ARTICLE pubs.acs.org/JPCA
Theoretical Study of Isomerism/Phase Dependent Charge Transport Properties in Tris(8-hydroxyquinolinato)aluminum(III) Hongze Gao,*,†,‡ Hongyu Zhang,§ Houyu Zhang,§ Yun Gen,‡ and Zhong-Min Su*,‡,§ †
Fundamental Department, Chinese People’s Armed Police Force Academy, Langfang 065000, Hebei, People's Republic of China Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, Jilin, People's Republic of China § State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, Jilin, People's Republic of China ‡
bS Supporting Information ABSTRACT: The charge carrier transporting ability in the polymorphism of tris(8-hydroxyquinolinato)aluminum(III) (Alq3) has been studied using density functional theory (DFT) and Marcus charge transport theory. R- and β-Alq3 composed of mer-Alq3 molecules have stronger electron-transporting property (n-type materials) compared with their hole-transporting ability. In contrast, γ- and δ-Alq3 formed by fac-Alq3 molecules possess stronger hole-transporting character than their electron-transporting ability. The detailed theoretical calculations indicate the reason lies in the differences of HOMO and LUMO distribution states of the two kinds of isomers, and the different molecular packing modes of charge-transporting pathways for different phases.
1. INTRODUCTION Followed by the pioneering work of tris(8-hydroxyquinolinato)aluminum(III) (Alq3) in organic light-emitting devices (OLEDs),1 Alq3 has drawn intensive attention in practical applications and has become one of the most successful candidates for electrontransporting layer, active emission layer, and even host material for tuning emission color from green to red.25 Because of its relatively low photoluminescent quantum yield (20%) in the solid state, Alq3 was often used as electron-transporting materials rather than active emissive materials. Generally, Alq3 was used in amorphous films and crystalline states with different polymorphs.621 So far at least four polymorphs (R, β, γ, and δ) of Alq3 have been identified, in which the R and β phases are composed of mer-Alq3 (mer = meridional) molecules and the γ and δ phases are formed by fac-Alq3 (fac = facial)14 (Figure 1). It was demonstrated that mer- and fac-Alq3 displayed green and blue emission, respectively,15 which exhibited isomerism/phase-dependent emission feature. And our experimental work has demonstrated the Alq3 molecule has isomerism-dependent charge-transporting property.22 Previous theoretical considerations on the charge transport in Alq3 were carried out only in β-crystalline phase and in disordered film; however, no direct theoretical investigation was given on the different polymorphs of Alq3. Herein, we perform detailed theoretical calculations on charge transport in different polymorphs of Alq3, aiming at the better understanding relationship between the structure and charge transport properties. The influence of molecular packing and intermolecular interactions in polymorphs on the charge transport properties in a microscopic r 2011 American Chemical Society
Figure 1. Molecular structure in R-, β-, γ-, and δ-Alq3 polymorphs. (Saturated H atoms are not shown.)
nature and to what extent the polymorphs determine the charge transport characteristics are described. Received: March 31, 2011 Revised: July 11, 2011 Published: August 03, 2011 9259
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Table 1. Reorganization Energy (λ) and Charge Carrier Mobilities (μ) for All Polymorphs R polymorphs
electron
β hole
γ
electron
hole
electron
δ hole
electron
hole
λ (eV)
0.276
0.229
0.276
0.229
0.140
0.106
0.139
0.095
μ (cm2/(V 3 s))
6.84 102
3.89 102
2.34 101
7.70 102
2.65 101
3.03
2.39 102
2.52
Figure 2. Hopping pathways of Alq3 in all polymorphs. (Note: the hopping pathways 1417 of R, 12 of β, 2123 of γ, and 18 of δ are not shown in the picture.)
2. THEORETICAL METHODOLOGY AND COMPUTATIONAL METHODS The theoretical studies of the charge transporting property were significant for understanding the performance of organic semiconducting materials, although the charge carrier transporting theories and calculation approaches remained to be improved.2326
The carrier mobilities in molecular organic materials can be described by the band theory27,28 and hopping model.29,30 Previous theoretical studies demonstrated that the hopping model was suitable to the Alq3 system.1012 In this work, we take our samples to be free of all impurities and model the charge carrier transport as a hopping-type motion process, coupled with the Marcus theory 9260
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Table 2. Charge Transfer Integrals Vh for Hole and Ve for Electron, Dimer Center Mass (CM) Distance in the DonorAcceptor Complexes of R- and β-Alq3 R-Alq3
β-Alq3 transfer integral/106 eV
pathway
dimer CM distance (Å)
Ve
Vh
transfer integral/106eV pathway
dimer CM distance (Å)
Ve
Vh
1
14.742
1.2704
0.94
1
8.057
736.800
6140.4502
2
10.204
158.0063
6023.28
2
8.443
842.800
3681.6026
3 4
12.914 18.072
151.4688 0.0054
124.38 0.01
3 4
12.778 12.237
0.1218 4.431
0.1519 1121.2903
842.8000
3681.6026
5
9.814
788.9320
408.92
5
8.443
6
14.742
1.2714
0.94
6
8.874
2705.00
4601.1468
7
14.127
26.5539
9.38
7
8.910
46523.7174
177.8103
8
12.914
151.4591
124.42
8
10.252
545.5584
110.1291
9
8.905
166.0803
3066.40
9
13.754
2.2329
0.2561
10
6.259
26214.4204
21078.60
10
8.022
2738.2082
22.1166
11 12
14.050 6.259
0.1006 26214.4204
13.86 21078.60
11 12
10.252 8.780
545.0532 9362.54
111.4371 21810.50
32319.6074
236.74
13
8.518
14
8.720
253.18
15
11.522
116.23
707.96
16
8.903
29.51
2321.00
17
11.276
1123.91
25.66
11326.30
Table 3. Charge Transfer Integrals Vh for Hole and Ve for Electron and Dimer Center Mass (CM) Distance in the DonorAcceptor Complexes of γ- and δ-Alq3 γ-Alq3
δ-Alq3 transfer integral/106 eV
pathway
dimer CM distance (Å)
Ve
Vh
transfer integral/106 eV pathway
796.87
dimer CM distance (Å)
Ve
Vh
1
9.801
100.1739
1
14.430
5.6696
2
14.381
3.8659
1.7662
2
18.256
0.0002
0.00
3
17.403
0.0319
0.0147
3
15.129
0.0078
0.09
4
14.381
0.8501
5
9.801
258.4032
1.8494 791.89
0.94
4
9.031
958.8942
1371.30
5
13.268
5.9585
137.74
6
14.381
2.8469
1.7930
6
14.832
95.5306
3.10
7
17.403
0.0111
0.0118
7
14.430
5.6676
0.93
8 9
14.381 9.801
2.3146 262.1754
1.7452 796.73
8 9
8.329 15.129
411.7398 0.0078
1071.72 0.09
10
14.381
1.1783
1.8218
10
16.457
0.0004
0.02
11
17.403
0.0220
0.0149
11
13.268
6.0050
138.28
12
10.167
21.6302
1440.02
13
10.925
7.7417
69.31
14
6.181
9379.0768
68714.29
1.79
12
14.381
1.8196
13
6.211
0.0347
14
10.999
398.8566
15
15.665
0.0122
15
9.031
958.8942
1371.30
16 17
9.801 6.211
262.6194 30447.7147
786.76 82389.90
16 17
6.181 10.167
9379.0768 21.6302
68714.29 1440.02
797.89
18
8.271
5902.2628
1109.05
18
9.801
259.6093
19
15.665
0.0053
20
22.576
0.0000
21
8.363
1274.6996
0.5716 216.98 0.0111
0.0110 0.0000 8025.19
22
8.363
1304.2459
8052.47
23
8.363
1241.4114
8098.10 9261
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energy λ consists of contributions from the inner reorganization energy λi and the external polarization λo. Norton et al. proved that the gas-phase reorganization energies of individual molecules constitute reasonable approximations for total reorganization energies for crystalline oligoacenes,32 thus λo was neglected in this study. λi can be obtained by quantum chemistry calculation because it is an intramolecular property in nature.9,3335 The reorganization energies were calculated based on the DFTB3LYP method employing a 6-31G(d) basis set using the Gaussian 03 program.36 Charge-transfer integrals have been obtained from the direct method35,3743 and the calculations were performed by using the PW91PW91/6-31G* method, which has been demonstrated to be an appropriate choice of functional for the DFT level.44 The charge carrier mobility (μ) was obtained from the Einstein relation μ = eD/kBT, where e is the electronic charge and D is the diffusion coefficient and can be approximately evaluated as4547 D ¼ lim
tf∞
1 ÆxðtÞ2 æ 1 ≈ 2d t 2n
∑i di 2 ki Pi
ð2Þ
where n = 3 is the dimensionality, ki is the hopping rate due to charge carrier to the ith neighbor, di is the distance to neighbor i, Pi is the relative probability for the ith pathway Pi ¼ ki =
∑i ki
3. RESULTS AND DISCUSSION Figure 3. Overlapping of orbitals in the part hopping pathways of all the polymorphs for the transport of hole and electron.
Figure 4. Intermolecular interaction in some charge hopping pathways.
for the electron transport rate (k) for a self-exchange reaction process28,31 4π2 1 λ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiV 2 exp ð1Þ k¼ 4kB T h 4πλkB T There are two major parameters that determine the selfexchange electron-transfer (ET) rate: the intermolecular transfer integral V and the reorganization energy λ. The reorganization
3.1. Reorganization Energy. The computed reorganization energies (λ) of Alq3 in all polymorphs are listed in Table 1 and the electron reorganization energy (0.276 eV) fac-Alq3 agrees with refs 7 and 9, but the hole reorganization energy does not (0.229 eV vs 0.242 in refs 7 and9). Our results are more reasonable because harmonic vibrational frequencies were calculated at the same theoretical level based on the optimized geometries to confirm they are the most stable. These datum indicated that the λ values of R (λhole = 0.229 eV, λelectron = 0.276 eV) and β (λhole = 0.229 eV, λelectron = 0.276 eV) phases are larger than those of γ (λhole = 0.106 eV, λelectron = 0.140 eV) and δ (λhole = 0.095 eV, λelectron = 0.139 eV) phases. This difference is attributed to the isomeric characteristic of Alq3 molecules in different phases, namely, R and β phases are composed of mer-Alq3, while γ and δ phases belong to fac-Alq3. The smaller λ value will benefit the charge carrier transport. From this sense, the relatively smaller λhole for all polymorphs suggests that Alq3 may favor transport holes, which is contrary to the previous observations.1012,48 This suggests that the charge transfer integral V may influence charge transport in a deterministic way. 3.2. Charge Transfer Integral. To achieve rational transfer integrals (V), all of the possible hopping pathways (dimers) of the four Alq3 polymorphs have been considered based on their crystal structures.13,1518,49 The hopping pathways of Alq3 in all polymorphs are shown in Figure 2. Due to the 2-fold degenerate LUMO and LUMO+1 levels and significant contributions from them, the “effective transfer integral”43 in which a linear combination of the degenerated LUMO and LUMO+1 as a basis set has been used in some γ-Alq3 (1, 5, 9, 14, 16, 18, 21, 22, and 23) and δ-Alq3 (4, 6, 8, 13, 15, and 18) hopping pathways. The dimer center mass (CM) distance and transfer integral are also presented in Tables 2 and 3. The magnitudes of calculated V values 9262
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Figure 5. Front molecular orbitals of Alq3 in all polymorphs.
Figure 6. Anionic and cationic singly occupied molecular orbital (SOMO) of Alq3 in all polymorphs.
are about one to several orders of magnitude less than the value of λ. For the hopping pathways with larger distance between the two molecular centers and less intermolecular overlap of orbitals, the V values are small, such as the pathways 3 and 9 of β-Alq3. The value of V is mainly determined by the orbital interaction between the two ligands from a pair of neighboring Alq3 molecules. Therefore, the orbital signs and distributions of the two ligands have a dramatic effect on the transfer integral. The two contacting orbitals with the same sign display a large transfer integral (Figure 3). The intermolecular noncovalent bonds often affect both Vh and Ve. The hopping pathways with π 3 3 3 π or CH 3 3 3 π interactions (such as pathway 10 of R-Alq3, 12 of β-Alq3, 17 of γ-Alq3, and 14 of δ-Alq3) generally exhibit larger charge-transfer integrals (Figure 4). 3.3. Charge Carrier Mobilities. The calculated charge carrier mobilities (μ) of four Alq3 polymorphs at room temperature (300 K) are listed in Table 1. The calculated mobilities of R- and β-Alq3 agree with the previous theoretical results8,9 in which the electron mobilities of Alq3 are about 2—3 times greater than those of holes, suggesting that the calculated results should be reliable. Surprisingly, γ- and δ-Alq3 show obviously higher hole transport ability compared with their electron carrier mobility. The calculated results show that the γ- and δ-Alq3 have higher hole mobility (3.03 cm2/(V 3 s) for γ-Alq3 and 2.52 cm2/(V 3 s) for δ-Alq3) compared with their electron mobility (2.65 101 cm2/(V 3 s) for γ-Alq3 and 2.39 101 cm2/(V 3 s) for δ-Alq3). And this unexpected result agrees with the findings in our experimental work.22 The electron carriers transfer through the LUMO of organic semiconducting molecules, while the hole carriers transfer through their HOMO. For R- and β-Alq3, the LUMO is more delocalized than HOMO, suggesting that R- and β-Alq3 favor transport of electrons. In the cases of γ- and δ-Alq3, HOMO is more delocalized than LUMO, indicating that γ- and δ-Alq3 favor transport holes (Figure 5). For the charged molecule, a more delocalized singly occupied molecular orbital (SOMO) would suggest that the electrons would be more mobile and could easily move among the molecule, which obviously increases the
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frequency of intermolecular charge hopping and the carrier mobility.5052 It also can be seen from Figure 6 that the SOMOs of R- and β-Alq3 are more delocalized upon negative charging than upon positive charging. Thus, the electron can easily move among the molecule which facilities electron transportation in R- or β-Alq3 anion when R- or β-Alq3 molecule carrying an extra electron. In contrast, the most important characteristic is that the electron clouds of SOMO in γ- or δ-Alq3 are distributed on three oxygen atoms upon positive charging while there is little contribution from three oxygen atoms upon negative charging. So the delocalization of the SOMO upon positive charging would be favorable for electron hopping to a neighboring molecule and thus more favorable for the hole transportation.
4. CONCLUSIONS In conclusion, we have revealed theoretically that Alq3 has phase- or isomerism-dependent charge-transporting properties. R- and β-Alq 3 composed of mer-Alq3 molecules have stronger electron-transporting property compared with their holetransporting ability. In contrast, γ- and δ-Alq3 formed by fac-Alq3 molecules possess stronger hole-transporting character than their electron-transporting ability. The different chargetransporting properties of mer- and fac-Alq3 should be mainly attributed to the differences of HOMO and LUMO distribution states of the two kinds of isomers and the different molecular packing modes of charge-transporting pathways for different phases. The above discovery might be beneficial for developing new applications of Alq3 in the optoelectronics. ’ ASSOCIATED CONTENT
bS
Supporting Information. The geometries of the hopping pathways of Alq3 in all polymorphs, optimized geometry of neutral Alq3, Alq3 cation, and Alq3 anion in all polymorphs. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected] (H. Gao);
[email protected]. cn(Z. M. Su).
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (20703008), the National Basic Research Program of China (973 Program 2009CB623605), the Chang Jiang Scholars Program (2006) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0714), and the Open Project Program of State Key Laboratory of Supramolecular Structure and Materials, Jilin University. ’ REFERENCES (1) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913–915. (2) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151–154. (3) Borek, C.; Hanson, K.; Djurovich, P. I.; Thompson, M. E.; Aznavour, K.; Bau, R.; Sun, Y. R.; Forrest, S. R.; Brooks, J.; Michalski, L.; Brown, J. Angew. Chem., Int. Ed. 2007, 46, 1109–1112. (4) Yan, B. P.; Cheung, C. C. C.; Kui, S. C. F.; Xiang, H. F.; Roy, V. A. L.; Xu, S. J.; Che, C. M. Adv. Mater. 2007, 19, 3599–3603. 9263
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