Recent progress in X-ray scattering techniques have provides unique insights into the characterization of polymers in energy applications of photovoltaics and photodiodes.  We presented using grazing incidence wide-/small angle X-ray scattering (GIWAXS/GISAXS) to study the bulk-heterojunction morphology of active layers in high-efficient organic photovoltaics and photodiodes. The active layer is mainly composed of donor–acceptor type conjugated polymer and n-type fullerene (or non-fullerene) small molecules. The GIWAXS is sensitive to the polymer’s crystalline and can determine the crystal structure and the orientation of the crystalline regions. By using GISAXS, the aggregation cluster of conjugated polymer/small molecules can be detected to a length scale range from nanometers to several hundred nanometers.