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_Z), 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-bis (trimethyl-stannyl)-N-methylpyrrole respectively, via Stille polymerzation, to form three donor-acceptor (D-A) type copolymers, poly {2,5-bis (2-octyldodecyl)-3-[5-(thiophen-2, 5-yl) thieno [2, 3-6] thiophen-2-yl] - 6-(thieno [2, 3-6] thiophen-2,5-yl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione} (pDPPTTZ-T),poly j 2,5-bis(2-octyldodecyl) - 3-[5-(selenophen-2,5-yl) thieno [2,3-6] thiophen-2-yl]-6-(thieno [2,3-b] thiophen-2,5-yl) pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione} (pDPPTTZ-Se) and poly 12,5-bis(2-octyldodecyl)-3-[5-(N-methylpyrrole-2, 5-yl) thieno[2,3-6] thiophen-2-yl]-6-(thieno[2,3-6] thiophen-2,5-yl) pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione} (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]-phenyl-C₇₁-butiric acid methyl ester (PC₇₁ 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.