Elimination of charge-carrier trapping by molecular design

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. Extrinsic impurities may trap electrons or holes leading to imbalanced charge transport in wide band gap organic semiconductors. Here the authors propose a bottom-up design strategy by spatially separating HOMO and LUMO orbitals to avoid charge trapping, enhancing charge transport by orders of magnitude.