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J. Phys. Chem. C 2009, 113, 13386–13390
Thermally Decomposable Lithium Nitride as an Electron Injection Material for Highly Efficient and Stable OLEDs Lian Duan,† Qian Liu,‡ Yang Li,§ Yudi Gao,| Guohui Zhang,⊥ Deqiang Zhang,# Liduo Wang,∇ and Yong Qiu* Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed: February 19, 2009; ReVised Manuscript ReceiVed: June 10, 2009
Improvement of electron injection is essential for highly efficient low-voltage organic light-emitting diodes (OLEDs). In this work, we report our study on thermally decomposable Li3N as the electron injection layer (EIL) for OLEDs. We use a quartz crystal microbalance method to investigate the decomposition process of Li3N during vacuum thermal evaporation in situ. Thermodynamics study also proves that Li3N decomposes in vacuum to form metallic lithium and nitrogen. OLEDs with Li3N as the EIL outperform those with conventional LiF in every respect of view. An optimized green OLED with a Li3N EIL shows 35% higher efficiency and more than doubled lifetime compared with the control device with a LiF EIL. Introduction Remarkable progress has been made since the initial discovery of the multilayer organic light-emitting diodes (OLEDs) by Tang et al. in 1987.1 It is realized that improvement of electron injection is of critical importance for obtaining highly efficient low-voltage OLEDs. Low work function metals such as Ca (2.9 eV), Mg (3.7 eV), Li (2.9 eV), or Cs (2.1 eV) have been intensively studied as the cathode for OLEDs to reduce the electron injection barrier and improve the device efficiency.1,2 However, reactive metals with a low work function are difficult to handle because they are always susceptible to atmospheric moisture and oxygen. The search for better cathode materials to complement the weakness of low work function metals has led to the discovery of bilayer cathode structures. In 1997, Hung et al. demonstrated that devices with a thin electron injection layer (EIL) of lithium fluoride (LiF) between the Al cathode and tris(8-hydroxyquinoline)aluminum (Alq3) could achieve lower driving voltage and higher electroluminescence efficiency as compared to a Mg0.9Ag0.1 cathode.3 As it is stable in air and the OLED performance is easily reproducible, LiF is widely used as the EIL in OLEDs and almost becomes an industrial standard. It has been reported that the improved electron injection of LiF is due to the metallic lithium liberated from the interfacial chemical reaction of Al and LiF in the presence of Alq3, which would in turn n-dope the underlying Alq3 layer.4 In this way, LiF can be viewed as a kind of “interfacial reactive precursor” to low work function alkali metal though its effectiveness is sensitive to the adjacent materials used. In the continuous research for better EILs, Cs2CO3 has been recently reported to be more effective than LiF in terms of facilitating electron injection.5a,b Owing to this, Cs2CO3 has been increasingly used in OLEDs and attracted many research * To whom correspondence should be addressed. Phone: +8610-01062779988. Fax: +8610-010-62795137. E-mail:
[email protected]. † E-mail:
[email protected]. ‡ E-mail:
[email protected]. § E-mail:
[email protected]. | E-mail:
[email protected]. ⊥ E-mail:
[email protected]. # E-mail:
[email protected]. ∇ E-mail:
[email protected].
interests in understanding its unique injection mechanism.5 In our previous work, we investigated the evaporation behavior of Cs2CO3 by using an improved quartz crystal microbalance (QCM) method.5f We elucidated that Cs2CO3 would decompose during vacuum vapor evaporation to liberate metallic cesium, which would be responsible for the injection improvement.5f Therefore, Cs2CO3 can be viewed a “thermal-decomposable precursor” to alkali metal. As cesium is liberated during the deposition process and no interfacial chemical reaction is required afterward, devices with a Cs2CO3 EIL have a wider choice of adjacent materials. The decomposition mechanism of Cs2CO3 opens a new way for screening high performance EILs based on “thermaldecomposable precursors”. It is worth noting that the “thermaldecomposable precursors” should be carefully chosen; otherwise, the harmful gases produced during decomposition and/or the possible residues left in the crucible might be a big problem for the OLED fabrication.6 From this point of view, alkali metal nitrides should be better than their carbonates, as only alkali metal and inert nitrogen would be released.7 Li3N is usually used as the electrode in lithium batteries or as a starting material for the synthesis of binary or ternary nitrides,8,9 and it has also been demonstrated to be one of the most promising systems for hydrogen storage recently.10 In this work, we report our study on Li3N as the EIL for highly efficient and stable OLEDs. We used the QCM method to investigate its decomposition process during vacuum thermal evaporation in situ. Thermodynamics study results agree well with the decomposition mechanism. OLEDs using a Li3N EIL were observed to outperform the control devices with a LiF EIL in both efficiency and lifetime. Experimental Section QCM Measurement. Li3N (purity >99.9%), LiF (random crystals), and CsF (purity 99.99%) were purchased from Aldrich. Samples of 10 mg, 20 mg, 30 mg, and 40 mg of LiF, CsF, and Li3N were put into a boron nitride (BN) crucible. The weighing of the materials was carried out in a glovebox filled with Ar with a Sartorius BP211D analytical balance. Then, the BN crucible containing the test materials was transferred into a vacuum chamber for evaporation. The base pressure of the
10.1021/jp901510j CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009
Lithium Nitride as an Electron Injection Material vacuum chamber was kept below 10-3 Pa. The source current through the BN crucible (0-160 A) was increased by 10 A every 2 min to generate the resistive heating. The QCM device used in the present work is a conventional thickness monitor operating at a resolution and frequency stability of (1 Hz. The QCM was placed about 300 mm above the crucible and was water cooled to lower the surface temperature. The piezoelectric quartz crystal wafer used in this study was gold-deposited ATcut with 5.99 MHz basic resonance frequency. Resistive heating of the bare BN crucible may lead to a positive frequency shift of the QCM when the source current was increased, but this frequency shift was measured to be less than 25 Hz, which can be negligible. All the test materials were used up after heating, and there was no residue in the crucible. OLED Fabrication and Characterization. Patterned ITO coated glass, as the substrate for the OLEDs, was cleaned with an ultrasonic process in deionized water containing lotion, with a temperature of about 60 °C, and then dried on a clean bench and finally positioned in a thermal evaporation chamber. The base pressure of the chamber was below 1.0 × 10-3 Pa. A 100nm-thick N,N′-di[4-(N,N′-diphenylamino)phenyl]- N,N′-diphenylbenzidine (DNTPD) as the hole injection layer and a 20nm-thick N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′diamine (NPB) as the hole transport layer were evaporated at the rate of 0.1 nm/s, respectively. As a light-emitting layer, a 30-nm-thick tris(8-hydroxyquinolato) aluminum (Alq3) doped with 0.7 wt % 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7tetrahydro-1H,5H,11H-[1]benzopyrano[6,7,8-ij]quinolizin-11one (C545T) was subsequently evaporated at the rate of 0.1 nm/s. Then a 20-nm-thick Alq3 was evaporated as the electron transport layer at the same rate. Without breaking the vacuum, Li3N was evaporated at the rate of 0.01 nm/s in the cathode chamber and then 150-nm-thick aluminum was deposited at the rate of 0.5-1.0 nm/s to complete the device fabrication. A control device with LiF (0.5 nm) as the EIL was fabricated for comparison. We also made a set of devices with a structure of NPB (50 nm)/Alq3 (50 nm)/LiF or Li3N or Li (0.5 nm)/Ag (200 nm). The I-V-L characteristics were measured simultaneously with a Keithley model 4200 source-measure unit and a calibrated silicon photodiode. The operation lifetime of devices was tested at a constant direct current density. The light intensity of the devices was monitored using calibrated photodiodes and continuously recorded using a computer-controlled data collection system (McScience 6100 lifetime test system). All measurements were carried out at room temperature. Results and Discussions Study on the Decomposition of Li3N. The QCM is a simple and extremely sensitive method for measuring in situ the mass increase on the quartz crystal wafer.6 The frequency shift of the QCM (∆f) is in direct proportion to the mass loss in the crucible (∆M).5f,6 So, we can estimate the decomposition products of Li3N by comparing the slope of ∆f Vs ∆M of Li3N with those of thermally stable references such as LiF and CsF. In our experiment, 10 mg, 20 mg, 30 mg, and 40 mg of LiF, CsF, and Li3N were put into the BN crucible in the vacuum chamber, respectively. The source current through the BN crucible (0-160 A) was increased by 10 A every 2 min. The frequency shift curves with increasing the applied source current for LiF, CsF, and Li3N are shown in Figure 1. The frequency shift remains unchanged when the source current is larger than a critical value during the evaporation. For LiF, CsF, and Li3N, the critical currents are 150 A, 100 A, and 120 A, respectively. All the materials are used up without any residuals in the
J. Phys. Chem. C, Vol. 113, No. 30, 2009 13387
Figure 1. Frequency shift vs source current characteristics of LiF (a), CsF (b), and Li3N (c).
Figure 2. Frequency shift Vs mass loss of the initial source materials of LiF, CsF, and Li3N.
crucible after heating. By using a thermocouple which is imbedded into the crucible, the temperature of onset of the frequency shift for Li3N was measured to be 382 °C at 10-3 Pa. The evaporation of Li3N causes a small decrease in vacuum level (the same phenomenon was observed during the deposition of Cs2CO3), and the main gas released was found to be nitrogen by the residue gas analyzer imbedded in the vacuum chamber. The ∆f Vs ∆M characteristics are shown in Figure 2. The ∆f value was calculated from the frequencies at the critical current and at 0 A. The experimental data of each material shown in Figure 2 can be fitted into straight lines. It is clear that the ∆f ∼ ∆M characteristics of all the three materials exhibit a good linear relationship, and the linear correlation coefficients of them are larger than 0.998. As expected, the slope of the fitted line for CsF is almost identical with that of LiF, as both are stable during thermal evaporation. We found that Li3N exhibits a much lower shift in frequency than LiF and CsF with the same amount of material in the crucible, indicating that Li3N decomposes during thermal evaporation. We get a slope ratio of 56.1 ( 2.4% by dividing the slope of Li3N with that of LiF, which is very close to the weight ratio of lithium atoms in Li3N (59.8%). It is evident that all the lithium can be liberated from Li3N in thermal vacuum evaporation according to the following reaction:
2Li3N (s) f 6Li (g) + N2 (g)
(1)
To investigate the possibility of the above reaction, a thermodynamic estimation was carried out. As is well-known in thermodynamics, the feasibility and spontaneity of a reaction can be evaluated by the value of the Gibbs free energy of the reaction (∆rGm(T,p)). Here we take the assumption that the reactions are taking place at constant temperature and pressure
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Duan et al.
TABLE 1: Thermochemical Data of the Materials Studieda Li3N (s) Li (g) N2 (g) a
θ ∆ fG m (298.15 K) (kJ · mol-1)
θ ∆ fH m (298.15 K) (kJ · mol-1)
Cp,m(298.15 K) (J · K-1 · mol-1)
a
b
c
-128.644 126.593 0.000
-164.557 159.300 0.000
75.275 20.786 29.123
17.154 20.753 27.865
194.933 0.000 4.268
8.159 0.000 0.000
Cp,m(T) ) a + b10-3T + c105T-2. Data from refs 17 and 18.
due to the extremely short reaction time. ∆rGm(T,p) is the sum of the standard Gibbs free energy (∆rGθm(T)) and a correction term related to pressure:17,18 θ ∆rGm(T,p) ) ∆rGm (T) + RT ln Jp
(2)
Here T, p, R, and Jp represent the thermodynamic temperature, the pressure, the molar gaseous constant (8.3145 J · K-1 · mol-1), and the pressure quotient of the reaction, respectively. ∆rGθm(T) is the stoichiometric sum of the standard Gibbs free energy of formation of the j-th reactant (or product) (∆fGθm,j(T)) and is depicted as θ ∆rGm (T) )
θ (T) ∑ γj∆f Gm,j
Figure 3. Reaction temperature versus pressure for Li3N decomposition by thermodynamic calculation.
(3)
j
where γj is the stoichiometric coefficient of the j-th reactant (or product). The correction term in eq 3 is influenced by the partial pressure of each reactant and product in the gaseous state (pi) as follows:
Jp )
∏ (pi/pθ)n
(4)
i
where pθ is the standard atmosphere pressure (105 Pa) and i represents the stoichiometric coefficient of the i-th reactant (or product) in the gaseous state. ∆fGθm,j(T) of a certain reactant or product can be expressed as θ θ ∆fGm,j (T)/T ) ∆f Gm,j (T0)/T0 +
θ (T)/T2) dT ∫TT (-∆f Hm,j 0
(5) with θ θ ∆f Hm,j (T) ) ∆f H m,j (T0) +
∫TT Cp,m,j(T) dT 0
(6)
where ∆fGθm,j(T0), ∆fHθm,j(T), ∆fHθm,j(T0), and Cp,m,j(T) represent the standard Gibbs free energy of formation at the temperature of T0 (298.15K), the standard enthalpy of formation, the standard enthalpy of formation at T0, and the heat capacity of the j-th reactant (or product), respectively. Cp,m,j(T) is usually expressed as a function of the temperature by the following empirical equation:17,18
Cp,m,j(T) ) a + bT + cT -2
(7)
where a, b, and c are empirical constants. The data used in eqs 2-7 are listed in Table 1.
Figure 4. I-V (open) and L-V (closed) curves of OLEDs with a structure of NPB (50 nm)/Alq3 (50 nm)/LiF or Li3N or Li (0.5 nm)/Ag (200 nm).
Then, the relationship of temperature Vs pressure (T ) f(p)) on the condition ∆rGm ) 0, the very moment when the reaction is initiated, can be calculated. The relationship of the reaction mentioned above is plotted in Figure 3. The temperature required for Li3N decomposition to form gaseous lithium and nitrogen was calculated to be 379.5 °C at 10-3 Pa, which is almost the same as the experimental data, indicating the decomposition process of Li3N as described by eq 1. Therefore, we have found a straightforward method to make thin films of lithium, without using highly reactive lithium2a or complicated multicomponent lithium dispensers.11 OLED Performances. As metallic Li is liberated during the evaporation of Li3N, OLEDs with Li3N EIL enjoy a wider choice of the cathode material. To ensure this, we made a set of OLEDs with inert Ag as the cathode with a device structure of NPB (50 nm)/Alq3 (50 nm)/LiF or Li3N or Li (0.5 nm)/Ag (200 nm). Figure 4 shows the current density-voltage (I-V) and luminance-voltage (L-V) characteristics of the OLEDs. The I-V and L-V curves of the Li3N/Ag device almost coincide with those of the Li/Ag device. The only difference in the I-V curves of the two is that the Li3N/Ag device has lower leakage currents than the Li/Ag device in the low voltage regions (
