1, University of Kentucky, Lexington, Kentucky, United States
3, Cornell University, Ithaca, New York, United States
In organic photovoltaics, fluorination of electron donor molecules or polymers at appropriate positions can lead to an improvement in photovoltaic (PV) device efficiency. Fluorination can change the energetics at the donor-acceptor interface and the morphology of the organic semiconductor blend, which can result in changes in charge separation efficiency, reduced charge-carrier recombination, and increased PV performance. Using ultraviolet photoemission spectroscopy (UPS) measurements of interfacial energetics and external quantum efficiency (EQE) measurements of charge-transfer (CT) states, we have investigated the effect of halogenation on the energetics of model anthradithiophene (ADT) compounds. We investigated bilayer ADT/fullerene C60 interfaces and ADT:C60 blends to emulate interfaces between pure donor-acceptor phases and mixed phases found in typical bulk-heterojunction photovoltaics. Non-halogenated ADT shows a lower energy CT state in ADT/C60 bilayers and higher energy CT states in ADT:C60 blends, while halogenated ADT shows the opposite trend. Surprisingly, in blend systems of halogenated ADTs with C60, the CT states display lower energies as compared to the bilayer system. In bulk-heterojunction photovoltaics, the lower energy CT states in the mixed phase for the halogenated derivatives will likely decrease the probability of charge separation and increase charge-carrier recombination. These CT energies combined with UPS measurements of the interfacial energy landscapes suggest that the beneficial effects often observed upon fluorination are not likely due to the affect of fluorination on energetics at the D-A interfaces.