On the Role of Spin States in Donor-Acceptor based Solar Cells and Light Emitting Diodes
Prof. Vladimir Dyakonov, Universität Würzburg
By tuning the energy levels of donors and acceptors it is possible to increase the power conversion efficiency in OPV devices. However, this opens up a new loss pathway from an interfacial charge transfer state (CTS) to a triplet exciton (TE) state, the so-called electron back transfer (EBT), which can be detrimental to device performance. To verify this, we study TE formation in the high performing polymer-fullerene and soluble small molecule-fullerene blend systems and determine the impact of the morphology-optimizing additives. Using spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs in pristine materials from via intersystem crossing (ISC), whereas in donor-acceptor blends via EBT mechanism from CTS. Using electrically detected spin resonance (EDMR), the presence of these TEs is confirmed even at room temperature, highlighting that TEs form during solar cell operation and influence the photocurrent and photovoltage. Surprisingly, the superior performing blends are found to have the largest triplet population. It is concluded, that the formation of triplets has no crucial impact on device performance in organic solar cells. 
Similar spin sensitive measurements were performed on Thermally Activated Delayed Fluorescence (TADF) light emitting devices, exploiting the mechanism of up-conversion of non-radiative triplet- to radiative singlet states in fluorescent donor-acceptor blends. We found a direct contribution of weakly bound exciplex states to electroluminescence being of reverse intersystem crossing (RISC) type. This process is strongly temperature dependent and we observed a crossover from recombination of weakly bound exciplex states to strongly bound TEs, i.e. from RISC to ISC mechanisms, by lowering the temperature and may thus exclude a direct contribution of triplet excitons to TADF. Our approach also allows the accurate determination of the singlet-triplet energy gap DEST for different material systems, which is the essential parameter governing device efficiency. 
1. a) H. Kraus, et al., Sci. Reps. 6, 29158 (2016); b) S. Väth, et al., Adv. Energy Mater. (2016). DOI: 10.1002/aenm.201602016.
2. S. Väth, et al., Adv. Optical Mater. (2016). DOI: 10.1002/adom.201600926.