Physics Letters A 364 (2007) 425–428 www.elsevier.com/locate/pla Dissociation of bipolaron in non-degenerate polymer chain at high electric fields ✩ Y.H. Yan ∗ , C.Q. Wu Department of Physics, Fudan University, Shanghai 200433, China Received 18 February 2006; received in revised form 8 December 2006; accepted 11 December 2006 Available online 23 December 2006 Communicated by R. Wu Abstract We investigate the dynamics of bipolaron in non-degenerate polymer (e.g. PPV) in an external electric field by using a nonadiabatic evolution method, which allows transition between instantaneous electronic states. When the applied electric field exceeds a critical value, a bipolaron is found to dissociate like the case of polaron due to the lattice distortion not being able to follow the fast moving electrons. The critical value is estimated to be of order 10 6 V/cm. This result is consistent with experiment in that a large increase in current in PPV occurs at high fields. At a given electric field, the saturation velocity of bipolaron will decrease with the increase of non-degenerate parameters t e .  2006 Elsevier B.V. All rights reserved. PACS: 72.20.Jv; 71.38.Mx; 72.80.Le Keywords: Polymer; Bipolaron; Nonadiabatic dynamics 1. Introduction Conjugated polymers have attracted much interest for their potential applications, e.g., organic light emitting diodes, field effect transistors, solar cells, etc. [1,2]. Due to the strong electron–lattice interactions, it is well known that additional electrons or holes in conjugated polymers will induce self- localized excitations, such as solitons [3] (only in trans- polyacetylene), polarons and bipolarons [4]. As a result, it has been generally accepted that the charge carriers in conjugated polymers are these excitations including both charge and lat- tice distortion [5]. Therefore, it is of fundamental importance to study the dynamics of charge carriers in polymers. There have been extensive studies on soliton and polaron dy- namics in conjugated polymers under the influence of external electric fields [6–12]. One of the major issues is the stabilities of ✩ Supported by National Natural Science Foundation of China (Nos. 90403110, 10374017, and 10321003) and the State Ministry of Education of China (No. 20020246006). * Corresponding author. E-mail address: yhyan@fudan.edu.cn (Y.H. Yan). those elementary excitations in electric fields. It has been shown that solitons and polarons can move with a constant speed along the polymer chain in electric field with strength below a criti- cal value. Solitons are shown to have a maximum velocity of 2.7v s [6,7], with v s being the sound velocity, while the max- imum velocity of polarons is about 4v s [8,9]. Furthermore, it has been shown that the polaron can survive under the field up to about 4 × 10 5 V/cm [9,10]. In an even higher electric field, polaron will dissociate due to the lattice distortion not being able to following the faster moving electron. Here, it should be pointed out that polaron is referred to as the optical polaron, which is much different from the acoustic one. Acoustic po- laron can only reach a maximum velocity near v s [13,14]. There is also much effort devoted to the study of bipolaron [5], the elementary excitation in non-degenerate polymers, such as poly(p-phenylene vinylene) (PPV), the polymeric material adopted mostly in organic opto-electronic devices [1]. In static case, it was shown that at the large electron–phonon coupling, the bipolaron remains stable up to relatively large Hubbard U [15,16]. As for the dynamical behavior, the motion of a bipo- laron (together with a polaron) under the influence of an electric field through a single-site impurity was discussed [17]. How- 0375-9601/$ – see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2006.12.053