Structural, optical, and morphological properties of thin films of PTB7-Th donor material at different spin speeds for organic solar cells

In this work, the morphology of the donor polymer Poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3- b ]dithiophene]{3-fluoro-2[(2 ethylhexyl)carbonyl]thieno[3,4- b ]thiophenediyl(PTB7-Th) was systematically studied at both solution and by preparing a thin film at different spin speeds. The thin films were prepared at 600, 800, 1500, and 3000 rotation per minutes (rpm) using spin coater, with approximate thicknesses of 85, 23, 14, and 11 nm, respectively, as measured by a surface profilometer. Transmission electron microscopy (TEM) was employed to examine the nanoscale morphology of PTB7-Th by dissolving it in chlorobenzene. Atomic force microscopy (AFM) was used to analyze the surface morphology of thin films of PTB7-Th coated on a glass substrate at different spin speeds. Furthermore, the optical and structural properties were investigated using UV–visible spectroscopy, photoluminescence (PL), and Raman spectroscopy to understand molecular interactions and film quality as a function of spin speed. Using these microscopy and spectroscopy techniques, it is inferred that the film prepared at 1500 rpm (thickness ∼ 14 nm) exhibited improved morphology, which facilitates better charge transport. Finally, an organic solar cell device using PTB7-Th and PCBM is fabricated by coating the active layer of the cell at different spin speeds and the device prepared at 1500 rpm exhibits the highest power conversion efficiency (PCE) among all the spin speeds.