Photoreactive low-bandgap 4H-cyclopenta[2,1-b:3,4-b′]dithiophene and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-based alternating copolymer for polymer solar cell

Polymer solar cells (PSCs) are often fabricated using a well-known 4H-cyclopenta[2,1-b:3,4-b′]dithiophene and 4,7-di(thiophen-2-yl)benzo[c] [1,2,5]thiadiazole-based polymer as a low-bandgap polymer. Further, PSCs are also fabricated by mixing methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PC₆₁BM) with the polymer. We prepared poly[4,4-bis(2- ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-4, 7-bis(2-thienyl)-2,1,3-benzothiadiazole-5′,5′′-diyl] after anchoring the penta-1,4-diene in the side chain of the cyclopentadithiophene and investigated the long-term performance stability of a photocrosslinked bulk heterojunction (BHJ) photovoltaic (PV) cell based on the polymer. The photocrosslinking reaction was monitored via infrared spectroscopy without the use of a photoinitiator, by carrying out a spontaneous radical coupling reaction. The polymer film has a broad absorption band extending from 300 to 850 nm with an optical bandgap as low as 1.52 eV. The polymer was employed to fabricate a PSC with PC₆₁BM. The resultant PSC device, which was treated by UV irradiation (λ = 254 nm, I = 40 mW/cm², 5 min), had good preliminary results with an open-circuit voltage of 0.62 V, a short-circuit current density of -5.37 mA/cm², a fill factor of 0.41, and an overall power conversion efficiency of 1.37%. All parameters of the UV-cured PSC device were more stable over the course of 300 h than those of P3HT-PC₆₁BM devices, indicating the long-term stability of the polymer.