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 useful as it provides direct information about the free charge carrier densities and the separation of the quasi-Fermi levels ΔEF. However, for organic solar cells the PL is strongly dominated by photogenerated excitons. To overcome this severe limitation, a new approach was developed which allows, to the best of our knowledge for the first time, disentangling the luminescence stemming from photogenerated excitons from the one originating from free charge carriers. Due to the large difference in the respective lifetimes - for photogenerated excitons it is ≤ ns whereas for free charge carriers it is in the μs-range - this is achieved by time-resolved PL measurements. It is found that ΔEF determined from these PL transients matches perfectly with the electrical voltage of the solar cells. Moreover, our new method also allows the determination of ΔEF from mere absorber films without electrodes.