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Article

Enhanced Emission by Accumulated Charges at Organic/Metal Interfaces Generated during the Reverse Bias of Organic Light Emitting Diodes

Department of Chemistry, Kumamoto University, Kumamoto, Kumamoto 860-8555, Japan
*
Author to whom correspondence should be addressed.
Appl. Sci. 2017, 7(10), 1045; https://doi.org/10.3390/app7101045
Submission received: 13 September 2017 / Accepted: 9 October 2017 / Published: 12 October 2017

Abstract

:
A high frequency rectangular alternating voltage was applied to organic light emitting diodes (OLEDs) with the structure ITO/TPD/Alq3/Al and ITO/CoPc/Alq3/Al, where ITO is indium-tin-oxide, TPD is 4,4′-bis[N-phenyl-N-(m-tolyl)amino]biphenyl, CoPc is cobalt phthalocyanine, and Alq3 is Tris(8-quinolinolato)aluminum, and the effect on emission of the reverse bias was examined. The results reveal that the emission intensity under an alternating reverse-forward bias is greater than that under an alternating zero-forward bias. The difference in the emission intensity (∆I) increased both for decreasing frequency and increasing voltage level of the reverse bias. In particular, the change in emission intensity was proportional to the voltage level of the reverse bias given the same frequency. To understand ΔI, this paper proposes a model in which an OLED works as a capacitor under reverse bias, where positive and negative charges accumulate on the metal/organic interfaces. In this model, the emission enhancement that occurs during the alternating reverse-forward bias is rationalized as a result of the charge accumulation at the organic/metal interfaces during the reverse bias, which possibly modulates the vacuum level shifts at the organic/metal interfaces to reduce both the hole injection barrier at the organic/ITO interface and the electron injection barrier at the organic/Al interface under forward bias.

1. Introduction

About 25 years after the first report of electroluminescence from a single crystal of anthracene under a high applied voltage [1], Tang proposed an organic light emitting diode (OLED) composed of a double-layer structure of organic thin films [2]. Since then, various OLEDs with different structures have been developed. The device structures are designed based on the work functions of the electrodes and molecular orbital energy levels of the organic materials, which are key factors of the OLEDs’ performance. Meanwhile, using ultraviolet photoemission spectroscopy and the Kelvin probe method, Ishii et al. discovered the vacuum level shift at various organic/metal interfaces, where band bending does not occur [3,4], meaning that the Mott–Schottky model is not applicable to OLEDs. Furthermore, by using a time-resolved optical second harmonic generation measurement while applying a rectangular alternating voltage to typical OLEDs, Iwamoto et al. reported that charges accumulate at the organic/organic interface under forward bias with high frequency, and they proposed a model of emission induced by the accumulated charges at the organic/organic interface [5,6,7,8]. Moreover, these studies signify that, to understand the OLED emission mechanism and control its performance, we must focus on the electronic states at the interfaces as well as the work functions and molecular orbital energy levels.
We have been attracted by the report that the emission intensity for an ITO/α-NPD/Alq3/Al device under an alternating reverse-forward bias becomes higher than that under an alternating zero-forward bias in the frequency region between 8 × 105 and 10 Hz [7]. The change in the emission intensity is reported to be due to suppression of the charge injection caused by the charge accumulation at the organic/organic interface under the alternating zero-forward bias.
In this study, we focused on the effect of reverse bias on the device properties. Zou et al. reported that a reverse bias with a long application time (101–104 s) induces the movement of ionic impurities in the active layer, leading to emission enhancement and lifetime improvement [9,10]; however, this mechanism is not acceptable in the high frequency region because the ionic impurities could not follow the frequency. To investigate the generality of the higher emission intensity under an alternating reverse-forward bias than that under an alternating zero-forward bias, OLEDs with the structure ITO/TPD/Alq3/Al and ITO/CoPc/Alq3/Al were fabricated, where TPD is 4,4′-bis[N-phenyl-N-(m-tolyl)amino]biphenyl, Alq3 is Tris(8-quinolinolato)aluminum, and CoPc is cobalt phthalocyanine. We propose another model in which an OLED works as a capacitor under reverse bias, and the accumulated charges at the organic/metal interfaces induce an enhancement of emission under the alternating reverse-forward bias.

2. Materials and Methods

CoPc was synthesized following the procedure reported in [11], and used as a hole injection and transport material. TPD was used as a hole injection and transport material and Alq3 was used as an electron injection, transport, and emitting material. Both are commercially available from Tokyo Kasei and were used without further purification.
The OLEDs were fabricated by the vacuum deposition of TPD or CoPc and Alq3 films (30 nm), followed by the Al cathode (30 nm) on an ITO (indium-tin-oxide) electrode. The base pressure for the organic materials and Al was less than 1.5 × 10−5 torr and 5.0 × 10−6 torr, respectively. The ITO substrate was cleaned by ultrasonic agitation for 15 min in acetone, 2-propanol, and methanol prior to film fabrications. Energy diagrams of the fabricated devices are shown in Figure 1.
An alternating rectangular voltage of various frequencies was applied using a Keithley 3390 waveform generator. The duty ratio for the applied alternating rectangular voltage signal was always 50%. The emission spectra were measured using an Ocean Optics QE65000 spectrometer with a 10 s exposure time. All measurements were performed in a vacuum (less than 1.0 × 10−4 torr) in an Oxford Optistat-CF cryostat.
Because of the deterioration of the devices, the measurements of the frequency dependence (Figure 2) and reverse-bias voltage dependence (Figure 3) were performed using different devices. However, the reproducibility of the tendency of the emission properties was ascertained several times using several devices for each measurement.

3. Results and Discussion

Figure 2 shows the emission intensity (I) as a function of the frequency (f), which alternated between −10 and 10 V (for reverse-forward bias) and 0 and 10 V (for zero-forward bias). In both devices, I decreased when the frequency increased under both reverse-forward and zero-forward biases. Under the reverse-forward bias, I is higher than under the zero-forward bias in the frequency region between 1 and 5 × 103 Hz and 1 and 1 × 105 Hz for the ITO/TPD/Alq3/Al and ITO/CoPc/Alq3/Al devices, respectively. These results resemble that reported in [7], suggesting that the higher emission intensity under an alternating reverse-forward bias than under an alternating zero-forward bias is generally observed in OLEDs.
We investigated how the reverse-bias voltage alters I. Figure 3 depicts I as a function of the reverse-bias voltage alternated with a 10 V forward bias with f = 1 × 103 Hz. In this figure, I linearly correlates with the reverse-bias voltage. Because the change in emission intensity (∆I) can be regarded as the difference of I under zero-forward bias and that under reverse-forward bias, ∆I is also proportional to the applied reverse-bias voltage, as shown on the right axis of Figure 3. This result signifies that ∆I depends on both the frequency and applied voltage of the reverse bias.
In [7], ∆I is rationalized by a model in which excess charges accumulate at the organic/organic interface: the accumulated charges under the alternating zero-forward bias suppress the charge injection, and induce the decrease in the emission intensity. The model is reasonable, however, the proportional dependence of the emission intensity change on the reverse-bias voltage, observed in this study is unclear in the model. Here, we focus on the proportional dependence, and propose another model in which an OLED acts as a capacitor during the reverse bias, where positive and negative charges respectively accumulate on the organic/Al and organic/ITO interfaces, and the accumulated charges at the organic/metal interfaces induce an enhancement of emission under the alternating reverse-forward bias. A series circuit of two parallel circuits of a resistor and capacitor is described to understand the charge accumulation at the organic/organic interface in a double-layer OLED [7,13]. In contrast, to estimate the charge accumulation at the organic/metal interfaces, the equivalent circuit shown in Figure 4, which is a series circuit of a resistor and a parallel circuit of a resistor and capacitor, is considered. The circuit equations are as follows
V = R 1 i R 1 + 1 C i c d t
i R 1 = i R 2 + i c
R 2 i R 2 = 1 C i c d t
where V is the absolute value of the applied reverse-bias voltage, i is current, C is capacitance, R1 is the internal resistance, R2 is the resistance of the OLED, and t is the time of the reverse-bias application, which is half the reciprocal of the frequency ( t = 1 2 f 1 ). These equations lead to the equation
R 2 1 R 1 + R 2 ( V R 1 i c ) = 1 C i c d t .
Because ic can be denoted as ic = dQc/dt using the quantity of electric charge (Qc), Equation (1) can be expressed as
R 2 1 R 1 + R 2 ( V R 1 d Q c d t ) = 1 C Q c ,
which can be transformed into
d Q c d t + R 1 + R 2 R 1 R 2 C Q c = V R 1 .
Considering an initial condition of Qc = 0, Equation (3) can be solved as
Q c = R 2 R 1 + R 2 C V { 1 exp ( R 1 + R 2 R 1 R 2 C t ) } ,
and supposing the emission intensity change induced by the accumulated charges (∆Ia) is proportional to Qc, we obtain
Δ I a = R 2 R 1 + R 2 A C V { 1 exp ( R 1 + R 2 R 1 R 2 C t ) } .
by introducing proportionality constant A. Here, because ∆Ia is the change in emission intensity during t, ∆I for an exposure time of 10 s can be denoted as
Δ I = 5 t Δ I a .
This equation adequately reproduces the observed proportional dependence of the emission intensity change on the reverse-bias voltage. With the parameter sets R1 = 50 Ω, R2 = 5.0 × 106 Ω, and C = 10 × 10−9 F, where R1 and R2 are the estimated resistances of the ITO electrode and OLED, respectively, and C is a typical value [14], Equation (6) produces the solid line shown in Figure 3 with a variable parameter of A = 2.88 × 106 and 6.35 × 105 count C−1 for the ITO/TPD/Alq3/Al and ITO/CoPc/Alq3/Al devices, respectively. Equations (5) and (6) clearly indicate that ∆I also depends on the time of the reverse-bias application t. Figure 5 depicts the t−1 (i.e., 2f) dependence of ∆I derived from Figure 2 together with the values calculated using the parameter sets in Figure 3. Clearly, they are not consistent. This might be because A is not a common constant for t but a function of t. At this time, we cannot evaluate A(t).
To understand the mechanism of the reverse-bias emission enhancement, we consider the vacuum level shift () at the organic/metal interface. When a reverse bias is applied to an OLED and it acts as a capacitor, the interfacial dipole at the organic/ITO interface with negative (positive) Δ decreases (increases), while that at the organic/Al interface with negative (positive) Δ increases (decreases), because of the accumulation of negative and positive charges at the organic/ITO and organic/Al interfaces, respectively. In any case, whether Δ at the organic/metal interfaces is negative or positive, this situation causes Δ at the organic/ITO interface to shift positively and Δ at the organic/Al interface to shift negatively. Therefore, the vacuum level shifts at the organic/metal interfaces were modulated to shift both the HOMO of a hole injection material and the LUMO of an electron injection material close to the Fermi levels of the ITO and Al electrodes, respectively, leading to a decrease in both the hole injection barrier at the organic/ITO interface and the electron injection barrier at the organic/Al interface. Consequently, we believe that both the hole and electron injections occur easily under forward bias after reverse bias has been applied, resulting in an increase of the number of carriers that can arrive at the organic/organic interface compared to ordinary DC or alternating zero-forward bias.
Based on this model, we can interpret the tendency of ΔI to depend on t by dividing Figure 5 into five regions, I, II, III, IV and V, according to t.
In region I, no emission was observed for both the alternating zero-forward and reverse-forward biases. In this study, only the ITO/CoPc/Alq3/Al device has a region I, meaning that no carriers can arrive at the CoPc/Alq3 interface under the high frequency bias.
In region II, emission was only observed under the alternating zero-forward bias. The carriers can arrive at the organic/organic interface during the forward-bias phase of the alternating zero-forward bias. In contrast, under the alternating reverse-forward bias, the carriers are consumed to cancel the accumulated charges. As a result, because few residual carriers occur during the forward bias, the emission under the alternating reverse-forward bias is not observed, leading to a negative value of ΔI in this region.
In region III, emission was also observed under alternating reverse-forward bias. Above t of region II, residual carriers exist that can arrive at the organic/organic interface during the forward-bias phase of the alternating reverse-forward bias. However, the emission intensity under the alternating reverse-forward bias is smaller than that under the alternating zero-forward bias because most carriers are also consumed to cancel the accumulated charges. Therefore, ΔI is still negative, even though it increases as t increases above 1.0 × 10−6 and 2.0 × 10−6 s (corresponding to the frequency below 5.0 × 105 and 2.5 × 105 Hz) for the ITO/TPD/Alq3/Al and ITO/CoPc/Alq3/Al devices, respectively.
In region IV, ΔI is positive and increases with increasing t. In this region, the number of carriers that can arrive at the organic/organic interface during the forward-bias phase of the alternating reverse-forward bias exceeds that of the alternating zero-forward bias because of the injection-barrier suppression by the modulation of the vacuum level shift Δ.
In region V, where t is above region IV, ΔI decreases with increasing t. Because the suppression of the carrier injection barrier caused by the reverse-forward bias works only until the modulation of Δ induced by the accumulated charges generated under the reverse bias is canceled by the forward bias, the number of the carriers that can arrive at the organic/organic interface during the forward bias after the cancelation is similar to that in the zero-forward bias. Therefore, during large t, the device cannot obtain the entire benefit from the reverse bias in one pulse, causing ΔI to decrease. For our device, the most efficient condition for the ITO/TPD/Alq3/Al and ITO/CoPc/Alq3/Al devices is the reverse-forward alternating bias at a frequency of 1.2 × 101 and 3.3 × 103 Hz, respectively, where the emission intensity is 20% and 40% larger than that under the zero-forward alternating bias.
The frequency region where ΔI increases is consistent with that of ordinary interfacial polarization [15], suggesting that the proposed model and mechanism for emission enhancement are appropriate. Furthermore, very recently, charge accumulation at the Alq3/cathode interface during the reverse-bias application has been reported [16], which supports our model. Here, we would like to emphasize that our model, in which accumulated charges at the organic/metal interfaces during the reverse bias enhance emission intensity, does not exclude the model in [7], in which accumulated excess charges at the organic/organic interfaces under the zero-forward bias suppress emission intensity; both types of accumulated charges should contribute to the device properties.

4. Conclusions

In conclusion, under the application of alternating reverse and forward rectangular biases, it was revealed that the emission properties of OLEDs depend on both the frequency and applied voltage of the reverse bias. The emission intensity under the alternating reverse-forward bias becomes larger than that under the alternating zero-forward bias as the level of reverse-bias voltage increases or as the frequency decreases. To interpret the change in the emission intensity (ΔI), we proposed a model in which the OLED works as a capacitor under reverse bias, and the accumulated charges at the organic/metal interfaces induce an enhancement of emission under the alternating reverse-forward bias. Based on the equivalent circuit of the model, an equation expressing the correlation between ∆I and the accumulated charges generated during reverse bias was derived. Experimental results for the applied reverse-bias voltage dependence of ∆I were adequately reproduced by the equation. Furthermore, our model can rationalize the emission enhancement, by considering that the accumulated charges at the organic/metal interfaces modulate the vacuum level shifts at the organic/metal interfaces to reduce both the hole injection barrier at the organic/ITO interface and the electron injection barrier at the organic/Al interface under forward bias.
The model proposed in this study might be applicable to any OLED, suggesting that the application of a rectangular alternating reverse-forward bias is a fruitful method for tuning the light emission efficiency of OLEDs, and there is a fair possibility of obtaining higher device performances than under the ordinary DC bias.

Acknowledgments

This study was supported in part by Grant-in-Aid for Scientific Research (C) (No. 25410097 and No. 16K05752) from the Japan Society for the Promotion of Science.

Author Contributions

S.N. and M.M. conceived and designed the experiments; S.N. performed the experiments; S.N. and M.M. analyzed the data; M.M. wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Energy diagram of the ITO/TPD/Alq3/Al device (a) and ITO/CoPc/Alq3/Al device (b). The energy levels of organic materials and electrodesare cited from [12].
Figure 1. Energy diagram of the ITO/TPD/Alq3/Al device (a) and ITO/CoPc/Alq3/Al device (b). The energy levels of organic materials and electrodesare cited from [12].
Applsci 07 01045 g001
Figure 2. Emission intensity I versus the frequency f of the voltage for the ITO/TPD/Alq3/Al device (a) and ITO/CoPc/Alq3/Al device (b). The solid and dashed lines indicate the emission intensity under −10 and 10 V (reverse-forward) bias and 0 and 10 V (zero-forward) bias, respectively.
Figure 2. Emission intensity I versus the frequency f of the voltage for the ITO/TPD/Alq3/Al device (a) and ITO/CoPc/Alq3/Al device (b). The solid and dashed lines indicate the emission intensity under −10 and 10 V (reverse-forward) bias and 0 and 10 V (zero-forward) bias, respectively.
Applsci 07 01045 g002
Figure 3. I (left axis) and ∆I (right axis) versus applied reverse-bias voltage alternated with forward 10 V with 1 × 103 Hz for the ITO/TPD/Alq3/Al device (a) and ITO/CoPc/Alq3/Al device (b). The solid line is ∆I calculated using Equation (6) (see main text).
Figure 3. I (left axis) and ∆I (right axis) versus applied reverse-bias voltage alternated with forward 10 V with 1 × 103 Hz for the ITO/TPD/Alq3/Al device (a) and ITO/CoPc/Alq3/Al device (b). The solid line is ∆I calculated using Equation (6) (see main text).
Applsci 07 01045 g003
Figure 4. Equivalent circuit based on a model in which the OLED acts as a capacitor during the reverse bias. Here, V is the absolute value of the applied voltage, R1 is the internal resistance, R2 is the resistance of the OLED, and C is capacitance.
Figure 4. Equivalent circuit based on a model in which the OLED acts as a capacitor during the reverse bias. Here, V is the absolute value of the applied voltage, R1 is the internal resistance, R2 is the resistance of the OLED, and C is capacitance.
Applsci 07 01045 g004
Figure 5. ∆I versus the reciprocal of t, i.e., 2f, for the ITO/TPD/Alq3/Al device (a) and ITO/CoPc/Alq3/Al device (b). The solid line is calculated by Equation (6) with the parameter sets used in Figure 3. To interpret the t dependence of ∆I, we divided t into five regions, I, II, III, IV, and V.
Figure 5. ∆I versus the reciprocal of t, i.e., 2f, for the ITO/TPD/Alq3/Al device (a) and ITO/CoPc/Alq3/Al device (b). The solid line is calculated by Equation (6) with the parameter sets used in Figure 3. To interpret the t dependence of ∆I, we divided t into five regions, I, II, III, IV, and V.
Applsci 07 01045 g005

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MDPI and ACS Style

Nozoe, S.; Matsuda, M. Enhanced Emission by Accumulated Charges at Organic/Metal Interfaces Generated during the Reverse Bias of Organic Light Emitting Diodes. Appl. Sci. 2017, 7, 1045. https://doi.org/10.3390/app7101045

AMA Style

Nozoe S, Matsuda M. Enhanced Emission by Accumulated Charges at Organic/Metal Interfaces Generated during the Reverse Bias of Organic Light Emitting Diodes. Applied Sciences. 2017; 7(10):1045. https://doi.org/10.3390/app7101045

Chicago/Turabian Style

Nozoe, Soichiro, and Masaki Matsuda. 2017. "Enhanced Emission by Accumulated Charges at Organic/Metal Interfaces Generated during the Reverse Bias of Organic Light Emitting Diodes" Applied Sciences 7, no. 10: 1045. https://doi.org/10.3390/app7101045

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