Molecular structure–property engineering of low‐band‐gap copolymers, based on fluorene, for efficient bulk heterojunction solar cells: A density functional theory study

Abstract

Abstract A comprehensive theoretical study of new donor (D)–acceptor (A) copolymers properties has been developed through quantum chemical calculations, on the basis of density functional theory (DFT). Fluorene and 2,1,3‐benzothiadiazole units were used as donor and acceptor units, respectively, in alternating D–A structures. The efficiency of D–A strength in the polymer was characterized, and their electronic properties are modulated through the insertion of thiophene, vinylene, or ethynylene as π‐spacer groups, in an attempt to reach the critical values, required for organic solar cells. Optoelectronic properties of these copolymers are rationalized on the basis of the patterns of their frontier orbitals. The analysis of the corresponding transition provides an efficient way for tracing the origin of various optical transitions of these copolymers and their photophysical properties. Bulk heterojunction photovoltaic cells, designed with the based fluorene‐copolymer as electron donor, blended with [6,6]‐phenyl‐C 61 ‐butyric acid methyl ester as an electron acceptor, were energetically elucidated. A model band diagram was established, simulating the energy behavior of this active layer. POLYM. ENG. SCI., 2013. © Society of Plastics Engineers