Japanese Journal of Applied
`Physics
`
`Improvement of Current-Voltage Characteristics in
`Organic Light Emitting Diodes by Application of
`Reversed-Bias Voltage
`
`To cite this article: Dechun Zou et al 1998 Jpn. J. Appl. Phys. 37 L1406
`
`View the article online for updates and enhancements.
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`Jpn. J. Appl. Phys. Vol. 37 (1998) pp. L 1406–L 1408
`Part 2, No. 11A, 15 November 1998
`c(cid:176)1998 Publication Board, Japanese Journal of Applied Physics
`
`Improvement of Current-Voltage Characteristics in Organic Light Emitting Diodes
`by Application of Reversed-Bias Voltage
`Dechun ZOU, Masayuki YAHIRO and Tetsuo TSUTSUI
`Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences,
`Kyushu University, Kasuga, Fukuoka 816, Japan
`
`(Received September 2, 1998; accepted for publication September 25, 1998)
`
`The effects of reversed-bias application on current-voltage and luminance-voltage characteristics of standard-type double-
`layer organic light emitting diodes [OLEDs], ITO/TPD(50 nm)/Alq3(50 nm)/Mg:Ag, were investigated. Both the magnitude
`of reversed-bias and the duration of reversed-bias application were systematically changed. Evident voltage shifts towards the
`lower voltage side in current-voltage and luminance-voltage characteristics were observed in the diodes which were treated
`under various reversed-bias condition. The longer the duration of reversed-bias application, the larger the voltage shift was.
`A maximum voltage shift of 0.7 V was observed in the diode treated under a ¡10 V bias for 3 h. Little change in luminance-
`current density relationships was observed for diodes which were treated under various reversed-bias conditions. The results
`were interpreted in terms of the movement of ionic impurities and orientational rearrangements of permanent dipoles in organic
`layers.
`KEYWORDS: organic electroluminescence, reverse bias, voltage shift, internal field, external field, enhancement
`
`One of the most important subjects related to the practi-
`cal applications of organic light emitting diodes (OLEDs) is
`the improvement of device durability. Durability of OLEDs
`is generally dependent on many factors, such as device struc-
`tures, organic materials, electrode materials, processing con-
`ditions, driving methods and so on. Extensive research has
`been performed on these various aspects.1–8) Among them,
`a driving scheme has recently been considered to be one of
`the most important factors for the improvement in the perfor-
`mances of OLEDs. This aspect was not seriously considered
`earlier, because DC driving was considered to be one of the
`major advantages of OLEDs. Indeed, DC driving is one of the
`essential features of OLEDs which discriminates them from
`inorganic AC electroluminescent devices.
`Two important issues, which cannot be disregarded con-
`cerning the driving modes of OLEDs, should be pointed out:
`one is the coexistence of a high electric resistance and a high
`charge mobility in organic materials used in OLEDs.9, 10) This
`class of organic materials are basically dielectric insulators.
`Even in such insulating materials, small amounts of ionic im-
`purities, which may migrate within insulating materials, ex-
`ist no matter how carefully they are synthesized and puri-
`fied. The other is that the electric field applied to OLEDs
`is extremely high, typically on the order of 106 V/cm. It is
`assumed, therefore, that ionic impurities move slowly in the
`presence of high applied electric field and hence form an in-
`ternal electric field in the opposite direction to the applied
`field, despite the fact that their mobilities are extremely low
`compared with those of holes and electrons.
`It is well known that a constant current driving mode can
`achieve a longer lifetime than a constant voltage driving
`mode. Especially, a pulsed driving mode combined with a
`reversed-bias component improves the lifetime of OLEDs.11)
`Little is known, however, about why the driving mode in-
`fluences the durability of OLEDs. We recently reported the
`reversed-bias induced recovery phenomenon of degradation
`in multilayer OLEDs and proposed that the movement of
`ionic impurities was a key factor which causes the degra-
`dation of OLEDs.12, 13) In particular, the initial degradation
`mechanism occurring within the first few minutes after a bias
`has been applied was understood to be closely related to the
`
`existence of ionic impurities. Ionic impurities are believed to
`be a dominant factor which causes recoverable degradation
`in OLEDs through the formation of an internal electric field
`in the opposite direction to the applied external field. The
`strength of the internal electric field may become comparable
`to that of the external electric field. The formation of an inter-
`nal field leads to a decrease in the effective electric field for
`charge injection and transport.
`In this letter, we will report our new experimental find-
`ings about the effects of the reversed-bias application on as-
`fabricated OLEDs. We will demonstrate the fact that the
`reversed-bias application not only accelerates the degradation
`recovery as reported by us earlier14) but also significantly im-
`proves the performances of as-fabricated OLEDs. We will
`also show that the observed effects of reversed-bias can be
`explained by the same model which we previously proposed
`for the interpretation of recoverable degradation in OLEDs.15)
`Standard double-layer OLEDs of ITO/TPD/Alq3/Mg:Ag
`were fabricated by a conventional vacuum-vapor deposition
`process. The size of the emitting area was 2 £ 2 mm2 and
`8 devices were formed on the same glass substrate at the
`same time. The OLEDs formed on the substrate were trans-
`ferred into a vacuum cryostat and measurements of all diodes
`on the same substrate were continuously performed without
`breaking the vacuum. This ensured reproducibility of our
`luminance-current density-voltage (L-J-V) data. A source-
`measure unit (Keithley 238) and luminance meter (Topcon
`BMW-5A) were used for L-J-V measurements. For rapid-
`mode measurements, relative luminance was directly detected
`using a photomultiplier. All measurements were controlled by
`a computer, and each L-J-V curve could be obtained in 40 ms
`only.
`First, L-J-V curves were repeatedly measured for several
`different diodes on a substrate by using the rapid-mode mea-
`surements. No detectable difference in L-J-V curves was ob-
`served. This observation ensured that no degradation process
`occurred during our rapid-mode 40 ms measurements, even
`though the voltage was scanned from 0 V to 10 V. Hence it
`was concluded that we can safely conduct our examination of
`the effect of reversed-bias application on device performance
`using the rapid-mode L-J-V measurements.
`
`L 1406
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`D. ZOU et al.
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`L 1407
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`observation exactly corresponds to the case where the time
`interval of reversed-bias application is increased.
`Figure 3 displays the plots of relative luminance against
`current density for all the diodes with different reversed-bias
`treatments. The open marks indicate data from diodes treated
`under constant reversed-bias voltage for different time inter-
`vals and closed marks indicate the data obtained from diodes
`treated with different bias voltages for a constant time inter-
`val. All data points fall on the same line, even though they are
`from different diodes which have suffered different bias con-
`ditions. This result is consistent with our previous report14)
`and indicates that the reversed-bias application does not cause
`a change in the quantum efficiency of electroluminescence
`in our observed luminance range (<270 Cd/m2), even though
`it significantly contributes to the improvement in luminance-
`voltage characteristics. In other words, the reversed-bias ap-
`plication produces no effect on charge injection and transport
`balance, but gives rise to a large influence on the drive voltage
`necessary for sufficient current flow through diodes.
`
`Jpn. J. Appl. Phys. Vol. 37 (1998) Pt. 2, No. 11A
`
`To each diode, the reversed-bias voltage of ¡10 V was ap-
`plied and maintained for a fixed duration. The time interval
`under the reversed-bias treatment was varied from 0 to 4 h on
`a logarithmic scale. Figures 1(a) and 1(b) show the J-V and L-
`V data obtained from the diodes with different reversed-bias
`time intervals. Due to a sensitivity saturation of the photomul-
`tiplier, relative luminance curves in the figure show saturation
`at the current density of about 6.0 mA/cm2. The luminance
`values at the saturation points correspond to a brightness of
`270 Cd/m2. All the J-V curves for the diodes with differ-
`ent treatments fall on the curve for the virgin diode if the
`curves are shifted horizontally along the increasing voltage
`axis. Moreover, all the L-V curves also fall on the original L-
`V curve with exactly the same amount of voltage axis shifting.
`In other words, the reversed-bias treatment caused a decrease
`in applied voltage for attaining fixed current density and lumi-
`nance values. A longer time interval under the reversed-bias
`treatment gives rise to a lower voltage to achieve the same
`current density and luminance. This observation suggests that
`the charge injection barrier decreases or effective charge in-
`jection/transport voltage increases by the reversed-bias treat-
`ment. The amount of voltage shift was as large as 0.7 V after
`a ¡10 V bias treatment for 3 h. It should be noted that this
`value is comparable to the reported values of charge injection
`barriers for conventional OLEDs.
`It should be emphasized
`that under the reversed-bias treatment better J-V characteris-
`tics can be achieved than those of an as-fabricated fresh diode.
`The L-J-V curves for a series of diodes treated with vari-
`ous magnitudes of the reversed-bias were examined and are
`shown in Fig. 2. When the reversed-bias voltage was in-
`creased, the amount of drive voltage shift was increased. This
`
`Fig. 2. Current-voltage-luminance characteristics of ITO/TPD/Alq3/Mg:Ag
`devices under various reverse bias levels.
`
`Fig. 1. Current-voltage (a) and light intensity-voltage (b) characteristics of
`ITO/TPD/Alq3/Mg:Ag devices under various reverse bias durations.
`
`Fig. 3. Plots of light intensity against current density for a low current
`(open marks: data from the test with variable reverse-bias durations, closed
`marks: data from the test with variable reverse-bias levels).
`
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`L 1408
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`Jpn. J. Appl. Phys. Vol. 37 (1998) Pt. 2, No. 11A
`
`D. ZOU et al.
`
`Fig. 4. A model for the formation of internal electric field based on the orientation of dipoles and movements of ions.
`
`The origin of voltage shift can be attributed to two factors,
`the decrease in carrier injection barrier (interfacial effect) and
`increase in effective electric field for charge transport (effect
`in bulk region). We have no experimental evidence at this
`stage for discriminating between these two factors or to de-
`termine which one is dominant. The point we emphasize here
`is that the internal field model, which we proposed in previous
`publications for explaining the degradation mechanism, espe-
`cially the phenomena related to the spontaneous and reversed-
`bias induced recoveries in OLEDs,13–17) is also applicable for
`the explanation of the improvement in J-V characteristics by
`the reversed-bias application. In contrast to the interpretation
`for the degradation due to a forward-bias application, the in-
`ternal field formed under the reverse bias treatment has the
`same direction as the forward-bias driving. Thus it causes an
`increase in the effective applied voltage, and this gives rise to
`the improvement in J-V and J-L characteristics of the diodes.
`Of course, in our devices, the movement of ions and orienta-
`tion of permanent dipoles could cause additional current just
`like the charge and discharge current in a condenser which
`does not contribute to the emission of the device. Fortunately,
`this fraction of current is far less than the injection current and
`its influence on emission efficiency can be ignored.
`Figure 4 shows a simplified model which explains the in-
`crease of effective electric field by the reversed-bias applica-
`tion. The model assumes that some kinds of ionic impurities
`and permanent dipoles are present in as-fabricated OLEDs.
`Ionic impurities and permanent dipoles are assumed to be ran-
`domly distributed in the organic layers (Fig. 3(a)). When a re-
`0
`versed bias is applied to an OLED, an internal field (E
`) in the
`forward direction is produced (Fig. 3(b)). On the other hand,
`when a forward external bias (E0) is applied, the effective
`electric field (Eeff D E0 C E
`0 > E0) is expected to be larger
`than the applied external field (Fig. 3(c)). Consequently, a
`larger current density and luminance can be obtained, and
`an enhancement of EL intensity is observable even under the
`same driving voltage.
`In summary, marked voltage shifts towards the lower volt-
`age side were observed both in L-V and I-V characteristics
`in the OLEDs treated under a reversed-bias. Decrease in the
`charge injection barrier or increase in effective electric field
`are possible origins. Reversed-bias treatment leads to im-
`provement in J-V characteristics and brings about an increase
`in energy efficiency but produces no change in quantum effi-
`
`ciency. Our finding is directly related to the fact that device
`performances are closely related to the driving mode. Con-
`stant current driving and pulse driving modes with a proper
`reversed-bias component can achieve better device perfor-
`mance than a constant voltage driving mode. An internal
`field model is proposed and is shown to be useful for qualita-
`tive interpretation of the observed phenomena. A quantitative
`analysis of voltage shifts due to reversed-bias application in
`OLEDs will be published elsewhere.
`
`Acknowledgment
`This work has been partly supported by the Core Research
`for Evolutional Science and Technology, Japan Science and
`Technology Cooperation (CREST/JST).
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