ARTICLE pubs.acs.org/JPCC
Hole and Electron Transport in Triarylamine-Based Charge-Transport Materials Investigated by the Time-of-Flight Method Richard A. Klenkler*,† and Galina Voloshin‡ † ‡
Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada ABSTRACT: For the organic charge-transport molecule NN0 -diphenyl-N,N0 bis(3-methlyphenyl)-[1,10 -biphenyl]-4,40 -diamine (TPD), theoretical predictions indicate that the mobility of holes should be significantly greater than that of electrons. In this report, a time-of-flight study of the hole and electron transport properties of TPD is presented. It is revealed that TPD has similar mobility for both holes and electrons, for example, a mobility of 1 103 cm2 V1 s1 for holes and 9 104 cm2 V1 s1 for electrons at 10 V μm1. The ability for the material to transport electrons is additionally demonstrated by the photoinduced discharge of a TPD-based photoreceptor with a positive electrostatically charged surface. The hole and electron charge transporting properties of TPD are of particular importance due to the widespread use of this material in a variety of organic electronic devices.
1. INTRODUCTION The mobility of holes versus electrons in amorphous organic electronic materials is a long-standing issue in the field of organic electronics.1 Experimental results indicate that some of these materials2,3 have similar hole and electron mobility, whereas for others,46 these values have been shown to differ by up to orders of magnitude. Several organic molecules have been examined on a theoretical basis, and predictions of relative hole versus electron mobility have been made.3,7,8 Some of these molecules are predicted to have similar hole and electron mobility, while others are predicted to have those that differ greatly. In some cases, the theoretical predictions agree with experimental results, for example, with N,N0 -di(naphthalene-1-yl)-N,N0 -diphenylbenzidine (NPB) and tris(8-hydroxyquinoline) aluminum (Alq3). However, few comparisons like this can be drawn due to the limited set of materials for which both theoretical predictions have been made and experimental results have been obtained. In the case of NN0 -diphenyl-N,N0 -bis(3-methlyphenyl)[1,10 -biphenyl]-4,40 -diamine (TPD), shown in Figure 1, it has been predicted that the mobility of holes should be distinctly greater than that of electrons.2,8 The argument for this is based on the Marcus electron-transfer theory, where the hopping rate is determined by two factors, the reorganizational energy (λ) and the charge-transfer integral.9 When applied to TPD, the theory indicates that the reorganizational energy for hole transport (λ+ = 0.28 eV) is less than that of electron transport (λ = 0.56 eV).8 On the basis of this difference, a hole to electron hopping rate ratio of 23 has been calculated.8 In addition, it has been predicted that the lowest unoccupied molecular orbital (LUMO) to LUMO charge-transfer integral for TPD further hinders its electron transporting ability.2,10 This is based on density functional theory calculations that indicate that the LUMO is concentrated on the biphenyl moiety at the center of the molecule and does not involve the phenyl and tolyl r 2011 American Chemical Society
groups on the periphery.8,10 Therefore, it has been concluded that the phenyl and tolyl groups effectively act as spacers that suppress electron charge transfer.2 However, the highest occupied molecular orbital (HOMO), according to density functional theory calculations,8 is less concentrated on the biphenyl moiety at the center of the molecule, thus giving rise to a relatively larger charge-transfer integral for holes than that for electrons.2,10 We are aware of only one report12 in which electron mobility in TPD was experimentally determined with success, whereas others report to have been unable to achieve an electron transient signal.13 For the group that was able to resolve electron transport, an electron mobility 4 orders of magnitude lower than hole mobility was reported.12 In this report, the results of a time-offlight (TOF) study of hole and electron transport in neat TPD and TPD- doped polycarbonate (PC) charge transporting layers (CTLs) is presented. It will be revealed that TPD can transport holes and electrons equally well, showing similar mobility for both modes of transport. It is worth noting that in the case of the similar charge transport material NPB, there too are conflicting reports of electron mobility differing by an order of magnitude.2,11
2. EXPERIMENTAL SECTION 2.1. Fabrication of Neat TPD Sample. The neat TPD sample was fabricated on a glass substrate. The substrate was first cleaned in a detergent solution, then rinsed with deionized water followed by isopropyl alcohol, and finally allowed to dry in a heated oven. Successive layers of patterned material were then deposited by vacuum thermal evaporation with the use of shadow masks. A thick layer of TPD was sandwiched between two thin Received: April 19, 2011 Revised: July 18, 2011 Published: July 18, 2011 16777
dx.doi.org/10.1021/jp2036586 | J. Phys. Chem. C 2011, 115, 16777–16781
The Journal of Physical Chemistry C electrodes by first depositing a 35 nm layer of Al, then a 15 μm thick layer of TPD, followed by a final 35 nm layer of Al (Figure 2a, inset). 2.2. Fabrication of the TPD-Doped PC Sample. The TPDdoped PC CTL sample was similarly configured with the CTL sandwiched between two Al electrodes (Figure 2b, inset). In the case of this sample, however, an aluminized mylar (DuPont) substrate was used. The substrate was first rinsed with isopropyl alcohol and then wiped dry with a paper tissue. A 20 μm thick CTL was then applied by blade coating a solution of 40 wt % TPD and 60 wt % bisphenol-Z-polycarbonate dissolved in dichloromethane (at a solid content of 18 wt %). The coated CTL was then allowed to dry in air for 1 h at room temperature and then was heat treated in an oven at 120 °C for 30 min to remove any remaining solvent. Finally, a 40 nm thick Al counter electrode was applied by vacuum thermal evaporation. The active area of both the neat TPD and TPD-doped PC samples was ∼1 cm2. 2.3. Time-of-Flight Measurement Method. The TOF setup and the measurement method were similar to that described by Melnyk and Pai.14 All measurements took place in air under ambient conditions (22 °C and 30% relative humidity). A Spectra-Physics (VSL-337ND-S) nitrogen laser emitting a 4 ns pulse at 337 nm served as the excitation light source. At this wavelength, the BeerLambert attenuation coefficient is 4.0 and 0.4 μm1 in the neat TPD and TPD-doped PC samples,
Figure 1. TPD chemical structure.
ARTICLE
respectively. Thus, 90% of the laser pulse is absorbed within the first ∼1.5 μm (10%) of the neat TPD sample and within the first ∼6 μm (15%) of the TPD-doped PC sample. In either case, the laser pulse is absorbed in a region that is much thinner than the overall thickness of the film. Measurements were performed over a range of electric fields (2 1042 105 V cm1), and the resultant transient signals were recorded on a GHz storage oscilloscope (Tektronics TDS 580D). For each measurement, the circuit resistance was adjusted to ensure that the RC time constant was no greater than 1/20th of the charge carrier transit time (ttr). However, for the neat TPD samples, when measuring electron transport at low fields (
