Measuring efficiency losses in quantum dot polymer solar cells

The interfacial charge transfer recombination processes under working conditions that limit the device performance in quantum dot/polymer bulk heterojunction solar cells have been investigated. We have employed spectroscopic techniques as charge extraction (CE), laser transient absorption spectroscopy (L-TAS) and transient photovoltage (TPV). From the CE technique, we have observed an exponential charge accumulation in the device when light intensity is increased, as it has been related to conventional molecular photovoltaic devices. The L-TAS showed that recombination dynamics, in thin films, are dominated by the presence of trap in both, the nanocrystals and the polymer materials. Finally, from the TPV experiments we observed a strong carrier lifetime, in complete devices, upon charge density. This strong dependency of charge concentration vs charge lifetime has been found to be higher than in organic photovoltaics based on polymer and fullerenes. The comparison between the recombination dynamics from TPV and LTAS, performed with and without electrodes, respectively, allow us to exclude any process related with the metal contacts in the strong dependency of the recombination with the charge density.