All-conjugated block copolymers for efficient and stable organic solar cells with low temperature processing

The low embodied energy within Organic Photovoltaics (OPVs) provides the technology with a characteristic that surpasses all other PV materials. In this work, all-conjugated block copolymers comprising of P3HT and PTB7-Th have been synthesized which enable even lower temperature processibility, thus reducing the embodied energy further. The all-conjugated block copolymers comprise of P3HT and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th). To synthesis these, a narrow-distributed, monobrominated P3HT (Mn = 7000, Mw/Mn = 1.31) is synthesized by Grignard metathesis polymerisation. This is further reacted with distannyl and dibromo monomers of PTB7-Th by Stille step-growth polycondensation to provide the block copolymers of P3HT-b-PTB7-Th. In these reactions, block ratios are adjusted to 1 to 2 and 1 to 10 based on the numbers of the repeating units of the monomers (i.e. 3-hexylthiophene unit: two monomers of PTB7-Th = 1:2 and 1:10). The block copolymer showed hole mobility of 5.9 × 10−5 cm2/Vs. The highest power conversion efficiency of 3.6%, which was achieved with the photoactive layer processed at 60 °C, which is substantially lower than the annealing temperature needed for standard P3HT-based solar cells. Furthermore, the stabilised lifetime of encapsulated devices is enhanced compared to P3HT and PTB7-Th devices, with no drop in efficiency noted for 7 days after initial burn in process.