Transient photoconductivity (σph) experiments have been carried out in single crystals of insulating La2CuO4+δ near the metal-insulator transition (δ≈ 1-2%). The time evolution of the σph changes dramatically with light intensity (IL). At low IL, σph is characterized by power law time decay, t-α, in the nanosecond time regime, and the exponent, α, decreases significantly with increasing IL. As IL increases to 5 × 1015 photons/cm2, σph reaches ∼ 15 S/cm, and the lifetime of the conducting state is enhanced by more than two orders of magnitude: σph exhibits a delayed peak centered at approximately 30 ns. followed by exponential decay with time constant of ∼ 360 ns. The data reveal a continuous distribution of localized electronic states above the (hole) mobility edge; the M-I transition is achieved when the Fermi level is shifted away from the localized states and across the mobility edge either by chemical dopiing or by photo-excitation. The time of σph at high IL implies the formation of metastable metallic droplets after photo-excitation. © 1992.

A resurgence in the use of the donor-acceptor approach in synthesizing conjugated polymers has resulted in a family of high-mobility ambipolar systems with exceptionally narrow energy bandgaps below 1 eV. The ability to transport both electrons and holes is critical for device applications such as organic light-emitting diodes and transistors. Infrared spectroscopy offers direct access to the low-energy excitations associated with injected charge carriers. Here we use a diffraction-limited IR microscope to probe the spectroscopic signatures of electron and hole injection in the conduction channel of an organic field-effect transistor based on an ambipolar DA polymer polydiketopyrrolopyrrole-benzobisthiadiazole. We observe distinct polaronic absorptions for both electrons and holes and spatially map the carrier distribution from the source to drain electrodes for both unipolar and ambipolar biasing regimes. For ambipolar device configurations, we observe the spatial evolution of hole-induced to electron-induced polaron absorptions throughout the transport path. Our work provides a platform for combined transport and infrared studies of organic semiconductors on micron length scales relevant to functional devices.

Charge carrier transport through organic solar cells is fundamentally dispersive due to the disordered structure and complex film morphology within the photoactive layer. A novel application of transient photocurrent and short-circuit variable time-delayed collection field measurements is used to reconstruct the complete charge carrier mobility distribution for the photogenerated carriers in optimized organic solar cells.