Correlating structure and morphology to device performance of molecular organic donor-acceptor photovoltaic cells based on diindenoperylene (DIP) and C₆₀

The performance of organic photovoltaic cells (OPVCs) shows a critical dependence on morphology and structure of the active layers. In small molecule donor/acceptor (D/A) cells fabrication parameters, like substrate temperature and evaporation rate, play a significant role for crystallization and roughening of the film. In particular, the fraction of mixed material at the interface between donor and acceptor is highly relevant for device performance. While an ideal planar heterojunction (PHJ) exhibits the smallest possible interface area resulting in suppressed recombination losses, mixed layers suffer strongly from recombination but show higher exciton dissociation efficiencies. In this study we investigate PHJ and planar-mixed heterojunction (PM-HJ) solar cells based on diindenoperylene (DIP) as donor and C₆₀ as acceptor, fabricated under different growth conditions. Grazing incidence small angle X-ray scattering (GISAXS), X-ray reflectometry (XRR) and atomic force microscopy (AFM) are used to obtain detailed information about in- and out-of-plane structures and topography. In that way we find that surface and bulk domain distances are correlated in size for PHJs, while PM-HJs show no correlation at all. The resulting solar cell characteristics are strongly affected by the morphology, as reorganizations in structure correlate with changes in the solar cell performance. The performance of small molecule organic photovoltaic cells shows a critical dependence on morphology and structure of the active layers. It is shown that fabrication parameters, such as substrate temperature and device architecture, play a significant role for crystallization and roughening of the film and that these features are related to characteristic parameters of planar and planar-mixed heterojunction donor/acceptor cells under operation.