Charge transport in vacuum-sublimed films of metal-free tetraphenylporphyrin and its relation to capacitance and photocurrent measurements

Drift mobilities (mu) of holes and electrons in vacuum-sublimed films of metal-free tetraphenylporphyrin (H2TPP) were measured by using a time-of-flight technique with a log-log analysis of photocurrent transients with an electric field (E) and temperature as parameters. Extrapolation of a plot of log mu vs. E-1/2 to E = 0 yielded a very small mu value of the order 10(-16) cm(2) V-1 s(-1) for hole and electron transport in the H2TPP film at 20 degrees C. Field and temperature dependencies of the hole mobilities, analyzed on the basis of the disorder formalism of Bassler and his coworkers, revealed that the extremely slow hole transport in the H2TPP films was characterized by the largest energetic disorder parameter of 0.22 eV among those reported for other media so far. In addition, measurements of capacitances and photocurrents due to an Al/H2TPP Schottky junction were carried out with photovoltaic 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 photocurrents with time paralleled a mobility increase, suggesting an important role of carrier mobilities in the mechanism of photocurrent generation in molecular semiconductors. (C) 1997 Elsevier Science S.A.