| Polyhedral oligomeric silsesquioxane (POSS) and its derivatives are used as examples to illustrate the tunability of electronic properties of organic semiconductors by density functional theory (DFT) and time-dependent density functional theory (TDDFT) studies. The results indicate that the POSS cage is quite rigid upon functionalization and thus provides a means for controlling the orientation of organic fragments attached to it. Moreover, the electronic properties such as frontier orbitals distribution and energy level, HOMO-LUMO gap, reorganization energy, and exciton binding energy can be tuned through the choice of functional groups and their positioning. By functionalizing the POSS cage with two pentacene molecules along the diagonal direction, we construct Dipentacene POSS with very high charge mobility predicted by multiscale modeling. Unlike the herringbone pattern in the pentacene single crystal, the Dipentacene POSS molecules adopt a parallel configuration in its single crystal that was predicted with molecular dynamics. This parallel configuration enhances the wavefunction overlap between pentacene segments, resulting in faster charge hopping between the molecules. The insight gained in the tuning of electronic properties and charge transport was used to design materials for organic light emitting diodes (OLEDs). We optimized the molecular structures of emitting materials based on first-principles calculations, resulting in a remarkable improvement in the maximum external quantum efficiency (EQE) of the undoped device from 2.0% to 4.99%. By doping the emitters in suitable host material, 4,4'-bis(carbazol-9-yl)biphenyl (CBP), at the optimal doping concentration, deep blue emission with extremely high maximum EQE of 10.7% and CIE coordinates of (0.151, 0.088) were achieved. The high EQE far exceeds the traditional upper limit of 5% if the singlet generation fraction in fluorescent OLEDs is 25%. A survey of the literature published in the last two decades indicates that singlet generation fraction can be higher than 25%. Using a model developed from Fermi's golden rule, we found that singlet generation fraction of most of the organic emitters for OLEDs is in the range of 40%--70%. The successful materials design for OLEDs suggests that we can prescreen and optimize molecular structures before synthesis in the lab, to avoid iterative and costly Edisonian approaches. |
