Solid-State Organization and Ambipolar Field-Effect Transistors of Benzothiadiazole-Cyclopentadithiophene Copolymer with Long Branched Alkyl Side Chains.

Wojciech Pisula,Don Cho,Hoi Tsao,Jie Shu,Sreenivasa Puniredd,Alexey Mavrinskiy,Michael Hansen,Yanfei Zhao,Klaus Müllen,Martin Baumgarten,Dmytro Dudenko

doi:10.3390/polym5020833

Abstract

The solid-state organization of a benzothiadiazole-cyclopentadithiophene copolymer with long, branched decyl-tetradecyl side chains (CDT-BTZ-C14,10) is investigated. The C14,10 substituents are sterically demanding and increase the π-stacking distance to 0.40 nm from 0.37 nm for the same polymer with linear hexadecyls (C16). Despite the bulkiness, the C14,10 side chains tend to crystallize, leading to a small chain-to-chain distance between lamellae stacks and to a crystal-like microstructure in the thin film. Interestingly, field-effect transistors based on solution processed layers of CDT-BTZ-C14,10 show ambipolar behavior in contrast to CDT-BTZ-C16 with linear side chains, for which hole transport was previously observed. Due to the increased π-stacking distance, the mobilities are only 6 × 10-4 cm²/Vs for electrons and 6 × 10-5 cm²/Vs for holes, while CDT-BTZ-C16 leads to values up to 5.5 cm²/Vs. The ambipolarity is attributed to a lateral shift between stacked backbones provoked by the bulky C14,10 side chains. This reorganization is supposed to change the transfer integrals between the C16 and C14,10 substituted polymers. This work shows that the electronic behavior in devices of one single conjugated polymer (in this case CDT-BTZ) can be controlled by the right choice of the substituents to place the backbones in the desired packing.

Highlights

Organic semiconductors are based on conjugated organic small molecules or polymers, which can be processed from solution into electronics devices, allowing a rapid inexpensive roll-to-roll fabrication [1]
The levels for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) versus vacuum were determined from the value of onset oxidation potential and onset reduction potential during cyclic voltammetry (CV) scans on thin films showing reversibility in both reduction and oxidation behavior
In comparison to the previously reported CDT-BTZ polymers with linear alkyl substituents, the poor device performance is related to increased π-stacking distance, while the ambipolarity is attributed to a lateral shift of the backbones towards each other

Summary

Introduction

Organic semiconductors are based on conjugated organic small molecules or polymers, which can be processed from solution into electronics devices, allowing a rapid inexpensive roll-to-roll fabrication [1]. This property can be realized by low-bandgap donor-acceptor copolymers, being highly attractive for solar cells, due to their broad light absorption profiles, and have revealed high ambipolar mobilities over 1 cm2/Vs for both charge-carrier species [12] In this context, benzothiadiazole-cyclopentadithiophene (CDT-BTZ) copolymers are one promising type of polymer leading to ultrahigh hole mobilities up to 5.5 cm2/Vs (in single-fiber FETs) when substituted with linear alkyl side chains and possessing higher molecular weight fractions [13,14]. CDT-BTZ-C14,10 shows ambipolar device characteristics under the same transistor geometry, as previously reported, despite unchanged energy levels in comparison to polymers with hexadecyl (C16) and 3,7-dimethyloctyl (C8,2) In this work, it is shown for the first time that the ambipolarity is related to a lateral shift of the conjugated backbones towards each other induced by the steric demand of the C14,10 chains. These results indicate that the electronic properties of a conjugated polymer might be dependent on the relative arrangement of the backbones, which can be well-controlled by the steric influence of the substituents

Experimental

Results and Discussion

Conclusions