An efficient hole transport material based on PEDOT dispersed with lignosulfonate: preparation, characterization and performance in polymer solar cells

Abstract

Inspired by the electron transfer process during the oxidation of electron-rich phenol derivatives and the serious aggregation properties of lignin, we studied the hole transporting properties of hole-only devices using water soluble lignosulfonate (SL) and an alkyl chain cross-linked lignosulfonate polymer (ASL) as active layers for the first time. SL with a higher phenolic group content shows higher hole mobility than ASL with a lower phenolic group content, which further suggests that phenolic groups are conducive to the hole transporting property. The maximum value of the hole mobility was achieved at 3.75 × 10⁻⁶ cm² V⁻¹ s⁻¹ for SL. This unexpected hole mobility provides a novel perspective for the application of SL as a potential polymeric p-type semiconductor. A water soluble and solution-processable PEDOT:SL material was prepared from the oxidation of EDOT dispersed in SL. As a hole-transport material in OPV with a PCE of 5.79%, PEDOT:SL showed comparable performance to that of the conventional PEDOT:PSS with a device structure of ITO/HTM/PTB7:PC₇₁BM/Al. In principle, amorphous lignosulfonate, which has a complex chemical structure, should show a worse hole transport property as a dopant for PEDOT than PSS, which has a regular molecular structure. In contrast, we found that PEDOT:SL showed comparable performance with that of PEDOT:PSS. The performance of PEDOT:SL is not very dependent on the mass ratio of PEDOT in PEDOT:SL. This also confirmed our proposal and results on the hole transport property of lignosulfonate. Our results provide a promising scaffold and concept for the design and feasible synthesis of HTMs based on the interconversion between phenol and benzoquinone for organic electronics. Moreover, our result also opens an application approach for lignin in the future.