Low band gap poly-thienopyrazines for solar cells—Introducing the 11-thia-9,13-diaza-cyclopenta[b]triphenylenes

The chemistry of the thienopyrazines has been explored with the aim of producing new low band gap polymers. 5,7-Di-(thiophen-2-yl)-thieno[3,4-b]pyrazines substituted in the pyrazine ring with alkyl groups, aryl groups and fused aromatic rings have been prepared and characterized. The electronic spectra show a great variation in the longest wavelength absorption band as a consequence of this substitution. A special case is the 11-thia-9,13-diaza-cyclopenta[b]triphenylene prepared by condensation of 3′,4′-diamino-[2,2′,5′,2″]terthiophene with phenanthrene-9,10-quinone. Alkyl substitution of the most promising monomers were carried out using the Kumada coupling and these were copolymerized with either 2,5-bis(trimethylstannyl)thiophene or 3-(3,7,11-trimethyl-dodecyl)-2,5-bis-trimethylstannyl-thiophene to form six new low band gap polymers: RISO-GREEN 1–3 and RISO-BROWN 1–3. The band gaps of these polymers were estimated from the UV–visible absorption spectra and found to be ca. 1.3 eV. Preliminary results from photovoltaic device fabrication with mixtures of the six polymers with either [60]PCBM or [70]PCBM gave modest efficiencies of max 0.2% with open circuit voltages Voc of 0.3 V and short circuit currents Jsc (1000 Wm−2 AM1.5) in the range of 2 mA cm−2.