Electroluminescence in Ion-Gel Gated Conjugated Polymer Field-Effect Transistors

We report electroluminescence from ion-gel gated, field-effect transistors based on the conjugated polymer, poly­(9,9′-dioctylfluorene-co-benzothiadiazole) gated by an 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide/poly (styrene-block-ethylene oxide-block-styrene) ion gel, and investigate the mechanism for light emission. The devices emit light from near the electron-injecting drain electrode when the drain source voltage exceeds approximately the energy gap of the polymer (<i>V</i>_ds <i>> E</i>_g<i>/e</i>). Charge accumulation spectroscopy is used to demonstrate the significant penetration of the negative TFSI^– ions into the F8BT assisted by the application of negative gate voltages, where they lead to significant p-type doping of the bulk of the F8BT film. In contrast, no evidence for diffusion of positive ions with positive gate voltages is observed, and this is consistent with the location of the recombination zone in the proximity of electron injecting electrode and the absence of a comparable electron current at positive gate voltages. We conclude that in the light-emission regime the devices operate more akin to a hole-current dominated light-emitting electrochemical cell than a transistor

Synthesis of High-Crystallinity DPP Polymers with Balanced Electron and Hole Mobility

We review the Stille coupling synthesis of P­(DPP2OD-T) (Poly­[[2,5-di­(2-octyldodecyl)­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4­(2<i>H</i>,5<i>H</i>)-dione-3,6-diyl]-<i>alt</i>-[2,2′:5′,2″-terthiophene-5,5″-diyl]]) and show that high-quality, high molecular weight polymer chains are already obtained after as little as 15 min of reaction time. The results of UV–vis spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and atomic force microscopy show that longer reaction times are unnecessary and do not produce any improvement in film quality. We achieve the best charge transport properties with polymer batches obtained from short reaction times and demonstrate that the catalyst washing step is responsible for the introduction of charge-trapping sites for both holes and electrons. These trap sites decrease the charge injection efficiency, strongly reducing the measured currents. The careful tuning of the synthesis allows us to reduce the reaction time by more than 100 times, achieving a more environmentally friendly, less costly process that leads to high and balanced hole and electron transport, the latter being the best reported for an isotropic, spin-coated DPP polymer

The Role of Regioregularity, Crystallinity, and Chain Orientation on Electron Transport in a High-Mobility n‑Type Copolymer

We investigated the correlation between the polymer backbone structural regularity and the charge transport properties of poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} [P­(NDI2OD-T2)], a widely studied semiconducting polymer exhibiting high electron mobility and an unconventional micromorphology. To understand the influence of the chemical structure and crystal packing of conventional regioregular P­(NDI2OD-T2) [RR-P­(NDI2OD-T2)] on the charge transport, the corresponding regioirregular polymer RI-P­(NDI2OD-T2) was synthesized. By combining optical, X-ray, and transmission electron microscopy data, we quantitatively characterized the aggregation, crystallization, and backbone orientation of all of the polymer films, which were then correlated to the electron mobilities in electron-only diodes. By carefully selecting the preparation conditions, we were able to obtain RR-P­(NDI2OD-T2) films with similar crystalline structure along the three crystallographic axes but with different orientations of the polymer chains with respect to the substrate surface. RI-P­(NDI2OD-T2), though exhibiting a rather similar LUMO structure and energy compared with the regioregular counterpart, displayed a very different packing structure characterized by the formation of ordered stacks along the lamellar direction without detectible π-stacking. Vertical electron mobilities were extracted from the space-charge-limited currents in unipolar devices. We demonstrate the anisotropy of the charge transport along the different crystallographic directions and how the mobility depends on π-stacking but is insensitive to the degree or coherence of lamellar stacking. The comparison between the regioregular and regioirregular polymers also shows how the use of large planar functional groups leads to improved charge transport, with mobilities that are less affected by chemical and structural disorder with respect to classic semicrystalline polymers such as poly­(3-hexylthiophene)