Thickness Dependence of Microstructure in Semiconducting Films of an Oligofluorene Derivative

Abstract

The measurement and optimization of microstructure development in organic semiconductor films is valuable because microstructure in many cases critically impacts electronic performance. We demonstrate a general method to measure microstructure thickness dependence in thin films using surface-sensitive near edge X-ray absorbance fine structure (NEXAFS) spectroscopy. The method is applied to an oligofluorene derivative DDFTTF, which consists of a fluorene-bithiophene-fluorene core that is end-substituted with linear dodecyl groups. The substrate-relative orientations of the aromatic core and the aliphatic end chains are independently determined, and comparing these orientations to terrace heights from atomic force micrographs proves that the end chains are interdigitated or folded. By measuring microstructure development from 6 to 150 nm, we find that DDFTTF exhibits two different preferential microstructures: one with large terraces within which molecules exhibit a strongly vertical orientation, and one with much smaller domains within which molecules exhibit a mildly horizontal orientation. The relative distribution of these two preferential microstructures depends on the distance of the domains from the substrate and the substrate temperature during deposition. The utility of this method is tested using a lamination technique to measure the saturation hole mobility at the top and bottom interface of DDFTTF films. We find that local microstructures with greater pi orbital alignment in the source-drain plane correlate directly to better local saturation hole mobilities.

Water-stable organic transistors and their application in chemical and biological sensors

“…The p-channel semiconductors pentacene, 5,5Ј-bis-(7-dodecyl-9H-f luoren-2-yl)-2,2Ј-bithiophene (DDFT TF) (26,27), and copper(II) phthalocyanine (CuPc), and an nchannel copper(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine (FCuPc) were thermally evaporated onto PVP-HDA (22 nm)-coated substrates. Top-contact OTFTs of each material exhibited excellent linear and saturation regime characteristics at low operating voltages.…”

Structural Characterization of Conjugated Polyelectrolyte Electron Transport Layers by NEXAFS Spectroscopy

“…p* resonance peaks. The quantification of molecular orientation and interchain p-interactions has unraveled the correlations of packing details in thin-film microstructure to field-effect mobilities in transistors or charge-carrier mobilities in organic thin-film transistors (OTFTs), which relies on regioregularity, [17] sidechain structures, [18] and molecular weights [19] of the materials, and processing variables of substrate surface chemistry, [20] film thickness, [21] thermal history, [22] and spin speed. [11] In this Communication, we use NEXAFS spectroscopy to examine the molecular orientation and composition of 10 nm thick CPE films as a function of molecular structure and underlying substrate.…”

Organic semiconductors for biological sensing

“…The structure, comprising the electron-donating moieties fluorene and bithiophene, gives the molecule strong p-type character. First presented in 2006 by the Bao group, 185 the straightforward synthesis of DDFTTF via a Suzuki-Miyaura cross-coupling is based upon that of a previous series of analogues by the same group in 2003. 186 DDFTTF is notable for being highly water-stable, allowing its usage in devices with analytes in aqueous media, a crucial property of effective sensors applied to biological media.…”