Design of (X-DADAD)_<i>n</i> Type Copolymers for Efficient Bulk Heterojunction Organic Solar Cells

We show that extended TBTBT structure (T = thiophene, B = benzothiadiazole) can be used as an electron-deficient building block for designing conjugated polymers with deeply lying HOMO energy levels and narrow band gaps. The first carbazole–TBTBT copolymer <b>P2</b> demonstrated power conversion efficiencies exceeding 6% in bulk heterojunction solar cells in combination with advanced operational stability, unlike conventional donor polymers such as PTB7, PBDTTT-CF, etc