High Conductivity and Photoelectrochemical Application

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Two-Dimensional Growth of Large-Area Conjugated Polymers on Ice Surfaces: High Conductivity and Photoelectrochemical Applications Dipankar Barpuzary, Kyoungwook Kim, and Moon Jeong Park ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07294 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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Two-Dimensional Growth of Large-Area Conjugated Polymers on Ice Surfaces: High Conductivity and Photoelectrochemical Applications Dipankar Barpuzary, Kyoungwook Kim, Moon Jeong Park* Department of Chemistry, Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea KEYWORDS: 2D growth, conducting polymer, ice template, high-conductivity, quantum dot, photoelectrochemical

ABSTRACT. Polymerizing monomers on atomically flat solid surface and air/water, solid/liquid or liquid/liquid interface is now a rapidly emerging frontier. Dimension-controlled synthesis of :

polymers is of particular interest, which can be achieved by precise control of

monomer distribution during the polymerization. Ice surface allows rapid polymerization of monomers in the plane direction along air–water interface to yield large-area two-dimensional sheet-like poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (2D sheet-like PEDOT:PSS) films with thickness of ca. 30 nm. The persuasive role of ice-chemistry is reflected in high degree of crystallinity and superior conductivity of resultant PEDOT:PSS films. Excellent photoelectrochemical (PEC) features were further disclosed when the ice-templated PEDOT:PSS films were coupled to quantum dots. Utilization of these polymer films in photovoltaic devices

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also resulted in excellent current density and power conversion efficiency. This work presents an innovative material technology that goes beyond traditional and ubiquitous inorganic 2D materials such as graphene and MoS2 for integrated electronic applications.

Ingenious scientific quest and steadfast efforts to soft electronics triggered a meteoric rise in

conducting

polymers—especially

poly(3,4-ethylenedioxythiophene)

(PEDOT)

for

optoelectronic, photo(electro)catalytic, bioelectronics, and thermoelectric applications, attributed to the intrinsic advantages of high electrical conductivity, optical transparency, chemical robustness and biocompatibility.1-5 Numerous reports delineate various strategies to improve the conductivity and processability of PEDOT.6-9 Most representative approach is to dope PEDOT with polystyrene sulfonate (PSS) to yield water-dispersible PEDOT:PSS having PEDOT-rich cores enclosed by PSS shells in colloidal particle form, amenable to fabricate onto various shapes and substrates.10 Conductive PEDOT:PSS films are often made by spin coating the dispersions onto a desired substrate wherein gel particles merge to form continuous film under heat treatment. However, the conductivity of resultant films is quite low due to the poor crystallinity of PEDOT oligomers linked to the PSS random coils.11,12 These films also break on mechanical deformation that makes them unsuitable for uses in flexible devices.13 To resolve these shortcomings, evolution of two-dimensional (2D) PEDOT:PSS thin films has thus been proliferated by fine-tuning their thickness in range of tens to hundreds of nanometer.14-16 Examples include growth of PEDOT:PSS films by layer-by-layer self-assembly, graphene composite, supramolecular assembly, hydrogel and vapor phase polymerization.17-19 Critical requisites to develop these films entail PEDOT:PSS deposition onto one template followed by detachment or template dissolution, and then capture onto another support.


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Despite they carry advantageous features over the typical spin coated films, i.e., conductivity of resultant 2D polymer films increases up to ca. 5 S cmD ,20-25 contrary to the low conductivity in the range of 0.1–1 S cmD obtained by spin-coating of water-dispersed pristine PEDOT:PSS, the outcomes are still not pleasing. Moreover, the aforementioned approaches require tedious synthetic steps including laborious removal of soft/hard templates (nonremovable in certain cases) and even the lateral dimension of obtained 2D polymers is limited in most cases.23,26 Incorporation of secondary additives and/or post-treatment with solvents have resolved this problem with enhanced conductivity by orders of magnitude.4-6 Key to this improvement is the alteration of PEDOT chain conformation from coil to linear by additives or PSS content reduction using post-treatment, thereby improving PEDOT crystallinity to enhance the intra and inter-particle charge transfer rate. Nevertheless, PEDOT:PSS without additive is yet most preferred for integrated electronic applications owing to the large-scale processability. Key challenges remaining for the synthesis of high-conductivity, large-area 2D PEDOT:PSS films are to (1) explore uniform and defect-free 2D template for polymerization, (2) fine-tune the distribution of monomers on the template, and (3) eliminate/minimize efforts to remove the template adhered to polymer.27-29 Overall, regardless in bulk or 2D film form, all the reports on conductivity amelioration of PEDOT:PSS acclaimed so far are limited to either secondary doping or post treatment. In this regard, the prospects for direct fabrication of freestanding PEDOT:PSS films having excellent conductivity, well-ordered crystallinity, preferred lateral dimension and easy transferability have not been explored yet. Here we report the ice-templated rapid synthesis of 2D sheet-like PEDOT:PSS thin films with several centimeters in diameter. The peculiar features of ice as hard template include easy


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removal after the synthesis via simple melting and tunable arrays of water molecules on surface by temperature, as recently reported by our group.30 2D lateral growth of PEDOT crystalline grains on ice surfaces led to a conductivity as high as 28 S·cmD with optical transparency and superior flexibility. This value is significantly higher than any pristine PEDOT:PSS thin films reported so far, displaying ca. 20 times higher conductivity than that of the spin-coated film. In contrast to typical template methods, to use ice as template at low temperature acts as the platform to ancestor PSS-mediated 2D propagation of PEDOT:PSS on top of ice surface. By coupling these PEDOT:PSS films to CdSe quantum dot (QD) sensitizers, hydrogen evolution up to ~86 µmol·cmD4 and markedly stable photocurrent density of ~2.3 mA·cmD4 were demonstrated. Further, utilization of these polymer films in photovoltaic devices even with simple configuration of ITO/PEDOT:PSS/P3HT:PCBM/Au results in excellent current density of 12.67 mA cm-2, open-circuit voltage of 0.66 V, fill factor of 63.2%, and power conversion efficiency of 5.34%.

RESULTS AND DISCUSSIONS Rapid Synthesis of 2D Sheet-Like PEDOT:PSS. The precursor solution of PEDOT:PSS was prepared by mixing EDOT in acetone and PSS/Na2S2O8/FeCl3/HCl in water under ice-cold conditions. As illustrated in Figure 1, the mixture was dropped on deeply frozen ice surface (D4- °C) where the presence of persulfate in precursor solution impedes etching of ice during the homogeneous distribution of reactant droplets. Light blue-colored PEDOT:PSS gradually appeared on the ice surface within 3 min. Upon increasing the temperature of ice to 0 C, ice at the air–water interface melted and rapid polymerization in the quasi-liquid layer for 5 min resulted in 2D sheet-like PEDOT:PSS thin film


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Optically Transparent, Large-Area, and High-Conductivity PEDOT:PSS Nanosheets. Transmission electron microscopy (TEM) image and optical photograph of icetemplated PEDOT:PSS films are shown in Figure 2a, representing that the large-area, uniform 2D nanosheet was formed. The resultant PEDOT:PSS nanosheets can be readily collected onto any types of substrates, i.e., fluorine doped tin oxide (FTO) or silicon/silicon oxide (Si/SiO2) substrates, flexible plastic substrates, and even onto liquid surfaces, via simple melting of ice template. Inset photograph of Figure 2a shows such example upon collecting PEDOT:PSS nanosheets onto PET substrate. AFM images of PEDOT:PSS nanosheets collected onto Si/SiO2 wafer are shown in Figure S2, displaying the smooth and uniform surface morphology of the films. It is noteworthy that the uniform 2D sheet-like PEDOT:PSS with large-area is formed only in presence of HCl in the precursor solution with unaffected flat-band potential (Figure S3). This is attributed to the reduced electrostatic attraction between PEDOT and PSS when PSS encloses hydrophobic EDOT oligomers in aqueous conditions upon HCl protonates PSS chains. The electrical conductivity of ice-templated PEDOT:PSS thin films are derived from current–voltage plot, measured at voltage sweep of D 9- V to +1.0 V at ambient conditions of 4AOL

and relative humidity of 22%. Figure 2b shows the result of 2D sheet-like PEDOT:PSS

loaded on top of gold-electrode coated Si/SiO2 substrate (see the schematic in inset). The distance between gold electrodes was 10PQ 9 To exclude any possible influence of residual water on electrical conductivity of ice-templated PEDOT:PSS, the changes in current values were carefully monitored while heating the sample at 120 °C and only the changes in the error range were observed.


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Notably, the high current value of 9.9 mA at 1.0 V is measured for 2D sheet-like PEDOT:PSS thin film developed on ice surface as compared to the low value of 0.9 mA determined for spin-coated film with similar thickness. The conductivity of ice-templated PEDOT:PSS film is 28 S·cmD , which is the highest value reported for any PEDOT films only with PSS dopant. The conductivity of spin-coated film was low as 1.8 S·cmD in good agreement with literature value. It should be noted that the conductivity of 2D sheet-like PEDOT:PSS developed on ice surface is a sensitive function of film thickness (Figure S4), which could be controlled by varying the concentration of reactants dropped on ice surface. We have observed that conductivity decreases as the film thickness increases, attributed to the increase in defects and decrease in crystallinity. Therefore, all experiments were conducted with a focus on the thinnest thickness of 30 nm. Figure 2c presents UV-vis spectra of PEDOT:PSS films, displaying the increment in near-infrared band of the film formed over ice. This indicates more conducting nature of icetemplated 2D PEDOT:PSS thin film with plausible coil-to-linear conformational change. Herein it should be noted that conductivity of PEDOT:PSS depends on various factors like ratio of PEDOT to PSS, type of oxidizing agent used, solvent effect and so on. Table 1 summarizes the representative conductivity values of PEDOT:PSS films reported in literature, concerning the preparation of PEDOT:PSS without additional dopants or solvent treatment. These results confirm the markedly high conductivity of 2D sheet-like PEDOT:PSS film, enabled by ice surface.


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Table 1. Conductivity 'R* values of PEDOT:PSS films fabricated by different methods without additional dopants or solvent treatment. Sample code

PEDOT:PSS ratio

Oxidizing agent

Fabrication Template

R Ref (S·cmD )

PEDOT:PSS film 1:2.5

Fe2(SO4)3, Na2S2O8

Spin coating

PDMS

3.5

15

Clavios PH 1000

Fe2(SO4)3, Na2S2O8

Spin coating

PET

0.3

20

PEDOT:PSS film 1:2

Fe2(SO4)3, Na2S2O8

Spin coating

Glass

0.2

21

PEDOT:PSS sheet

FeCl3, Na2S2O8

Peel off

Glass

5

22

PEDOT:PSS film 1:2

Fe2(SO4)3, Na2S2O8

Spin coating

Glass

0.2

32

PEDOT:PSS film 1:2

Fe(III)-pSpin toluenesulfonate coated

PVDF

3.2

23

PEDOT:PSS film 1:2.5

Fe2(SO4)3, Na2S2O8

Spin coated

PDMS

2

24

PEDOT:PSS

Fe2(SO4)3, K2S2O8

Spin coating

Glass

5

25

FeCl3, Na2S2O8

Peel off

Ice surface

28

This work

1:2.5

-

1:6 (monomer)

2D sheet-like 1:1.7 PEDOT:PSS

Mechanisms Underlying the Development of High-Conductivity 2D Sheet-Like PEDOT:PSS. Phase purity and crystallinity of ice-templated PEDOT:PSS nanosheets are investigated by combining selected area electron diffraction (SAED), high-resolution TEM (HRTEM), powder X-ray diffraction (XRD), and grazing incident X-ray scattering (GIXS) experiments. Figure 3a shows a representative SAED pattern of ice-templated PEDOT:PSS, revealing 2D crystal formation to be indexed to monoclinic crystal of the P2 space group. HRTEM image in


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Figure 3b confirms markedly well-defined crystalline phases based on :–: stacked PEDOT molecules with d spacing of 3.4 Å. XRD profile in Figure 3c depicts strong appearance of (200), (-101), (101), (400), (002), (-501), (202), and (010) diffraction peaks for ice-templated PEDOT:PSS at 4T = 5.7°, 8.7°, 10.5°, 14.5°, 17.9°, 18.4°, 20.5°, and 25.9°, respectively. Because XRD samples were prepared by crushing multiple PEDOT:PSS nanosheets, we see the polycrystalline characteristics. A series of diffraction peaks elucidate the formation of monoclinic crystals with the unit cell parameters a = 24.7 Å, b = 3.4 Å, c = 10.1 Å,

= = 90.00°, and

= 103.1°.

The projected molecular arrangement of PEDOT in PEDOT:PSS nanosheets is illustrated in inset of Figure 3c. Sharp (200) diffraction peak is ascribed to the second order diffraction of the lamella stacking of PEDOT (linked PSS chains are invisible in the scheme). A d100 lattice spacing of 24.7 Å was assessed. High intensity (002) peak is assigned to the horizontal packing of PEDOT oligomers with d002 lattice spacing of 5.1 Å. The :–: stacking distance (d010) of thiophene units of PEDOT is 3.4 Å, lesser than that of pure PEDOT in literature (ca. 3.5 Å). This indicates strong interaction of conjugate backbones via quinoid-structure formation creating more rigid and planar polymer backbones. The crystalline grain size analyzed by Debye– Scherrer relation was large as 48 nm, indicative of strong interaction among quinoid PEDOT chains favoring markedly increased crystalline order. It is worthwhile to note that such well-defined crystalline phases have not been reported in literature for any PEDOT:PSS. The 2D growth of PEDOT:PSS with large grain size upon the introduction of ice template is schematically depicted in the right panel of Figure 3c. This peculiar type 2D ordering of PEDOT:PSS core–shell grains is not observed in commonly fabricated PEDOT:PSS films, where these grains slide over each other in their colloidal


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particulate mixture. The broad and ill-defined diffraction pattern of bulk PEDOT:PSS prepared by oxidative polymerization is shown in Figure 3c as a reference.


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Figure 3. (a) SAED pattern and (b) HRTEM image that indicate highly crystalline ice-templated PEDOT:PSS with a monoclinic structure, as indexed in (a). (c) Powder XRD patterns for 2D sheet-like PEDOT:PSS and bulk polymerized PEDOT:PSS, indicating dissimilar crystalline morphologies, as schematically depicted in the right panels. The proposed molecular arrangement of PEDOT:PSS nanosheets developed on ice surface is depicted in the inset. (d) 2D GIXS patterns obtained by placing ice-templated PEDOT:PSS films on top of Si/SiO2 wafer. Two patterns were acquired by rotating the films in the plane direction, as illustrated in the schemes.

Figure 3d depicts the molecular arrangement of PEDOT chains in ice-templated PEDOT:PSS films. 2D GIXS patterns were acquired by placing the films on top of Si/SiO2 wafer, indicating that the PEDOT backbones are predominantly found to be tiled in the vertical direction with long-range order. Upon rotating the samples in the plane direction, it was further revealed that the PEDOT main chains are periodically positioned with a predominant edge-on :– : stacking of thiophene units. All of the d spacings obtained by GIXS experiments were in good agreement with the powder XRD results. The low temperature-controlled ordering of PSS over quasi-ordered dangling OH groups on ice surface at D4- °C should be responsible for the well-defined 2D growth of PEDOT:PSS. Figure 4a shows attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectra of PSS exposed to different surroundings. IR absorptions at 896 cmD (

S-O)

and at 1350 cmD (

S=O,

symmetric) of PSS vanish when PSS is coated on ice surface or dissolved in water, indicative of hydrogen-bonding interactions of sulfonic acid groups with water molecules. The appearance of


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broad peak at 1175 cmD (

S=O,

asymmetric) for PSS in contact with water signifies the removal

of degeneracy of PSS–SO3H groups upon forming PSS–(SO3)D H3O+.33,34

Figure 4. (a) ATR-FTIR spectra of PSS in dry condition, in contact with water, and on ice surface. (b) Schematic illustration showing PSS adsorption on ice surfaces via predominant interchain hydrogen bonding interactions.

The

S=O (asym.)

peak is less extensive and blue-shifted to 1154 cmD for PSS on ice

surface, accompanied by red-shifted and broadened IR peaks at 1001 cmD and 1123 cmD (aromatic ring in-plane deformation). Lower degree of proton dissociation of PSS on ice surface


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and predominant intermolecular hydrogen-bonding interactions among adjacent PSS–SO3H groups can thus be inferred. As schematically depicted in Figure 4b, this embodies planar orientation of PEDOT linked to PSS chains through self-arranged PSS–PSS interlinks on ice surface. Rapid polymerization in the quasi-liquid layer on top of ice surface further facilitates ordered lateral packing of PEDOT:PSS at air–water interface by confinements.35 This is in sharp contrast to the typical efforts to synthesize PEDOT:PSS based on the sliding of PEDOT-linked PSS grains dispersed in water. In general, to obtain PEDOT:PSS thin films, heat-treated water removal is intentionally done after spin coating PEDOT:PSS to retard the extent of hydrogen bonding between SO3H groups of PSS and water. It stimulates hydrogen bonding interactions among SO3H groups of adjacent PSS chains, leading to interconnect PEDOT:PSS core–shell grains. Interestingly, our ice-assisted route facilitates the spontaneous and rapid formation of hydrogen-bonded PSS-PSS networks because most water molecules are deeply frozen. It is thus inferred that the fast synthesis of highly conducting 2D sheet-like PEDOT:PSS films on ice surface has become possible by virtue of PSS-mediated lateral alignment.

Photoelectrochemical Applications. To assess the mechanistic influence of highly conducting 2D polymers, ice-templated PEDOT:PSS films were transferred onto FTO substrates and utilized as the host to scaffold CdSe QD photosensitizers for use as photoanodes. CdSe QDs were loaded onto 2D sheet-like PEDOT:PSS films by chemical bath deposition (CBD) method in aqueous media. This is to rule out the possibility of solvent post-treatment occurring on ice-assisted PEDOT:PSS nanosheets.


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Figure 5a shows the changes in X-ray photoelectron spectroscopy (XPS) spectrum of 2D sheet-like PEDOT:PSS with incorporated CdSe QDs. QD loading onto PEDOT:PSS films resulted in upshifts in the binding energies of S2p3/2 and S2p1/2 for PSS while those for thiophene rings of PEDOT remain unchanged. This indicates selective anchoring of QDs to sulfonate groups. The specific binding energy of 3d peaks for PEDOT:PSS/QD are provided in Figure S5. It should also be noted that the PEDOT:PSS surface composition ratio of 1:1.7 is obtained from integral area of S2p peaks. This is relatively low to satisfy the well-ordered crystalline arrangement of PEDOT chains.5-6 As far as the water-dispersion of PEDOT:PSS core–shell particles are used to fabricate thin films, secondary doping is post-treated to reduce PSS concentration that reinforces the overall alignment of PEDOT. In contrast, ice-assisted route preferably lowers this PSS content in the shells due to the restricted motion of PSS chains over ice surface at deep-frozen conditions.36 The PEDOT:PSS ratio is further endured (1:1.6) for PEDOT:PSS/CdSe film, indicating no significant loss of PSS during the CBD process. As shown in Figure 5b, the morphology of composite film was investigated by TEM in which uniformly distributed QDs with average size of 4.3 nm (size histogram is provided in inset) were observed throughout the film. XRD data in Figure 5c further indicate high crystallinity of CdSe QDs grown on 2D sheet-like PEDOT:PSS, where typical diffraction peaks for (111), (220), (311), (331) and (422) lattice planes of CdSe nanocrystals were shown. The retention of crystallinity of PEDOT:PSS (shown by inverted arrows) in the composite film is also noteworthy.


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Normalized intensity

a

PEDOT:PSS/QD PEDOT:PSS

S2p3/2 S2p1/2

S2p3/2 S2p1/2

162

164

166

168

170

172

Binding energy (eV)

b

2

4

6 nm

20 nm

c Intensity (a.u.)

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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PEDOT:PSS/QD (111) (220)

(311)

CdSe QD (331) (422)

10

20

30

40

50

60

70

80

2 (degree) Figure 5. (a) XPS spectra showing binding energies of S2p for PEDOT:PSS film and PEDOT:PSS/CdSe QD. (b) Bright-field TEM image of PEDOT:PSS/CdSe QD. Particle size histogram indicating the average diameter (4.3 ± 0.3 nm) of CdSe QDs is shown in inset. (d) Powder XRD patterns of CdSe QD and PEDOT:PSS/CdSe QD.


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We now investigate the photoelectrochemical (PEC) dynamics of ice-templated PEDOT:PSS/CdSe QD photoanodes under the standard solar illumination (AM 1.5 G, 100 mW cm–2). Figure 6a shows a photocurrent value of ~1.6 mA·cmD4 for ice-templated PEDOT:PSS/QD photoanode, which is superior to 2D graphene/QD counterparts reported in literature, as marked in the figure.37-40 The ice-assisted route to make self-supporting PEDOT:PSS films with high conductivity, high lateral dimension, and minial defects is the key to the high performance, which is not trival for graphene counterparts. In other words, the exfoliation of graphene by ultrasonic bath is typically required to obtain 2D nanosheets, practically limiting their production in high lateral dimension. The corresponding chronoamperometry plot exhibits stable photocurrent response of icetemplated PEDOT:PSS/QD under light on–off conditions as shown in Figure 6b, which is significantly higher than that of spin-coated counterparts. This indicates more facile charge transport properties of PEDOT:PSS in 2D morphology. From the low current values of bare 2D sheet-like PEDOT:PSS and polymer-free QD based electrodes, it is also inferred that 2D sheetlike PEDOT:PSS films act as efficient host to anchor QDs by forming an interconnected network among QDs and polymer for efficient generation of photocurrent. Note that while the data presented in Figure 5 and Figure 6 were obtained from PEDOT:PSS/QD composite at a fixed QDs loading, the loading of QDs onto polymer film was varied to optimize the PEC properties. Figure S6 and Figure S7 depict the examples of such optimization.


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a [37]

2

Current (mA cm )

Graphene/QD

Ice-templated PEDOT:PSS/QD

1.6 1.2

[37]

0.8 [38]

0.4

[39]

(High lateral dimension) 0.0

0.4

0.6

0.8

[40]

1.0

1.2

Exfoliated, 2D sheet-like (low lateral dimension)

Voltage (V vs. RHE)

b

Ice-templated PEDOT:PSS/QD

ON

1.6

-2

Current (mA cm )

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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1.2 0.8

Spin-coated PEDOT:PSS/QD

0.4

ON OFF

QD Ice-templated PEDOT:PSS

0.0 0

30

60

90

120

150

180

210

240

270

300

Time (s)

Figure 6. (a) Photocurrent density of ice-templated PEDOT:PSS/QD photoanode, compared with that of 2D graphene/QD counterparts reported in literature. (b) Time-profile photocurrent generation curve of ice-templated PEDOT:PSS/QD photoanode, compared with those of spincoated PEDOT:PSS/QD, bare 2D sheet-like PEDOT:PSS, and polymer-free QD based electrodes. All measurements were carried out under AM 1.5 G (100 mW·cm–2) solar illumination.


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In order to further improve the PEC properties of PEDOT:PSS/QD photoanodes, surface charge recombination of photoanode was lowered by electrodeposition of cobalt-phosphate (CoPi) on top of the electrodes.41 Figure 7a displays schematic drawings of the photoanode structure and charge transport by sulfide/polysulfide electrolyte for photocatalytic hydrogen production. The monochromatic incident photon-to-current conversion efficiency (IPCE) of PEDOT:PSS/CdSe/CoPi was improved to 35%, as shown in Figure 7b. The new photoanode demonstrated hydrogen gas evolution up to 86 µmol·cmD4 over 2 h against Pt counter electrode of two-electrode configuration at applied bias of 1.2 V, as presented in Figure 7c. This is also in good agreement to the superior photocurrent characteristics of PEDOT:PSS/QD electrodes over the spin-coated PEDOT:PSS/QD and polymer-free QD counterparts. Notably the stable photocurrent density of 2.3 mA·cmD4 is also presented in the inset, demonstrating that the iceassisted synthesis of 2D conducting polymers is the future avenue towards future PEC technologies that goes beyond conventional inorganic 2D materials. Further utilization of this 2D sheet-like PEDOT:PSS film in photovoltaic devices was demonstrated by fabricating polymer solar cells with simple ITO/PEDOT:PSS/P3HT:PCBM/Au configuration. Their photovoltaic features were monitored by measuring current density (Jsc) as a function of applied voltage (V) and the critical role of ice-templated PEDOT:PSS in device performance was assessed by comparing the parameters to typically spin-coated PEDOT:PSS counterparts.


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Figure 7. (a) Schematic showing the components of PEDOT:PSS/CdSe QD photoanode and charge transport with sulfide/polysulfide electrolyte. (b) Incident photon-to-current conversion efficiency (IPCE) plots of PEDOT:PSS/QD and PEDOT:PSS/QD/CoPi electrodes. (c) Hydrogen evolution kinetics of fabricated photoanodes, as denoted by ice-templated PEDOT:PSS/QD/Co-


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Pi, ice-templated PEDOT:PSS/QD, spin-coated PEDOT:PSS/QD, ice-templated PEDOT:PSS and polymer-free QD. Chronoamperometry plot in the inset shows photostability of bestperformed ice-templated PEDOT:PSS/ QD/CoPi photoanode during gas evolution. All measurements were carried out under AM 1.5 G.

As evident from Figure 8a, ice-templated PEDOT:PSS based devices exhibited Jsc of 12.67 mA cm-2, open-circuit voltage (Voc) of 0.66 V and fill factor (FF) of 63%, affording a maximum power conversion efficiency (PCE) of 5.34%. In contrast, identical devices with spincoated PEDOT:PSS having identical thickness showed Jsc of 9.8 mA/cm2, Voc of 0.61 V, FF of 52%, and low PCE of 3.1%. This indicates the increases in PCE by 42% and FF by 18% of the solar cells comprising ice-assisted PEDOT:PSS films, ascribed to the improved internal ordering and high conductivity of ice-templated PEDOT:PSS as compared to those of spin-coated polymer film. The generation of 2D charge transport pathways in 2D sheet-like PEDOT:PSS thin films with low surface roughness (Rq < 1 nm, Figure S2) is also key to such improvement. In Table S1, a brief overview of reported PEDOT:PSS/P3HT:PCBM type devices with varied components and structural modulations for enhancing device performance is provided. We note here that the solar cell performance can be further improved if the modification of P3HT:PCBM active layer is made with the optimized contact angles of ice-templated PEDOT:PSS, but this is beyond the scope of current work. IPCE of solar cells was measured against wavelength under short-circuit conditions. As can be seen in Figure 8b, IPCE curves show wide plateau in the visible range 'G>A-DCA- nm) for the devices, confirming efficient solar harvesting by P3HT:PCBM active layer. Notably, the devices endowed improved IPCE profile when fabricated with ice-templated PEDOT:PSS rather


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than spin-coated component. Keeping in mind both types of devices to be identical except for the PEDOT:PSS layer, presence of 2D polymer enhances overall charge migration, leading to better device performance. The overall photovoltaic parameters of devices are summarized in Table 2.

b

a -12

60

-9

IPCE (%)

2

Current (mA cm )

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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-6 -3

Spin-coated Ice-templated

0 0.0

0.2

0.4

40 20

Spin-coated Ice-templated

0 300

0.6

400

Voltage (V) Figure 8. (a) Current

D"

500

600

700

Wavelength (nm) (JscDV) and (b) incident photon-to-current conversion

efficiency (ICPE) plots for polymer solar cells fabricated with ice-templated PEDOT:PSS (red line) and spin-coated PEDOT:PSS (black line) under AM 1.5 G solar illumination.

Table 2. Short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), power conversion efficiency (PCE) and maximum IPCE (IPCEmax) values for fabricated solar cellsa Device

Jsc

Voc

FF

PCE

IPCEmax

code

(mA/cm2)

(V)

(%)

(%)

(%)

Ice-templated Spin-coated aData

12.67 (11.91±0.7) 0.66 (0.66±0.01) 63.2 (63.0±0.2) 5.34 (5.22±0.08) 9.83 (9.02±0.8)

0.61 (0.60±0.02) 51.9 (51.6±0.1) 3.08 (3.01±0.14)

72 61

reported are for the highest value out of five devices for each configuration. Average

values of five devices and standard deviations/errors are given in parentheses.


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CONCLUSIONS We propose a platform for rapid synthesis of high-conductivity self-supporting 2D sheetlike PEDOT:PSS thin films. Confinements provided by ice surface at the air-water interface enabled the 2D lateral growth of PSS-linked PEDOT crystalline grains in large-area with conductivity of 28 S cm-1 without any secondary doping or post-treatment process. Further intriguing features of 2D conducting network are revealed through high power conversion efficiency and excellent PEC hydrogen evolution by coupling CdSe QDs to the PEDOT:PSS nanosheets. These polymer films are also capitalized as the component in P3HT:PCBM based polymer solar cells that demonstrate power conversion efficiency as high as 5.34%.

METHODS AND EXPERIMENTAL Materials. 3,4-Ethylenedioxythiophene (97%), poly-(4-styrenesulfonic acid) solution (Mw ~75,000, 18 wt.% in water), acetone (99.5%), sodium persulfate (98%), ferric chloride (97%), hydrochloric acid (37%), cadmium acetate dihydrate (98%), sodium sulfite (98%), selenium powder (~100 mess, 99.99%), ammonium hydroxide solution (30% in water), sodium sulfide (99%), sulfur (99.5%), potassium phosphate monobasic (99%), potassium phosphate dibasic (99%), cobalt chloride (98%), and potassium chloride solution (3 M) were purchased from Sigma Aldrich and used without purification. Fluorine-doped tin oxide (FTO) coated glass substrates (surface resistivity ~7 X8 5$ Sigma Aldrich), indium tin oxide (ITO) coated glass substrates (surface resistivity ~10 X8 5$ SN Science), and 0.2 µm PTFE syringe filter (Whatman) were used. Regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT; ~95%), [6,6]-phenyl-C61butyric acid methyl ester (PCBM; >99%) and encapsulation epoxy were purchased from Ossila, UK. Distilled water (18.2 %X cm) was used for all the experiments.


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Synthesis of 2D Sheet-Like PEDOT:PSS Thin Films. Self-supporting 2D sheet-like PEDOT:PSS films were synthesized by employing ice template. In order to formulate the flat and smooth surface of ice template, water in petri dish was frozen at D4- °C and turned the ice upside down. 1 mL of Na2S2O8 (0.1 M) and 1 mL of FeCl3 (1.5 M) aqueous solutions were added to 1 mL of PSS solution (1.8 wt.% in H2O), followed by the addition of 3 mL of HCl (25 mM) aqueous solution. 0.11 mL of EDOT was dissolved in 10 mL of acetone and 1 mL of the solution was added to aforementioned aqueous mixtures under ice-cold condition, shaken for 10–15 seconds until forming transparent light green-colored solution. The solution was immediately drop-cast over ice surface at –20 °C by passing through PTFE syringe filter. Light blue-colored droplets gradually appear on top of the ice surface within 3 min. The temperature of ice was raised to 0 C and ice melts at the ice-air interface (liquid-like layer) in which polymerization occurs to form 2D sheet-like PEDOT:PSS. Distilled water was carefully poured from the edges of ice piece to float and wash the polymer film. The film was rinsed with distilled water plenty of times, immediately collected while cold onto desired substrates, dried at 120 °C for 10 min to remove any traces of water and stored under vacuum. Removal of impurities (FeCl3, Na2S2O8) in the film was confirmed by energy-dispersive X-ray spectroscopy. As a control sample, identical amounts of reactants were mixed in a round bottom flask and allowed to stir for 3 h at room temperature. The blue-colored precipitate was filtered, washed with distilled water, and dried at 120 °C for 10 min. Loading of CdSe QDs to PEDOT:PSS Thin Films. 0.5M aqueous solutions of Cd(CH3COO)2 2H2O and Na2SeSO3 (obtained by refluxing Se and Na2SO3 at 120 °C for 5 h in water) were prepared. To load CdSe QDs onto PEDOT:PSS films, five CdSe QDs precursor solutions were prepared at the fixed concentration of 4.0 mM Cd2+ and 10 µL of NH4OH while the


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concentration of Se precursor solution was varied into 0.5, 1.0, 1.5, 2.0 and 2.5 mM. The PEDOT:PSS film on FTO coated glass substrate was dipped inside vessels containing the CdSe precursor solutions by facing FTO side down. The vessels were sealed, maintained at 90 °C for 1 h, and then allowed to cool down to room temperature. The substrates were collected, rinsed with water and dried at 120 °C for 10 min. The photoanodes based on PEDOT:PSS/CdSe were termed as the concentrations of Se precursor solutions, i.e., Se[0.5], Se[1.0], Se[1.5], Se[2.0], and Se[2.5]. Identical procedure was followed to fabricate spin-coated PEDOT:PSS/CdSe and polymer-free CdSe photoanodes on FTO substrates for the control experiments. Morphology and Molecular Characterization. Morphologies of 2D sheet-like PEDOT:PSS thin films with and without CdSe quantum dots were characterized by combining TEM (JEOL JEM1011 for TEM at POSTECH Biotech Center and FEI Tecnai F20 for HRTEM at Molecular Foundry of Lawrence Berkeley National Lab.), operated with a 200kV accelerating voltage, powder XRD, and GIXS experiments. SAED pattern of each sample was recorded using FEI/Philips CM200 with a 200kV accelerating voltage at Molecular Foundry. XRD profile and 2D GIXS pattern of each sample were recorded at 5A beamline and 3C beamline, respectively, at Pohang Accelerator Laboratory (PAL) using the synchrotron radiation. X-ray photoelectron spectroscopy (XPS) was conducted by ESCALAB 250XI (Thermo Fisher Scientific). Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra was recorded using a PerkinElmer instrument. Solid-state UV–vis diffused reflectance spectra were recorded using Shimadzu UV-2600 spectrophotometer with BaSO4 as an internal reference. Photo-Assisted Electrodeposition of Cobalt Phosphate (CoPi) Layer. Cobalt phosphate (CoPi) surface layer was electrodeposited onto the photoanodes by three-electrode system. For this, PEDOT:PSS/CdSe@FTO (working), Ag/AgCl (reference) and Pt loop (counter) were fitted in


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three-neck cell with quartz window containing 0.1 M phosphate buffer pH 7 and 0.5 mM CoCl2. Prior to electrodeposition, Ar gas was purged through the electrolyte for 30 min. The working electrode was dipped in electrolyte and constant bias of 0.59 V (vs. Ag/AgCl) was immediately applied by Gamry potentiostat (Reference 600) for 10 min with 100 mW·cmD2 backside illumination and continuous Ar flow through the reactor headspace. The resultant PEDOT:PSS/CdSe/CoPi electrode was vacuum dried to eliminate the traces of water. Photoelectrochemical (PEC) Performance. PEC properties of the photoanodes were measured by employing standard three-electrode cell under 100 mW·cmD2 solar irradiation (AM 1.5G, 300 W Xe lamp) from FTO backside. A light-blocking mask was used to make an irradiation area to 0.25 cm2 for all the photoanodes. Photocurrents were collected using PEDOT:PSS/CdSe@FTO (working), Ag/AgCl (reference) and Pt wire (counter) in 0.1 M sulfide/polysulfide electrolyte under a voltage sweep from D0.5 V to 1.5 V at scan rate of 10 mV·sD1. For the other sets of photocurrent measurements, the working electrode was changed with corresponding configuration. Acquired photocurrent parameters were converted to RHE. Incident photon-to-current conversion efficiency were measured using Oriel IQE-200 instrument (300 W quartz tungsten halogen lamp), equipped with chopper, Cornerstone monochromator, and Merlin lock-in amplifier radiometry unit. Flat-band potential of the polymer films was determined from MottDSchottky plot recorded at 1 kHz frequency in 0.1 M phosphate buffer (pH 7) at 50 mW·sD1 scan rate. To measure the PEC gas evolution, PEDOT:PSS/CdSe/CoPi@FTO photoanode and Pt cathode were placed in parallel at 1.5 cm distance inside two-electrode PEC reactor containing 0.1 M phosphate buffer (pH 7). The reactor was sealed and electrolyte was bubbled with Ar for 1 h prior to the measurement to remove the traces of O2. An absolute bias of +1.2


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V under AM 1.5G solar irradiation (100 mW·cmD2) was applied and gas evolved inside the reactor was extracted by glass syringe at an interval of 15 min. The gas evolution was analyzed using Ar as carrier in gas chromatograph (GC, HP6890A) fitted with thermal conductivity detector and 5Å molecular sieve column. Same procedure was repeated to acquire data for the other electrodes. Solar Cell Fabrication and Characterization. The solar cells were fabricated on commercially available pre-patterned ITO-coated glass substrates. These were cleaned under ultrasonic bath in detergent solution, distilled water, acetone, ethanol and isopropanol consecutively for 15 min each. ITO substrates were dried by blowing argon followed by UV-ozone treatment for 20 min. Next, pre-washed PEDOT:PSS films were transferred on top of patterned ITO and dried at 120 °C for 15 min. After inserting these substrates inside glove box, P3HT:PCBM active layer was spin coated onto PEDOT:PSS films at 1000 rpm (thickness ~110 nm) and annealed at 150°C for 30 min. P3HT:PCBM (1:1.08) blend was prepared in 1,2dichlorobenzene by stirring at 60 °C for 48 h and then filtered through 0.2 µm PTFE syringe filter prior to use. The cathode contacts were made by thermal evaporation of Au (~60 nm) layer on top of active layer and the devices were encapsulated by UV-curable epoxy. A set of control devices were fabricated by same procedure using spin-coated PEDOT:PSS (3000 rpm, ~30 nm). Solar cell parameters were recorded under 100 mW·cmD2 (AM 1.5G) solar irradiation on active cell area of 0.04 cm2 for each device.

ASSOCIATED CONTENT Supporting Information. EDS spectra, AFM image, Mott-Schottky plots, I-V plots, XPS spectra, UV–vis spectra, photocurrent generation and time-profile, literature data on


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PEDOT:PSS/P3HT:PCBM type solar cells. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *Moon Jeong Park (E-mail: [email protected]). ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. NRF-2017R1A2B3004763), by the Korea government (MSIT) (No. NRF-2017R1A5A1015365), and the Creative Materials Discovery Program through the NRF funded by Ministry of Science and ICT (2018M3D1A1058624). Prof. Park also acknowledges financial support from LG Yonam Foundation of Korea. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Dr. Guan-Woo Kim for assistance with solar cell experiments and Prof. Taiho Park (POSTECH) for helpful discussions. REFERENCES 1. Ganji, M.; Kaestner, E.; Hermiz, J.; Rogers, N.; Tanaka, A.; Cleary, D.; Lee, S. H.; Snider, J.; Halgren, M.; Cosgrove, G. R.; Carter, B. S.; Barba, D.; Uguz, I.; Malliaras, G. G.; Cash, S. S.; Gilja, V.; Halgren, E.; Dayeh, S. A., Development and Translation of PEDOT:PSS Microelectrodes for Intraoperative Monitoring. Adv. Funct. Mater. 2018, 28, 1700232. 2. Sinha, S. K.; Noh, Y.; Reljin, N.; Treich, G. M.; Hajeb-Mohammadalipour, S.; Guo, Y.; Chon, K. H.; Sotzing, G. A., Screen-Printed PEDOT:PSS Electrodes on Commercial


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Films:

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Morphological, Mechanical and Electrical Analyses. J. Mater. Chem. C 2014, 2, 9903– 9910. 37. Lightcap, I. V.; Kamat, P. V., Fortification of Cdse Quantum Dots with Graphene Oxide. Excited State Interactions and Light Energy Conversion. J. Am. Chem. Soc. 2012, 134, 7109–7116. 38. Li, H.; Wen, P.; Hoxie, A.; Dun, C.; Adhikari, C.; Li, Q.; Lu, C.; Itanze, D. S.; Jiang, L.; Carroll, D.; Lachgar, A.; Qiu, Y.; Geyer, S. M., Interface Engineering of Colloidal CdSe Quantum Dot Thin Films as Acid-Stable Photocathodes for Solar-Driven Hydrogen Evolution. ACS Appl. Mater. Interfaces 2018, 10, 17129–17139. 39. Wang, M.; Shang, X.; Yu, X.; Liu, R.; Xie, Y.; Zhao, H.; Cao, H.; Zhang, G., GrapheneCdS Quantum Dots-Polyoxometalate Composite Films for Efficient Photoelectrochemical Water Splitting and Pollutant Degradation. Phys. Chem. Chem. Phys. 2014, 16, 26016– 26023. 40. Zhu, N.; Zheng, K.; Karki, K. J.; Abdellah, M.; Zhu, Q.; Carlson, S.; Haase, D.; Žídek, K.; Ulstrup, J.; Canton, S. E.; Pullerits, T.; Chi, Q. Sandwiched Confinement of Quantum Dots in Graphene Matrix for Efficient Electron Transfer and Photocurrent Production. Sci. Rep. 2015, 5, 9860. 41. Zhong, D. K.; Cornuz, M.; Sivula, K.; Grätzel, M.; Gamelin, D. R., Photo-Assisted Electrodeposition of Cobalt–Phosphate (Co–Pi) Catalyst on Hematite Photoanodes for Solar Water Oxidation. Energy Environ. Sci. 2011, 4, 1759–1764.


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ACS Nano

Two-Dimensional Growth of Large-Area Conjugated Polymers on Ice Surfaces: High Conductivity and Photoelectrochemical Applications Dipankar Barpuzary, Kyoungwook Kim, Moon Jeong Park*

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High Conductivity and Photoelectrochemical Application

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