BODIPY-Based Conjugated Polymers for Use as Dopant-Free Hole

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Organic Electronic Devices

BODIPY-Based Conjugated Polymers for Use as DopantFree Hole Transporting Materials for Durable Perovskite Solar Cells: Selective Tuning of HOMO/LUMO Levels Minkyu Kyeong, Jinho Lee, Kwanghee Lee, and Sukwon Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05956 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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ACS Applied Materials & Interfaces

BODIPY-Based Conjugated Polymers for Use as Dopant-Free Hole Transporting Materials for Durable Perovskite Solar Cells: Selective Tuning of HOMO/LUMO Levels Minkyu Kyeong,†,⊥ Jinho Lee,‡,⊥ Kwanghee Lee*,†‡§ and Sukwon Hong*,†§∥

†

School of Materials Science and Engineering, Gwangju Institute of Science and Technology,

123 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea ‡

Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology, 123

Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea §

Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and

Technology, 123 Cheomdan-gwagiro, Buk-gu, Gwangju 61005, Republic of Korea ∥

Department of Chemistry, Gwangju Institute of Science and Technology, 123 Cheomdan-

gwagiro, Buk-gu, Gwangju 61005, Republic of Korea

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ABSTRACT: Recently perovskite solar cells (PSCs) have emerged as an excellent photovoltaic device owing to the outstanding power conversion efficiency (PCE). Nevertheless, device instability remains a critical issue in this field. To overcome device instability without deteriorating PCE, dopant-free hole transporting materials (HTMs) is needed to separate the air-sensitive perovskite layer from extrinsic factors which induce its degradation. Herein we developed novel conjugate polymers of benzo[1,2-b:4,5b′]dithiophene (BDT) and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) for use as HTMs without dopants. The pBDT-BODIPY polymer allows individual “dialing” of the HOMO or LUMO levels with small modifications to the molecular structure, enabling study of the impact of the frontier molecular orbital on PSC performance. Different alkyl chains on BDT can minutely adjust the HOMO level and meso-substituents on BODIPYs can selectively set the LUMO level of the resulting polymers. Application of BODIPY-containing polymer into the perovskite solar cell as an HTM leads to a high PCE value (16.02%) and exceptional solar cell stability shown by the fact that over 80% of its original PCE value maintained after 10 days under ambient air conditions.

KEYWORDS: dopant-free, BODIPY, perovskite solar cell, hole transporting material, energy level tuning


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INTRODUCTION Recently, perovskite solar cells (PSCs) have made immense progress as an emerging photovoltaic device to resolve issues of low light absorption efficiency and high charge recombination.1,2 PSCs feature the power conversion efficiency (PCE) surpassing 22%3, thanks to strong light absorption, broad absorption in the visible area, low charge recombination, and balanced hole and electron diffusion length in the perovskite active layer. After Miyasaka and co-workers first reported photovoltaic performances of perovskites which had a PCE value of 3.8% in 2009, PSCs were developed in various ways, for instance, regarding cell architectures, perovskite materials and interfacial materials.4,5 Among these components of PSCs, interfacial materials are vital to achieve highly efficient device performance by tailoring the energy level, passivation of traps in the perovskites, and the enhancement of long-term stability.6-11 To enhance the charge transport of HTMs, dopants can be incorporated such as 4-tertbutylpyridine (tBP) and Li-bis(trifluoromethanesulfonyl)-imide (Li-TFSI). In general, doped HTMs are so vulnerable to O2 and moisture that device stability can be damaged.12 Interestingly, there are a few reports that high device stability and performance were maintained despite the use of a HTM with dopants. When Grätzel and co-workers used 2,2,7,7-tetrakis(N,N-di-p-methoxyphenyl-amine) 9,9-spirobifluorene (Spiro-OMeTAD) as a representative HTM, PSC device stability and performance could be improved by the use of poly(methyl methacrylate) or α-bis-[6,6]-phenyl C61-butyric acid methyl ester as a template.13,14 Alternatively, PEDOT:PSS as a representative HTM shows high conductivity without dopants. Nevertheless, the hygroscopicity and acidity tend to deteriorate PSC device stability.8 Therefore, new dopant-free HTMs have been required to be developed to improve long-term stability. 3 ACS Paragon Plus Environment


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Even though a number of dopant-free HTMs were researched,15-20 there is a room for improvement for new dopant-free HTMs of which the energy level can be selectively tuned to block electron transport from perovskites and maintain hole transport.5 Considering high PCE values, hole transport from perovskites to an anode electrode should be promoted if the highest occupied molecular orbital (HOMO) of HTMs is slightly higher than that of the perovskites. Simultaneously, the difference between the lowest unoccupied molecular orbital (LUMO) of HTMs and that of the perovskites ought to be large to efficiently block electron transport from perovskites to the anode electrode.6,7,21 Despite the significance of tailoring energy level of HTMs, the research has not been vigorously explored.22-24 Furthermore, previous studies demonstrated that both HOMO and LUMO levels of HTMs were changed simultaneously by chemical modifications of HTMs.22–24 Thus, it would be highly desirable if a new core unit is developed where its HOMO/LUMO values can be individually modulated. To modulate the HOMO/LUMO values systematically, 4,4-Difluoro-4-bora-3a,4a-diaza-sindacene (BODIPY) can be an interesting candidate. Density functional theory (DFT) calculations show that the HOMO level of BODIPY is less affected, because there is a nodal plane at the meso-position. However, the energy of the LUMO is affected more due to a large molecular orbital coefficient at this position (Figure 1c).25 In addition, BODIPY has many attractive features such as facile modification of chemical structures, π-stacking ability, structural rigidity and outstanding photostability.26 Despite these advantages, BODIPYcontaining polymers have scarcely been used in electronic applications.27-35


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Figure 1. Characteristics of (a) BODIPY and (b) new designed BODIPY-containing polymers. (c) DFT model of BODIPY. (d) HTM in PSCs.

Herein we report that new polymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and BODIPY are prepared and that the use of easily modifiable monomers enabled minute control of the polymers’ physical properties. To conduct systematic research on tailoring the energy level of our designed polymers, DFT calculations were conducted. Based on the theoretical investigation, a total of six different polymers (K1–K6) were synthesized with different lengths of alkyl side chain on BDT (HOMO tuning) and different meso-substitutions on BODIPY (LUMO tuning) (Scheme 1). Subsequently, fundamental analyses such as optical, electrochemical and thermal characterization were performed to study the HOMO/LUMO levels, optical bandgaps and thermal stability. Based on these fundamental characterization data, the inverted PSC devices employing the BODIPY-containing polymers as HTMs without any chemical dopants achieved a maximum PCE value of 16.02%. In addition, these dopant-free PSCs offered high device stability in ambient air compared to one



with hygroscopic PEDOT:PSS as an HTM by retaining higher than 80% of its initial PCE, even after 10 days.

Scheme 1. Synthesis of final polymers with combination of various monomers.

RESULTS AND DISCUSSION Materials design, synthesis and characterization To demonstrate selective tuning of the HOMO/LUMO, DFT calculations of BODIPY and BODIPY-containing polymers were conducted at B3LYP/6-31G(d) level as a feasibility study. First of all, Figure 1c shows that the HOMO has a nodal plane at the meso-position, while the LUMO has a large molecular orbital coefficient at the same position, which suggests that chemical substitution on the meso-position of BODIPY can modulate the LUMO energy level selectively. Likewise, BDT–BODIPY polymers had the similar result in DFT calculations (Figure 2a). Surprisingly, in case of the BDT–BODIPY polymers, their electron density distribution of the HOMO was primarily located on the BDT, which means 6 ACS Paragon Plus Environment

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that the HOMO value can be determined by the BDT. The DFT-calculated HOMO values of the meso-CH3 BODIPY polymer and the meso-CF3 BODIPY polymer are -4.39 eV and -4.61 eV respectively.

The DFT-calculated LUMO values of the meso-CH3 BODIPY polymer

and the meso-CF3 BODIPY polymer are -2.75 eV and -3.97 eV, respectively. Overall, these polymers can be an adequate candidate to achieve the selective tuning of the HOMO/LUMO levels. Additionally, coplanarity between the plane of BDT and that of BODIPY is low as shown in Figure 2a. Dihedral angles between the plane of BDT and that of meso-CH3 substituted BODIPY, and between the plane of BDT and that of meso-CF3 substituted BODIPY are 54.8° and 50.4°, respectively, which demonstrate an analogous tendency of BODIPY–thiophene alternating copolymers.32 These non-planar structures help themselves to be flexible and conformal, thus the HTM polymers can fully cover the perovskite as a thin layer.36 For this reason, the thin films of BODIPY-containing polymers could be attractive to enhance PSC stability by providing a good coverage on perovskite layer (Figure 1). Based on the theoretical calculations, we chose BDT and BODIPY as monomers of newly designed donor–acceptor (D–A) copolymers. BDT has a planar structure and an electron donating characteristic. Furthermore, it is easy to introduce solubilizing groups at two positions on the central benzene core to ensure good solubility and to control HOMO/LUMO energy levels.37,38 BODIPY also has a planar structure, and it is easy to introduce substituents at the α,β,meso-positions so as to influence physical properties such as solvent solubility and frontier orbital energy levels. Moreover, α,β-substitution of BODIPY increases structural stability,39 thus all of the BODIPY derivatives were functionalized with four methyl groups. Therefore, a use of BDT as a donating subunit and BODIPY as an accepting subunit would



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provide enhanced rigidity, considerable hydrophobicity, and the possibility of fine adjustment of the energy levels in the polymers. A series of BDT-BODIPY copolymers (K1–K6) were prepared by palladium-catalyzed cross-coupling polymerizations. To ensure a sufficiently high molecular weight and good solubility in processing solvents for device fabrication, the polymerization was carried out for longer reaction times (up to 2 days). All of the synthesized polymers were verified by nuclear magnetic resonance spectroscopy (NMR) (See the Supporting Information). In case of K1, a peak at a chemical shift of 7.5 ppm in 1H NMR was detected, which is caused by homocoupling of BDT (approximately 13% of BDT-BDT connectivity determined by NMR).40 This phenomenon could intermittently happen when P(ο-tolyl)3 or Pd2(dba)3 are used in Stille coupling.41 When Pd(PPh)4 was used as a catalyst, attempted polymerization for K1 yielded an insoluble product. While K1 includes a small portion of BDT-BDT coupled region, K2–K6 show good solubility and do not have a homocoupled part, probably owing to usage of Pd(PPh)4. Molecular weight information was obtained by high temperature gel permeation chromatography. As shown in Table 1, the longer the alkyl chains on the BDTs in K1, K3, and K6 were, the higher the molecular weights and the broader the molecular weight distributions were. The meso-CF3 BODIPY-contained polymers with better solubility also exhibited higher molecular weights and broader molecular weight distributions than mesoCH3 BODIPY-containing polymers. In particular, linear alkyl chain-substituted BODIPYcontaining polymer K5 presented the highest molecular weight of our synthesized polymers. Cyclic voltammetry studies reveal the electrochemical characteristics of the polymeric materials (Figure S1a). After calibrating with ferrocene as a standard in every electrochemical experiment, the HOMO level for each polymer was calculated according to its onset oxidation potential. The HOMO values of K1–K6 were estimated to -5.29, -5.35, 5.40, -5.31, -5.40 and -5.42 eV, respectively. As expected, alteration of the alkyl chain length 8 ACS Paragon Plus Environment

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on BDT had little effect on the HOMO levels. This means that the HOMO values are only minutely changed by the alkyl chains. Using the UV–Visible absorption spectra of synthesized copolymers K1–K6 (Table 1, Figure 2c and Figure S1b), the optical bandgaps were estimated by the edge of absorption spectra. The LUMO values were estimated by a simple sum of the corresponding optical bandgaps and the HOMO values. Even though the optical bandgap does not include the binding energy of electron,42 the aforementioned method is broadly acceptable.19,

43-45

It is interesting to note that there are remarkable

differences in optical features between the meso-CF3 BODIPY-containing polymers (K2 and K4) and the rest (K1, K3, K5 and K6). K2 and K4 showed narrow optical bandgaps (c.a. 1.48 eV and 1.53 eV, respectively), which originated from the deep LUMO levels without much difference in the HOMO values. The electron withdrawing character of CF3 in the mesoposition of BODIPY appears to have deepen their LUMO levels, in accord with the DFT calculations. All of the synthesized polymers (K1-K6) are thermally stable (Table 1 and Figure S1c) and the degradation temperatures (Td) of K1–K6 are 255, 290, 281, 235, 300 and 308 °C, respectively, at the weight loss of 5%.



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Figure 2. (a) Molecular orbital of BDT and meso-CH3/CF3 BODIPY alternating structures. (b) Absorption spectra in solid-state films. (c) Energy level of HOMO/LUMO calculated from the optical bandgaps and the onset oxidation potentials of each polymer vs Ag/AgCl (1M KCl) calibrated by the onset oxidation potentials of ferrocene.


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Table 1. Optical/electrochemical characteristics and molecular weight information. a)

HTM

λmax (nm)

K1

485

K2

b)

EHOMO (eV)

c)

d)

e)

f)

f)

ELUMO (eV)

Eg (eV)

Td (°C)

Mn (Da)

Mw (Da)

-5.29

-3.33

1.96

255

3400

4500

1.29

645

-5.35

-3.87

1.48

290

11800

21000

1.78

K3

489

-5.40

-3.43

1.97

281

6400

8300

1.30

K4

649

-5.31

-3.78

1.53

235

12400

21900

1.77

K5

545

-5.40

-3.48

1.92

300

47900

106200

2.22

K6

511

-5.42

-3.50

1.92

308

20300

68100

3.35

g)

PDI

a)

λmax is the wavelength at the local maximum value of absorption peak; b) EHOMO is calculated by the onset oxidation potential; c) ELUMO = EHOMO + Eg; d) Eg = 1240/λedge; e) Td is the degradation temperature at weight loss of 5%; f) Mn and Mw are determined by GPC; g) PDI=Mw/Mn.

Device fabrication and performance In order to investigate the applicability of our new polymers for use as HTMs in PSCs, inverted (p-i-n) PSCs was fabricated (Figure 3a). The perovskite film was introduced through a one-step solvent engineering method (see the Experimental Section for the details). We note that the perovskite precursor solution containing ionic species was incompatible with the hydrophobic surface of the hole-transporting layer (HTL); the significant difference in polarities causes a severe dewetting problem, resulting in incomplete perovskite film coverage. To solve this problem, we employed a conjugated polyelectrolyte, PFN (poly[(9,9bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)])

as

an

amphiphilic interfacial compatibilizer between the HTL and the perovskite layer.46 After depositing the PFN layer, the surface energy of the HTL was effectively modified without any significant change in the surface electronic properties of the HTL (Figures S2 and S3). 11 ACS Paragon Plus Environment


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We confirmed that improved wetting of the perovskite precursor solution enabled us to fabricate homogeneous and compact perovskite polycrystalline films successfully (Figures S4–S6). As a result, the HTL/PFN bilayer configuration provided not only hole selective contact but also an intimate interface with the perovskite layer. To complete the device, PCBM and zirconium acetylacetonate (ZrAcac) were introduced as an electron transport layer and cathode interfacial layer, respectively.47 Figure 3b and Figure S7 present the current density–voltage (J–V) characteristics of the devices with only PFN and various polymers/PFN for use as HTMs under the standard air mass of 1.5 global (AM 1.5G) irradiation from a calibrated solar simulator with an irradiation intensity of 100 mW cm-2. To validate whether the PFN plays a role as an HTM, the J–V characteristic of the PFN-only device was measured to show a poor performance (PCE value of around 6%). However, compared to the PFN-only device, the devices with our synthesized polymers exhibited different aspects (Table 2). The PSCs with K3, K5, and K6 HTMs had maximum PCE values of 12.50%, 14.65%, and 13.77%, respectively, whereas the K2 and K4-based PSCs yielded poor PCE values of 3.85% and 6.45%, respectively. This significant difference in device performance could be ascribed to the HOMO/LUMO levels and electrical properties (e.g., mobility) of the HTMs by considering the fact that the light absorption region had less impact due to their thinness (approximately 10 nm, Figure S8). In the K3, K5, and K6 cases, a similar HOMO level to the valence band maximum (VBM) of perovskite (-5.4 eV) facilitated hole transfer, and a higher LUMO level than the conduction band minimum (CBM) of perovskite (-3.9 eV) blocked electron flow from the perovskite to the ITO electrode. The major difference among K3, K5, K6 and K1 is the electron blocking efficiencies resulted from the positions of their LUMO levels. In contrast, a LUMO level of K2 (-3.87 eV) and K4 (-3.78 eV) was too low to suppress the electron transfer, which could have resulted in interfacial electron-hole recombination and thereby would have significantly 12 ACS Paragon Plus Environment

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deteriorated the PSC performance. Notably, the K1 PSC outperformed the other devices by exhibiting the highest PCE value with an open-circuit voltage (Voc) of 1.06 V, a short-circuit current density (Jsc) of 19.22 mA cm−2, and a fill factor (FF) of 0.78 due to its favorable energy levels. We note that Jsc values are in good accordance with the integrated Jsc from external quantum efficiency (EQE) spectra, and all of the PSCs showed highly reliable device operation with negligible hysteresis and rapid stabilization of power output (Figure 3c and Figure S9). Despite of absorption and charge generation capabilities, photocurrent contribution from polymeric HTMs might be negligible due to their thin thicknesses. The difference in the shape of the EQE spectra can be attributed to the sensitive morphological change depending on the fabrication conditions. To confirm the effect of K1 on the excited-state properties of the active layer, steady-state photoluminescence (PL) measurements were conducted (Figure 3d). According to the PL spectra of ITO/perovskite and ITO/polymer/perovskite films, the PL signals of the perovskite films grown on the polymeric HTMs (K1–K6) showed weaker intensities than that of the ITO/perovskite film, which indicates that the polymeric HTMs help to quench PL efficiently and this can lead to fast interface charge separation.48-51 We note that strong PL quenching of perovskite on K2 might be ascribed to the energy transfer from perovskite to K2. From the space charge–limited current (SCLC) data, the hole mobility of the K1 was measured as 2.96 x 10-5 cm2V-1s-1 which was the highest value among the HTMs (Figure S10 and Table S2). Considering the PCEs along with mobility values of HTMs, mobility can be a crucial factor for determining the device performance.



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Figure 3. (a) Schematic diagram of inverted perovskite solar cell structures, ITO/HTL/PFN/CH3NH3PbI3/PCBM/ZrAcac/Al. (b) J–V curve and (c) EQE spectra and integrated Jsc values of the corresponding devices using different HTMs. (d) PL spectra of ITO/perovskite and ITO/HTL/perovskite films.


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Table 2. Statistic photovoltaic performance parameters of the PSCs. The data were averaged from 10 devices. Numbers in parentheses are the best values of each PSCs. Voc (V)

Jsc (mA cm-2)

FF

PCE (%)

1.05 ± 0.01

18.61 ± 0.61

0.76 ± 0.02

15.19 ± 0.83

(1.06)

(19.22)

(0.78)

(16.02)

0.90 ± 0.02

7.06 ± 0.63

0.51 ± 0.04

2.9 ± 0.95

(0.92)

(7.69)

(0.55)

(3.85)

1.01 ± 0.01

16.18 ± 0.76

0.72 ± 0.01

11.98 ± 0.52

(1.02)

(16.94)

(0.73)

(12.50)

0.92 ± 0.02

10.59 ± 0.48

0.60 ± 0.02

5.84 ± 0.61

(0.94)

(11.07)

(0.62)

(6.45)

1.04 ± 0.01

17.70 ± 0.62

0.73 ± 0.03

13.83 ± 0.82

(1.05)

(18.32)

(0.76)

(14.65)

1.03 ± 0.01

16.91 ± 0.84

0.73 ±0.02

13.06 ± 0.71

(1.04)

(17.75)

(0.75)

(13.77)

HTM K1

K2

K3

K4

K5

K6

Device stability test Because the hydrophobicity of organic material can help to inhibit the ingress of moisture into the perovskite layer, we expect that the PSCs with our polymeric HTMs exhibited improved stability to air exposure compared to conventional PSCs with PEDOT:PSS as an HTM, which caused device instability due to its hygroscopic and acidic nature. Based on the device performance of the PSCs with PEDOT:PSS (Figure S11) and K1, to explore the impact of the hydrophobicity of HTLs on device stability, we conducted a durability test by storing the unsealed PSCs composed of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/ZrAcac/Al and ITO/K1/PFN/CH3NH3PbI3/PCBM/ZrAcac/Al under ambient conditions, and measured 15 ACS Paragon Plus Environment


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the performance as a function of exposure time. Figure 4a and Figure S12a show that the reference devices with PEDOT:PSS HTL degraded rapidly, losing more than half of its initial PCE after 8 days of storage, whereas K1/PFN HTL provided enhanced stability by maintaining over 80% of its original PCE after 10 days of exposure to air. To gain insight into the origin of the degradation, we investigated the degradation behaviors of each performance parameter (Figures 4b–d and Figures S12b–d). Although the Voc maintained nearly constant values in both devices, the Jsc and FF values of the reference PSC with PEDOT:PSS showed significant decreases. This result might be ascribed to the following reasons: (i) decomposition of the perovskite caused by the hygroscopicity and acidity of PEDOT:PSS accelerating the degradation of the moisture-labile perovskite and (ii) interfacial instability because of ionic species (e.g., PbI2 and CH3NH3I) originating from the decomposition of the perovskite forming an insulating contact and chemically reducing the PEDOT:PSS, resulting in work function reduction.52,53 In contrast, the K1/PFN-based PSCs exhibited prolonged lifetime with only a slight performance decline. These results indicate that interfacial hydrophobic passivation might have retarded the extrinsic degradation of PSCs by protecting the perovskite layer from moisture. To further explore the operational stability, we measured the PCE decay profiles of PSCs by tracking maximum power point (MPP) under continuous light exposure in an N2 atmosphere. Notably, PEDOT:PSS-based PSC exhibits rapid degradation with a short lifetime (the time for reaching PCE/PCE0 = 80%) of less than 25 hours, whereas K1-based PSC shows stable device operation with a prolonged lifetime over 200 hours; we speculate that unwanted chemical reaction between perovskite and PEDOT:PSS is responsible for the device instability.53


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Figure 4. Solar cell devices stability evaluation data for PEDOT:PSS and K1/PFN as HTMs in ambient condition without solar irradiation. Normalized (a) PCE, (b) Voc, (c) Jsc and (d) FF was measured as a function of exposure time.

CONCLUSIONS In conclusion, a new series of pBDT–BODIPY copolymers were synthesized in order to apply them as HTMs without dopants in PSCs. Based on the DFT calculations of our designed polymers, we note that the HOMO energy level was minutely adjusted by various alkyl side chains on BDT and the LUMO energy level was predominately tailored by mesosubstitution on BODIPY, which indicates the LUMO value can be selectively tunable. These 17 ACS Paragon Plus Environment


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theoretical results were proven through optical and electrochemical measurements. Furthermore, our synthesized polymers have good thermal properties. With these features of the polymers in hand, inverted PSCs were fabricated using pBDT-BODIPY polymers as HTMs. The higher LUMO level of the HTMs compared to the CBM of perovskite assists in efficiently blocking electron transfer from the perovskite to the ITO electrode, resulting in high PCE values in PSCs only if the HOMO levels of HTMs relatively well match with the VBM of perovskite. Consequently, K1-based PSCs without dopants yielded over 16% of PCE with high durability under ambient condition (over 80% of its original PCE, even after 10 days) by preventing humid air ingress. These easily modified polymers can be an interesting candidate for use as dopant-free HTMs in PSCs. Moreover, more sophisticated tuning of the polymer structure could improve PCE values of over 20% in the near future.

EXPERIMENTAL SECTION Materials. All reactions were proceeded in flame-dried glassware under inert gas. Methylene chloride was dried over solvent purification column (Flinn Scientific). Toluene and triethylamine was used after removing water under distillation. Pd(PPh3)4 was purchased from Strem Chemicals. n-Butyllithium solution (2.5M in hexane), 2-Decyl-1-tetradecanol were purchased by Sigma Aldrich. 2-ethylhexyl bromide, N-iodosuccinimide, 2,4Dimethylpyrrole were purchased from Acros Organics. Trimethyl tin chloride, thiophene-3carboxylic acid, oxalyl chloride, Boron trifluoride diethyl etherate, Dodecanoyl chloride, Trifluoroacetic anhydride were purchased from Alfa Aesar. 2-hexyl-1-decanol, [[(3,5Dimethyl-1H-pyrrol-2-yl)(3,5-dimethyl-2H-pyrrol-2ylidene)methyl]methane](difluoroborane) (BODIPY(CH3)) were purchased from TCI. Silica 18 ACS Paragon Plus Environment

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gel (230-400 mesh) was purchased from Merck. All reagents were used without any further purifications. Materials which were not commercially available were synthesized as following literatures: 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene (BDT(EH)),54 4,8-bis((2-hexyldecyl)oxy)benzo[1,2-b:4,5-b′]dithiophene

(BDT(HD)),54

4,8-bis((2-

decyltetradecyl)oxy)benzo[1,2-b:4,5-b′]dithiophene (BDT(DT)),54 pre–functionalization of BDT derivatives, meso-CF3-substituted BODIPY (BODIPY(CF3)),55 meso-C11H23-substituted BODIPY (BODIPY (C11)),56 iodination of BODIPY derivatives.57

Materials characterization. Molecular weight average and polydispersity were measured by GPC, Malvern. The polymer solutions were prepared as 1mg ml-1, and were injected at 80 °C under toluene as an eluent and BHT as a stabilizer in flow of N2 after using polystyrene as standard with different molecular weights. Calibration curve was obtained by means of retention times and molecular weights of polystyrene, after that, molecular weight of our synthetic polymers was measured. Electrochemical analysis was performed by Eco Chemie Autolab PGSTAT 30 with 0.1 M tetrabutylammonium perchlorate (TBAP) in acetonitrile as three electrode configuration; Pt wire as a counter electrode and Ag/AgCl (1M KCl) as a reference electrode. UV–VIS spectroscopy was conducted by Perkin Elmer Lambada 750. The polymer films were spin-coated on glasses and heated at 100 °C for 5 min. The polymer solutions were dissolved in chloroform. TGA was performed by LABSYS EVO (Setaram Instrumentation) in flow of N2. Ultraviolet photoelectron spectroscopy (UPS) and the PL emission spectra of the perovskite films were determined by the same method in the previous paper.46 To study the hole mobility of polymer, hole-only device was fabricated [ITO/PEDOT:PSS (20nm) / HTMs /MoO3 (2 nm, thermally evaporated in high vacuum condition)/Ag]. The mobility was determined by following Mott-Gurney law in the range of the space-charge-limited current regime. 19 ACS Paragon Plus Environment


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Solar Cell Fabrication and Characterization. The perovskite solar cells were constructed on the patterned ITO electrodes [ITO/HTL/PFN/CH3NH3PbI3/PCBM/ZrAcac/Al]. The ITO substrates were washed with detergent, ultrasonicated in water, acetone, and isopropyl alcohol (IPA). After UV–ozone treatment, a thin HTL was spread by the spincoating with a diluted HTM solution (0.25 wt% in chlorobenzene) at 5,000 rpm for 20 s and then heated on a hot plate at 80 °C for 10 min. The resulting film thicknesses were approximately 10 nm. Subsequently, the PFN solution (0.1 wt% in methanol) was spincoated at 5,000 rpm for 20s. The perovskite precursor solution was prepared by dissolving PbI2 (Alfa Aesar 99.9985%, 692 mg) and CH3NH3I3 (Dyesol, 238 mg) at 1:1 (molar ratio) in 1 ml of N,N-dimethylformamide (DMF)/dimethylsulfoxide (a volume ratio of 9:1). For the fabrication of perovskite layer, the completely dissolved perovskite precursor solution was spin-coated at 5,000 rpm for 40 s and 1 ml of Et2O was dropped on the rotating substrate after 10 s. The obtained films were annealed at 100 °C for 10min. A solution of PCBM (40 mg ml1

in chlorobenzene) was spin-casted on the perovskite layer at 2,000 rpm for 20 s.

Subsequently, ZrAcac solution (1 mg ml-1 in methanol) was spin-casted at 5,000 rpm for 20 s. In the end, the Al (100 nm) electrode was coated by thermal evaporation. The Al electrode area (10 mm2) determined the active area of the device. The J–V characteristics of the devices and the EQE spectra were performed by the same method in the previous paper.46 The MPP data was measured that the power output of PSC was continuously tracked under N2-filled glove box with a custom-built LabView program (National Instruments, Austin, TX). Surface characterization. The scanning electron microscopy (SEM) images, the X-ray diffraction (XRD) patterns of the perovskite films and the contact angles on water and DMF


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were obtained by the same method in the previous paper.46 The thicknesses of films were measured using a Kosaka ET-3000 surfcorder.

ASSOCIATED CONTENT Supporting Information. Material characterizations such as cyclic voltammetry, UV–VIS spectroscopy, TGA, water contact angle, UPS, SEM and XRD and device performance data of our synthesized polymers. The following files are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (K.L.). *E-mail: [email protected] (S.H.). Author Contributions ⊥

M.K. and J.L. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Global Research Laboratory Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning



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(NRF-2017K1A1A2013153), and by the GIST Research Institute (GRI) grant funded by the GIST in 2018.

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Table of Contents (TOC)


BODIPY-Based Conjugated Polymers for Use as Dopant-Free Hole

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