Interplay of Processing, Morphological Order, and Charge-Carrier Mobility in Polythiophene Thin Films Deposited by Different Methods: Comparison of Spin-Cast, Drop-Cast, and Inkjet-Printed Films

Abstract

The dependence of morphology and polymer-chain orientation of regioregular poly(3-hexylthiophene) (rrP3HT) thin films on processing conditions have been widely studied. However, their possible variation across the film thickness direction remains largely unknown. We report here a marked difference in the optical dielectric (n,k) spectra between the top and bottom interfaces of spin-cast (sc) rrP3HT films deposited from chlorobenzene solutions. These spectra were obtained from reflection variable-angle spectroscopic ellipsometry using a self-consistent graded optical model with self-imposed Kramers-Krönig consistency. The top interface shows a red-shifted absorption that is characteristic of better order than at the bottom, across a wide range of film thicknesses. This disparity diminishes in drop-cast (dc) and multipass inkjet-printed (ijp) films, and disappears in amorphous films such as those of polystyrene and of a green-emitting phenyl-substituted poly(p-phenylenevinylene). The (n,k) spectra also reveal that crystallinity increases across sc < dc < ijp films. This is supported by cross section scanning electron microscopy of the cleaved edges and measurement of the microroughness of both the film interfaces. Furthermore, optical anisotropy decreases across sc > dc > ijp films. Finally, near-edge X-ray absorption fine structure spectroscopy also shows the frontier chains in ijp and dc films are more isotropically oriented than those in sc films. These results suggest that semicrystalline conjugated polymer films can be produced far from equilibrium. This explains the marked variation in their (opto)electronic properties between the top and bottom surfaces that has sometimes been found depending on the film deposition method. In particular, an unusually pronounced crystallization is induced by ijp. We label this marked ijp-induced crystallization the "ijp morphology", which appears to be general, as it is found also in single-inkjet-droplet films. It appears also to be responsible for the lower field-effect mobility measured for ijp films deposited on a variety of linear and circular electrode arrays. This however can fortuitously be reversed by annealing in solvent vapor. As all films were deposited in the low Peclet-number regime, we can rule out surface skin formation. We attribute the extensive crystallization to the non-uniform drying of picoliter droplets, further promoted by repeated film swelling-deswelling cycles in multipass-ijp films.

Inkjet‐Printing: A New Fabrication Technology for Organic Transistors

“…Wong et al fabricated single‐droplet OFETs using a solution of P3HT in chlorobenzene. Their inkjet‐printed devices showed relatively poor electrical performances, with a carrier mobility of about 0.0005 cm 2 V −1 s −1 , even if an increase of one order of magnitude could be observed after a solvent vapor annealing treatment.…”

Inkjet Printing

“…The drying time is determined by the boiling point of the solvents used and the processing technique applied (Dang et al, 2011;Wong et al, 2010). Inkjet printing techniques require longer film drying time than the spin coating process.…”

Influence of the linkage type between the polymer backbone and side groups on the surface segregation of methyl groups during film formation

“…[1][2][3][4] The control of the surface composition is of paramount importance in many applications of materials. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] In general, the surface structure formation is governed by thermodynamics, but the kinetics for segregation and array processes also play an important role in the ultimate surface structure formation, which is mainly determined using fixed process methods and conditions. [5][6][7][8][9] Due to the complexity of the film-forming process, the relationship between the surface structures of polymer thin films and the corresponding molecular structure of the polymer have been little studied and are accordingly not clearly understood.…”