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

Drift mobilities (μ) 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 μ vs. E^1/2 to E = 0 yielded a very small μ value of the order 10^-16 cm² V^-1 s^-1 for hole and electron transport in the H₂TPP film at 20°C. Field and temperature dependencies of the hole mobilities, analyzed on the basis of the disorder formalism of Bässler 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.