Synthesis And Optoelectronic Properties Of Organic Small-Molecule Photovoltaic Materials Based On N,S-Aromatic Heteroacene

Organic solar cells(OSCs)have attracted more and more attention due to their advantages of easy modification,light weight,and flexible devices.In recent years,the emergence of small molecule acceptors(SMAs)materials has enabled the energy conversion efficiency(PCE)of OSCs to exceed 18%.The regulation strategies of molecular structure,photoelectric properties and device performance of A-D-A type small molecule acceptor and donor materails,mainly focus on two aspects:the central electron donating unit(D unit)and the terminal electron withdrawing unit(A unit).Therefore,in this paper,a series of A-D-A-type small molecule acceptor or donor materials based on N,S-aromatic heterocyclic ring,including N-heteroindeno dithiophene(TC),5,10-dihydroindolo[3,2-b]indole(DBPP)and pyrrolo[2,3-b:4,5-b']diindole(DIP),were designed and synthesized with the fluorinatedornon-fluorinated2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile(IC and ICF)as the A unit.The effects of different N,S-aromatic heterocyclic D units and A units on the thermal stability,absorption spectrum,electronic energy level and photovoltaic performance of small molecule acceptor and donor materials have been systematically studied.1.Two multi-asymmetric SMAs(TC-IC and TCT-IC)with asymmetric TC and its thiophene substituted analog TCT as the D unit and asymmetric side chain(alkyl vs alkylphenyl)and IC as the electron withdrawing terminal A unit were designed and synthesized.The effects of multiple asymmetric factors(D unit and side chain)and the expansion of the conjugated framework of D unit on the photophysical,electrochemical properties and photovoltaic performance of small molecule acceptors were studied in detail.The results showed that the absorption spectra of TC-IC both in solution and film state were redshifted and the LUMO energy level also was upshifted as compared with the analogues based on indenodithiophene.In addition,TCT-IC exhibited a red-shifted absorption spectrum,a reduced optical bandgap,and upshifted HOMO/LUMO energy levels relative to TC-IC.The medium bandgap polymer PBDB-T was used as the donor material to pair with two synthesized SMAs to prepare OSCs.After optimization,the TC-IC-and TCT-IC-based OSC devices delivered a PCE of 6.65%and 6.24%,respectively.2.Two new A-D-A small molecules,DBPT-IC and DBPT-ICF,were designed and synthesized by using DBPP-based thiophene-fused octacyclic N,S-aromatic heterarene(DBPT)as the D unit,and IC and ICF as the A unit,respectively.The influences of the introduction of pyrrole unit and different A units on the molecular structure,photophysics,electrochemistry,and photovoltaic performance of these small molecular materials were systematically studied.The results showed that compared with the octacyclic analogues based on thiophene,the introduction of pyrrole unit can significantly broaden the absorption spectrum of small molecules,greatly reduce the bandgap and significantly upshift the HOMO/LUMO energy levels.The absorption edge bands of the two small molecules are both over 1000 nm.Compared with DBPT-IC,the fluoro-terminal DBPT-ICF has a lower optical bandgap(1.16 vs 1.12 e V)and a lower-lying HOMO/LUMO energy levels(-5.21/-3.87 vs-5.22/-3.98 e V).OSCs were prepared using synthetic small molecules as donor materials and fullerene PC₇₁BM as an acceptor material.After optimization,OSCs based on DBPT-IC showed a relatively high PCE of 3.65%.3.Two new A-D-A small molecules,DIPT-IC and DIPT-ICF,were designed and synthesized by using DIP-based thiophene-fused nonacyclic N,S-aromatic heterohydrocarbon(DIPT)as the D unit,and IC and ICF as the A unit,respectively.The effects of the increase of pyrrole rings(two to three)and different A units on the molecular structure,photophysics,electrochemical properties and photovoltaic performance of these small molecular materials were systematically studied.The results showed that the absorption spectra of DIPT-IC and DIPT-ICF containing three pyrrole units are further red-shifted compared with the analogs containing two pyrrole units(DBPT-IC and DBPT-ICF),and the absorption edge bands both exceeded 1100nm.Compared with DIPT-IC,DIPT-ICF showed a downshifted HOMO/LUMO energy levels(-5.17/-3.93 vs-5.19/-4.02 e V)and a reduced bandgap(1.12 vs 1.06 e V).OSCs were prepared by using synthetic small molecules as donor materials and fullerene PC₇₁BM as an acceptor material.After optimization,OSCs based on DIPT-IC showed a higher PCE of 4.42%than that of DIPT-IC.