Electron drift mobility of oxadiazole derivatives doped in polycarbonate

Electron drift mobility of oxadiazole derivatives doped in polycarbonate Hiroaki Tokuhisa, Masanao Era, Tetsuo Tsutsui, and Shogo Saito Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyusyu University, Kasuga-shi, Fukuoka 816, Japan

~Received 12 December 1994; accepted for publication 29 March 1995! Charge drift mobilities of five oxadiazole derivatives doped in polycarbonate ~PC! were evaluated with the time-of-flight technique. It is demonstrated that oxadiazoles incline to having electron-transport characteristics. In particular, an oxadiazole with naphthyl substituent ~BND! was found to possess high potential of electron transport; the electron drift mobility of 50 wt % BND doped PC was 2.2310 25 cm2 V21 s21 at an electric field of 7.5310 5 V cm21 at room temperature. In addition, incorporating strong electron-releasing substituents into oxadiazoles was demonstrated to add hole transport characteristics to oxadiazoles. © 1995 American Institute of Physics.

Extensive studies on organic photoconductors in the past decade have provided various kinds of hole conductors; the highest values of their charge drift mobilities have been at the level of 10 24 cm2 V21 s21 for molecularly doped polymer films,1–3 10 23 cm2 V21 s21 for amorphous solids4 and 10 21 cm2 V21 s21 for liquid crystals.5,6 In contrast with organic hole conductors, there are only a few studies on organic electron conductors.7–10 Furthermore, few electron transporting compounds with drift mobility comparable to those of hole-transporting compounds have been developed. Development of electron conductors with high charge drift mobility is of fundamental importance to the applications of organic compounds not only to photoconductors for photocopiers but to other advanced electronic devices, such as electroluminescent ~EL! devices11–16 and photovoltaic devices.17 Saito et al. pointed out that vacuum-sublimed thin films of oxadiazoles have electron-transporting capability based on their studies on multilayered organic thin-film EL devices.18,19 They demonstrated that large numbers of electrons were injected from a cathode and transported through oxadiazole layers in double-layer-type EL devices composed of a hole transport layer with emissive capability and an oxadiazole layer. Direct evidence of electron transport in the oxadiazole films, however, has not been given from the experiments on the multilayered EL devices. The demonstration of electron-transport capability of oxadiazoles in the multilayered EL device gave us a strong motivation to evaluate directly the electron drift mobility of oxadiazoles. Thus, we tried direct determination of the electron drift mobility of oxadiazoles using the conventional time-of-flight ~TOF! technique. Figure 1 shows five oxadiazoles employed in this work: 2-~4-biphenyl!-5-~4-tertbutylphenyl!-1,3,4-oxadiazole ~PBD!, 2,5-bis~4-naphthyl!-1,3,4-oxadiazole ~BND!, 2,5bis~3-methoxyphenyl!-1,3,4-oxadiazole ~BMD!, 2,5-bis~4diethyl-aminophenyl!-1,3,4-oxadiazole ~BDD!, and 2,5bis~4-diphenylaminophenyl!-1,3,4-oxadiazole ~BAPD!. Four oxadiazoles, PBD, BND, BMD, and BDD were commercially available. BAPD was prepared by arylation of 2,5-~4diamino!-1,3,4-oxadiazole with iodebenzene in the presence of a copper catalyst. BAPD was used after purification with Appl. Phys. Lett. 66 (25), 19 June 1995

the train-sublimation method.20 Polycarbonate ~PC! was used as an electrically inert polymer binder. In preliminary TOF measurements, we used simple sandwich-type samples of semitransparent Al/oxadiazole doped film/Al. Oxadiazole doped films were obtained by spin coating from the dichloromethane solution of oxadiazole and PC mixture onto fused quartz substrates with semitransparent aluminum electrodes. Aluminum electrodes were prepared by vacuum deposition at a pressure of about 10 25

FIG. 1. Molecular structures of oxadiazoles, a perylene derivative ~PV! and a polycarbonate ~PC! used in this study.

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FIG. 2. Absorption spectra of 30 wt % oxadiazole doped PC films: 1; BND, 2; BMD, 3; PBD, 4; BDD, and 5;BAPD. Curve 6 shows absorption spectrum of a PV vacuum-deposited film.

Torr. For carrier generation, a N2 laser @wavelength5337 nm, pulse energy5about 120 mJ and pulse width ~FWHM!510 ns# was used as a light source, and oxadiazole molecules were directly excited by irradiation of the N2 laser. Under this measurement condition, however, oxadiazole doped PC films gave small photocurrent signals. The small signals are most likely to be due to poor carrier generation in the oxadiazoles by photoexcitation. Accordingly, the employment of the carrier generation layer was necessary to obtain large photocurrent signal. For enhancement of photocurrent signal, a perylene derivative ~PV! was employed as a carrier generation material; PV is assumed to possess n-type semiconducting characteristics and high electron mobility.10,19,21 The PV was prepared by condensation of 3,4,9,10-perylenetetracarboxylic dianhydride and 1,2-phenylenediamine22 and was used after purification by the train-sublimation method. We fabricated twolayer-type samples of semitransparent Al/PV carrier generation layer/oxadiazole doped film/Al as follows. First, the PV carrier generation layer ~thickness5100 nm! was vacuum deposited on fused quartz substrates with semitransparent aluminum electrodes at about 10 25 Torr. Then, oxadiazole doped films were spin coated on the PV carrier generation layer, and last aluminum top electrodes were vacuum deposited. For carrier generation, a dye laser was used as a light source and its wavelength was tuned at 504 nm. As shown in Fig. 2, oxadiazoles do not absorb the dye laser light while PV film has large absorbance around a laser wavelength of 504 nm. By irradiation with the dye laser, accordingly, carriers are generated only in the PV carrier generation layer. Figure 3 shows the transient photocurrent profiles due to electron transport of 30 wt % BND doped PC films ~profile a; two-layer type sample with PV carrier generation layer, profile b; simple sandwich-type sample without PV carrier generation layer!. Profiles a and b were obtained by irradiation of the dye laser ~pulse energy510 mJ! and the N2 laser ~pulse energy5120 mJ!, respectively. The applied field was 7.5310 5 V cm21. Photocurrent signal is greatly enhanced by use of PV carrier generation layer; the photocurrent value of the two layer-type sample is more than 10 times larger than that of the simple sandwich-type sample although the pulse 3434

FIG. 3. Transient photocurrent profiles due to electron transport for 30 wt % BND doped PC films with PV carrier generation layer ~profile a! and without PV carrier generation layer ~profile b!.

energy of the dye laser is less than one tenth that of the N2 laser. The enhancement of photocurrent is most likely to be caused by injection of carriers generated in the PV carrier generation layer to the BND doped PC film. An apparent plateau part followed by a broadened and decreasing tail was observed in the enhanced photocurrent profile, indicating nondispersive carrier transport. Then, we successfully determined the transit time t T from the inflection point from plateau to tail. The transit time t T means the time when the leading part of the carrier distribution reaches the collecting electrode. Drift mobility m was calculated according to the equation m 5L/t T F, where L is film thickness and F applied field. To prove that the transit times reflect electron transport in BND doped PC films, the film thickness dependence of transit time was examined under a constant field. When the thickness of the BND doped PC film was increased from 2.1 to 4.4 mm, transit time of the BND doped PC film increased from 4.3310 25 to 1.2310 24 s. From the transit time, the electron drift mobility was determined to be 6.5310 26 cm2 V21 s21 at a thickness of 2.1 mm and 4.9310 26 cm2 V21 s21 at a thickness of 4.4 mm, respectively. The transit time was roughly proportional to film thickness, and the electron drift mobility was found to be basically independent on film thickness. From the result, one can say that the photocurrent profiles represent electron transport in BND doped PC film and that the transit time is not influenced by the insertion of PV carrier generation layer. The concentration dependence of electron drift mobility of BND doped PC film is shown in Fig. 4, where dopant concentration varied in the range from 10 to 50 wt %. The measurements were carried out under an applied field of 7.5310 5 V cm21 at room temperature. The electron drift mobility m e of BND doped PC film increase with increasing dopant concentration, indicating that electrons transport on BND molecules as hopping sites. The m e of BND doped PC film reached a value of 2.2310 25 cm2 V21 s21 at a dopant concentration of 50 wt %. The value is several times larger than those of electron conductors previously reported, for example, diphenoquinone doped polycarbonate ~m e 5about 4310 26 cm2 V21 s21 at a dopant concentration of 50 wt %

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FIG. 4. The concentration dependencies of electron drift mobilities of oxadiazoles doped in PC: open circles; BND, open square; BMD, open triangle; PBD, and solid circle; BDD at an applied field of 7.5310 5 V cm21 at room temperature.

On the other hand, incorporating strong electronreleasing substituents, diethylamino, and diphenylamino groups, into oxadiazole skeleton gave hole-transport characteristics to oxadiazoles. BDD was found to possess both electron-transport and hole-transport properties, in other words, bipolar carrier transport properties; m e 56.5310 28 cm2 V21 s21 and m h 59.8310 28 cm2 21 21 V s in a 30 wt % BDD doped PC film. In a BAPD doped PC film, furthermore, only hole drift mobility of moderately high value was observed ~m h 51.6310 27 cm2 V21 s21 at a dopant concentration of 30 wt %!. In conclusion, we succeeded in evaluating electron drift mobility of oxadiazoles with the TOF technique by using a perylene derivative PV film as a carrier generation layer. From the TOF measurements, it was demonstrated that oxadiazoles have high potential for electron transport. In addition, incorporating strong electron-releasing groups into the oxadiazole skeleton was demonstrated to give hole-transport properties to oxadiazoles. D. M. Pai, J. F. Yanus, and M. Stolka, J. Phys. Chem. 33, 4714 ~1984!. J. X. Mack, L. B. Schein, and A. Peled, Phys. Rev. B 39, 7500 ~1988!. 3 P. M. Borsenberger, E. H. Magin, and J. J. Fitzgerald, J. Phys. Chem. 97, 9213 ~1993!. 4 M. Stolka, J. F. Yanus, and D. M. Pai, J. Phys. Chem. 88, 4707 ~1984!. 5 D. Adam, F. Closs, T. Frey, D. Haarer, H. Ringsdorf, P. Schumacher, and K. Siemensmeyer, Phys. Rev. Lett. 70, 457 ~1993!. 6 D. Adam, P. Schumacher, J. Simmerer, L. Haessling, K. Siemensmeyer, K. H. Etzbach, H. Ringsdorf, and D. Haarer, Nature ~London! 371, 141 ~1994!. 7 Y. Yamaguchi, T. Fujiyama, and M. Yokoyama, J. Appl. Phys. 70, 855 ~1991!. 8 P. M. Borsenberger, T. M. Kung, and W. B. Vreeland, J. Appl. Phys. 70, 4100 ~1990!. 9 W. D. Gill, , J. Appl. Phys. 43, 5033 ~1972!. 10 E.H. Magin and P. M. Borsenberger, J. Appl. Phys. 73, 787 ~1993!. 11 C. W. Tang and S. A. VanSlyk, Appl. Phys. Lett. 51, 913 ~1987!. 12 C. W. Tang, S. A. VanSlyk, and C. H. Chen, J. Appl. Phys. 65, 3610 ~1989!. 13 C. Adachi, T. Tsutsui, and S. Saito, Appl. Phys. Lett. 56, 799 ~1990!. 14 C. Adachi, T. Tsutsui, and S. Saito, Appl. Phys. Lett. 57, 531 ~1990!. 15 J. H. Borroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature ~London! 347, 539 ~1990!. 16 P. L. Burns, A. B. Holmes, A. Kraft, D. D. C. Bradley, A. R. Brown, R. H. Friend, and R. W. Gymer, Nature ~London! 356, 47 ~1992!. 17 C. W. Tang, Appl. Phys. Lett. 48, 183 ~1986!. 18 C. Adachi, T. Tsutsui, and S. Saito, Appl. Phys. Lett. 55, 1489 ~1989!. 19 Y. Hamada, C. Adachi, T. Tsutsui, and S. Saito, Jpn. J. Appl. Phys. 31, 1812 ~1992!. 20 H. J. Wagner, R. O. Loutfy, and C.-K. Hsiao, J. Mater. Sci. 17, 2781 ~1982!. 21 C. Adachi, S. Tokito, T. Tsutsui, and S. Saito, Jpn. J. Appl. Phys. 27, L269 ~1988!. 22 T. Maki and H. Hashimoto, Bull. Chem. Soc. Jpn. 25, 411 ~1952!. 1 2

at a field strength of 5310 5 V cm21!,7 fluorenylidene malononitrile doped polyester ~m e 53.6310 27 cm2 V21 s21 at a dopant concentration of 30 wt % at a field strength of 7.8310 5 V cm21!8 and trinitrofluorenone doped polyvinylcarbazole ~m e 5about 10 26 cm2 V21 s21 at a dopant concentration of 50 wt % at a field strength of 5310 5 V cm21!.9 In addition, the absolute value is comparable to that of a typical hole conductor, N,N 8 -diphenyl-N,N 8 -bis~3-methylphenyl!-@1,18-biphenyl#-4,48-diamine ~TPD!; hole drift mobility m h of 50 wt % TPD in polycarbonate film was reported to be about 3310 25 cm2 V21 s21 at a field strength of 6310 5 V cm21.4 The results quantitatively verify that BND is a potential electron-transporting compound with high drift mobility. The electron drift mobilities of other three oxadiazoles, BMD, PBD, and BDD, were also successfully evaluated by using PV carrier generation layer. The concentration dependencies of m e in their PC solid solution films are also shown in Fig. 4. The m e values were 9.7310 26 cm2 V21 s21 for BMD, 1.9310 26 cm2 V21 s21 for PBD, and 7.2310 27 cm2 V21 s21 for BDD, respectively, at a dopant concentration of 50 wt % at a field strength of 7.5310 5 V cm21. From these results, it was confirmed that oxadiazoles incline to having electron-transport characteristics, as expected from the studies on the multilayered EL devices using oxadiazoles.

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