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The storage of charges and its optical application in organic light-emitting diodes measured by a transient electroluminescence method
Organic Electronics 27 (2015) 114e118
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
The storage of charges and its optical application in organic lightemitting diodes measured by a transient electroluminescence method Chengwen Zhang a, b, Suling Zhao a, b, *, Zheng Xu a, b, Xiaoxia Hong a, b, Zhijuan Long a, b, Peng Wang a, b, Yuening Chen c, Xurong Xu a, b a b c
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, China Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, China Physics Department, Liaoning University, Shenyang 110036, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2015 Received in revised form 4 September 2015 Accepted 13 September 2015 Available online 25 September 2015
Organic light-emitting diodes (OLEDs) device capable of stored charges with poly (methyl methacrylate) (PMMA) layer is studied by transient electroluminescence measurements. The mechanism and optical application of stored carriers are elucidated. A spike after a driven pulse is found in the device with PMMA layer, which is attributed to the drifting back of accumulated electrons and trapped ones in shallow states, and the detrapping of latter may result in a long decay tail. A reversed post-pulse is applied to release the electrons in deep traps as they are immobile unless under a strong reversed field. Since the stored charges can lead to a great loss of carriers and weaken the performance of device, we find a way to use them in the form of light emitting with an enhanced intensity more than 3 times as against steady-state. So we have a good reason to believe if in a proper way, we can make full use of the stored charges in optical application. © 2015 Elsevier B.V. All rights reserved.
Keywords: OLEDs Stored charges Transient EL method
1. Introduction Since Tang et al. firstly developed the multilayer Organic lightemitting diodes (OLEDs) [1], much great progress has taken place during the past decades [2e5]. To perform well, the device should possess the characteristics such as low operating voltage, high efficiency and good stability. Efficient OLEDs are crucially dependent on the balanced injection of charge carriers, but the carriers in device are imbalanced commonly. As a result, it will lead to the increase of leak current and the quenching near the electrode, which can weaken the performance of device. To overcome these problems, enhancing either charge injecting ability or charge confinement for bipolar recombination will be achieved [6]. As to the former, it can be realized by inserting some buffer layers like carrier injection layers [7e9], SAM layers [10e12], insulator layers [13e16] etc. The charge confinement in the emission layer can be assisted by applying the carrier blocking layers [17,18] or quantum well structure [19e22]. However, any effort to enhance the charge
* Corresponding author. Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044. China. E-mail address: [email protected] (S. Zhao). http://dx.doi.org/10.1016/j.orgel.2015.09.019 1566-1199/© 2015 Elsevier B.V. All rights reserved.
balance of OLEDs will result in the storage of carriers in OLEDs [23e25]. The stored carriers, including accumulated carriers and trapped ones, can take place in many OLEDs structures more or less, which are inevitable and suspected to decrease the efficiency of device. First, excitons may be quenched by the extra polarons [26]. Second, it will lead to field-induced quenching because of the additional electric field created by these charges [27]. Third, the trap-assisted carriers will result in the nonradiative recombination to compete with the radiative one, and eventually decrease the external quantum efficiency [25,28]. Finally, the storage of charges in the interface may lead to the local overheating and even the degradation [29]. Now that these stored charges are not favorable to the performance of devices, to solve the problem may be a great challenge. However, there are few papers related to measure and use the stored charges in electroluminescence. In this paper, we prepared a structure capable of the stored charges. With the help of transient measurement, we not only analyzed the mechanism on them, but significantly made use of them in the form of light emitting, so as to realize the optical application of the stored charges.
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2. Experimental details The device was fabricated on patterned indium tin oxide (ITO) with a sheet resistance 20 U/,. The device structure is ITO/poly (methyl methacrylate) (PMMA)/Tris-(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/LiF (0.6 nm)/Al (80 nm), as shown in Fig. 1(a). ITO was treated by UV-ozone for 10 min. PMMA was dissolved in chloroform and spin-coated on ITO, and other parts were thermally evaporated sequentially. In order to investigate the rule of PMMA in storing carriers, devices with different thickness of PMMA were prepared and measured. Among them, device with 10 nm PMMA showed the best performance in storing charges. Thus, we show the devices with PMMA thickness of 0(device A) and 10 nm (device B), respectively. The active area of the device was 0.09 cm2. The current densityevoltageeluminance (JeVeL) characteristics were measured with a programmable Keithley Source Meter 2410 and Newport 1830 Optical Power Meter. The electroluminescence (EL) spectra were detected by a charge-coupled device (CCD) spectrometer. The transient EL was performed under a series of forward and reversed pulses. The forward pulse was generated by RIGOL DG1022 Function/Arbitrary Waveform Generator, and the reversed pulse was generated by Agilent 8114A High Power Pulse Generator. Both the generators were controlled by DG535 Digital Delay/Pulse generator. The characteristics of transient EL were detected by a Zolix Instruments Model PMTH-S1C1-CR131 Photomultiplier Tube and recorded with a Tektronix Model DPO 4104 digital phosphor oscilloscope. All measurements were carried out at room temperature under ambient atmosphere.
Fig. 2. Current densityevoltageeluminance (JeVeL) curves for device A (without PMMA) and device B (with PMMA). The inset is the EL spectra of both devices.
3. Results and discussion 3.1. EL characteristics and transient EL measurements The EL spectra and JeVeL characteristics of both devices are shown in Fig. 2. The emission of devices is from the exciton recombination of Alq3 without any change. Compared to device A, the current density of device B with 10 nm PMMA decreases sharply and the luminance enhances much, which may result from the blocking and storage of carriers to increase the carrier recombination. In order to study whether there are stored charges, the transient electroluminescence measurements under a pulse bias with the period of 1ms were carried out and shown in Fig. 3. In Fig. 3(a), two devices were driven by a single pulse with the amplitude of 10 V and the width of 250 ms. The electroluminescence intensity of device A decayed directly when the pulse ended. However, there is a remarkable spike and a much longer emission tails, i.e., a much slower emission decay taking place in device B when the pulse bias was removed. The spike and the longer emission decay are resulted from the recombination emission of excitons formed by stored
Fig. 1. (a) Device structure of the OLEDs (b) Energy level diagram of the OLEDs.
Fig. 3. (a) The transient EL under a single driven pulse of 10 V, 250 ms (b) the transient EL followed by a reversed post-pulse of 6 V/ 8 V, 250 ms (c) the transient EL under a reversed post-pulse of 8V, 250 ms with a delay interval of 250 ms.
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carrier in device B with 10 nm PMMA layer. Then a post reversed pulse of 250ms width following the driving pulse immediately is applied on two devices to confirm whether there are still trapped carriers in these two devices, as shown in Fig. 3(b). The voltages on device B are -6 V and -8 V, respectively, and a great change has taken place. The peak height of the spike is enhanced greatly, and the long tail of the decay is approximately reduced by a factor of 5 with respect to that without the post reversed pulse. With the bias increasing from 6V to 8V , EL intensity of the spike enhances correspondingly. The result of device A is still at no variance except the faster decay compared with Fig. 3(a), so we just show its transient EL under -8 V. In Fig. 3(c), an identical reversed post-pulse is also applied to the devices 250ms later the first pulse ends so as to verify the existence of trapped charges after a long decay. The phenomenon of device A is same as shown in Fig. 3(a), and nothing has appeared in this condition. Nevertheless, there are two spikes in device B. The first is similar to that in Fig. 3(a), and the second occurs at the beginning of the reversed post-pulse and then decays rapidly. 3.2. Mechanism analysis The different results between the two devices are originated from the presence of PMMA with a large band gap, as shown in Fig. 1(b). As we have expected, PMMA layer plays the role of blocking electron in the device. As a result, the storage of charges will takes place. The stored charges, which are revealed in the schematic diagram of Fig. 4, can be divided into three parts. Some are accumulated nearby the interface of PMMA and Alq3 (region 1), some are trapped in shallow states (region 2) and others are trapped in deep states (region 3). When applied the driven pulse, carriers are injected from two electrodes. Due to the PMMA layer, electrons as the majority carriers drifted to anode under the driven pulse are blocked by PMMA. As a result, the leaked electrons of device B will decrease more remarkably compared to device A, just as shown in Fig. 2. Some electrons may be accumulated nearby the interface between the layer of PMMA and Alq3, and some may be trapped in the layer of PMMA, while only a few can pass through the blocking layer.
Meanwhile, some minority carriers, holes, may be trapped in both devices under driven bias thanks for the high energy offsets between Alq3 and cathode and the trap sites for holes [30]. When the driven pulse ends, the electric field disappear and the accumulated electrons at the interface of PMMA/Alq3 and the trapped holes will be suddenly released to Alq3 and recombined (as the process Ⅰin Fig. 4) to give emissions as the spike in Fig. 3(a). Notably, the higher intensity of spike than steady-state is due to the existence of exciton formed under driven pulse. In other words, the spike is at the basis of the emission prior. Also, the electrons trapped in shallow states can be released (process Ⅱ), and the long decay tail is attributed to them. Unlike the accumulated charges, the trapped electrons can not release at once, and it takes some release time to detrap, which leads the recombination of the trapped carriers to dominate the decay after the peak, and thus results in a slow decay. However, there are not enough accumulated electrons or holes in device A. Therefore, there is no spike emission when the driven pulse ends. When followed by a reversed postpulse in Fig. 3(b), the detrapping of electrons is facilitated. Electrons in the deep trap can also be set free (process Ⅲ), as these charges are immobile with strong binding energy unless under a strong reversed field, eventually leading to a larger spike. With the reversed voltage increasing from -6 V to -8 V, more trapped electrons will be released, which results in an enhanced spike and can prove the dependence of deeply trapped charges on voltage. In addition, the singlet exciton will be dissociated under the reversed bias. Therefore, the emission decay is faster, as shown in Fig. 3(b) than (a). As to Fig. 3(c), the attribution of the first spike at the ending edge of the driven pulse is same with Fig. 3(a). The second spike results from the electrons in deep traps, as the electrons accumulated and in shallow traps have been consumed. Notably, after a long time interval of 250ms, there are still many trapped carriers according to the intensity of spike, and it will be a great loss of carriers. Also, the trapped charges as well as accumulated charges in OLEDs may induce the quenching, nonradiative recombination, local overheating and even degradation. Otherwise, the spike and long decay tail, for display applications, are the limitations for their performance. Now that the stored charges are not favorable to device, to solve it will be a challenging and meaningful work. 3.3. Optical application of stored charges
Fig. 4. The schematic diagram of stored charges resulting in the spike. Region 1, 2, 3 represent the electrons accumulated nearby the interface, trapped in shallow states and in deep states, respectively. Process Ⅰ, Ⅱ& Ⅲ represent the recombination of the electrons in three regions with holes.
The ideal treatment is to use it rather than to eliminate it. In electronic application, the trapped charges have been used in bistable device [31]. As to OLEDs, the optical application is more important. So the usage of stored charges in optical application needs to be carried out. Weichsel et al. [25] found when applying a forward post-pulse followed by the driven pulse, which should not lead to injection of carriers, the spike would disappear and delay to the end of post-pulse. This can be accounted for the forward postpulse provides an applied field to overcome the reversed internal field. As a result, the charges accumulated and trapped in shallow states can not drift back until the forward bias is removed. With this method, we applied forward post-pulses from 1 V to 3 V between the driven pulse and the delayed reversed post-pulse, and the results are shown in Fig. 5. As can be seen in Fig. 5(a), with the second forward bias increasing, the first spike vanishes gradually and its decay is also reduced. However, the second spike gets larger and larger. The enhanced spike is due to the recombination of stored charges, which can include three parts. One is the light emitting from the accumulated electrons nearby the interface, one results from the electrons trapped in shallow states, and another is from the ones in deep traps, which are immobile unless under a strong
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driven by a single pulse and the second one is accounted for the electrons in the deep traps. As can be seen, there are still many trapped charges and it will lead to a great loss of carriers. Also, the trapped charges as well as accumulated charges will weaken the performance of device. Otherwise, for display applications, the spike and long decay tail will limit the performance of displays. As a result, by applying a forward bias between driven pulse and reversed post-pulse, we take advantages of stored charges in the form of light emitting. The intensity of the spike even exceeds 3 times as against that of the steady-state. So we have a good reason to believe if in a proper way, we can make full use of the stored charges in optical application. Acknowledgments The authors express the thanks to the 863 Program (2013AA032205), the NSFC (51272022 and 11474018), RFDP (20120009130005), and FRFCU (2012JBZ001). References
Fig. 5. (a) The transient ELs with applying forward bias 0 V, 1 V, 2 V, 3 V between driven pulse and delayed reversed pulse. (b) The transient ELs with increasing reversed pulse from 8V to 12V when applying forward post-pulse of 3 V.
reversed field. As a result, the stored charges can be used in the form of light emitting. In Fig. 5(b), the forward post-pulse keep to 3 V and the reversed pulse is increased to 12 V. As can be seen, the spike is also enhanced for more deeply trapped electrons are released. Its intensity even exceeds 3 times more than that of the steady-state. If permitting, we can continue to obtain light emitting of stored charges by increasing bias. In our opinions, the light will eventually saturate for the depletion of carriers, which will be under further investigation. So we have a good reason to believe if in a proper way, we can make full use of the stored charges in optical application.
4. Conclusions In summary, here an OLED device capable of stored charges with PMMA is prepared. A series of pulses are applied to the devices and the mechanism is elucidated according to the transient ELs. Compared to the device absent of PMMA, which stays nearly invariant with different pulses, the presence of PMMA can result in the appearance of EL spike. For the single driven pulse, the spike is attributed to the recombination of electrons accumulated and trapped in shallow state with holes. And the long decay tail can be explained in the term of the releasing of electrons trapped in shallow states. When applying a reversed post-pulse following the driven pulse, the releasing of electrons trapped in deep states is accelerated by the reversed field also with dissociation of excitons, which will lead to a strong spike and a rapid decay, and it is proportional to the voltage. If the reversed pulse is delayed for a time interval, there will be two spikes. The first one is same with that
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
The storage of charges and its optical application in organic lightemitting diodes measured by a transient electroluminescence method Chengwen Zhang a, b, Suling Zhao a, b, *, Zheng Xu a, b, Xiaoxia Hong a, b, Zhijuan Long a, b, Peng Wang a, b, Yuening Chen c, Xurong Xu a, b a b c
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, China Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, China Physics Department, Liaoning University, Shenyang 110036, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2015 Received in revised form 4 September 2015 Accepted 13 September 2015 Available online 25 September 2015
Organic light-emitting diodes (OLEDs) device capable of stored charges with poly (methyl methacrylate) (PMMA) layer is studied by transient electroluminescence measurements. The mechanism and optical application of stored carriers are elucidated. A spike after a driven pulse is found in the device with PMMA layer, which is attributed to the drifting back of accumulated electrons and trapped ones in shallow states, and the detrapping of latter may result in a long decay tail. A reversed post-pulse is applied to release the electrons in deep traps as they are immobile unless under a strong reversed field. Since the stored charges can lead to a great loss of carriers and weaken the performance of device, we find a way to use them in the form of light emitting with an enhanced intensity more than 3 times as against steady-state. So we have a good reason to believe if in a proper way, we can make full use of the stored charges in optical application. © 2015 Elsevier B.V. All rights reserved.
Keywords: OLEDs Stored charges Transient EL method
1. Introduction Since Tang et al. firstly developed the multilayer Organic lightemitting diodes (OLEDs) [1], much great progress has taken place during the past decades [2e5]. To perform well, the device should possess the characteristics such as low operating voltage, high efficiency and good stability. Efficient OLEDs are crucially dependent on the balanced injection of charge carriers, but the carriers in device are imbalanced commonly. As a result, it will lead to the increase of leak current and the quenching near the electrode, which can weaken the performance of device. To overcome these problems, enhancing either charge injecting ability or charge confinement for bipolar recombination will be achieved [6]. As to the former, it can be realized by inserting some buffer layers like carrier injection layers [7e9], SAM layers [10e12], insulator layers [13e16] etc. The charge confinement in the emission layer can be assisted by applying the carrier blocking layers [17,18] or quantum well structure [19e22]. However, any effort to enhance the charge
* Corresponding author. Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044. China. E-mail address: [email protected] (S. Zhao). http://dx.doi.org/10.1016/j.orgel.2015.09.019 1566-1199/© 2015 Elsevier B.V. All rights reserved.
balance of OLEDs will result in the storage of carriers in OLEDs [23e25]. The stored carriers, including accumulated carriers and trapped ones, can take place in many OLEDs structures more or less, which are inevitable and suspected to decrease the efficiency of device. First, excitons may be quenched by the extra polarons [26]. Second, it will lead to field-induced quenching because of the additional electric field created by these charges [27]. Third, the trap-assisted carriers will result in the nonradiative recombination to compete with the radiative one, and eventually decrease the external quantum efficiency [25,28]. Finally, the storage of charges in the interface may lead to the local overheating and even the degradation [29]. Now that these stored charges are not favorable to the performance of devices, to solve the problem may be a great challenge. However, there are few papers related to measure and use the stored charges in electroluminescence. In this paper, we prepared a structure capable of the stored charges. With the help of transient measurement, we not only analyzed the mechanism on them, but significantly made use of them in the form of light emitting, so as to realize the optical application of the stored charges.
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2. Experimental details The device was fabricated on patterned indium tin oxide (ITO) with a sheet resistance 20 U/,. The device structure is ITO/poly (methyl methacrylate) (PMMA)/Tris-(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/LiF (0.6 nm)/Al (80 nm), as shown in Fig. 1(a). ITO was treated by UV-ozone for 10 min. PMMA was dissolved in chloroform and spin-coated on ITO, and other parts were thermally evaporated sequentially. In order to investigate the rule of PMMA in storing carriers, devices with different thickness of PMMA were prepared and measured. Among them, device with 10 nm PMMA showed the best performance in storing charges. Thus, we show the devices with PMMA thickness of 0(device A) and 10 nm (device B), respectively. The active area of the device was 0.09 cm2. The current densityevoltageeluminance (JeVeL) characteristics were measured with a programmable Keithley Source Meter 2410 and Newport 1830 Optical Power Meter. The electroluminescence (EL) spectra were detected by a charge-coupled device (CCD) spectrometer. The transient EL was performed under a series of forward and reversed pulses. The forward pulse was generated by RIGOL DG1022 Function/Arbitrary Waveform Generator, and the reversed pulse was generated by Agilent 8114A High Power Pulse Generator. Both the generators were controlled by DG535 Digital Delay/Pulse generator. The characteristics of transient EL were detected by a Zolix Instruments Model PMTH-S1C1-CR131 Photomultiplier Tube and recorded with a Tektronix Model DPO 4104 digital phosphor oscilloscope. All measurements were carried out at room temperature under ambient atmosphere.
Fig. 2. Current densityevoltageeluminance (JeVeL) curves for device A (without PMMA) and device B (with PMMA). The inset is the EL spectra of both devices.
3. Results and discussion 3.1. EL characteristics and transient EL measurements The EL spectra and JeVeL characteristics of both devices are shown in Fig. 2. The emission of devices is from the exciton recombination of Alq3 without any change. Compared to device A, the current density of device B with 10 nm PMMA decreases sharply and the luminance enhances much, which may result from the blocking and storage of carriers to increase the carrier recombination. In order to study whether there are stored charges, the transient electroluminescence measurements under a pulse bias with the period of 1ms were carried out and shown in Fig. 3. In Fig. 3(a), two devices were driven by a single pulse with the amplitude of 10 V and the width of 250 ms. The electroluminescence intensity of device A decayed directly when the pulse ended. However, there is a remarkable spike and a much longer emission tails, i.e., a much slower emission decay taking place in device B when the pulse bias was removed. The spike and the longer emission decay are resulted from the recombination emission of excitons formed by stored
Fig. 1. (a) Device structure of the OLEDs (b) Energy level diagram of the OLEDs.
Fig. 3. (a) The transient EL under a single driven pulse of 10 V, 250 ms (b) the transient EL followed by a reversed post-pulse of 6 V/ 8 V, 250 ms (c) the transient EL under a reversed post-pulse of 8V, 250 ms with a delay interval of 250 ms.
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carrier in device B with 10 nm PMMA layer. Then a post reversed pulse of 250ms width following the driving pulse immediately is applied on two devices to confirm whether there are still trapped carriers in these two devices, as shown in Fig. 3(b). The voltages on device B are -6 V and -8 V, respectively, and a great change has taken place. The peak height of the spike is enhanced greatly, and the long tail of the decay is approximately reduced by a factor of 5 with respect to that without the post reversed pulse. With the bias increasing from 6V to 8V , EL intensity of the spike enhances correspondingly. The result of device A is still at no variance except the faster decay compared with Fig. 3(a), so we just show its transient EL under -8 V. In Fig. 3(c), an identical reversed post-pulse is also applied to the devices 250ms later the first pulse ends so as to verify the existence of trapped charges after a long decay. The phenomenon of device A is same as shown in Fig. 3(a), and nothing has appeared in this condition. Nevertheless, there are two spikes in device B. The first is similar to that in Fig. 3(a), and the second occurs at the beginning of the reversed post-pulse and then decays rapidly. 3.2. Mechanism analysis The different results between the two devices are originated from the presence of PMMA with a large band gap, as shown in Fig. 1(b). As we have expected, PMMA layer plays the role of blocking electron in the device. As a result, the storage of charges will takes place. The stored charges, which are revealed in the schematic diagram of Fig. 4, can be divided into three parts. Some are accumulated nearby the interface of PMMA and Alq3 (region 1), some are trapped in shallow states (region 2) and others are trapped in deep states (region 3). When applied the driven pulse, carriers are injected from two electrodes. Due to the PMMA layer, electrons as the majority carriers drifted to anode under the driven pulse are blocked by PMMA. As a result, the leaked electrons of device B will decrease more remarkably compared to device A, just as shown in Fig. 2. Some electrons may be accumulated nearby the interface between the layer of PMMA and Alq3, and some may be trapped in the layer of PMMA, while only a few can pass through the blocking layer.
Meanwhile, some minority carriers, holes, may be trapped in both devices under driven bias thanks for the high energy offsets between Alq3 and cathode and the trap sites for holes [30]. When the driven pulse ends, the electric field disappear and the accumulated electrons at the interface of PMMA/Alq3 and the trapped holes will be suddenly released to Alq3 and recombined (as the process Ⅰin Fig. 4) to give emissions as the spike in Fig. 3(a). Notably, the higher intensity of spike than steady-state is due to the existence of exciton formed under driven pulse. In other words, the spike is at the basis of the emission prior. Also, the electrons trapped in shallow states can be released (process Ⅱ), and the long decay tail is attributed to them. Unlike the accumulated charges, the trapped electrons can not release at once, and it takes some release time to detrap, which leads the recombination of the trapped carriers to dominate the decay after the peak, and thus results in a slow decay. However, there are not enough accumulated electrons or holes in device A. Therefore, there is no spike emission when the driven pulse ends. When followed by a reversed postpulse in Fig. 3(b), the detrapping of electrons is facilitated. Electrons in the deep trap can also be set free (process Ⅲ), as these charges are immobile with strong binding energy unless under a strong reversed field, eventually leading to a larger spike. With the reversed voltage increasing from -6 V to -8 V, more trapped electrons will be released, which results in an enhanced spike and can prove the dependence of deeply trapped charges on voltage. In addition, the singlet exciton will be dissociated under the reversed bias. Therefore, the emission decay is faster, as shown in Fig. 3(b) than (a). As to Fig. 3(c), the attribution of the first spike at the ending edge of the driven pulse is same with Fig. 3(a). The second spike results from the electrons in deep traps, as the electrons accumulated and in shallow traps have been consumed. Notably, after a long time interval of 250ms, there are still many trapped carriers according to the intensity of spike, and it will be a great loss of carriers. Also, the trapped charges as well as accumulated charges in OLEDs may induce the quenching, nonradiative recombination, local overheating and even degradation. Otherwise, the spike and long decay tail, for display applications, are the limitations for their performance. Now that the stored charges are not favorable to device, to solve it will be a challenging and meaningful work. 3.3. Optical application of stored charges
Fig. 4. The schematic diagram of stored charges resulting in the spike. Region 1, 2, 3 represent the electrons accumulated nearby the interface, trapped in shallow states and in deep states, respectively. Process Ⅰ, Ⅱ& Ⅲ represent the recombination of the electrons in three regions with holes.
The ideal treatment is to use it rather than to eliminate it. In electronic application, the trapped charges have been used in bistable device [31]. As to OLEDs, the optical application is more important. So the usage of stored charges in optical application needs to be carried out. Weichsel et al. [25] found when applying a forward post-pulse followed by the driven pulse, which should not lead to injection of carriers, the spike would disappear and delay to the end of post-pulse. This can be accounted for the forward postpulse provides an applied field to overcome the reversed internal field. As a result, the charges accumulated and trapped in shallow states can not drift back until the forward bias is removed. With this method, we applied forward post-pulses from 1 V to 3 V between the driven pulse and the delayed reversed post-pulse, and the results are shown in Fig. 5. As can be seen in Fig. 5(a), with the second forward bias increasing, the first spike vanishes gradually and its decay is also reduced. However, the second spike gets larger and larger. The enhanced spike is due to the recombination of stored charges, which can include three parts. One is the light emitting from the accumulated electrons nearby the interface, one results from the electrons trapped in shallow states, and another is from the ones in deep traps, which are immobile unless under a strong
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driven by a single pulse and the second one is accounted for the electrons in the deep traps. As can be seen, there are still many trapped charges and it will lead to a great loss of carriers. Also, the trapped charges as well as accumulated charges will weaken the performance of device. Otherwise, for display applications, the spike and long decay tail will limit the performance of displays. As a result, by applying a forward bias between driven pulse and reversed post-pulse, we take advantages of stored charges in the form of light emitting. The intensity of the spike even exceeds 3 times as against that of the steady-state. So we have a good reason to believe if in a proper way, we can make full use of the stored charges in optical application. Acknowledgments The authors express the thanks to the 863 Program (2013AA032205), the NSFC (51272022 and 11474018), RFDP (20120009130005), and FRFCU (2012JBZ001). References
Fig. 5. (a) The transient ELs with applying forward bias 0 V, 1 V, 2 V, 3 V between driven pulse and delayed reversed pulse. (b) The transient ELs with increasing reversed pulse from 8V to 12V when applying forward post-pulse of 3 V.
reversed field. As a result, the stored charges can be used in the form of light emitting. In Fig. 5(b), the forward post-pulse keep to 3 V and the reversed pulse is increased to 12 V. As can be seen, the spike is also enhanced for more deeply trapped electrons are released. Its intensity even exceeds 3 times more than that of the steady-state. If permitting, we can continue to obtain light emitting of stored charges by increasing bias. In our opinions, the light will eventually saturate for the depletion of carriers, which will be under further investigation. So we have a good reason to believe if in a proper way, we can make full use of the stored charges in optical application.
4. Conclusions In summary, here an OLED device capable of stored charges with PMMA is prepared. A series of pulses are applied to the devices and the mechanism is elucidated according to the transient ELs. Compared to the device absent of PMMA, which stays nearly invariant with different pulses, the presence of PMMA can result in the appearance of EL spike. For the single driven pulse, the spike is attributed to the recombination of electrons accumulated and trapped in shallow state with holes. And the long decay tail can be explained in the term of the releasing of electrons trapped in shallow states. When applying a reversed post-pulse following the driven pulse, the releasing of electrons trapped in deep states is accelerated by the reversed field also with dissociation of excitons, which will lead to a strong spike and a rapid decay, and it is proportional to the voltage. If the reversed pulse is delayed for a time interval, there will be two spikes. The first one is same with that
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