Dynamics of photoexcitations in electric fields in poly(p-phenylenevinylene) diodes

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Synthetic M e t a l s 67 (1994) 169-172

Dynamics of photoexcitations in electric fields in poly(p-phenylenevinylen e) diodes U. L e m m e r ", S. Karg b, M. Scheidler a, M. Deussen a, W. Riel3 b, B. Cleve a, P. Thomas a, H. Bfissler ~, M. Schwoerer b, E.O. G6beP "Fachbereich Physik und Fachbereich Physikalische Chernie und Zentrum fiir Materialwissenschafien der Philipps-Universitiit, Renthof 5, 35032 Marburg, Germany bphysikalisches Institut und Bayreuther Institut fiir Makrornolekfilforschung, Universitiit Bayreuth, 95440 Bayreuth, Germany

Abstract

We report on a study of the dissociation of neutral photoexcitations in electric fields in ITO/poly(p-phenylenevinylene) (PPV)/AI diodes, This process leads simultaneously to photoconductivity and field-induced quenching of the photoluminescence+ By illuminating the samples through the different contacts we show that the high electric fields are restricted to a region close to the AI layer as characteristic for a Schottky diode. Time-resolved luminescence measurements show that the dissociation takes place on a picosecond time scale. The experimental data are compared with a Monte-Carlo simulation of the dynamics of photoexcitations created in a disorder-broadened density of states, Kcywords: Dynamics; Photoexcitations; Diodes; Poly(p-phenylenevinylene)

1. Introduction The physics of photoexcitations in conjugated polymers is currently attracting much interest from both a fundamental and an application oriented point of view. Among the conjugated systems poly(p-phenylene vinylene) (PPV) and its derivatives are of particular importance due to their promising prospects for lightemitting devices ( L E D @ Since the first report of LED operation in PPV [1], much effort has been devoted to the characterization of the optoelectronic properties of PPV and devices based on this material [2-5]. Key issues for the understanding are the nature of the primary photoexcitations and suitable models for charge transport and device operation. Both are controversially discussed. It is still heavily debated whether an excitonic or a free carrier description is more appropriate for photoexcitations in conjugated polymers [6,7]. Further insight into the nature of optical excitations is expected from optical experiments under the influence of electric fields since free carriers and excitons should respond differently to strong electric fields. Photocurrent (PC) experiments directly address the transition from neutral optical excitations to charge carriers. An alternative approach is to perform photoluminescence (PL) quenching experiments under the influence of electric fields.

0379-6779/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0 3 7 9 - 6 7 7 9 ( 9 4 ) 0 2 2 3 5 - Q

In principle, these methods are complementary since PC probes the number of mobile charge carriers and PL quenching probes the decrease of the number of neutral excitations that can recombine radiatively. Timeresolved PL experiments additionally allow a study of the dynamics of the dissociation process. Here we use both methods for the investigation of photocarrier generation in electric fields of an ITO/ PPV/A1 diode. We show that the dissociation of neutral excitations occurs on a picosecond time scale. On the base of a Monte-Carlo study we explain our experimental results by a competition between relaxation and dissociation processes in the disordered conjugated polymer. It is found that the electric field is concentrated to a thin layer close to the A1 contact. Thus, our measurements give direct evidence that the diodes consist of a junction and a bulk region and can be described within a Schottky-like model.

2. Experimental PPV was synthesized according to a precursor polymer route [8]. Films of a thickness of approximately 300 nm were deposited on an ITO-coated substrate via the doctor-blade technique. As the other electrode we have

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evaporated a semi-transparent A1 layer (about 10 nm) allowing optical excitation of the PPV layer through the contact. The PC experiments were performed with a monochromatized tungsten lamp as excitation source. For the PL quenching experiments we have used a frequency-doubled mode-locked Ti:sapphire laser delivering 100 fs laser pulses at 3.1 eV (400 nm). In the time-integrated PL experiments the luminescence was detected either with a photomultiplier or a diode array. The streak-camera technique with a time resolution of about 20 ps was used for the time-resolved detection. All experiments were performed at room temperature.

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Room-temperature PL spectra, excited through the A1 contact, for various values of the reverse bias voltage (A1 positive, ITO negative) are shown in Fig. 1. As the main result a drastic decrease of the luminescence intensity for increasing reverse bias voltages from 0 to - 1 9 V is observed. The luminescence quenching is almost spectrally independent, i.e., no change of the spectral shape of the emission is observed even for the highest fields. This indicates that the emitting species are not subject of a Stark shift since a significant red shift of several meV is expected for the high electric fields applied here [9]. We attribute this behaviour to the fact that the polymer chains are predominantly oriented parallel to the substrate and, thus, vertical to the applied field. The spectrally integrated luminescence quenching Z~[/I as a function of the applied voltage is plotted in Fig. 2. The circles represent the luminescence quenching

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observed for illumination through the A1 contact, whereas the squares were measured for illumination through the ITO layer. Due to the high absorption coefficient the photoexcitations are created within a 50 nm layer close to the illuminated contact. In the case of excitation through the A1 layer the luminescence is strongly reduced under reverse bias conditions. Quenching values - A I / I of up to 80% are observed for the highest reverse bias voltage of - 2 0 V. In the case of excitation through the ITO contact the luminescence is only weakly affected. These results give direct evidence for inhomogeneous electric field distribution within the polymer layer, and can be explained in terms of a Schottky model for the PPV diode. Previous impedance spectroscopy experiments [4] have revealed that a depletion layer with a thickness of the order of 100 nm is formed at the interface between PPV and Al at zero bias. An applied bias voltage in reverse direction drops almost completely at this layer and furthermore increases the depletion width. Hence, in the case of excitation through the AI contact, the photoexcitations are created within the depletion layer and undergo dissociation due to the present high electric fields. On the other hand, illumination through the ITO layer creates photoexcitations in the bulk region with low electric fields and, thus, the luminescence is only weakly affected. These experimental findings concur with the PC spectra shown in Fig. 3. The PC spectra are completely different depending on the direction of illumination. In the case of excitation through the AI contact, the spectra are very similar to the absorption spectrum while the photocurrent response for illumination through the ITO layer peaks in a narrow spectral window at the low energy side of the absorption spectrum

U. Lern,ner et al. / Synthetic Metals 67 (1094) 160-172

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10° (b) of PPV. Photons penetrating the A1 layer are absorbed within the depletion layer near the contact. The builtin electric field separates the charges leading to a photocurrent. The more photons are absorbed in the thin depletion layer the higher is the photocurrent. In the case of illumination through the ITO layer only those photoexcitations can be dissociated which pass through the PPV layer and get close to the AI contact. Thus, only photons at energies with a low absorption coefficient contribute to the photocurrent. The dynamics of the dissociation process can be investigated via time-resolved luminescence spectroscopy under application of electric fields. Fig. 4 shows the decay of the spectrally integrated luminescence with and without reverse bias voltage. For clarity, Fig. 4(a) shows the data on a linear scale, whereas Fig. 4(b) depicts the normalized traces in a semilogarithmic plot. Without external voltage the 1/e decay time is 94 ps. This decay is mainly due to fast nonradiative processes, which concomitantly reduce the PL quantum efficiency [10]. The application of a reverse bias voltage leads to a significant reduction of the intensity already on a time scale comparable to the time resolution of the setup (about 20 ps). However, the decay is also accelerated, as can be clearly seen in Fig. 4(b). The decay time in this case is reduced to 66 ps. This behaviour is attributed to the dynamic dissociation of neutral excitonic excitations. A quantitative description of the exciton dissociation has to account for several competing relaxation processes: After excitation at 400 nm into the vibronic progression of the S~-So transition, rapid vibronic relaxation leads to a random population within the disorder-broadened density of states (DOS). Spectral relaxation of the neutral excitations associated with hopping to longer conjugated polymer segments leads to a red shift of the transient luminescence spectrum

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[11]. Dissociation of neutral photoexcitations into weaker bound geminate e-h pairs also occurs and leads to a decrease of the fluorescence intensity since their recombination probability is small. This process is enhanced by electric fields. Competing with the radiative decay, which occurs on a time scale of about 1 ns, the trapping of mobile neutral excitations leads to fast luminescence quenching [10]. Due to the complexity of the problem, a MonteCarlo study is one suitable way to obtain further insight into the dynamics. Fig. 5 shows the results of a MonteCarlo study using a Gaussian width of the DOS of G= 80 meV, a Coulomb binding energy of E B = 0.4 eV and a fundamental hopping rate of uo=6× 1014 s The typical intersite distance was chosen to be 30 with a localization radius of 6 A. Spatial disorder was

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U. Lernmer et al. / Synthetic Metals 67 (1994) 169-172 '

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We have shown that the dissociation of neutral photoexcitations in ITO/PPV/A1 diodes leads to a photocurrent and simultaneous PL quenching. The dissociation occurs only in a thin layer close to the A1 contact, thereby showing that the diodes can be described within a Schottky-like model. The quenching dynamics can be explained in the framework of spectral relaxation and dissociation within an disorder-broadened density of states.

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We thank J. Gmeiner for the polymer synthesis and M. Preis for technical assistance. We acknowledge R.F. Mahrt and K. Lips for fruitful discussions. The work was supported by the Stiftung Volkswagenwerk, the Deutsche Forschungsgemeinschaft and the Bayerische Forschungsstiftung (FOROPTO). Special thanks to Dr Jestel (Flachglas AG) for the delivery of the ITO substrates.

References included. Details of the calculation will be published elsewhere [12]. Curve (a) shows the transient number of neutral excitations for the pure radiative recombination with a rate of 1 ns -1. For the calculation of curve (b) spontaneous dissociation into geminate e-h pairs was taken into account. The lower the exciton binding energy the higher the probability that the neutral excitons dissociate even under zero field conditions. In our case this process leads already to an accelerated luminescence decay. However, the experimental data can only be fitted if nonradiative traps are taken into account which are located energetically well below the DOS and therefore act as quenching centres. Curve (c) was calculated for a trap concentration of 15%. This simulated curve fits the experimental zero-voltage luminescence transient. Curve (d) is calculated with the same parameters but with an electric field of 1.5 MV/cm corresponding to our experimental situation with a reverse bias of - 1 5 V. Again the simulated and measured luminescence transient agree quite well,

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