Concentration effect of glycerol on the conductivity of

Materials Science and Engineering B104 (2003) 26–30

Concentration effect of glycerol on the conductivity of PEDOT film and the device performance S.L. Lai, M.Y. Chan, M.K. Fung, C.S. Lee∗ , S.T. Lee Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China Received 2 April 2003; accepted 1 July 2003

Abstract Poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT)-based polymeric electroluminescent (EL) devices using glycerol-modified poly (styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (GPEDOT) films as anodes have been fabricated. By comparing the devices made with unmodified commercial PEDOT anode, glycerol doping effectively reduces the operating voltage. At a current density of 60 mA/cm2 , the operating voltage for device with 0.28 g/ml GPEDOT anode greatly reduces to 7.8 V; while that for device with unmodified PEDOT anode is about 9.9 V. This is mainly attributed to the significant enhancement of conductivity by incorporating a suitable amount of glycerol into polymeric PEDOT layer. It was further found that the conductivity of PEDOT film increases with the concentration of glycerol doping. Therefore, this highly conductive polymeric anode can be finely tuned to have an optimized conductivity such that a better balance of electron and hole currents, and thus a better device efficiency, can be achieved. © 2003 Elsevier B.V. All rights reserved. Keywords: PLED; Electroluminescence; PEDOT

1. Introduction Polymeric light-emitting devices (PLEDs) have attracted considerable attention due to its potential applications in flat panel displays since the discovery of electroluminescence (EL) from a thin-film polymeric layer in 1990 [1]. A lot of efforts have been made to improve their performance by modifying their structures to achieve an effective and balanced injection of the carriers. At the cathode side, the use of low work function metals with a stable cap, such as Ca/Al, LiF/Al, LiF/Ca/Al, CsF/Yb/Ag [2–5] and the introduction of electron transporting materials with high electron affinity [6] have been investigated. At the anode side, since the highest occupied molecular orbitals (HOMO) of the most conjugated polymers, such as poly(p-phenylene vinylene) (PPV) and polyfluorene (PFO) lie more than 5 eV below the vacuum level, so that there is a significant energy barrier to hole injection into the polymer from indium-tin oxide (ITO) anode, which is responsible for an increase in the driving ∗ Corresponding author. Tel.: +86-852-2788-7826; fax: +86-852-2788-7830. E-mail address: [email protected] (C.S. Lee).

0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(03)00262-9

voltage of the devices. Possible strategies to ease the problem are to insert a hole transporting polymeric layer (HTL) [7–10], an insulating layer [11], and by various treatments of ITO [12,13]. The introduction of poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) is one of the most commonly used anode modification approach [8–10]. The major advantage that the PEDOT:PSS layer brings into the device design is a 0.5 eV reduction of the barrier height at the anode/polymer interface, independent of the work function of the underlying ITO [8], yielding an increase in device efficiency and lifetime and a reduction in the operating voltage. PEDOT has been doped with PSS mainly to enhance the conductivity of the film from about 300 to 10 S/cm [14], which can further improve current–voltage–luminance (I–V–L) characteristics of PLEDs and EL quantum efficiency. Recent studies have demonstrated that the conductivity of PEDOT:PSS films can be dramatically increased by adding a high boiling liquid additives such as glycerol or sorbitol [15–17]. Kim et al. reported that this highly conductive polyalcohol modified anode greatly decreases the surface sheet resistance to about 1850 /sq, comparing to

S.L. Lai et al. / Materials Science and Engineering B104 (2003) 26–30

the value of about 7150 /sq for PEDOT:PSS. This leads to a reduction of operating voltage and a better EL efficiency, compared to that using the commercial PEDOT:PSS anode, in molecular OLEDs [17]. Furthermore, Fung et al. also demonstrated this improvement in PFO-based PLEDs [18]. However, the influence of the amount of glycerol on the device performance has not been investigated yet. In this letter, we fabricated and characterized poly(9,9dioctylfluorene-co-benzothiadiazole) (F8BT)-based PLEDs using glycerol-modified and commercial conducting polymeric PEDOT:PSS anode (hereafter, abbreviated as GPEDOT and PEDOT, respectively). In addition, the effects of glycerol concentration on the device performance were also studied in order to optimize the device performance. The results suggest that devices with an optimum amount of glycerol doping consistently shows higher current density and brightness at the same driving voltage, and thus better device efficiency. This significant improvement is accompanied by the dramatic increase in conductivity of polymeric PEDOT layer.

2. Experimental The device configuration of PLEDs discussed in this letter was single layer sandwich structure consisting of a top calcium electrode with an additional protective layer of silver, a F8BT emitting layer and either a PEDOT or a GPEDOT anode on an ITO glass substrate. 30 /sq patterned ITO glass was used as substrate, which was subjected to a routine cleaning process with rinsing in Decon 90, deionized water and then drying in an oven and finally treating in an UV-ozone chamber. Conducting polymer dispersions of PEDOT used in this work was purchased from Bayer AG, Leverkusen, Germany. A 60 nm thick unmodified PEDOT or GPEDOT (prepared by mixing 1.7 g glycerol in 3 ml of methanol into 6, 7, 12 ml PEDOT forming 0.28, 0.24, 0.14 g/ml GPEDOT) anode was spin-coated onto the ITO glass substrates. After baking at 150 ◦ C for 20 min, a thin layer of the emissive polymer, F8BT supplied by the Dow Chemical Company, from a xylene solution (15 mg/ml), was then spun onto the PEDOT or GPEDOT to form a 70 nm thick film and the device was transferred to an evaporation chamber for the cathode deposition after further baking. A 50 nm thick Ca cathode layer had then been deposited and following by 200 nm Ag cap. Deposition rates of the Ca and the Ag metal layers were controlled to be 6–7 and 1–2 Å/s, respectively. All films were deposited at pressures below 5× 10−6 mbar. The deposition rates were controlled by a quartz oscillating thickness monitor. A shadow mask was used to define the cathode and to make four 0.1 cm2 devices on each substrate. Luminance–current–voltage characteristics were measured simultaneously with a programmable Keithley model 237 power source and a Photoresearch PR650 spectrometer in air at room temperature.

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3. Results and discussion Fig. 1(a) and (b) show the current density and luminance as a function of operating voltage for a set of devices fabricated using PEDOT, 0.14, 0.24, and 0.28 g/ml GPEDOT layers as anodes. The chemical structure of F8BT is shown in the inset of Fig. 1a. Apparently, J–V–L characteristics clearly indicate that the device performance is significantly improved by employing glycerol-modified anodes. Both the J–V and L–V curves are shifted towards lower voltage. For instance, the voltage at a current density of 60 mA/cm2 is 8.6 and 9.9 V for devices using 0.14 g/ml GPEDOT and PEDOT as anodes, respectively. Further increase of the glycerol concentration to 0.28 g/ml GPEDOT would further decrease the operating voltage to 7.8 V. Similarly, the voltages to obtain a luminance of 3000 cd/m2 for the devices using 0.28, 0.14 g/ml GPEDOT and PEDOT as anodes are, respectively, 7.6, 8.2 and 9.4 V. This clearly indicates that the presence of glycerol in the conducting PEDOT film significantly reduces the operating voltage of the PLEDs. Besides, a maximum luminance of 8400 cd/m2 for the device using 0.28 g/ml GPEDOT anode was achieved. It is almost 2.6 times higher than that for the device using undoped PEDOT anode. This clearly demonstrates that after the addition of glycerol the device can effectively sustain a higher current density and correspondingly raises the brightness. The luminance versus current density curves for devices using GPEDOT and PEDOT anodes is depicted in Fig. 2. It is clear that at a current density of 40 mA/cm2 the device fabricated with 0.24 g/ml GPEDOT as anode displays the highest luminance (2955 cd/m2 ) and current efficiency (7.39 cd/A) comparing to the corresponding values of 2500 cd/m2 and 6.25 cd/A in the device using unmodified PEDOT film. The corresponding EL efficiencies of the devices with 0.28 and 0.14 g/ml GPEDOT anodes are 6.06 and 5.31 cd/A, respectively. As suggested by Mäkinen [15], the work function of PEDOT film does not change but its conductivity dramatically increases upon glycerol doping. In order to explore the effect of glycerol concentration on the work function we have studied several completely dried PEDOT and GPEDOT films with different concentration glycerol 0.28, 0.24 and 0.14 g/ml by ultraviolet photoemission spectroscopy (UPS), the experimental as mentioned in [18]. The results are consistent with those from other studies that the addition of glycerol into PEDOT film has no observable effect on the work function [15,18]. In order to understand the effect of glycerol amount on the conductivity of this conducting polymeric layer, we had measured the conductivities of the pristine PEDOT and GPEDOT films. Fig. 3 shows the I–V characteristics of these polymeric films and the inset shows the chemical structure of PEDOT and glycerol. Both films show a linear I–V characteristics and the resistance of commercial PEDOT film is about 16.9 M (corresponding to the conductivity of 9.8 S/cm), which is consistent with that

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Fig. 1. (a) J–V and (b) L–V characteristics of the devices using different concentrations of glycerol in PEDOT anodes. The inset in part a shows the chemical structure of F8BT.

reported [14,18], while those of 0.14 and 0.28 g/ml GPEDOT films were 3.57 and 1.92 M, respectively (corresponding to the conductivity of 46.6 and 86.8 S/cm). This apparently demonstrates that glycerol doping significantly increases the conductivity of polymeric PEDOT film, and is consistent with the shift in I–V characteristics of the presented devices. Ghosh and Inganäs suggested that such increase in conductivity of PEDOT film is attributed to the rearrangement of the PEDOT at high temperature (150 ◦ C in this case), with the high boiling liquid additives such as glycerol acting as plasticisers, to form a better connection between the conducting PEDOT chains [16]. With the work function and conductivity data, the effect of glycerol concentration on the device performance can be understood. As the glycerol doping has no effect on the work function, it is believed that the performance enhancement is not related to a change of the hole injection barrier. Instead, the enhancement is attributed to the increase in

the conductivity of the PEDOT layer and thus the current density. It is also interesting that while the GPEDOT with highest concentration of glycerol gives the highest current density, it does not lead to the highest current efficiency. As the concentration of glycerol in PEDOT is increased, the conductivity of the anode and thus the hole current density are increased. Thus, the relative amount of hole and electron currents in the PLEDs can be finely tuned by modifying the glycerol concentration in the PEDOT layer. For polymer-based devices, a much higher hole injection barrier generally exists that limits the EL efficiency of these PLEDs. An improvement on hole injection or transport, either by increasing the work function or increasing the conductivity of the anode, can significantly enhance current efficiency. In the present case, glycerol doping can increase the conductivity of the anode, giving a better hole transport into emissive polymer. In the devices with low and high glycerol concentrations, there is a surplus of electron and hole currents,


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Fig. 2. Luminance plotting against the current density for the devices using different concentrations of glycerol in PEDOT anodes.

Fig. 3. I–V characteristics of pristine and glycerol doped PEDOT films. The inset shows the chemical structure of PEDOT (upper) and glycerol (lower).

respectively. Consequently, lower EL efficiencies for such devices are obtained. For an optimum glycerol concentration (0.24 g/ml GPEDOT in the present case), the hole and electron current are best balanced and gives the highest current efficiency. It is also important to mention that due to the high sheet resistance and poor conductivity of the commercial PEDOT film, the devices using unmodified PEDOT failed at a relatively low current density of about 60 mA/cm2 ; while GPEDOT anodes can sustain to a considerably high current density without failure.

4. Conclusions We have fabricated F8BT-based PLEDs using glycerolmodified anodes with different concentrations. At a fixed operating voltage, glycerol doping effectively enhances the current density and corresponding brightness of devices. This is attributed to the dramatic increase of conductivity of PEDOT layer, which increases almost by eight times for 0.28 g/ml GPEDOT film comparing to that of the unmodified one. Furthermore, it was found that the glycerol concentration in PEDOT film critically influences its conductivity.

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This shows that the glycerol-modified anode can be finely tuned for an optimized concentration of glycerol (0.24 g/ml) in order to achieve a better balance of electrons and holes at the F8BT interface and thus a higher current efficiency. Acknowledgements This work was supported by the Research Grants Council of Hong Kong (Project No. CityU 1028/00E). References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackey, R.H. Friend, P.K. Burns, A.B. Holmes, Nature 347 (1990) 539. [2] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477. [3] T.M. Brown, R.H. Friend, I.S. Millard, D.J. Lacey, J.H. Burroughes, F. Caciali, Appl. Phys. Lett. 79 (2001) 174–176. [4] M.Y. Chan, S.L. Lai, M.K. Fung, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 374 (2003) 215–221. [5] L.C. Palilis, D.G. Lidzey, M. Redecker, D.D.C. Bradley, M. Inbasekaran, E.P. Woo, W.W. Wu, Synth. Met. 111–112 (2000) 159–163.

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