The structure and electronic properties of polyhedral oligomeric silsesquioxane (POSS) cages functionalized with different organic groups have been studied using density functional theory and time-dependent density functional theory calculations. Accordingly, the POSS-T8 cage is quite rigid upon functionalization and thus provides a means for spatially separating conjugated organic fragments, which is useful for the realization of novel organic molecular architectures for light-emitting diodes. Moreover, electronic properties can be tuned through the choice of functional groups and their positioning on or within the POSS cage. Attaching an electron-donating group, such as 4-carbazolephenyl, to the silicon atom at the corner of the cage raises the HOMO level, while attaching an electron-withdrawing group, such as 4-cyanophenyl, or inserting an inert molecule, such as N(2), into the POSS cage lowers the LUMO level. Frontier orbital analysis indicates that the POSS cage is partially conjugated and serves a role as electron acceptor. Carrier transport rates are discussed in the frame of Marcus' electron hopping theory. On the basis of the calculated reorganization energies, these POSS compounds can be used as carrier transporting or blocking materials, depending on the functionalization. Exciton binding energies strongly depend on the spatial arrangement of frontier orbitals rather than on molecular sizes.