The charge generation layer incorporating two p-doped hole transport layers for improving the performance of tandem organic light emitting diodes

Transcription

1 Eur. Phys. J. Appl. Phys. (2014) 67: DOI: /epjap/ The charge generation layer incorporating two p-doped hole transport layers for improving the performance of tandem organic light emitting diodes Dashan Qin, Mingxia Wang, Yuhuan Chen, Lei Chen, Guifang Li, and Wenbo Wang

2 Eur. Phys. J. Appl. Phys. (2014) 67: DOI: /epjap/ Regular Article THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS The charge generation layer incorporating two p-doped hole transport layers for improving the performance of tandem organic light emitting diodes Dashan Qin a, Mingxia Wang, Yuhuan Chen, Lei Chen, Guifang Li, and Wenbo Wang Institute of Polymer Science and Engineering, School of Chemical Engineering, Hebei University of Technology, Tianjin , P.R. China Received: 30 November 2013 / Received in final form: 30 March 2014 / Accepted: 18 July 2014 Published online: 20 August 2014 c EDP Sciences 2014 Abstract. We report the charge generation layer (CGL) structure comprising of Li 2CO 3 doped bathocuproine (BCP:Li 2CO 3)/MoO 3 doped 4,4-N,N-bis [N-1-naphthyl-N-phenyl-amino]biphenyl (NPB:MoO 3)/MoO 3 doped 4,4 -N,N -dicarbazole-biphenyl (CBP:MoO 3) for tandem organic light emitting diodes (TOLEDs). Compared to the TOLED using the conventional CGL structure of BCP:Li 2CO 3/20 nm CBP:MoO 3, the one using the CGL structure of BCP:Li 2CO 3/5 nm NPB:MoO 3/15 nm CBP:MoO 3 showed increased electrical and luminous properties, mostly because the introduction of the higher-conductivity NPB:MoO 3 relative to CBP:MoO 3 could improve the current conduction in the CGL structure. Whereas, the performance of the CGL structure of BCP:Li 2CO 3/x nm NPB:MoO 3/20-x nm CBP:MoO 3 decreased with x increasing, mostly due to the fact that the CBP:MoO 3 became depleted of mobile holes upon contacting p-doped NPB: the smaller thickness of CBP:MoO 3, the worse conductivity for it. We provide some in-depth insights on designing the high-performance CGLs for TOLEDs. 1 Introduction Organic light emitting diodes (OLEDs) have been changing the markets of flat-panel display and general solid-state lighting due to their remarkable developments during the past two and a half decades. In order to cultivate OLEDs into one leading pillar of the modern industry in the near future, the tremendous efforts are needed to advance OLEDs towards the practical levels that inorganic LEDs are approaching. In particular, it is of significance to optimize the power efficiency and loss of OLEDs, relying on the prospective breakthroughs in the structural designs and physical thoughts of OLEDs. Tandem OLEDs (TOLEDs) are a class of luminous devices containing two or more electroluminescent (EL) units stacked in series connection, which are being intensely studied due to their improved stability and efficiency than the single-unit OLEDs [1 10]. The key component in TOLEDs is the charge generation layer (CGL), which resides between the neighboring EL units and undertakes the role of electrical connection. The charge carrier generation and transport inside the CGLs are thought to determine the TOLED performance [4, 8]. The n-doped electron transport layer (n-etl) can combine with MoO 3 to form a kind of promising CGL a qindashan06@aliyun.com for TOLEDs, due to the nature of MoO 3 possessing a deep-lying conduction band and a high work function. It is proposed that an electric-field-assisted charge-generation process generally takes place at the interface between MoO 3 and hole transport layer (HTL), creating hole current in one EL subunit and electron current in MoO 3 [11,12]; the role of the adjacent n-etl is to facilitate the electron injection from MoO 3 into another EL unit [12, 13]. It is noteworthy that the other high-work function transition metal oxides (TMOs), e.g., WO 3 and ReO 3, can be used to substitute MoO 3 in such a kind of CGLs [14, 15]. Recently, the p-doped HTLs (p-htls) by TMOs have been well applied in OLEDs [16 18]. Due to the formation of the charge transfer (CT) complex between the dopant and matrix, the p-htls own markedly increased density of hole carriers, thereby improving the hole-electron balance in the recombination zone of device, compared to the undoped HTLs [19,20]. Thus, it is desirable to use p-htl in the CGL structure. It has been proven that the reverse doped p-n heterojunction, i.e., the combination of n-etl and p-htl, may function as an efficient CGL structure [7,15]. In this case, the holes and electrons are simultaneously generated in the p-htl and n-etl, respectively, via the field-induced charge-carrier separation at the doped heterointerface [21], and then transported into the corresponding EL units p1

3 The European Physical Journal Applied Physics 10 3 Device Fig. 2. The I-V characteristics of devices 1 and 2. resistance of 10 Ω per square. After cleaned in acetone, alcohol and de-ionized water sequentially by an ultrasonic horn, the patterned ITO substrates were blown dry by a nitrogen gun and then treated in UV-ozone for 15 min. The tris(8-quinolinolato)aluminum (Alq3), 4,4 -N,N - dicarbazole-biphenyl (CBP), 4,4-N,N-bis [N-1-naphthyl- N-phenyl-amino]biphenyl (NPB) and bathocuproine (BCP) were purchased from Jilin OLED material company. The Li 2 CO 3 and MoO 3 were used as n- andp-typed dopants. All the materials were used as received. The base pressure of the device fabrication via the thermal evaporation was Pa. Two non-emissive devices were fabricated as follows in order to compare two kinds of the CGLs: Device 1: ITO(+)/CGL-1/CBP 80 nm/moo 3 10 nm/al( ), Device 2: ITO(+)/CGL-2/CBP 80 nm/moo 3 10 nm/al( ), Fig. 1. The structure schemes for the T-devices 1 3. The 1st EL unit is ITO/CBP:MoO 3 10 nm/cbp 70 nm/alq3 60 nm. The 2nd EL unit represents CBP 70 nm/alq3 60 nm/bcp:li 2CO 3 5nm/Al. Form a theoretical point of view, the CGLs would be optimized by the usage of higher-conductivity n-etl or p-htl, because they can reduce the ohmic losses in the current conduction through the CGLs. Recently, it was shown the adoption of higher-conductivity n-etl in the CGL improved the performance of TOLEDs [22]. Here, the higher-conductivity p-htl was introduced in the CGL to increase the TOLED performance, and the relevant mechanism was also discussed. 2 Experimental details 100 nm thick indium tin oxide (ITO) thin film coated glass substrates were commercially available with a sheet where the CGL-1 and CGL-2 represented the 4:1 BCP: Li 2 CO 3 10 nm/2:1 CBP:MoO 3 20 nm and the 4:1 BCP: Li 2 CO 3 10 nm/2:1 NPB:MoO 3 5 nm/2:1 CBP:MoO 3 15 nm structures, respectively. Considering the BCP: Li 2 CO 3, NPB:MoO 3 and CBP:MoO 3 are nearly transparent in the visible-light range [18, 22], the optical absorption measurements of the CGLs-1 and -2 were not done. A single-unit OLED with structure of ITO/2:1 CBP: MoO 3 10 nm/cbp 70 nm/alq3 60 nm/4:1 BCP:Li 2 CO 3 5 nm/al was made and denoted as the. Three TOLEDs were designed as shown in Figure 1 and denoted as the T-devices 1 3. It should be stressed that some part of MoO 3 degraded into lower valance oxides in the vacuum deposition, but for simplifying the device expression, MoO 3 was used as the nominal notation. Note that, the 2:1 doping ratio of organic material to MoO 3 was an optimized value [23, 24]. The current versus voltage (I-V) characteristics and luminance of the devices were measured using a programmable Keithley 2400 sourcemeter and an ST-86LA spot photometer, respectively, under the air condition p2

4 D. Qin et al.: The CGL incorporating two p-doped hole transport layers for improving the performance of TOLED (a) (c) Current efficiency (cd/a) (b) (d) Luminance (cd/m 2 ) 10 3 Power efficiency (lm/w) Fig. 3. The I-V (a), luminance versus voltage (b), current efficiency versus current density (c), and power efficiency versus current density (d) characteristics of the, T-devices 1 and 2. 3 Results and discussion 3.1 The performance comparison between the CGLs-1 and -2 In devices 1 and 2, since the ITO anode was unable to inject holes into BCP:Li 2 CO 3 and the MoO 3 /Al composite cathode provided very infirm electron injection into CBP, the current conduction in these two devices was understood as follows. Firstly, the holes and electrons were simultaneously generated in the p-htl and n-etl, respectively, via the field-induced charge-carrier separation at the doped heterointerfaces [21]; then, the electrons were conducted through BCP:Li 2 CO 3 into the anode, and at the same time the holes were transported through the p-doped areas and CBP into the composite cathode. It is reasonable to consider that the charge-carrier separations at the n-etl/p-htl were not the CGL-limiting steps, since the relevant underlying processes, i.e., the electron tunneling through the doped organic/organic heterointerfaces by the external electric field, were very efficient [21]. Hence, the I-V characteristics of these two devices are thought to mainly reflect the difference between the hole conduction in the CGLs-1 and -2. As shown in Figure 2, device 2 showed increased current density than device 1 at a given voltage. For instance, device 2 reached a current density of ma/cm 2 at a driving voltage of 8.0 V, in contrast to that of ma/cm 2 achieved in device 1. Hence, it is inferred that the hole conduction in the CGL-2 was much more efficient than that in the CGL-1, mostly resulting from the higher conductivity of NPB:MoO 3 than that of CBP:MoO 3 [18]. Thus, it can be concluded that the intervention of higher-conductivity p-htl might reduce the ohmic loss across the CGL structure and thereby improve the performance of TOLEDs. 3.2 The performance comparisons between the T-devices 1, 2 and Figure 3 shows the electrical and luminous characteristics of the T-devices 1, 2 and. As predicted, the device gave higher current density than the at a given voltage >7.5 V, mostly attributed to the lowered ohmic loss across the CGL-2 than that across the CGL-1. Accordingly, the was also brighter p3

5 The European Physical Journal Applied Physics Table 1. The performance comparison of the, T-devices 1 and 2. The I, L, CF and PF represent current density, luminance, current efficiency and power efficiency, respectively. Cell Voltage (V ) I (ma/cm 2 ) L (cd/m 2 ) CF (cd/a) PF (lm/w) than the. At a driving voltage of 16.5 V, the current density and luminance of the were 85.3 ma/cm 2 and 6208 cd/m 2, superior to those (38.2 ma/cm 2 and 2634 cd/m 2 ) of the. The current efficiency of the appeared greater than that of the in the current density range from 0.5 to 90 ma/cm 2, demonstrating that the CGL-2 might offer improved hole-electron balance than the CGL-1. The maximum current efficiencies of the T-devices 1 and 2 were 7.0 and 7.3 cd/a, respectively, both almost equaling twice that of the (3.6 cd/a), indicating that both the CGLs-1 and -2 could effectively connect the two EL units in the TOLEDs. When the current density increasing from 0.5 to 90.0 ma/cm 2,the power efficiency of the varied between 1.31 and 1.40 lm/w, that of the changed between 1.42 and 1.57 lm/w, and that of the went between 1.14 and 1.47 lm/w. It is worth noting that, compared to the, the gave decreased power efficiency at current density <21.8 ma/cm 2, while almost same power efficiency at current density 21.8 ma/cm 2. Table 1 compares the performance of the at a driving voltage of 8.0 V to those of the T-devices 1 and 2 at a driving voltage of 16.0 V. The current densities of these two TOLEDs were much lower than that of the, and their luminance was also far less than twice that of the, mostly due to the fact that the thicknesses of the T-devices 1 and 2 were 20 nm larger than twice that of the, thereby causing some inevitable, additional ohmic losses. Nevertheless, the current efficiencies of the T-devices 1 and 2 were almost twice that of the and their power efficiencies were comparable to that of the. Owing to the smaller hole-conducting resistance in the CGL-2 than that in the CGL-1, the outperformed the. 3.3 The influence of the CBP:MoO 3 thickness on the hole conduction in the CGL structure using two p-htls Figure 4 compares the performance of the T-devices 2 and 3. It can be seen that the exhibited higher current density at a given voltage >7.5 V but nearly identical current efficiency, relative to the T-device 3. It is therefore considered that the hole current in the CGL structure of 4:1 BCP:Li 2 CO 3 10 nm/2:1 NPB:MoO 3 (a) (b) Current efficiency (cd/a) T-device T-device 3 Fig. 4. The I-V (a) and current efficiency versus current density (b) characteristics of the T-devices 2 and x nm/2:1 CBP:MoO 3 x nm became lowered with the x decreasing. Figure 5 describes the electronic states of CBP:MoO 3 before and after forming a junction with NPB:MoO 3. When the CBP:MoO 3 is in contact with the NPB: MoO 3, because of the higher work function of CBP:MoO 3 (5.6 ev) than that of NPB:MoO 3 (4.9 ev) [17,25], a certain number of the electrons are spontaneously transferred from NPB:MoO 3 to CBP:MoO 3 so that the Fermi level across the NPB:MoO 3 /CBP:MoO 3 interface is aligned. Then, part of these transferred electrons become immobilized in the CBP:MoO 3 side of the interface, forming an interfacial dipole with their counterparts (holes) in the NPB:MoO 3 side of the interface; the rest neutralize the same-amount mobile holes, thereby reducing the hole carrier concentration and the conductivity of the CBP:MoO 3. It can be thought that the decrease in the hole carrier concentration in the CBP:MoO 3 is inversely proportional to the thickness of the CBP:MoO 3, since the number of the electrons transferred from NPB:MoO 3 to CBP:MoO 3 remains almost fixed regardless of the CBP:MoO 3 thickness. Thus, the CBP:MoO 3 in the CGL-2 is less depleted p4

6 D. Qin et al.: The CGL incorporating two p-doped hole transport layers for improving the performance of TOLED VL efficient hole injection from the ITO anode [18], therefore, in this case the conductivity of the CBP:MoO 3 is not sensitive to the hole carrier depletion proposed in Figure 5. E F (a) NPB:MoO 3 (5.35 ev) VL Δ e CBP:MoO 3 VL E F (6.25 ev) 4 Conclusion The CGL structure using the BCP:Li 2 CO 3 /5 nm NPB: MoO 3 /15 nm CBP:MoO 3 was demonstrated to outperform the conventional one using the BCP:Li 2 CO 3 /20 nm CBP:MoO 3, because the 5 nm NPB:MoO 3 and 15 nm CBP:MoO 3 combination offered enhanced hole current than the single 20 nm CBP:MoO 3 due to the higher conductivity of NPB:MoO 3 than that of the CBP:MoO 3. The effects of the NPB:MoO 3 /CBP:MoO 3 interface on the conductivity of the CBP:MoO 3 were also discussed. The current research opens a easy, practical way to minify the ohmic loss across the CGLs for the TOLEDs. The authors are grateful for the financial supports from the National Science foundations of China (Grant No ) and Hebei province (Grant No. E ). CBP:MoO 3 References (b) NPB:MoO 3 E F (5.35 ev) (6.25 ev) Fig. 5. The schematic diagrams describing the electronic states of the CBP:MoO3 before (a) and after (b) in contact with the NPB:MoO3. The Δ, VL, EF and stand for interfacial dipole, vacuum level, quasi Fermi level and lowest unoccupied molecular orbital, respectively. In order to align the E F of NPB:MoO 3 and CBP:MoO 3, a certain amount of the electrons need to be transferred from NPB:MoO 3 to CBP:MoO 3 as seen in (a). As a result, an interfacial dipole is formed at the p-doped interface as shown in (b). The free holes and the localized electrons are represented by the red circled plus signs and green minus signs, respectively. Note that, the band bending is neglected at the NPB:MoO 3/CBP:MoO 3 interface for the simplicity. of hole carriers and thereby more conductive than that in the CGL-3, causing that the CGL-2 provides more efficient electrical connection than the CGL-3. It needs to be pointed out that the hole carrier concentration of the CBP:MoO 3 in the ITO/NPB:MoO 3 / CBP:MoO 3 structure can be recovered by the very 1. J. Kido, T. Matsumoto, T. Nakada, J. Endo, K. Mori, N. Kawamura, A. Yokoi, SID Int. Symp. Digest Tech. Papers 34, 964 (2003) 2.Y.H.Chen,J.S.Chen,D.G.Ma,D.H.Yan,L.X.Wang, F.R. Zhu, Appl. Phys. Lett. 98, (2011) 3. M.Y.Chan,S.L.Lai,K.M.Lau,M.K.Fung,C.S.Lee,S.T. Lee, Adv. Funct. Mater. 17, 2509 (2007) 4. L.S. Liao, K.P. Klubek, Appl. Phys. Lett. 92, (2008) 5. X. Qi, N. Li, S.R. Forrest, J. Appl. Phys. 107, (2010) 6. J.X. Sun, X.L. Zhu, H.J. Peng, M. Wong, H.S. Kwok, Appl. Phys. Lett. 87, (2005) 7. T.Y. Cho, C.L. Lin, C.C. Wu, Appl. Phys. Lett. 88, (2006) 8. S. Lee, J.H. Lee, J.H. Lee, J.J. Kim, Adv. Funct. Mater. 22, 855 (2012) 9. H.M. Zhang, Y.F. Dai, D.G. Ma, J. Phys. D: Appl. Phys. 41, (2008) 10. T. Tsutsui, M. Terai, Appl. Phys. Lett. 84, 440 (2004) 11. M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, A. Kahn, Appl. Phys. Lett. 95, (2009) 12. S. Hamwi, J. Meyer, M. Kroger, T. Winkler, M. Witte, T. Riedl, A. Kahn, W. Kowalsky, Adv. Funct. Mater. 20, 1762 (2010) 13. J.P. Yang, Y. Xiao, Y.H. Deng, S. Duhm, N. Ueno, S.T. Lee, Y.Q. Li, J.X. Tang, Adv. Funct. Mater. 22, 600 (2012) 14. J. Meyer, M. Kroger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, A. Kahn, Appl. Phys. Lett. 96, (2010) 15. D.S. Leem, J.H. Lee, J.J. Kim, J.W. Kang, Appl. Phys. Lett. 93, (2008) p5

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