Low temperature transient response and electroluminescence characteristics of OLEDs based on Alq3

Accepted Manuscript Title: Low temperature transient response and electroluminescence characteristics of OLEDs based on Alq3 Authors: Chao Yuan, Min Guan, Yang Zhang, Yiyang Li, Shuangjie Liu, Yiping Zeng PII: DOI: Reference:

S0169-4332(17)31059-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.041 APSUSC 35716

To appear in:

APSUSC

Received date: Revised date: Accepted date:

3-1-2017 5-4-2017 5-4-2017

Please cite this article as: Chao Yuan, Min Guan, Yang Zhang, Yiyang Li, Shuangjie Liu, Yiping Zeng, Low temperature transient response and electroluminescence characteristics of OLEDs based on Alq3, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Low temperature transient response and electroluminescence characteristics of OLEDs based on Alq3 Chao Yuana,b, Min Guan* a, Yang Zhang a, Yiyang Li a, Shuangjie Liu a, Yiping Zeng a,b a

Key Laboratory of Semiconductor Material Sciences, Beijing Key Laboratory of Low Dimensional

Semiconductor Materials and Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China b

College of materials science and optoelectronic devices, University of Chinese Academy of Sciences,

Beijing 100049, People’s Republic of China 

Corresponding author: Tel: +86-10-82304101, Fax: +86-10-82304232

E-mail: [email protected];

Highlights 

The dependency relation between transmission rate and electron transport layer is revealed



The critical temperature points for the influence of luminescent materials and injection barriers on delay time are found.



The influence of light-emitting material and injection layer on carrier accumulation is quantified.

Abstract In this work, the organic light-emitting diodes (OLEDs) based on Alq3 are fabricated. In order to make clear the transport mechanism of carriers in organic light-emitting devices at low temperature, detailed electroluminescence transient response and the current-voltage–luminescence (I–V–L) characteristics under different temperatures in those OLEDs is investigated. It founds that the acceleration of brightness increases with increasing temperature is maximum when the temperature is 200K and it is mainly affected by the electron transport layer (Alq3). The MoO3 injection layer and the electroluminescent layer have great influence on the delay time when the temperature is 200K. Once the temperature is greater than 250K, the delay time is mainly affected by the MoO3 injection layer. On the contrary, the fall time is mainly affected

by the electroluminescent material. The Vf is the average growth rate of fall time when the temperature increases 1K which represents the accumulation rate of carriers. The difference between Vf caused by the MoO3 injection layer is 0.52 us/K and caused by the electroluminescent material Ir(ppy)3 is 0.73 us/K.

Keywords: low temperature; turn-on voltage; electron mobility; electroluminescence transient response

1. Introduction Since 1983, the first OLED was developed by Dr. Tang [1], it has been the subject that is widely noted and continuously researched because of its fast response speed, good color performance and application in Organic/Inorganic Hybrid Near-Infrared Light up converter [2-4]. In recent years, many new developments have been made in the exploration of new materials in order to continuously improve the performance of OLEDs[5-7]. Meanwhile, a series of studies on the transport process of the devices are carried out including the capacitance characteristics and the transient response characteristics of OLEDs [8-16]. Our research group has reported the influence of injection barrier, electroluminescent material and the driving voltage on carriers transport. Temperature，as an important parameter which affects the carriers interface transition and the mobility of the material also has an important impact on the carriers transport in the OLEDs. Some studies have reported the temperature dependence of electron mobility, quantum efficiencies，electroluminescence and carriers dynamic behaviors of OLEDs [17-24]. However those studies are only for the device with a particular structure and have no detailed conclusions about the influence of the temperature on the carriers transport in the devices. So, it is very necessary to find out the influence of temperature on the carriers transport in the OLEDs with different electroluminescent materials and structures. A detailed explanation has been made by our previous studies about the carrier transport in the interface layer by the method of transient electroluminescence. All this work was done on the room temperature（300K）. In order to improve our research，the temperature is introduced as a variable. In this paper, the current-voltage–luminescence (I–V–L) characteristics and transient characteristics of the OLEDs based on Alq3 are tested under different temperatures. According to the analysis results of

temperature dependence of turn-on voltage, delay time and fall time in these OLEDs, the transport process of the carriers is revealed. 2. Experimental The OLEDs are prepared on indium tin oxide (ITO) glass substrate. The multilayer OLEDs consist of ITO as the anode that is treated with O3 for 15 min, MoO3 as hole injection layer (HIL), N,N´enyl-N,N´-bis(1naphtyl)- 1,1´-biphenyl-4,4´-diamine (NPB) as hole injection layer (HIL) and hole transport layer (HTL), Fac-tris(2-phenylpyri-dine) iridium doped 4,4´-Bis(N-carbazolyl)-1,1´-biphenyl [Ir(ppy)3:CBP] (12wt%) as emitting layer for phosphorescent OLED and Tris (8-hydroxyquinoline) aluminum (Alq3) as emitting/ electron transport layer (ETL) for fluorescent OLED, 2,9-dimethyl-4,7 diphenyl-1, 10-phenanthro-line (BCP) as hole blocking layer. LiF /Al semitransparent cathode is deposited on the top of the organic stack finally. The emission area of the devices is 1mm2 determined by the overlap area of the anode and the cathode. All the devices are made in an organic molecular beam deposition system (OMBD) with a vacuum of 5.0 × 10−8 Torr for OLED layers deposition. The device structures and materials are shown in the table1. The currentvoltage–luminescence (I–V–L) characteristics and the EL response characteristics of these OLEDs are measured immediately when the OLEDs are removed from the OMBD. In the test of I-V-L, a Keithley electrometer 2400 and an ST-86LA spot photometer are used to control voltage and to detect brightness. To investigate EL response characteristics of OLEDs under different temperatures, a measurement system is set up as depicted in figure 1. The SYSTRON DONNER MODEL 110D, as a Pulsed Generator, is used to drive the OLED with the parameters of a 33 Hz pulsed voltage, 100μs duration and a less than 20ns rise time. The photo multiplier tube (PMT) is used to detect the EL signals and then transfer it to a RIGOL DS1062CA oscilloscope. The response property of the pulsed generator, PMT and the oscilloscope are sufficiently fast, so measurements of the OLED’ transient EL excited by pulsed voltage would not be restricted by the instruments. Both tests of current-voltage–luminescence (I–V–L) characteristics and the EL response characteristics use the same temperature-controlled system. The temperature controlled from 77K to 300K is achieved by the way of cold and hot equilibrium between the liquid nitrogen refrigeration and electric heating. SHIMADEN-SR92, as the temperature controller, is used to control the temperature which guarantees one centigrade error at most. The experimental results show that the signal to noise is relatively low when the temperature is lower than 200K. Therefore, the temperature range of transient EL measurement is selected as 200K-300K. 3. Results and discussion

3.1. Investigations on turn-on voltage in different temperatures The current-voltage–luminescence (I–V–L) characteristic of device 1 is depicted in figure 2. As shown in the Inset（b）of figure 2, the current is 800nA when the device is turn-on (L=1cd/m2). Because of the oneto-one match relation between the current and brightness, the current is 800nA corresponding to the voltage that is the turn-on voltage. The Inset（a） of figure 2 is the I-V characteristic of device 1 under different temperatures. When taking I=800nA, the corresponding voltage is the turn-on voltage at each temperature. In the same way, we can get the turn-on voltage of three other devices at each temperature .As shown in the table 2, the turn-on voltage of device 1 and device 3 without MoO3 is distinctly larger than the device 2 and device 4 with MoO3. It means that the injection layer plays a decisive role in the turn-on voltage for its function on reducing the injection barrier. Compared to the injection layer, the light emitting material has a smaller influence on turn-on voltage. Figure 3 shows the temperature dependent turn-on voltage of the OLEDs from the 77K to 300K. The first observation is the dramatic decrease in turn-on voltage with increasing temperature. This is reasonable since mobility in small-molecule organics is essentially a thermally driven process, the increase of the temperature leads to the increase of energy of thermally activated hopping speed which can make carriers over the interfaces more easily to combine. Meanwhile，according to the Poole-Frenkel formula：µ=µ0exp （-θ/kT）exp(гE1/2) [25] , the material mobility has a strong dependence on the temperature and it will increase with the increase of temperature. To further study this phenomenon, the derivatives of the measured data (dV/dT) is calculated. The results are shown in the Inset diagram of Figure 3. The V represents the turn-on voltage which is the corresponding voltage when the luminescence of the device is 1cd/m2. dV/dT represents the acceleration of the turn-on voltage decreases with increasing temperature, there is a positive correlation between the turn-on voltage and the number of recombination luminescence carriers, so dV/dT can also represent the acceleration of the number of recombination luminescence carriers increasing with temperature increasing. As shown in the Inset diagram of figure 3, the dV/dT initially increases with temperature and after a certain temperature (200K) it drops down. This fall off is seen on all of the several devices tested over temperature. So, the inflection point（200K）is not influenced by the light-emitting material and interface barrier. The hole mobility in the OLED is much larger than the electron mobility, so the radiative recombination growth rate mainly depends on electron transport layer (Alq3). The increase of temperature leads to the increase in mobility, so more and more carriers are injected into the device at faster and faster

speed to form exciton and emit light when the temperature is less than 200K. Once the temperature exceeds 200K, the number of exciton is still increasing, but the rate began to drop down due to the accumulation of carriers in the Alq3 interfacial layer. So the temperature dependence of the turn-on voltage results from the electron mobility of Alq3 which leads to the space charge effect. 3.2. Investigations on EL transient response characteristics in different temperatures A typical transient EL response intensity under rectangular pulsed voltage and the fitted curve are depicted in figure 4 [21]. As shown in the figure 4, EL intensity achieves a transient high in a short delay time after the pulsed voltage switched on and then decayed to a saturated level quickly. When the pulsed voltage is switched off, the EL will decay for a period of time. There two main parameters are defined as follows: delay time required before light emission onset after the pulsed voltage switched on is defined as td; fall time (tf) is defined as the time taken until a 10% EL intensity is measured. The dependence of the values of delay time and fall time on the amplitudes of rectangular pulsed voltage has been discussed in detail in our previous work [21]. The measured td and tf under different temperature of four devices are listed in table 3. The amplitudes of rectangular pulsed voltage are 10V that is used to drive the transient EL responses under different temperatures. 3.2.1. Investigations on EL delay time As shown in figure 4（a），the only difference between device 2 and 1 is the utilization or absence of the hole injection layer MoO3 (HIL).The td of device 1 and device 2 decrease with increasing temperature and the td of device 2 is significantly less than device 1. Meanwhile △td12 also decreases with increasing temperature as depicted in the inset of figure 4(a). When temperature reaches 300K, the td of device 1 and 2 are the same and the △td12 drops to zero. The td is mainly caused by the space charge effect as a result of the carrier accumulation and the baffle of the injection barrier. Compared with the device 1，the injection hole layer of device 2 can drop the injection barrier and the holes in device 2 are more easily to cross the barrier to combine, so the td of the device 2 is significantly less than the device 1. As we discussed above in the temperature dependence of the turn-on voltage, due to smaller carrier thermal motion and low organic layer mobility in the low temperature, the numbers of the holes injected to cross the barrier are less and the migration velocity in the organic layer is small. Therefore, there will a large number of carriers accumulate in the interface layer and the internal material at low temperature, the injection barrier becomes the main factor that hinders the recombination luminescence of carriers. It can be said that it is the key factor that affects the delay time of the device. However, with the increase of temperature, the

carriers are more likely to cross the barrier by the way of thermionic emission leading to the accumulation amount are much less. The influence of injection barrier to carrier injection is getting smaller. So △td12 caused by the injection barrier is getting smaller. When the temperature reaches 300K, the carrier can easily cross the injection barrier into the devices and is no longer hindered. Eventually two devices have the same delay time. In conclusion, HIL influences the td by affecting the injection capacity of carriers and the influence degree can be obviously observed from △td12 under different temperatures. The influence is more obvious at low temperature and disappears in 300K. To further study decay characteristics, the td of phosphorescent device 1 and fluorescence device 3 are compared. As depicted in figure 4(b), the delay time is basically the same in the several higher temperature points in addition to the temperature in the 200K. From this, it can be concluded that the influence of electroluminescent materials on the delay time is more obvious at low temperature and basically disappears with the increase of the temperature. This phenomenon is consistent with the effect of the electroluminescent materials to turn-on voltage mentioned in the above T-V characteristics. It can be attributed to the luminescence efficiency of electroluminescent materials. When the temperature is 200K, the EL intensity of the two devices is very weak. For the lower internal quantum efficiency of fluorescence device than phosphorescent devices, if the fluorescence device want to be detected by a photomultiplier, it need take longer time to form more exciton to achieve sufficient brightness. So the td of device 3 is larger than device 1. As the temperature increases to a certain value, the EL intensity from device 1 and device 3 at the same length of time is enough to be detected, so the td of device 3 and device 1 turn to the same. As shown in the inset of figure 4(b)，the certain value is 250K. Once the temperature is greater than 250K，the influence from electroluminescent materials on td is very small. 3.2.2. Investigations on EL fall time In the process of carriers injected to the emitting layer to produce light, the redundant carrier will accumulate in the emitting layer if the transmission speed of carriers is greater than the recombination speed. Once the pulse voltage is disconnected, the redundant carrier in the emitting layer will be recombined to produce light until all the redundant carriers are depleted, so we can observe the phenomenon of decay [26]. The tf of several OLEDs based on Alq3 are shown in figure 6. For devices having the same injection structures，the fall time of phosphorescent devices 1 and 2 is significantly larger than the fluorescent devices 3 and 4. As it can be seen from the table 3, the fall time value of phosphorescent device 1 is five

times of the fluorescent devices 3 when the temperature is 250K. This is mainly due to the different light emitting mechanisms. The radiation luminescence of triplet exciton in phosphorescent device 1 and 2 need more time than the radiation luminescence of singlet exciton in fluorescence device 3 and 4, resulting in a large number of excitons and carriers accumulation at the EML/HBL. It will take a longer time to consume the accumulated excitons and carriers after the pulsed voltage in phosphorescent devices 1 and 2, so the tf is larger. For devices having the same light emitting materials, the fall time of phosphorescent devices 2 with the injection layer is 1.5 times of the device 1 without injection layer when the temperature is 250K. The difference is mainly caused by the injection barrier which the device 2 and is lower than device 1, so device 2 can generate more carriers accumulation and have a longer tf. The average growth rate (Vf) of fall time with increasing temperature are calculated: Vf= (tf300-tf200) / (300200). As it is mentioned above that tf represents the accumulation quantity of carriers. According to the formula, we can see that Vf and tf are positively correlated, so the Vf can also represents accumulation quantity of carriers when the temperature increases 1K. The calculation results are: Vf1=0.81 us/K, Vf2=1.33 us/K, Vf3=0.08 us/K and Vf4=0.60 us/K. In order to further quantify the injection layer and the light emitting materials impact on the fall time, △Vf is calculated as shown in Fig.6 inset. The difference between device 1 and 2 or device 3 and 4 lies in whether there is a MoO3 injection layer，so the same values（0.52 us/K） between △Vf12 and △Vf34 represent the difference of accumulation rate caused by the MoO3 injection layer. In the same way, the same values（0.73 us/K）between △Vf13 and △Vf24 represent the difference of accumulation rate caused by the light emitting materials. For △Vf13=△Vf24 >△Vf12=△Vf34, It shows that the light emitting materials have greater impact on the accumulation rate of carriers. Under the comprehensive influence of the light emitting materials and the injection layer, the difference value of accumulation rate between device 2 and 3(△Vf23) is 1.25 us/K which is just the sum of the △Vf13/△Vf24 plus △Vf12/△Vf34. It proves that injection layer and phosphorescent materials will promote the accumulation rate of the carriers. On the contrary, the difference value of accumulation rate between device 1 and 4(△Vf14) is 0.21 us/K which is just the difference of the △Vf13/△Vf24 minus △Vf12/△Vf34. The promoting effect of injection layer and phosphorescent materials on accumulation rate will cancel each other out，the 0.21 us/K is the net rate of phosphorescent material beyond the interfacial layer. Therefore, we can quantitatively understand the effects of carriers transport in different OLEDs based on △Vf。 4. Conclusion

In conclusion，the current–voltage–luminescence (I–V–L) characteristics and EL transient characteristics of the OLEDs based on Alq3 are tested under the different temperatures. It founds that the acceleration of brightness increases with increasing temperature is mainly affected by the electron transport layer (Alq3). The td is influenced by the injection layer and the electroluminescent layer. At high temperature, the delay time is mainly affected by the injection layer. The tf is mainly due to the accumulation of carriers. The small injection barrier and phosphorescent mechanism are easier to make the carriers accumulate in the electroluminescent layer. Compared with the MoO3 HIL, the electroluminescent materials have a greater impact on the accumulation quantities. We can use the method of calculating △Vf to quantify the size of influence of each parameter on fall time. Considering the impact factors on OLEDs electroluminescence, we can improve OLED response speed significantly by adjusting the exciton lifetime, device structure and temperature. Acknowledgments The authors acknowledge the support from National Natural Science Foundation of China (Grant Nos. 61274049, 61404130 and 61574140), Beijing Nova Program (Grant No. xxhz201503), and Open Research Fund Program of the State Key Laboratory of Virology of China (2017IOV002).

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Figure captions Fig. 1. Schematic of variable temperature transient EL measurement system setup. Fig. 2. I-L characteristic of device 1 at 300K. Inset（a）：I-V characteristic under different temperatures. Inset（b）：Schematic diagram of partial parameters in I-L characteristic of device 1 at 300K. Fig. 3. Turn-on voltage of several devices under different temperatures. Inset: dV/dT under different temperatures. Fig. 4. A typical pulsed voltage driven OLED transient EL response signal fitted curve and the definition of delay time and fall time. Fig.5. Delay time of several devices. (a) Td of device 1 without MoO3 (HIL) and device 2 with MoO3 (HIL )under different temperatures. Inset：The difference of delay time between device 1 and device 2（△td12 ）under different temperatures; (b) Td of phosphorescent device 1 and fluorescent device 3 with different emitting layers under different temperatures. Inset：the difference of delay time between device 1 and device 3（△td31）under different temperatures; Fig.6. Fall time of several devices under different temperatures. Inset：The difference of the average growth rate of fall time between several devices.

Table captions Table 1. Summary of the device structure. Device No. 1

Device structure

ITO/NPB（75nm）/Ir（ppy）3:CBP（20nm）/BCP（10nm）/Alq3（20nm）/LiF（1nm）/Al（ 120nm）

2

ITO/MoO3（5nm）/NPB（70nm）/Ir（ppy）3:CBP（20nm）/BCP（10nm）/Alq3（20nm） /LiF（1nm）/Al（120nm）

3

ITO/NPB（75nm）/Alq3（60nm）/LiF（1nm）/Al（120nm）

4

ITO/MoO3（5nm）/NPB（70nm）/Alq3（60nm）/LiF（1nm）/Al（120nm）

Table 2. Turn-on voltage in different temperatures of several devices Device

Turn-on voltage

No.

77K

1

14.2V

13.7V

11.4V

7.7V

5.2V

3.6V

2

13.8V

13.2V

10.6V

7.1V

4.8V

3.1V

3

14.6V

13.8V

11.7V

8.8V

5.8V

3.9V

4

13.9V

13.3V

11.4V

7.9V

5.3V

3.4V

100K

150K

200K

250K

300K

Table 3. Summary of td and tf of devices 1 to 4 under different temperatures. Device No.

Delay time (td) and Fall time (tf) 200K td

tf

250K td

tf

273K td

tf

300K td

tf

1

3.2us 9.0us

2.0us 40.0us

1.0us 52.0us

0.4us 76.0us

2

2.2us 11.6us

1.4us 64.0us

0.8us 78.0us

0.4us 144.0us

3

5.5us 3.4us

2.1us 6.2us

4

2.0us 10.0us

1.6us 32.6us

1.1us 9.8us 1.0us 49.0us

0.4us 11.4us 0.4us 68.0us