Impact of Competing Crystallization Processes on the Structure of All-Conjugated Donor–Acceptor Block Copolymers P3HT-b-PNDIT2 in Highly Oriented Thin Films

Abstract

Mastering orientation and crystallization of all-crystalline, all-conjugated donor-acceptor diblock copolymers is of high interest for photovoltaic applications, as electronically beneficial nanostructuring of thermodynamically stable bulk heterojunctions can be achieved. Diblock copolymers composed of a regioregular poly(3-hexylthiophene) (P3HT) as donor and PNDIT2 as acceptor block were crystallized and oriented by epitaxy and high-temperature (HT) rubbing. The resulting highly oriented thin film structures were investigated in detail by transmission electron microscopy (electron diffraction and high resolution) and temperaturedependent polarized UV-vis absorption spectroscopy. Epitaxy and HT rubbing allow for selective crystallization and orientation of the blocks to obtain structures not accessible by standard spin coating and annealing processes. Epitaxy on trichlorobenzene allows for orientation and crystallization of P3HT, whereas additional annealing is necessary to crystallize the PNDIT2 block also. By contrast, HT rubbing aligns both blocks. The rubbing temperature controls the polymorphism (form I versus form II of PNDIT2), the orientation and the contact plane (edge-on versus face-on) of the backbones on the substrate. Crystallization of PNDIT2 constraints stacking of P3HT and modifies its unit cell parameters especially along the alkyl side chain direction. The lattice mismatch between the P3HT and PNDIT2 blocks hampers the growth of long-range highly ordered lamellar domains and results in highly disordered interfaces at the block junction. Overall, this study underlines the necessity to adapt layer spacings along alkyl side chains between donor and acceptor blocks to minimize unit cell mismatches and to enhance long range self-assembly into ordered lamellar donor-acceptor phases.

Enhancing Optoelectronic Properties of Conjugated Block Copolymers through Crystallization of Both Blocks

“…Comparing with GIWAXS data from PFTBT homopolymer films (Figure S6 of the Supporting Information), this peak in scattering from block copolymers can be attributed to the crystallization of the PFTBT block and corresponds to a 1.3 nm backbone spacing across the side chains (i.e., likely a 100 peak). Crystallization of both blocks within a conjugated block copolymer has also been observed in P3HT- b -polyfluorene and in P3HT- b -poly­(naphthalene- alt -dithiophene) block copolymers.…”

“…Both liquid crystallinity and crystallization can promote the formation of a mesoscale structure by creating nanoscale ordered domains within an amorphous matrix. In some cases, both blocks can crystallize and even perturb the crystallinity and unit cell when compared to the homopolymer. , …”

Insights into the structural complexity of semi-crystalline polymer semiconductors: electron diffraction contributions

“…They can form spontaneously nanostructured bulk heterojunctions via phase separation of donor and acceptor blocks. [77][78][79][80] Preliminary studies show that the alignment methods used for the individual blocks are relevant to orient and crystallize such conjugated copolymers and ultimately study their organization by electron diffraction. In particular, TEM studies demonstrated that the crystallization of the P(NDI2ODT2) block can hamper substantially that of the P3HT block due to a mismatch of lattice parameters of the two blocks at the block junction.…”

Tailoring Cocrystallization and Microphase Separation in Rod–Rod Block Copolymers for Field-Effect Transistors

“…Among the various conjugated rod–rod BCPs, BCPs containing poly­(3-alkylthiophene) (P3AT) have been mostly studied as model systems owing to their remarkable electronic activity and straightforward synthesis. By a facile Grignard metathesis (GRIM) polymerization, , a range of P3AT-based rod–rod BCPs have been prepared, which are classified into three types: thiophene–thiophene BCPs differing solely in their side alkyl chain length or topology, − thiophene–thiophene BCPs with functionalized side chains, − and BCPs consisting of various arene backbones. − In principle, the competition between microphase separation and crystallization in BCPs has a profound effect on their final morphology and resulting properties. For example, poly­(3-butylthiophene)- block -poly­(3-octylthiophene) (P3BT- b -P3OT), with varying side-chain lengths, self-assembled and crystallized into nanowires in solution and microphase-separated into lamellae during the melt phase .…”