Conjugated random copolymers consisting of pyridine- and thiophene-capped diketopyrrolopyrrole as co-electron accepting unit for efficient polymer solar cells

Abstract

One of the most successful approaches to achieve high power conversion efficiency (PCE) of polymer solar cells (PSCs) is to develop new alternating push-pull type copolymers, which consist of electron-rich (D) and electron-poor (A) unit in polymer backbone. Although intensive research efforts have been devoted to developing new D and A moieties, a few D-A alternating copolymers have shown high PCE. Random copolymers composed of one D unit and two different A units can be used as a promising donor material for high performance PSCs, if the absorptions of two electron accepting units are complementary to each other and therefore the resulting copolymer shows broad absorption. Both thiophene-capped (T) and pyridine-capped (Py) diketopyrrolopyrrole (DPP) have been used as electron accepting units for D-A type conjugated polymers for PSCs and OFETs: A low bandgap polymer (pTDPP2T) composed of TDPP and bithiophene (2T) shows high short circuit current due to its low bandgap, while the polymer composed of PyDPP and 2T exhibits high open circuit voltage (VOC) due to its low-lying HOMO energy level. In this work, a new series of conjugated random copolymer was synthesized by copolymerization of 2T (an electron donating unit) with TDPP and PyDPP (co-electron accepting units). The VOC of random copolymer can systematically be controlled by varying the feed ratio of PyDPP to TDPP for polymerization. The VOC was increased with increasing the PyDPP content in the random copolymer, since electron withdrawing power of pyridine is stronger than that of thiophene and thus lower the HOMO energy level: The HOMO energy level becomes deeper as the PyDPP content in the random copolymer is increased. Consequently, the solar cell device based on the random copolymer with the feed ratio of 1:1 shows higher PCE of 7.1% with higher VOC of 0.70 V as compared with those (6.6%, 0.62 V) of the reference homopolymer (pTDPP2T).