In Situ Real Time Characterization Of Micro-structure Change In The Thin Film Of SMDPPEH During The Process Of Annealing

With the development of organic solar cells, new materials are constantly emerging, a solution-processable conjugated small molecules materials are received full attention because of their molecule weight well-fixed and molecule structures, simple purification and batch-to-batch reproducibility, which are better than polymer materials. A small molecule derivatives of diketopyrrolopyrrole(DPP)-containing oligothiophene —SMDPPEH, with a conjugated planar conjugated bicyclic structure, high charge carrier mobilities and the broadness of absorption in visible region characteristics,which has become a kind of promising organic semiconductor materials. Annealing treatment on thin films has been proved to be an efficient method to improve power conversion efficiency on organic solar cell.In this paper, we characterized the films of SMDPPEH and blended films of SMDPPEH with PCBM in the process of annealing to study the variation of crystal structure and morphology. The in situ thermal annealing of UV-Vis spectroscopy shown that the second and the third absorption maximum(λmax) was blued shifted with intensities increased with annealing higher than 100 ℃. The results of 1D out-of-plan grazing-incidence X-ray diffraction of SMDPPEH thin films shown that a phase transition around 100℃, and the crystallinity of the films of SMDPPEH and SMDPPEH blended with PCBM were in creasing by the increase of annealing temperature. While the crystal orientations of SMDPPEH were changed. The crystallinity of blended films can be improved by this phase transition of SMDPPEH, which is a reason for the improvement of power conversion efficiency of diketopyrrolopyrrole(DPP)-based bulk heterojunction solar cells of a blend of SMDPPEH and PCBM. The blended thin films shown a large-size phase segregation when annealing up to a certain temperature, which presumably is the primary reason for the decrease of the PCEs in device performance. This paper also studied the morphology of SMDPPEH in the process of melting and recrystallization. And we found that three different stereoisomers of SMDPPEH can be formed different morphologies and structures and the order of appearance of the crystallization.