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ANNUAL REPORT 2010
AC-biased organic light-emitting devices
Xuhai Liu and Jakob Kjelstrup-Hansen
General introduction
Organic light-emitting transistors (OFETs) are under intensive investigations due to their promising capability
to surpass organic LEDs in terms of output quantum efficiency [1]. Besides, OFETs are also ideal platforms for
fabricating organic laser diodes by electrically exciting organic materials [2]. In these organic light-emitting
devices, charge carrier injection from metal electrodes to organic materials is of vital importance to achieve
high current density. Biasing OFET platforms utilizing AC voltages [3] provides a valuable method to investigate carrier injection and could finally lead to electrically pumped organic laser diodes [2].
Research results
Figure 1: Schematic diagram of the electrical circuit along with a bottom
contact/bottom gate OLET substrate. Thin films of four different organic
oligomers act as active materials (shown on the right-hand side; the color of
the materials is indicated by the color of the text).
In this project, four organic molecules with various HOMO-LUMO gaps were investigated as active materials
based on a bottom contact/bottom gate OLET structure, as illustrated in figure 1. The OLET substrates composing of highly n-doped Si and thermally grown SiO2 were processed in the cleanroom into a configuration
with interdigitated Au electrodes on top and a rectangular contact window connecting with the bottom gate.
Different organic materials were then deposited directly onto substrates in a high vacuum system at room
temperature. Figure 2 shows the electroluminescence of four organic materials excited by the AC gate voltage with frequency 200 kHz and suitable voltage amplitudes ranging from 62.5 V to 87.5 V, while keeping
drain Vd and source voltage Vs at Vd = -Vs < 30 V.
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Figure 2: Optical microscope images of electroluminescence from four different organic materials excited
by an AC gate voltage.
Figure 3: Schematic diagram of the device operating
mechanism. (a) During the negative semi-period of the
gate voltage, holes are injected forming space charge
field. (b) During the subsequent positive semi-period,
electrons are injected assisted by the previously formed
positive space charge field.
Since electroluminescences of various organic active materials can be observed from individual electrodes (e.g.
from the grounded drain electrode where the source
electrode was floating) and from both sides of the transistor channel [3], a model based on space-charge field
assisted charge carrier injection involving an individual
metal electrode was proposed, as shown in figure 3 to
the right.
This model is different from previous reports that electrons and holes with comparable mobilities from opposite electrodes could recombine at the center of the transistor channel under AC gate voltage biasing [2].
In order to elucidate the underlying operating mechanism of an AC voltage biased OLET, time-resolved measurements with photo multiplier tube and investigations on various charge injection theories are under way.
Upcoming research 2011
The metal electrodes of the OLET will be modified with self-assembled monolayers (SAMs) so as to change
the barrier height of the charge carriers and help investigating carrier traps near the metal electrodes. Low
temperature experiments are promising to better understand the underlying operating mechanisms. Moreover, crystalline organic structures, such as nanofibers, could also act as active materials after being transferred onto OLET substrates using novel transferring techniques.
Papers
[1] [2] [3] Organic Light-Emitting Transistors with an Efficiency that Outperforms the Equivalent Light-Emitting Diodes. R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti, and M. Muccini, Nature Materials,
9(2010)496-503
Current-Injected Spectrally-Narrowed Emissions from an Organic Transistor. T. Yamao, Y. Sakurai, K. Terasaki, Y. Shimizu, H. Jinnai, and S. Hotta, Advanced Materials, 22(2010)3708-3712
Electroluminescence of Naphthyl End-Capped Oligothiophenes by Alternating-Current Voltage in an Organic Light-Emitting Field Effect Transistor Configuration. X. Liu, I. Wallmann, H. Boudinov, J. Kjelstrup-Hansen, M. Schiek, A. Lützen, and H.-G. Rubahn, Organic Electronics, 11(2010)1096-1102.
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