Organic devices are generally processed via solution approaches, e.g., roll-to-roll printing and inkjet printing. In this respect, what directly determines the final electrical properties is the microstructure formed from solution. The affinity of solvents and evaporation rates are important factors influencing the final structure. Meanwhile, the structure is not always stable and may undergo multiple transitions. Recently, a new AABB type thiophene copolymer, poly[5,5′-bis(2-butyloctyl)-

(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT), was synthesized and had shown superior electric properties in organic solar cell applications. A systematic investigation on the structural transitions of solution-cast PDCBT film was carried out by a team from the Institute of Chemistry, the Chinese Academy of Sciences (ICCAS). The research was published on November 2016 in Macromolecules.

The team studied the structure transitions by multiple techniques. Thermal transition temperatures were determined, including the glass transition temperature (T_g), melting (T_m), and crystallization (T_c) temperatures. The structural evolution in the crystalline region was measured by In-situ WAXS at the beamline 1W2A at the Beijing Synchrotron Radiation Facility (BSRF). The (100) diffraction peak became sharper and more intense in the heating process, suggested that the material reorganized into a more ordered structure along the side chain direction. The structure reorganization has a nontrivial influence on the power conversion efficiency (PCE) and fill factor (FF) of polymer solar cells.

Molecular structure, DSC curves and DMA curves of PDCBT solution cast film.

Article: Haiming Chen, Guoming Liu^*, Yunpeng Qin, Alejandro J. Müller, Jianhui Hou, Dujin Wang^* Structural Transitions in Solution Cast Films of a New AABB Type Thiophene Copolymer. Macromolecules 49(2016), 8653？8660.