Article pubs.acs.org/JPCC
Low-Temperature Evaporable Re2O7: An Efficient p‑Dopant for OLEDs Yifu Jia, Lian Duan,* Deqiang Zhang, Juan Qiao, Guifang Dong, Liduo Wang, and Yong Qiu* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China ABSTRACT: Transition-metal oxides (TMOs) are one of the most promising kinds of p-doping materials for organic semiconductors. However, to be compatible with organic materials, low-temperature evaporable TMOs are highly desirable. Rhenium(VII) oxide with a very low melting temperature of only 225 °C, which is the lowest among all TMO dopants, is first investigated as a p-dopant in N,N′-bis(1naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4-diamine (NPB). Systematic studies are performed compared with ReO3, a different valence state oxide of rhenium. Hole mobility improvement from 5.38 × 10−4 to 5.88 × 10−3 cm2/(V s) at an electric field of 3 × 105 V/cm is achieved by doping Re2O7 into NPB. Lower valence states of Re species in Re2O7-doped NPB than ReO3 are observed by XPS study, indicating stronger charge transfer between Re2O7 and NPB. Temperature-dependent I− V study reveals lower hole injection barrier of Re2O7 than ReO3 in hole-only devices. Crystallinity of NPB films is found to be the same before and after doping by XRD study. Absorption spectrum study reveals higher stability of Re2O7-doped NPB than ReO3 in air. Hole current is enhanced by three orders of magnitude at 2 V when utilizing both rhenium-oxide-doped NPBs in hole-only devices. OLED devices with both rhenium-oxide-doped NPBs as hole injection layer (HIL) show a similar efficiency of 3.3 cd/A at 300 mA/cm2. Also, driving voltage is reduced from 2.6 V for pure NPB to 2.5 and 2.4 V for Re2O7 and ReO3 doped NPB, respectively.
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INTRODUCTION Organic semiconductors (OSCs) have shown increasing potential in a wide variety of optoelectronic devices such as organic light-emitting diodes (OLEDs),1−3 organic photovoltaic cells (OPVs),4,5 and organic thin-film transistors (OTFTs).6 Because of weak molecular interaction and heterogeneous stack, however, performances of pure OSCs are often poor compared with inorganic semiconductors. Thus, as a means to improve charge density, film conductivity, or charge injection, various concepts of p-type doping have been presented in previous reports, that is, Lewis acids (FeCl3, SbCl5),7 aromatic acceptor molecules,8,9 and transition-metal oxides (TMOs).10−13 In particular, TMOs are one of the most promising kinds of p-doping materials because of their deeplying electronic states.14,15 Despite the success of TMO doping, one of the main concerns is its high evaporating temperature, that is, 795 °C for MoO3 and 850 °C for WO3, which needs to be avoided for compatibility reasons during coevaporation with low-meltingpoint organic materials.11 Up to now, TMO with low evaporating temperature was reported only by Kim et al.,16 who doped rhenium trioxide (ReO3) with a melting point of 340 °C into NPB. Another interesting fact is that the metal elements of TMOs usually have different valence states and thus may affect the doping performance of the corresponding TMOs.17 In 2008, Adachi et al.18 compared doping performance of MoO2- and MoO3-doped α-6T by measuring the J−V © 2013 American Chemical Society
characteristics and VIS-NIR absorption spectra. The results indicated a superior behavior of MoO2 than MoO3 in terms of current magnitude and absorbance. Therefore, it is suggested that the valence states of TMOs are important parameters. In the compound ReO3, rhenium is not at its highest valence state and the corresponding oxide containing higher valence state of rhenium is Re2O7. Because Re2O7 was never used in p-doping of OSCs, it is very interesting how the two oxides with different valence states of rhenium differ from each other. Re2O7 whose melting-point is as low as 225 °C is utilized to dope the widely used amorphous hole transporting material N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB). A systematic study is carried out to compare the doping performance of Re2O7 and ReO3. Superior performances in stability and mobility are observed for Re2O7 compared with ReO3 as verified by absorption spectra and time-of-flight measurement. XPS study is used to characterize the difference of charge transfer, and the results reveal that Re species in Re2O7-doped NPB exhibit lower valence states than those of ReO3-doped NPB. Finally, hole-only and OLED devices doped with each rhenium oxide are fabricated and similar performance improvements are recorded. Received: January 1, 2013 Revised: April 1, 2013 Published: June 13, 2013 13763
dx.doi.org/10.1021/jp400003m | J. Phys. Chem. C 2013, 117, 13763−13769
The Journal of Physical Chemistry C
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Article
EXPERIMENTAL METHODS
Film Preparation and Characterization. ITO substrates were cleaned in a series of lotions, then rinsed in deionized water and baked for 2 h before use. Rhenium oxides and NPB were coevaporated inside a high vacuum chamber at 2 × 10−4 Pa. The coating rate of NPB was 4 Å/s, and for TMO, the coating rate varied from 0.02 to 0.5 Å/s according to the doping concentration. A quartz crystal sensor monitored the thickness of organic layers in situ. The thermal properties of rhenium oxides were determined by thermogravimetric analysis (TGA, TA Instruments, Q5000 IR). Valence states of Re species of rhenium oxides in 25 mol % doped NPB were characterized by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, PHI Quantera SXM). The crystallinities of 25 mol % doped NPB films on silicon substrates were determined by X-ray diffraction (XRD, D/max-IIIA 3KW, Cu-Ka). The absorption spectra of 25 mol % doped NPB films on quartz substrates were recorded before and after exposure to air for 24 h with a UV/vis spectrometer (PerkinElmer Lambda 950). Device Fabrication and Characterization. The structures of the TOF samples was ITO/NPB: p-dopants (x) (1 μm)/Ag (150 nm), where x was the mol percentage of the dopants. After doping, the TOF samples were transferred to a N2 glovebox without being exposed to air and then sealed with glass plates. A nitrogen-pulsed laser (pulse width 10 ns, wavelength 337 nm, beam size 3.14 mm2), directing at the ITO side, was used as the excitation light source to generate a thin sheet of excess carriers near the ITO/organic interface. The photocurrent was recorded by a digital storage oscilloscope with a current sensor resistor (R) of 50 Ω. Hole-only and OLED devices were fabricated on ITO with a sheet resistance of 15 Ω/□ after UV-ozone treatments. The structures of hole-only and OLED devices were ITO/NPB: pdopants (25 mol %) (5 nm)/NPB (100 nm)/Al and ITO/ NPB: p-dopants (25 mol %) (5 nm)/NPB (50 nm)/Alq3 (50 nm)/Bphen (10 nm)/ Mg:Ag, respectively. The structures of temperature-dependent hole-only devices were ITO/ReOx (5 nm)/NPB (60 nm)/Al. All organic layers were sequentially deposited at a rate of 2−4 Å/s. The current density− luminance−voltage (J−V−L) characteristics were measured by a Keithley 4200 semiconductor characterization system.
Figure 1. TGA curve of Re2O7 (a) and ReO3 (b).
if ReO3 decomposes following the first step of eq 2, then the weight loss will be 68.9%, instead of ∼30% in our case, as depicted by TGA curve in Figure 1b, indicating a deviation from the expected decomposition behavior (small weight loss at 100 °C of both TGA curves referred to vaporization of absorbed water). We note that by carefully tuning the evaporation temperature we always used the vapor from the first decomposition step of ReO3 as dopant into NPB. Nevertheless, the two different forms of rhenium oxides exhibited different doping behaviors that will be discussed below.
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RESULTS AND DISCUSSION TGA was used to examine the thermal properties of Re2O7 and ReO3. The result is shown in Figure 1. Re2O7 started to vaporize at ∼225 °C, and its weight decreased rapidly until 300 °C, when all Re2O7 completely vaporized. The data of previous sublimation and evaporation of Re2O7 were also in good agreement19,20 and indicated an evaporation behavior, as shown in eq 1. To our knowledge, this is the lowest evaporating temperature for TMOs used in organic doping, which is rather beneficial to be compatible with low-melting-point organic materials. As for ReO3, a two-stage decomposition could be observed. It is generally reported21 that ReO3 decomposes by a disproportionation reactions, as shown in eq 2. However, previous studies regarding the vapor composition were not in good consistency.22 Some suggested that vapor species consisted of pure Re2O7,23 while others reported various mixed vapor species of Re2O7/ReO3,24 Re2O7/Re, or Re2O7/ ReO3/Re.25 Meanwhile, preparations of ReO3 thin film by thermal evaporation of ReO3 were also reported,26 suggesting a direct vaporization of ReO3 without any decomposition. In fact,
498K
Re2O7 (s) ⎯⎯⎯⎯→ Re2O7 (g)
(1)
673K ⎧ ⎪ 3ReO3(s) ⎯⎯⎯⎯→ ReO2 (s) + Re2O7 (g) ⎨ ⎪ 7ReO (s) ⎯1123K ⎯⎯⎯⎯→ 3Re(s) + 2Re2O7 (g) ⎩ 2
(2)
Mobilities of Re2O7- and ReO3-doped NPB under various fields are depicted in Figure 2. Hole mobilities of pure NPB were ∼5 × 10−4 cm2/(V s), which were well-consistent with previously reported value. Mobilities of doped NPB exhibited negative correlations with electric fields, which was in sharp contrast with pure NPB, whose mobility increased monotonically with electric fields. This was probably because after doping-induced charge transfer TMOs became negatively charged and would hinder the move of holes,27 and thus the increase in velocity of holes (ν) could not catch up with the increase in electric fields (E), causing mobility (μ = ν/E) to decrease. It was worth noticing that after doping with Re2O7 the highest mobility was improved by one magnitude from 5.38 13764
dx.doi.org/10.1021/jp400003m | J. Phys. Chem. C 2013, 117, 13763−13769
The Journal of Physical Chemistry C
Article
Figure 2. Hole mobilities of Re2O7- (a) and ReO3- (b) doped films under various fields.
Figure 3. XPS Re 4f7/2 spectra of 25 mol % Re2O7- (a) and ReO3- (b) doped NPB films.
× 10−4 to 5.88 × 10−3 cm2/(V s) at an electric field of 3 × 105 V/cm due to filling of traps by charge-transfer-generated carriers.28 Also, for 25 mol % doping, mobility of Re2O7-doped NPB was much higher than ReO3-doped NPB, whose mobility became even lower than pure NPB. To investigate doping effects between TMO and NPB, we performed XPS studies. A nonlinear least-squares fitting algorithm was applied using peaks with a mix of Gaussian and Lorentzian shape and a Shirley baseline. Figure 3 shows the XPS spectra of Re 4f7/2 of 25 mol % Re2O7- and ReO3-doped NPB films. The distribution of the Re oxidation states in doped films was estimated by the curve fit of the XPS spectra,29 as is presented in Table 1. Because zero valence state corresponded to a binding energy of 40.0 eV, Re with a lower binding energy could be attributed to a negative valence state, indicating a very strong charge transfer between rhenium oxides and NPB that was sufficient enough to reduce Re from +7 valence state to negative. Note that Lee et al.30 also reported that in MoOxdoped pentacene Mo0+ and Moδ+ (δ58% of Mo. It may seem surprising that TMOs can be reduced to such low valence states. However, according to previous reports,31,32 TMOs tended to form nanoclusters of a few nanometers scale, and one nanocluster contained ∼100 molecules. Thus it was the surface TMO molecules of nanoclusters that interacted initially with adjacent organic molecules and drew electrons from them, while inner molecules tended to remain their original form. Considering the penetration depth of XPS, it was very likely that information of valence states that we received mostly came from surface TMO molecules of nanoclusters, which explained the relatively lower valence states compared
with their original high oxidation states. Also, because work functions of TMOs tended to increase with increasing cation oxidation state and higher work functions were more beneficial for charge transfer,17 it was reasonable that Re2O7 with the highest oxidation state of Re exhibited higher capability of charge transfer than ReO3 (although the vapor composition of ReO3 was not clear, the possibility of pure Re2O7 vapor coming out from ReO3 was foreclosed, as discussed before), as depicted in Table 1. We could see that all of the binding energies of the species of Re in Re2O7 were lower than those in ReO3, indicating a more sufficient charge transfer of Re2O7, which might explain the superior performance of mobility in 25 mol % Re2O7-doped NPB. To know the difference of the hole-injection barrier heights between the devices using different rhenium oxides as hole injection layer, we fabricated two hole-only devices with the structure of ITO/ReOx (5 nm)/NPB (60 nm)/Al. The temperature-dependent J−V relationship could be fitted by a Schottky thermal emission model:33 ⎡ ⎛ ⎢ −q⎜φB − ⎝ 2 J = A*T exp⎢ ⎢ kT ⎢⎣
qV ⎞ ⎤ ⎟⎥ 4πεid ⎠
⎥ ⎥ ⎥⎦
(3)
where A* is the effective Richardson constant, T is the temperature, φB is the barrier height at the interface, q the electronic charge, V is the applied voltage, εi the dielectric permittivity of the mixed organic layer, d is the thickness of organic layer, and k is the Boltzmann constant. Figure 4a,b 13765
dx.doi.org/10.1021/jp400003m | J. Phys. Chem. C 2013, 117, 13763−13769
The Journal of Physical Chemistry C
Article
Table 1. Distribution of the Re Oxidation States in Doped Films Re2O7
ReO3
binding energy (eV)
percentage (%)
valence state
binding energy (eV)
percentage (%)
valence state
39.9 39.3 37.4
55.69 21.01 23.29
0 negative negative
41.5 40.5 39.9
30.72 36.56 32.71
+ξ (0 < ξ
