Determination of Free Charge Carrier Luminescence and Quasi-Fermi Level Separation in Organic Solar Cells via Transient Photoluminescence Measurements

The detection of photoluminescence (PL) is extremely valuable for photovoltaic technologies as it provides direct information about the free charge carrier densities and therefore about the separation of the quasi-Fermi levels (& UDelta;E-F). However, for organic solar cells the PL is strongly dominated by photogenerated excitons that in part decay radiatively before they form free charge carriers via dissociation at a donor/acceptor interface. For this reason, it is impossible to deduce the free charge carrier densities and & UDelta;E-F directly from the PL signal. To overcome this severe limitation, a new approach is developed that allows disentangling the luminescence of photogenerated excitons from the one of free charge carriers. Due to the large difference in respective lifetimes-for photogenerated excitons it is & LE; ns whereas for free charge carriers it is in the & mu;s-range-this is achieved by time-resolved PL measurements. For highly efficient organic solar cells, it is found that & UDelta;E-F determined from these PL transients matches perfectly with the electrical voltage. Moreover, the new method also allows for the determination of & UDelta;E-F from mere absorber films without electrodes and thus paves the way for a better understanding of organic solar cells.