Patterning Conjugated Polymers by Laser: Synergy of Nanostructure

Subscriber access provided by READING UNIV

Article

Patterning conjugated polymers by laser: Synergy of nanostructure formation in the all-polymer heterojunction P3HT/PCDTBT Álvaro Rodriguez-Rodriguez, Esther Rebollar, Tiberio A. Ezquerra, Marta Castillejo, Jose Vicente Garcia-Ramos, and Mari Cruz García-Gutiérrez Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03761 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 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

Langmuir

Patterning conjugated polymers by laser: Synergy of nanostructure formation in the all-polymer heterojunction P3HT/PCDTBT Álvaro Rodríguez-Rodríguez,† Esther Rebollar,‡ Tiberio A. Ezquerra,† Marta Castillejo,‡ Jose Vicente Garcia-Ramos,† and Mari-Cruz García-Gutiérrez*,† †

Instituto de Estructura de la Materia (IEM-CSIC), Serrano 121, 28006 Madrid, Spain

‡

Instituto de Química Física Rocasolano (IQFR-CSIC), Serrano 119, 28006 Madrid, Spain

ABSTRACT

In this work we report a broad scenario for the patterning of semiconducting polymers by laserinduced periodic surface structures (LIPSS). Based on the LIPSS formation in the semicrystalline poly(3-hexylthiophene) (P3HT), we have extended the LIPSS fabrication to an essentially amorphous semiconducting polymer like poly[N-90-heptadecanyl-2,7-carbazole-alt5,5-(40,70-di-2-thienyl-20,10,30 benzothiadiazole)] (PCDTBT). This polymer shows a good quality and well-ordered nanostructures not only at the 532 nm laser wavelength, as in the case of P3HT, but also at 266 nm providing gratings with smaller pitch. In addition, we have proven

1 ACS Paragon Plus Environment

Langmuir 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

Page 2 of 37

the feasibility of fabricating LIPSS in the P3HT/PCDTBT (1:1) blend, which can be considered as a model bulk-heterojunction for all-polymer solar cells. In spite of the heterogeneous roughness, due to phase separation in the blend, both P3HT and PCDTBT domains present well defined LIPSS as well as a synergy for both components in the blend when irradiating at wavelengths of 532 nm and 266 nm. Both, P3HT and PCDTBT in the blend require lower fluence and less pulses in order to optimize LIPSS morphology than in the case of irradiating the homopolymers separately. Near edge X-ray absorption fine structure and Raman spectroscopy reveal a good chemical stability of both components in the blend thin films during LIPSS formation. In addition Scanning Transmission X-ray Spectro-Microscopy shows that the mechanisms of LIPSS formation do not induce a further phase segregation neither a mixture of the components. Conducting atomic force microscopy reveals a heterogeneous electrical conductivity for the irradiated homopolymer and for the blend thin films, showing higher electrical conduction in the trenches than in the ridge regions of the LIPSS.

1. INTRODUCTION Conjugated polymers are widely used in organic electronics

1-4

. Organic photovoltaics (OPVs)

have emerged as a challenging new research field during the past decades. Compared with classical inorganic systems, OPVs offer many potential advantages, such as mechanical flexibility, applicability for “roll-to-roll” processing, low-cost fabrication and mass production, among others

5-6

. The active layer in OPV bulk heterojunction (BHJ) devices is usually

composed of a blend of π-conjugated electron-donor polymers and electron-acceptor small molecules. Polymer/fullerene BHJ solar cells have proven power conversion efficiencies (PCEs) of over 11% now reported

7-11

. Despite the success of the fullerene derivatives acceptors, they


Page 3 of 37 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

Langmuir

suffer from several limitations which significantly constrain the development of new donor materials; these include limited optical absorption, costly production, and fixed band alignment 12-16

. Therefore, recent studies have focused on developing and understanding alternative

acceptors. Among them π-conjugated electron-transporting polymers have been investigated and achieved excellent PCEs > 8%

15-18

. All-polymer solar cells (all-PSCs) with photoactive layers

composed of a donor−acceptor polymer blend have proved many advantages over polymer−fullerene system: tunable chemical and electronic properties of the blend components, enhanced light absorbance and thermal and mechanical properties

18-21

. Polymer donor and

acceptor materials with complementary absorption spectra in the vis-NIR region should be chosen for improving PCEs of the all-PSCs. Additionally, the morphology, the phase separation and the interfacial area between the donor and the acceptor polymers in the blend forming the BHJ, are critical factors for the performance of the OPV devices

16, 18, 22-24

. The fabrication of

active layers with periodic structures has been implemented as an effective way to improve light harvesting

25

. Polymer micro- and nano-structures for OPV applications have been prepared by

several strategies

25-30

. Laser-induced periodic surface structures (LIPSS)

31

under ambient

conditions is a cheap and versatile alternative to other lithography methods. LIPSS in the form of ripples develop on the polymer surface as a result of irradiation with a linearly polarized laser beam due to a heterogeneous intensity distribution produced by the interference between the incoming and the surface scattered waves, which together with a feedback mechanism, ends in the enhancement of the modulation intensity

32

. The ripple period is determined by the laser

wavelength, the incidence angle and by the effective refractive index of the material 31. Recently we have reported the formation of LIPSS on P3HT as a model semiconducting polymer

33-34

. It

was shown that LIPSS fabricated at 266 nm present structures with a lower degree of order than


Langmuir 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

Page 4 of 37

those created with the irradiation wavelength of 532 nm. Differences in the quality of the structures, in terms of regular lengths and size of the ripples, are related to the light absorption coefficient of the material at each wavelength. In addition, it was shown the heterogeneous electrical conductivity of the structured P3HT films, comprising alternation of conducting valleys and nonconducting hills. A reduction of the crystallinity of the hills was proposed as the reason of these effects based on Raman spectroscopy and X-ray scattering measurements. In this work, we extend the fabrication of LIPSS to a mainly amorphous semiconducting polymer

like

poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30

benzothiadiazole)] (PCDTBT) with the main aim of investigating the effect of the absorption coefficient of the material at the laser wavelength and the crystallinity on LIPSS quality and on the electrical conductivity, respectively. In addition, after proving the formation of LIPSS in semiconducting homopolymers like P3HT and PCDTBT we have investigated the feasibility of fabricating LIPSS in the P3HT/PCDTBT (1:1) blend, as a model BHJ for all-PSCs. 2. EXPERIMENTAL 2.1 Materials and sample preparation Poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30

benzothiadiazole)]

(PCDTBT) and poly(3-hexylthiophene) (P3HT) were purchased from Ossila. PCDTBT (Mw = 35400 g/mol, PDI = 2.4), P3HT (Mw = 34100 g/mol, PDI = 1.7; regioregularity = 94.7%). The chemical structures of both materials are shown in Figure S1. Solutions of P3HT and PCDTBT in chlorobenzene (24 mg/mL) were prepared and stirred during several hours until complete dissolution. Afterwards solutions were blended with a weight


Page 5 of 37 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

Langmuir

ratio of 1:1. Thin polymer films were prepared by spin-coating on the polished surface of silicon wafers (100) (ACM, France). The wafers were previously cleaned with acetone and isopropanol. A fixed amount of 0.1 mL of polymer solution was dropped by a syringe on a silicon substrate placed in the center of a rotating horizontal plate. A rotation rate of 2400 rpm was kept during 60 s. For the C-AFM measurements, thin films were prepared on arsenic n-doped silicon substrates (resistivity ≈ 0.001 Ωcm; Wafer World, Inc.) under the same conditions. For NEXAFS measurements, polymer solutions were spin-coated on silicon substrates, then floated off into a very dilute NaOH/water solution (0.25 wt %), and finally picked up with transmission electron microscopy (TEM) grids. An averaged thickness of 165 nm for the P3HT/PCDTBT blend was estimated by measuring thicknesses of about 140 nm and 190 nm for P3HT and PCDTBT domains respectively. 2.2 Laser irradiation Laser irradiation was carried out under ambient air conditions at normal incidence with a linearly polarized laser beam of a Q-switched Nd:YAG laser (Lotis TII LS-2131M, pulse duration of 8 ns) at two different wavelengths, i.e., using the second (532 nm) and fourth harmonic (266 nm) of the fundamental laser emission at a repetition rate of 10 Hz. These wavelengths were selected for the experiments because P3HT, PCDTBT and the blend of both components absorb efficiently as it is shown in Figure 1. While the absorption coefficient at 532 nm is almost the same for the blend and its components with a value of about 6.0 x 104 cm-1, the absorption coefficient at 266 nm has a value of 1.0 x 104 cm-1 for P3HT, 6.2 x 104 cm-1 for PCDTBT and 3.2 x 104 cm-1 for the blend. The irradiation fluences were determined by measuring the laser energy in front of the sample and considering 5 mm as the diameter of the laser spot. The fluences used for fabrication of LIPSS were well below the ablation threshold of the polymer films. 5 ACS Paragon Plus Environment

Langmuir

532 nm

266 nm 4

8,0x10

-1

Absorption coefficient (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

Page 6 of 37

4

6,0x10

4

4,0x10

4

2,0x10

0,0 200

400

600

800

Wavelength (nm) Figure 1. UV-visible absorption spectra of thin films of P3HT (red the blend P3HT/PCDTBT (green

), PCDTBT (blue

) and

).

2.3 Scanning probe measurements Atomic Force Microscopy (AFM) measurements were carried out using a commercial scanning probe microscope (MultiMode 8 equipped with a Conductive AFM module (C-AFM) and the Nanoscope V controller, Bruker). The topography AFM images were collected in tapping mode using silicon probes (NSG30 probes by NT-MDT). Heights and periods were measured in three different zones of the sample and analyzed by Nanoscope Analysis 1.50 software (Bruker). Electrical measurements were performed by C-AFM with conductive tips (Pt−Ir covered Si probes with a low spring constant, k = 0.2 N m−1, SCM-PIC by Bruker) in contact mode by measuring simultaneously both topography and electrical images. For out-of plane current 6 ACS Paragon Plus Environment

Page 7 of 37 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

Langmuir

measurements, the sample was attached with conductive epoxy (CW2400, Chemtronics) onto a metal support. In these measurements, the conducting probe makes contact with the sample, acting like a nanoelectrode, and maps a current image at a fixed bias. The bias was applied to the conducting substrate, and the current was measured by a preamplifier 35. 2.4 Ultraviolet-Visible Spectroscopy Absorption spectra of polymer thin films spin-coated on quartz substrates were recorded in transmission geometry using a UV-visible spectrophotometer (UV-3600, Shimadzu). 2.5 Raman Spectroscopy Raman spectroscopy was accomplished by using a Renishaw Raman InVia Reflex Spectrophotometer, equipped with a Leica Microscope and an electrically refrigerated CCD camera, with excitation lines at 442 nm (HeCd laser), 532 nm (Nd:YAG laser), and 785 nm (diode laser). The spectra were acquired with a spectral resolution of 2 cm−1 using a 50× magnification objective to a spot with a diameter of about 1 µm on the samples. Laser power conditions were those that ensured the integrity of the polymer. 2.6 Grazing Incidence X-ray Scattering Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) measurements were performed at the synchrotron beamline BW4 at HASYLAB (DORIS, DESY) in Hamburg. The experimental setup has been described in a previous study 36. An X-ray wavelength of λ= 0.13808 nm, with a beam size (horizontal x vertical) of 40 x 20 µm2 and an exposure time of 4 min was used. A MarCCD detector of 2048 x 2048 pixels with a resolution of 79.1 µm per pixel was used for recording the scattered intensity. A sample−detector distance of 10.6 cm was selected to 7 ACS Paragon Plus Environment

Langmuir 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

Page 8 of 37

investigate the inner film structure. The incident angle (αi) was set to 0.40º, which is above the critical angles of P3HT and PCDTBT, in order to probe the structure of the full film thickness 32. Patterns were analyzed by the Fit2D software 37. 2.7 X-ray Microscopy Scanning Transmission X-ray Spectro-Microscopy (STXM), with image contrast based on NearEdge X-ray Absorption Fine Structure (NEXAFS) spectromicroscopy, was performed at the PolLux beamline

38-40

at the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland.

Thin films supported on TEM grids were placed on the sample chamber which was evacuated to low vacuum. The transmitted X-ray intensity through the film was recorded using a scintillator and photo-multiplier tube and measured as a function of energy (275.0-345.0 eV with a resolution of 0.1 eV) and position. The X-ray focus was ~ 25 nm. Transmitted X-ray intensity was converted to an X-ray optical density (defined as OD= -ln (I/I0)) by recording the X-ray intensity (I0) through an empty TEM grid. The aXis2000 software package

41

was used for the

analysis of images. 3. RESULTS AND DISCUSSION 3.1 LIPSS fabrication on PCDTBT thin films We have studied the influence of the number of pulses and laser fluence on the LIPSS morphology at laser wavelengths of 532 and 266 nm by means of AFM. 3.1.1 Dependence on fluence and number of pulses


Page 9 of 37 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

Langmuir

As-cast PCDTBT films show quite smooth surfaces (with mean surface roughness Ra = 1.2 nm) without morphological signs of crystallization, as revealed by AFM (Figure 2a). Nevertheless, the GIWAXS pattern (Figure S2) of the as-cast film reveals a small degree of crystallinity, as two weak reflections the 100 and the 010 are observed. Ripples parallel to the polarization vector of the laser are observed after laser irradiation under certain fluence conditions, well below the ablation threshold. Figures 2 a) and b) show, together with the AFM image of the non-irradiated sample, selected height images of PCDTBT films irradiated at λ = 532 nm with different fluences and number of pulses respectively.

Figure 2. AFM height images (5 × 5 µm2) of PCDTBT: a) non-irradiated and irradiated at 532 nm with 3600 pulses at different fluences. b) Irradiated at 532 nm with a fluence of 26 mJ/cm2 as


Langmuir 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

Page 10 of 37

a function of number of pulses. The height profile along a 4 µm line perpendicular to the ripples is shown below every image. The arrows denote the polarization vector of the laser.

The dependence of the period and depth of LIPSS with the fluence, at λ = 532 nm and 3600 pulses as derived from AFM analysis, is represented in Figure 3a. It shows that the period increases up to a fluence of ~31.2 mJ/cm2 and remains practically constant afterward with a value slightly smaller than the irradiation wavelength. The depth of ripples follows a similar tendency as the period, increasing up to 120 nm for ~36.4 mJ/cm2 and reaching a plateau. The dependence of period and depth of LIPSS at a constant fluence of 26 mJ/cm2 with the number of pulses is represented in Figure 3b. It shows that the period increases slightly up to ~3600 pulses and reaches a steady level for higher number of pulses. The depth of the ripples increases up to ~6000 pulses reaching a plateau of about 150 nm.


Page 11 of 37

a)

Z (nm)

L (nm)

450 400 350 100 50 0

15

30

45

Fluence (mJ/cm2) b)

L (nm)

450 400 350

Z (nm)

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

Langmuir

100 50 0 0

2500

5000

7500

10000

Number of pulses Figure 3. Variation of periods (L, filled circles) and depths (Z, empty circles) of LIPSS in PCDTBT as a function of (a) fluence at a constant number of pulses of 3600 and (b) number of pulses at a constant fluence of 26 mJ/cm2 for the laser irradiation wavelength λ = 532 nm.

LIPSS of smaller dimensions were fabricated by irradiating the PCDTBT thin films at λ = 266 nm. Selected height AFM images of PCDTBT films irradiated with different fluences and number of pulses are shown in Figure S3 of the supplementary information.


Langmuir 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

Page 12 of 37

The dependence of the period and depth of LIPSS with the fluence, at λ = 266 nm and 3600 pulses is represented in Figure 4a. It is shown that the period remains practically constant with a value of about 250 nm in the investigated fluence range of 13.4 – 21.7 mJ/cm2. The depth of ripples follows a similar tendency as the period, remaining almost constant at 30 nm in the investigated fluence range. The dependence of period and depth of LIPSS at a constant fluence of 13.4 mJ/cm2 with the number of pulses is shown in Figure 4b. It shows that the period increases slightly up to ~3600 pulses and reaches a steady level for higher number of pulses. The depth of the ripples increases up to 100 nm for ∼9000 pulses reaching a plateau and decreasing above ∼15000 pulses when the ripples start to deteriorate. Finally, it has been proved that under certain conditions of fluence and number of pulses LIPSS morphology is highly reproducible with an averaged error of about 5-10% in period and depth.


Page 13 of 37

260

L (nm)

a)

240

Z (nm)

220 90 60 30 0 12

15

18

21

24

Fluence (mJ/cm2)

L (nm)

b)

Z (nm)

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

Langmuir

260 240 220 90 60 30 0 0

4000

8000 12000 16000 20000

Number of pulses Figure 4. Variation of periods (L, filled circles) and depths (Z, empty circles) of LIPSS in PCDTBT as a function of (a) fluence at a constant number of pulses of 3600 and (b) number of pulses at a constant fluence of 13.4 mJ/cm2 for the laser irradiation wavelength λ = 266 nm.

3.1.2 Chemical stability and structural modification in nanostructured PCDTBT thin films 3.1.2.1 Near Edge X-ray Absorption Fine Structure


Langmuir 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

Page 14 of 37

NEXAFS experiments were performed in order to gain further information about the chemical stability of the PCDTBT thin films after laser irradiation at 532 nm. A NEXAFS spectrum in the K-edge reveals the excitation of 1s electrons to unfilled molecular orbitals. The NEXAFS spectra in the carbon K-edge of pristine thin films and of those with LIPSS are presented in Figure 5. The π*(C=C) bands in both cases are similar, suggesting that the π-backbone is not affected by laser irradiation. The main difference is found in the band σ*(C-H) with the apparition of a new band near 288 eV, probably related to a weak modification of the polymer lateral chains. Overall, NEXAFS results suggest the absence of significant changes in the chemical structure of PCDTBT after LIPSS formation.

Figure 5. NEXAFS spectra in the carbon K-edge of PCDTBT thin film (blue) and of PCDTBT film with LIPSS fabricated at 532 nm, 26 mJ/cm2, and 9000 pulses (black). The spectra have been vertically shifted for clarity.


Page 15 of 37 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

Langmuir

3.1.2.2 Raman Spectroscopy Figure 6 shows the Raman spectra of PCDTBT thin films (blue) and PCDTBT with LIPSS fabricated at 532 nm, 26 mJ/cm2, and 7500 pulses (black) at the different excitation wavelengths used (λexc = 785, 532, and 442 nm). As it is shown in Figure 1 and also reported in the literature 42-43

, PCDTBT thin films present optical absorption between 250 and 600 nm. The UV/Vis

absorption band at 395 nm is attributed to π–π* transition, whereas the absorption band at 570 nm is known to be due to charge transfer from donor carbazole unit to acceptor DTBT unit

42

.

This indicates that excitation at 785 nm occurs under nonresonance conditions, whereas excitation at 532 and 442 nm leads to measurements under resonance conditions (Figure 6).


Langmuir 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

Page 16 of 37

Figure 6. Raman spectra of PCDTBT thin films (blue) and LIPSS in PCDTBT (black) at different excitation wavelengths, λexc labelled in the upper left corners. Stars (*) in the top panel correspond to silicon bands.

The PCDTBT Raman spectra show several characteristic bands. The one present with high intensity at every excitation wavelength and located at 1444 cm-1 is a mode assigned to a broad ring stretch focused on the DTBT acceptor unit that is delocalized across the benzothiadiazole and the two thiophenes

43

. The 1541 cm-1 mode corresponds to the benzothiadiazole ring

stretching mode, and the band at 1623 cm-1 is associated with the ring stretching mode of the carbazole. Peaks at 1349 and 1373 cm-1 are attributed to the C–C stretching mode of the carbazole and DTBT unit, respectively 42, 44-45. The modes for lower frequency Raman bands at 844 and 874 cm-1, present under resonance conditions but almost completely absent under nonresonance conditions, are difficult to assign unambiguously but are tentatively assigned as inplane bending modes of the DTBT group 42. No significant changes are observed in PCDTBT spectra upon laser irradiation (Figure 6). There is no evidence of new bands and only slight changes of the intensity ratio between few bands can be detected. These differences are difficult to interpret but the main bands are maintained, suggesting that the PCDTBT thin films evidence a weak impact on its chemical structure under the irradiation conditions used for fabricating LIPSS. 3.1.3 Electrical properties of nanostructured PCDTBT thin films


Page 17 of 37 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

Langmuir

The electrical properties at the nanoscale of LIPSS fabricated by irradiating a PCDTBT thin film at 532 nm were investigated by C-AFM. Figure 7 shows the C-AFM current images (electric current map), obtained in contact mode, of a PCDTBT thin film sample before irradiation (Figure 7a) and after irradiation with a fluence of 26 mJ/cm2 and 9000 pulses (Figure 7b) by applying a constant voltage of -5 V on the conducting substrate. Before irradiation the current image is almost homogeneous throughout the film, whereas the current image of the structured PCDTBT presents stripes with conductivity comparable to that of the as-cast thin film separated by nonconductive ones. Comparing the topography and current images measured simultaneously, we can conclude that conductive regions correspond to trenches and nonconductive regions to ridges.

Figure 7. C-AFM current images of a PCDTBT thin film measured at a constant bias of -5 V. (a) Non-irradiated film, (b) film with LIPSS fabricated at 532 nm, 26 mJ/cm2, and 9000 pulses.

We explained a similar behavior found for LIPSS in P3HT

33

, based on Raman spectroscopy

and in situ GIWAXS experiments, considering that during irradiation the surface melts, leading to a ripple morphology characterized by the existence of low crystallinity and nonconducting 17 ACS Paragon Plus Environment

Langmuir 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

Page 18 of 37

ridges over a continuous and more-conducting P3HT residual layer whose initial crystallinity seems to be unaffected in comparison to that of the pristine P3HT thin film. Although the as-cast PCDTBT thin film presents a very low crystallinity (Figure S2) and no differences between the Raman spectra of non-irradiated films and those with LIPSS are observed (Figure 6), it is known that the annealing of PCDTBT thin films at temperatures close to its melting point improves side-chain order while the π−π stacking is reduced

35, 46

. Accordingly, we suggest that the

temperature reached during LIPSS formation has a similar effect in the surface of PCDTBT thin films. Thus, due to the distortion of π−π stacking the conductivity of the ridges in the nanostructured sample is reduced. 3.2 LIPSS fabrication on P3HT/PCDTBT (1:1) thin films 3.2.1 Dependence on fluence and number of pulses After proving the formation of LIPSS in semiconducting homopolymers like P3HT

33

and

PCDTBT we have investigated the feasibility of fabricating LIPSS in the P3HT/PCDTBT (1:1) blend, which has been used as bulk-heterojunction acting as the active layer in organic solar cells 47

. As-cast films of P3HT/PCDTBT (1:1) present the characteristic morphology of lateral phase

separation (Figure 8a), indicating immiscibility of the two polymers

33

. The blend thin films

present a 50 nm difference in height between P3HT and PCDTBT domains for an average film thickness of 165 nm. Previous NEXAFS experiments

35

allowed us to identify the lowest

domains as the P3HT ones surrounded by higher domains of PCDTBT. Figures 8 a) and b) show, together with the AFM image of the initial nonirradiated sample, selected height images of P3HT/PCDTBT (1:1) films irradiated at λ = 532 nm with different fluences and number of pulses respectively. Both, P3HT and PCDTBT domains present well defined LIPSS. 18 ACS Paragon Plus Environment

Page 19 of 37 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

Langmuir

Figure 8. AFM height images (5 × 5 µm2) of P3HT/PCDTBT: a) Nonirradiated and irradiated at 532 nm with 3600 pulses at different fluences. b) Irradiated at 532 nm with a fluence of 26 mJ/cm2 as a function of number of pulses. The height profile along a 4 µm line perpendicular to the ripples is shown below every image. The arrows indicate the polarization vector of the laser.

The dependence of the period and depth of LIPSS with the fluence, at λ = 532 nm and 3600 pulses, is represented in Figure 9a. It is observed that the period increases up to a fluence of ~16 mJ/cm2 and reaches a plateau with a value slightly smaller than the irradiation wavelength. While the measured period is similar for both phases, the depth of ripples follows a similar tendency as the period but they appear for PCDTBT at lower fluence than for P3HT and the depth of ripples is higher for PCDTBT in the range of fluences studied. The dependence of 19 ACS Paragon Plus Environment

Langmuir

period and depth of LIPSS at a constant fluence of 26 mJ/cm2 with the number of pulses is represented in Figure 9b. It shows that the period increases slightly up to ~900 pulses and reaches a steady level for higher number of pulses. The depth of the ripples increases up to ~2400 pulses reaching a value of about 150 nm for PCDTBT and about 90 nm for P3HT. For higher number of pulses up to 6000, the depth of the ripples for PCDTBT decreases matching the depth of P3HT ripples.

a)

L (nm)

450

Z (nm)

500

100

400

50 0 10

20

30

40

Fluence (mJ/cm2) b)

L (nm)

500 450

Z (nm)

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

Page 20 of 37

100

400

50 0 0

1500

3000

4500

6000

Number of pulses Figure 9. Variation of periods L (filled circles) and depths Z (P3HT phase red empty squares and PCDTBT phase blue empty triangles) of LIPSS as a function of (a) fluence at a constant


Page 21 of 37 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

Langmuir

number of pulses of 3600 and (b) number of pulses at a fluence of 26 mJ/cm2 for the laser irradiation wavelength λ = 532 nm.

LIPSS in the blend were also fabricated by irradiating at λ = 266 nm. Selected height AFM images of P3HT/PCDTBT (1:1) films irradiated with different fluences and number of pulses are shown in Figure S4 of the supplementary information. The dependence of the period and depth of LIPSS with the fluence, at λ = 266 nm and 3600 pulses is represented in Figure 10a. It is shown that the period remains practically constant with a value of about 250 nm in the investigated fluence range. The ripples appear for PCDTBT at lower fluence than for P3HT and the depth of ripples is higher for PCDTBT in the range of fluences studied. In addition the depth of the ripples increases up to ~13 mJ/cm2 reaching a value of about 100 nm for PCDTBT and about 90 nm for P3HT, for higher fluences the depth of the ripples for P3HT decreases slightly while for PCDTBT decreases abruptly. The dependence of period and depth of LIPSS at a constant fluence of 13.4 mJ/cm2 with the number of pulses is represented in Figure 10b. It shows that the period increases up to ~3600 pulses and reaches a plateau for higher number of pulses. The depth of the ripples also increases up to ~3600 pulses reaching a similar value of about 90 nm for both P3HT and PCDTBT, for higher number of pulses up to 6000 the depth of the ripples decreases faster for PCDTBT than for P3HT. This behavior could be explained by a faster process of LIPSS formation for PCDTBT than for P3HT, PCDTBT needs lower fluence and number of pulses in order to develop the ripple morphology and therefore they start to deteriorate (depth decrease) also at lower fluence and number of pulses compared to P3HT in the blend.


Langmuir

Z (nm)

L (nm)

a)

270 240 210 90 60 30 0 11

12

13

14

15

Fluence (mJ/cm2) L (nm)

b)

Z (nm)

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

Page 22 of 37

270 240 210 90 60 30 0 0

2000

4000

6000

8000

Number of pulses Figure 10. Variation of periods L (filled circles) and depths Z (P3HT red empty squares and PCDTBT blue empty triangles) of LIPSS as a function of (a) fluence at a constant number of pulses of 3600 and (b) number of pulses at a fluence of 13.4 mJ/cm2 for the laser irradiation wavelength λ = 266 nm.

It is interesting to point out the synergy found for both components in the P3HT/PCDTBT blend when irradiating at wavelengths of 532 nm and 266 nm. P3HT and PCDTBT in the blend need, at a constant number of pulses, less fluence than in the case of irradiating the homopolymer films in order to optimize LIPSS morphology. The same behavior is found when irradiating the 22 ACS Paragon Plus Environment

Page 23 of 37 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

Langmuir

blend at a constant fluence, P3HT and PCDTBT components need less number of pulses in order to optimize LIPSS morphology than in the case of irradiating the homopolymer films. 3.2.2 Quantitative chemical composition mapping The STXM technique provides quantitative chemical composition of organic films with nanoscale resolution (~20 nm)

48-49

by the mixture of spectroscopy and imaging capacities.

Providing information on the domain sizes, shapes, and purities of thin film polymer blends 50.

Figure 11. Reference NEXAFS spectra for films of P3HT (red line) and PCDTBT (blue line) irradiated at 532 nm with a fluence of 26 mJ/cm2, 3600 and 9000 pulses respectively. Dashed arrows point out the photon energies used to acquire the raw STXM images (5 µm x 5 µm) of the 23 ACS Paragon Plus Environment

Langmuir 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

Page 24 of 37

same sample area of a P3HT/PCDTBT (1:1) thin film irradiated at 532 nm with a fluence of 26 mJ/cm2 and 3600 pulses, situated at the top. The inset emphasizes differences in the NEXAFS spectra of P3HT and PCDTBT in the C-1s / π* region.

In a previous work we have used the STXM technique in order to investigate the composition and phase separation in P3HT/PCDTBT (1:1) blends as a function of thickness 35. In the present work we are interested in extending the use of the STXM technique to investigate whether the mechanisms of LIPSS formation may induce either a further phase segregation or a mixture of the components. We have utilized the singular value decomposition (SVD) method

48, 51

to

acquire quantitative chemical composition of the blend thin film irradiated at 532 nm. Fig. 11 shows the mass absorption coefficients, required by the SVD procedure, as a function of energy for P3HT and PCDTBT homopolymers irradiated at 532 nm of known thickness measured before irradiation. The transmitted X-ray intensity measured was converted to an X-ray optical density (defined as = − ⁄ ), where I and I0 are the transmitted and incident intensity, respectively. Since = , where µ is the mass absorption coefficient, ρ is the density and t the sample thickness, we have considered density values of 1.13 and 1.33 g/cm3 for P3HT and PCDTBT respectively

52

in order to calculate the mass absorption coefficient of each material.

STXM images (Fig. 11 top) of the same area of the P3HT/PCDTBT (1:1) thin film irradiated at 532 nm were taken at the following energy values: 280 eV (preedge), 284.2 eV (PCDTBT resonance), 287.8 eV (P3HT resonance) and 320 eV (chemically insensitive). By using the aXis2000 software package 41 where the SVD method is implemented, the information contained within the series of STXM images acquired at different energies can be converted into maps quantifying the composition and thickness of the sample in each pixel 48, 51. The composition and 24 ACS Paragon Plus Environment

Page 25 of 37 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

Langmuir

thickness maps of the P3HT/PCDTBT (1:1) irradiated at 532 nm are shown in Fig. 12, revealing the chemical nature of the different domains and the morphology of thinner P3HT-rich domains surrounded by a thicker PCDTBT-rich matrix. The thicknesses of domains and the height of ridges and trenches are in agreement with the AFM measurements, considering that STXM takes into account the complete sample thickness and AFM only the structured surface. In addition it seems that the mechanisms of LIPSS formation do not induce a further phase segregation compared to the blend before laser irradiation.

Figure 12. (5 µm x 5 µm) images of quantitative composition (a) and thickness (b) maps of the P3HT/PCDTBT (1:1) blend irradiated at 532 nm, calculated from the series of raw images in Fig. 11 top.


Langmuir 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

Page 26 of 37

3.2.3 Chemical stability and structural modification in nanostructured P3HT/PCDTBT (1:1) thin films The Raman spectra for the P3HT/PCDTBT (1:1) blend is practically an arithmetic average of the individual components, as it is shown in Figure S5 of the supplementary information for the different excitation conditions used (λexc = 785, 532, and 442 nm). No new bands are observed.

Figure 13. (a) Raman spectra of non-irradiated P3HT/PCDTBT thin films (green) and films with LIPSS fabricated at 532 nm, 26 mJ/cm2 and 2400 pulses (black) under different excitation wavelengths. Stars (*) in the top panel correspond to silicon bands. (b) Comparison of the ν(C=C) band region of P3HT before (green) and after laser irradiation (black).


Page 27 of 37 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

Langmuir

Figure 13 shows the Raman spectra of P3HT/PCDTBT (1:1) blend thin films (green) and P3HT/PCDTBT (1:1) blend with LIPSS fabricated at 532 nm, 26 mJ/cm2 and 2400 pulses (black) at the different excitation wavelengths employed. We focus on the 1400−1500 cm−1 spectral range (enlarged in Figure 13b) where the band centered at 1445 cm-1 is the convolution of the PCDTBT and the P3HT bands. As we have pointed out no evidence of new bands is observed for PCDTBT non-irradiated films and those with LIPSS (Figure 6), hence the changes observed in Figure 13b between the spectra of P3HT/PCDTBT (1:1) non-irradiated films and those with LIPSS can be attributed to the P3HT component. It seems that in the blend the P3HT behaves in a similar way that it does as pure homopolymer. In a previous work

33

, we have

reported that for the pure P3HT with LIPSS, focusing on the 1400− 1500 cm−1 spectral range, under resonance conditions (λexc = 442 nm), corresponding to a region of high absorption for disordered P3HT chains, the ν(C=C) band shifts toward higher wavenumbers where it presents a shoulder, marked by an arrow in Figure 13b (bottom). This shoulder can be assigned to a relative increase of the amorphous phase after laser irradiation

33, 53

. The Raman spectra collected at an

excitation wavelength of 532 nm, for the P3HT with LIPSS, also present the mentioned shoulder but in less extension. This effect may be related to the fact that under these resonance conditions (λexc = 532 nm), P3HT segments located in ordered regions absorb more strongly than those in the amorphous phase 54. Under nonresonance conditions (λexc = 785 nm) for both the P3HT thin film and the P3HT with LIPSS, the Raman spectra do not present noticeable variations, only a small increase of the width of the ν(C=C) band located at 1445 cm-1 is appreciated. However, we have previously observed

33

that the pure P3HT with LIPSS, focusing on the 1400− 1500 cm−1

spectral range, presents a shoulder at 1420 cm−1 on the lower wavenumber region of the ν(C=C) 27 ACS Paragon Plus Environment

Langmuir 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

Page 28 of 37

band located at 1445 cm-1. This shoulder is assigned to the stretching of the C=C bond of the quinoid form of P3HT because of the oxidation of the aromatic backbone

55-56

. The absence of

the mentioned shoulder in the blend could be explained either by the prevention of the oxidation of P3HT aromatic backbone due to the presence of high stable PCDTBT, or by the limit of detection of Raman taking into account that in the blend we have 50% of P3HT. 3.2.4 Electrical properties of nanostructured P3HT/PCDTBT (1:1) thin films Current imaging by C-AFM was utilized to investigate conductivity variations in the P3HT/PCDTBT (1:1) thin films structured by LIPSS. Figure 14 shows the C-AFM current images, obtained in contact mode, of an as-cast P3HT/PCDTBT thin film (Figure 14a) and a structured one with a fluence of 26 mJ/cm2 and 3600 pulses (Figure 14b) by applying a constant bias of -5 V on the conducting substrate. The P3HT/PCDTBT (1:1) thin film shows conductive strips (dark region) separated by nonconductive ones (bright region). In combination with the compositional maps extracted from STXM experiments, we can assign the conductive strips to the P3HT-rich domains and the nonconductive ones to PCDTBT 35. On the other hand, the image showing the current map of the structured thin film polymer blend presents a similar trend followed by the irradiated homopolymer thin films, consisting in valleys with almost the same conductivity that the pristine thin film separated by nonconductive hills.


Page 29 of 37 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

Langmuir

Figure 14. C-AFM current images of a P3HT/PCDTBT thin films measured at a constant bias of -5 V. (a) Nonirradiated film, (b) film with LIPSS fabricated at 532 nm, 26 mJ/cm2, and 3600 pulses.

The C-AFM results obtained for the irradiated P3HT/PCDTBT (1:1) thin film can be explained by the same basis considered for the irradiated homopolymer thin films. During irradiation, melting of the blend surface takes place leading to ripple morphology into the P3HT and PCDTBT domains, characterized by the existence of low crystallinity and nonconducting ridges over a continuous and more-conducting either P3HT or PCDTBT residual domain layer whose initial crystallinity, in the case of P3HT, and order of π−π stacking, in the case of PCDTBT, seems to be unaffected in comparison to that of the non-irradiated P3HT/PCDTBT (1:1) thin film. 4. CONCLUSIONS In summary, patterning by LIPSS has been extended to the amorphous conjugated polymer PCDTBT as well as to the P3HT/PCDTBT (1:1) photovoltaic blend. The high absorption


Langmuir 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

Page 30 of 37

coefficient of PCDTBT at the laser wavelengths used for LIPSS fabrication is the reason for the good quality and well-ordered nanostructures obtained at both, 266 and 532 nm. As-cast films of P3HT/PCDTBT (1:1) present the characteristic morphology of lateral phase separation, with 50 nm difference in height between P3HT and PCDTBT domains for an average film thickness of 165 nm. In spite of the heterogeneous roughness of the blend both, P3HT and PCDTBT domains present well defined LIPSS. It is interesting to point out not only the feasibility to fabricate LIPSS in an all-polymer bulk heterojunction, but also the synergy found for both components in the P3HT/PCDTBT blend when irradiating at wavelengths of 532 nm and 266 nm. P3HT and PCDTBT in the blend need, at a constant number of pulses, less fluence than in the case of irradiating the homopolymers in order to optimize the LIPSS morphology. The same behavior is found when irradiating the blend at a constant fluence: P3HT and PCDTBT components need less number of pulses in order to optimize LIPSS morphology than in the case of irradiating the homopolymers. NEXAFS and Raman spectroscopy measurements reveal good chemical stability of PCDTBT and the blend thin films under the laser irradiation conditions used for LIPSS formation. Concerning the heterogeneous electrical conductivity shown by the irradiated P3HT/PCDTBT (1:1) blend, it can be explained by the same basis considered for the irradiated homopolymer thin films. During irradiation the surface of the blend melts leading to a ripple morphology into the P3HT and PCDTBT domains, characterized by the existence of low crystallinity and nonconducting ridges over a continuous and more-conducting either P3HT or PCDTBT residual domain layer whose initial crystallinity in the case of P3HT and order of π−π stacking in the case of PCDTBT seems to be unaffected in comparison to that of the nonirradiated P3HT/PCDTBT (1:1) thin film.


Page 31 of 37 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

Langmuir

Finally, on one hand the patterning by LIPSS of one of the components of the active layer with periodic structures will increase the interfacial area between the donor and the acceptor in bilayer all-polymer solar cells, being in this case more convenient to produce ripples with 266 nm period compared to 532 nm. On the other hand, patterning by LIPSS the bulk heterojunction active layer will increase the interfacial area between the active layer and the top electrode and also can increase the light harvesting due to internal multiple reflections into the periodic nanostructures. ASSOCIATED CONTENT Supporting Information PDF file with 5 figures. Figure S1: Chemical structures of P3HT and PCDTBT. Figure S2: 2D GIWAXS pattern of the as-cast PCDTBT thin film. Figure S3: AFM high images of PCDTBT irradiated at 266 nm with different fluences and number of pulses. Figure S4: AFM high images of the P3HT/PCDTBT blend irradiated at 266 nm with different fluences and number of pulses. Figure S5: Raman spectra of thin films of P3HT, PCDTBT and P3HT/PCDTBT excited at different conditions. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT 31 ACS Paragon Plus Environment

Langmuir 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

Page 32 of 37

The authors gratefully acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness (MINECO) through Projects MAT2014-59187-R, MAT2015-66443-C02 1-R and CTQ2016-75880-P. A.R.-R. and E.R. are indebted to MINECO for a FPI (BES-2013062620) and Ramón y Cajal (RYC-2011-08069) contracts, respectively. We thank the Swiss Light Source for beamtime at PolLux and B. Watts for assistance during the NEXAFS and STXM experiments. REFERENCES (1) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Twodimensional charge transport in self-organized, high-mobility conjugated polymers. Nature 1999, 401 (6754), 685-688. (2) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Liquid-crystalline semiconducting polymers with high charge-carrier mobility. Nat Mater 2006, 5 (4), 328-333, DOI: http://www.nature.com/nmat/journal/v5/n4/suppinfo/nmat1612_S1.html. (3) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A. A high-mobility electron-transporting polymer for printed transistors. Nature 2009, 457 (7230), 679-686, DOI: http://www.nature.com/nature/journal/v457/n7230/suppinfo/nature07727_S1.html. (4) Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature 2004, 428 (6986), 911-918. (5) Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Advanced Materials 2014, 26 (1), 10-28, DOI: 10.1002/adma.201304373. (6) Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chemical Reviews 2014, 114 (14), 7006-7043, DOI: 10.1021/cr400353v. (7) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat Photon 2012, 6 (9), 591-595, DOI: http://www.nature.com/nphoton/journal/v6/n9/abs/nphoton.2012.190.html#supplementaryinformation. (8) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.; So, F.; Reynolds, J. R. Dithienogermole As a Fused Electron Donor in Bulk Heterojunction Solar Cells. Journal of the American Chemical Society 2011, 133 (26), 10062-10065, DOI: 10.1021/ja204056m. (9) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells. Nature Communications 2014, 5, 5293, DOI: 10.1038/ncomms6293 32 ACS Paragon Plus Environment

Page 33 of 37 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

Langmuir

https://www.nature.com/articles/ncomms6293#supplementary-information. (10) He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y. Single-junction polymer solar cells with high efficiency and photovoltage. Nat Photon 2015, 9 (3), 174-179, DOI: 10.1038/nphoton.2015.6 http://www.nature.com/nphoton/journal/v9/n3/abs/nphoton.2015.6.html#supplementaryinformation. (11) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient organic solar cells processed from hydrocarbon solvents. Nature Energy 2016, 1, 15027, DOI: 10.1038/nenergy.2015.27 https://www.nature.com/articles/nenergy201527#supplementary-information. (12) Zang, Y.; Li, C.-Z.; Chueh, C.-C.; Williams, S. T.; Jiang, W.; Wang, Z.-H.; Yu, J.-S.; Jen, A. K. Y. Integrated Molecular, Interfacial, and Device Engineering towards High-Performance Non-Fullerene Based Organic Solar Cells. Advanced Materials 2014, 26 (32), 5708-5714, DOI: 10.1002/adma.201401992. (13) Earmme, T.; Hwang, Y.-J.; Subramaniyan, S.; Jenekhe, S. A. All-Polymer Bulk Heterojuction Solar Cells with 4.8% Efficiency Achieved by Solution Processing from a CoSolvent. Advanced Materials 2014, 26 (35), 6080-6085, DOI: 10.1002/adma.201401490. (14) Mu, C.; Liu, P.; Ma, W.; Jiang, K.; Zhao, J.; Zhang, K.; Chen, Z.; Wei, Z.; Yi, Y.; Wang, J.; Yang, S.; Huang, F.; Facchetti, A.; Ade, H.; Yan, H. High-Efficiency All-Polymer Solar Cells Based on a Pair of Crystalline Low-Bandgap Polymers. Advanced Materials 2014, 26 (42), 7224-7230, DOI: 10.1002/adma.201402473. (15) Lin, Y.; Zhang, Z.-G.; Bai, H.; Wang, J.; Yao, Y.; Li, Y.; Zhu, D.; Zhan, X. Highperformance fullerene-free polymer solar cells with 6.31% efficiency. Energy & Environmental Science 2015, 8 (2), 610-616, DOI: 10.1039/C4EE03424D. (16) Jung, J. W.; Jo, J. W.; Chueh, C.-C.; Liu, F.; Jo, W. H.; Russell, T. P.; Jen, A. K. Y. FluoroSubstituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%. Advanced Materials 2015, 27 (21), 3310-3317, DOI: 10.1002/adma.201501214. (17) Li, H.; Hwang, Y.-J.; Courtright, B. A. E.; Eberle, F. N.; Subramaniyan, S.; Jenekhe, S. A. Fine-Tuning the 3D Structure of Nonfullerene Electron Acceptors Toward High-Performance Polymer Solar Cells. Advanced Materials 2015, 27 (21), 3266-3272, DOI: 10.1002/adma.201500577. (18) Gao, L.; Zhang, Z.-G.; Xue, L.; Min, J.; Zhang, J.; Wei, Z.; Li, Y. All-Polymer Solar Cells Based on Absorption-Complementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%. Advanced Materials 2016, 28 (9), 1884-1890, DOI: 10.1002/adma.201504629. (19) Schubert, M.; Collins, B. A.; Mangold, H.; Howard, I. A.; Schindler, W.; Vandewal, K.; Roland, S.; Behrends, J.; Kraffert, F.; Steyrleuthner, R.; Chen, Z.; Fostiropoulos, K.; Bittl, R.; Salleo, A.; Facchetti, A.; Laquai, F.; Ade, H. W.; Neher, D. Correlated Donor/Acceptor Crystal Orientation Controls Photocurrent Generation in All-Polymer Solar Cells. Advanced Functional Materials 2014, 24 (26), 4068-4081, DOI: 10.1002/adfm.201304216. (20) Kim, T.; Kim, J.-H.; Kang, T. E.; Lee, C.; Kang, H.; Shin, M.; Wang, C.; Ma, B.; Jeong, U.; Kim, T.-S.; Kim, B. J. Flexible, highly efficient all-polymer solar cells. Nature Communications 2015, 6, 8547, DOI: 10.1038/ncomms9547


Langmuir 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

Page 34 of 37

https://www.nature.com/articles/ncomms9547#supplementary-information. (21) Bin, H.; Zhang, Z.-G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. NonFullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2D-Conjugated Polymers Reach 9.5% Efficiency. Journal of the American Chemical Society 2016, 138 (13), 4657-4664, DOI: 10.1021/jacs.6b01744. (22) Guo, S.; Wang, W.; Herzig, E. M.; Naumann, A.; Tainter, G.; Perlich, J.; MüllerBuschbaum, P. Solvent–Morphology–Property Relationship of PTB7:PC71BM Polymer Solar Cells. ACS Applied Materials & Interfaces 2017, 9 (4), 3740-3748, DOI: 10.1021/acsami.6b14926. (23) Zhou, N.; Dudnik, A. S.; Li, T. I. N. G.; Manley, E. F.; Aldrich, T. J.; Guo, P.; Liao, H.-C.; Chen, Z.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Olvera de la Cruz, M.; Marks, T. J. AllPolymer Solar Cell Performance Optimized via Systematic Molecular Weight Tuning of Both Donor and Acceptor Polymers. Journal of the American Chemical Society 2016, 138 (4), 12401251, DOI: 10.1021/jacs.5b10735. (24) Li, Z.; Xu, X.; Zhang, W.; Meng, X.; Ma, W.; Yartsev, A.; Inganäs, O.; Andersson, M. R.; Janssen, R. A. J.; Wang, E. High Performance All-Polymer Solar Cells by Synergistic Effects of Fine-Tuned Crystallinity and Solvent Annealing. Journal of the American Chemical Society 2016, 138 (34), 10935-10944, DOI: 10.1021/jacs.6b04822. (25) Na, S.-I.; Kim, S.-S.; Jo, J.; Oh, S.-H.; Kim, J.; Kim, D.-Y. Efficient Polymer Solar Cells with Surface Relief Gratings Fabricated by Simple Soft Lithography. Advanced Functional Materials 2008, 18 (24), 3956-3963, DOI: 10.1002/adfm.200800683. (26) Aryal, M.; Trivedi, K.; Hu, W. Nano-Confinement Induced Chain Alignment in Ordered P3HT Nanostructures Defined by Nanoimprint Lithography. ACS Nano 2009, 3 (10), 3085-3090, DOI: 10.1021/nn900831m. (27) Pfadler, T.; Coric, M.; Palumbiny, C. M.; Jakowetz, A. C.; Strunk, K.-P.; Dorman, J. A.; Ehrenreich, P.; Wang, C.; Hexemer, A.; Png, R.-Q.; Ho, P. K. H.; Müller-Buschbaum, P.; Weickert, J.; Schmidt-Mende, L. Influence of Interfacial Area on Exciton Separation and Polaron Recombination in Nanostructured Bilayer All-Polymer Solar Cells. ACS Nano 2014, 8 (12), 12397-12409, DOI: 10.1021/nn5064166. (28) Chen, D.; Zhao, W.; Russell, T. P. P3HT Nanopillars for Organic Photovoltaic Devices Nanoimprinted by AAO Templates. ACS Nano 2012, 6 (2), 1479-1485, DOI: 10.1021/nn2043548. (29) Sarkar, K.; Schaffer, C. J.; Gonzalez, D. M.; Naumann, A.; Perlich, J.; Muller-Buschbaum, P. Tuning the pore size of ZnO nano-grids via time-dependent solvent annealing. Journal of Materials Chemistry A 2014, 2 (19), 6945-6951, DOI: 10.1039/C4TA00489B. (30) Steirer, K. X.; Reese, M. O.; Rupert, B. L.; Kopidakis, N.; Olson, D. C.; Collins, R. T.; Ginley, D. S. Ultrasonic spray deposition for production of organic solar cells. Solar Energy Materials and Solar Cells 2009, 93 (4), 447-453, DOI: https://doi.org/10.1016/j.solmat.2008.10.026. (31) Rebollar, E.; Castillejo, M.; Ezquerra, T. A. Laser induced periodic surface structures on polymer films: From fundamentals to applications. European Polymer Journal 2015, 73, 162174, DOI: http://dx.doi.org/10.1016/j.eurpolymj.2015.10.012. (32) Martín-Fabiani, I.; Rebollar, E.; Pérez, S.; Rueda, D. R.; García-Gutiérrez, M. C.; Szymczyk, A.; Roslaniec, Z.; Castillejo, M.; Ezquerra, T. A. Laser-Induced Periodic Surface Structures Nanofabricated on Poly(trimethylene terephthalate) Spin-Coated Films. Langmuir 2012, 28 (20), 7938-7945, DOI: 10.1021/la300833x. 34 ACS Paragon Plus Environment

Page 35 of 37 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

Langmuir

(33) Rodríguez-Rodríguez, Á.; Rebollar, E.; Soccio, M.; Ezquerra, T. A.; Rueda, D. R.; GarciaRamos, J. V.; Castillejo, M.; Garcia-Gutierrez, M.-C. Laser-Induced Periodic Surface Structures on Conjugated Polymers: Poly(3-hexylthiophene). Macromolecules 2015, 48 (12), 4024-4031, DOI: 10.1021/acs.macromol.5b00804. (34) Cui, J.; Rodríguez-Rodríguez, Á.; Hernández, M.; García-Gutiérrez, M.-C.; Nogales, A.; Castillejo, M.; Moseguí González, D.; Müller-Buschbaum, P.; Ezquerra, T. A.; Rebollar, E. Laser-Induced Periodic Surface Structures on P3HT and on Its Photovoltaic Blend with PC71BM. ACS Applied Materials & Interfaces 2016, 8 (46), 31894-31901, DOI: 10.1021/acsami.6b09053. (35) Rodríguez-Rodríguez, Á.; Soccio, M.; Martínez-Tong, D. E.; Ezquerra, T. A.; Watts, B.; García-Gutiérrez, M.-C. Competition between phase separation and structure confinement in P3HT/PCDTBT heterojunctions: Influence on nanoscale charge transport. Polymer 2015, 77, 7078, DOI: http://dx.doi.org/10.1016/j.polymer.2015.09.012. (36) Perlich, J.; Rubeck, J.; Botta, S.; Gehrke, R.; Roth, S. V.; Ruderer, M. A.; Prams, S. M.; Rawolle, M.; Zhong, Q.; Körstgens, V.; Müller-Buschbaum, P. Grazing incidence wide angle xray scattering at the wiggler beamline BW4 of HASYLAB. Review of Scientific Instruments 2010, 81 (10), 105105, DOI: doi:http://dx.doi.org/10.1063/1.3488459. (37) Hammersley, A. FIT2D Website. http://www.esrf.eu/computing/scientific/FIT2D 2010. (38) Raabe, J.; Tzvetkov, G.; Flechsig, U.; Böge, M.; Jaggi, A.; Sarafimov, B.; Vernooij, M. G. C.; Huthwelker, T.; Ade, H.; Kilcoyne, D.; Tyliszczak, T.; Fink, R. H.; Quitmann, C. PolLux: A new facility for soft x-ray spectromicroscopy at the Swiss Light Source. Review of Scientific Instruments 2008, 79 (11), 113704, DOI: doi:http://dx.doi.org/10.1063/1.3021472. (39) Flechsig, U.; Quitmann, C.; Raabe, J.; Böge, M.; Fink, R.; Ade, H. The PolLux Microspectroscopy Beam line at the Swiss Light Source. AIP Conference Proceedings 2007, 879 (1), 505-508, DOI: doi:http://dx.doi.org/10.1063/1.2436109. (40) Frommherz, U.; Raabe, J.; Watts, B.; Stefani, R.; Ellenberger, U. Higher Order Suppressor (HOS) for the PolLux Microspectroscope Beamline at the Swiss Light Source SLS. AIP Conference Proceedings 2010, 1234 (1), 429-432, DOI: doi:http://dx.doi.org/10.1063/1.3463232. (41) Hitchcock, A. P. aXis 2000—analysis of X-ray images and spectra. http://unicorn.mcmaster.ca/aXis2000.html 2000. (42) Reish, M. E.; Nam, S.; Lee, W.; Woo, H. Y.; Gordon, K. C. A Spectroscopic and DFT Study of the Electronic Properties of Carbazole-Based D–A Type Copolymers. The Journal of Physical Chemistry C 2012, 116 (40), 21255-21266, DOI: 10.1021/jp307552z. (43) Jha, P.; Koiry, S. P.; Saxena, V.; Veerender, P.; Gusain, A.; Chauhan, A. K.; Debnath, A. K.; Aswal, D. K.; Gupta, S. K. Air-stability and bending properties of flexible organic fieldeffect transistors based on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole)]. Organic Electronics 2013, 14 (10), 2635-2644, DOI: http://dx.doi.org/10.1016/j.orgel.2013.06.031. (44) Provencher, F.; Bérubé, N.; Parker, A. W.; Greetham, G. M.; Towrie, M.; Hellmann, C.; Côté, M.; Stingelin, N.; Silva, C.; Hayes, S. C. Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions. Nat Commun 2014, 5, DOI: 10.1038/ncomms5288. (45) Martin, E. J. J.; Berube, N.; Provencher, F.; Cote, M.; Silva, C.; Doorn, S. K.; Grey, J. K. Resonance Raman spectroscopy and imaging of push-pull conjugated polymer-fullerene blends. Journal of Materials Chemistry C 2015, 3 (23), 6058-6066, DOI: 10.1039/C5TC00847F. 35 ACS Paragon Plus Environment

Langmuir 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

Page 36 of 37

(46) Wang, T.; Pearson, A. J.; Dunbar, A. D. F.; Staniec, P. A.; Watters, D. C.; Yi, H.; Ryan, A. J.; Jones, R. A. L.; Iraqi, A.; Lidzey, D. G. Correlating Structure with Function in Thermally Annealed PCDTBT:PC70BM Photovoltaic Blends. Advanced Functional Materials 2012, 22 (7), 1399-1408, DOI: 10.1002/adfm.201102510. (47) Liu, X.; Huettner, S.; Rong, Z.; Sommer, M.; Friend, R. H. Solvent Additive Control of Morphology and Crystallization in Semiconducting Polymer Blends. Advanced Materials 2012, 24 (5), 669-674, DOI: 10.1002/adma.201103097. (48) Ade, H.; Hitchcock, A. P. NEXAFS microscopy and resonant scattering: Composition and orientation probed in real and reciprocal space. Polymer 2008, 49 (3), 643-675, DOI: 10.1016/j.polymer.2007.10.030. (49) Ade, H.; Stoll, H. Near-edge X-ray absorption fine-structure microscopy of organic and magnetic materials. Nature Materials 2009, 8 (4), 281-290, DOI: 10.1038/nmat2399. (50) Watts, B.; McNeill, C. R.; Raabe, J. Imaging nanostructures in organic semiconductor films with scanning transmission X-ray spectro-microscopy. Synthetic Metals 2012, 161 (23–24), 2516-2520, DOI: http://dx.doi.org/10.1016/j.synthmet.2011.09.016. (51) Koprinarov, I. N.; Hitchcock, A. P.; McCrory, C. T.; Childs, R. F. Quantitative Mapping of Structured Polymeric Systems Using Singular Value Decomposition Analysis of Soft X-ray Images. The Journal of Physical Chemistry B 2002, 106 (21), 5358-5364, DOI: 10.1021/jp013281l. (52) Lu, X.; Hlaing, H.; Germack, D. S.; Peet, J.; Jo, W. H.; Andrienko, D.; Kremer, K.; Ocko, B. M. Bilayer order in a polycarbazole-conjugated polymer. Nat Commun 2012, 3, 795, DOI: http://www.nature.com/ncomms/journal/v3/n4/suppinfo/ncomms1790_S1.html. (53) Gao, Y.; Grey, J. K. Resonance Chemical Imaging of Polythiophene/Fullerene Photovoltaic Thin Films: Mapping Morphology-Dependent Aggregated and Unaggregated C═C Species. Journal of the American Chemical Society 2009, 131 (28), 9654-9662, DOI: 10.1021/ja900636z. (54) Shrotriya, V.; Ouyang, J.; Tseng, R. J.; Li, G.; Yang, Y. Absorption spectra modification in poly(3-hexylthiophene):methanofullerene blend thin films. Chemical Physics Letters 2005, 411 (1–3), 138-143, DOI: http://dx.doi.org/10.1016/j.cplett.2005.06.027. (55) Shi, G.; Xu, J.; Fu, M. Raman Spectroscopic and Electrochemical Studies on the Doping Level Changes of Polythiophene Films during Their Electrochemical Growth Processes. The Journal of Physical Chemistry B 2002, 106 (2), 288-292, DOI: 10.1021/jp013023o. (56) Yu, W.; Zhou, J.; Bragg, A. E. Exciton Conformational Dynamics of Poly(3hexylthiophene) (P3HT) in Solution from Time-Resolved Resonant-Raman Spectroscopy. The Journal of Physical Chemistry Letters 2012, 3 (10), 1321-1328, DOI: 10.1021/jz3003298.


Page 37 of 37 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

Langmuir

SYNOPSIS


Patterning Conjugated Polymers by Laser: Synergy of Nanostructure

Recommend Documents