Efficient polymer solar cells based on a cathode interlayer of dicyanomethylenated indacenodithiophene derivative with large π-conjugation and electron-deficient properties

Abstract

Two indacenodithiophene (IDT) based water/alcohol-soluble small molecular cathode interlayers (CILs), namely TBIDTD and TBIDTCN, were designed, synthesized and applied in typical PTB7:PC₇₁BM-based conventional structural single-junction polymer solar cells (PSCs). This is the first report where the IDT core was used as a large π-conjugated backbone for CILs. These two molecules adopt the same polar end-groups. However, compared to TBIDTD, TBIDTCN exhibits a lower LUMO (lowest unoccupied molecular orbital) level due to the introduction of dicyanomethylene groups, which is beneficial for electron transport and extraction at the interface. Systematic optimizations were carried out to confirm the interfacial modification ability of the CILs. The results indicate that both CILs can significantly improve the performance of PSCs, and the TBIDTCN-modified devices presented better performance because of the further improved electron mobility and further reduced R_s (series resistance). A PCE (power conversion efficiency) of 9.19% was achieved by the TBIDTCN-modified devices, which was nearly 1.66 times that of the bare cathode device, due to the simultaneously enhanced V_oc (circuit voltage), J_sc (short-circuit current) and FF (fill factor). Our work demonstrates that the IDT-based CILs are effective in improving the performance of PSCs and presents an attractive design strategy for CILs with large π-conjugation and electron-deficient properties.