Polaron formation is central to charge injection and transport in organic semiconductors. The flexibility of the organic molecule, the deformability of the van der Waals bonded lattice, and the narrowness of the electronic bands all favor the self-trapping of charge carriers and the formation of small polarons. This is in addition to the prevalence of structural and chemical defects that form the bulk of charge carrier traps in organic semiconductors. We take two spectroscopic approaches to probe charge carrier trapping and self-trapping in organic semiconductors. The first approach relies on in situ infrared absorption spectroscopy to directly monitor molecular vibrations and electronic transitions associated with charge carriers in gate-doped organic semiconductors. These experiments allow us to establish quantitatively the polaron-bipolaron equilibrium in gate-doped poly(3-hexylthiophene) (P3HT), the electron injection barrier and LUMO density-of-state distribution in N-N’dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8), and the de-trapping kinetics in these materials. The second approach relies on femtosecond time-resolved two-photon photoemission (TR-2PPE) spectroscopy to follow the formation and decay of small polarons in model systems. We photo-inject an electron from the metal or semimetal contact into the conduction band of a crystalline thin film and record the energy and parallel dispersion of the transient electron en route to localization in real time. These experiments provide unprecedented insight into charge carrier trapping and self-trapping in organic semiconductors.

We studied the processes of charge injection and discharging of semiconductor conjugated polymer thin film devices using Fluorescence-Voltage Time Resolved (FV-TR) spectroscopy. The poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) was studied in current work. This polymer is in extensive research for its potential applications in Organic Light Emitting Diodes (OLED) and Photovoltaic devices. The structure of the capacitor-like, “holes only” device used in our study consists of the following layers, ITO/PEDOT:PSS/F8BT/SiO2/Al. The photogenerated carriers, originated from excitons dissociation, were found to play a dominant role in the fluorescence quenching mechanism. The evidence of relatively slow discharging rate suggests trapping of the holes in deep trap sites in studied polymer. These deep traps are formed at the interface of the polymer film and hole injection layer, PEDOT:PSS. We show that the presence of oxygen in the device strongly affects device characteristics. Aged samples demonstrate more fluorescence quenching at positive biases than freshly prepared. We believe that the candidate for the photobleached form of the polymer is F8BT+/anion complex. Such complex is formed by photoinduced electron transfer between F8BT molecules and oxygen forming F8BT+/O2-. Both processes of charging and discharging of the device were found to be bias dependant. However, we show that the presence of light (laser excitation) is required for more efficient hole-injection and discharging of the device.

Recently, there has been considerable interest in the development of semiconducting polymers for use in printable electronic devices, such as transistors for display backplanes. These materials offer a cost effective alternative to conventional semiconductors, with key advantages of organic materials being their low processing temperature and possibility of solution processing in ambient, leading to roll-to-roll printing. Performance over the past few years has improved, but charge transport in these materials is not yet fully understood. These conjugated polymers generally form a semicrystalline microstructure, consisting of lamellar crystalline regions separated by amorphous grain boundaries. A mobility edge model has been proposed that suggests charge delocalization in the crystallite regions where mobile states exist, with the effective mobility limited by the localized states within the disordered grain boundaries and at defects. We have used a means of controlling the orientation and size of crystallites in the plane of the substrate to explore the relationship between trap density within grain boundaries and charge transport. Regioregular poly-3-hexylthiopene (P3HT) is the material under investigation. We have fabricated anisotropic films on glass and silicon substrates by taking advantage of the needle-like growth of 1,3,5-tricholorobenzene (TCB). We use TCB first as a solvent at an high temperature and then as a nucleating agent and substrate for epitaxy once cooled. When TCB is removed from the P3HT/TCB film, an oriented single-component polymer film is left behind, consisting of large (mm2) domains where the film extinguishes uniformly under crossed polarizers, suggesting long range orientation of the polymer chain axis. Characterization with an AFM reveals crystalline lamellae stacked along the fiber direction. Films were further characterized at the Stanford Synchrotron Radiation facility. We have collected 2D diffraction patterns, specular diffraction patterns, and grazing incidence diffraction patterns. Charge transport in the directionally crystallized films was probed by measuring in plane mobilities using thin film transistors with the oriented film as the active layer. Devices were made with different relative orientations between the channel and the polymer film. Transport measurements as a function of charge density and temperature for different orientations of our film were used to explore the effect of microstructure on trap distribution.

Understanding the conduction mechanism in organic semiconductors is essential for the materials development for plastic electronics. The role of lattice phonon scattering is particularly important for organic semiconductors because low energy intermolecular vibrations are heavily excited at room temperature. It sets the upper limit of the carrier mobility in highly ordered crystalline devices. We measured the in-plane band dispersion of standing ‘thin film phase” pentacene monolayer prepared by epitaxial growth on α-√3×√3 Bi-Si(111) with a nanoscale morphological template of bunched steps. Using this quasi-single crystalline monolayer film, the band dispersion along Γ-X, Γ-Y and Γ-M directions was obtained at different temperatures. Sinusoidal band dispersion of upper HOMO band observed at 140K was lost in Γ-Y and Γ-M directions while it remained in Γ-X direction at