Key parameters for device performance are derived from excited-state properties of crystalline organic semiconductors. The optical gap, which corresponds to the lowest singlet exciton energy, is of fundamental importance for any device based on absorption or emission of light. The triplet exciton energy is important for devices that involve conversion between singlet and triplet states. To increase the efficiency of OLEDs, it is desirable to convert electrically generated triplet excitons into emissive singlet excitons via thermally activated delayed fluorescence (TADF). In OPV, the thermalization loss may be reduced by converting high-energy sin-glet excitons into two triplet excitons, via singlet fission (SF). Conversely, the transmission loss of photons with energies below the optical gap of the absorber may be reduced by up-converting two low-energy triplet excitons into one singlet exciton via triplettriplet annihilation (TTA). TTA also plays a crucial role in the detection of X-rays, gamma rays, and neutrons in scintillators. Another important parameter for organic devices is the exciton binding energy, which is the electrostatic attraction between the electron and hole that needs to be overcome in order to separate an exciton into free charge carriers. The band lineup e.g., between the donor and acceptor in organic solar cells, is often engineered to overcome the exciton binding energy and provide the driving force for charge separation.