Orientation Control of Poly(3-alkyl thiophene) via Microphase Separation of Block Copolymer for High Vertical Charge Mobility

Abstract

Conjugated polymers (CPs) have been widely used in light-emitting diodes (LEDs), organic field-effect transistors (OFETs), and organic photovoltaics (OPVs) due to their mechanical flexibility, low fabrication coasts, solution processability, and the ability to tailor material properties through molecular design. Among various CPs, poly(3-alkyl thiophene) (P3AT) has attracted great interest because of its high charge mobility, good solubility in various organic solvents, and relatively easy synthetic process via the quasi- living Grignard metathesis (GRIM) method. To improve the performance of P3AT-based organic electronic devices, the control of the nanostructure of the P3AT and the polymer chain arrangement are very important because those affect the pathway of holes. P3AT can crystallize into three different arrangements: edge-on (the main chain is parallel, and the π-plane is vertical to the substrate), face-on (the main chain and the π-plane are parallel to the substrate), and end-on (the main chain and the π-plane are vertical to the substrate). It is known that P3AT thin films prefer to have edge-on orientation due to the low surface energy of the alkyl side chain. Charge transport along the main chain is faster than that in the π−π stacking and alkyl side chain direction, while the alkyl side chain direction shows the slowest charge transport. Edge-on orientation is useful for OFETs, where the charge transport occurs parallel to the substrate. However, for applications such as LEDs and OPVs, where the charge transport occurs vertically on the substrate, the polymer backbone should be oriented perpendicularly to the substrate (that is, end-on orientation). Obtaining the end-on orientation is not easy due to a low surface energy of the alkyl side chain in P3AT. In this thesis, I utilized the microphase separation of block copolymers to obtain the end-on orientation of P3AT. For P3AT, particularly poly(3-hexyl thiophene) (P3HT), a strong rod-rod interaction prevents the microphase separation. Therefore, by adjusting the length of the alkyl side chain or controlling the regioregularity, the rod-rod interaction can be modified to fabricate the end-on orientation. In chapter 2, I used amphiphilic diblock copolymers consisting of hydrophobic poly(3-dodecylthiophene) (P3DDT) and hydrophilic poly(3-(2-(2-(2- methoxyethoxy)ethoxy)ethoxy)-methylthiophene) (P3TEGT) blocks to increase the incompatibility between two blocks to form well-ordered lamellar microdomains. P3DDT- b-P3TEGTs with two different weight fractions of P3DDT block (wP3DDT = 0.48 and 0.65) showed lamellar microdomains in bulk. All thin films showed parallel oriented lamellar microdomains to a substrate. This is because of a large difference of surface tension between hydrophobic P3DDT and hydrophilic P3TEGT. We found via grazing incidence wide-angle X-ray scattering (GIWAXS) that P3DDT and P3TEGT chains had end-on orientation. Because of this orientation, the hole mobility of P3DDT-b-P3TEGTs along the film thickness direction was greatly enhanced (more than 10 times) compared with P3DDT neat film with edge-on orientation. However, the value of hole mobility (μ, ∼10−5 cm2 V−1 s−1) was still low due to a low crystallinity of the P3TEGT block in addition to the existence of an edge-on orientation of the P3DDT block on the top film surface. Therefore, to obtain higher μ of P3AT polymers with the end-on orientation, the side chain length in P3AT should be shortened to increase crystallinity. In chapter 3, I synthesized the poly(3-hexylthiophene)-block-poly(3-(2- methoxyethoxy)methylthiophene) copolymer (P3HT-b-P3MEGT) with a regioregularity (RR) of the P3HT block having 82% to obtain the end-on orientation throughout the entire film thickness. P3HT-b-P3MEGT with RR = 82% of P3HT exhibited a well-defined lamellar morphology induced by microphase separation between P3HT and P3MEGT blocks. P3HT- b-P3MEGT thin films showed parallel-oriented lamellar nanostructures on a silicon substrate. Parallel-oriented lamellar nanostructures induced the end-on orientation of P3HT and P3MEGT chains across the entire film. End-on-oriented P3HT-b-P3MEGT thin films showed much enhanced vertical hole mobility, 30 times higher than that of edge-on-oriented P3HT homopolymer thin films with a high RR of 95%. But, hydrophilic P3MEGT block contain many polar groups, which promote the binding of water molecules. When water molecules are close to the polymer chain, they induce charge localizations (or traps), resulting in a reduced hole mobility. Therefore, an effective method to enhance the hole mobility of the hydrophilic CP block is to suppress water- induced traps. To reduce water-induced traps, dopants with fluorine or nitrile groups, such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), have been extensively employed due to its strong dopant/polymer interaction, which prevents direct contact of water molecules to the polymer backbone. In chapter 4, I minimized water-induced traps using sequential doping of F4TCNQ into a P3HT-b-P3MEGT thin film. F4TCNQ in acetonitrile (ACN) was sequentially doped into the amphiphilic P3HT-b-P3MEGT thin film with the end-on orientation. Due to the use of the polar solvent (ACN), F4TCNQ was predominantly doped in the amorphous region of P3HT-b-P3MEGT. At low F4TCNQ concentration (c) in ACN, the μ was slightly lower than that of the neat P3HT-b-P3MEGT film owing to the increase in the distance between the polymer backbones. However, with increasing c, it increased and reached the maximum at c = 0.8 mg mL−1. The maximum μ was (8.04 ± 0.14) × 10−2 cm2 V−1 s−1, which is twice larger than that of the neat P3HT-b-P3MEGT film. This is attributed to the effective suppression of water-induced traps, generated from the hydrophilic P3MEGT containing an ethylene glycol unit.