Organic semiconductors are a promising class of materials for the next generation of light emitting diodes (OLED). Our research on electronic excitations in perfectly ordered molecular single crystals, crystalline thin films and even single molecules is completed by studies on disordered molecular donor-acceptor systems based on so-called thermally activated delayed fluorescence (TADF), in which non-emissive triplet states are converted to emissive singlet states. Operational principles of these OLEDs are based on the spin properties of triplet excitons and charge transfer states that are optically or electrically generated and thus can influence the radiative recombination efficiency. In other words, spin is a crucial parameter controlling the physical limits of device quantum efficiency. Therefore, we apply novel electrically and optically based electron spin resonance (ESR) techniques to control the spin state and to study its influence on the device performance in operando. Understanding these fundamental OLED processes is also essential for the practical realization of novel PV device concepts, such as up- and down-conversion (triplet fusion, singlet fission), which may redefine the physical limits for organic optoelectronics and photovoltaics in future.