## 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 \(5\times 10^{3}\) Hz and \(1\) and \(1\times 10^{5}\) Hz for the ITO/TPD/Alq\({}_{3}\)/Al and ITO/CoPc/Alq\({}_{3}\)/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 \(f\) = \(1\times 10^{3}\) Hz. In this figure, \(I\) linearly correlates with the reverse-bias voltage. Because the change in emission intensity (\(\Delta I\)) can be regarded as the difference of \(I\) under zero-forward bias and that under reverse-forward bias, \(\Delta I\) is also proportional to the applied reverse-bias voltage, as shown on the right axis of Figure 3. This result signifies that \(\Delta I\) depends on both the frequency and applied voltage of the reverse bias. In [7], \(\Delta I\) 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  
Figure 3: \(I\) (left axis) and \(\Delta I\) (right axis) versus applied reverse-bias voltage alternated with forward \(10\) V with \(1\times 10^{3}\) Hz for the ITO/TPD/Alq\({}_{3}\)/Al device (**a**) and ITO/CoPc/Alq\({}_{3}\)/Al device (**b**). The solid line is \(\Delta I\) calculated using Equation (6) (see main text). Figure 2: Emission intensity \(I\) versus the frequency \(f\) of the voltage for the ITO/TPD/Alq\({}_{3}\)/Al device (**a**) and ITO/CoPc/Alq\({}_{3}\)/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. 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  
\[V=R_{1}i_{R_{1}}+\frac{1}{C}\int i_{c}dt\] \[i_{R_{1}}=i_{R_{2}}+i_{c}\] \[R_{2}i_{R_{2}}=\frac{1}{C}\int i_{c}dt\]  
where \(V\) is the absolute value of the applied reverse-bias voltage, \(i\) is current, \(C\) is capacitance, \(R_{1}\) is the internal resistance, \(R_{2}\) is the resistance of the OLED, and \(t\) is the time of the reverse-bias application, which is half the reciprocal of the frequency (\(t=\frac{1}{2}f^{-1}\)).