Alcohol-soluble polyfluorenes containing dibenzothiophene-S,S-dioxide segments for cathode interfacial layer in PLEDs and PSCs

Abstract

A series of alcohol-soluble amino-functionalized polyfluorene derivatives (PF-N-S, PF-N-SC8 and PF-N-SOC8) comprising various ratios of dibenzothiophene-S,S-dioxide segments (S/SC8/SOC8) in the main chains, respectively, were synthesized and utilized as cathode interfacial layer (CIL) in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) with high-work-function Al (or Au) electrode. The polymers possess LUMO/HOMO levels at −2.78 to −3.53eV/−5.69 to −6.32eV. Multilayer PLEDs and PSCs with device configurations of ITO/PEDOT:PSS (40nm)/P-PPV or PFO-DBT35:PCBM=1:2 (80nm)/CIL (3–15nm)/Al (or Au) (100nm) were fabricated. The PF-N-S-10/Al (or Au) cathode PLEDs displayed maximum luminous efficiency of 24.4cdA−1 (or 11.9cdA−1), significantly higher than bare Al (or Au) cathode device, exceeding well-known Ba/Al and poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)/Al (or PFN/Au) cathode devices. The enhanced open-circuit voltages (Vocs), electron reflux and reduced work functions clarify that the electron injection barrier from the Al (or Au) electrode can be lowered by inserting the polymers as CIL. The resulted PSCs also show device performances exceeding Al and PFN/Al cathode devices. The results indicate that PF-N-S, PF-N-SC8 and PF-N-SOC8 are excellent CIL materials for PLEDs and PSCs with high-work-function Al or Au electrode.