Green dioxythiophene-benzothiadiazole donor-acceptor copolymers for photovoltaic device applications

With the perspective of producing power-generating displays of various colors based on π-conjugated semiconducting polymers, we have developed a synthetic design aimed at addressing color states commonly difficult to attain. Herein, we report on the structure property relationships and performance in photovoltaic devices of a series of green-colored donor acceptor (DA) π-conjugated polymers comprised of electron-rich 3,4-dioxythiophenes (DOTs) and the electrondeficient 2,1,3-benzothiadiazole (BTD). In particular, the synthesis and chemical polymerization of two DOT-BTD regiosymmetric oligomers (pentamers M2 and M3), that can be chemically oxidized to yield two-band absorbing polymers with a transmission window in the 480-560 nm range hence reflecting/transmitting the color green (P2 and P3), is reported. The optical and electrochemical properties of P2 and P3 are described and compared to those of a blue-colored parent polymer (P1) obtained via the polymerization of a smaller DOT-BTD oligomeric precursor (trimer M1). The photovoltaic (PV) properties of P1-P3 were investigated in DA bulk heterojunction (BHJ) devices with PC₆₀BM as the acceptor. P2 and P3 afforded green-colored devices with up to 1.9% power conversion efficiency (PCE) under AM 1.5 G solar illumination. Taking into account the differences in polymer energy band structure, we have replaced PEDOT:PSS by MoO₃ and optimized the solarcell device configuration for the most efficient polymer derivative (P3), demonstrating up to a 2.71 % PCE. Insight into the morphology and charge transport of these polymers in blends with PCBM is provided and related to the synthetic design and PV device performance.