The conversion of donor to acceptor and rational design for diketopyrrolopyrrole-containing small molecule acceptors by introducing nitrogen-atoms for organic solar cells

Abstract

For the purpose of improving the power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) devices, non-fullerene small acceptors have been presented by modifying desirable donors, such as a diketopyrrolopyrrole (DPP)-based acceptor with two thiazoles, namely poly{3,6-bis(5-hexyldecyl-2thenyl)-2,5-dihydro-2,5-di(alkyl)pyrrolo[3,4]pyrrolo-1,4-dionethiazole} (PDPP2TzT), synthesized by introducing two nitrogens onto the copolymer based on DPP and terthiophene (poly[{2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl}]-alt-{[2,2′:5′,2′′-terthiophene-5,5′′-diyl}]) (PDPP3T). In order to clarify the transfer mechanism, we put forward a comparative theoretical study of PDPP3T and PDPP2TzT using density functional theory/time-dependent density functional theory (DFT/TD-DFT) calculations. Subsequently, we determined that the mechanism of the conversion from a donor to an acceptor was that the insertion of nitrogen could reduce the LUMO energy by changing the electron density and intramolecular interactions. Based on this mechanism, a series of acceptors were designed with the advantages of PDPP2TzT. The results illustrate that b4 showed the best performance when blended with PDPP5T as the donor for BHJ devices. Finally, we hope our investigations will provide guidelines for further rational design of acceptor materials for BHJ devices.