On the influence of physical and chemical structure on charge transport in disordered semiconducting materials and devices
Creators: name: Foster, Samuel
Publisher: name: Imperial College London
Year: 2013

description: Achieving fast charge carrier transport in disordered organic semiconductors is of great importance for the development of organic electronic devices. Disordered organic materials generally show low charge carrier mobilities due to their inherent energetic and configurational disorder, and the presence of chemical and physical defects. Efforts to improve mobility typically involve chemical design and materials processing to control macromolecular conformation and/or induce greater crystalline or liquid crystalline order. Whilst in many cases fruitful, these approaches have not always translated into higher bulk mobilities in devices. Addressing the adverse effect on mobility of specific types of disorder or specific defects has proven difficult due to problems distinguishing the many such features spectroscopically and controlling their formation in isolation. In the three experimental Chapters following, we attempt to make clear links between the charge carrier mobility and the presence of specific structural defects or sources of energetic or configurational disorder. In the first experimental study, we investigate hole transport in a family of polyfluorenes based on poly(9,9-dioctylfluorene) (PFO). By controlling the phase formation of the materials through processing and by virtue of their chemical design, we examine the effect on transport of distinct material phases. Remarkably, we are able to isolate the effect of the single chain conformation of PFO known as the beta-phase and show that when embedded in a glassy PFO matrix it acts as a strong hole trap, reducing the mobility of the bulk material by over two orders of magnitude. By fabricating a device with negligible beta-phase, we demonstrate the highest time-of-flight mobility in PFO to date, at over 3 10-2 cm2/Vs. This study provides the first clear and unambiguous example of the effect on transport of a distinct conformational defect in a conjugated polymer. We also demonstrate the adverse effect on mobility of crystallinity in the polyfluorenes. We suggest that our findings may generalise to other systems in the sense that the mobility may be limited by a minority population of structural traps, which may include highly ordered, crystalline regions. Significant mobility improvements may then be more easily achieved by removing the minority ordered phases than by increasing their concentration. We believe that this approach offers an alternative paradigm by which higher mobilities may be obtained in general, and in particular in systems where crystallinity is undesirable. In the second experimental study, we study charge transport in the fullerene derivatives [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), bis-PCBM and tris-PCBM. The fullerene multi-adducts bis-PCBM and tris-PCBM are of interest as alternative OPV acceptor materials with the potential to increase open-circuit voltage. However, most OPV blends employing the multi-adducts have failed to improve upon those employing PCBM. This is thought to be a result of the inferior electron transport properties of the multi-adducts, due to either (i) higher energetic disorder in the multiadducts due to the presence of isomers with varying LUMO energies or (ii) higher con gurational disorder due to a lower degree of order in molecular packing in the multi-adducts than in PCBM. We distinguish the e ects of energetic and con gurational disorder using temperature-dependent ToF and FET measurements. We find that differences in configurational disorder appear negligible, and that the reduced mobility in the multi-adducts is due predominantly to the energetic disorder resulting from the presence of a mixture of isomers with varying LUMO energies. In the third and final experimental study, we examine the charge transport properties of polymer: PCBM blends for OPV, focusing on the PTB7:PCBM and P3HT:PCBM systems. In particular, we address the question of why state-of-the-art OPV systems such as PTB7:PCBM perform so much worse at large active layer thicknesses than P3HT:PCBM. We find that low electron mobility is the main cause of this di erence. The electron mobility in PTB7:PCBM blends, at 10-5 { 10-4 cm2/Vs, is 1-2 orders of magnitude lower than the electron mobility in annealed P3HT:PCBM, at over 10-3 cm2/Vs. The hole mobility, in contrast, is the same to within a factor of approximately three. We hypothesise that the low tendency of PTB7 to order leads to a low degree of phase separation in the blend and to a poorly connected, disordered PCBM phase. We find that increasing the PCBM fraction is very effective in improving electron transport and electrical Fill Factor, but strongly reduces absorption. We suggest that a key challenge for OPV researchers is thus to achieve better connectivity and ordering in the fullerene phase in blends without relying on either (i) a large excess of fullerene or (ii) strong crystallisation of the polymer.