The incorporation of rigid and planar, π-expanded diketopyrrolopyrrole (EDPP) units into alternating donor–acceptor copolymers is reported.

A common obstacle of many organic semiconductors is that they show highly unipolar charge transport. This unipolarity is caused by trapping of either electrons or holes by extrinsic impurities, such as water or oxygen. For devices that benefit from balanced transport, such as organic light-emitting diodes, organic solar cells and organic ambipolar transistors, the energy levels of the organic semiconductors are ideally situated within an energetic window with a width of 2.5 eV where charge trapping is strongly suppressed. However, for semiconductors with a band gap larger than this window, as used in blue-emitting organic light-emitting diodes, the removal or disabling of charge traps poses a longstanding challenge. Here we demonstrate a molecular strategy where the highest occupied molecular orbital and lowest unoccupied molecular orbital are spatially separated on different parts of the molecules. By tuning their stacking by modification of the chemical structure, the lowest unoccupied molecular orbitals can be spatially protected from impurities that cause electron trapping, increasing the electron current by orders of magnitude. In this way, the trap-free window can be substantially broadened, opening a path towards large band gap organic semiconductors with balanced and trap-free transport.

Diblock copolymer electrolytes based on a π-conjugated polyfluorene (PF) backbone were synthesized comprising nanodomains of a polymerized ionic liquid (PIL) and of a solid polymer electrolyte (SPE). The former consists of a single-ion conductor based on an imidazolium alkyl chain with a [Br]− counteranion grafted on the PF backbone. The latter consists of short ethylene oxide (EO) chains, grafted on the PF backbone and further doped with LiTFSI. The two nanophases support ionic conductivity, whereas the rigid PF backbone provides the required mechanical stability. In the absence of LiTFSI, ionic conductivity in the PIL nanophase is low and exhibits an Arrhenius temperature dependence. LiTFSI substitution enhances ionic conductivity by about 3 orders of magnitude and further changes to a Vogel–Fulcher–Tammann temperature dependence. However, at ambient temperature, ionic conductivity is lower than in the corresponding PEO/LiTFSI electrolytes. X-ray studies and thermal analysis revealed that the conjugated backbone imparts liquid-crystalline order that can be fine-tuned through the EO side group length. Ionic conductivity measurements performed as a function of pressure identified local jumps of [Li]+ and [Br]− ions in the respective SPE/PIL nanophases as responsible for the ionic conductivity. Between the two ions, it is [Li]+ that has the major contribution to the ionic conductivity. The current results provide designing rules for new copolymers that comprise two different ionic nanodomains (PIL and SPE) and a conjugated backbone that can further support electronic conduction.

Efficient organic light‐emitting diodes (OLEDs) commonly comprise a multilayer stack including charge‐transport and charge‐ and exciton‐blocking layers, to confine charge recombination to the emissive layer. Here, a highly simplified single‐layer blue‐emitting OLED is demonstrated based on thermally activated delayed fluorescence with the emitting layer simply sandwiched between ohmic contacts consisting of a polymeric conducting anode and a metal cathode. The single‐layer OLED exhibits an external quantum efficiency of 27.7% with minor roll‐off at high brightness. The internal quantum efficiency approaches unity, demonstrating that highly simplified single‐layer OLEDs without confinement layers can achieve state‐of‐the‐art performance, while greatly reducing the complexity of the design, fabrication, and device analysis.