Control of interface order by inverse quasi-epitaxial growth of squaraine/fullerene thin film photovoltaics

It has been proposed that interface morphology affects the recombination rate for electrons and holes at donor–acceptor heterojunctions in thin film organic photovoltaic cells. The optimal morphology is one where there is disorder at the heterointerface and order in the bulk of the thin films, maximizing both the short circuit current and open circuit voltage. We show that an amorphous, buried functionalized molecular squaraine donor layer can undergo an “inverted” quasi-epitaxial growth during postdeposition processing, whereby crystallization is seeded by a subsequently deposited self-assembled nanocrystalline acceptor C60 cap layer. We call this apparently unprecedented growth process from a buried interface “inverse quasi-epitaxy” where the crystallites of these “soft” van der Waals bonded materials are only approximately aligned to those of the cap. The resulting crystalline interface hastens charge recombination, thereby reducing the open circuit voltage in an organic photovoltaic cell. The lattice ...