Effect of aggregation behavior and phenolic hydroxyl group content on the performance of lignosulfonate doped PEDOT as a hole extraction layer in polymer solar cells

Using lignosulfonate (LS) and alkyl chain-coupled lignosulfonate-based polymer (ALS) as the raw materials, the aggregation behavior of LS and ALS was investigated, and they both showed a unique aggregation behavior to form a block-like self-assembly for the first time. The aggregation behavior and mechanism of LS and ALS were investigated by SEM, TEM and DLS. The block-like aggregates prepared from ALS (micron size) were larger than that of LS (nano size). The unique aggregates were also further confirmed by XPS, meanwhile, SAXS was applied to explore the regular intrinsic characteristics of the block-like aggregates. Inspired by the aggregation behavior of LS and ALS, the electron transfer properties of LS and ALS were also studied including the electrochemical properties and hole mobility measurements. The oxidation peaks at 1.2 V and 1.4 V were observed at the LS and ALS modified electrode, respectively. We studied the hole transport properties of LS and ALS using the space-charge-limited current method (SCLC). Average hole mobilities of 2.95 × 10⁻⁶ cm² V⁻¹ s⁻¹ and 3.18 × 10⁻⁷ cm² V⁻¹ s⁻¹ were estimated for LS and ALS, respectively. The above results indicated that LS and ALS are potential water soluble polymeric p-type semiconductors, and the hole transport property of LS is better than that of ALS. Based on the unique aggregation behavior and hole mobility property described above which will facilitate charge transport, water soluble PEDOT:LS and PEDOT:ALS were prepared and applied as the hole extraction layer (HEL) in polymer solar cells. The PCE decreased with a decrease of the phenolic hydroxyl group content (–OH), which suggested that –OH is important for the strength of the PCE. The application properties were consistent with the results of the aggregation behavior and electron transfer properties. The power conversion efficiency (PCE) of 5.19% from PEDOT:LS-1 : 1 as the HTL was achieved with a device structure of ITO/HEL/PTB7:PC71BM/Al in our study. Our results showed that the phenolic hydroxyl group content and conjugation structure of amorphous LS contribute to its promising potential as a dopant of semiconductors, such as PEDOT in organic electronics. Our results provide a novel perspective for the design of dopants for semiconductive polymers. In summary, the phenolic hydroxyl group of the polymer will provide hole transport capability due to its oxidation during device operation.