Photocurrent Spectroscopy of Pentacene Thin Film Transistors

We investigate charge transport in pentacene thin-film transistors by means of photocurrent spectroscopy. The photocurrent spectrum in these devices reveals contributions from the Davydov-split singlet excitons at 1.85 eV and 1.96 eV, inter-pentacene molecular charge-transfer at 2.1 eV and 2.3 eV, and a broad continuum above 2.25 eV associated with the HOMO- LUMO transitions. By using photon excitations at these characteristic energies, we obtained the optical current- voltage characteristics of the pentacene TFTs by measurements of the source-drain photocurrent as a function of the source- drain and gate voltages, allowing a spectroscopic probe of the various transport mechanisms in these devices. The implications of these results to models of the current-voltage characteristics of pentacene TFTs will be discussed.