Carbon Nanosheet from Polyethylene Thin Film as a Transparent

Subscriber access provided by University of South Dakota

Article

Carbon Nanosheet from Polyethylene Thin Film as a Transparent Conducting Film: ‘Upcycling’ of Waste to Organic Photovoltaics Application Dalsu Choi, Jun-Seok Yeo, Han-Ik Joh, and Sungho Lee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b03066 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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 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 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.

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 28 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

ACS Sustainable Chemistry & Engineering

Carbon Nanosheet from Polyethylene Thin Film as a Transparent Conducting Film: ‘Upcycling’ of Waste to Organic Photovoltaics Application

Dalsu Choi†,1, Jun-Seok Yeo†,1, Han-Ik Joh§, Sungho Lee*,†,‡

AUTHOR ADDRESS †

Carbon Composite Materials Research Center, Korea Institute of Science and Technology, 92

Chudong-ro, Bongdong-eup, Wanju-gun, Jeonbuk 55324 Republic of Korea ‡

Department of Nano Material Engineering, Korea University of Science and Technology, 217

Gajeong-ro, Yuseong-gu, Daejeon 34113 Republic of Korea §

Department of Energy Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu,

Seoul 05029 Republic of Korea

Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 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 28

KEYWORDS Transparent Conducting Film, Upcycling, PE, Graphitic Carbon, Organic Photovoltaics,

ABSTRACT

In this article, we demonstrate the successful transformation of a polyethylene (PE) thin film into an effective transparent conducting film (TCF). Through thermal oxidative stabilization, an innately non-carbonizable PE thin film could survive carbonization process and converted into a carbon nanosheet (CNS). Interestingly, unlike other CNSs fabricated from pricey sources, those from PE, one of the cheapest polymers, included highly ordered graphitic moieties confirmed by combination of Raman spectra analyses and transmission electron microscopy (TEM). Inclusion of graphitic moieties endowed superior electrical conductivity to PE based CNSs over other CNSs and TCFs without harshly compromising other figures of merits as TCFs. Effectiveness of CNSs as TCFs was assessed by fabricating organic photovoltaic cells (OPVs). OPVs structured on PE CNSs successfully operated and exhibited sound efficiencies. More significantly, PE wastes could be equivalently transformed into CNSs and OPVs as pristine PE did, suggesting possible pathway providing extra financial value to PE wastes; it is worthy to note that PE waste is the largest polymer waste in volume and is mostly buried or burnt. Thus, the technology introduced in this article has profound potential as an effective solution to deal with environmental problems associated with PE wastes by envisioning viable ‘upcycling’ pathway of the wastes.


2

Page 3 of 28 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


Introduction Transparent conductive film (TCF) is a quintessential component of many modern electronic devices such as touch panels and displays.1–4 Currently, indium tin oxide (ITO) is the most abundantly used material as TCF due to its exceptional electrical conductivity and stability.5–7 However, the high-cost manufacturing process of ITO requiring high vacuum and scarcity of indium urge research efforts for low cost substitutes of ITO.2,3 Moreover, the paradigm shift in electronic device configuration from conventional rigid form toward flexible/stretchable analogue also drives significant efforts for ITO substitutes.8–10 ITO is too brittle to be used for next-generation flexible electronic devices. Therefore, new types of TCFs, which promise material integrity upon deformation, have garnered lots of traction. Especially, those can be processed without vacuum process, more specifically solution-proccessible TCFs enabling lowcost large area fabrication, have received a major attention.1–4 Several classes of solution processed flexible TCFs including films from dispersions of metal nanowires, CNTs and graphene derivatives (i.e. graphene/GO/rGO) have been reported as viable flexible substitutes of ITO.1,3–5,8,10 Percolated network of nanowires, CNTs, graphene, GO or rGO served well as TCFs with sound mechanical flexibility. However, these dispersion based TCFs rely upon pricey conductive particles and still bear cost related problems. Recently, a new type of TCF other than dispersion based TCFs has been introduced. Carbon nanosheeet (CNS), a several nanometerthick carbon thin film derived from a relatively cheap polymeric thin film, exhibited good electrical, optical and mechanical properties as a flexible TCF.2,11–13 Various materials such as polymer with intrinsic microporosity (PIM), coal-tar pitch and polyacrylonitrile (PAN) have been utilized as precursors for CNS fabrication and electronic devices including flexible organic


3


Page 4 of 28

photovoltaic cells (OPV) and organic field effect transistors (OFET) were successfully built based on CNSs. Here, we demonstrate CNS fabrication based on PE, the most widely used polymer and one of the cheapest polymers.14,15 Solution processed PE thin films were successfully transformed into CNSs with graphitic moieties, which in turn envisioned the fabrication of TCFs based on lowcost materials. Interestingly, PE CNSs processed under relatively low temperature carbonization (1200 oC) comprised graphitic structures, which were not observed in other CNSs fabricated with the exactly same condition.2,12,13,16,17 These well-ordered carbon features contributed to high electrical conductivity of PE CNSs reaching ~ 1100 S/cm, which is superior to CNSs from other precursors and films with percolated rGO or GO network. Operable OPVs with power conversion efficiency of ~ 2 % were successfully structured based on PE CNS with graphitic structures. Furthermore, we confirmed that even PE wastes can be equivalently utilized for CNS production as pristine PE did, implicating that the proposed technique endows extra economic values to PE waste and provides viable ‘upcycling’ pathway.

Result and Discussions Scheme 1 describes the fabrication process of PE CNS. In this paper, linear low density PE was selected and used as a representative PE derivative. First, PE solutions were prepared in toluene with varied PE weight percent. Next, respective solutions were spin casted on quartz


4

Page 5 of 28 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


substrates and PE thin films with thickness ranging from 0.64 µm to 1.6 µm were prepared (Table S1). Thickness of pristine polymer films and their carbonized analogues are provided in Table S1. Resulting thin films were thermally oxidized in a convection oven at 270 oC for 4 hrs. Samples were then carbonized under nitrogen at 1200 oC. Upon carbonization, CNSs with a high carbon atomic percent of ~ 97 % (Figure S1) were successfully fabricated from the PE thin films. We reported successful transformation of bulk PE into a carbonaceous material through thermal oxidative pre-treatment. Mechanistic details about structural changes during conversion were discussed in the previous study.18 Here, we focused on chemical structural changes of PE in the thin film configuration.

Scheme 1. Sketch depicting fabrication process of PE based carbon nanosheet (CNS)

Raman spectra of the thin films after thermal oxidation and carbonization were sequentially analyzed to track the structural changes (Figure 1-a). While pristine PE thin films simply showed Raman bands at ~ 2800 cm-1 corresponding to C-H single bond,19,20 thermal oxidation resulted in the appearance of two peaks at 1400 and 1600 cm-1, which are respectively known as D and G bands from carbon structure.21–23 However, since thermally oxidized PE did not yet form carbon structure, the Raman band at 1400 cm-1, which is commonly related to defects in


5


Page 6 of 28

carbon structure, should be carefully interpreted. The origin of Raman band at 1400 cm-1 is known as emergence of the band from the breathing vibrational mode of six-membered benzene ring; The relationship between defects in carbon structure and 1400 cm-1 Raman band was also established based on this physical origin.24,25 Therefore, even without formation of carbon structure, the presence of benzene rings in the chemical structure itself gives a rise of the peak at 1400 cm-1. This interpretation discerned the peak found in thermally oxidized PE films as a consequence of formation of polyaromatic moieties. Similarly, occurrence of the G band was related to stretching vibrational mode of carbon double bond rather than associating it as a fingerprint of carbon structure.26,27 In sum, Raman bands at 1400 and 1600 cm-1 provided clues about chemical structure of thermally oxidized PE thin films. Formation of benzene rings and carbon double bonds were apparent from Raman spectra analysis and confirmed that the chemical structural changes during thermal oxidation in thin films were similar to those reported in the previous study on thermal oxidation of bulk state PE.18 These structural developments provide thermal stability to molecules and direct rearrangement into carbon materials, rather than break down into volatile species, when processed under harsh carbonization condition. Most carbon precursors require such chemical structural transformation for successful carbonization and the process in charge of the transformation is called ‘stabilization’.28,29 As thermal oxidation was adopted as a pre-treatment protocol for effective carbonization in this study, the term ‘stabilization’ and ‘thermal oxidation’ will be interchangeably used in this study. Next, from Xray photoelectron spectroscopy (XPS) analyses, the attachment of oxygen functional groups as another characteristic structural changes upon thermal oxidation was investigated. Figure 1-b represents deconvoluted XPS spectra of thermally oxidized PE thin film. Both C1s and O1s scans (Figure 1-b, inset) suggested the attachment of oxygen functional groups including C-O


6

Page 7 of 28 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


and C=O, resembling chemical structural changes observed in a bulk analogue. Coupled with XPS spectra analysis, the atomic composition of the film was quantified. The result indicated introduction of large amount of oxygen reaching 23.5 atomic %. Furthermore, the information about quantified atomic composition of oxygen functional groups was also assessed (Table S2).

Figure 1. a) Raman spectra of thin films from their pristine state to respectively processed state b) Deconvoluted C1s XPS spectra of stabilized PE thin film. Inset is O1s scan of the sample

Subsequently, carbonized films were examined via Raman spectroscopy (Figure 1-a). Interestingly, unlike the thermally oxidized thin film, which experienced the same structural changes as bulk samples did, structural changes during carbonization process in the thin film conformation were substantially different from those appeared in bulk samples. During the conversion of stabilized PE thin film into CNSs, highly ordered graphitic structures, distinguishable by the Raman band at 2600 cm-1,24,25,29,30 were organized at relatively low temperature where stabilized bulk PE could not develop graphitic structure. It is worthy to note that for Raman analysis, laser power was restricted to 5 % of maximum output to avoid a laser induced structural evolution. 5 % of maximum output was mild enough to examine PE thin film


7


Page 8 of 28

without damage. For a better understanding of the phenomenon, prepared CNSs were thoroughly probed in microscopic point of view using optical microscopy (OM). OM images revealed development of dark dots embedded within the plain background. These black dots were irrelevant to contamination from the furnace or outer sources (Figure S2). Larger black dots were observed in CNS prepared from the higher polymer content solution with the thicker film (Figure 2-a). Image analysis (Figure S3) also concluded the concomitant trend. An average area of black dots became larger from ~ 54 µm2 to ~ 126 µm2 as CNSs became thicker. Structural information of both regions, dark dots and background, was studied based on the Raman spectra analysis. When the laser was manually focused on dark dots relying on OM feed, obtained Raman spectra exhibited fingerprints of highly graphitic carbon with a relatively small D band at 1400 cm-1 and emergence of 2D band at 2600 cm-1, supported highly ordered structural characteristic of dark dots (Figure 2-b).23,25,28,29 In contrast, background regions, all the other areas except dark dots, appeared to be amorphous (Figure 2-c). ID/IG ratio, a value representing the degree of structural defects of carbon materials,24,25 was calculated based on obtained Raman spectra to quantitatively differentiate graphitic dark dots from amorphous background. The ID/IG ratios of black dots ranged from 0.6 to 0.8 while those of background areas were more than two times higher, ranging from 1.5 to 1.65 (Figure 2-d). It is worthy to note that the solution weight percent affected the size of graphitic moieties, but there was no a significant influence on the development of carbon structure. The ID/IG ratios of both dark dots and background stayed at almost similar level regardless of the solution weight percent for CNSs fabrication. To support the conclusions based on Raman spectra from the local area, OM images of CNSs were mapped based on ‘pixel by pixel’ Raman signals to assure that areas where observed as dark dots via the manual laser spotting were truly graphitic. With a step size of ~ 100 nm, Raman


8

Page 9 of 28 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


signals from the area spanning 40 µm by 40 µm were collected and the data were reconstructed by normalizing the 2D band signal by intensity of G band. The processed signals were then color-coded from blue to red as a function of normalized relative intensity (Figure 2-e). The dark dots under OM and green-red islands in the reconstructed mapping image, where 2D band signals were high, exactly matched to each other. On top of Raman mapping image, TEM further articulated formation of graphitic moieties under low carbonization temperature in CNSs.

Figure 2. a) OM images of CNSs from PE solution with varied polymer weight content b) Raman spectra obtained when the laser was manually focused on areas appeared as darks dots in


9


Page 10 of 28

OM images c) Raman spectra from background areas d) ID/IG ratios of dark dots and background area calculated from deconvoluted Raman spectra e) Color mapped OM images based on pixel by pixel acquisition of Raman signals

Both amorphous (Figure 3-a) and graphitic (Figure 3-b) regions were observed when PE CNSs were interrogated under TEM. It is speculated that formation of graphitic carbon structure at low carbonization temperature is mainly due to the development of relatively small basic structural units (BSUs) compared to other polymeric carbon precursors. The BSU is a polyaromatic stacking, which can be referred as seeds for carbon structure growth during carbonization. In the previous studies about carbonization of pitch, specifically when small molecules were extracted from isotropic pitches, formation of graphitic structure at low carbonization temperature was reported,29,31–33 which is similar to carbonization of PE. The phenomenon was rationalized in a way that small BSUs of stabilized pitch play critical role in easier organization of graphitic structures. Pitch is a polyaromatic small molecule with a broad range of moieties. Pitch organizes small BSUs upon thermal stabilization while long molecular chains of polymeric carbon precursors such as PAN are destined to organize BSUs with a larger size. Melting point of most polymeric carbon precursors is higher than stabilization temperature.28,29 However, PE melts and more importantly, even partially decomposes at the temperature below stabilization temperature of ~ 270 oC under air. It is suggested that stabilization and oxidative chain scission simultaneously take place in the heating of PE, thus BSUs with smaller size than other polymeric carbon precursors could be structured. To verify the hypothesis, Raman spectra of stabilized PAN and PE films were compared in Figure S4. The intensity of Raman band at 1400 cm-1, corresponding to breathing vibration mode of benzene ring qualitatively, indicates the relative size of polyaromatic moieties,24,25 which are BSUs in stabilized polymer thin films. The band at 1400 cm-1 was much more eminent in stabilized PAN


10

Page 11 of 28 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


thin film than in stabilized PE counterpart, proving that BSUs in stabilized PE thin films are much smaller than those in stabilized PAN. Further details why smaller BSUs are formed when PE is thermal oxidatively stabilized and how the small BSUs reorganize into more graphitic structure during carbonization will be elaborated in the future report.

Figure 3. TEM images of PE based CNSs showing areas where are a) amorphous and b) graphitic

Electrical, optical and morphological properties of PE based CNSs with the graphitic structure were examined to determine applicability of newly introduced CNSs as an effective TCF. Electrical properties of CNSs were measured with a 4-point probe (Figure 4-a). Sheet resistance of CNSs steeply decreased as PE weight percent of solution, which is positively related to thickness of resulting films, increased. The sheet resistance of PE based CNSs reached 0.3 kOhm/square when the thickness of CNSs was ~ 25 nm. Then, electrical conductivity of CNSs was calculated based on the thicknesses and the sheet resistances. Surprisingly, regardless of the thickness of CNSs, values averaged to ~ 1100 S/cm, reaching maximum of 1150 S/cm in case of 25 nm thick CNSs fabricated from 5 wt. % solution. The electrical conductivity is not only higher than that of other previously reported CNSs12,13,16, but also higher than those of TCFs


11


Page 12 of 28

based on rGO34–36 or GO37 dispersion and almost matched that of Graphene dispersion TCF38 (Figure 4-b). The superior electrical conductivity of PE derived CNSs to other CNSs can be attributed to the presence of highly ordered graphitic moieties embedded in amorphous carbon matrix. Even though graphitic moieties were randomly spread and did not construct the percolation network, it is important to recall that amorphous carbon background is also conductive. PE based CNS introduced here should not be interpreted as common insulatorconductive filler system. Whole CNS is electrically conductive system, thus inclusion of graphitic moieties, which obviously have higher electrical conductivity, simply assists more efficient transport of electrons.

Figure 4. a) Plot presenting changes in thickness, sheet resistance and conductivity of CNSs when solution polymer content was varied b) Plot comparing electrical conductivity of PE CNS to that of other previously reported CNSs.

Optical transparency, another important parameter to be considered for effective TCF application, was examined by measuring transmittance of the CNSs at 550 nm using an UV-Vis spectrometer (Figure 5-a). Transmittance of thicker CNSs fabricated from the PE solution with a higher polymer content was lower than thinner CNSs. Overall, the transmittance of CNSs were in the range of 50 % ~ 60 %, which are not the best value but still applicable as TCFs. However,


12

Page 13 of 28 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


the value suddenly dropped to 35 % as the thickness of the CNS reached ~ 25 nm when CNSs were fabricated using 5 wt. % solution. Moreover, digital holographic microscopy (DHM) analysis provided quantified surface roughness information of CNSs over the large area spanning 0.5 mm by 0.5 mm (Figure 5-b). The surface roughness values of CNSs ranged from 4 nm to 4.5 nm. The values are higher than those of previously reported CNSs with the roughness of subnanometers.2,12,13,17 However, the surface roughness of PE CNSs is quite low for a heterogeneous film comprising both graphitic structure and amorphous background. Recalling other types of TCFs, TCFs consisted of nanowires, CNTs, GO or rGO have the similar (GO and rGO) or much higher (nanowires and CNTs) surface roughness than that of PE CNSs.3,4 One of major applications of TCF is as a transparent electrode for photovoltaic cells. Flexibility of thin carbon films, especially those under 20 nm thickness, and an effective application of those on flexible electronic device fabrication were well confirmed in previous reports.2,13 In this study, organic photovoltaic cells (OPVs) were built on PE based CNSs. Before OPV fabrication, work function of PE CNSs were tabulated by ultraviolet photoelectron spectroscopy (UPS) (Figure 5-c). When compared to ITO, a reference transparent electrode with work function of 4.4 eV, the work function of PE CNSs was slightly higher irrespective of CNS thickness (~ 4.7 eV). Therefore, hole collection was expected to be more efficient because of better energy-level matching between CNSs and PEDOT:PSS (~ 5.2 eV), which is a common hole extracting layer used for OPV fabrication (Figure S5).


13


Page 14 of 28

Figure 5. a) UV-Vis spectra of CNSs for determining transmittance of the CNSs b) Surface roughness of CNSs measured by digital holographic microscopy (DHM) c) Work function of CNSs measured by UPS OPV cells were fabricated as depicted in Scheme 2. PEDOT:PSS was spin casted on PE CNSs prepared on quartz substrates. P3HT:PCBM heterojunction was adopted as an active layer and was deposited through spin coating. Lastly, aluminum electrode was thermally evaporated on the top of active layer and completed fabrication of P3HT:PCBM OPV cells with the PE CNS transparent electrode.

Scheme 2. Scheme depicting fabrication process of PE CNS based organic photovoltaic (OPV) cells

We directly evaluated the performance of PE based CNSs as a new class of transparent electrode substituting ITO for OPV application. Figure 6-a presents the representative


14

Page 15 of 28 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


photocurrent density (J)-voltage (V) curves of the CNS-based OPVs with variable PE concentrations, and the corresponding photovoltaic parameters including open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and power conversion efficiency (PCE) are summarized in Figure 6-a. As the polyethylene concentration increased, the FF values of respective devices were sharply enhanced from 45.0 % to 54.5 %, which is comparable to that of the ITO-based cell (60.9 %). The FF increase could be well elucidated by a decrease of the device series resistance (RS), which was calculated from the J-V curves. Considering that the photo-generated charge carriers from the active layer should travel the lateral distance of the transparent electrode, i.e. the CNS film, the low sheet resistance of CNS from 5 wt. % solution is mainly attributed to the low device RS as shown in the inset of Figure 6-b. Thus, thicker CNS film from higher PE concentration guarantees the lower RS, thereby leading to the high FF. However, as confirmed in Figure 4-a, the thickness of CNS films and their optical transparency are negatively related, which in turn adversely affected the JSC of CNSs based OPVs as the number of incident photons decreased. It indicates the complementary relationship between the sheet resistance of CNS for FF and the optical transmittance for JSC, so that the control of CNS thickness is closely related to the optimization of the resultant device efficiency. Accordingly, the device using 4 wt. % CNS exhibited the highest performance with VOC of 0.60 V, JSC of 6.34 mA/cm2, FF of 52.2 % and PCE of 1.98 %. Notably, despite the values still lower than ITO based OPVs, the performance of PE CNS based OPVs is superior to OPVs based on solution processed graphene derivatives. Moreover, we further explored the practical potential of PE precursors as transparent electrodes by remediating one of PE plastic wastes, ‘poly-glove’ waste in a laboratory. Interestingly, CNS films converted from ‘poly-glove’ could efficiently act as a transparent


15


Page 16 of 28

electrode without any chemical additives or additional treatments. As shown in Figure 6-c, OPV from the ‘poly-glove’ transparent electrode exhibited a high PCE of 1.33% with VOC of 0.60 V, a JSC of 6.24 mA/cm2, and a FF of 35.7 %. The successful demonstration of ‘up-cycling’ PE waste into a functional TCF manifests profound impact of the technology as follows; i) introducing a highly cost-effective precursor, PE waste which is nearly free, for TCF fabrication ii) providing an economically profitable recycling pathway, ‘upcycling’ pathway in other words, thereby initiating consumption of PE wastes rather than simply burying or burning them and finally protecting environment in an indirect way.

Figure 6. a) J-V curves of OPVs structured on PE based CNSs. Inset shows series resistance of CNSs. b) J-V curve of OPVs structured on CNSs from PE waste c) Table describing performance of OPVs from various PE CNSs and reference

Conclusion


16

Page 17 of 28 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


Here, we demonstrated the successful fabrication of TCF derived from PE thin film through carbonization following thermal oxidative stabilization. Upon thermal oxidation, as bulk PE did in the previous study, PE thin film was converted into a thermally stable chemical structure with benzene rings and oxygen functional groups. However, structural evolution during carbonization process in the thin film configuration was significantly differed from the bulk PE counterpart. CNSs fabricated from stabilized PE thin films included graphitic moieties while bulk analogue18 only organized amorphous carbon when processed under the same carbonization condition. Microscale dark dots were observed in OM images of PE CNSs and structural characteristics of the features were examined through Raman spectra analysis coupled with an image mapping and TEM observation. The concomitant analysis results concluded that the dark dots from the OM images were graphitic carbon moieties and their surroundings were amorphous carbon. The inclusion of highly ordered graphitic moieties facilitated higher electrical conductivity of PE CNSs than previously reported CNSs with amorphous characteristics, also even higher than other types of TCFs including rGO or GO based films. The presence of graphitic moieties, however, negatively affected the optical and morphological properties of CNSs. The transmittance and surface roughness of PE CNSs were not as superior as ultrasmooth CNSs from other polymeric precursors, but the values were comparable to other types of TCFs with heterogeneous nature. Given the sound properties of PE CNSs as TCFs, OPVs were fabricated using the PE based CNSs as transparent electrodes. PCE of cells was not as high as OPV cells fabricated from ITO, but the value was comparable to that of OPVs based on other CNSs or rGO/GO derived TCFs. More significantly, we explored the promising environmental and economic potential of the technology. PE wastes could be equivalently used as a precursor for fabricating high performance CNSs with graphitic features, endowing an extra economic


17


Page 18 of 28

value to the wastes. Therefore, PE wastes, which provoke serious environmental issues as they are mostly buried or burnt, might be actively consumed for TCFs fabrication. We believe that the technology demonstrated in this study not only allows ‘cost-effective’ fabrication of TCFs by introducing a new class precursor but also provides a viable ‘upcycling’ pathway of PE waste which in turn resolves the environmental issues surrounding PE wastes.

Materials and Methods PE Thin Film Preparation LLDPE pellets (UL 814, Lotte Chemical, Republic of Korea) with melt index of 20 g /10 min (measured at 190 °C) and density of 0.93 g/cm3 (measured via density gradient column, Ray-Ran, UK) were dissolved in toluene (99.5 % purity, Sigma Aldrich, US) to prepare PE solution with varied polymer content. Density of reclaimed poly-glove was 0.91 g/cm3 (density gradient column, Ray-Ran, UK). For proper dissolution of PE, solution was heated at 80 °C with stirring for 1 hr. Resulting solutions were spin coated on quartz substrate using spin coater with heating capability (Bespoke spin coater, Intech System, Republic of Korea). Solidification of PE solution during spin coating was suppressed by maintaining chamber temperature at 80 °C. For comparison purpose, PAN films were fabricated from PAN (Mw = 150,000) purchased from Sigma Aldrich, US. Using dimethyl sulfoxide (DMSO, 99 % purity, purchased from Daejung Fine Chemical, Republic of Korea) as a solvent, 3 wt. % PAN solution was made. Prepared PAN solution was spin casted on quartz substrate.


18

Page 19 of 28 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


Stabilization and Carbonization of Thin Films Prepared precursor thin films were thermally oxidized using convection oven (Heratherm, Thermo Scientific, US). Ramping rate was fixed at 5 °C/ min and samples were held at 270 °C for 4 hours. Stabilized samples were carbonized at 1200 °C under N2 condition with a flow rate of 2000 sccm using tube furnace (ThermVac Engineering, Republic of Korea). Final carbonization temperature was reached with ramping rate of 5 °C/min. Characterizations Raman Spectra of samples were obtained using Renishaw inVia Raman spectrometer (Renishaw, UK) equipped with 514 nm laser having 0.15 mW output. Laser was focused through 20X optical lens with 5 % power of maximum output and exposure time was set as 30 sec. For Raman mapping, Renishaw WIRETM software was used to control the micro-stage and process the obtained signals. X-ray Photoelectron Spectroscopy (XPS) was measured using K-Alpha XPS (Thermo Scientific, US). Survey scan spanning 0 eV to 1350 eV was followed by high resolution scans of C 1s and O 1s. Transmission Electron Microscopy (TEM) characterization was performed using Tecnai G2 F20 TEM (FEI, US) with acceleration voltage of 200 kV. Sheet resistance of CNSs was measured using 4-point probe (Loresta-GP, Mitsubishi Chemical Analytech, Japan). Thickness of CNSs were determined using atomic force microscopy (AFM, Dimension 3100, Veeco, US). Detail experimental procedure is as follows; before stabilization and carbonization process, some portion of spin-coated PE films on top of the quartz substrates was etched using sharp razor. Then, the area including boundary between bare quartz substrate and carbonized portion was scanned through AFM to determine the thickness of CNSs. Transmittance of the CNSs were measured using UV-Vis spectrometer (V-570, Jasco, Japan).


19


Page 20 of 28

For surface roughness measurement, CNSs were interrogated using DHM (R-1000, Lyncee Tec, CH). For measuring work function of CNSs and ITO, UPS (ESCALAB 250Xi, Thermo Scientific, US) was used.

Image Analysis OM images were analyzed using ‘ImageJ’ software. Color threshold of images were first adjusted using software’s internal function. Adjusted images were then processed through particle analysis algorithm. Among captured areas by particle analysis algorithm, areas with circularity less than 0.2 were rejected for counting.

OPV Cell Fabrication Firstly, for the fabrication of the CNS electrode-based OPVs, the hole-transporting layer of the PEDOT:PSS (CleviosTM HTL Solar, Heraeus) was spin-coated (5000 rpm/40 s) onto the prepared substrate, and then thermally treated (150 °C/10 min). Next, to deposit photoactive layer, a blended solution composed of 25 mg of P3HT (Rieke Metals) and 25 mg of PC61BM (Nano-C) in 1 ml of 1,2-dichlorobenzene (DCB) was spin-coated (700 rpm/60s) under N2 environment. Then, a slow evaporation of the DCB solvent was performed by keeping the photoactive layers inside a covered glass jar for 2 h, followed by baking (110 °C/10 min).


20

Page 21 of 28 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


Finally, cathodes of Ca (20 nm)/Al (80 nm) were deposited through a shadow mask (active area: 4.64 mm2) by thermal evaporation at a base pressure of 1 × 10−6 Torr. Photocurrent density– voltage (J–V) characteristics of the resultant OPVs were measured using a Keithley 2400 source meter under a simulated AM 1.5 G illumination of 100 mW/cm2. For the accurate characterization, the light intensity was precisely calibrated by using a KG5 filtered silicon reference cell certified by the National Renewable Energy Laboratory (NREL).

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website Thickness of CNSs and pristine PE films; XPS survey scan of PE CNSs; Quantified atomic weight percent of carbon chemical bonds based on deconvoluted XPS C1s scan; OM images and Raman spectra of bare quartz before and after carbonization as a contamination check-up; Image analysis result of OM images; Raman spectra of stabilized PAN film and stabilized PE film; Band diagram of OPVs demonstrated in this paper (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions


21


Page 22 of 28

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 1These authors contributed equally.

Funding Sources This work was supported by a grant from the Korea Institute of Science and Technology (KIST) Institutional program (2Z05360, 2Z05400), the Industrial Core Technology Development Program (10052760), and the Civil-Military technology cooperation program funded by the Ministry of Trade, Industry and Energy, Republic of Korea.

REFERENCES

(1)

Du, J.; Pei, S.; Ma, L.; Cheng, H.-M. 25th Anniversary Article: Carbon Nanotube and Graphene-Based Transparent Conductive Films for Optoelectronic Devices. Adv. Mater. 2014, 26 (13), 1958–1991, DOI 10.1002/adma.201304135.

(2)

Fan, H.; Shen, W. Carbon Nanosheets: Synthesis and Application. ChemSusChem 2015, 8 (12), 2004–2027, DOI 10.1002/cssc.201500141.

(3)

Yu, L.; Shearer, C.; Shapter, J. Recent Development of Carbon Nanotube Transparent Conductive Films. Chem. Rev. 2016, 116 (22), 13413–13453, DOI 10.1021/acs.chemrev.6b00179


22

Page 23 of 28 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


(4)

Sannicolo, T.; Lagrange, M.; Cabos, A.; Celle, C.; Simonato, J.-P.; Bellet, D. Metallic Nanowire-Based Transparent Electrodes for Next Generation Flexible Devices: A Review. Small 2016, 12 (44), 6052–6075, DOI 10.1002/smll.201602581.

(5)

Sung, H.; Ahn, N.; Jang, M. S.; Lee, J.-K.; Yoon, H.; Park, N.-G.; Choi, M. Transparent Conductive Oxide-Free Graphene-Based Perovskite Solar Cells with over 17% Efficiency. Adv. Energy Mater. 2016, 6 (3), 1501873, DOI 10.1002/aenm.201501873

(6)

Liu, Z.; Parvez, K.; Li, R.; Dong, R.; Feng, X.; Müllen, K. Transparent Conductive Electrodes from Graphene/PEDOT:PSS Hybrid Inks for Ultrathin Organic Photodetectors. Adv. Mater. 2015, 27 (4), 669–675, DOI 10.1002/adma.201403826.

(7)

Kim, J.-Y.; Jeon, J.-H.; Kwon, M.-K. Indium Tin Oxide-Free Transparent Conductive Electrode for GaN-Based Ultraviolet Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2015, 7 (15), 7945–7950, DOI 10.1021/am509116s.

(8)

Jang, H.; Park, Y. J.; Chen, X.; Das, T.; Kim, M.-S.; Ahn, J.-H. Graphene-Based Flexible and Stretchable Electronics. Adv. Mater. 2016, 28 (22), 4184–4202, DOI 10.1002/adma.201504245.

(9)

Bao, Z.; Chen, X. Flexible and Stretchable Devices. Adv. Mater. 2016, 28 (22), 4177– 4179, DOI 10.1002/adma.201601422.

(10)

Liu, Z.; Xu, J.; Chen, D.; Shen, G. Flexible Electronics Based on Inorganic Nanowires. Chem. Soc. Rev. 2015, 44 (1), 161–192, DOI 10.1039/C4CS00116H.


23


(11)

Page 24 of 28

Son, S.-Y.; Yeo, J.-S.; Jung, G. Y.; Lee, S.; Joh, H.-I. Micromesh Carbon Nanosheet Electrodes Fabricated by Phase-Separation of Immiscible Polymer Blends. J. Ind. Eng. Chem. 2018, DOI 10.1016/j.jiec.2018.01.039.

(12)

Son, S.-Y.; Noh, Y.-J.; Bok, C.; Lee, S.; Kim, B. G.; Na, S.-I.; Joh, H.-I. One-Step Synthesis of Carbon Nanosheets Converted from a Polycyclic Compound and Their Direct Use as Transparent Electrodes of ITO-Free Organic Solar Cells. Nanoscale 2014, 6 (2), 678–682, DOI 10.1039/C3NR04828D.

(13)

Son, S.-Y.; Yun, J.-M.; Noh, Y.-J.; Lee, S.; Jo, H.-N.; Na, S.-I.; Joh, H.-I. Highly Flexible and Bendable Carbon Nanosheets as Transparent Conducting Electrodes for Organic Solar Cells. Carbon. 2015, 81, 546–551, DOI 10.1016/J.CARBON.2014.09.089.

(14)

Plastics Europe, Plastics - the Facts 2014/2015; 2015.

(15)

Siebertz, M. The European Market for Plastic Sorting and Recycling: Locations, Plants, Backgrounds and Market Estimations; 2015.

(16)

Na, S.-I.; Noh, Y.-J.; Son, S.-Y.; Kim, T.-W.; Kim, S.-S.; Lee, S.; Joh, H.-I. Efficient Organic Solar Cells with Solution-Processed Carbon Nanosheets as Transparent Electrodes. Appl. Phys. Lett. 2013, 102 (4), 043304, DOI 10.1063/1.4789980.

(17)

Joh, H.-I.; Lee, S.; Kim, T.-W.; Hwang, S. Y.; Hahn, J. R. Synthesis and Properties of an Atomically Thin Carbon Nanosheet Similar to Graphene and Its Promising Use as an Organic Thin Film Transistor. Carbon. 2013, 55, 299–304, DOI 10.1016/J.CARBON.2012.12.067.


24

Page 25 of 28 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


(18)

Choi, D.; Jang, D.; Joh, H.-I.; Reichmanis, E.; Lee, S. High Performance Graphitic Carbon from Waste Polyethylene: Thermal Oxidation as a Stabilization Pathway Revisited. Chem. Mater. 2017, 29 (21), 9518–9527, DOI 10.1021/acs.chemmater.7b03737.

(19)

Kageyama, K.; Tamazawa, J.; Aida, T. Extrusion Polymerization: Catalyzed Synthesis of Crystalline Linear Polyethylene Nanofibers Within a Mesoporous Silica. Science. 1999, 285 (5436), DOI 10.1126/science.285.5436.2113.

(20)

Kanamoto, T.; Tsuruta, A.; Tanaka, K.; Takeda, M.; S. Porter, R. On Ultra-High Tensile Modulus by Drawing Single Crystal Mats of High Molecular Weight Polyethylene. Polym. J. 1983, 15 (4), 327–329, DOI 10.1295/polymj.15.327.

(21)

Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett. 2010, 10 (3), 751–758, DOI 10.1021/nl904286r.

(22)

Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Raman Spectroscopy in Graphene. Phys. Rep. 2009, 473 (5–6), 51–87, DOI 10.1016/j.physrep.2009.02.003.

(23)

Tuinstra, F.; Koenig, J. L. Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53 (3), 1126–1130, DOI 10.1063/1.1674108.

(24)

Ferrari, A. C.; Basko, D. M. Raman Spectroscopy as a Versatile Tool for Studying the Properties of Graphene. Nat. Nanotechnol. 2013, 8 (4), 235–246, DOI 10.1038/nnano.2013.46.


25


(25)

Page 26 of 28

Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B 2000, 61 (20), 14095–14107, DOI 10.1103/PhysRevB.61.14095.

(26)

Ferrari, A. C.; Robertson, J. Raman Spectroscopy of Amorphous, Nanostructured, Diamond–like Carbon, and Nanodiamond. Philos. Trans. R. Soc. London A Math. Phys. Eng. Sci. 2004, 362 (1824), DOI 10.1098/rsta.2004.1452.

(27)

Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; et al. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97 (18), 187401, DOI 10.1103/PhysRevLett.97.187401.

(28)

Donnet, J.-B. Carbon Fibers; Marcel Dekker, 1998, ISBN 0824701720.

(29)

Morgan, P. Carbon Fibers and Their Composites; Taylor & Francis, 2005, ISBN 0824709837.

(30)

Yao, L.; Yang, W.; Li, S.; Sha, Y.; Tan, J.; An, Y.; Li, H. Graphitization of PAN-Based Carbon Fibers by CO2 Laser Irradiation. Carbon Lett. 2017, 24, 97–102, DOI 10.5714/CL.2017.24.097.

(31)

Lee, J.-S.; Kim, Y.-K.; Hwang, J. Y.; Joh, H.-I.; Park, C. R.; Lee, S. Carbon Nanosheets by the Graphenization of Ungraphitizable Isotropic Pitch Molecules. Carbon. 2017, 121, 479–489, DOI 10.1016/J.CARBON.2017.06.010.


26

Page 27 of 28 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


(32)

Vix-Guterl, C.; Saadallah, S.; Vidal, L.; Reda, M.; Parmentier, J.; Patarin, J. Template Synthesis of a New Type of Ordered Carbon Structure from Pitch. J. Mater. Chem. 2003, 13 (10), 2535, DOI 10.1039/b305295h.

(33)

Frank, E.; Steudle, L. M.; Ingildeev, D.; Spörl, J. M.; Buchmeiser, M. R. Carbon Fibers: Precursor Systems, Processing, Structure, and Properties. Angew. Chemie 2014, 53 (21), 5262–5298, DOI 10.1002/anie.201306129.

(34)

Yin, Z.; Sun, S.; Salim, T.; Wu, S.; Huang, X.; He, Q.; Lam, Y. M.; Zhang, H. Organic Photovoltaic Devices Using Highly Flexible Reduced Graphene Oxide Films as Transparent Electrodes. ACS Nano 2010, 4 (9), 5263–5268, DOI 10.1021/nn1015874.

(35)

Geng, J.; Jung, H.-T. Porphyrin Functionalized Graphene Sheets in Aqueous Suspensions: From the Preparation of Graphene Sheets to Highly Conductive Graphene Films. J. Phys. Chem. C 2010, 114 (18), 8227–8234, DOI 10.1021/jp1008779.

(36)

Geng, J.; Liu, L.; Yang, S. B.; Youn, S.-C.; Kim, D. W.; Lee, J.-S.; Choi, J.-K.; Jung, H.T. A Simple Approach for Preparing Transparent Conductive Graphene Films Using the Controlled Chemical Reduction of Exfoliated Graphene Oxide in an Aqueous Suspension. J. Phys. Chem. C 2010, 114 (34), 14433–14440, DOI 10.1021/jp105029m.

(37)

Kymakis, E.; Savva, K.; Stylianakis, M. M.; Fotakis, C.; Stratakis, E. Flexible Organic Photovoltaic Cells with In Situ Nonthermal Photoreduction of Spin-Coated Graphene Oxide Electrodes. Adv. Funct. Mater. 2013, 23 (21), 2742–2749, DOI 10.1002/adfm.201202713.


27


(38)

Page 28 of 28

Ricciardulli, A. G.; Yang, S.; Feng, X.; Blom, P. W. M. Solution-Processable HighQuality Graphene for Organic Solar Cells. ACS Appl. Mater. Interfaces 2017, 9 (30), 25412–25417, DOI 10.1021/acsami.7b09702.

TOC

SYNOPSIS We demonstrated ‘upcycling’ of PE wastes, which are mostly buried or burnt therefore provoke environmental problems, into transparent electrodes and they were successfully applied for organic photovoltaic cells.


28

Carbon Nanosheet from Polyethylene Thin Film as a Transparent

Recommend Documents