DSpace at VNU: Green light-emitting diodes based on a hybrid TiO2 nanoparticle-conducting polymer blend

Home Search Collections Journals About Contact us My IOPscience Green light-emitting diodes based on a hybrid TiO2 nanoparticle-conducting polymer blend This content has been downloaded from IOPscience Please scroll down to see the full text 2011 Adv Nat Sci: Nanosci Nanotechnol 035012 (http://iopscience.iop.org/2043-6262/2/3/035012) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 128.151.244.46 This content was downloaded on 25/05/2014 at 22:57 Please note that terms and conditions apply IOP PUBLISHING ADVANCES IN NATURAL SCIENCES: NANOSCIENCE AND NANOTECHNOLOGY Adv Nat Sci.: Nanosci Nanotechnol (2011) 035012 (4pp) doi:10.1088/2043-6262/2/3/035012 Green light-emitting diodes based on a hybrid TiO2 nanoparticle-conducting polymer blend Phuong Hoai Nam Nguyen and Nang Dinh Nguyen Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University in Hanoi, 144 Xuan Thuy Road, Cau Giay District, Hanoi, Vietnam E-mail: namnph@vnu.edu.vn Received 25 April 2011 Accepted for publication 16 June 2011 Published 15 July 2011 Online at stacks.iop.org/ANSN/2/035012 Abstract The energy transfer in a blend of conducting polymers, poly[9-vinylcarbarzole] (PVK) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), was investigated Energy transfer from PVK to MEH-PPV leads to enhanced emission of MEH-PPV Photoluminescence (PL) of the nanocomposite of TiO2 and the blended polymer films was enhanced as the relative content of TiO2 was increased, and, in particular, the most improved PL was observed for the optimal ratio of TiO2 and these films emitted green light These results provide further insight into the photophysics of conjugated polymers and the improvement of the current–voltage (I–V) characteristics of the devices based on blended conducting polymer systems Keywords: nanoparticle, conducting polymer, blend polymer, light-emitting diode Classification numbers: 4.02, 5.11 it is processed into EL devices, and different quantum efficiencies were obtained by changing the work function of the electrode [6, 7] In this work, blended films of PVK and MEH-PPV with different weight ratios of TiO2 nanoparticles were fabricated and characterized The photoluminescence (PL) measurement indicated that there is efficient energy transfer between PVK and MEH-PPV The energy-transfer process from PVK to MEH-PPV was observed, and thus the emission of MEH-PPV was exclusively observed when the blended polymer film was photoexcited by light whose energy corresponded to the absorption of PVK Moreover, the relative PL quantum efficiency increased as the weight ratio of TiO2 nanoparticles in the blended polymer increased The current–voltage (I–V) characteristics were also studied Introduction Organic light-emitting devices (OLEDs) have been applied to flat-panel displays due to their ease of manufacturing, all solid-state, faster switching speed and wider viewing angle, etc Along with developing new technology, OLEDs have the potential to replace liquid-crystal display (LCDs) and to become the pacemaker in the display market However, enhancing the brightness, efficiency and stability of OLEDs is still an important area of research [1, 2] In order to achieve more satisfying results, a lot of new materials and methods have been used in fabricating OLEDs Kido et al [3] demonstrated that molecularly doped polymers provide a high level of flexibility in designing electroluminescence (EL) devices, in which poly(9-vinylcarbazole) (PVK) was used both as a matrix and as a hole transport material Poly[2-methoxy-5(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) has been the subject of intense investigation because it can be dissolved in many organic solvents, which allows MEH-PPV to be easily processed into various types [4, 5] It is well known that this polymer shows yellowish-red light when 2043-6262/11/035012+04$33.00 Experimental Figure shows the molecular structures of PVK and MEH-PPV used in this study PVK and MEH-PPV were purchased from Aldrich Chemical Company and used as received Indiumtinoxide (ITO) and Al were used as the anode © 2011 Vietnam Academy of Science & Technology Adv Nat Sci.: Nanosci Nanotechnol (2011) 035012 P H N Nguyen and N D Nguyen CH2 O CH n N n CH3O (a) (b) Figure Molecular structures of the compounds used in this study Figure Normalized optical absorption spectra of (a) PVK and (c) MEH-PPV and photoluminescence spectra of (b) PVK and (d) MEH-PPV thin films Figure SEM of the film (PVK : MEH-PPV) : TiO2 = (100) : 15 and cathode, respectively The sheet resistance of the ITO was 25 cm−1 Before use, the ITO substrate and glass were cleaned carefully according to the method of Tang and Van Slyke [1] The blended polymers were obtained by mixing PVK with MEH-PPV (PVK : MEH-PPV = 100 : 15) and then TiO2 nanoparticles were added in the weight ratios of 100 : 5; 100 : 10; 100 : 15 and 100 : 20, in 1,2-dichloroethane (about 13 mg ml−1 ) The nanocomposite blends were spin-coated onto the substrates and dried in a vacuum at 80 ◦ C for h The thickness of the polymer layer was controlled both by the spin speed and by the concentration of polymer in the solvent The thicknesses of the films were measured to be around 120 nm Atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) images showed no indication of phase separation or layer formation due to the immiscibility of the two polymers (figure 2) The surface morphology of the samples was investigated by using a Hitachi FE-SEM S-4800 AFM images were obtained using an NT-MDT AFM operating in a tunnel current mode The absorption spectra of the coated films were recorded using a Jasco UV–Vis–NIR V570 spectrometer The PL spectra were carried out using a FL3-2 spectrophotometer and I–V characteristics were measured on an Auto-Lab Potentiostat PGS-30 All the photophysical measurements were performed at room temperature in air Figure UV–Vis spectra of PVK, PVK : MEH-PPV and (PVK : MEH-PPV) +TiO2 thin films of MEH-PPV, and thus an efficient Förster energy transfer can be anticipated [8] According to figure 3, PVK absorbs from 200 to 350 nm with two peaks at 295 and 339 nm, respectively, and emits at 410 nm In contrast, MEH-PPV reveals a broad absorption in the longer wavelength region of 420–520 nm and maximum absorption at 490 nm arising from the π-conjugated structure [9] MEH-PPV is the yellowish-red light emitter, and the PL spectra of the thin film show an emission peak at 593 nm and the other at 640 nm, and a small shoulder at 707 nm reflecting excimer formation In figure 4, the nanocomposite blended polymer films show absorption peaks corresponding to both PVK and MEH-PPV Figure shows the PL spectra of the nanocomposite blended films, which were excited at 325 nm From this figure, we see that the peaks at 410 and 555 nm are due to PVK and MEH-PPV emissions, respectively Though the PVK concentration is much higher than that of MEH-PPV, the PL spectra are dominated by the green-yellow MEH-PPV emission With an increase in TiO2 concentration in the blends, the PVK fluorescence decreases while the MEH-PPV Results and discussion Figure compares the UV–Vis and PL spectra of bulk films of PVK and MEH-PPV The PL emission from PVK film excited at 325 nm overlaps with the absorption peaks Adv Nat Sci.: Nanosci Nanotechnol (2011) 035012 P H N Nguyen and N D Nguyen Figure Photoluminescence spectra of the nanocomposite blended polymer thin films (PVK : MEH-PPV) : TiO2 = (100) : 5, (100) : 10, (100) : 15, (100) : 20 All PL spectra were recorded with excitation at 442 nm Figure Normalized photoluminescence spectra of the nanocomposite blended polymer thin films (PVK : MEH-PPV) : TiO2 = (100) : 5, (100) : 10, (100) : 15, (100) : 20 All PL spectra were recorded with excitation at 325 nm emission increases, indicating that more energy transfer occurs from PVK to MEH-PPV At the doping ratio of PVK + MEH-PPV : TiO2 = 100 : 15 and 100 : 20, the efficient Förster energy transfer from PVK to MEH-PPV becomes saturated, which results in the appearance of MEH-PPV emission in the green region and the disappearance of the small shoulder at 707 nm Although PL enhancement has rarely been mentioned, one could suggest that the increase in PL intensity for such a composite film can be explained by the large absorption coefficient for TiO2 particles Indeed, this phenomenon would be attributed to the non-radiative Förster resonant energy transferfrom TiO2 nanoparticles to polymer with excitation of wavelength of less than 350 nm [10] Figure shows the PL spectra of the nanocomposite blended polymer films with excitation at 442 nm to examine the TiO2 ratio effect on the MEHPPV emission MEH-PPV strongly absorbs the light at 442 nm but PVK does not (figure 2) From figure 6, we can see that a blue shift from 588.4 to 560.6 nm was observed for the PL peak, similar to the result in [11] reporting on the PL spectra for a hybrid MEH-PPV/nc-MoO3 film This result is consistent with the currently obtained result from polymeric nanocomposites [12] where the blue shift was explained by a reduction in the chain length of the polymer, when nanoparticles were embedded latter [10] The single EL devices based on the blended polymer films were fabricated, consisting of a transparent ITO conducting electrode, the (PVK : MEH-PPV) +TiO2 film and an aluminium (Al) electrode: ITO/(PVK : MEH-PPV) +TiO2 /Al The thickness of the nanocomposite blended film was estimated to be around 120 nm For labeling the samples, device (D1), device (D2), device (D3) and device (D4) are abbreviated notations of the samples with (PVK : MEH-PPV) : TiO2 = (100) : 5; (100) : 10; (100) : 15 and (100) : 20, respectively Figure shows the I–V characteristic of the single-layer devices The nanocomposite blends increase the threshold field of the devices According to figure 7, doping with TiO2 , however, appears to stabilize the device to a significant extent Figure Electrical properties of the single EL-devices: (D1) ITO/(PVK : MEH-PPV) : TiO2 = (100) : 5/Al; (D2) ITO/(PVK : MEH-PPV) : TiO2 = (100) : 10/Al; (D3) ITO/(PVK : MEH-PPV) : TiO2 = (100) : 15/Al; (D4) ITO/(PVK : MEH-PPV) : TiO2 = (100) : 20/Al Device D3 reveals threshold field around 2.5 V and good stability The results suggest that the optimal ratio of the TiO2 in the blend of PVK + MEH-PPV is a promising material for a high-efficiency EL device as well as organic solar cell applications Conclusion A new emitting layer based on the blending of PVK and MEH-PPV doping with TiO2 was fabricated The investigation into the photophysics of the blended polymers suggests that the excitons in PVK produced upon photoexcitation rapidly transfer to MEH-PPV The energy-transfer process enhances not only the PL quantum efficiency of the blended polymers but also the emission in the green region The blended film with optimal ratio will be a candidate for a green light-emitting diode Adv Nat Sci.: Nanosci Nanotechnol (2011) 035012 P H N Nguyen and N D Nguyen Acknowledgments [4] Brown A R, Greenham N C, Burroughes J H, Bradley D D C, Friend R H, Burn P L, Kraft A and Holmes A B 1992 Chem Phys Lett 200 46 [5] Smilowitz L, Hays A, Heeger A J, Wang G and Bowers J E 1993 J Chem Phys 98 6504 [6] Yang Y and Heeger A 1994 J Appl Phys Lett 64 1245 [7] Parker I D 1994 J Appl Phys 75 1656 [8] Nam N P H, Cha S W, Kim B S, Choi S-H, Choi D S and Jin J-I 2002 Synth Met 130 271 [9] Chung S J, Jin J-I and Kim K K 1997 Adv Mater 551 [10] Dinh N N, Chi L H, Thuy T T C, Trung T Q and Truong V V 2009 J Appl Phys 105 1255 [11] Dinh N N, Chi L H, Thuy T T C, Thanh D V and Nguyen T P 2008 J Korean Phys Soc 53 802 [12] Heliotis G, Itskos G, Murray R, Dawson M D, Watson I M and Bradley D D C 2006 Adv Mater 18 578 This work was supported in part by the Vietnam National University in 2010 (Project Code: CN.10.08) and by the NAFOSTED in 2010 (Project Code: 103.02.88.09) References [1] Tang C W and Van Slyke S A 1987 Appl Phys Lett 51 913 [2] Burroughes J H, Bradley D D C, Brown A R, Marks R N, Mackay K, Friend R H, Burns P L and Holmes A B 1990 Nature 347 608 [3] Kido J, Hongawa K, Okuyama K and Nagai K 1994 Appl Phys Lett 64 815 ... organic solar cell applications Conclusion A new emitting layer based on the blending of PVK and MEH-PPV doping with TiO2 was fabricated The investigation into the photophysics of the blended polymers... images showed no indication of phase separation or layer formation due to the immiscibility of the two polymers (figure 2) The surface morphology of the samples was investigated by using a Hitachi... Though the PVK concentration is much higher than that of MEH-PPV, the PL spectra are dominated by the green- yellow MEH-PPV emission With an increase in TiO2 concentration in the blends, the PVK