Transient charging of copper phthalocyanine: model and experiment
Introduction
Organic molecular materials are becoming a promising subject of not only scientific interest but also as components for potential applications such as organic light emitting diodes (OLEDs) or solar cells, the motivation coming from the reports on enhanced efficiency of opto-electronic devices based on organic layers [1], [2]. Early studies of metal phthalocyanines were mainly devoted to their xerographic applications as charge generation materials, due to their very intense absorption bands at approximately 400 and 700 nm [3].
Now, particular attention is being paid to the engineering of charge injecting contacts to OLEDs, directed towards lowering the driving voltages and achieving longer lifetimes of prototype devices. The use of hole-only copper phthalocyanine (CuPc), interlayers for the injection of positive charge carriers from indium–tin oxide (ITO) into an active hole transporting material led to a significant enhancement of the device performance [4]. The role played by the thin CuPc interlayer resides in an enhancement of hole injection efficiency of the ITO anode, thereby, improving recombination kinetics with electrons injected from the cathode [4], [5].
When extending the considered organic based OLEDs application towards operation in a dynamic mode while modulating the intensity of the generated light, it is mandatory to optimise transient response of such devices to the alternating bias. Following this goal, it is of particular importance to understand first the transient behaviour of individual components of such devices. The primary concern is the capture at and release of charge carriers from deep traps, an issue reviewed by Brütting et al. [6] and Riess et al. [7]. Apart from reducing the mobility of injected charge carriers, ionised (charged) traps may act as unintentionally introduced dopants.
The present contribution is aimed at providing a brief report on the results of a time-domain study of transient charging of CuPc thin films, as obtained by combining an innovative version of the charge transient spectroscopy (charge deep-level transient spectroscopy – Q-DLTS) developed originally by Farmer et al. [8] and the feedback charge capacitance–voltage method (FCM) described in detail by Mego [9]. It is demonstrated that a parabolic band bending is not a prerequisite for detecting signals from bulk traps when using the pure time-domain techniques, while concentrating on the spatial profiling of deep traps in a hole-only material. Model calculations will be confronted with original experimental data obtained on Ag/CuPc/ITO diodes in UHV at ambient temperature (300 K). Finally, the origin of the acceptor-like traps at the CuPc/ITO interface will be the subject of the concluding remarks from the point of view of electrochemistry of these type of interfaces [10].
Section snippets
Sample preparation
The devices for electrical measurements were prepared in an Ag/CuPc/ITO configuration. ITO coated glass substrates were first cleaned in an ultrasonic bath of acetone followed by isopropanol for 30 min each, boiling in these solvents for 5 min. Then the ITO surfaces were dried in a nitrogen flux. After this chemical cleaning procedure the substrates were transferred into an UHV chamber with a nominal pressure below 2×10−10 mbar. Prior to deposition, the cell was degassed at approximately 200 °C
Model of excitation of a trap level
A few comments are necessary on the conceptual difficulties met when trying to apply the DLTS transient spectroscopy to undoped wideband organic semiconductors in general. First, it is not clear where the transport band is, one may even expect either a dispersive or a non-dispersive hopping transport within a band of electron states between HOMO and LUMO to take place. Now it is well known that the capture at and emission from traps in darkness are governed by the principle of detailed balance
Results and discussion
We start the presentation of experimental results by looking at the result of applying the FCM to the Ag/CuPc/ITO diodes. A representative example is reproduced in Fig. 3, which is instructive for distinguishing between different contributions to the excess capacitance, the contributions being of different origins. Note that on the x-axis the potential of ITO is plotted, nevertheless, the course of the correlated total charges belonging to the longer pulse durations and longer delays t2
Conclusions
An attempt has been presented to reconstruct the shape of the spatial profile of deep acceptors detected by the charge DLTS and the feedback charge capacitance (FCM) experiments in Ag/CuPc/ITO diodes. Within the framework of the model presented, the density of the deep acceptors is found to increase when moving from the Ag gate to the ITO substrate. There is a linear relationship between the energy stored in the trap-limited capacitance CT(U) at a given bias U and the local trap density. The
Acknowledgements
The authors gratefully acknowledge the financial support provided by the Graduiertenkolleg ‘Dünne Schichten und nichtkristalline Materialien’ at Technical University Chemnitz.
References (16)
- et al.
Chem. Phys. Lett.
(2001) - et al.
Org. Electron.
(2001) - et al.
Sol. Energy Mater. Sol. Cells
(2000) - et al.
Chem. Phys. Lett.
(2001) Appl. Phys. Lett.
(1986)- et al.
Appl. Phys. Lett.
(1987) - et al.
Molecular Semiconductors
(1985) - et al.
Appl. Phys. Lett.
(1996)
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