPDT2FBT HTMs with different BCP contents, as shown in Figure 4b and 4c. From IMPS, we could obtain information about the dynamics of charge transport under short-circuit conditions. In Figure 4b, the device with 4 wt% BCF in the HTL shows overall higher diffusion coefficients (Dn = 1.45~3.28 × 10-6 cm2/s at current densities of 0.71~1.31 mA/cm2) than other devices, indicating fast and efficient hole transport. Meanwhile, IMVS provides information on the charge carrier lifetime under open-circuit conditions. The PVSC device with 4 wt% BCF also exhibits longer lifetimes (τn = 3.09~7.04 × 10-4 ms at current densities of 0.71~1.31 mA/cm2) than the other devices at the same current densities. These results clearly confirm that the charge transport and the recombination characteristics are improved by adding BCF dopants into the PPDT2FBT HTL. Therefore, the FF and VOC are also improved from 1.06 V and 77.6% (undoped) to 1.08 V and 79.1% with 4 wt% BCF. Accordingly, the charge carrier diffusion length (Ln = (Dn·τn)1/2) also increases from ~230 nm without BCF to ~320 nm with 4 wt% BCF (Table S1).


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Figure 5 2D-GIXRD images of (a) pristine PPDT2FBT, (b) 2 wt%, (c) 4 wt% and (d) 6 wt% of BCF doped PPDT2FBT and line-cut profiles of (e) out-of-plane and (f) in-plane direction. The PPDT2FBT film morphology was also investigated with increasing BCF dopant contents by GIXRD44 to correlate the morphology, electrical properties, and resulting device performance. The 2D GIXRD images and the corresponding line-cut profiles are shown in Figure 5, and the corresponding packing parameters are summarized in Table S2. The pristine and doped films display similar GIXRD scattering images, showing a strong face-on orientation with a (100) lamellar peak in the xy direction (qxy = 0.30 Å−1, d-spacing of 2.1 nm) and (010) peak in the z direction (qz ~1.68 Å−1) with a corresponding π-π stacking distance of ~3.7 Å.32 Interestingly, the addition of BCF dopants (up to 4 wt%) negligibly disrupts the crystalline interlamellar and π-π packing structures, and the resulting crystal coherence length (see Table S2).



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Figure 6 Device stability of planar type MAPbI3 perovskite solar cells with PPDT2FBT HTL with and without BCF (4 wt%), compared to reference device with spiro-OMeTAD HTL under light soaking (1 sun) at 85 oC and 85% humidity without encapsulation.

In addition, we also investigated the temporal device stability of the conventional MAPbI3 PVSCs with the Li-TFSI doped spiro-OMeTAD, pristine PPDT2FBT, and BCF (4 wt%) doped PPDT2FBT as HTLs (Figure 6). To examine the effect of different HTLs on the device stability, we monitored the PCE of devices (without encapsulation) under lightsoaking conditions. The J-V curves were measured every 5 min for 500 h under 1 sun (AM 1.5G illumination) at 85 °C and 85% relative humidity. As shown in Figure 6, the reference device with a spiro-OMeTAD HTL immediately degrades under the given light-soaking conditions because of a low resistance to moisture.44,45 However, the devices with PPDT2FBT HTLs show significantly improved long-term stabilities under the same light soaking conditions (retains 60% of initial PCE after 500 h). The BCF dopants in the 18 ACS Paragon Plus Environment

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PPDT2FBT HTL do not deteriorate the long-term stability, unlike that observed for the conventional Li-TFSI in spiro-OMeTAD or polytriarylamine, which must be related to the crystalline nature of PPDT2FBT (with a strong resistance toward moisture) and the hydrophobic properties of BCF and PPDT2FBT.

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Figure 7 (a) J-V curves and (b) EQE curve of FAPbI3-xBrx based PVSCs employing PPDT2FBT as an HTL.

Finally, to check if the applicability of PPDT2FBT HTL can be extended to other types of PVSCs, we also fabricated a formamidinium lead iodide (FAPbI3-xBrx)-based PVSC with a device architecture of FTO/TiO2/FAPbI3-xBrx/PPDT2FBT with BCF (4 wt%)/Au (Figure 7). The FAPbI3-xBrx PVSC with a PPDT2FBT HTL exhibits the best PCE of 18.8% with a JSC of 22.4 mA/cm2, VOC of 1.07 V and FF of 78.5% in the reverse scan. The EQE spectrum covers up to ~850 nm and the broader light absorption of FAPbI3-xBrx is expected to increase the JSC compared to the MAPbI3 PVSC. The FAPbI3-xBrx devices with PPDT2FBT HTL with BCF (4 wt%) also show similar long term stability with MAPbI3 PVSCs, confirming the roles of crystalline PPDT2FBT HTL and non-hygroscopic BCF to improve thermal and humidity 19 ACS Paragon Plus Environment


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stability (Figure S6). This suggests the great potential of the semi-crystalline conjugated polymer, PPDT2FBT-based HTL which can be widely utilized for different types of PVSCs.

4. CONCLUSIONS A solution-processed, highly-crystalline and face-on packed polymer (PPDT2FBT) was studied as an HTL in planar type MAPbI3 PVSCs and we achieved a PCE of 16.8 %, which is comparable to that of PVSCs with a Li-TFSI doped spiro-OMeTAD HTL. This remarkable result is attributed to the polymer’s suitable HOMO level (5.35 eV), high vertical hole mobility (7.3×10-3 cm2/V⋅s), homogeneous thin film formability, and good adhesion properties on the perovskite surface, facilitating charge transport and extraction to the anode. By doping 4 wt% non-hygroscopic BCF (Lewis acid) in the PPDT2FBT HTL, we could obtain better PCE (17.7 %) than the Li-TFSI doped spiro-OMeTAD based devices (17.3%). By IMPS and IMVS measurements, we found that the device with 4 wt% BCF shows a higher carrier diffusion coefficient (diffusion length > 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 condition (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 showed similarly high device stability with the undoped device, because of the moisture-insensitive nature of BCF. Accordingly, the non-hygroscopic BCF dopant had a synergistic effect on both of the device 20 ACS Paragon Plus Environment

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efficiency and stability by improving the conductivity of HTL without degrading the device stability against humidity, heat and light. Finally, we also tested the BCF-doped PPDT2FBT as an HTL for the planar type FAPbI3-xBrx PVSCs to show 18.8 % PCE, confirming a wide applicability of this new HTL with non-hygroscopic dopants.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publication website at DOI: SCLC mobility, photovoltaic characteristics, AFM, IMPS, IMVS, ESR and 2D-GIXRD packing parameters. AUTHOR INFORMATION Corresponding Authors *E-mail for H. Y. Woo: [email protected], S. H. Im: [email protected] Author Contributions ¶

C. W. Koh and J. H. Heo contributed equally to this work.

Notes The authors declare no competing financial interest.

Acknowledgment This work was supported by the National Research Foundation (NRF) of Korea (2016M1A2A2940911, 2012M3A6A7055540, 20100020209). This research was also supported by the Technology Development Program to Solve Climate Changes of the



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National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M1A2A2054991).

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Table of Contents


Enhanced Efficiency and Long-Term Stability of Perovskite Solar Cells

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