Showing papers by "Dean M. DeLongchamp published in 2008"

The Effect of Interfacial Roughness on the Thin Film Morphology and Charge Transport of High‐Performance Polythiophenes

Abstract: We control and vary the roughness of a dielectric upon which a high-performance polymer semiconductor, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) is cast, to determine the effects of roughness on thin-film microstructure and the performance of organic field-effect transistors (OFETs). pBTTT forms large, well-oriented terraced domains with high carrier mobility after it is cast upon flat, low-surface-energy substrates and heated to a mesophase. Upon dielectrics with root-mean square (RMS) roughness greater than 0.5 nm, we find significant morphological changes in the pBTTT active layer and significant reductions in its charge carrier mobility. The pBTTT films on rough dielectrics exhibit significantly less order than those on smooth dielectrics through characterization with atomic force microscopy and X-ray diffraction. This critical RMS roughness implies that there exists a condition at which the pBTTT domains no longer conform to the local nanometer-scale curvature of the substrate.

Molecular Basis of Mesophase Ordering in a Thiophene-Based Copolymer

Abstract: The carrier mobility of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) semiconductors can be substantially enhanced after heating through a thermotropic mesophase transition, which causes a significant improvement in thin film structural order. By directly measuring film structure throughout a heating and cooling cycle, we identify the molecular origin of this mesophase transition as the melting of interdigitated linear alkane side chains, in this case quaterdecyl. The morphology and phase behavior throughout the thermal cycle are controlled by the changing conformation of the side chains. Surprisingly, the melting of the side chains allows increases in the backbone order, p-p stacking, and carrier mobility. Upon cooling, the side chains recrystallize to preserve the excellent mesophase order and enhanced electrical performance.

Structural Characterization of Conjugated Polyelectrolyte Electron Transport Layers by NEXAFS Spectroscopy

Abstract: NEXAFS spectroscopy of conjugated polyelectrolytes used as the electron transport layer in polymer light-emitting diodes reveals a substrate-dependent accumulation of charged groups at the surface (see figure). The measurements are also consistent with a preferential orientation of tetrahedral counterions at the surface.

Thin Film Microstructure of a Solution Processable Pyrene-Based Organic Semiconductor

Abstract: Semiconducting 6,7,15,16-tetrakis(dodecylthio)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine (TQPP-12) has been synthesized as a candidate solution-processable semiconductor for organic electronics applications. We characterize the microstructure of TQPP-12 in films using a combination of polarized photon absorption spectroscopies (X-ray, vis, and infrared), X-ray diffraction, and scanning probe techniques. This characterization strategy allowed for the determination of molecular orientation and packing style within thin films of this complex molecule. The TQPP-12 molecules are arranged within layers, and the aromatic cores are separated from the alkane side chains. Both the core long axes and side chains are highly tilted with respect to surface normal, and the conjugated planes of the core are cofacially packed.

Microstructure of Oligofluorene Asymmetric Derivatives in Organic Thin Film Transistors

Abstract: In organic thin film transistors (OTFT), modifying the molecular chemical structure affects the molecular packing and thin film morphology, which both sensitively influence the charge carrier mobility. A detailed understanding of the interplay of molecular chemical structure, molecular packing, and thin film morphology is therefore necessary to improve OTFT performance. Fluorene-bithiophene-fluorene (FTTF) derivatives have demonstrated great potential for use as the active layer for OTFTs. A series of FTTF asymmetrically substituted derivatives were synthesized to fine-tune the film properties. In this study, the thin film microstructure details of FTTF and several FTTF derivatives with asymmetrically substituted alkyl-chains of different lengths are studied and compared with their electrical performance in thin film transistor devices. The respective unit cells were characterized using grazing incidence X-ray diffraction (GIXD), and for FTTF the detailed molecular packing was determined from the GIXD int...

Conjugation Effects on Carrier Mobilities of Polythiophenes Probed by Time-Resolved Terahertz Spectroscopy

Abstract: Optically generated carrier conductivity of thiophene-based spin-cast polymer films, as cast poly(3-hexylthiophene) (P3HT) and both as cast and above liquid crystalline temperature annealed poly(2,5-bis(3-tetradecylthiophen-2yl)thieno[3,2-b]thiophene) (PBTTT), were measured by time-resolved THz spectroscopy (TRTS) and compared to reported thin-film transistor (TFT) device measurements. The relative mobilities of the samples measured by TRTS agree well with reported TFT mobilities. Since TRTS is sensitive to short-range carrier mobility, this study suggests that increased mobility of PBTTT relative to P3HT films arises from improved ordering of the local structure, increased effective conjugation length, and planarity of PBTTT molecules in the solid state.

The Impact of the Dielectric/Semiconductor Interface on Microstructure and Charge Carrier Transport in High-Performance Polythiophene Transistors

Abstract: The performance of organic field-effect transistors (OFETs) significantly depends on the properties of the interface between the semiconductor and gate dielectric. Here, we study the impact of chemically modified and morphologically controlled dielectrics on the performance of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) semiconductors. We find that the molecular packing, domain size, and carrier mobility of pBTTT are highly sensitive to dielectric chemistry and dielectric roughness. The large and well-oriented terraced domains that are the origin of pBTTT's high performance can develop well on certain dielectrics, but can be disrupted on others.