Materials Chemistry and Physics 77 (2002) 476–483 PVK-based organic light emitting diodes with low turn on voltage S. Ouro Djobo a,∗ , J.C. Bernède a , K. Napo b , Y. Guellil a a LPSE Université de Nantes, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France b Laboratoire d’Energie Solaire, Université du Bénin, Lomé, Togo Received 10 July 2001; received in revised form 11 December 2001; accepted 14 January 2002 Abstract Poly(N-vinylcarbazole) (PVK) thin films deposited by evaporation have been used in electroluminescent organic diodes. The turn on voltages of the current–voltage (I–V) and electroluminescence–voltage (EL–V) characteristics are not very different and they are smaller than the one obtained in such PVK-based devices, when the PVK film is obtained by spin coating. It is shown that the carrier injection process is dominated by tunneling field effect. During evaporation the chain length of the polymer is shortened and the PVK films contain a high density of dangling bonds. It is proposed that these dangling bonds introduce a broad distribution of localized states in the band gap. Therefore, the carrier injection occurs via hopping among an energetic distribution of localized states rather than by band motion, which is equivalent to a reduction of the energy barriers as experimentally measured. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Poly(N-vinylcarbazole); Evaporation; Electroluminescence; Hopping tunneling 1. Introduction During the last 10 years organic light emitting diodes (OLEDs) have drawn wide attention due to their potential application in multicolor flat panel displays. The discov- ery that various polymers can serve as the active layer in OLEDs has stimulated interest in these devices, in such a way that more than 20 reviews on the subject have been written since 1993 [1]. The study of the electroluminescence (EL) of organic thin films has been the subject of increasing worldwide in- terest since the description of the EL of poly(1,4-phenylene vinylene) (PPV) thin film-based diodes by the Cambridge group in 1990 [2]. It should be noted that the EL of poly(N-vinylcarbazole) (PVK) had been reported a few years earlier [3]. PVK is promising as a polymer for blue EL device because of its wide energy gap. Their luminescence properties have been carried out first by Gill [4], then Partridge in his pioneering work [3], has developed the PVK device in which EL emission is based on double injection from the electrodes. Such PVK-based struc- tures have been widely studied and their EL performances have been improved significantly [5–13]. However, the turn on voltage of the PVK OLED stays quite large (>10 V) [12]. Usually the simplest structures are constituted of a PVK thin film sandwich between a trans- ∗ Corresponding author. parent conductive oxide (TCO, In 2 O 3 ) and metallic elec- trode (Al, Mg, Ag). It is generally accepted that efficient carrier injection from both electrodes is a key factor in the performance of OLEDs. The band structure of the whole device should be optimized in order to decrease the bar- riers at the electrode/polymer interface. It has been shown that even for interface constituted of the same materials the barrier height depends strongly on thin film processing conditions [14]. Usually the PVK layer is deposited by spin coating and some impurity and/or conformational heterogeneity could be present at the TCO/polymer interface. An alternative ap- proach, which can avoid such difficulties, is vacuum evap- oration. Vapor deposition under vacuum provides a clean environment, high coverage efficiency, and it is solvent free. Earlier we have studied the properties of evaporated PVK thin films [15]. We have shown that if the vinylcar- bazole function is preserved during the heating process, the chain length of the PVK is strongly decreased and the films are essentially composed of oligomers. These films are photoluminescent and when they are introduced in diode structures an electroluminescent effect is obtained [16]. Moreover, in these structures the turn on voltage is only 3 V for a PVK thickness of 100 nm. In the present paper we proceed to systematical study of these evaporated PVK-based structures in order to discuss such low turn on voltage value. 0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0254-0584(02)00093-7