A strategy for increasing the conversion efficiency of organicphotovoltaics has been to increase the voltage by tuning the energylevels of donor and acceptor components. However, while increasing thepower conversion efficiency, this opens up a new loss pathway frominterfacial charge transfer (CT) states to energetically lower-lyingtriplet excitons (TE), which is detrimental to device performance andfurthermore a precursor to device degradation. To test thishypothesis, we study triplet formation in high performing solar cellson the basis of various polymer and small-molecule donors incombination with fullerene derivative acceptors and next generationso-called non-fullerene acceptors (NFA). Using photo- andelectroluminescence in combination with spin-sensitive magneticresonance techniques, we find that fullerene containing solar cellsmay form triplet excitons either via intersystem crossing (ISC) fromsinglet excitons or via electron back transfer from CTstates. Remarkably, in state-of-the-art NFA- based solar cells, thisloss mechanism is absent, which is in line with their superiorefficiency and stability. Similar spin sensitive measurements were performed on donor:acceptor OLEDs, exploiting the mechanism of thermally activated delayed fluorescence (TADF). Here, non-radiative triplets are up-converted to radiative singlets to boost efficiency. We found a direct contribution of interfacial triplet exciplex states in both electrically driven OLEDs and optically excited films. Under optical excitation, however, additional localized molecular TEs can be observed, that are absent in electrically driven OLEDs. Hence, we conclude that only the combination of optical and device-based experiments delivers a comprehensive picture of molecular photophysics.