Solution Sequential Doping of Lewis-paired Dopants for Conjugated polymer-based Semiconductors

P-type chemical doping (p-doping) plays a crucial role in modulating the optical, electrical, and electronic properties of conjugated polymer (CP)-based semiconductors, making them suitable for various organic (opto)electronic devices.[1] One successful doping method is the solution sequential doping process (SqP), where dopants are diffused into printed CP films, while preserving the crystallinity and morphology of the films, leading to enhanced charge transport and overall device performance.[2] P-doping typically occurs through charge transfer between the lowest unoccupied molecular orbital (LUMO) of the dopants and the highest occupied molecular orbital (HOMO) of the CPs.[3] However, commercial organic p-dopants have shallow LUMO levels (–5.4 eV) to efficiently p-dope various CPs with a wide range of HOMO levels (> –5.9 eV), making it challenging to apply doping techniques in this field.[4]
Recently, novel organic p-dopants based on Lewis-paired –CN groups are developed, which offer strong oxidation properties (–5.93 eV) and excellent doping stability under thermal condition.[5,6] In detail, when tris(pentafluorophenyl)borane (BCF) coordinates with the –CN groups of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) (–5.24 eV), the resulting F4TCNQ:BCF shows unprecedently deep LUMO level (–5.87 eV) and their large size benefits to doping stability under thermal condition. However, in polar solutions suitable for SqP, BCF coordinates with the solvent molecules rather than the –CN group, significantly reducing doping efficiency. For non-polar solutions, it dissolves the polymer matrix, decreasing the charge transport properties.[5,7] Therefore, the challenge lies in enhancing the doping efficiency of Lewis-paired dopants during SqP, without compromising the pre-coated CPs films.
Herein, a novel doping method is introduced for enhancing the charge transport properties of CPs by investigating the impact of solvent polarity on the formation mechanism of the Lewis-paired dopants. For six orthogonal solvents to CPs with various dielectric constants, the doping efficiency is closely linked to the chemical equilibrium between BCF and the solvent molecules. In low-polar solvents, the stability of the BCF–solvent complex decreases, leading to an increased dissociation rate that facilitates the formation of Lewis-paired dopants and enhances doping efficiency. For high-polar solvent, increased doping times and concentrations are required to overcome the low production of Lewis-paired dopants, thereby inactive dopants remain in the films that hindered charge transport. Consequently, the CP films doped with low-polar solvent show more planar conjugated backbone conformation, promoting delocalized charge carriers, resulting in 4-fold higher electrical conductivity compared with high-polar solvent. This work provides valuable insights into the solvent-dopant interactions and presents an effective strategy for fabricating the thermally stable and highly conductive CPs films, without the limitations of doping efficiency.

[1] N. Sakai, … H. J. Snaith Nat. Mater. 2021, 20, 1248-1254.
[2] I. E. Jacobs, … A. J. Moulé J. Mater. Chem. C, 2016, 4, 3454-3466.
[3] A. D. Scaccabarozzi, … T. D. Anthopoulos Chem. Rev. 2022, 122, 4420-4492.
[4] Y. Karpov, … A. Kiriy Adv. Mater. 2016, 28, 6003-6010.
[5] E. H. Suh, … J. Jang Angew. Chem. Int. Ed. 2023, 62, e202304245.
[6] A. E. Mansour, … N. Koch ACS Appl. Mater. Interfaces, 2023, 15, 46148-46156.
[7] O. Zapata-Arteaga, … M. Campoy-Quiles ACS Energy Lett. 2024, 9, 3567-3577.