An all‐polymer blend for photovoltaic applications: A combined photoabsorption, photoemission and ab initio multi‐configurational study

We have investigated the electronic structure and morphology of thin films of the electron‐donor poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexylthio)‐4‐fluorothiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b']dithiophene))‐ alt ‐(5,5(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PBDB‐T‐SF), the non‐fullerene acceptor (NFA) poly[[1,2,3,6,7,8‐hexahydro‐2,7‐bis(2‐octyldodecyl)‐1,3,6,8‐dioxobenzo[ lmn ][3,8]phenanthroline‐4,9‐diyl][2,2′‐bithiophene]‐5,5′‐diyl] (P(NDI2OD‐T2)) and their blend. Angle‐resolved near‐edge X‐ray absorption fine structure (NEXAFS) in total electron yield detection mode was employed to probe the C 1s, N 1s and S 2p absorption edges. The results indicate a preferential face‐on orientation of the naphthalene diimide unit in the acceptor, and an edge‐on orientation for the thiophene rings. A preferential edge‐on orientation was also observed for the electron‐donor polymer. Analysis of the blend's NEXAFS spectra suggests an accumulation of the NFA component on the surface of the film. The investigated films were also characterized by X‐ray photoelectron spectroscopy (XPS), attesting the presence of thiophene in all the samples and imide in the films containing the NFA polymer. The XPS results further indicate that the acceptor material is accumulated in the surface of the blend film. Finally, the analysis was complemented by ab initio calculations at the multi‐configurational level for transitions at C 1s, N 1s and S 2p absorption edges, with spin–orbit effects included for the latter.