Using Femtosecond Laser Irradiation to Enhance the Vertical

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Using femtosecond laser irradiation to enhance the vertical electrical properties and tailor the morphology of a conducting polymer blend film Sangmin Chae, Ahra Yi, Cheolmin Park, Won Seok Chang, Hyun-Hwi Lee, Jiyeon Choi, and Hyo Jung Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05937 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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ACS Applied Materials & Interfaces

Using femtosecond laser irradiation to enhance the vertical electrical properties and tailor the morphology of a conducting polymer blend film

Sangmin Chae12, Ahra Yi1, Cheol Min Park2, Won Seok Chang2, Hyun Hwi Lee3 *, Jiyeon Choi2*, and Hyo Jung Kim1* 1. Department of Organic Material Science and Engineering, Pusan National University, Busan 46241, South Korea 2. Korea Institute of Machinery and Materials (KIMM), 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, South Korea 3. Pohang Accelerator Laboratory, POSTECH , Pohang 37673, South Korea

e-mail to: [email protected], [email protected], [email protected],

keywords: conducting polymer, orientation, morphology, charge transport, laser direct writing, polymer solar cells

ACS Paragon Plus Environment


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Abstract We report femtosecond infrared laser induced selective tailoring of carrier transport as well as surface morphology on a conducting polymer blend thin film. Maximal 2.4 times enhancement on vertical current transport in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, was achieved by this irradiation. The laser irradiation induced a photo expansion without deteriorating its molecular structure and the film morphology could be customized in micron scale by adjusting the laser writing parameters. In the photo-expanded region, the face-on populations were about 2.2 times larger in comparison with the pristine region, which was a major contributor to the enhanced carrier transport.


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Organic solar cell (OSC)-based conjugated polymers have attracted a great deal of attention due to their low-cost processing and flexible application characteristics.1,2 OSCs are usually fabricated using bulk hetero-junction (BHJ) structures, which are mixtures of donor and acceptor semiconductors. For high device efficiency, suitable phase separation and nanoscale ordering of molecules are required for highly interpenetrating networks and superior charge transport.3-6 In vertical devices such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs), charges are injected and extracted in the vertical direction, and these devices hence require enhanced levels of charge transport in this direction. In recent years, various strategies such as annealing processing, additive solvent treatment, use of a block copolymer, layer-by-layer deposition, the template method, and nano-imprint lithography (NIL) have been proposed to better control the morphology as well as structure of the organic solar cell.7-10 Among them, NIL is a particularly successful emerging method, which can well define such morphology because of its high patterning resolution and good fidelity. Charge carrier transport has been reported to be enhanced by NIL-induced alignment of the polymer chains and improved light trapping within the nanoimprinted active layer.11-14 However, NIL also has certain disadvantages, such as mask contamination and complicated processing due to the high temperature and pressure required for this technique. Femtosecond laser direct writing is a versatile maskless technique for microprocessing delicate materials such as (in)organic thin films, brittle glasses, and biomaterials due to its nearly non-invasive nature, which results from its extremely short pulse width preventing unnecessary heat transfer alongside the irradiated area.15-18 Direct writing with a micrometer-sized focused beam enables rapid (i.e., within a few minutes) structuration on square-inch films.19 In addition, a compatible technology for OSC has been described with



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optimal formation of laser-induced periodic surface structures (LIPSS). Ripples created by LIPSS with multiple laser pulses in P3HT and PCBM have been successfully formed and shown the possibility into organic photovoltiacs.20,21 Recently, it has been reported that a single pulse femtosecond laser irradiation could induce alkyl chain alignment along a polarization direction of the laser in a poly(3-hexylthiophene) (P3HT) : [6,6]-phenyl-C61butyric methyl ester (PC61BM) blend film, which is an archetypal system in conjugated polymers for OSCs.22 The chain alignment in a certain direction could be advantageous to increase electrical properties in conjugated polymer systems. However, the effects of the femtosecond laser irradiation on the electrical properties of the film have not been entirely undiscovered. In addition, the detail of the laser process on the conjugated polymer systems has not been reported yet. Herein, we firstly report a systematic tailoring and enhanced electrical properties in the P3HT:PC61BM film by the femtosecond laser. The tailoring of the morphology and vertical charge current of the film after the laser irradiation was confirmed by taking conductive atomic force microscopy (c-AFM), and the structural alteration of the irradiated film was elucidated by micro-Raman and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. P3HT:PCBM blend films with thicknesses of 180 nm (which is a common bulkheterojunction structure for basic studies) deposited on ITO/glass substrates were used for the direct laser writing. The wavelength and the pulse duration of the laser were 1030 nm and 300 fs, respectively. An Yb-doped fiber amplifier was used in this experiment. Figure 1 shows the AFM images with height profiles of a surface of P3HT:PCBM subjected to a laser fluence of 0.076 J/cm² followed by a laser fluence of 0.095 J/cm² and finally a laser fluence of 0.100 J/cm². During this three-step treatment, the surface was observed to correspondingly


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expand in three distinct steps. At the low applied fluence, the P3HT:PCBM surface began to expand and form irregularly shaped dots, as shown in Figure 1a. The dots were much smaller than the diameter of the focused beam, indicating that only the central part of the Gaussianshaped beam had a sufficient intensity to initiate the modification. This process was denoted as Step 1 or initiation. As the laser fluence was increased, the irradiated surface showed more dots that combined to form a line, as shown in Figure 1b. This process was denoted as Step 2 or expansion. The height of the photo-expanded line pattern grew as the laser fluence was increased, and the maximum height achieved was almost the original P3HT:PCBM film thickness. The maximum height exhibiting a Gaussian bump was produced at a laser fluence of 0.095 J/cm², which is about 95% of the ablation threshold fluence. When the laser irradiation power was increased to the ablation threshold fluence, the bump apparently started to collapse and to yield two split lines, as shown in Figure 1c. This stage was denoted as Step 3 or ablation.

Debris was only observed to form during Step 3 due to the removal of the

polymer by the laser ablation. Meanwhile, the photo-expansion process did not generate such debris during Step 2, as no removal of material was involved. The laser fluence in Step 2, ranging from approximately 80~95% of threshold fluence, was thus used throughout the investigation to fabricate photo-expanded Gaussian-shaped structures. The photo-expanded morphology resembled a Gaussian shape, reflecting the transverse laser beam profile. Therefore, when using the laser photo-expansion process, the morphology of the irradiated material may be tailored by designing both customized transverse profiles of the incident laser beam and by designing the writing pattern.



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Figure 1.

The photo-expansion process as a function of laser fluence. (a) During the

initiation stage, when the laser fluence was 0.076 J/cm², dot-shaped structures formed and expanded on the surface. (b) During the photo-induced expansion stage, when the laser fluence was 0.095 J/cm², the dot-shaped structures formed a line that then thickened. (c) During the ablation stage, when the fluence was 0.100 J/cm², split lines formed. The molecular structural stability of the photo-expanded area of P3HT:PCBM film was elucidated using micro-Raman analysis, which is widely used to analyze the chemical composition and intramolecular structure by detecting the vibrational modes of the molecules. Figure 2a shows Raman spectra between 600 and 1600 cm-1 of a pristine area of the P3HT:PCBM film and of irradiated areas of this film under three different laser powers of 0.076 J/cm2, 0.095 J/cm2, and 0.100 J/cm2, respectively. A beam size of 1μm was used to obtain sufficient spatial resolution. Two major peaks were observed in these Raman spectra, at 1381 cm-1 and 1445 cm-1, which corresponded to symmetric C-C intra-stretch and C=C stretch modes, respectively.23 Figure 2b shows magnified views of these spectra at the locations of these peaks. The shapes and positions of the peaks of the C-C and C=C modes in the Raman spectrum of the area of the film irradiated with the low laser power (black line, bottom panel of Fig. 2b) were nearly identical to those of the Raman spectrum of the pristine


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(non-irradiated) area (blue line), indicating that the chemical composition and intramolecular structure of the film during the initiation stage were not affected by the laser process. The very similar Ic-c/Ic=c intensity ratios resulting from these irradiated and pristine areas indicated that the planarity of the polymer backbones was preserved.23 The peaks resulting from the area irradiated with medium laser power, 0.095 J/cm2 were also fairly similar to the abovedescribed peaks, indicating the film in the expansion stage displayed a similar behavior to the sample in the initiation stage. On the other hand, the film irradiated with the highest laser power, 0.100 J/cm2 showed a significantly decreased C-C peak intensity, implying that the P3HT backbone ruptured due to the intense laser irradiation over the ablation threshold fluence. These results suggest that a fabrication based on the photo-expansion process can be a clean and nondestructive process, and hence be valuable for organic optoelectronic materials and devices.

Figure 2. (a) Micro-Raman spectra of a pristine (non-irradiated) area of the P3HT:PCBM thin film and of irradiated areas of this film under three different laser powers of (initiation) 0.076 J/cm2, (expansion), 0.095 J/cm2, and (ablation) 0.100 J/cm2, respectively. (b) The normalized two major Raman modes (C-C and C=C modes) of the P3HT:PCBM blend films at three



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different stages (ablation at top side, expansion at middle side, and initiation at bottom side, respectively). To demonstrate the ability to tailor the morphology of a film using direct laser writing with the photo-expansion process, a P3HT:PCBM thin film with micro-dots was prepared as shown in Fig. 3. Each dot had a dome-like shape and was formed by the photoexpansion process with a single laser pulse. The prepared structure was obviously different from the above thick line, which illustrates that one can tailor the surface morphology to include features with various sizes and shapes by simply tuning the laser writing process parameters. To examine local electrical characteristics of this embossed film, we took conductive AFM (c-AFM) measurements, which are suitable for creating a vertical current mapping of patterned samples in the nanometer-to-micron scale. For the c-AFM measurement depicted in Fig 3, a conductive Pt tip and the ITO film underneath the blend film were used as top and bottom electrodes, respectively. Figure 3a shows a topological height image of the photoexpanded P3HT:PCBM film showing distinct dot structures reaching as high as 100 nm. Figure 3b shows an image of the simultaneously obtained current map of this same film. The photo-expanded region (red line) apparently yielded a lower current value than the pristine region (blue line) in the raw current map, as shown in Fig 3d. However, this decrease did not imply a deterioration of electrical properties in the photo-expanded region, as these electrical properties were instead enhanced in this region. According to an analytic model derived for a conductive AFM experiment on an intrinsic semiconductor, the current density (J) is ଷ

௎మ

ଶ

௅య

inversely proportional to the cube of the film thickness (L) as described by J = ߳ߤ

, where

ε and µ are the permittivity and the charge-carrier mobility, respectively, and U is the applied voltage, with a value of 5 V applied for this experiment.24 Therefore, the expected current


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value at photo-expanded area can be calculated from the measured height and current density. Then, this expected value can be compared to a pristine region with equivalent height at the same position, as shown in figures 3c and 3d. The average calculated current density of the expanded region at the ridges (green square) have exhibited 136(±38) pA. In figure 3e, the maximal calculated current density of the expanded region at the ridge was ~2.4 times higher than the average current density of the pristine region. This current enhancement at photoexpanded area showed a trend quite similar to that of the height profile. These results illustrated that the photo-expansion process enhanced the vertical current flow.

Figure 3. (a) Topological height image of the P3HT:PCBM film with photo-expanded microdots and (b) simultaneously obtained current AFM map of this same film. (c,d,e) Line-cut graphs of c) the height images, (d) measured current images, and (e) calculated current images derived from the pristine and photo-expanded regions of the P3HT:PCBM film. GIWAXS measurements were conducted in order to determine whether and to what extent molecular orientation would be associated with the vertical current enhancement after the laser irradiation because crystal orientation is one of main factors affecting charge carrier transport. The GIWAXS data has been carried out according to the procedure described in the



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ref.25 Figure 4a and 4b show 2D GIWAXS images of the pristine and photo-expanded areas using incidence angles of 0.10°. The pristine area exhibited strong (h00) diffraction along the merdian direction, as shown in figure 4a, with this diffraction originating from the P3HT edge-on configuration. On the other hand, the expanded area yielded a slight decrease in outof-plane (h00) diffraction intensity compared to that from the pristine area, but a significant appearance of both out-of-plane (010) and in-plane (100) peaks, which corresponded to a face-on configuration. To quantitatively compare the face-on configurations, figures 4c shows the fractional angular populations plotting of P3HT (100) crystals by integration process in a spherical coordinate.26-28 The tilt angular range could be divided by three sections (two angular regimes of the 0°~30° and 60°~90° as those corresponding to the populations of edge-on and face-on crystallites). Notably, the photo-expanded area yielded 54% increased the face-on populations than that resulting from the pristine area (24%). On the other hands, the edge-on populations of the photo-expanded area was decreased into 16% from 63%. These results suggest that the photo-expansion process efficiently transformed the P3HT orientation from edge-on to face-on. The details of crystal information are summarized in table 1. Interestingly, the intensities of the peaks derived from PCBM were also largely greater for the expanded area than for the pristine area, which may have been due to both the ordering and increase in quantity of the PCBM clusters. Therefore, a transformation of P3HT to a face-on orientation and a phase separation in the blend film resulted from the photoexpansion process, which in turn was induced by irradiation of our film with a femtosecond laser. Both this transformation and phase separation may have contributed to vertical carrier transport and hence resulted in the outstanding current enhancement.


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Figure 4. 2D GIWAXS images of the pristine (a) and expanded (b) regions at incidence angles of 0.10˚. (c) Angular volume fractions of the P3HT crystals in the pristine (blue line) and expanded (red line) films, as a function of the tilt angle.

Samples

Crystal

Fractional

Size (nm)

Population

Vertical

Pristine

15.9

62.8%

(edge-on)

Expanded

13.9

16.3%

Horizontal

Pristine

12.3

24.2%

(face-on)

Expanded

12.2

53.7%

Table 1. Summary of the GIWAXS results measured at 0.1° incidence angles in the pristine and photo-expanded films.



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In summary, a photo-expansion process resulting from femtosecond laser irradiation was demonstrated. By adjusting laser parameters, this process could be used to easily modify and tailor the morphology of the P3HT:PCBM blend film. The photo-expansion induced the P3HT face-on orientation and improved the vertical charge transport of the polymer semiconductor, which is desirable for vertical-direction optoelectronics. We believe that the photo-expansion process would be attractively applied for BHJ organic solar cells which are highly demanded appropriate phase separation and vertical charge transport. Moreover, direct laser writing scheme is readily adoptable not only in fabrication process of OPV, but also in post process to mend or enhance the local properties of given OPVs.

Acknowledgements This work was supported by the Center for Advanced Meta-Materials(CAMM) funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CAMM- No. 2014M3A6B3063707), the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean Government (NRF2015M2A2A6A03044907) and was also supported by the World Class 300 project (S2317456) of the Small and Medium Business Administration (SMBA, Korea).


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ToC figure


Using Femtosecond Laser Irradiation to Enhance the Vertical

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