Star-like n-type conjugated polymers based on naphthalenediimide for all-polymer solar cells

Abstract

We developed n-type conjugated polymers that contain a naphthalene diimide moiety with star-like molecular geometry that are synthesized by incorporating 1,3,5-trisphenyl-benzene as the branched unit into the linear copolymer N2200. The developed copolymers, NDIPh1 and NDIPh2, exhibited frontier molecular orbital energy levels and optical properties similar to those of N2200, whereas the melting and crystallization temperatures were simultaneously decreased due to the incorporated branched central unit. Both copolymers showed good miscibility with the widely used electron-donating copolymer poly[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th). All-polymer solar cells (All-PSCs) based on PTB7-Th:NDIPh1 presented an impressive power conversion efficiency of 6.87%, which obviously exceeded the 5.75% efficiency obtained from the device based on N2200. The enhanced photovoltaic performance, by virtue of the improved short-circuit current density, can be attributed to the slightly enhanced electron mobility, higher exciton dissociation rates, and more efficient charge collection of NDIPh1-based device. These observations indicate that the developed star-like NDIPh copolymers can be promising n-type acceptors for the construction of high-performance all-PSCs.