Ultrafast Dynamics Group
Frédéric Laquai's Group

Photo-generated carriers

Photo-generated carriers lose energy during extraction from polymer-fullerene solar cells

Photo-generated carriers lose energy during extraction from polymer-fullerene solar cells
Armantas Melianas, Fabian Etzold, Tom J. Savenije, Frédéric Laquai, Olle Inganäs, Martijn Kemerink
Nature Communications
 
Article number:
 
8778
doi:10.1038/ncomms9778
Armantas Melianas, Fabian Etzold, Tom J. Savenije, Frédéric Laquai, Olle Inganäs, Martijn Kemerink
Photo-generated Charges, Thermal Equilibrium, Thermalization, time-resolved optical spectroscopy
2015
In photovoltaic devices, the photo-generated charge carriers are typically assumed to be in thermal equilibrium with the lattice. In conventional materials, this assumption is experimentally justified as carrier thermalization completes before any significant carrier transport has occurred. Here, we demonstrate by unifying time-resolved optical and electrical experiments and Monte Carlo simulations over an exceptionally wide dynamic range that in the case of organic photovoltaic devices, this assumption is invalid. As the photo-generated carriers are transported to the electrodes, a substantial amount of their energy is lost by continuous thermalization in the disorder broadened density of states. Since thermalization occurs downward in energy, carrier motion is boosted by this process, leading to a time-dependent carrier mobility as confirmed by direct experiments. We identify the time and distance scales relevant for carrier extraction and show that the photo-generated carriers are extracted from the operating device before reaching thermal equilibrium.