Effect of Molecular Weight on the Structure and Morphology of Oriented Thin Films of Regioregular Poly(3‐hexylthiophene) Grown by Directional Epitaxial Solidification

Abstract

Regioregular head‐to‐tail (HT)‐coupled poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) with a weight‐average molecular weight (Mw) in the 7.3–69.6 kDa range is crystallized by directional epitaxial solidification in 1,3,5‐trichlorobenzene (TCB) to yield highly oriented thin films. An oriented and periodic lamellar structure consisting of crystalline lamellae separated by amorphous interlamellar zones is evidenced by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both the overall crystallinity as well as the orientation of the crystalline lamellae decrease significantly with increasing Mw. The total lamellar periodicity is close to the length of “fully extended” chains for Mw = 7.3 kDa (polystyrene‐equivalent molecular weight, eq. PS) and it saturates to a value of ca. (25–28) ± 2 nm for Mw ≥ 18.8 kDa (eq. PS). This behavior is attributed to a transition from an oligomeric‐like system, for which P3HT chains are essentially in a fully extended all‐trans conformation and do not fold, to a semicrystalline system that involves a periodic alternation of crystalline lamellae separated by extended amorphous interlamellar zones, which harbor chain folds, chain ends, and tie molecules. For P3HT with Mw of ca. 7.3 kDa (eq. PS), epitaxial crystallization on TCB allows for the growth of both “edge‐on” and “flat‐on” oriented crystalline lamellae on the TCB substrate. The orientation of the lamellae is attributed to 1D epitaxy. Because of the large size of the “flat‐on” crystalline lamellae, a characteristic single‐crystal electron diffraction pattern corresponding to the [001] zone was obtained by selected area electron diffraction (SAED), indicating that P3HT crystallizes in a monoclinic unit cell with a = 16.0 Å, b = 7.8 Å, c = 7.8 Å, and γ = 93.5°.

Structure and morphology control in thin films of regioregular poly(3‐hexylthiophene)

“…As pointed out in various studies, the width of nanofibrils as well as the lamellar period in epitaxied layers tends first to increase with M n and reach a plateau when M n > 10 kDa. 37,38 This is a typical fingerprint of the onset of chain folding as observed for instance in the case of oligo(urethane)s or polyethylene/ parafins. 39 A second hint for the possibility of P3AT chains to fold lies in the variation of the a-axis parameter (layer period) with increasing M n [see Fig.…”

Impact of Thermal Annealing on the Semicrystalline Nanomorphology of Spin-Coated Thin Films of Regioregular Poly(3-alkylthiophene)s as Observed by High-Resolution Transmission Electron Microscopy and Grazing Incidence X-ray Diffraction

“…20 As yet, various studies have clearly demonstrated that P3AT chains tend to fold above a certain M w of 10 kDa. 28,29 Chain folds in P3AT are modeled by a sequence of 7−8 monomer units in cis conformation leading to a 2.0 nm wide fold. Whereas in a crystal structure like PE the interchain interactions with the next-neighbor chains are uniform in the plane perpendicular to the chain, the situation is quite different for P3ATs.…”

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

“…The relationship between polymer M W and PCE has been exhaustively studied in 301 P3HT and mixtures with PCBM [49][50][51][52][53][54][55][56][57][58][59][60][61]. We will attempt to briefly summarize the 302 lessons here.…”