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

Abstract

The impact of thermal annealing on the growth of crystalline nanodomains of poly(3-alkylthiophene) (P3AT) in thin films (15−20 nm thick) was investigated as a function of length of alkyl side chain by combined low-dose highresolution transmission electron microscopy (HR-TEM) and grazing incidence X-ray diffraction (GIXD). Statistical analysis of the data yields the characteristic dimensions of the face-on oriented P3AT nanocrystals, i.e., average stem length l c along backbones and lateral dimension perpendicular to the stems l a along side chains. The following trends were identified: (i) in as-spin-coated films, the proportion of face-on oriented nanocrystals increases with the number of carbon atoms in the side chain, (ii) annealing favors the lateral in-plane growth of the nanocrystals along the side chain direction (a P3AT axis), (iii) for a given P3AT, the proportion of face-on oriented domains increases with annealing temperature, (iv) lateral growth along the a P3AT axis is most efficient for the longer octyl side chains, and (v) thermal annealing induces only modest lamellar thickening which is limited by the poor slide diffusion of π-stacked P3AT chains as opposed to lateral growth favored by weak van der Waals interactions between layers of n-alkyl side chains. The increase in the population of face-on oriented crystallites, observed when the length of the side chain increases, coincides with a corresponding decrease in the field effect mobility in annealed P3AT thin films.

Local structure of semicrystalline P3HT films probed by nanofocused coherent X-rays

“…3(b). 20, where the face-on oriented domains were mainly observed, we conclude that the face-on morphology is preferential in this region. 3(a) The first region [red points in Fig.…”

“…1(a)], and perpendicular in the case of edge-on domains [ Fig. 4,15,20 P3HT films can be prepared using various methods, for example, drop-casting, spin-coating, dip-coating or directional crystallization, leading to a different degree of crystallinity and preferred orientation of P3HT domains. The shortest distance between P3HT layers, called p-p stacking distance, is equal to b/2, where b is a unit cell parameter [ Fig.…”

“…The shortest distance between P3HT layers, called p-p stacking distance, is equal to b/2, where b is a unit cell parameter [ Fig. 1,5,10,12,13,15,[20][21][22][23][24][25][26][27][28] It has been demonstrated that thermal annealing of the grown films results in better crystallinity and improved charge carrier mobility. Mixed orientation of domains also occurs, especially in non annealed samples.…”

“…1(a) and (b)]. 13,14,20,25,29 Together with pristine P3HT, its blends of various compositions emerge as another important class of hybrid materials with attractive structural and electronic properties. Generally, it is assumed that the face-on orientation is favorable for photovoltaic applications, while the edge-on orientation of domains enhances OFET performance.…”

“…1,5,10,12,13,15,[20][21][22][23][24][25][26][27][28] It has been demonstrated that thermal annealing of the grown films results in better crystallinity and improved charge carrier mobility. 5,12,20,30 To study structural variations in semicrystalline P3HT films on the nanoscale we performed spatially-resolved coherent X-ray scattering experiment with a nanofocused X-ray beam, in combination with the X-ray cross-correlation analysis (XCCA). 26,30,31 In this work we analyzed structural variations of the P3HT host matrix upon small addition of gold nanoparticles stabilized with fluorene derivatives (AuNPs-SFL) [see Fig.…”