## 3 Results and Discussion
Here, \(V\) is the absolute value of the applied voltage, \(R_{1}\) is the internal resistance, \(R_{2}\) is the resistance of the OLED, and \(C\) is capacitance. Figure 3:4 \(\Delta I\) versus the reciprocal of \(t\), i.e., \(2f\), for the ITO/TPD/Alq\({}_{3}\)/Al device (**a**) and ITO/CoPc/Alq\({}_{3}\)/Al device (**b**). The solid line is calculated by Equation (6) with the parameter sets used in Figure 3:5 To interpret the \(t\) dependence of \(\Delta I\), we divided \(t\) into five regions, I, II, III, IV, and V.  
In region I, no emission was observed for both the alternating zero-forward and reverse-forward biases. In this study, only the ITO/CoPc/Alq\({}_{3}\)/Al device has a region I, meaning that no carriers can arrive at the CoPc/Alq\({}_{3}\) 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 \(\Delta I\) 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, \(\Delta I\) is still negative, even though it increases as \(t\) increases above 1.0 \(\times\) 10\({}^{-6}\) and 2.0 \(\times\) 10\({}^{-6}\) s (corresponding to the frequency below 5.0 \(\times\) 10\({}^{5}\) and 2.5 \(\times\) 10\({}^{5}\) Hz) for the ITO/TPD/Alq\({}_{3}\)/Al and ITO/CoPc/Alq\({}_{3}\)/Al devices, respectively. In region IV, \(\Delta I\) 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 \(\Delta\). In region V, where \(t\) is above region IV, \(\Delta I\) decreases with increasing \(t\). Because the suppression of the carrier injection barrier caused by the reverse-forward bias works only until the modulation of \(\Delta\) 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 \(\Delta I\) to decrease. For our device, the most efficient condition for the ITO/TPD/Alq\({}_{3}\)/Al and ITO/CoPc/Alq\({}_{3}\)/Al devices is the reverse-forward alternating bias at a frequency of 1.2 \(\times\) 10\({}^{1}\) and 3.3 \(\times\) 10\({}^{3}\) Hz, respectively, where the emission intensity is 20% and 40% larger than that under the zero-forward alternating bias. The frequency region where \(\Delta I\) 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 Alq\({}_{3}\)/cathode interface during the reverse-bias application has been reported [16], which supports our model.