Side-chain engineering by thymine groups enables hydrogen bond in P-type donor-acceptor polymers with enhanced optoelectronic properties

Abstract

Realizing ordered structures at the molecular level is a key approach to increase the charge mobility of organic semiconductors. However, the typical solution-based methods employed for the processing make the achievement of well-organized organic nanostructures difficult with the favored formation of disordered assemblies. To realize well-ordered thin films, one needs to design conjugated materials with the ability of self-assembling. In this work, two alternating donor-acceptor copolymers based on the carbazole and diketopyrrolopyrrole (DPP) units, namely P1 and P2, are successfully synthesized. Compared to P1, the polymer P2 contains an extra thymine group at the end of the alkyl chain, through which the hydrogen bonding (CO⋯NH) can be formed between the amide (NH) and carbonyl units (CO), as verified by the fourier transform infrared (FTIR) and differential scanning calorimetry (DSC) study. The electrochemical analysis shows that the hydrogen bonding association influences the carbazole donor ability, resulting in a higher HOMO energy level of P2 compared to P1. In addition, the as-formed hydrogen bonds promote the assembling of the molecules into highly ordered structures by enabling strong aggregation as well as a narrow distance between adjacent molecules with long-range ordering packing and large nanocrystalline grains. As a result, the intermolecular charge transfer is enhanced. Consequently, the organic field-effect transistors (OFETs) constructed with P2 as the semiconductor layer present a p-type behavior with maximum hole mobility up to 1.32 cm2 V−1 s−1 upon annealing, which is almost 5 times higher than that of the pristine P2 (0.26 cm2 V−1 s−1), while the hole mobility of the annealed P1 is only 0.23 cm2 V−1 s−1. Our results indicate that the alkyl chain engineering, with the introduction of a thymine group into the donor units of the polymer, is responsible for the formation of hydrogen-bonded superstructures. This is a facile approach to enhance the optoelectronic properties of organic semiconductors.