Photo- and electroluminescence efficiency in poly(dialkoxy-p-phenylenevinylene)

SYliilTlUI|TIIC BITRILS ELSEVIER

Synthetic Metals 66 (1994) 75-79

Photo- and electroluminescence efficiency in poly(dialkoxy-pphenylenevinylene) D. Braun, E.G.J. Staring, R.C.J.E. Demandt, G.L.J. Rikken, Y.A.R.R. Kessener, A.H.J. Venhuizen Philips Research Laboratories, Professor Holstlaan 4, 5656 A A Eindhoven, Netherlands

Received 23 February 1994; accepted 9 March 1994

Abstract

This paper presents the dependence of luminescence efficiency on the degree of conjugation for a soluble poly(dialkoxyp-phenylenevinylene),comparing the photoluminescence yields with the electroluminescence yields. Photoluminescence efficiency increases with the fraction of non-conjugated units for polymers both in the solid state and in solution. The highest external electroluminescence efficiency measured is 1.4% (photons/electron) for a polymer having approximately 10% non-conjugated units. Electroluminescence efficiency depends not only on the radiative recombination efficiency of the emitting material but also on carrier transport into and within the material. K~3~words: Luminescence; Poly(dialkoxy-p-phenylcnevinylene)

1. Introduction

The discovery of electroluminescence (EL) in poly(pphenylenevinylene) (PPV) [1] was followed by a great deal of interest in EL from PPV derivatives [2-10] and other materials [11]. PPV derivatives offer opportunities to control emission colour, to facilitate solution processing and to enhance emission efficiency. This article pursues the latter goal by focusing on the emitting layer. For example, we do not vary the electrodes, even though the use of alternative electrode materials changes carrier injection efficiency and can strongly influence device efficiency [12,13]. EL efficiency depends both on the radiative recombination efficiency of the emitting material and the carrier transport to and within the material. Strategies used to enhance radiative efficiency in polymer lightemitting diodes (LEDs) include copolymers that incorporate a wider bandgap or insulating segments, incomplete conversion of a non-conjugated precursor [5-9,14,15] and blends that dilute the emitting material in a host matrix [10,16,17]. The goal of both these techniques can be expressed from the perspective of a guest-host description: optimal EL efficiency occurs when the emission centres have been diluted enough to allow high photoluminescence (PL) efficiency, but 0379-6779/94/$07.(10 © 1994 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02132-I

not so dilute as to impede carrier transfer to the emission centres. Alternatively, a bandgap engineering description can be used: optimal radiative carrier recombination occurs in an appropriately designed system of potential wells separated by barriers. Both descriptions suggest that the increase in PL efficiency with the fraction of non-conjugated segments and the decrease in intrachain mobility should cause the EL efficiency to peak at an intermediate value. This article presents the dependence of both PL and EL efficiency on degree of conjugation for a soluble poly(dialkoxy-pphenylenevinylene) (O-PPV).

2. Material and device preparation

Fig. 1 shows the synthesis of the polymers used in this study, where RI=CH3 and R2---CloHzl [18]. A controlled C1/OCH3 exchange, followed by conjugation, adjusts the fraction of non-conjugated units. The fractions n of conjugated units and m of non-conjugated units are determined by proton NMR. Devices are prepared on glass substrates covered with patterned indium-tin oxide (ITO) electrodes (R<20 fZ/V1) using a previously reported technique [12]. The polymer films are spin-coated from toluene

76

D. Braun et al. / Synthetic Metals 66 (1994) 75-79

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or toluenefrHF solutions with concentrations of 5-20 mg/ml. Calcium contacts (2500 /~) are deposited on top of the polymer films by vacuum evaporation at pressures below 2 × 10 -6 mbar yielding active areas of 4-81 mm 2. All processing steps are carried out in a nitrogen atmosphere. Films for PL are prepared on glass substrates without ITO.

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3. Characterization

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Fig. 2 compares the room temperature PL spectra for copolymers with various degrees of conjugation• Spectra recorded using a Perkin-Elmer LS50 luminescence spectrometer have the broad emission band with vibronic features characteristic of non-oriented samples of these materials [2,19]. Spectra are normalized to the amplitude of the zero-phonon peak. Copolymers with m < 15% non-conjugated segments have a zerophonon peak at approximately 590 nm and the onephonon peak at 634 nm. With increasing conjugation, there is a noticeable increase in oscillator strengthin the red, seen in the increased strength of the onephonon peak. Nevertheless, the position of the zerophonon peak remains constant. For the least conjugated sample measured (m = 37 + 7%), there is a noticeable blue shift associated with the decrease in conjugation length, as the zero-phonon peak shifts to 560 nm. Fig. 3 shows the smoothest rather than a typical current versus voltage characteristic. Current and EL 1.2

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intensity are plotted as a function of voltage applied to the ITO electrode with the calcium electrode grounded. The voltage was swept from 0 to + 20 to - 2 0 V and back to 0 V using a Hewlett-Packard 4145 semiconductor parameter analyser. A 1 cm 2 Si photodiode (Hamamatsu S1227-1010BQ) detects the EL intensity, and a Keithley 617 electrometer monitors the photocurrent. This device was prepared from a polymer with less than 5% non-conjugated segments and has an active area of 4 mm 2. In forward bias, there is a sharp exponential turnon below 2 V accompanied by the onset of light emission. At higher biases, the increase becomes more gradual, following either an exponential or power law behaviour consistent with tunnelling or space charge limited transport [2,20-22]. This device has rectification ratios in the range 1 0 5 - 1 0 6 , whereas devices made from less completely conjugated material have ratios in the range 1-104 and more pronounced hysteresis. The change in rectification ratios with the fraction of non-conjugated segments is not entirely understood. The reduced rectification does result from larger reversed bias current flow. The increased flexibility of the less conjugated polymer chains can enhance the conductivity by forming a morphology with increased filamentary conduction or greater interchain mobility. Fig. 4 compares the EL intensity as a function of current flow under increasing forward bias for LEDs made with four different concentrations of non-con-

D. Braun et aL / Synthetic Metals 66 (1994) 75-79 107

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equatorial distribution is almost constant over 360 °, with a standard deviation less than 4%. Fig. 5 shows that the angular dependence of the luminescence does not strictly follow the cosine dependence of a Lambertian emitter [24]. For EL, external quantum efficiencies in photons per electron are measured with a calibrated silicon photo diode located at 0 = 0 °. The non-Lambertian geometry of the emission pattern is accounted for by calibration against measurements performed in a calibrated integrating sphere. Each data point in Fig. 6 represents the maximum efficiency measured on a given substrate with up to nine LEDs tested per substrate. The data of Fig. 6 indicate that peak efficiencies above 1% photons/electron occur for m near 10%. The values are comparable to those reported in the literature for other O-PPV and PPV LEDs [3,4,8,12,13,15,25,26]. Note that the relative error for the efficiency measurements not performed in the integrating sphere is 7%, with the bulk of the error due to the uncertainty in the detector position. Errors in the non-conjugated fraction m are _+2% f o r m < 2 0 % and + 7 % for r n > 3 0 % . PL efficiencies recorded for solid polymer films and dilute polymer solutions appear in Fig. 7. The 514 and 458 nm lines of an argon ion laser are used as excitation sources. The luminescence is collected with a calibrated integrating sphere, corrected for the spectral dependences of the luminescence, collection optics and reflection at the sample surface. In order to gauge the accuracy of the test set-up, a solution of Rhodamine 101 laser dye was also measured. The PL efficiency of 94_+ 3% compares quite well with the literature values of 95-100% [27]. PL efficiencies ranged from 20 to 40% for solutions and from 5 to 25% for solid films. The PL efficiencies tend to increase with m, with the solid state values lower than those recorded for solutions. PL quantum efficiency can also be estimated from PL lifetime measurements. The values measured here are comparable to that reported for PPV in the solid

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jugated segments. The data follow a power law with exponents ranging from 1.2 to 1.5. Such superlinear behaviour can be attributed to singlet recombination accompanied by triplet-triplet annihilation [9,23] or field-enhanced quenching of non-radiative recombination, for example, by trap filling [12]. Fig. 5 plots the azimuthal dependence of the luminescence intensity along arcs at various equatorial angles. Data are recorded with a luminance meter (Licht Mesz Techniek L10009) mounted on a 366 mm radius goniometer. In order to ensure that the luminance measured normal to the sample ( 0 = 0 °) remains steady at 12 cd/m 2 during the 5 min duration of the measurement, the device operates at the relatively low current density of 1.82 mA/cm 2 and a bias of 9 V. The

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D. Braun et al. / Synthetic Metals 66 (1994) 75-79

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state (8-25%) [1,28] and lower than that reported for another O-PPV in solution (60-70%) [29,30].

4. Discussion

In viewing the luminescence efficiency results as a function of non-conjugated fraction m, we begin with the completely conjugated material. The room temperature luminescence spectrum in Fig. 2 has the orange colour typical of O-PPV. No higher bandgap elements are present on the polymer chain, so no additional exciton confinement occurs beyond that due to the conjugated chain. The long conjugated sequences allow increased exciton motion that facilitates non-radiative recombination by helping excitons move to non-radiative recombination sites and to other excitons where nonradiative scattering can occur [7]. Fig. 8 depicts the carrier confinement as a function of non-conjugated fraction rn. At the highest fraction of non-conjugated segments m, the PL efficiency increases as the non-conjugated segments block non-radiative processes. This increase in PL efficiency could also result in an increase in EL efficiency. However, sufficiently high concentrations of non-conjugated segments also interfere with intrachain mobility of electrons and holes, so exciton formation becomes more difficult and EL efficiency ultimately drops. The presence of a large fraction of non-conjugated segments to the chain, rn > 30%, for example, confines excitons to shorter conjugated segments, leading to a significant blue shift in the luminescence spectrum (see Fig. 2). The PL efficiency increases dramatically (to 23%) as non-radiative processes, such as diffusion to non-radiative sites and exciton-exciton scattering, diminish. The EL efficiency decreases as the non-conjugated segments reduce not only the migration of excitons but also the mobility of the charged carriers, thereby impeding exciton formation.

(c) Fig. 8. Carrier confinement as a function of non-conjugated fraction m: (a) m = 0%; (b) m = 10%; (c) m = 40%. The lines represent potential energy along the chain and the ellipses portray the spatial extent of the excitons.

Somewhere in between the two limiting cases outlined above, apparently near m = 10%, there is an optimum for EL efficiency. The presence of non-conjugated segments does improve the PL efficiency by reducing nonradiative recombination. However, such a small fraction is apparently not severe enough to prevent exciton formation or to induce quantum confinement which would cause a blue shift. There is a slight loss of oscillator strength in the red, perhaps because of increased disorder [19], resulting from the enhanced flexibility of chains containing non-conjugated segments. The reduced rectification ratios and improved solubility of the incompletely conjugated materials may also arise from the enhanced flexibility. The EL efficiencies are sufficient for devices to produce light visible in well-lit ambient conditions, yet they are more than an order of magnitude lower than the PL efficiencies. A number of obstacles that injected carriers must surmount explain the discrepancy. Trapping and an imbalanced injection ratio will prevent carriers from finding their oppositely charged mates. For such a simple sandwich device structure, the injection ratio depends strongly on the potential barriers at the electrodes, so an imbalance can greatly decrease EL efficiency [12,21]. If a pair does form an exciton, then spin statistics dictate that only 25% can form the singlet state responsible for radiative recombination [31]. Even if spin statistics proves the limiting factor, more sophisticated device structures [13,32,33] and the use of material with fewer non-radiative recombination sites promise a factor two to five improvement in EL efficiency. Other results present in the literature offer a variety of strategies to increase EL efficiency of PPV derivatives. [5-10,14-17] Varying the fraction of conjugated segments, the fraction of low bandgap segments or the fraction of luminescent centres leads to a maximum in the EL efficiency. In most cases, the luminescence spectra also shift during this process [5,6,8,9,14-16].

D. Braun et al. / Synthetic Metals 66 (1994) 75-79

The variation in EL efficiency reported here is less than the order of magnitude effects reported in the literature [7,15,16], but the maximum external EL efficiency of 1.4% is the highest value yet reported for a PPV derivative sandwiched directly between a metal cathode and an anode.

[8]

[9]

[10]

5. Conclusions Incorporating wider bandgap or insulating segments offers one strategy to enhance the radiative efficiency of polymer LEDs. We have directly measured the PL and EL efficiencies for a series of O-PPV polymers, with controlled fractions of non-conjugated segments (m < 50%). The PL efficiency increases with the fraction of non-conjugated segments, and the EL efficiency peaks at an intermediate fraction. Optimizing carrier injection and confinement for radiative recombination can lead to further improvements in device efficiency.

[11]

[12] [13]

[14]

[15) [16]

Acknowledgements We are grateful to A.R. Brown, D.M. de Leeuw, L.W. Molenkamp and A. Tol for valuable comments. E. Vossen and H. Zegers supplied and patterned the ITO substrates, and J. van de Bilt and G. Rinzema performed the gonioluminescence measurements. Monomers were synthesized by H. Wynberg and W. ten Hoeve from Syncom B.V. in Groningen, The Netherlands.

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