Investigation of the magnetic field effects on the electron mobility in tri-(8-hydroxyquinoline)-aluminum based light-emitting devices Qiming Peng, Jixiang Sun, Xianjie Li, Mingliang Li, Feng Lia) State Key Lab of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, People’s Republic of China
Abstract: We investigated the mganetic field effects (MFEs) on electron mobility in tri-(8-hydroxyquinoline)-aluminum based light-emitting devices by transient-electroluminescence method upon application of various offset voltages. It is found the rising edges of EL pulses are well overlapped and the falling edges of EL pulses are separated for the magnetic field on and off when Voffset=0 V and Voffset﹥ Vturnon of the devices. The results suggest the bipolaron model and the triplet-polaron interaction model related to the carriers mobilities are not the dominant mechanisms to explain the MFEs under our experimental conditions, and the magnetic field affects the carriers recombination process is confirmed.
a)
Author to whom correspondence should be addressed; Electronic mail:
[email protected]
1
Since the first observation of magnetic field effects (MFEs) on emission and current in organic light-emitting diodes (OLEDs) with nonmagnetic electrodes in 2003 was reported by kalinowski et al.,1 the MFEs on OLEDs have been studied extensively2-27 due to the potential to enhance the efficiency of organic devices or produce the next-generation semiconductor device.7 Despite the large progress in the research of MFEs on organic light-emitting diodes, the microscopic mechanism still deserves further investigation. To the current, a number of models have been proposed to explain the MFEs.1,4,6,9,13,15,24,28 In these models, there are two mechanisms that are correlated to the carrier mobilities. One is the bipolaron model15 suggesting that the mobilities are a function of inter-charge (or inter-polaron) spin interaction which can be affected by the magnetic field. Our previous results demonstrate that the bipolaron model is not the dominant mechanism in our devices under the pulse driving voltage.17 The other is the triplet-polaron interaction (TPI) model28 suggesting that the TPI could reduce the carriers mobilities and since the external magnetic field could shift the ratio of the triplets to singlets thus the magnetic field can affect the carrier mobilities. Recently, Song et al.13 reported their results about
the
MFEs
on
the
hole
mobility
(TPI
model)
in
films
of
N,N'-diphenyl-N,N'-bis(3-methylphenyl)- (1,1'-biphenyl)-4,4'-diamine (TPD) by the dark injection method.29 Motivated by their works, we carried out this experiment to investigate the MFEs on the electron mobility in tri-(8-hydroxyquinoline)-aluminum (Alq3) based light-emitting diodes and test whether the TPI model is appropriate to interpret the MFEs in our experimental conditions. 2
In this work, a transient electroluminescence (EL) method was used.30 Similar with the work of Song et al.,13,29 an offset voltage (Voffset) was added to the pulse voltage (Vpulse) to test the TPI. The detailed definition of Voffset , Vpulse and Vtotal is shown in inset of Fig. 4. Different with the dark injection method which acquires the transient current signals, the transient EL method in our experiment acquires the transient EL signals. We used an Agilent 8114A pulse generator (100v/2A) to apply rectangular pulses with repetition of 1 kHz and width of 7 μs or 10 μs to our devices, and the Voffset were obtained by setting the baseline nonzero. The devices were fixed to a teflon stage with the magnetic field from an electrical magnet perpendicular to its current direction. The light emission was collected by a lens coupled with an optic fiber connected to a Hamamatsu photomultiplier (H5783-01) with time resolution of 0.78 ns. Then the photomultiplier was connected to a digital oscilloscope (Tektronix DPO7104, band width of 1 GHz) with the input resistance as 50Ω. The RC time of the circuit was less than 0.1μs. All measurements were carried out at room temperature under ambient condition. The
structure
of
the
devices
was
ITO/
N,
N′-di-1-naphthyl-N,
N′-diphenylbenzidine (NPB) (60 nm)/tri-(8-hydroxyquinoline)-aluminum (Alq3) (80 nm)/LiF (0.8 nm)/Al. The NPB was the hole-transport layer while the Alq3 functioned as both emissive and electron-transport layer. Because the hole mobility in NPB layer is at least one order of magnitude larger than the electron mobility in Alq3 layer. The mobility of electron in Alq3 layer can be calculated by μe=L/(tdE), approximately, where L is the thickness of Alq3 layer, E is the electric field in Alq3 layer and td is the 3
delay time from the moment when the pulse voltage is applied to the device to the appearance of the rising edge of the pulse EL signal.30 Fig. 1 shows the transient EL response of the device to the magnetic field (150 mT) on and off driven by different Vpulse with the Voffset, that is the baseline, is zero. As can be seen, the rising edges are well overlapped and the falling edges are separated from each other for the curves with the magnetic field on and off. In addition, the magnetoelectroluminescence (MEL) defined as MEL=(EL(B) − EL(0))/EL(0) turns from positive to negative for the EL pulses at flat period when the pulse voltage increases from 7 V to 18 V. These results are consistent with our previous work17,18 and suggest that the bipolaron model is not the dominant mechanism in our devices. What we are more interested is the transient EL responses with the magnetic filed on and off under the condition that the Voffset is larger that the turn-on voltage (Vturnon) of the device. When the Voffset is larger than the Vturnon, the triplets should be pre-exist in Alq3 layer before the pulse voltage arrives because of the ambipolar-current injection and the long lifetime of triplet excitons. Assuming the triplets could reduce the carrier mobility by TPI, the mobility of electron in Alq3 layer with magnetic field on should larger than that with magnetic field off owing to that the magnetic field could decrease the concentration of triplets. So the obtained EL pulses with magnetic field on should ahead of those with magnetic field off, leading to the rising edges of EL pulse are separated with the magnetic field on and off. Thus whether the td is changed with the magnetic on and off under the condition that the Voffset﹥Vturnon can be used to check the TPI mode. Fig. 2 shows the transient EL responses under the 4
condition that the Voffset﹥Vturnon of the device. The EL signals can be observed before the pulse voltage is applied, meaning the ambipolar current-injection and the creation of the excitons. The values of MELs are positive and decline with the Voffset increases from 5 V to 7 V. After Vpulse of 17 V is applied, the MEL changes to be negative. The positive and negative MELs are observed in the time regions of Voffset and Vpulse, respectively, in one EL pulse tested with the magnetic on and off. This further convinces our previous result that the values of MELs decline with the increase of driving voltage and turn from positive to negative sign when the driving voltage is big enough.18 Fig. 3 depicts the normalized transient EL responses of the device driven by Vpulse from 3 V to 19 V and Voffset of 5 V with the magnetic field on and off. As can be seen, the rising edges of the EL pulses with the magnetic field on and off overlap perfectly. This suggests that the td is not changed by the magnetic field. Thus it can be inferred that the magnetic field does not affect the electron mobility in Alq3 layer when the Alq3 is full of triplet exctions. That is, the TPI mode is not the dominant mechanism for the MFEs under our experimental conditions. On the contrary, the falling edges of the EL pulses are separated with the magnetic field on and off, which confirms that the magnetic field actually affects the carriers recombination process.9,17 The same conclusion can be drawn when the offset voltage changes from 4 V to 7 V. Fig. 4 shows the normalized transient EL responses of the device upon application of various offset voltages from 1 V to 9 V, and the total voltage is kept constant as 14 V. If the TPI plays a major role, the electron drift velocity should drop 5
because of the site blocking effect of triplet excitons when Voffset is larger than Vturnon (～2.5 V here), that is, the ambipolar injection is achieved and the triplet excitons are created. This would induce that the td increase with the Voffset. However, from Fig.4 we can see that the td decreases with the Voffset rather than increases. This further verifies that the TPI mode is not the dominant mechanism for the MFEs under our experimental conditions. Here the td reflects the time when the two leading fronts of injected carriers, electrons and holes, meet in Alq3 layer.30 The holes in Alq3 layer are minority carrier and the penetrating distance of holes in Alq3 layer increases with the Voffset, which means the meeting position of the leading fronts of electrons and holes moves toward the cathode with the increase of the Voffset, thus inducing the decrease of td. We changed the light-emitting layer of the device from Alq3 to Alq3 doped Rubrene
(2
wt
%)
and
Alq3
doped
DCJTB
(4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-py ran) (2 wt %). The similar results can be obtained. In summary, we investigated the MFEs on electron mobility in Alq3 based light-emitting devices by transient EL method upon application of various offset voltages. It is found that the rising edges of EL pulses are well overlapped and the falling edges of EL pulses are separated for the magnetic field on and off when Voffset=0 V and Voffset﹥Vturnon of the devices. The results implies that the magnetic field has no effect on electron mobility in Alq3 under the condition of Voffset=0 V and Voffset﹥Vturnon. Thus the bipolaron model and the TPI model related to the carriers 6
mobilities are not the dominant mechanisms to explain the MFE under our experimental conditions, and the magnetic field affects the carriers recombination process is confirmed. We are grateful for financial support from National Natural Science Foundation of China (grant numbers 60878013 and 60706016).
7
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9
Figure captions FIG.1. The transient EL responses of the device driven by various rectangular pulse voltage (the repetition and the width are 1 KHz and 7 μs, respectively. The offset voltage is zero) with the magnetic field of 150 mT on (red line) and off (black line). (a), (b), (c) and (d) shows the details of the EL curves at rising edges, flat period and falling edges, respectively. FIG.2. The transient EL responses of the device driven by rectangular pulse voltage of 17 V (the repetition and the width are 1 KHz and 10 μs, respectively, and the offset voltage increases from 5 V to 7 V) with the magnetic field of 150 mT on (red line) and off (black line). The values of MELs correlated to the offset voltages and the pulse voltages are shown in the texts, respectively. FIG.3. The normalized transient EL responses of the device driven by rectangular pulse voltage from 3 V to 19 V (the repetition and the width are 1 KHz and 10 μs, respectively, and the offset voltage is 5 V) with the magnetic field of 150 mT on (red line) and off (black line). FIG.4. The normalized transient EL responses of the device upon application of various offset voltages from 1 V to 9 V. The total voltage is kept constant as 14 V. Insert: the sketch of Voffset, Vpulse and Vtotal,, respectively.
10
FIG.1.
1.4 black:B=0 red:B=150mT
EL Intensity (a.u.)
1.2
18V
(a)
1.0 0.8
15V (c)
12V
0.6
9V
0.4
7V
0.2
(d)
(b)
0.0 -0.2
-9
-6
-3
0
3
6
9
12
15
18
Time (s)
11
FIG.2.
0.35 offset MEL V % 7 1.388 6 1.567 5 1.923
EL Intensity (a.u.)
0.30 0.25
pulse MEL v % 17 -0.26 17 -0.27 17 -0.23
0.010
0.20 0.15 0.10 0.05
0.008
black:B=0 red:B=150mT
0.006
0.004
0.00 -5
0
5
10
15
20
Time (s)
12
FIG.3.
Pulse 19v 15v 11v 7v 5v 3v
EL Intensity (a.u.)
1.0 0.8 0.6
black:B=0 red:B=150mT
0.4 0.2 0.0 -5
0
5
10
15
20
Time (s)
13
FIG.4.
1.4
1.0
8 Voltage (v)
1.2 EL Intensity (a.u.)
10
VoffsetVtotal td 0v 14v 0.23s 1v 14v 0.22s 3v 14v 0.20s 5v 14v 0.18s 7v 14v 0.15s 9v 14v 0.12s
6
Vtotal
4 2 0
0.8
Vpulse Voffset
-2 -10 -5
0.6
0 5 10 15 20 Time (s)
0.4 0.2 0.0 -5
0
5
10
15
20
25
Time (s)
14
