Johan Reynaert

Ambipolar injection in a submicron-channel light-emitting tetracene transistor with distinct source and drain contacts

We report on organic light-emitting transistors with a submicron-channel length, gold source, and calcium drain contacts. The respective contact metals allow efficient injection of holes and electrons in the tetracene channel material. Transistor characteristics were measured in parallel with electroluminescence being recorded by a digital camera focused on the transistor channel. In the case of submicron-channel lengths, the transistor source-drain current at higher gate voltages was determined by the source-drain voltage. At larger channel lengths, the source-drain current was limited by the injection of electrons from the calcium contact, as hole ejection to this contact was fully blocked. The hole blocking is explained in terms of a chemical reaction occurring at the Ca/tetracene interface.

Current-voltage characteristics of a tetracene crystal: Space charge or injection limited conductivity?

We show that current–voltage characteristics, measured on a tetracene crystal sandwiched between two gold electrodes, are in quantitative agreement with the concept of dopant-assisted charge injection across a metal–organic interface. This notion explains the wild range of carrier mobilities recently found by space-charge limited current measurements in apparently identical tetracene crystals [de Boer et al., J. Appl. Phys. 95, 1196 (2004)] and the difference between these data and the mobility measured in the time-of-flight experiments.

Effect of surface localized states on conductivity in an organic crystal

Current-voltage characteristics of perylene crystals have been measured at different temperatures both with coplanar Au contacts and in the sandwich configuration in between an indium tin oxide bottom contact and a Au top contact. The voltage and temperature dependences of the current turned out to be very different in coplanar and sandwich structures. A strong increase of the conductivity in coplanar contact structures was observed upon oxygen-plasma treatment, indicating a strong influence of surface states on electronic properties of organic crystals. The effect of crystal-growth conditions on electronic parameters of the crystal is discussed. The obtained experimental data are interpreted in terms of dopant-assisted charge injection across a metal-organics interface.

Current-voltage characteristics in organic semiconductor crystals: space charge vs. contact-limited carrier transport

Numerous experimental studies, mostly based on the time-of flight (TOF) technique, showed that the conductivity in organic crystals can be analysed in terms of (trap-controlled) band transport. However, recent comparative studies of TOF signals and space charge limited currents (SCLCs) in tetracene crystals revealed a striking difference in carrier mobilities estimated from TOF current transients and from SCLC curves. The analysis of the SCLC curves yielded the mobilities wildly varying within 6 orders of magnitude. Therefore, it is not always clear whether the measured current-voltage (IV) device characteristics are controlled by charge injection or by transport in the bulk. In this work, we formulate a model of dopant-assisted carrier injection across a metal/organics interface and use this model for the analysis of IV curves measured on a tetracene and perylene crystal. The model suggests the occurrence of an energetically disordered layer at the surface of an organic crystal. This might be either an amorphous phase of the same material or a crystalline layer with a high density of defects and/or impurities. Since, at variance with bulk properties, the surface of an organic crystal is poorly controlled and can be strongly modified upon the contact deposition, the model of injection-controlled IV characteristics can explain the striking difference between the TOF mobility and the apparent ‘SCLC mobility’ measured in tetracene crystals. In order to give more credence to the role of surface defects states in the dark charge transport, we compare IV characteristics measured on sandwich and coplanar structures. In the latter structure, surface states show a major contribution to the conductivity.