Incorporation of a liquid crystal to enhance the luminescence properties of Langmuir BlodgettfilmsofOC1OC6-PPV

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1 Universidade de São Paulo Biblioteca Digital da Produção Intelectual - BDPI Departamento de Física e Ciências Materiais - IFSC/FCM Artigos e Materiais de Revistas Científicas - IFSC/FCM Incorporation of a liquid crystal to enhance the luminescence properties of Langmuir BlodgettfilmsofOC1OC6-PPV Journal of Luminescence,Amsterdam : Elsevier,v. 129, n. 11, p , Nov Downloaded from: Biblioteca Digital da Produção Intelectual - BDPI, Universidade de São Paulo

2 Journal of Luminescence 129 (29) Contents lists available at ScienceDirect Journal of Luminescence journal homepage: Incorporation of a liquid crystal to enhance the luminescence properties of Langmuir Blodgett films of Marystela Ferreira a,,debora T. Balogh b, Jose A. Giacometti c, Clarissa A. Olivati d a Universidade Federal de S~ao Carlos, Campus de Sorocaba, , Sorocaba, SP, Brazil b IFSC/USP, CP 369, , S~ao Carlos/SP, Brazil c FCT/UNESP, CP 467, 196-9, Presidente Prudente/SP, Brazil d Depto de Física/IGCE/UNESP, CP 178, , Rio Claro/SP, Brazil article info Article history: Received 7 November 28 Received in revised form 23 June 29 Accepted 2 July 29 Available online 8 July 29 Keywords: Langmuir Blodgett films Photoluminescence Properties abstract The control of molecular architecture provided by the Langmuir Blodgett (LB) method allows one to obtain enhanced luminescence properties in polymer films, which can be exploited in polymer light emitting diodes (PLEDs). In this study, we show that incorporating a liquid crystal (LC) (PCH32/34) into poly-(2-methoxy-5-hexyloxy-p-phenylene-vinylene) () in LB films leads to a 4-fold increase in photoluminescence and 2-fold increase in electroluminescence for (ITO/(LC5%)/ Al) devices for emission at 585 nm, with the current vs. voltage (I vs. V) characteristics exhibiting a typical behavior of rectifying diode. The enhanced luminescence was attributed to an efficient separation of emitting units induced by the LC in the LB film, which were less ordered than LB films from neat. & 29 Elsevier B.V. All rights reserved. 1. Introduction A key factor for obtaining efficient organic devices is the ability to control the molecular architecture and finding synergy in properties of distinct materials that are combined in the same device, as is the case of heterojunctions in organic solar cells [1]. In electroluminescent devices, in particular, film architecture plays an important role [2] and advantage may be taken of filmforming methods allowing for molecular control of the final properties, such as the electrostatic layer-by-layer (LbL) [3] and the Langmuir Blodgett (LB) [3] techniques. In LB films one major challenge is to obtain sufficiently robust films that withstand application of the electric field without short-circuiting, which then allows for a low operation voltage owing to the ultra-thin nature of the films. Indeed, suitable LB films from poly(pphenylene vinylene (PPV)) derivatives could be used in electroluminescent devices where charge injection was enhanced by adding ionomer layers, with much superior performance than devices made with cast films [4]. An added advantage of LB films may be the possible emission of polarized light [5]. Further improvements in polymer electroluminescent devices using LB films may be envisaged through a fine control of the molecular architecture and combination with distinct materials. An important issue in this regard is to control the radiative energy Corresponding author. Tel.: ; fax: address: marystela@ufscar.br (M. Ferreira). transfer in these layered films, similarly to what has been achieved in LbL films [6]. One needs to find a balance between increasing organization and avoiding a close molecular packing that would favor non-radiative energy transfer in the LB films. In this study, we combine a PPV derivative, namely poly-(2- methoxy-5-hexyloxy-p-phenylene-vinylene) () and a liquid crystal (LC), in a strategy similar to that of Carter et al. [7] who obtained enhanced photoluminescence and electroluminescence by adding a surfactant to MEH-PPV polymer films. Optoelectronic devices such as LEDs benefit from high photoluminescence (PL) yield of their active layer, and several routes have been tried to enhance PL [8,1]. While the initial motivation was to improve order in the LB films, we found out that the incorporation of the LC actually induced disorder, but nevertheless enhanced the optical properties of the films, probably owing to an efficient separation of emitting units. For the sake of comparison, we also produced cast films. 2. Experimental Poly-(2-methoxy-5-hexyloxy-p-phenylene-vinylene) (OC 1 OC 6 - PPV) was synthesized with a modified procedure to that patented by Wudl et al. [9], and LC (PCH32/34) was purchased from Merck. The cast and Langmuir films were obtained by spreading a solution prepared by mixing chloroform solutions of (.2 mg ml 1 ) and LC (1 mg ml 1 ), with relative concentrations of 1, 2 and 3 wt% of LC in for cast films, and for /$ - see front matter & 29 Elsevier B.V. All rights reserved. doi:1.116/j.jlumin

3 1382 M. Ferreira et al. / Journal of Luminescence 129 (29) Langmuir films. Cast films were obtained as described elsewhere [4]. Langmuir and LB films were produced with a KSV5 Langmuir trough housed in a class 1, clean room. Langmuir films were spread onto ultra pure water supplied by a Millipore Milli-Q system (resistivity 18.2 M O cm). All experiments were conducted at room temperature (22 1C). Film compression was carried out at a barrier speed of 1 mm min 1. p-a isotherms were conducted with a Wilhelmy plate provided by KSV. The molecular weight of the polymer-repeating unit (232 g mol 1 ) was adopted for all areas calculated. Y-Type LB films of (LC 3%) were transferred onto ITO (41 layers) and glass (31 layers) at a surface pressure of 2 mn m 1 with a dipping speed of 3 mm min 1 for the upstrokes and 3 mm min 1 for the downstrokes, in the dark at room temperature of 22 1C. The LB films from neat were prepared as described elsewhere [1]. The UV Vis absorption and the emission spectra of the films were recorded in a Hitachi U-21 spectrometer and Shimadzu 531 PC spectrofluorimeter, respectively. Devices were fabricated for an opto-electrical characterization of the films. Cast and LB films from identical solutions and under identical conditions as those to measure absorbance and PL were used as active layers of the devices. The current vs. voltage (I vs. V) measurements were carried out with a Keithley 238 unit and the active area of the device was ca.12 cm 2. The device was fabricated as follows: the indium-tin-oxide (ITO) anode was covered by a cast or LB film as the active layer. The cathode was formed by vacuum evaporating aluminum (Al) onto the organic film (ITO/(LC)/Al). For the sake of comparison, devices with neat films were also fabricated, as described elsewhere [5]. All measurements were carried out at room temperature, under vacuum and in the dark. Surface pressure / mn m neat LC 1.5% LC 2.5% LC 5% Area per molecule / Å 2 3. Results and discussion For a systematic investigation of the mixed films, we compare in Fig. 1 the isotherms with the published curves for neat OC 1 OC 6 - PPV monolayers [1]. The area per molecule at the condensed phase, extrapolated to zero pressure from the high-pressure part of the curve, is ca. 22 Å 2 for the neat monolayer. For the mixtures, we note that LC affects the monolayer already in an expanded phase, as LC molecules are incorporated into the monolayer. The isotherms display two plateaus, the first of which occurs with LC incorporation, while the second is probably due to expulsion of the LC molecules. Indeed, the extrapolated area per molecule for LC1% is much smaller than for pure, which is consistent with a smaller number of molecules at the interface. Though for the other two mixtures we cannot determine an extrapolated area, it is likely to be small as well. Since we cannot be sure whether the mixed monolayers were compressed to a condensed state compression was stopped before that it is difficult to determine the collapse pressure. It is clear, nevertheless, that the mixed films are likely to be less stable. The transfer of LC along with in the LB films was confirmed with FTIR spectroscopy. The spectra of neat OC 1 OC 6 - PPV and mixed (LC) (3%) in both cast and LB films are similar to each other (results not shown), with the exception of a band at 825 cm 1 assigned to vinylic C H in LC. However, the band is relatively less intense than in cast films, which may be attributed to an incomplete transfer of the LC along with OC 1 OC 6 - PPV. Indeed, by comparing FTIR data for cast and LB films, we estimated that for a mixture containing 3% of LC only 5% of the LC are transferred. The incomplete transfer should be expected from the surface pressure isotherms that indicated that some LC molecules may not remain at the interface at high pressures. Fig. 1. Surface pressure area isotherms for pure and the mixtures of and LC spread on water (ph ¼ 5.6) sub-phases, with the area per molecule being calculated per monomer. Fig. 2 shows the UV Vis spectra for neat and (LC) mixtures, featuring l max at 5 and 47 nm for cast and LB films, respectively, which are related to transitions between delocalized p p* states. There is practically no effect from adding the LC, and the red shift in absorbance for the cast films is probably to a larger degree of aggregation in comparison with the LB films. In subsidiary experiments, we observed in the polarized absorption spectra that (LC 3%) LB films are anisotropic, with polymer chains oriented preferentially along the dipping direction, analogously to the results for LB films from neat [5]. Fig. 3 shows the PL spectra of neat and OC 1 OC 6 - PPV (LC) of cast and LB films. The PL spectrum of neat cast film displays a zero-phonon peak (I ) at 591 nm and one electron-vibrational coupling mode (phonon replica I 1 ) at 645 nm (Fig. 3a). For the (LC) cast film, two peaks appeared: one related to zero-phonon transition at 591 nm and the phonon replica at 629 nm, with decreased line width and increased intensity of the zero-phonon peak when compared with neat film. A similar effect is observed for LB films where the intensity of zero-phonon peak increases with addition of LC, while the electron-vibrational coupling mode (phonon replica I 1 ) was suppressed. The relative intensities of the vibronic structures are directly related to the coupling between the electronic transitions and vibrational modes, which can be characterized by the Huang Rhys factor (SI 1 /I ). Experimental studies reveal that the Huang Rhys factor is correlated with disorder; that is, the more localized the electronic states the more

4 M. Ferreira et al. / Journal of Luminescence 129 (29) cast films 1. LB films.8.8 Absorbance Fig. 2. UV Vis spectra of (A) neat (U) and (3%LC) (o) cast films and (B) neat (U) and (5%C) (o) LB films. 8 (LC3%) 8 (LC5%) 6 6 PL (unid. arb.) λ (nm) λ (nm) Fig. 3. Photoluminescence spectra of (A) neat ( ) and (3%LC) ( ) (o) cast films and (B) neat ( ) and (5%LC) ( ) LB films. pronounced the vibronic progression. The disorder causes the conjugation length of this kind of polymer to decrease. Therefore, the LC causes disorder in cast and LB films, since S is lower for (LC) than for neat films. The density current vs. voltage measurements, depicted in Fig. 4 for devices made with LB films, reveal a classical behavior of a polymer light emitting diodes (PLED) [11]. The threshold value is about 5 V, and the onset of visible light occurs at ca. 8 V. The electroluminescence (EL) spectra for the and (LC5%) PLED, at 12 V forward bias, are centered at 585 nm, similarly to the photoluminescence spectra, which indicates that the radiative decay of the same singlet exciton is responsible for both emissions. The EL intensity of (LC5%) PLED was 1.6 the value for PLED at 585 nm. Devices with cast films were not produced, for in analogy to the results for neat [4] the performance is expected to be inferior to those with LB films. 4. Conclusions The incorporation of a LC into LB films led to an enhanced performance in optical properties, as demonstrated in PLED devices. This enhancement cannot be attributed to an increased order in the films. On the contrary, the LC appears to induce disorder in the LB films, as indicated by the analysis of the PL profiles, while keeping the same anisotropic nature as the neat LB films. Therefore, the enhanced luminescence is probably due to a separation of emitting units in the LB films, which reduces non-radiative energy transfer processes [7,12].

5 1384 M. Ferreira et al. / Journal of Luminescence 129 (29) J (A/cm 2 ) 1-4 EL (arb. units.) Voltage (V) Fig. 4. (A) Current density vs. voltage curves and (B) electroluminescence spectrum at 1 V of applied voltage for ITO/(LB)/Al (U) and ITO//LC5% (LB)/Al (o) devices. Acknowledgments This work had the financial support from FAPESP, CNPq and IMMP/MCT (Brazil). We also thank Prof. Osvaldo N. Oliveira Jr. for helpful discussions. References [1] J. Peet, J.Y. Kim, N.E. Coates, W.L. Ma, D. Moses, A.J. Heeger, G.C. Bazan, Nature Materials 6 (27) 497. [2] E. Arias-Marin, J.C. Arnault, D. Guillon, T. Maillou, J. Le Moigne, B. Geffroy, J.M. Nunzi, Langmuir 16 (2) 439. [3] M. Ferreira, V. Zucolotto, M. Ferreira, O.N. Oliveira Jr., K. Wohnrath, in: S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, Vol. 4, American Scientific Publishers, Los Angeles, 23, p [4] C.A. Olivati, M. Ferreira, A.J.F. Carvalho, D.T. Balogh, O.N. Oliveira Jr., H. von Seggern, R.M. Faria, Chemical Physics Letters 48 (25) 31. [5] C.A. Olivati, M. Ferreira, T. Cazati, D.T. Balogh, F.E.G. Guimar~aes, O.N. Oliveira Jr., R.M. Faria, Chemical Physics Letters 381 (23) 44. [6] H.R. Favarim, D. Spadacio, A.D. Taceto, V. Zucolotto, O.N. Oliveira Jr., F.E.G. Guimar~aes, Advanced Functional Materials 17 (27) [7] V. Sholin, E.J. Lopez-Cabarcos, S.A. Carter, Macromolecules 39 (26) 583. [8] T.-Q. Nguyen, I.B. Martini, J. Liu, B.J. Schwartz, Journal of Physical Chemistry B 14 (2) 237. [9] F. Wuld, G. Srdanov, USPO (1993). [1] M. Ferreira, C.J.L. Constantino, C.A. Olivati, L. Vega, D.T. Balogh, R.F. Aroca, R.M. Faria, O.N. Oliveira Jr., Langmuir 19 (23) [11] H. Burroughes, D.D.C. Bradely, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (199) 539. [12] N.C. Greenham, R.H. Friend, Solid State Physics, Vol. 49, Academic Press Inc, 1995, p. 1.