Add time:08/11/2019         Source:sciencedirect.com

This work emphasizes on the synthesis of two new acceptor units for donor (D)-acceptor (A)-based polymers used in photovoltaic devices, namely NT-Th and Th-NT-Th. Both the NT-based acceptors have been designed using 5,6-positions of benzothiadiazole (BT) and synthesized by Wittig coupling reaction. Newly synthesized acceptor units distinguishes itself from existing literature of naphtho[2,3-c][1,2,5]thiadiazole (NT) acceptor in having additional two Thiophene (Th) units attached at 6,7 position of NT, imparting into it a unique 2-dimensional (2D) type of conjugated structure. Alternate D-A co-polymer of new NT-based acceptors with commonly used BDT donor was synthesized via Stille coupling, resulting in new conjugated polymers (CPs) namely P(BDT-NTTh) and P(BDT-ThNTTh). Both the CPs were characterized by GPC, UV–vis, 1H-NMR, 13C-NMR, TGA and CV. Dihedral angle for optimized geometry and ICT of newly synthesized NT-based polymers have also been investigated using DFT. Theoretical calculations suggest that incorporating two Th-units, separates the donor and acceptor as in case of P(BDT-ThNTTh) leading to lowering of dihedral angle, with maximum value of 42.9˚ resulting in more planar geometry. PSC performance of synthesized CPs have been examined by fabricating BHJ solar cells with configuration ITO/PEDOT:PSS/Polymer-PC71BM/LiF/Al, which resulted in a PCE of 0.34% and 1.31% with P(BDT-NTTh) and P(BDT-ThNTTh) respectively. Hole-mobility for blend of P(BDT-NTTh) and P(BDT-ThNTTh) with PC71BM were calculated using SCLC (space charge limited current) method by fabricating hole only devices of configuration ITO/PEDOT:PSS/Polymer-PC71BM/Cu and mobility of 2.2 × 10−6 and 1.5 × 10-5 cm2 V-1 s-1 were established respectively. Morphology of active layer of fabricated PSC has been studied with atomic force microscopy (AFM) and is correlated with their performances.

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