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Alcohol-Soluble Conjugated Polymers as Cathode Interlayers for All-Polymer Solar Cells Kim Bini, Xiaofeng Xu, Mats R. Andersson, and Ergang Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00225 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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ACS Applied Energy Materials

Alcohol-Soluble Conjugated Polymers as Cathode Interlayers for AllPolymer Solar Cells Kim Bini, Xiaofeng Xu, Mats R. Andersson & Ergang Wang*

K. Bini, Dr. X. Xu, Dr. E. Wang, e-mail: [email protected] Department of Chemistry and Chemical Engineering/Applied Chemistry, Chalmers University of Technology, SE-412 96, Göteborg, Sweden Prof. Mats. R. Andersson, Chemical & Physical Sciences, Flinders University, Adelaide 5001, South Australia, Australia

Keywords: all-polymer solar cells, alcohol-soluble, conjugated polymers, interfacial layers, imidazole pendant group

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Abstract Conjugated polymers with polar side chain components have successfully been used as cathode interfacial materials (CIM)s to improve the performances of polymer solar cells with fullerenes as acceptors. However, their uses and functions in all polymer solar cells (all-PSCs) have not been well investigated. Therefore, in this work, four conjugated polymers bearing different functional side chains, including dimethylamine, diethanolamine and imidazole, were used as CIMs to study their effects on all-PSCs. With a combination of high-performing acceptor and donor polymers as active layers, the performances of the devices based on the four CIMs were compared with conventional devices with LiF/Al and Al as cathode. Compared to the devices with only Al as cathode, the devices comprising conjugated polymers as CIMs presented large improvement in power conversion efficiency from 2.7% to around 5.3%, which is comparable to the devices with LiF/Al as cathode. The encouraging results demonstrated that the use of conjugated interfacial polymers is a robust way to improve the performances of all-PSCs and can elegantly circumvent the use of low work function metals as cathodes. This is very important for roll-to-roll processing of flexible and large scale solar cells.


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1. Introduction The rapid rise in the efficiencies of polymer solar cells (PSCs) in the last few years has renewed the hope of achieving commercially viable PSCs. Just four years ago, the common view was that PSCs were mainly suitable for niche application and off-grid power generation.1 While the active layer materials are important for device performance, much effort has also been directed at improving overall device structures and interfacial materials. A favorable energy level alignment between the active layer and the electrode is crucial for effective charge extraction. Low work function (WF) metals such as Ca and Ba can be used to achieve this, but their high reactivities tend to reduce the stabilities of the devices.2-3 Furthermore, the difference between the hydrophilic metal surface and the hydrophobic active layer leads to wettability-problems, reducing interfacial contact.4-5 Device performance can be enhanced in several ways by placing a cathode interfacial material (CIM) between the active layer and the metal cathode. Primarily the CIM modifies the WF of the metal electrode, aligning the energy-levels of the electrode and active layer.6-8 This enables the use of less reactive metals as electrodes thereby increasing the long-term stability of the device. A CIM also serves as a diffusion barrier to stop metal ions from diffusing into the active layer.8 Some commonly used CIMs are lithium fluoride (LiF), zinc oxide (ZnO) and certain compounds containing aliphatic tertiary amine units.9-12 The most common types of amine-containing CIMs are polymers with amine pendant groups such as polyethylenimine (PEI) and polyethylenimine ethoxylated (PEIE)13 and conjugated polymers with pendant amine groups such

as

poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-

dioctylfluorene)] (PFN).14 These compounds can be ionized to create even more water/alcohol soluble materials with better charge transporting properties.6,15 In some cases they have been crosslinked to further improve stability.16 An advantage of polymeric CIMs is that they are solvent processable, while LiF is vacuum deposited, which is an energy-intensive process and 3 ACS Paragon Plus Environment


unsuitable for large scale roll-to-roll production.17 Furthermore, polymer-based interlayers are more mechanically stable, making them well-suited for flexible substrates.18 The active layers of PSCs consist of donor- and acceptor molecules, intermixed in bulkheterojunction (BHJ) structures. The most commonly used acceptor molecules have long been fullerene derivatives such as [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), which is still widely used. But it has several drawbacks. PC71BM is a very expensive compound19. It has a relatively low molecular absorption coefficient, which means it does not participate much in photon absorption20. Furthermore, it has a tendency to slowly diffuse through active layers and crystallize, shortening device lifetime.21-22 Acceptor polymers, on the other hand, can be designed to have comparable absorption coefficients and complementary absorption spectra to the donor, due to their readily tunable energy levels.23 They are also more stable under mechanical stress, which leads to more stable devices when printed on flexible substrates.24 The advantages of conjugated polymers with amine functional groups as CIMs have been well established for polymer:PCBM-based devices25. One of the functions served by aminecontaining interfacial polymers in PCBM-based solar cells is to n-dope the fullerene, increasing its conductivity and reducing interfacial resistance.25 This effect has not been observed in all-PSCs.26 Although the efficiencies of all-PSCs have been largely improved from 2% to 10% in only five years, the functions of the cathode interfacial layers on the performances of all-PSCs have not been well investigated. Therefore, we chose four polymers with three different types of functional side chains and two backbone configurations to test how they function as CIMs in all-PSCs. The three different nitrogen-containing pendant groups include dimethylamine, diethanolamine and imidazole. Dimethylamine, in the form of PFN, has been used extensively in PCBM-based solar cells.27 Diethanolamine-containing polymers have been used mainly for organic light emitting diodes but have also found 4 ACS Paragon Plus Environment

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applications in organic photovoltaics.28-29 Polymers with imidazole pendant groups have been reported in chemosensory applications, but to our knowledge, they have not been utilized as interlayers in PSCs previously.30-31 2. Results and Discussion Chart 1 shows the structures of the three amine-containing polymers used as interfacial layers in this study. These polymers were prepared following modified polymerization procedures and details about their syntheses are given in Figure S1 and Figure S2 in the supporting information. The post polymerization reactions were performed according to previous publications, with altered procedures in some cases.28,31 Three of the interfacial polymers share the same polyfluorene backbone structure but have different pendant groups. PFN has simple dimethylamine groups and is one of the most common conjugated interfacial layers. In this work, PFN was used as a reference.8 PFN(EtOH)2 contains diethanolamine amine units and PFIm incorporates imidazole groups. The fourth polymer, PBzFN, was designed to study if the backbone structure can influence the performance and has an alternating fluorenephenylene backbone bearing the same dimethylamine groups on the side chains as PFN. The photoactive layers in the all-PSCs were composed of two well-performing donor and acceptor materials previously used in fullerene-free PSCs. Poly[(2,6-(4,8-bis(5-(2ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T) was the donor and PNDI-T10 was the acceptor. Chart 1 shows the chemical structures of PBDB-T and PNDIT10. The combination of PBDB-T:PNDI-T10 selected in this work should be promising according to related reports published recently.32-33 In the past, PBDB-T was mainly used in conjunction with the small molecule acceptor ITIC34-35 while PNDI-T1023,36 and other PNDI derivatives were mainly used as acceptors with poly[4,8-bis(5-(2-ethylhexyl)thiophen-2yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,45 ACS Paragon Plus Environment


b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) as donor.37-38 The donor polymer PBDB-T is a high band gap polymer, while the acceptor polymer PNDI-T10, which demonstrated improved performance compared to N2200 in our previous reports,23,36 is a low band gap polymer, giving the combination of suitable complementary absorption.

Chart 1. Chemical structures of the active layer polymers (top) and interfacial polymers (bottom)

The molecular weights of the polymers were analyzed using high temperature gel permeation chromatography (GPC). The active layer polymers showed high number average molecular weights (Mn) of 92 kDa for PBDB-T and 170 kDa for PNDI-T10. Three of the interfacial layer polymers were produced from the same precursor polymer, called PFBr (Figure S2, 6 ACS Paragon Plus Environment

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supporting information), which had Mn of 28 kDa with a dispersity of 3.4. PFN and similar polymers were designed to have orthogonal solubility with other device layers, in order to be stable during sequential solvent processing. This means that their solubilities in the commonly used GPC eluent 1,2,4-trichlorobenzene are limited, and it is not possible to determine their molecular weights in these systems. Since PBzFN was synthesized from the amine-containing monomer directly, not from the precursor polymer, the molecular weight could not be determined. The thermal stability of the precursor polymer and the interfacial polymers were studied by thermogravimetric analysis (TGA) (Figure S7, supporting information). PFN(EtOH)2 started to degrade at a low temperature, with an onset of 190 °C. Since the precursor polymer did not exhibit this low temperature degradation, the degradation of PFN(EtOH)2 at low temperature is likely due to the diethanolamine pendant groups. PBzFN also had a low temperature degradation with an onset of 220 °C. The main degradation mode of the polymers due to backbone decomposition was between 350 °C to 420 °C. PFIm in particular exhibited a high degree of stability with a degradation onset of 420 °C without any side chain degradation.

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the active layer and interfacial layer polymers were determined by square wave voltammetry (SWV). Figure 1 shows the energy levels of the polymers with PC71BM as a reference23. Table 1 summarizes all electrochemical and optical properties of the polymers. Figure S3-S5 (supporting information) show the UV-Vis spectra and SWV voltammograms of the polymers. The four polymers exhibited similar electrochemical and optical properties, with the largest deviation being for PBzFN due to the altered backbone composition. The energy levels of the three polymers synthesized from the precursor PFBr were similar. A small difference was observed for PFN(EtOH)2 that exhibited a two-step oxidation with peaks at 0.72 V and a larger one at 0.85 V. The first oxidation peak was 7 ACS Paragon Plus Environment


attributed to the oxidation of the pendant amine group, which was verified by comparison with the oxidation of triethanolamine (N(EtOH)3) as shown in Figure S6. None of the other polymers exhibited this oxidation peak. Therefore, the second oxidation peak was believed to represent the backbone oxidation which gave a HOMO level of 5.98 eV, which matches the other polymers well. These amine pendant group oxidations are sometimes observed in electrochemical measurements of similar polymers, but generally not when electron-deficient units are present.39 All four interlayer polymers exhibited relatively large band gaps, which can be attributed to the backbone structures. The absorbances of these polymers are not highly important for the device performances, since they are placed behind the photoactive layer. However, a limited absorbance overlap between the interfacial polymers and the active layers is still desirable.

-2.38 -2.44 -2.52 -2.45 -2.57

-2.5 -3.0

-5.0 -5.5

-5.63 -6.0 -6.5

-6.09 -6.02 -5.98 -6.36 -6.44

PBzFN

PFIm

PFBr

PC71BM

-4.5

-4.05 -4.14

PNDI-T10

-4.0

PFN

-3.5

PFN(EtOH)2

-3.34

PBDB-T

Energy Level VS Vacuum (eV)

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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-5.94 -6.14

Figure 1. Energy levels for the active and interfacial polymers with PC71BM as a reference.


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Table 1. Electrochemical and optical properties electrochemistry polymer

absorption solution

film

HOMO LUMO Egec (eV) (eV) (eV)a

λmaxb (nm)

λmaxc λonset Egopt (nm) (nm) (eV)d

PBDB-T

−5.63

−3.34

2.29

616

578

678

1.83

PNDI-T10

−6.36

−4.02

2.31

639

688

798

1.55

PFBr

−6.09

−2.38

3.71

381

377

420

2.95

PFN

−6.02

−2.44

3.58

389

385

425

2.92

PFN(EtOH)2

−5.98

−2.52

3.46

381

386

424

2.92

PFIm

−6.14

−2.45

3.69

390

381

424

2.92

PBzFN

−5.94

−2.57

3.37

375

370

415

2.99

a

b

Electrochemical band gap; Absorption maximum in chloroform solution; cAbsorption maximum in film; dOptical band gap All-PSCs were fabricated with the conventional device structure: ITO/PEDOT:PSS(40 nm)/active layer(~90 nm)/CIM/Al(90 nm). Measurements of the photovoltaic performances of the solar cells were carried out under an illumination of AM 1.5G simulated solar light at 100 mW/cm2. The optimized ratio of PBDB-T:PNDI-T10 was 1:1 and the blend was spincoated from chlorobenzene solution with a thickness of around 90 nm on top of the PEDOT:PSS layer. The interfacial layer polymer (0.2 mg/mL) was spin-coated from an acetic acid:methanol solution (0.5%, v/v) onto the active layer. PFN and PBzFN showed good solubilities, while PFIm and PFN(EtOH)2 had limited solubilities and had to be filtered before spin-coating. The photovoltaic parameters are summarized in Table 2 and the current densityvoltage (J−V) curves, dark current, external quantum efficiency (EQE) and electron mobility curves are depicted in Figure 2. The devices without CIMs had low performances where a combination of low open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF) resulted in a PCE of 2.7%. The low performances of these devices can be explained by the



mismatch of WF between the active layer and the electrode, as well as poor charge extraction due to high contact resistance.8,27 This is clearly reflected in the J-V-curves where significant reductions in both Jsc and Voc, as well as FF were observed compared to the other systems. When LiF was inserted between the active layer and Al electrode, the performance was dramatically improved and the end result was a doubled PCE of 5.3%. When the three CIMs bearing tertiary amine and diethanolamine units (PFN, PFN(EtOH)2 and PBzFN) are compared, their similar performance improvement suggests that they probably share the same working mechanism. The mechanism behind interfacial layer improvement is generally attributed to several factors. The most important is the formation of an interfacial dipole which tunes the WF of the metal to a more well-aligned level.40 This reduces the barrier height of the cathode interface, which improves the charge extraction.27,41 The Jsc and FF can be improved by a more efficient charge extraction. Furthermore, interfacial layers improve the diode character of devices by reducing current leakage, which improves the Voc.27 All of these improved factors will significantly increase the PCE of devices. For PFIm, which had imidazole-functionalized side chains, a slightly lower performance was obtained. The Voc and FF were unchanged while the Jsc was reduced. PFIm was the only polymer that deviated significantly in performance and this may be attributed to a smaller interfacial dipole, which is hard to determine. In the case of PBzFN, it was noted that the backbone does not affect the performance. J-V-curves under dark conditions, presented in Figure 2b, show minor differences in current leakage between devices with the different CIMs. The current leakage was lower compared to the bare Al-devices, which is consistent with previously published studies.27


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103

2

a) Current density (mA/cm2)

Current density (mA/cm2)

100

Al LiF/Al PFN/Al PFN(EtOH)2/Al

10-1

PFIm/Al PBzFN/Al

102

0 Al LiF/Al PFN/Al PFN(EtOH)2/Al

-2 -4

PFlm/Al PBzFN/Al

-6 -8 -10

10

10

b)

1

-2

10-3 10-4 10-5 10-6 10-7

-12 -0.2

0.0

0.2

0.4

0.6

0.8

10-8 -2.0

1.0

-1.5

-1.0

Voltage (V)

-0.5

0.0

0.5

1.0

1.5

Voltage (V) 104

Al LiF/Al PFN/Al PFN(EtOH)2/Al

c)

50

d) 103

Current density (mA/cm2)

60

PFlm/Al PBzFN/Al

40

EQE (%)

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


30 20 10 0 300

102 101

LiF/Al PFN/Al PFN(EtOH)2/Al

100 10-1

PFlm/Al PBzFN/Al Fitting lines

10-2

400

500

600

700

800

900

10-3

Wavelength (nm)

0.1

1

10

Voltage (V)

Figure 2. J−V characteristics a) under illumination, b) in dark c) EQE curves and d) Electron mobility in electron-only devices.

To evaluate the spectral responses of the all-PSCs with the different cathode treatments, external quantum efficiency (EQE) measurements were performed. The spectra, shown in Figure 2c, exhibit identical range of photoresponse between 300 and 800 nm and have wellbalanced double-peak shapes. These double-peak shapes correspond well with the absorption profiles of the two active layer polymers, shown in Figure S4. The peak around 350 nm can be attributed to the acceptor polymer PNDI-T10, while the peak between 600 and 700 nm is a combination of both acceptor and donor polymer absorption. The EQE values were around 55% at the 365 nm peak for LiF, PFN(EtOH)2, PFN and PBzFN as interfacial layers. The low wavelength maximum for PFIm shifted slightly from 365 nm to 401 nm, where it exhibited an EQE of 48%. The Jsc values calculated from integrating the EQE with the AM1.5G spectrum are close to the measured values from the J-V curves, with an error of ±10%. Electron 11 ACS Paragon Plus Environment


mobility (µe) was measured in electron-only devices with the structure: ITO/ZnO/Active layer/CIM/Al. The electron mobilities of the CIMs were extracted using the space-chargelimited current (SCLC) model and the values are presented in Table 2.23 Devices with polymer CIMs had close to identical electron mobilities around 7.5×10−5 cm2 V−1 s−1, slightly lower than that for devices with LiF/Al. PFIm showed slightly lower mobility, which could explain part of the overall lower performance of the polymer. Table 2. Device parameters of the all-PSCs. µe PCE 2 −1 −1 (%) (cm V s )

Cathode

Voc (V)

Jsc (mA/cm2)

FF

Al

0.79

7.9a (7.1)b

0.43

2.7

-

LiF/Al

0.90

9.9 (10.1)

0.60

5.3

7.94×10−5

PFN/Al

0.87 10.2 (10.2) 0.60

5.3

7.45×10−5

PFN(EtOH)2/Al 0.86

10.6 (9.8)

0.58

5.3

7.50×10−5

PFlm/Al

0.89

8.4 (8.5)

0.60

4.5

7.16×10−5

PBzFN/Al

0.87 10.4 (10.1) 0.60

5.4

7.55×10−5

a

Average values from ten devices; bCurrent density calculated by integrating the EQE with the

AM1.5G spectrum

Figure 3. AFM images (2.5×2.5 µm) of (a) the naked active layer and (b-e) interlayer on top of the active layer. To investigate how the CIMs influence the morphology of the surface, atomic force microscopy (AFM) was performed on the active layers with and without CIM. This measurement shows the very fine morphology achieved by the suitable polymer donor and acceptor. The AFM images shown in Figure 3 have fine morphologies in all five cases, where the roughnesses are more or less unchanged for all five films, with and without CIMs. The 12 ACS Paragon Plus Environment

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unchanged morphology indicates the CIMs are very thin and did not dissolve the active layer during deposition.

Figure 4. Contact angle measurements for (a) the naked active layer and (b-e) with different interlayers As the presence of CIMs could not be clearly verified by AFM images, contact angle measurements were performed. The contact angles were measured for the naked active layer, and the active layers with CIMs on top. The average angle from five measurements was calculated for each film. A representative image for each film is shown in Figure 4. The variation between the four films with interlayers was quite small. The largest angle was for observed for PFN(EtOH)2 which had an average of 92.0° while the lowest angle was observed for PBzFN with an average of 88.6°. PBzFN does not have any hydrophobic octyl side chains which could explain the slightly better wettability of the surface. All of the angles from the films with CIMs on top were significantly smaller than the angles observed from the naked active layer film with an average of 105.4°. These measurements confirm that the CIMs are present on top of the active layer and that their polar pendant groups made the surface more hydrophilic. Since the aluminum electrode was not in place in these films, its exact effect on the interfacial contact was not determined, but the increased polarity presumably improves contact, as it does in inverted structures.3 This is also supported by the J-V-characteristics of the devices, which showed improved charge extraction for all devices with interlayers, when compared to the device with bare aluminum. 3. Conclusion



In this work, we proved that several conventional interfacial layers, previously used mainly for PCBM-based solar cells, work well in improving the performances of all-PSCs. All four interlayer polymers showed similar electrochemical and optical properties, as well as surface energy and surface morphology when applied on top of active layers. Comparison of the device performances of PFN- and PBzFN-bearing all-PSCs, suggested that the backbone played a minor role in influencing the performance. The influences of the pendant aminecontaining groups on device performances were similar for dimethylamine and diethanolamine and both could perform comparably to LiF. The imidazole pendant group was less effective, which resulted in slightly worse device performance. On the other hand, the imidazole-functionalized polymer showed the highest degree of thermal stability. This would make imidazole-functionalized polymers attractive for devices where high temperature annealing has to be performed. Since LiF/Al electrodes are a significant source of device degradation over time, and metal evaporation is unsuitable for roll-to-roll processing, the interfacial polymers are viable contenders for commercial applications. These interfacial polymers have the advantages of easy production, solution processability and suitability for flexible device applications. Since these conventional CIMs perform well in all-PSCs, we believe that they will find applications in the large-scale production of all-PCSs using roll-toroll processing techniques. 4. Experimental Section Materials All solvents and reagents were purchased from Sigma Aldrich, VWR or Solarmer Materials Inc., and were used without further purification unless otherwise specified. All reactions with air-/moisture-sensitive components were performed under an atmosphere of dry nitrogen. Material characterization


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For Gel Permeation Chromatography, an Agilent PL-GPC 220 Integrated High Temperature GPC/SEC System was used, along with refractive index and viscometer detectors using 3×PLgel 10 µm MIXED-B LS, 300×7.5 mm columns with 1,2,4-trichlorobenzene as mobile phase, heated to 150 °C. Relative calibration using a polystyrene standard was used to calculate the molecular weights. Absorption spectroscopy was performed with a PerkinElmer 900 UV−Vis−NIR absorption spectrometer. Solutions were prepared from stock solution of 10 mg/mL polymer in chloroform and diluted until suitable absorbance was reached. Films were prepared from the same stock solutions by spin-coating at 2000 RPM for 1 min. Square Wave Voltammetric measurements were done on a CH-Instruments 650A Electrochemical Workstation. A 0.1 M solution of tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile, degassed with nitrogen, was used as supporting electrolyte. Platinum wires were used as reference- and working electrodes, and Ag/Ag+ was used as reference electrode. Potentials were referenced to the ferrocenium/ferrocene (Fc/Fc+) couple by using ferrocene as internal standard. Polymer films were deposited onto the working electrode from a 10 mg/mL polymer solution in chloroform. The HOMO and LUMO energy levels were calculated from the peak potentials by setting the oxidative peak potential of Fc/Fc+ vs. the normal hydrogen electrode (NHE) to 0.63 V, and the NHE vs. the vacuum level to 4.5, in total 5.13 V. The HOMO level was then calculated according to HOMO = −(Eox + 5.13) eV and LUMO = −(Ered + 5.13) eV, where the Eox and Ered were determined from the oxidation and reduction peaks. 1H NMR (400 MHz) and

13

C NMR (100 MHz) spectra were recorded on a Varian

Inova 400 MHz NMR spectrometer with tetramethylsilane (TMS) as internal reference. Matrix assisted laser desorption ionization - time of flight (MALDI-TOF) mass spectrometric measurements were conducted on a Bruker Autoflex Speed spectrometer. Atomic Force Microscopy (AFM) analysis was performed in tapping mode with an ND-MDT NTEGRA Prima inside an AEK-2002 acoustic enclosure, using HQ:NSC15/Al BS-15 tips with



resonance frequencies of 300 kHz. The samples were prepared by spin coating the active layers from a 12 mg/mL oDCB solution at 1000 RPM onto PEDOT:PSS coated glass. Contact angle measurements were performed on films prepared in identical fashion for AFM measurements, and an Attension Theta tensiometer was used, with the software One Attension v 2.6 used for data analysis. An average contact angle was taken from 5×4 µL drops on different parts of the films. TGA analysis was performed on a Mettler Toledo TGA/DSC 3+ STARe System. Acknowledgements: We would like to acknowledge the Swedish Research Council, the Swedish Research Council Formas, the Wallenberg Foundation and National Natural Science Foundation of China (21728401) for funding this study. Supporting Information Available: Synthesis procedures and additional characterization details, UV-Vis absorption, Electrochemical Properties and TGA.

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