New paths to efficient organic solar cells

Currently, the performance of OPV blend films — binary mixtures of electron-donor and electron-acceptor materials — has reached approximately 10 %. Several loss mechanisms that prevent the extraction of the photogenerated charges account for their low power conversion efficiency. These are mainly attributed to the unfavourable nanomorphology of the OPV blends that instead of promoting free-charge extraction favour free-charge recombination.

Scientists initiated the EU-funded project 'Delayed luminescence spectroscopy of organic photovoltaic systems' (DELUMOPV) to provide further insight into the processes that currently limit the power conversion efficiency of solution-processed organic OPV devices. To this end, focus was placed on developing a methodology for tuning the layer nanomorphology to favour charge extraction and increase photocurrent generation. Furthermore, DELUMOPV sought to identify alternative electron acceptors to the presently used expensive fullerene derivatives.

Scientists confirmed that the delayed luminescence intensity of delayed excimer (delplex) states in the microsecond timescale is intertwined with the photocurrent generation efficiency of OPV devices. In particular, the decay dynamics of the delayed luminescence of the charge-transfer states provided useful information regarding the charge transport of the photoactive layers under investigation. Results from the electric field–induced photoluminescence quenching experiments showed that free-charge recombination and trapped charges in OPV devices account for the delayed charge-transfer luminescence.

Scientists designed a state-of-the-art spectroscopic rig to study delayed luminescence from different material combinations. This involved using several non-fullerene–based electron-acceptor materials and polymer electron-donor matrices. Through microscopic techniques, scientists observed a previously unidentified memory effect in OPV polymeric composites. The use of a thin interlayer in OPV devices made of a polymer photoactive layer helped in tuning its morphology to ultimately optimise charge extraction.

DELUMOPV activities aimed to maintain the EU's competitive advantage in the organic electronics and OPV fields. Project findings are expected to serve as a basis for comparisons between fullerene and non-fullerene–based OPV systems. This should provide valuable feedback to scientists, enabling the design of next-generation molecular structures that will combine the advantageous characteristics of both fullerenes and other polymers.