Competitive role between conformational lock and steric hindrance in D-A copolymers containing 1,4-bis(thieno[3,2-b]thiophen-2-yl)benzene unit

We report the design and synthesis of two donor-acceptor (D-A) conjugated polymers based on a 2,2'-(2,5-dialkoxy-1,4-phenylene)dithieno [3,2-b] thiophene (DTTP) donor subunit having highly planar confirmation via S center dot center dot center dot O noncovalent bonding and electron-rich property. Introducing the DTTP subunit improves the interchain ordering and of resultant polymers, without losing solution processability. In order to promote the charge separation from photo-induced excitons at D/A interfaces, we control the position of side chain and investigate their effect on D/A phase separation in bulk-heterojunction films. We have systematically investigated the effects of noncovalent bonding and side chain position on the chain planarity and interaction packing of polymers, bulkheterojunction thin film morphologies, and relevant photovoltaic devices. The thin film morphologies composed of conjugated polymers (PDTTP-2FBT or PDTTP-2FBTA) and with PC71BM are found to be critically dependent upon the side chain position and processing additive treatments. The resulting photovoltaic energy conversion efficiency of the polymer solar cells reached 6.79% and a comparative study on the physical, molecular modeling, and thin film characteristics was performed through DFT, GIXD, TPC, TPV, etc. The results obtained here provide an important guideline of tailoring the polymer chemical structure to optimize the molecular packing behavior and thin film morphology for high-performance organic electronics.