Direct Observation of Interfacial Morphology in Poly(3-hexylthiophene) Transistors: Relationship between Grain Boundary and Field-Effect Mobility

Abstract

We investigated the effects of microstructural (crystallization and molecular orientation) and morphological alternation (grain boundary) of poly(3-hexylthiophene) (P3HT) films on the field-effect mobility (µ) before (as-spun P3HT) and after (melt-crystallized P3HT) melting of P3HT films. Although grazing incidence X-ray scattering shows that melt-crystallized P3HT has a more highly ordered edge-on structure than as-spun P3HT, the melt-crystallized P3HT reveals µ ) 0.003 cm 2 V -1 s -1 ; this is an order of magnitude lower than that of as-spun P3HT (µ ) 0.01 cm 2 V -1 s -1 ). In addition, the interfacial morphologies of the bottom surfaces of P3HT films, which are attached to the gate dielectric, were investigated using a film transfer technique. The melt-crystallized P3HT at this interface consists of well-developed nanowire crystallites with well-defined grain boundaries that act as trap states, as verified by analysis of the temperature-dependence of µ. The remarkable reduction of µ in low-molecular-weight P3HT film (8 kg/mol) that results from melt-crystallization is due to the increased number of well-defined grain boundaries.

Characterization and Distribution of Poly(3‐hexylthiophene) Phases in an Annealed Blend Film

“…In the SVA process (Figure S1), molecules in the upper layer contact solvent vapor ad-equately, which results in a high crystallization degree of polymer and high phase separation degree between donor and acceptor; these results are shown by SEM and AFM images of annealed films. [21,24] However, molecules in middle and bottom layers contact solvent vapor inadequately because of hindrance by the upper-layer molecules, which results in insufficient self-organization among molecules; consequently disordered aggregate is the predominant phase in the middle and bottom layers of a blend film.…”

“…[4,5] In annealed P3HT/PCBM blend films, ordered and disordered P3HT phases co-exist. [17][18][19][20] The ordered phase with its regularly ranked main chain of polymer backbone and interchain separation of 1.3-2.7 nm [21][22][23][24][25][26] facilitates a lower excitation energy for exciton dissociation and high transport efficiency for holes. [27,28] The primary charge species in the ordered phase are defined as delocalized polarons with long lifetime and good long-distance transport properties as well, which are crucial for higher PCE of organic solar cells.…”

Structure difference of sorbitol derivatives influences the crystallization and performance of P3OT/PCBM organic photovoltaic solar cells

“…Stingelin's group added DMDBS and tris-tert-butyl-1,3,5-benzenetrisamide (BTA) into poly(3-alkylthiophene)s (P3ATs) and found that the crystallization temperature (T c ) and crystallization rate of P3ATs increased and the carrier mobilities of them improved [21]. They also added DMDBS into 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo [3,2- [1,2,5] thiadiazole) (p-DTS-(FBTTh 2 ) 2 )/phenyl-C 71 -butyric acid methyl ester (PC 71 BM) small molecule BHJ OPV devices and found that DMDBS decreased the annealing time for the BHJ OPV devices and reduced the size of donor crystallites. The J SC and PCE of the devices with DMDBS were increased dramatically [22].…”

Experimental evidence for an optical interference model for vibrational sum frequency generation on multilayer organic thin film systems. I. Electric dipole approximation

“…43 In other instances, buried interfaces have been studied by physically removing the film from the substrate at low temperatures and carrying out X-ray or SPM measurements on the newly exposed surface. [44][45][46] Of course, this destructive approach renders the device unviable for further testing and likely perturbs the (formerly) buried interface. Many of these techniques are complementary, and a common strategy is to use some combination of experiments to impart some confidence in data interpretation.…”