Abstract
Two narrow band-gap copolymers consisting of 2,7-bis(5-(trimethylstannyl)thiophen-2-yl)-4,5,9,10-tetrakis(2-ethylhexyloxy)-pyrene (M1) as an electron-rich unit and diketopyrrolopyrrole (DPP) as an electron-deficient unit have been synthesized and characterized for polymer solar cells. The two polymers were prepared by Stille coupling reactions. Two solubilizing alkyl chains (ethylhexyl and octlydodecyl) were attached to the DPP unit in order to evaluate their impact upon the optical and electrochemical characteristics of the two polymers. Poly[4,5,9,10-tetrakis[(2-ethylhexyl)oxy]pyrene-alt-3,6-bis(thiophen-2-yl)-2,5- bis(2-octyldodecyl)pyrrole[3,4-c]pyrrole- 1,4(2H,5H)-dione] (PP(EH)DT-DPP(ODo) ) and poly[4,5,9,10-tetrakis((2-ethylhexyl)oxy)pyren-alt-3,6-bis(thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrole[3,4-c]pyrrole-1,4(2H,5H)-dione] (PP(EH)DT-DPP(EH) ) exhibited high thermal stability with decomposition temperatures over 300 °C. Optical properties showed that PP(EH)DT-DPP(ODo) and PP(EH)DT-DPP(EH) have optical band gaps of around 1.40 eV. It is believed that both polymers adopt high planar structures in the thin film, leading to more electronic conjugation along the backbone of the conjugated polymers. Powder X-ray diffraction revealed that PP(EH)DT-DPP(ODo) and PP(EH)DT-DPP(EH) seem to have an amorphous nature. The HOMO energy levels of the two polymers are clearly affected by changing alkyl chains. The HOMO levels of PP(EH)DT-DPP(ODo) and PP(EH)DT-DPP(EH) were found to be at -5.27 and -5.38 eV, respectively. PP(EH)DT-DPP(ODo) showed a HOMO energy level approximately 0.11 eV shallower than that of PP(EH)DT-DPP(EH) , which is probably a consequence of attaching a larger alkyl chain to the DPP moiety reducing its electron accepting ability.