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The impact of additives on the nanoscale structures of spin‐cast polymer composite films, particularly in polymer solar cells, is a topic of significant interest. This study focuses on the blend film comprising poly(thieno[3,4‐b]thiophene‐alt‐benzodithiophene) (PTB7) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM), exploring how additives like 1,8‐diiodooctane (DIO) influence the film structures spin‐cast from chlorobenzene solution. Combined results of specular X‐ray and neutron reflectivity, grazing‐incidence small‐ and wide‐angle X‐ray scattering (GISAXS and GIWAXS), and X‐ray photoelectron spectroscopy indicate that DIO could significantly enhance the dispersion of PC71BM and reduce composition inhomogeneity in the film. Time‐resolved GISAXS–GIWAXS with 100 ms resolution further captures a rapid spinodal decomposition of the mixture within 1 s in the constant‐evaporation stage of spin‐casting. Further combined with parallel analysis of time‐resolved UV–Vis reflectance, these findings reveal that DIO mitigates the spinodal decomposition process by accelerating solvent evaporation, which, in turn, decelerates phase segregation, leading to a nucleation‐driven process. These observations provide mechanistic insights into the role of additives in controlling the nanostructural evolution of spin‐cast films by altering the kinetics of solvent evaporation and phase separation during the spin‐coating process. [ABSTRACT FROM AUTHOR]

A kinetic analysis was formulated for electromigration enhanced intermetallic evolution of a Cu–Sn diffusion couple in the Sn-based Pb-free solder joints with Cu under bump metallurgy. The simulated diffusion couple comprised the two terminal phases, Cu and Sn, as well as the two intermetallic phases, Cu3Sn and Cu6Sn5, formed between them. The diffusion and electromigration parameters were obtained by solving the inverse problem of the electromigration enhanced intermetallic growth, and they were compatible with the literature values. Finite difference method was applied to the whole simulated domain to solve for the mass transport kinetics within the intermetallic phases and across each interface of interest. Simulation showed that, when electromigration effect was absent (zero current), intermetallic growth followed a parabolic law, suggesting a diffusion controlled mechanism for thermal aging. However, under significant current stressing (4×104 A/cm2), the growth of the dominant intermetallic Cu6Sn5 clearly followed a linear law, suggesting a reaction controlled mechanism for electromigration. Simulation results were consistent with the experimental observations. The analysis of vacancy transport was also incorporated with the model, and the results showed substantial increase in vacancy concentration at the Cu6Sn5 phase near the Cu3Sn/Cu6Sn5 interface. The peaking of the vacancy concentration explains the substantial Kirkendall void formation under electromigration at this region. [ABSTRACT FROM AUTHOR]