Synthetic Metals 101 (1999) 409-412 Temperature dependence of charge carrier creation in poly(p-phenylene vinylene) [PPV] Bertrand Dulieu, Jany Why, Eric Faulaues, and Jacques Bullet Laboratoire de Physique Cristalline, Institut aks Matdriaux de Nantes, Unit& Mixte de Recherche CNRS / Universite’ de Nantes no6502 2 rue de la HoussinZre, BP 32229,44322 Nantes Cedex 3, France. Abstract The photocurrent decays measured in poly@-phenylene vinylene) [PPV] from room temperature (RT) down to 77 K in the pre- sence of air traces have been studied. We have observed long-lived decays, between RT and -200 K, which can be fitted to the well known Kohlrausch’s law: i&t) = i&O) exp-(t/T)a characteristic of dispersive transport. A detailed temperature dependence study has been carried out to investigate the behavior of the exponent p and the relaxation time 7 in order to understand the origin of the observed complex transport mechanism. At lower temperature, especially below 160 K, a fast component is detected alone, where the slow one is frozen. The associated interpretation is based on a recent work of simulated yield of geminate pair dissociation in an energetically random disorder. Modulated experiments show that both components are superimposed at enough high temperature. In our model, the long-lived contribution is interpreted as being extrinsic due to dissociation of polaron pairs assisted by electronegative defects and the fast one as being intrinsic due to dissociation of excitons assisted by the intrinsic energetic disorder, created by the distribution of conjugation length in PPV. Keywords: photoconductivity; PPV; Kohlrausch’s law; dispersive transport. 1. Introduction Poly@-phenylene vinylene) or PPV has attracted much attention during the past decade owing to its extraordinary phototransport and photoluminescence properties which make this polymer or its derivatives good candidates for various photonic devices [l] such as light-emitting diodes. However, little research, at our knowledge, was done on the systematic temperature dependence of the phototransport for long relaxation times and the decay kinetics of the photocurrent i&t) after illumination cut-off. These experiments could shed light on the origin of the photogenerated charge carriers in PPV to better understand the transport properties. In this work we show that i&t) follows at high temperature a nonexponential time-dependent relaxation law namely a stretched exponential Kohlrausch’s law [2]. The following experimental data suggest that at high temperature the full photocurrent & is composed of a fast and a slow components. The slow one dominates at high temperature and the fast one is isolated below 160 K. Modulated experiments have proved that the fast component in 4 exists in the whole temperature range and that both components have some different physical natures. This work is therefore devoted to understanding of the creation and transport processes of long-lived charge carriers in that conjugated polymer. Two approaches may account for the experimental results. The fist theory suggests the existence of photogenerated bipolarons which migrate from the bulk to the surface of the film and then dissociate into polarons [3]. The second is based on an excitonic mechanism with geminate bound electron-hole pairs decaying into mobile positive polarons [4]. We will show that our results are in agreement with the latter scenario, because it is possible to built a coherent model on the coexistence of an intrinsic photoconductivity and an extrinsic one. Although time- resolved photoconductivity, photoinduced absorption and magnetoresistance experiments could help to discriminate these mechanisms, we present nevertheless a set of data which are well interpreted in the frame of the excitor& theory [5]. After describing experimental details in Part 2, we present in Part 3 the main experimental results of photoconductivity on neutral PPV, and we propose a detailed model of charge carrier creation and transport in PPV. We summarize the results in Part 4. 2. Experimental The studied films of conjugated polymer PPV are deposited onto-a silica substrate from a methanolic solution containing the tetrahydrothiophenium polyelectrolyte precursor [6,7]. After thermal conversion under vacuum, the films are attached to the cold finger of a cryostat immersed in a static atmosphere. Photocurrent measurements are made using the 457.9 nm argon laser line with light power density of about 6 pW.cm-’ onto the 1 x 8 mm interelectrode spacing and under an electric tield of 2x lo3 V.cm”. An electrometer, and a light chopper coupled to a lock-m amplifier, are used for dc and modulation experiments, respectively. 3. Results and discussion The variation of the photocurrent under continuous wave (c.w.) illumination vs. time for a sample respectively kept under helium and afterwards in presence of air traces is depicted in Fig. 1. In air the total measured current increases, whereas it 0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved PII: SO379-6779(98)003 17-8