On the influence of physical and chemical structure on charge transport in disordered semiconducting materials and devices

摘要

Achieving fast charge carrier transport in disordered organic semiconductors is of great importanceudfor the development of organic electronic devices. Disordered organic materials generally show lowudcharge carrier mobilities due to their inherent energetic and configurational disorder, and the presenceudof chemical and physical defects. Efforts to improve mobility typically involve chemical designudand materials processing to control macromolecular conformation and/or induce greater crystallineudor liquid crystalline order. Whilst in many cases fruitful, these approaches have not always translatedudinto 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 manyudsuch features spectroscopically and controlling their formation in isolation.udIn the three experimental Chapters following, we attempt to make clear links between the chargeudcarrier 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 theudmaterials through processing and by virtue of their chemical design, we examine the effect on transportudof distinct material phases. Remarkably, we are able to isolate the effect of the single chainudconformation of PFO known as the beta-phase and show that when embedded in a glassy PFOudmatrix it acts as a strong hole trap, reducing the mobility of the bulk material by over two ordersudof magnitude. By fabricating a device with negligible beta-phase, we demonstrate the highestudtime-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 inuda conjugated polymer. We also demonstrate the adverse effect on mobility of crystallinity in theudpolyfluorenes. We suggest that our findings may generalise to other systems in the sense that theudmobility may be limited by a minority population of structural traps, which may include highlyudordered, crystalline regions. Significant mobility improvements may then be more easily achievedudby removing the minority ordered phases than by increasing their concentration. We believe thatudthis approach offers an alternative paradigm by which higher mobilities may be obtained in general,udand 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-adductsudbis-PCBM and tris-PCBM are of interest as alternative OPV acceptor materials with the potentialudto increase open-circuit voltage. However, most OPV blends employing the multi-adducts haveudfailed to improve upon those employing PCBM. This is thought to be a result of the inferior electronudtransport properties of the multi-adducts, due to either (i) higher energetic disorder in the multiadductsuddue to the presence of isomers with varying LUMO energies or (ii) higher con gurationaluddisorder due to a lower degree of order in molecular packing in the multi-adducts than in PCBM.udWe distinguish the e ects of energetic and con gurational disorder using temperature-dependentudToF and FET measurements. We find that differences in configurational disorder appear negligible,udand that the reduced mobility in the multi-adducts is due predominantly to the energetic disorderudresulting from the presence of a mixture of isomers with varying LUMO energies.udIn the third and final experimental study, we examine the charge transport properties of polymer:udPCBM blends for OPV, focusing on the PTB7:PCBM and P3HT:PCBM systems. In particular,udwe address the question of why state-of-the-art OPV systems such as PTB7:PCBM perform soudmuch worse at large active layer thicknesses than P3HT:PCBM. We find that low electron mobilityudis the main cause of this di erence. The electron mobility in PTB7:PCBM blends, at 10-5 { 10-4udcm2/Vs, is 1-2 orders of magnitude lower than the electron mobility in annealed P3HT:PCBM, atudover 10-3 cm2/Vs. The hole mobility, in contrast, is the same to within a factor of approximatelyudthree. We hypothesise that the low tendency of PTB7 to order leads to a low degree of phase separationudin the blend and to a poorly connected, disordered PCBM phase. We find that increasingudthe PCBM fraction is very effective in improving electron transport and electrical Fill Factor, butudstrongly reduces absorption. We suggest that a key challenge for OPV researchers is thus to achieveudbetter connectivity and ordering in the fullerene phase in blends without relying on either (i) a largeudexcess of fullerene or (ii) strong crystallisation of the polymer.