Optoelectronic processes in polyfluorene ambipolar transistors

This thesis describes the use of charge modulation spectroscopy to investigate the negative and positive charge-induced absorptions in conjugated semiconducting polymers as a way to experimentally compare the wavefunctions of electrons and holes. Interactions between light and charges including fluorescence quenching and photocurrent are also explored. Conjugated polymers have an electronic structure with an energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In the neutral ground state, there are no optical transitions at photon energies lower than this gap. When an excess charge is added to a conjugated polymer, the charge couples with a local structural reorganisation forming a localised entity known as a polaron. The polaron has two new electronic states within the energy gap symmetrically spaced about the midgap energy. Typically two new optical transitions between the polaronic states are allowed and can be accessed with sub gap energies. In order to probe the sub gap polaron absorptions charges can be added by electrical injection. Electrical injection in a transistor configuration provides a controlled way to measure the absorption of a known number of charges in the solid state and without triplet or singlet absorptions complicating the spectra as observed in photo-induced absorption. By taking advantage of recently developed ambipolar transistors where both holes and electrons can be accumulated in the same device a comparison can be made between the negative and positive polaron wavefunctions. Two polyfluorene polymers were chosen as examples where quantum chemical calculations predict either the same or different wavefunctions for the electron and hole. Poly(9,9-di-n-octylfluorene) (F8) is a hydrocarbon-only polymer which is expected to have similar electron and hole wavefunctions, whereas the related co-polymer, poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) is expected to have an electron wavefunction that is more localized that the hole. The ambipolar transistors used in this thesis are typified by a dominant contact resistance which introduces difficulties in the charge modulation spectroscopy experiment. New techniques for simultaneous electrical and optical characterisation are developed and new device structures and fabrication processes are introduced in order to overcome a number of artifacts and improve the accuracy of the measurement allowing quantitative comparisons to be made. The increase in transistor or diode current with energy gap illumination and the quenching of fluorescence in the presence of charges is also investigated and a new method for imaging charge trapping and device operation in transistors with luminescent semiconductors is introduced.EPSRC, Clare College, Philosophical Society, Board of Graduate Studie