Investigation of fullerene concentration profile in polymer based solar cell by using magnetic contrast neutron reflectivity

Poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b] thiophene) (pBTTT) has recently caused great interest as the electron donor in organic photovoltaics. It was demonstrated that fullerene molecules intercalate between side-chains of this semiconducting polymer creating a stable crystalline structure, so, a large concentration of fullerene must be used relative to the polymer to promote phase separated electron conductive pathways. We used state-of-the-art neutron reflectivity methods, with the application of magnetic contrast variation, to investigate the concentration profile of [6,6]-phenyl-C$_{71}$- butyric acid methyl ester(PC$_{71}$BM) in order to understand the internal structure within the active layer. The PC$_{71}$BM concentration profile greatly depends on the weight ratio of polymer to fullerene. XRD and other data are also used to show the morphology change of active layer.