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 V-oc of 0.3 V and short circuit currents J(sc) (1000 Wm(-2) AM 1.5) in the range of 2mA cm(-2). (c) 2007 Elsevier B.V. All rights reserved.