layers share a similar bulk heterojunction (BHJ) structure with a well-controlled level of phase separation, consisting of a conjugated polymer as the electron donor and another polymer or small molecule as the electron acceptor. [2] It is worth noting that the micro-and nanostructures of these organic semiconductors, as well as the resulting blend morphology and the subsequent device performance, are extremely sensitive to processing conditions. Such changes in microstructure and morphology naturally occur during the film formation from solution, since the final structure of a BHJ film is determined by the interplay between solutes and solvent, including solubility and miscibility of molecules, solvent evaporation, molecular packing, phase separation, etc. [3] Over the past decades, this has led to the efforts to measure the optical properties of these materials during processing, in order to monitor, understand, and finally manipulate and optimize the film formation process.Several methods are well established to investigate the morphology formation of polymer:fullerene BHJ films. The first attempts to monitor the drying kinetics of BHJ films were done using in situ white-light or laser reflectometry, which can measure the thickness and solid content variation during drying. [4] Later on, spectroscopic ellipsometry was introduced into the in situ studies, allowing the real-time studies of the optical constant, thin-film thickness, surface roughness, optical anisotropy, and compositional change. [5] In the meantime, in situ X-ray scattering and X-ray diffraction were used, either