Synthesis of Thieno[2, 3-b] thiophene-diketopyrrolopyrrole-based Polymers for Application in Bulk-heterojunction-type Solar Cells

Thieno [2, 3-b] thiophene-diketopyrrolopyrrole ( DPPTT,), a novel cross-conjugated electrondeficient unit, has been synthesized. This unit was brominated and then copolymerized with 2, 5-bis ( trimethylstannyl) thiophene, 2, 5 -bis ( trimethylstannyl) selenophene and 2, 5-his ( trimethyl-stannyl) -N methylpyrrole respectively, via Stille polymerzation, to form three donor-acceptor ( D-A) type copolymers, poly 2,5-his ( 2-octyldodecyl) -3-[5 -( thiophen-2, 5-y1) thieno [2, 3-h] thiophen-2-yl] -6-( thieno [2, 3-h] thiophen-2,5-y1) pyrrolo [3, 4-c] pyrrole-1, 4 ( 2H,5H) -dione1 ( pDPPTTZ-T), poly I 2,5-his ( 2 -octyldodecyl) 3 -[5 -( selenophen-2, -yl) thieno [2, 3 -b] thiophen-2 -y1] -6-( thieno [2, 3 -b] thiophen-2, -yl) pyrrolo [3, 4 -c pyrrole-1,4 ( 2H, 5H) -dionel ( pDPPTTZ-Se) and poly I 2, 5 -bis ( 2 -octyldodecyl) -3 -[5 -( N-methylpyrrole-2, 5 -yl) thieno [2, 3 -b] thiophen-2-y1] -6-( thieno [2, 3 -b] thiophen-2, 5 -y1) pyrrolo [3,] pyrrole-i,4 ( 2H, 5H) dione1 ( pDPPTTZ-Py). These polymers exhibit deep HOMO energy levels ( 5. 26 5. 37 eV),leading to high open-circuit voltage ( 0. 68 0. 81 V) in OPV devices, and good stability of the devices towards atmospheric oxygen. The UV-Vis spectra show broad features with long-wavelength absorption maximum around 620 nm and optical band gap of 1. 86, 1. 83 and 1. 85 eV for pDPPTTZ-T, pDPPTTZ-Se and pDPPTTZ-Py respectively, which give polymers a good overlap of solar spectrum. Bulk heterojunction solar cells were fabricated by blending these polymers with [6, 6] -butiric acid methyl ester ( PC.7, MB), and the power conversion efficiency ( PCE) reaches 3. 05% for the as cast pDPPTTZ-T: PCBM-based device without optimization, under simulated AM 1. 5 G irradiation of 100 mW/cm2.