Optical probes of electronics states injected into poly(p-phenylenevinylene) electroluminescent devices

Synthetic Metals, 55-57 (1993) 4117--4122

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OPTICAL PROBES OF ELECTRONIC STATES INJECTED INTO POLY(P-PHENYLENEVINYLENE) ELECTROLUMINESCENT DEVICES

A. R. BROWN, K, PICHLER, N. C. GREENHAM, D. D. C. BRADLEY, and R. H. FRIEND Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, UK P. L. BURN and A. B. HOLMES University Chemical Laboratory, Lensfield Road, Cambridge, CB2 1EW, UK

ABSTRACT We have been working to characterise the electronic excitations created by charge injection into conjugated polymers such as poly(p-phenylenevinylene)in electroluminescentdevices. We report on the optical probing of electronic states injected into an indium-tin oxide/PPV/calcium electroluminescent device. We monitor changes in absorption in the polymer film due to charge cartier injection into the device by detecting the reflection of a probe beam off the back metal contact layer. These injected carriers are expected to reside in states with energy levels within the band gap and hence give rise to a change in the sub-band gap optical properties. We observe absorption features centred around 1.38 eV, 1.55 eV, 0.65 eV. These states closely resemble those seen in photoinduced absorption and have been assigned to optical transitions involving bipolarons (0.65 and 1.55 eV) and a triplet-exciton (1.38 eV).

INTRODUCTION Electroluminescentdevices constructed from conjugated polymers are now very actively researched [1- 6]. They offer potential as a large-area flat-panel display technology, since they can be fabricated over large areas using solution-processing techniques, and also they provide a useful probe of the fundamental physics of these semiconducting polymers. Recent interest has been directed to the nonlinear electronic response of the coupled electron-lattice [7,8]. One consequence of this nonlinearity is that injected charges are expected to reside in polaron-like self-localised excitations in non-degenerate ground state conjugated polymers [7,8]. Three experimental techniques for charge generation have been used previously : chemical doping, photoexcitation [9] and MIS field-effect devices [10]. Electrical injection is particularly attractive as a method of charge injection as charges are introduced without associated counter-ions and charge concentration is controllable with an externally applied electric field. Electroluminescent devices operate by electron and hole injection at negative and positive electrodes, with electron-hole capture to form excitons, which can then decay radiatively. Such 0379-6779/93/$6.00

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devices thus offer a new electrical method of charge injection into conjugated polymers. A deeper understanding of the electrical excitations in these devices is required if their potential is to be fully exploited. In particular, the role of bipolarons is of interest since such excitations are not considered to contribute to the electroluminescence output but rather act as quenching sites [11]. In this paper characteristic sub-gap optical absorptions are used to detect the presence of the non-linear excitations.

SAMPLE PREPARATION Indium-tin oxide (ITO)-coated glass substrates were thoroughly cleaned with acetone and subsequently with propan-2-ol, both in an ultrasonic bath. The THT-leaving precursor polymer to PPV [12,13] was spin-coated from methanolic solution within a nitrogen-filled glovebox (02 and I-I20 content <10 ppm), wherein all subsequent processing steps were also performed. The polymer was thermally converted at 200 °C in vacuo (< 5x10 -6 torr) for 12 hours. Top calcium contacts were then vacuum-deposited. Typical sample area was 4 mm 2. PPV f'dm thicknesses were of the order of 400 rim. A sample of polymer was prepared simultaneously on a silica substrate for photoinduced absorption studies.

EXPERIMENTAL DETAILS For the modulated absorption measurement the sample was mounted in an optical-access helium cryostat, allowing temperature control between 10 K and 350 K. A square-wave voltage pulse (from 0 V to a positive voltage V, above the voltage threshold for light emission) with unity duty cycle was applied to the ITO contact, keeping the calcium contact grounded. The pre-monochromated output from a tungsten lamp was used as the optical probe. The probe was passed through the semitransparent ITO contact, At energies below the optical bandgap, the probe largely passes through the polymer layer, is reflected at the calcium contact and travels back through the polymer layer and the ITO contact. This reflected beam was detected with a silicon photodiode, cooled germanium detector, or cooled indium arsenide detector. Cut-off filters were used to reduce the spectrally-constant background electroluminescence. Phase-sensitive detection was performed using a lock-in amplifier referenced to the voltage pulse modulation frequency. Both the "in-phase" and "quadrature" response of the amplifier were recorded, permitting phase manipulation of the data, and hence the separation of overlapping signals with different time constants. A decrease in the intensity of the reflected beam corresponds to an electrically-induced absorption. The signal is normalised by the unmodulated reflected beam, (-AR/R). Due to a constant reflection at the device surface this normalisation underestimates the magnitude of the signal, (-AoJ0t), though not by a large factor. The integrated EL signal was monitored with the silicon diode to both observe any sample deterioration and to measure the EL offset on signals detected with the silicon diode. For experimental details of photoinduced absorption see Pichler et al. [14].

RESULTS The electroluminescent devices were driven at drive voltages above 20 V; this threshold is set by the low temperatures and thicknesses of the devices. The characteristic electroluminescent spectrum of PPV was observed as we have reported previously [1,15].

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Energy (eV) Figure 1 The photoinduced absorption signal at 20 K and 67 Hz (solid circles, left axis), and electroluminescent induced absorption signals recorded at 15 K and 74 Hz, phase rotated to show signal size and shape of the characteristic triplet signal (open triangles, fight axis) and characteristic bipolaron signal (open circles, signal amplified by 5, right axis).

Figure 1 shows the photoinduced absorption signal obtained of the PPV used in the EL devices. Only one contribution was found with a peak at 1.36 eV. This is at a lower value than in previous reports of photoinduced absorption in PPV [9] but in line with recent reports [ 14] and is indicative of a larger extent of conjugation. The magnitude and position of the peak are characteristic of a triplettriplet transition [9,16,17]. The observed reflectance modulation signal in the range 1.1-1.8 eV had two clear contributions which were separated by their significantly different frequency response. We attribute the larger of the two signals which peaks at 1.38 eV to the triplet-triplet transition observed in photoinduced absorption. We attribute the smaller signal centred at 1.55 eV to an optical transition involving a bipolaron by comparison with previous photoinduced absorption reports [9,16,17]. Figure 2(a) shows the frequency dependence of the triplet signal at 15 K. The signal is still rising at 14 Hz, indicating that some triplet states have lifetimes in excess of 70 ms. In the high frequency regime (cox >> 1) the signal decays as co-n where n = 0.84. Monomolecular decay predicts n = 1. Previously monomolecular decay and characteristic lifetimes of 1 msec have been observed in photoinduced absorption [9]. In this study there appears to be some dispersion of the lifetimes. Figure 2b shows the temperature dependence of both the photoinduced absorption and reflectance modulation signals. For the latter, the rate of production of triplets was maintained constant by

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Figure 2 (a) The frequency dependence of the signal at 1.38 eV normalised with the electroluminescent signal (15 K), (b) The temperature dependence of the same signal (open circles) and of the photoinduced absorption signal (solid circles). The photoinduced absorption has been scaled to have the same magnitude as the reflectance modulation at 30 K. reducing the drive voltage with increasing temperature to keep the electroluminescence output constant (the electroluminescence output is directly proportional to the radiative decay of singlet excitons and it is assumed that over the experimental range that the ratio of triplet production to singlet production by polaron-polaron capture remains constant). Both these signals fall off faster with temperature than previously observed [9], but in line with recent reports [14]. The reflectance modulation signal in the IR is complicated by various interference effects which are only dependent on the magnitude of the applied electric field and the frequency of the drive voltage but independent of temperature. The development of the reflectance modulation signals is most clearly observed in differences in (-AR/R). Figure 3 shows two difference spectra obtained from spectra recorded at three different voltages over which the total electroluminescent signal increased in the ratio h 2.0: 4.6. The clear peak centred at 0.65 eV we attribute to an optical transition involving a bipolaron in comparison with previous photoinduced absorption reports [9,16,17]. We note that the signal saturates in this regime. The characteristic electroabsorption of PPV at the optical band edge at 2f, twice the modulation frequency was observed [18].

DISCUSSION The first significant feature of these results is the observation of the induced absorption features characteristic of triplet excitons and charged states. The charged states are as found in photoinduced absorption measurements and are usually considered to be bipolarons. We have evidence, now, that these long-lived excited states are extrinsically stabilised [14], though we refer to them here as bipolarons. We have previously reported the observation of triplet states in an EL device in an electroluminescence-detected magnetic resonance (ELDMR)experiment [15]. Whilst the states are relatively long-lived at 15 K, the rapid decrease in signal with increasing temperature indicates the lifetime of the state decreases equally rapidly, and thus it may be expected that at room temperature electroluminescence is 'prompt' due to direct decay of singlet excitons and has a negligible

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contribution from 'delayed' singlets produced by triplet-triplet collisions. In our ELDMR study the large narrow quenching resonance was attributed to polaron-to-bipolaron conversion. This present study is the ftrst direct observation of the presence of bipolarons in an EL device. 6 10-5

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Secondly the differences between photoinduced absorption and reflectance modulation are of note. The absence of observed bipolarons in photoinduced absorption compared with previous reports of photoinduced absorption in PPV [9,16] is discussed by Pichler et al. [ 14]. We find that bipolaronic states are less easily observed by photoinduced absorption in our PPV. We attribute this to a smaller quantity of extrinsic defects due to impurities which act to stabilise bipolarons and to increase their lifetime and enable their observation. The observation of the bipolaronic states in the reflectance modulation experiment is thus of interest. Under conditions of photoexcitation, cartier generation is geminate and thus the space-charge density is expected to be zero throughout the sample, as equal numbers of positive and negative free carriers are generated. However under conditions of charge injection in an electroluminescent device all carrier recombination to give electroluminescence is nongeminate. Holes (positive polarons) are injected at the positively biased contact and electrons (negative polarons) are injected at the negatively biased contact and the carriers meet within an electron-hole capture zone. As a consequence there is a space charge distribution throughout the device and in particular near the positive (negative) contact there will only be positive (negative) polarons. As a result the presence of bipolarons due to conversion of two like-charged polarons is significantly more probable under charge injection than photoexcitation. A similar conclusion was drawn in explanation of the ELDMR results [ 15]. The observed bipolaron saturation at higher current densities was also observed under ELDMR and may be explained as (i) at higher current densities electron and hole injection imbalance is reduced and thus decreasing the space charge or (ii) higher charge-injection rates may be saturating bipolaron generation if it occurs at charge-trapping sites. The

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two triplet signals have significantly diferent lineshapes. The lineshape observed in reflectance modulation is similar to that observed previously [9], and that observed in photoinduced absorption is similar to that recently observed in photoinduced absorption [ 14] which suggests that the long-lived triplets observed in reflectance modulation are stabilised by defects. ACKNOWLEDGEMENTS We thank the Science and Engineering Research Council and Cambridge Research and Innovation Limited for support for this work, and in particular Roger Jackson for synthesising the polymer used in this study. REFERENCES 1. J.H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Bum and A. B. Holmes, Nature. 347 £1990) 539-541. 2. A . R . Brown, D. D. C. Bradley, P. L. Burn, J. H. Burroughes, R. H. Friend, N. C. Greenham, A. B. Holmes and A. Kraft, Aonl. Phvs. Lett.. (submitted) 3. A.R. Brown, P. L. Burn, D. D. C. Bradley, R. H. Friend, A. Kraft and A. B. Holmes, Mol. Crvst. Liq. Crvst, 216 (1991) 111-116 4. D. Braun and A. J. Heeger, Appl. Phys. Lett.. 58 £1991) 1982-1984. 5. P.L. Burn, A. B. Holmes, A. Kraft, A. R. Brown, D. D. C. Bradley and R. H. Friend,. in L. Y. Chiand, A. F. Garito, and D. J. Sanman (ed.), Electrical, Ootical. and Maznetic oroDerties of Organic State Solid Materials. 247 (MRS Symposium, Fall Meeting, Boston, 1991), p 647-654 6. P.L. Burn, A. B. Holmes, A. Kraft, A. R. Brown, D. D. C. Bradley, R. H. Friend and R. W. Gymer, Nature. 356 ~1992) 47-49. 7. S.A. Brazovskii and N. N. Kirova, JETP Lett. 33 ~1981) 4-8. 8. K. Fesser, A. R. Bishop and D. K. Campbell, Phys. Rev. B, 27 (1983) 4804-4825. 9. N.F. Colaneri, D. D. C. Bradley, R. H. Friend, P. L. Burn, A. B. Holmes and C. W. Spangler, Phvs. Rev. B. 42 f1990) 11670-11681. 10. K. E. Ziemelis, A. T. Hussain, D. D. C. Bradley, R. H. Friend, J. Riahe and G. Wegner, Phvs. Rev. Letts.. 66 ~1991) 2231-2234. 11. D. D. C. Bradley and R. H. Friend, J. Phvs.:Condens Matter. 1/1989) 3671-3678. 12. P. L. Burn, D. D. C. Bradley, A. R. Brown, R. H. Friend, D. A. Halliday, A. B. Holmes, A. Kraft and J. H. F. Martens, in Springer Series in Solid State Sciences. Electronic Properties of polymer~ .107 (1992). 13. P. L. Burn, D. D. C. Bradley, A. R. Brown, R. H. Friend and A. B. Holmes, Synthetic Metals. 41-43 1~1991) 261-264. 14. K. Pichler, D. A. Halliday, D. D. C. Bradley, R. H. Friend, P. L. Burn, and A. B. Holmes, these Proceexlin~s. 15. L. S. Swanson, J. Shinar, A. R. Brown, D. D. C. Bradley, R. H. Friend, P. L. Burn, A. Kraft and A. B. Holmes, Phvs. Rev. B.. (in Dress) 16. R. H. Friend, D. D. C. Bradley and P. D. Townsend, J. Phys. D: Appl. Phys., 20/1987) 1367-1384. 17. X. Wei, B. C. Hess, Z. V. Vardeny and F. Wudl, . ~ t~1992) 666-669. 18. O. M. Gelsen, D. A. Halliday, D. D. C. Bradley, P. L. Burn, A. B. Holmes, H. Murata, N. Takada, T. Tsutsui and S. Saito, Mol. Cryst. Liq. Cry st..216 (1992) 117-121