Ultrapure, high mobility organic photoconductors

170 W. Warta et al.
3. D.C. Hoesterey, G.M. Letson: J. Phys. Chem. Sol. 24, 1609
(1963)
4. N. Karl: In Festktrperprobleme/Advances Solid State Phys.
14, 261 (Vieweg, Braunschweig 1974)
5. L.B. Schein, D.W. Brown: Mol. Cryst. Liq. Cryst. 87, 1 (1981)
6. K.H. Probst, N. Karl: Phys. Stat. Solidi A27, 499 (1975)
7. L.B. Schein, A.R. McGhie: Phys. Rev. B20, 1631 (1979)
8. Z. Burshtein, D.F. Williams: Phys. Rev. B 15, 5769 (1977)
9. W. Tuffentsammer: Stuttgarter Kristallabor (unpublished)
10. N. Karl: High purity organic molecular crystals, in Crystals 4
(Springer, Berlin, Heidelberg, New York 1980)
11. N. Karl: J. Crystal Growth 51, 509 (1981)
12. J.F. Nye: Physical Properties of Crystals, Their Represen-
tation by Tensors and Matrices (Clarendon Press,
Oxford 1972)
13. J. Berrthar, M. Schott: Mol. Cryst. Liq. Cryst. 46, 223 (1978)
14. Electric field-dependent electron mobilities #~. reported for
anthracene at 140K by S.Nakano, Y. Maruyarna: Solid
State Commun. 35, 671 (1980) could not be reproduced by
L.B. Schein, R.S. Narang, R.W. Anderson, K.E. Meyer, A.R.
McGhie: Chem. Phys. Lett. 100, 37 (1983) even not at still
higher electric fields
15. K. Seeger: Semiconductor Physics, Springer Ser. Solid-State
Phys. 40 (Springer, Berlin, Heidelberg 1982)
16. E.M. Conwell: In Solid State Physics, Suppl. 9 (Academic,
New York 1967)
17. R. Stehle, N. Karl: To be published
18. Y. Maruyama, T. Kobayashi, H. Inokuchi, S. Iwashima: Mol.
Cryst. Liq. Cryst. 20, 373 (1973)
19. For the fit an underlying microscopic mobility #o(T) was
assumed which was obtained from an extrapolation of the
T>45K mobilities using the (E-~0) tangent, n=-1.87,
drawn in Fig. 9. It turns out that the trap activation energy
obtained from the data between 14 and 20K is rather
insensitive to the exact go(T) dependence. Even assuming a
constant value, #o = 100 cm2/Vs, would cause only a minor
change of the resulting trap depth (to 16.1 meV). Between
these two extreme assumptions, the resulting trap concentration
changes from 5 x 10 - 4 to 2 10- 4 mol/mol. - The
discrepancy between the calculated and the measured mobility
curves in the range between 20 and 45 K results from
the fact that the measured mobifities in this high-mobility
region are strongly dependent on the applied electric field
strength. - For very shallow traps, as found here, the
possibility of field-activated detrapping (Pool-Frenkel effect)
can require a revision of the simple Hoesterey-Letson
description. However, this effect can be estimated to decrease
the thermal activation energy by only a few meV for the fields
used here