Thermochromic and Photovoltaic Properties of an Alternating Copolymer of Dithieno[3,2-b :2′,3′-d ]thiophene and Thieno[3,4-c ]pyrrole-4,6-dione

The synthesis of a new alternating conjugated polymer, PDTTTPD, based on electron-donating dithieno[3,2-b:2′,3′-d]thiophene (DTT) and electron-withdrawing thieno[3,4-c]pyrrole-4,6-dione (TPD) units is reported. This polymer shows strong thermochromic effect in chlorobenzene with a peak shift up to 170 nm. This phenomenon is studied with fluorescence spectroscopy and related to the steric hindrance along the polymer main chain. Polymer solar cells are fabricated from its blends with PC71BM, and shows modest power conversion efficiency up to 2.1%. The low efficiency is due to the low short-circuit current, which is also attributed to the steric effect. Based on these results, a general design rule for polymer photovoltaic material with controlled backbone conformation is proposed. A new alternating conjugated polymer, PDTTTPD, based on electron-rich dithieno[3,2-b:2′,3′-d] thiophene (DTT) and electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) building blocks is designed and synthesized. PDTTTPD shows strong thermochromic effect in chlorobenzene solution and modest PCE of 2.1% in organic solar cell devices due to the steric hindrance induced twisting of the conjugated main chain. A general design rule is then suggested to control the main chain conformation of polymer materials for photovoltaic applications. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.