Unexpected fluorescent emission of graft sulfonated-acetone-formaldehyde lignin and its application as a dopant of PEDOT for high performance photovoltaic and light-emitting devices

In this study, grafted sulfonated-acetone-formaldehyde lignin (GSL) was prepared via graft sulfonation using alkali lignin from pulping black liquor as a raw material and studied by GPC, functional group content measurement and FTIR to confirm its fundamental structure. Unexpected cluster-induced green fluorescent emission of sulfonated-acetone-formaldehyde polymer in GSL, an unconventional chromophore, was investigated for the first time. Moreover, inspired by the phenolic hydroxyl group of GSL, we studied the electron transfer process during the oxidation of GSL by cyclic voltammetry and hole transporting mobility test. Electrochemical behaviour test showed an oxidation potential at 1.1 V. Moreover, a hole mobility of 2.27 × 10^-6 cm² V^-1 s^-1 was detected with GSL as the hole transport material. The results reveal that GSL is a potential water soluble polymeric p-type semiconductor. Taking advantage of the hole transporting property of GSL, PEDOT:GSLs with controllable work functions were developed and applied as hole transport layers (HTLs) in organic electronic devices, including organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). As a result, a maximum power efficiency of 14.67 lm W^-1 was achieved with PEDOT : GSL-1 : 6 as HTL in OLEDs, which was 1.78 times that with PEDOT:PSS (8.25 lm W^-1). Moreover, a high power conversion efficiency (PCE) of 8.47% using PEDOT : GSL-1 : 4 as HTL was achieved in the OPV devices structure of ITO/HTL/PTB7-Th:PC₇₁BM/PFN/Al. These results show that amorphous polymer GSL might be a potential dopant of PEDOT in organic electronic devices and it provides a novel perspective to adjust the work function of PEDOT by doping with sulfonated lignin, which is a cheap and renewable biomass.