Y. Harima et al., CHARGE-TRANSPORT IN VACUUM-SUBLIMED FILMS OF METAL-FREE TETRAPHENYLPORPHYRIN AND ITS RELATION TO CAPACITANCE AND PHOTOCURRENT MEASUREMENTS, Thin solid films, 300(1-2), 1997, pp. 213-217
Drift mobilities (mu) of holes and electrons in vacuum-sublimed films
of metal-free tetraphenylporphyrin (H2TPP) were measured by using a ti
me-of-flight technique with a log-log analysis of photocurrent transie
nts with an electric field (E) and temperature as parameters. Extrapol
ation of a plot of log mu vs. E-1/2 to E = 0 yielded a very small mu v
alue of the order 10(-16) cm(2) V-1 s(-1) for hole and electron transp
ort in the H2TPP film at 20 degrees C. Field and temperature dependenc
ies of the hole mobilities, analyzed on the basis of the disorder form
alism of Bassler and his coworkers, revealed that the extremely slow h
ole transport in the H2TPP films was characterized by the largest ener
getic disorder parameter of 0.22 eV among those reported for other med
ia so far. In addition, measurements of capacitances and photocurrents
due to an Al/H2TPP Schottky junction were carried out with photovolta
ic cells of Al/H2TPP/Au. The lack of dependencies of the capacitances
on the frequency and bias voltage was interpreted in terms of the low
mobilities of holes in the H2TPP films. A marked increase of photocurr
ents with time paralleled a mobility increase, suggesting an important
role of carrier mobilities in the mechanism of photocurrent generatio
n in molecular semiconductors. (C) 1997 Elsevier Science S.A.