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Synthesis and characterization of transparent and flexible polymer clay substrate for OLEDs
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 3 (2016) 2409–2412
www.materialstoday.com/proceedings
Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)
Synthesis and characterization of transparent and flexible polymer clay substrate for OLEDs D.Shanmuga Sundara, A.Sivanantharajaa, C.Sanjeevirajaa, D.Jeyakumarb,* a b
Alagappa Chettiar College of Engineering & Technology, Karaikudi, Tamilnadu CSIR-Central Electrochemical Research Institute, Karaikudi, Tamilnadu, INDIA
Abstract Organic Light Emitting Diode (OLED) has major applications in the field of displays, lighting and visible light communication. Due to the advantages of low cost, light weight, low power and high efficiency, flexible organic light emitting diode have attracted the interest of researchers all over the World. Flexible OLEDs are considered to be the best suitable candidate for next generation lighting devices. In this research, a new flexible and transparent substrate with an average transmittance of 60-70% in the visible region (300-700nm) is synthesized with the help of organic materials such as tetraphenylphosphonium modified lithium saponite (TPP-LiSA), Synthetic Saponite (SA) and N, N-dimethylformamide (DMF). The as-synthesized substrate exhibits refractive index as close as glass which makes this substrate to be used for flexible displays. Characteristics such as extinction coefficient (k), absorption of the synthesized substrate are also calculated. © 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords:Flexible displays; Thin film; Organic Light Emitting Diode, Refractive Index, Polymer Clay Substrate.
* Corresponding author. E-mail address:[email protected] 2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].
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D.Shanmuga Sundar, et al./ Materials Today: Proceedings 3 (2016) 2409–2412
1. Introduction Organic light-emitting devices (OLED) have attracted considerable interest for many applications such as flat panel displays and interior lighting source. Due to ultra-thin thickness, light weight and environmental protective nature makes OLEDs to be suitable candidate for next generation planar lighting source and display system [1]. Organic electroluminescence was first experimentally demonstrated by Pope et al. in 1963 [2]. In this case, electroluminescence was observed for an applied voltage of 400 V. However, as-prepared device exhibited low luminance efficiency. The first practical OLED was developed in 1987 at Eastman Kodak by Tang and VanSlyke and the device turn-on voltage was around 10 Volts [3]. Flexible displays are the one which bends during the use but it has certain limit, apart the limit it acts as rolling displays. Recently literature review on flexible candidates has been expanding. It now includes a book on flexible flat panel displays written by Crawford [4] and a special edition of the Proceedings of the IEEE on flexible displays [5]. Eventhough the consequences for flexible displays are good, there is still some difficult on technical and manufacturing developments [6, 7]. Production of flexible displays is still in laboratory level due to the unresolved problems in backplane and high temperature fabrication process but the recent advancement in mobile phones and displays leads to the development of displays with high degree of flexibility and rollability [8]. Another major drawback of plastic as a flexible displays are refractive index mismatch of the substrate and the Indium Tin Oxide (ITO). But this can be overcome by using anode stack which will increase the efficiency of the device [9]. However polyesters such as PET and PEN have advantages such as optical transparency, better thermal coefficient, chemical resistance and moisture absorption, but their upper operating temperature and surface roughness are not so good [10]. Advantages of being as transparent, light weight, flexible and robust, polymers are considered to be a suitable substitute for both glass and plastic in the applications of OLEDs and OPVs. Polymers have mechanical properties that vary from strong rigidity, such as in engineering plastics, to softness, such as in rubber or polyethylene films. Due to low cost, mass production via roll-to-roll (RTR) process, polymers have various application areas such as transparent substrates, electrodes and active materials for organic light emitting devices (OLEDs), LCDs and organic thin-film transistors (OTFTs) which makes the researchers to start the investigation on all polymer-based flexible devices [11]. In this paper, a new transparent and flexible substrate named Polymer Clay substrate for OLED application is synthesized by using mineral clay and tetraphenylphosphonium bromide. The as-synthesized film exhibits transparency of about 60-70% in the visible region. Also the characteristics such as absorption (A), refractive index (n) and extinction coefficient (k) are calculated. Optical band gaps for both direct and indirect transitions are also calculated. The refractive index of the as-prepared film is found to be close as glass. 2. Experimental In this work, a thin flexible flim with improved characteristics is synthesized by means of using materials such as Tetraphenylphosphonium bromide (TPP-Br) purchased from Alfa-Aesar with an assay of 98%, Synthetic Saponite (SA) from Mineral Clay Society of Japan and N,N-Dimethylformamide (DMF) from Sigma Aldrich with 99% assay. The cation exchange capacity (CEG) of the synthetic saponite is found to be around 76meq/100g. The TPPBr of twice the CEG is added to the synthetic saponite and allowed to dissolve in a mixture and then the mixture is stirred for 24 hours. Then the mixture is washed with the solvent (ethanol: water) in the ratio of 1:5 and then it is centrifuged to collect the samples, above process is repeated for atleast three times to obtain a final product. Then the final product is mixed with DMF in the ratio of 40:60 and it is stirred for 24 hours and the final product called organo clay is obtained and coated in a Teflon sheet of uniform thickness by using doctor blade method. In order to increase the characteristics of the film, Poly (ethylene glycol) (PEG) from Fluka analytical and Poly (acrylic acid) (PA) from Alfa Aesar are used. PEG and PA are mixed in the ratio of 40:60 to from the solution at a temperature of 150oC in a hot plate. Then the polymer mixture is added in the organo clay in the ratio of 90:10 and the mixture is stirred for 24 hours to form Polymer clay (P-clay). Then the P-clay materials is degassed and then it is
D.Shanmuga Sundar, et al./ Materials Today: Proceedings 3 (2016) 2409–2412
2411
coated in the Teflon sheet of uniform thickness using doctor blade method and it is allowed to dry at room temperature for few hours and finally a free self-standing flim is peeled off from the Teflon sheet. Characterizations such as Transmittance (T), Absorbance (A) are measured using UV 3000+ UV-VIS spectrophotometer from LABINDIA. From the obtained values, the other parameters such as refractive index (n), extinction coefficient (k) and optical bandgap for both direct and indirect transitions are also calculated. 3. Results and Discussion Figure 1 shows the photographic images of Polymer clay substrate and its flexibility which indicates the flexible nature of the substrate.
Fig.1. (a) Photographic Image of Polymer Clay substrate; (b) Flexibility of the substrate.
Figure 2 shows the changes of transmittance of the substrate with respect to the wavelength using UV 3000+ UVVIS spectrophotometer from LABINDIA. Transmittance of about 60-70% is obtained in the region of 300-700nm (visible region), still we are working on the substrate to increase the transmittance which make suit of the film to be more applicable for display applications.
100
Transmittance (%)
80
60
40
20
0 400
600
800
Wavelength (nm)
Fig.2. Variation of Transmittance with wavelength of P-clay film
1000
2412
D.Shanmuga Sundar, et al./ Materials Today: Proceedings 3 (2016) 2409–2412 2.8
(a)
0.00012
Extinction coefficient (k)
Refractive index (n)
2.4
2.0
1.6
1.2
(b)
0.00010 0.00008 0.00006 0.00004 0.00002 0.00000
400
600
800
1000
1200
400
600
800
1000
1200
Wavelength (nm)
Wavelength(nm)
Fig.3. Plot of Extinction Co-efficient (K) and Refractive Index (n) vs. Wavelength (nm) of P-clay film
Optical parameters such as refractive index (n), extinction coefficient (k) are calculated for the flim using the following equations n=
(1 + R )
(1 − R )
+
4R − k2 (1 − R )2
k=αλ/4π)
(1) (2)
where R is a reflectance, α is an absorption coefficient, λ is the wavelength and π is 3.14. The wavelength dependent of extinction coefficient and refractive index is shown in figure 3a and 3b. The refractive index of the substrate is found to be approximately close to glass which makes it to be suitable for opto-electronic device applications. From figure 3b it can be concluded that the dispersion of extinction coefficient decreased lightly in the visible region and starts to increase after the visible region 4. Conclusion In summary, we had synthesized and characterized a flexible and transparent substrate named P-clay with a transmittance of about 60-70% in the range of 300-700nm (visible region). Also the optical characteristics such as transmittance (T), absorption (A), refractive index (n) and extinction coefficient (k) are also calculated and which shows that the substrate can be used for flexible OLEDs applications. Further, thermal characteristics of the flim have to be improved in order to support high temperature manufacturing process. References [1]. T.-H. Han, Y. Lee, M.-R.Choi, S.-H. Woo, S.-H. Bae, B. H. Hong, J.-H.Ahn, and T.-W. Lee: Nat. Photonics 6 (2012) 105. [2]. M. Pope, H. P. Kallmann, and P. Magnante: J. Chem. Phys. 38 (1963) 2042. [3]. W. C. Tang and S. A. VanSlyke: Appl. Phys. Lett. 51 (1987) 913. [4]. Crawford GP. Flexible flat panel display technology. New York: Wiley; 2005. [5]. Nathan A, Chalamala BR. Special issue on: flexible electronics technology. ProcIEEE 2005;93(7–8). [6]. Gasman L. OLED and paper-like display markets. Veritas et Visus, Flex Substr 2006;2(6):22–5. [7]. Allen KJ. Reel to real: prospects for flexible displays. ProcIEEE 2005;93:1394–9. [8]. Hill P. iSuppli flexible displays report. Veritas et Visus, Flex Substrate 2006;2(5):21–5. [9]. DhanapalanShanmugasundar andA.Sivanantharaja, Opt Quant Electron DOI 10.1007/s11082-012-9604-x [10].D.Shanmugasundar and A.Sivanantharaja, International Journal of Applied Engineering Research, 10 (2015) 25. [11].Myeon-CheonChoia, Youngkyoo Kim, Chang-Sik Ha, Prog. Polym. Sci. 33 (2008) 581–630.
ScienceDirect Materials Today: Proceedings 3 (2016) 2409–2412
www.materialstoday.com/proceedings
Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)
Synthesis and characterization of transparent and flexible polymer clay substrate for OLEDs D.Shanmuga Sundara, A.Sivanantharajaa, C.Sanjeevirajaa, D.Jeyakumarb,* a b
Alagappa Chettiar College of Engineering & Technology, Karaikudi, Tamilnadu CSIR-Central Electrochemical Research Institute, Karaikudi, Tamilnadu, INDIA
Abstract Organic Light Emitting Diode (OLED) has major applications in the field of displays, lighting and visible light communication. Due to the advantages of low cost, light weight, low power and high efficiency, flexible organic light emitting diode have attracted the interest of researchers all over the World. Flexible OLEDs are considered to be the best suitable candidate for next generation lighting devices. In this research, a new flexible and transparent substrate with an average transmittance of 60-70% in the visible region (300-700nm) is synthesized with the help of organic materials such as tetraphenylphosphonium modified lithium saponite (TPP-LiSA), Synthetic Saponite (SA) and N, N-dimethylformamide (DMF). The as-synthesized substrate exhibits refractive index as close as glass which makes this substrate to be used for flexible displays. Characteristics such as extinction coefficient (k), absorption of the synthesized substrate are also calculated. © 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords:Flexible displays; Thin film; Organic Light Emitting Diode, Refractive Index, Polymer Clay Substrate.
* Corresponding author. E-mail address:[email protected] 2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].
2410
D.Shanmuga Sundar, et al./ Materials Today: Proceedings 3 (2016) 2409–2412
1. Introduction Organic light-emitting devices (OLED) have attracted considerable interest for many applications such as flat panel displays and interior lighting source. Due to ultra-thin thickness, light weight and environmental protective nature makes OLEDs to be suitable candidate for next generation planar lighting source and display system [1]. Organic electroluminescence was first experimentally demonstrated by Pope et al. in 1963 [2]. In this case, electroluminescence was observed for an applied voltage of 400 V. However, as-prepared device exhibited low luminance efficiency. The first practical OLED was developed in 1987 at Eastman Kodak by Tang and VanSlyke and the device turn-on voltage was around 10 Volts [3]. Flexible displays are the one which bends during the use but it has certain limit, apart the limit it acts as rolling displays. Recently literature review on flexible candidates has been expanding. It now includes a book on flexible flat panel displays written by Crawford [4] and a special edition of the Proceedings of the IEEE on flexible displays [5]. Eventhough the consequences for flexible displays are good, there is still some difficult on technical and manufacturing developments [6, 7]. Production of flexible displays is still in laboratory level due to the unresolved problems in backplane and high temperature fabrication process but the recent advancement in mobile phones and displays leads to the development of displays with high degree of flexibility and rollability [8]. Another major drawback of plastic as a flexible displays are refractive index mismatch of the substrate and the Indium Tin Oxide (ITO). But this can be overcome by using anode stack which will increase the efficiency of the device [9]. However polyesters such as PET and PEN have advantages such as optical transparency, better thermal coefficient, chemical resistance and moisture absorption, but their upper operating temperature and surface roughness are not so good [10]. Advantages of being as transparent, light weight, flexible and robust, polymers are considered to be a suitable substitute for both glass and plastic in the applications of OLEDs and OPVs. Polymers have mechanical properties that vary from strong rigidity, such as in engineering plastics, to softness, such as in rubber or polyethylene films. Due to low cost, mass production via roll-to-roll (RTR) process, polymers have various application areas such as transparent substrates, electrodes and active materials for organic light emitting devices (OLEDs), LCDs and organic thin-film transistors (OTFTs) which makes the researchers to start the investigation on all polymer-based flexible devices [11]. In this paper, a new transparent and flexible substrate named Polymer Clay substrate for OLED application is synthesized by using mineral clay and tetraphenylphosphonium bromide. The as-synthesized film exhibits transparency of about 60-70% in the visible region. Also the characteristics such as absorption (A), refractive index (n) and extinction coefficient (k) are calculated. Optical band gaps for both direct and indirect transitions are also calculated. The refractive index of the as-prepared film is found to be close as glass. 2. Experimental In this work, a thin flexible flim with improved characteristics is synthesized by means of using materials such as Tetraphenylphosphonium bromide (TPP-Br) purchased from Alfa-Aesar with an assay of 98%, Synthetic Saponite (SA) from Mineral Clay Society of Japan and N,N-Dimethylformamide (DMF) from Sigma Aldrich with 99% assay. The cation exchange capacity (CEG) of the synthetic saponite is found to be around 76meq/100g. The TPPBr of twice the CEG is added to the synthetic saponite and allowed to dissolve in a mixture and then the mixture is stirred for 24 hours. Then the mixture is washed with the solvent (ethanol: water) in the ratio of 1:5 and then it is centrifuged to collect the samples, above process is repeated for atleast three times to obtain a final product. Then the final product is mixed with DMF in the ratio of 40:60 and it is stirred for 24 hours and the final product called organo clay is obtained and coated in a Teflon sheet of uniform thickness by using doctor blade method. In order to increase the characteristics of the film, Poly (ethylene glycol) (PEG) from Fluka analytical and Poly (acrylic acid) (PA) from Alfa Aesar are used. PEG and PA are mixed in the ratio of 40:60 to from the solution at a temperature of 150oC in a hot plate. Then the polymer mixture is added in the organo clay in the ratio of 90:10 and the mixture is stirred for 24 hours to form Polymer clay (P-clay). Then the P-clay materials is degassed and then it is
D.Shanmuga Sundar, et al./ Materials Today: Proceedings 3 (2016) 2409–2412
2411
coated in the Teflon sheet of uniform thickness using doctor blade method and it is allowed to dry at room temperature for few hours and finally a free self-standing flim is peeled off from the Teflon sheet. Characterizations such as Transmittance (T), Absorbance (A) are measured using UV 3000+ UV-VIS spectrophotometer from LABINDIA. From the obtained values, the other parameters such as refractive index (n), extinction coefficient (k) and optical bandgap for both direct and indirect transitions are also calculated. 3. Results and Discussion Figure 1 shows the photographic images of Polymer clay substrate and its flexibility which indicates the flexible nature of the substrate.
Fig.1. (a) Photographic Image of Polymer Clay substrate; (b) Flexibility of the substrate.
Figure 2 shows the changes of transmittance of the substrate with respect to the wavelength using UV 3000+ UVVIS spectrophotometer from LABINDIA. Transmittance of about 60-70% is obtained in the region of 300-700nm (visible region), still we are working on the substrate to increase the transmittance which make suit of the film to be more applicable for display applications.
100
Transmittance (%)
80
60
40
20
0 400
600
800
Wavelength (nm)
Fig.2. Variation of Transmittance with wavelength of P-clay film
1000
2412
D.Shanmuga Sundar, et al./ Materials Today: Proceedings 3 (2016) 2409–2412 2.8
(a)
0.00012
Extinction coefficient (k)
Refractive index (n)
2.4
2.0
1.6
1.2
(b)
0.00010 0.00008 0.00006 0.00004 0.00002 0.00000
400
600
800
1000
1200
400
600
800
1000
1200
Wavelength (nm)
Wavelength(nm)
Fig.3. Plot of Extinction Co-efficient (K) and Refractive Index (n) vs. Wavelength (nm) of P-clay film
Optical parameters such as refractive index (n), extinction coefficient (k) are calculated for the flim using the following equations n=
(1 + R )
(1 − R )
+
4R − k2 (1 − R )2
k=αλ/4π)
(1) (2)
where R is a reflectance, α is an absorption coefficient, λ is the wavelength and π is 3.14. The wavelength dependent of extinction coefficient and refractive index is shown in figure 3a and 3b. The refractive index of the substrate is found to be approximately close to glass which makes it to be suitable for opto-electronic device applications. From figure 3b it can be concluded that the dispersion of extinction coefficient decreased lightly in the visible region and starts to increase after the visible region 4. Conclusion In summary, we had synthesized and characterized a flexible and transparent substrate named P-clay with a transmittance of about 60-70% in the range of 300-700nm (visible region). Also the optical characteristics such as transmittance (T), absorption (A), refractive index (n) and extinction coefficient (k) are also calculated and which shows that the substrate can be used for flexible OLEDs applications. Further, thermal characteristics of the flim have to be improved in order to support high temperature manufacturing process. References [1]. T.-H. Han, Y. Lee, M.-R.Choi, S.-H. Woo, S.-H. Bae, B. H. Hong, J.-H.Ahn, and T.-W. Lee: Nat. Photonics 6 (2012) 105. [2]. M. Pope, H. P. Kallmann, and P. Magnante: J. Chem. Phys. 38 (1963) 2042. [3]. W. C. Tang and S. A. VanSlyke: Appl. Phys. Lett. 51 (1987) 913. [4]. Crawford GP. Flexible flat panel display technology. New York: Wiley; 2005. [5]. Nathan A, Chalamala BR. Special issue on: flexible electronics technology. ProcIEEE 2005;93(7–8). [6]. Gasman L. OLED and paper-like display markets. Veritas et Visus, Flex Substr 2006;2(6):22–5. [7]. Allen KJ. Reel to real: prospects for flexible displays. ProcIEEE 2005;93:1394–9. [8]. Hill P. iSuppli flexible displays report. Veritas et Visus, Flex Substrate 2006;2(5):21–5. [9]. DhanapalanShanmugasundar andA.Sivanantharaja, Opt Quant Electron DOI 10.1007/s11082-012-9604-x [10].D.Shanmugasundar and A.Sivanantharaja, International Journal of Applied Engineering Research, 10 (2015) 25. [11].Myeon-CheonChoia, Youngkyoo Kim, Chang-Sik Ha, Prog. Polym. Sci. 33 (2008) 581–630.