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

Abstract

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 (ΔEF). 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 ΔEF 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 ≤ ns whereas for free charge carriers it is in the µs‐range—this is achieved by time‐resolved PL measurements. For highly efficient organic solar cells, it is found that ΔEF determined from these PL transients matches perfectly with the electrical voltage. Moreover, the new method also allows for the determination of ΔEF from mere absorber films without electrodes and thus paves the way for a better understanding of organic solar cells.

Nanoscale spatial and time-resolved mapping in perovskite and organic solar cells: a multimodal technique to visualize the photoinduced charge dynamics

“…Photoluminescence (PL) spectroscopy provides information about the radiative recombination processes and the efficiency of charge carrier generation in active materials. 18–20…”

“…Transient absorption spectroscopy (TAS) allows the study of excited states and charge carrier dynamics by monitoring changes in the absorption of the material following photoexcitation. 20–22 Some recently deployed techniques for evaluating multiple nanoscale properties simultaneously in OSC and PSC devices are termed multimodal techniques. 23,24 Such techniques include transmission electron microscopy (TEM) 25,26 and photoinduced force microscopy (PiFM), 27–30 etc.…”