Heterojunction effect on the charge injection and transport

ARTICLE IN PRESS Physica B 405 (2010) 469–471

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Heterojunction effect on the charge injection and transport ShanYu Quan a,, Linmei Yang a, Feng Teng b, Zheng Xu b, Yongsheng Wang b a b

ShenYang University of Technology, ShenYang 130021, PR China Institute of Optoelectronics, Northern Jiaotong University, Beijing 100044, PR China

a r t i c l e in fo

abstract

Article history: Received 15 October 2008 Received in revised form 15 July 2009 Accepted 1 September 2009

The charge injection and transport across an organic heteroinjection of is investigated. The heteroinjection is formed by a poly-p-phenylene vinylene derivative (MEH-PPV) hole injection layer with a poly(9,9-dioctylfluorene) (PFO) hole accepting layer on top. The electric field in the accepting layer is obtained after correcting the applied voltage for the voltage drop across the injecting layer due to the buildup of space charge. The distribution of electric field in the accepting layer has effect on the transport of charge. At high electric fields, the current across the heterojunction exhibits only a weak dependence on the field. The strong dependence at low fields can be explained by taking the increase of the Fermi level into account, which effectively modifies the barrier for charge carriers waiting for a jump across the heterojunction. & 2009 Elsevier B.V. All rights reserved.

Keywords: Electrical properties Polymers Thin films Electroluminescence

1. Introduction Organic light-emitting devices (OLEDs) are presently considered as suitable candidates for large-area applications due to their potential advantages in low-power, emissive, flexible, costcompetitive, flat panel displays [1–3]. Attention has especially been focused on poly-p-phenylene vinylene derivative which have an external conversion efficiency of larger than 1% photons/ electron. The hole conduction in the PPV derivatives proved better than the electron conduction, which was attributed to the presence of traps [4] or lower electron mobility [5]. In order to study the injection and transport properties of hole in poly-pphenylene vinylene derivative, hole only device is obtained by the organic heterojunction. The use of heterojunctions has proven to be very useful, as has been demonstrated in light-emitting diodes (LED) based on evaporated small molecules (OLED). In these multilayer OLEDs, the active part consists of various layers with various functions, leading to highly efficient devices [6]. These layers are chosen to have properties, such as hole and electron transport, hole or electron blockage, and high emission. Seeking to improve the OLED performance, it is important to address a device’s internal field distribution, space-charge distribution, and the rate of electron-hole recombination. In the present study, the charge injection across an organic heterojunction is investigated. The heterojunction is formed by a poly [2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene vinylene]

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E-mail address: [email protected] (S. Quan). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.09.001

(MEH-PPV) as hole injecting layer with a poly (9,9-dioctylfluorened ) (PFO) hole accepting layer on top. Fig. 1 shows the schematic energy band diagram of the double-layer device. For such a system, an interface energy barrier for hole transport is formed between the MEH-PPV and the PFO due to the offset between the highest occupied molecular orbits (HOMO) of both polymers. MEH-PPV has a HOMO of 5.3 eV [7] while for PFO the HOMO is about 5.8 eV [8] resulting in an interface energy barrier of fB  0.5 eV. It is demonstrated that such a large injection barrier strongly limits the hole current across the heterojunction. Heterojunction effect on the charge injection and transport is investigated.

2. Experiment The devices consist of an indium tin oxide (ITO) bottom electrode, covered with a layer of MEH-PPV, which has been spin coated from chloroform (6 mg/ml). The top layer consists of PFO, which has been spin coated from toluene (20 mg/ml). The thickness of the layer was measured with a step profilometer (XP-2, AmBios TECHNOLOGY). Several batches have been used. For one, the bottom layer is dMEH-PPV = 100 nm, with a top layer of dPFO =70 nm, others have a bottom layer dMEH-PPV = 140 nm, dPFO =90, 120, and 210 nm. For such a device shown in Fig. 1, the bottom ITO electrode forms an Ohmic contact on MEH-PPV, while the Au top contact blocks electron injection into the PFO layer, and the current throughout the device is carried by holes (hole only device). As a reference, single layer hole only devices have been made, where the ITO bottom electrode has been covered with the MEH-PPV, and on top of the PFO, an Au contact has been

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S. Quan et al. / Physica B 405 (2010) 469–471

MEH-PPV

PFO Au

Fig. 1. Shows the schematic energy band diagram of the double-layer device.

evaporated at pressures less than 10  6 Torr. The current–voltage (I–V) was measured using a source meter (Keithley 2400 source meter) and a luminometer (Minolta LS110 Luminometer) with a close-up lens (Minolta no. 110). All the measurements were carried out at room temperature in air.

3. Results and discussion Fig. 2 shows the current–density voltage (J–V) characteristic of a MEH-PPV/PFO double-layer device. The thicknesses of the MEHPPV and PFO are 140 nm and 210 nm respectively. It is observed that the current density of the two-layer MEH-PPV/PFO devices is indeed strongly reduced with respect to the MEH-PPV single-layer device. The dashed line is the space-charge limited (SCL) hole current calculated p with a field-dependent mobility of the form ffiffiffi mh ðEÞ ¼ mh ð0Þ exp ðg EÞ [9]. A zero-field mobility mh(0) of m0 =3  10 10[m2/Vs] [10], and a field activation factor g of 5  10  3[cm/V]1/2 have been obtained [11]. As a reference, the maximum attainable current for the double layer device is calculated: This current is reached when the energy barrier is not present and the current is only limited by the buildup of space charge in the two layers. The solid line shows the calculated spacecharge limited current (SCLC) for the two-layer device. For the mobility of PFO, mh(0)=1 10  9 m2/Vs and g =5  10  5(m/V)1/2 have been used [12,13]. It is observed that at low voltages, the measured current density of the two-layer device (triangles) is more than three orders of magnitude lower than the calculated space-charge limited current (solid line), which indicates that the current across the heterojunction is indeed strongly injection limited. From this observation it is expected that the field distribution across the PFO layer is uniform, since the amount of charge carriers entering the PFO is too small to locally change the field. It should be noted that this constant electric field in the PFO layer, EPFO, determines the charge transport across the heterojunction [14]. In order to analyze the dependence of the injection current on the field across the PFO, the applied voltage needs to be corrected for the voltage drop across the MEH-PPV layer. For very thin PFO layers, an eventual voltage drop across the MEH-PPV layer can have a relatively large influence on the electrical characteristics. It is important to realize that the charge transport through the MEH-PPV injecting layer is SCL. As a result, this layer only becomes conductive when charge is injected into it. Since this charge is not neutralized, it will lead to a buildup of electric field in the MEH-PPV, and subsequently to a substantial voltage drop across this layer. For SCL transport, the voltage drop across the MEH-PPV (VMEH-PPV) for a given current density J of the double-layer device is simply given by 2 VM-PPV ¼

8 Jd3M-PPV ðe ¼ the dielectric permittivityÞ 9 em

ð1Þ

In the case of a field dependent mobility, the voltage drop can be solved numerically from the one-carrier SCLC model [15]. It should be noted that such a procedure correctly provides the

Current Density (mA/mm2)

1

ITO

0.1

0.01 Device a Device b

1E-3

Cal Device a Cal Device b

1E-4

5

10

15 20 Voltage (V)

25

30

Fig. 2. Current density as a function of voltage for Device a and Device b. The dashed line is the calculated SCLC through the single layer device; the solid line depicts the calculated SCLC through the double layer in the absence of an interface barrier. Device a: ITO/MEH-PPV/Au dMEH-PPV = 140 nm; Device b: ITO/MEH-PPV/ PFO/Au dMEH-PPV = 140 nm, dPFO = 210 nm.

voltage drop across the MEH-PPV layer of the heterojunction device, since both current and voltage drop are related to the total amount of charge inside the layer. However, the field and carrier density distribution of the single carrier SCLC model are not applicable to the heterojunction device; in the heterojunction device, there will be a large buildup of charge carriers at the blocking junction. With VM-PPV known, the voltage drop across the PFO layer follows directly from VPFO ¼ V  VM-PPV þ Vbi

ð2Þ

Vbi is the average voltage drop due to the build electric field. The actual electric field, Eactual_PFO , in the PFO top layer now becomes Eactual_PFO ¼

VPFO V  VM-PPV V ¼ þ bi dPFO dPFO dPFO

ð3Þ

If omitting the effect of build-in electric field of PFO layer, then: EPFO ¼

VPFO V  VM-PPV  dPFO dPFO

ð4Þ

which means: EPFO ¼

VPFO V  VM-PPV  oEactual_PFO dPFO dPFO

ð5Þ

Fig. 3 shows the resulting J–EPFO plots for the various double-layer devices. It is observed that current density scales with the electric field in the PFO layer. At high electric fields, the current across the heterojunction exhibits only a weak dependence on the field. Thus Fig. 3 represents the J–EPFO relation of the experimental injected limited current across the MEH-PPV/PFO heterojunction. The injected carrier density in the MEH-PPV layer plays an important role in the J–EPFO relation. Fig. 4 shows the theoretical model of the injection across an organic heterojunction. For injection of charges from a metal contact into an organic dielectric, the starting point for a charge carrier jump is always the metal Fermi level, independent of the injection current density. However, for an organic heterojunction, we believe the Fermi level does depend on the injection current density because the charge carrier concentration at the interface changes with current. In a disordered semiconductor, the charge carrier mean energy is at s2/kT [16] below the center of the Gaussian DOS for low carrier concentrations, with s as the width of the energy distribution of the transport sites. This means the injection barrier height is fb + s2/kT for low carrier concentration.

ARTICLE IN PRESS S. Quan et al. / Physica B 405 (2010) 469–471

4. Conclusions

1 Current Density (mA/mm2)

471

It has been demonstrated that a polymeric heterojunction strongly limits the hole current due to an interface energy barrier. The potential drop across the injecting layer, due to the buildup of space charge, has to be investigated. At high electric fields, weak field dependence of the injection current is due to the change of effective barrier height. The strong field dependence of the injection current at low fields can be explained by a rapid rise of the Fermi level out of the tail of the MEH-PPV DOS, due to the increase in the charge carrier concentration.

0.01

210nm 120nm 90nm 70nm

1E-4

0

5

10

15

20

EPFO=VPFO/dPFO (V/nm) Fig. 3. Current density J versus the electric field EPFO in the PFO layer, with the applied voltage being corrected for the voltage drop across the MEH-PPV layer. One double-layer device consists of a MEH-PPV layer with dMEH-PPV = 100 nm and a top PFO layer of dPFO = 70 nm, the other devices have a MEH-PPV bottom layer of dMEH-PPV = 140 nm and top PFO layers of dPFO =90, 120, and 210 nm, respectively.

Acknowledgments The authors would like to thank Mr. Yanbin Hou from Institute of Optoelectronics, Northern Jiaotong University, Beijing for the supply of the PFO. This work has been supported by National Natural Science Foundation of China (no. 90301004), Education Department Foundation of Liaoning Province (no. 20086495) and Initial Foundation for PH.D from ShenYang University of Technology (no. 521101302). References

Fig. 4. Schematic representation of the injection across an organic heterojunction for low field and high field.

However, with increasing carrier concentration, the effective barrier height has been changed due to the filling of states at the injecting interface. At high applied voltages, the Fermi level passes the equilibration energy,eF 4  s2 =kT and the energy level from which the charge carriers are injected now rises with eF .

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