Highly efficient tandem organic light-emitting diodes based on SubPc:C60 bulk heterojunction as charge generation layer

Journal of Luminescence 169 (2016) 29–34

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Highly efficient tandem organic light-emitting diodes based on SubPc:C60 bulk heterojunction as charge generation layer Zhu Ma a, Shengqiang Liu a, Song Hu b, Junsheng Yu a,n a State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, PR China b Chengdu Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 9 July 2015 Received in revised form 16 August 2015 Accepted 21 August 2015 Available online 11 September 2015

High efficiency tandem organic light-emitting diodes (OLEDs) were realized using an organic bulk heterojunction (BHJ) as charge generation layer (CGL) consisted of boron subphthalocyanine chloride (SubPc) and fullerene (C60). The results showed that the SubPc:C60 based CGL is a promising connecting unit for the fabrication of high efficiency tandem OLEDs with 2.64 folds enhancement in a current efficiency of 63.6 cd/A, compare to the corresponding single-unit OLEDs. The efficiency enhancement of tandem OLEDs is attributed to better charge carrier balance derived from the optimized charge transport pathways and reduced interface energy barrier. It showed that the charge generation ability and charge transport characteristics of CGL were strongly dependent on SubPc:C60 mixing ratio and corresponding film morphology, which was verified by the charge extracting analysis. Additionally, the highest occupied molecular orbital energy level of SubPc is compatible for most of hole transporting layer to minimize the hole injection barrier and suppress the hole accumulation and charge recombination in tandem OLEDs. & 2015 Elsevier B.V. All rights reserved.

Keywords: Tandem organic light-emitting diodes Charge generation layer SubPc:C60 Charge transport

1. Introduction Organic light-emitting diodes (OLEDs) have been thoroughly investigated due to their high efficiency, low-power consumption, large-area potential, and mechanical flexibility for display applications and general solid state lighting [1–3]. To date, the efficiency of OLEDs have been developed to compete with other light sources, future research tends to focus on the device stability and lifetime to make OLEDs as competitive alternative for commercial application [4,5]. The degradation of OLEDs performance mainly derives from excessive operational current, therefore, it is desirable to obtain high brightness and efficiency at low current density in OLEDs. Tandem OLEDs have emerged as a promising architecture to reduce the device current while preserving the device performance [6–8]. Tandem OLEDs are accomplished by vertically stacking several individual electroluminescent (EL) units and electrically connected in series by inserting a charge generation layer (CGL) between adjacent EL units. The CGL is required to act as an effective anode and a cathode to generate intrinsic charge carriers, and facilitates opposite electrons and holes being injected into the adjacent emissive units. Thus, the development of high performance CGL and the exploration of the intrinsic properties of n

Corresponding author. Tel.: þ 86 28 8320 7157. E-mail address: [email protected] (J. Yu).

http://dx.doi.org/10.1016/j.jlumin.2015.08.040 0022-2313/& 2015 Elsevier B.V. All rights reserved.

CGL are the critical issues for designing high performance tandem OLEDs [9–11]. Recently, organic bulk heterojunction (BHJ) with co-evaporated n-type and p-type materials was proposed as an effective CGL in tandem OLEDs, instead of the common CGLs incorporated with ultrathin metal, transparent conductive oxides, or transition metal oxides [12–18]. For instance, highly efficient phosphorescent tandem OLEDs were achieved using BHJ based CGLs consisted of phthalocyanine based donor, e.g., ZnPc, CuPc and H2Pc (or pentacene) and acceptor C60, showing two folds enhancement in a maximum current efficiencies of ∼35 cd/A than the single-unit devices [19,20,6]. Although the BHJ based CGLs exhibited bipolar characteristics demonstrated in double channel organic fieldeffect transistors, the performance of tandem OLEDs was still limited by the high operational current, which mainly originated from unoptimized charge generation and transportation of BHJ based CGLs [21,22]. As well known, the energy level alignment of BHJ based CGLs have close relationship with the processes of charge generation and transportation [23]. The highest occupied molecular orbital (HOMO) of conventionally used donor materials ranges from 4.9
5.1 eV, e.g., ZnPc, CuPc, H2Pc and pentacene [19,6,24]. Thus, there is an obvious energy barrier (ΔHOMO) between the HOMO levels of above donor materials and traditional hole transport materials, e.g., MoO3, TAPC, NPB, NPD and TCTA ranged from 5.3
5.8 eV [10,25,26]. The high hole injection barriers of ΔHOMO

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within 0.2–0.8 eV easily result in the accumulation of holes at the interface of CGLs and hole transport layer, which directly affect the performance of tandem OLEDs. Therefore, it is an important issue to eliminate or diminish the energy barrier and fabricate effective CGLs via the design or usage of p-type materials with proper HOMO level. In this work, a p-type organic semiconductor of boron subphthalocynine chloride (SubPc) was used to fabricate CGL for the first time, and the performance of tandem OLEDs exhibited a mixing ratio dependence of SubPc:C60 based CGL. The atomic force microscopy (AFM) measurement, current-limited devices (CLDs), and EQE characteristics of tandem OLEDs were performed to investigate the effect of the mixing ratio of SubPc:C60 on device performance. It was found that the charge generation, charge injection and transportation abilities were improved by optimizing the mixing ratio of SubPc:C60, leading to the highly efficient devices. These results indicated that the SubPc:C60 BHJ could be a promising CGL for high performance tandem OLEDs.

2. Experimental Indium-tin-oxide (ITO) glasses with a sheet resistance of 10 ohm/sq and film thickness of 150 nm were used as substrates. The substrates were ultrasonically cleaned with detergent, acetone, ultra-purified water and ethanol, and then were treated using a 15 min ultraviolet ozone. Organic layers were deposited under the vacuum of 10  4 Pa, inorganic and metallic layers were deposited at a pressure of 10  3 Pa. A 30 nm N,N’-diphenyl-N,N’-bis (1-napthyl)-1,1’-biphenyl-4,4’-diamine (NPB) was used as holetransporting layer (HTL) on the ITO anode side and a 35 nm 4,7diphenyl-1,10-phenanthroline (BPhen) was used as the electrontransporting layer (ETL) on the Mg:Ag cathode (100 nm) side. The 15 nm emitting layer between HTL and ETL was created by combining a phosphorescent dopant of tris(phenylpyridine) iridium (Ir(ppy)3) and a host of 4,4’-N,N’- dicarbazole-biphenyl (CBP). The tandem OLEDs were fabricated by stacking two single units via using a 5 nm SubPc:C60 based CGL. The mixing ratio of SubPc:C60 blend film was realized by controlling the individual deposition rate of SubPc and C60. Meanwhile, the interface modification layer of LiF (1 nm) and MoO3 (5 nm) were located at the two sides of CGL, which are used to enhance the injection of the generated charges into the corresponding EL units. The typical area of the test OLEDs is 28 mm2. Control device A0 is the single unit OLEDs with a structure of ITO/NPB/CBP: 8 wt% Ir(ppy)3/BPhen/Mg:Ag. Tandem devices with an architecture of ITO/NPB/CBP: 8 wt% C60 (x:y)/MoO3/NPB/CBP: 8 wt% Ir(ppy)3/BPhen/LiF/SubPc: Ir(ppy)3/BPhen/ Mg:Ag, where, the mixing ratios of x to y are 8:1, 4:1, 2:1, 1:1 and 1:2, corresponding to device A, B, C, D and E,

respectively. The schematic of control device and tandem OLEDs are shown in Fig. 1(a), the energy level diagram of tandem OLEDs and charge carriers movement in tandem OLEDs are shown in Fig. 1(b). The current density–voltage–luminance (J–V–L) curves were measured with a Keithley 4200 semiconductor characterization system and a ST-86LA luminance meter at room temperature under ambient atmosphere without encapsulation. The EL spectra were recorded with an OPT-2000 spectrophotometer. The transmittance spectra were measured by using a Shimadzu UV-1700 ultraviolet-visible spectrophotometer. The surface morphology of CGL with different mixing ratios was measured by using an AFM system (SPA 300 HV, Seiko Instrument Inc.).

3. Results and discussion 3.1. EL characteristics of T-OLED The effect of SubPc:C60 mixing ratio on the EL performance of tandem OLEDs is studied. The luminance–voltage (L–V) and current density–voltage (J–V) characteristics of single-unit OLEDs and tandem OLEDs are illustrated in Fig. 2(a) and (b). The turn-on voltages (Von) of device A0
E are 3.4 V, 10 V, 9.8 V, 7.9 V, 8.4 V and 8.8 V, respectively. It can be seen that all tandem devices show the higher turn-on voltage compared to the single-unit devices, and the mixing ratios of SubPc:C60 have significant influence on the Von in the tandem OLEDs. It is also noteworthy that the luminance in the resulting tandem OLEDs exhibit a prominent dependence on the mixing ratio of CGL. The luminance gradually increases with the increasing C60 ratio in CGL, and then decreases at a mixing ratio of 2:1. The current density–luminance (J–L) performance is shown in Fig. 2(c) and summarized in Table 1. Device A0 has the operational current density of 0.08 mA/cm2, 0.18 mA/cm2, 0.45 mA/cm2 and 4.7 mA/cm2 at 1 cd/m2, 10 cd/m2, 100 cd/m2 and 1000 cd/m2, respectively. However, for the tandem device C with ratio optimized CGL can achieve the same luminance as device A0 at low current densities of 0.004 mA/cm2, 0.02 mA/cm2, 0.17 mA/cm2 and 1.65 mA/cm2, respectively. The dramatic reduction of operational current density indicates that the SubPc:C60 organic BHJ works well as CGL in tandem OLEDs, owing to the effective charge generation and enhanced transportation ability. Additionally, the current density of tandem OLEDs also exhibits a notable ratio dependence of SubPc and C60. The current density of tandem devices A
E at 1000 cd/m2 decreases from 3.0 mA/cm2 to 1.65 mA/cm2, and then increase to 2.1 mA/cm2. It demonstrates that SubPc:C60 with optimized mixing ratio exhibits more charge balance and efficient hole–electron recombination.

Fig. 1. (a) The three dimensional architecture of tandem OLEDs, and the molecular structures of C60 and SubPc. (b) Energy level diagram of tandem OLEDs (bottom) and emitting units (top). Schematic of the charge generation, injection and transport processes of electrons and holes from charge generation layer.

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Fig. 2. EL performance of single-unit OLEDs and tandem OLEDs with SubPc:C60 based CGL of different mixing ratios. (a) The luminance–voltage, (b) current density–voltage, (c) current density–luminance, (d) current efficiency–current density and (e) power efficiency–luminance characteristics. (f) Normalized EL spectra of devices at 13 V (bottom) and the transmittance of SubPc:C60 films with different mixing ratios (top).

The utilization of SubPc:C60 based CGL significantly improves the efficiency of tandem OLEDs, as shown in Fig. 2(d) and (e). The tandem OLEDs also exhibits a dependence of efficiency on the ratio of SubPc to C60. The maximum efficiencies are slightly increased from 34.3 cd/A and 6.9 lm/W to 63.6 cd/A and 15.4 lm/W, and then

decrease to 47.6 cd/A and 9.3 lm/W, respectively. In contrast, the tandem OLEDs shows 2.64 folds enhancement in current efficiency compared to the corresponding single-unit OLEDs, which mainly attributes to the low current density of tandem OLEDs. As depicted in Fig. 1(b), the HOMO of SubPc (5.6 eV) is 0.3 eV higher than the

Table 1 EL performance of single-unit OLEDs and tandem OLEDs. J, Von, CEMax, PEMax, and B present current density, turn-on voltage, maximum current efficiency, maximum power efficiency, luminance of single-unit OLEDs at 15.1 V and tandem OLEDs at 24.6 V, respectively. Device

SubPc:C60

J at 1 cd/m2

J at 10 cd/m2

J at 100 cd/m2

J at 1000 cd/m2

Von (V)

B (cd/m2)

CEMax (cd/A)

PEMax (lm/W)

A0 A B C D E

8:1 4:1 2:1 1:1 1:2

0.08 0.11 0.05 0.004 0.005 0.01

0.18 0.18 0.10 0.02 0.03 0.05

0.45 0.44 0.32 0.17 0.23 0.30

4.7 3.0 2.4 1.65 2.1 2.1

3.4 10.0 9.8 7.9 8.4 8.8

48000 18667 21150 33358 43794 37293

24.1 34.3 43.3 63.6 46.9 47.6

15.0 6.9 9.1 15.4 11.6 9.3

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work function of MoO3 (5.3 eV) [27]. Eliminated energy barrier facilitates the hole injection from SubPc to the next emitting unit and improves the charge carrier balance, leading to the ameliorative current density and improved efficiency of tandem OLEDs. Fig. 2(f) depicts the EL spectra of the single-unit OLEDs and tandem OLEDs at a bias of 13 V. In device A0, there is a peak at 510 nm with a shoulder at 534 nm, corresponding to the EL characteristics of Ir(ppy)3 in conventional OLEDs [28]. For tandem devices A, B, C, D and E, most devices exhibit an obvious shoulder peak at 543 nm. With the decreasing of SubPc ratio in the blend films, the relative intensity between peak and shoulder shows steadily variation, and the shoulder intensity is gradually weaken. As shown in Fig. 2(f), the transmittance of 5 nm SubPc:C60 films with different mixing ratios is measured. It can be seen that the optical transparency can be up to 97% in the green light region, which is good for the application of CGL in tandem OLEDs. The SubPc:C60 blend films exhibits an obvious absorption from 500 nm to 650 nm, which originates from the absorption of SubPc. With decreasing SubPc ratio in the blend films, the transmittance of SubPc:C60 films increases continuously. With the increasing of transmittance, the intensity of shoulder peak is gradually weaken, thus, it can be deduced that the spectra variation is not affected by the absorption properties of the SubPc:C60 blend films. However, the absorption band of SubPc:C60 films inevitably affect the luminance of tandem OLEDs, which is evidently demonstrated by the good accordance of the mixing ratio dependence of luminance and transmittance. 3.2. Electrical characteristics of CGL To elucidate the observed trends of mixing ratio dependence in tandem device performance, CLDs were used for the comparison of charge generation ability. In the CLDs, the Bphen and NPB layer worked as hole blocking layer and electrons blocking layer, respectively. Without inserting CGLs, the J–V curves of CLDs

exhibits non-diode characteristics, the injection and transportation of charge carriers are forbidden. After utilizing the CGLs, holes and electrons are generated at the SubPc/C60 interface and eventually collected by corresponding electrodes, resulting in distinct diode characteristics in CLDs [17]. The structure of CLDs is ITO/BPhen/LiF/SubPc:C60 (2:1, 1:2)/MoO3/NPB/Mg:Ag, and the thicknesses of all functional layers are identical with tandem OLEDs. Fig. 3(a) shows the J–V characteristic of the CLDs with different mixing ratio of SubPc:C60 and three dimensional (3D) diagram of CLDs. The J–V characteristic of the CLDs demonstrates the efficient charge generation ability of SubPc:C60 blend film. Meanwhile, the operating voltages of the CLDs with 2:1 and 1:2 ratio are 16 V and 17.7 V at 0.5 mA/cm2. The lower operating voltage with 2:1 ratio of SubPc:C60 indicates that the optimized SubPc:C60 based CGL promotes the charge generation in the CLDs. To gain deeper insight into the influence of SubPc:C60 mixing ratio on the electrical properties of tandem OLEDs, the EQE-J measurement was used (Fig. 3(b)). From the previous work, the external quantum efficiency ( ηex ) can be expressed as Eq. (1),

ηex = e⋅π⋅A

780

∫380

I (λ )⋅λ dλ Ie 683⋅hc

(1)

Where e is the charge of an electron, A is a constant, λ is wavelength, I (λ ) is the relative EL intensity at each wavelength, h is Planck's constant and c is the velocity of light [29,30]. The formula shows positive correlation between EQE and luminance, and inverse relationship between EQE and current density. As shown in Fig. 3(b), a maximum EQE of tandem device C is 18.7% at current density of 1.14 mA/cm2, which is higher than tandem device E with a maximum EQE of 14.4% at 3.6 mA/cm2. On the basis of the theory of thermal emission of electrons, the charge transfer from SubPc to C60 can be achieved, the interfacial dipoles originate from interfacial charge transfer and dipoles are separated into holes and electrons in electric filed (Fig. 3(c)). Therefore, it is significant to optimize the ratio of SubPc:C60 to achieve effective interface for

Fig. 3. (a) J–V characteristics of current-limited devices with different mixing ratio of CGL. Inset: three dimensional diagram of current-limited device. (b) External quantum efficiency–current density curves of the tandem OLEDs. (c) Charge generation mechanism of CGL.

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Fig. 4. (a) and (b) three dimensional diagram of AFM of ITO/Bphen (30 nm)/LiF (1 nm)/SubPc:C60 (10 nm, 2:1 and 1:2). (c) Proposed operating processes of SubPc:C60 CGL with different ratios.

interfacial dipole formation and charge balance, which will dramatically improve the generation ability of charge carriers, the balance of charge transportation and recombination of tandem OLEDs.

3.3. Surface morphology of CGL The morphology of CGL is another significant factor in determining the efficiency of tandem OLEDs [31]. Charge transport characteristics and charge generation ability of SubPc:C60 are strongly dependent on the mixing ratios of p-type and n-type materials and corresponding film morphology. Distinct roughness of CGL could affect the charge generation and transport ability. The surface morphology of 10 nm thick SubPc and C60 blend films with 2:1 and 1:2 mixing ratios were investigated by AFM as shown in Fig. 4(a) and (b). In order to keep same situation as in the tandem OLEDs, the SubPc and C60 blend films are fabricated on ITO/Bphen(30 nm)/LiF(1 nm). The SubPc:C60 blend film with a 2:1 ratio exhibits a smooth and uniform surface with a root mean square (RMS) roughness of 0.8 nm. While the SubPc:C60 blend film with a 1:2 ratio shows a rough surface and nonuniform film with a RMS roughness of 2.28 nm, and island-like growth can be clearly seen in the 3D diagram. Normally, neat films of SubPc are amorphous, smooth surface and characterized with high hole mobility [32]. The neat C60 films confirm polycrystalline nature with the crystallite size ranging from 10 to 20 nm, and C60 films exhibit high electron mobility [33,34]. When the SubPc is the main part of CGL, the blend film of CGL is relatively smooth and uniform. When the C60 becomes the main part of CGL, the blend film is rough and non-uniform, which attributes to the formation

of large aggregates of polycrystalline C60. In the tandem OLEDs, 5 nm thin SubPc:C60 blend films were inserted as CGL. Therefore, it is very important to obtain uniform blend film with smooth surface and proper ratio to avoid the leak current and enhance the charge generation, which leads to the optimized device performance. For different mixing ratio of SubPc:C60, as shown in Table 1, when increases the proportion of C60 in the CGL, the current density decreases firstly, and then increases at the same luminance. The ratio dependence of the current density attributes to the morphology and charge generation ability of CGL. Detail electrical comparisons of the SubPc:C60 surface morphology with different mole ratio were demonstrated in Fig. 4(c). Here, the weight ratio of 2:1 and 1:2 are transferred to mole ratio of 1.2:1 and 1:3.4. When the ratio of SubPc is higher, adequate holes transferred into electrons and plenty of pathways was generated for hole transport in CGL, leading to brilliant charge generation ability. Meanwhile, 4.9 cm2/V s electron mobility of C60 is five orders of magnitude higher than the hole mobility of SubPc (
10  5 cm2/V s) [35,36]. However, the hole and electron mobility of the blend film with mole ratio of 1.2:1 is at the same magnitude, leading to charge generation and charge transport balance in the CGL as shown in the left of Fig. 4(c). The blend film forms the identical amount of pathways for electron and hole transport. With the mole ratio of 1:3.4 (the right of Fig. 4(d)), less SubPc molecules limit the charge transfer from SubPc to C60, the hole and electron transport balance is broken. The electron mobility of blend film is three orders of magnitude higher than hole mobility [32], leading to poor EL performance of the tandem OLEDs.

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4. Conclusion In summary, we demonstrate that SubPc combined with C60 has effectively functioned as charge generation and transportation for tandem OLEDs. The influence of the mixing ratio of SubPc:C60 blend film on the performance of resulting tandem OLEDs was systematically studied. The results showed that the utilization of SubPc:C60 is very beneficial for obtaining higher performance tandem OLEDs. Through optimizing the mixing ratio of SubPc:C60 based CGL, the tandem OLEDs exhibited a optimized current efficiency, EQE, power efficiency and current density of 63.6 cd/A, 18.7%, 15.4 lm/W and 1.65 mA/cm2 at 1000 cd/cm2, respectively. The charge generation and transport ability of SubPc:C60 were illuminated by investigating the electrical characteristics of CLDs and the morphology of BHJ films. It has been proven that the enhanced device performance was due to good charge carrier balance caused by the improved charge transport pathways in CGL, and the reduced barrier at the interface of CGL and holetransporting layer. These results reveal the potential of SubPc:C60 blend films as an effective CGL, which provides an alternative for the architecture design of highly efficient tandem OLEDs.

Acknowledgments This research was funded by the Foundation of the National Natural Science Foundation of China (NSFC) (Grant no. 61177032), the Foundation for Innovation Research Groups of the NSFC (Grant no. 61421002).

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