Naphthalenediimide‐alt‐Fused Thiophene D–A Copolymers for the Application as Acceptor in All‐Polymer Solar Cells

Abstract Three n‐type alternating D–A copolymers based on a naphthalenediimide (NDI) acceptor (A) unit and three different donor (D) units with varied electron‐donating strength including thiophene ( P(NDI‐T) ), thieno[3,2‐b]thiophene ( P(NDI‐TT) ), and thieno[3,2‐b;4,5‐b]dithiophene ( P(NDI‐TDT) ), were synthesized, for the application as acceptor materials in all‐polymer solar cells (all‐PSCs). The effect of the donor units of thiophene, thienothiophene (TT) and thienodithiophene (TDT) on the physicochemical and photovoltaic properties of the n‐type D–A copolymers was systematically investigated. It was found that the absorption spectrum is red‐shifted and the energy band gap ( E g ) is reduced for the NDI‐based D–A copolymers with increasing number of thiophene rings in the thiophene or fused thiophene donor units. All‐PSCs were fabricated with the medium band gap conjugated polymer J51 ( E g of ca 1.9 eV) as polymer donor and the n‐type D–A copolymers as acceptor. The power conversion efficiency reached 2.59 %, 3.70 % and 5.10 % for the all‐PSCs with P(NDI‐T) , P(NDI‐TT) , and P(NDI‐TDT) as acceptor, respectively. The results indicate that a larger conjugated fused molecular plane with more thiophene rings as donor units in the NDI‐based D–A copolymers is beneficial to reduce the band gap, broaden the absorption and enhance the photovoltaic performance of n‐type D–A copolymer acceptors.