Paul Blom1,Oskar Sachnik1,Xiao Tan1,Constantin Haese1,Naomi Kinaret1,Kun-Han Lin1,Denis Andrienko1,Robert Graf1,Gert-Jan Wetzelaer1,Jasper Michels1
Max-Planck-Institute for Polymer Research1
Paul Blom1,Oskar Sachnik1,Xiao Tan1,Constantin Haese1,Naomi Kinaret1,Kun-Han Lin1,Denis Andrienko1,Robert Graf1,Gert-Jan Wetzelaer1,Jasper Michels1
Max-Planck-Institute for Polymer Research1
A disadvantage of many organic semiconductors is that they exhibit highly unipolar charge transport, meaning that they predominantly conduct either electrons or holes. This unipolarity is caused by trapping of either electrons or holes by external impurities such as water or oxygen. Besides unbalanced transport, charge traps give rise to unwanted nonradiative recombination events in devices. We have recently demonstrated that an energetic window with a width of 2.5 eV exists inside which organic semiconductors are not susceptible to charge trapping, leading to trap-free charge transport of both carriers.<sup>[1] </sup>For devices that benefit from balanced transport such as OLEDs, organic solar cells and organic ambipolar transistors the energy levels of the organic semiconductors are ideally situated within this energetic window. However, for blue-emitting OLEDs with a band gap larger than this window, removal or disabling charge traps poses a significant challenge. One option is to use the concept of trap dilution, where the organic semiconductor is mixed with an insulator, leading to a reduced occupation of trap states.<sup>[2] </sup>A disadvantage of this concept is that for small molecules this also implies a reduction of the charge-carrier mobility due to the increased distance between the molecules. In the present study, we demonstrate an alternative molecular strategy where the HOMO and LUMO orbitals are spatially separated on different parts of the molecules. By tuning the stacking, the LUMO 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 transport.<br/>[1] N. B. Kotadiya, A. Mondal, P. W. M. Blom, D. Andrienko, G-J. A. H. Wetzelaer, <i>Nature Mater.</i> 2019, 18, 1182.<br/>[2] D. Abbaszadeh, A. Kunz, G.-J. A. H. Wetzelaer, J. J. Michels, N. I. Crăciun, K. Koynov, I. Lieberwirth, P. W. M. Blom, <i>Nature Mater.</i> 2016, 15, 628.