Energy Level Engineering in Organic Thin Films by Tailored Halogenation

Abstract

In modern electronics, it is essential to adapt band structures by adjusting energy levels and band gaps. At first sight, this “band structure engineering” seems impossible in organic semiconductors, which usually exhibit localized electronic states instead of Bloch bands. However, the strong Coulomb interaction in organic semiconductors allows for a continuous shift of the ionization energy (IE) over a wide range by mixing molecules with halogenated derivatives that exhibit different quadrupole moments. Here, this effect of energy level engineering on blends of pentacene and two fluorinated derivatives, in which the position but not the number of fluorine atoms differ, is studied. Structural investigations confirm that pentacene forms intermixed phases in blends with the fluorinated species. The investigation of electronic properties and simulations reveals a much larger shift of the ionization energy (1.5 eV) than in previous studies, allowing to test this model in a range not investigated so far, and emphasizing the role of the position of the halogen atoms. The tuning effect is preserved in electronic devices such as field‐effect transistors and significantly influences device characteristics.

Permanent Dipole Moments Enhance Electronic Coupling and Singlet Fission in Pentacene

“…This has led to increased interest in the engineering of the properties of single molecules and molecular aggregates via chemical modification. In the context of a rational design of SF chromophores, several key parameters have been identified, such as electronic coupling strength, molecular arrangement, and the energetic difference of singlet and triplet states. − Current approaches toward an optimization of SF chromophores employ, for example, the addition of bulky side groups to modify the molecular packing in aggregates ,− or the use of dimers of SF chromophores for a controlled variation of the relative molecular arrangement and, thus, of the electronic coupling strength. − Alternatively, the energy difference between singlet and triplet states has been modified by the substitution of carbon atoms of the SF chromophore backbone with heteroatoms, such as nitrogen or sulfur, − or by fluorination of the SF chromophore. − The latter is in the general context of the optimization of organic semiconductors for optoelectronic devices, a well-established approach to increase stability against oxidation or to control energy levels, molecular packing, and charge transport. − …”

Spectroelectrochemically determined energy levels of PM6:Y6 blends and their relevance to solar cell performance

“…The difference in the absolute values compared to the CV/UV-vis results could be caused by different electrostatic interactions, e.g., dipole, quadrupole or higher order moments, polarization and screening effects etc., as has been demonstrated before. [70][71][72]…”

Aromatic Ring Overlap Pedals the Nature of Exciton Coupling and Carrier Transport in a Series of Electron-Deficient Anthracenes

“…Organic semiconductors with ambipolar carrier mobility possess an inevitable role as active layer materials have been widely utilized in smart electronic devices. − The limiting factor of the performance of these devices is solely subject to either the optical (in terms of various radiative transitions) or the charge transport properties (in terms of electron and hole mobilities) of the active layer materials. Hence, fine-tuning the optical and charge transport behaviors of such materials, especially small-molecular-weight organic semiconductors, will ensure better performance of the derived electronic devices. − Interestingly, the radiative transitions from the electronically excited states and the complementing ground state process of the electronic transport characteristics of such bulk materials can be effectively correlated with known molecular exciton coupling models. − ,, The advantage of such an effort is to use the optical signals as an effective readout to predict the carrier transport behavior of the bulk semiconductor layer. There have been efforts to understand the mechanism in the direction mentioned above.…”