Shaofeng Lu

Large modulation of carrier transport by grain-boundary molecular packing and microstructure in organic thin films

Solution-processable organic semiconductors are central to developing viable printed electronics, and performance comparable to that of amorphous silicon has been reported for films grown from soluble semiconductors. However, the seemingly desirable formation of large crystalline domains introduces grain boundaries, resulting in substantial device-to-device performance variations. Indeed, for films where the grain-boundary structure is random, a few unfavourable grain boundaries may dominate device performance. Here we isolate the effects of molecular-level structure at grain boundaries by engineering the microstructure of the high-performance n-type perylenediimide semiconductor PDI8-CN2 and analyse their consequences for charge transport. A combination of advanced X-ray scattering, first-principles computation and transistor characterization applied to PDI8-CN2 films reveals that grain-boundary orientation modulates carrier mobility by approximately two orders of magnitude. For PDI8-CN2 we show that the molecular packing motif (that is, herringbone versus slip-stacked) plays a decisive part in grain-boundary-induced transport anisotropy. The results of this study provide important guidelines for designing device-optimized molecular semiconductors.

Anthracenedicarboximide-based semiconductors for air-stable, n-channel organic thin-film transistors: materials design, synthesis, and structural characterization

A family of six n-channel organic semiconductors (1–6) based on the N,N′-dialkyl-2,3:6,7-anthracenedicarboximide (ADI) core was synthesized and characterized. These new semiconductors are functionalized with n-octyl (-n-C8H17), 1H,1H-perfluorobutyl (-n-CH2C3F7), cyano (–CN), and bromo (–Br) substituents, which results in wide HOMO and LUMO energy variations (∼1 eV) but negligible optical absorbance (λmax = 418–436 nm) in the visible region of the solar spectrum. Organic thin-film transistors (OTFTs) were fabricated via semiconductor vapor-deposition, and the resulting devices exhibit exclusively electron transport with good carrier mobilities (μe) of 10−3 to 0.06 cm2 V−1 s−1. Within this semiconductor family, cyano core-substitution plays a critical role in properly tuning the LUMO energy to enable good electron transport in ambient conditions while maintaining a low level of ambient doping (i.e., low Ioff). Core-cyanated ADIs 3 and 6 exhibit air-stable TFT device operation with electron mobilities up to 0.04 cm2 V−1 s−1 in air. Very high current on/off ratios of >107 are measured with positive threshold voltages (Vth = 5–15 V) and low off currents (Ioff = 10−9 to 10−12 A). Single-crystal structures of N,N′-1H,1H-perfluorobutyl ADIs 5 and 6 exhibit slipped-stack cofacial crystal packing with close π–π stacking distances of ∼3.2 A. Additionally, close intermolecular interactions between imide-carbonyl oxygen and anthracene core-hydrogen are identified, which lead to the assembly of highly planar lamellar layers. Analysis of the air-stability of 1–6 thin films suggests that air-stability is mainly controlled by the LUMO energetics, and an electrochemical threshold of Ered1 = −0.3 to −0.4 V is estimated to stabilize n-channel transport in this family of materials.

Tailoring the molecular structure to suppress extrinsic disorder in organic transistors

In organic field-effect transistors, the structure of the constituent molecules can be tailored to minimize the disorder experienced by charge carriers. Experiments on two perylene derivatives show that disorder can be suppressed by attaching longer core substituents - thereby reducing potential fluctuations in the transistor channel and increasing the mobility in the activated regime - without altering the intrinsic transport properties.

Synthesis and thin-film transistor performance of benzodipyrrolinone and bithiophene donor-acceptor copolymers

Three novel benzodipyrrolinone–bithiophene based donor-acceptor polymers are synthesized and used as p-type semiconductors in organic thin-film transistors (OTFTs). Hole mobility of up to 0.03 cm2 V−1 s−1 is achieved. A new acetal-type branched substituent is introduced to significantly improve the solubility of the resulting polymer.

Influence of the contact metal on the performance of n-type carbonyl-functionalized quaterthiophene organic thin-film transistors

Organic thin-film transistors using 5, 5‴-diperfluorohexylcarbonyl-2,2′:5′,2″:5″,2‴-quaterthiophene (DFHCO-4T) as the electron conducting organic semiconductor are fabricated and the performance of these transistors with different top-contact metals is investigated. Transistors with Au source-drain top contacts attain an apparent saturation mobility of 4.6 cm2/V s, whereas this parameter is 100 times lower for similar transistors with Al/LiF top contacts. We explain this lower performance by the formation of a thin interfacial layer with poor charge injection properties resulting from a redox reaction between Al and DFHCO-4T.

Empirically based device modeling of bulk heterojunction organic photovoltaics

We develop an empirically based optoelectronic model to accurately simulate the photocurrent in organic photovoltaic (OPV) devices with novel materials including bulk heterojunction OPV devices based on a new low band gap dithienothiophene-DPP donor polymer, P(TBT-DPP), blended with PC70BM at various donor-acceptor weight ratios and solvent compositions. Our devices exhibit power conversion efficiencies ranging from 1.8% to 4.7% at AM 1.5G. Electron and hole mobilities are determined using space-charge limited current measurements. Bimolecular recombination coefficients are both analytically calculated using slowest-carrier limited Langevin recombination and measured using an electro-optical pump-probe technique. Exciton quenching efficiencies in the donor and acceptor domains are determined from photoluminescence spectroscopy. In addition, dielectric and optical constants are experimentally determined. The photocurrent and its bias-dependence that we simulate using the optoelectronic model we develop, wh...

Realization of dual-channel organic field-effect transistors and their applications to chemical sensing

We report on the realization of dual-channel organic field-effect transistors (FETs). These devices have a four-terminal configuration with a polymeric semiconductor p-channel, a small molecule semiconductor n-channel, and a polymeric gate dielectric. The polymeric p-channel and the small molecule n-channel are coupled across the gate dielectric. Both the p-FET and the n-FET exhibit acceptable device characteristics at ∣VDS∣⩽50V and ∣VG∣⩽50V, in which the performances of the p-FET and the n-FET are comparable. The p-FET and n-FET respond to isopropyl alcohol and ethanol vapors with significant sensitivities.

Organic gate insulator materials for amorphous metal oxide TFTs

Numerous solution-processed dielectrics were studied in metal oxide thin film transistors (TFTs) to understand their potential application in flexible display technology. A series of dielectrics was synthesized and systematically formulated to deposit organic gate insulating thin film layers that demonstrate low leakage current (≤10-8 A/cm2 at 2 MV/cm), high breakdown voltages (>150 V), film flexibility, photopatternability (5-10 μm via hole size), increased temperature stability (up to temperatures of 300 °C), and resistance to common chemicals used in the indium gallium zinc oxide (IGZO) TFT fabrication process. In particular, surface modification improved the stability of bottom-gate organic gate insulators during the sputtering, patterning, and annealing processes for the IGZO active layer on the dielectric surface. The best IGZO TFT performance was achieved when certain Polyera organic gate insulators were incorporated in top-gate top-contact IGZO TFT devices, yielding excellent mobility (~15 cm2/V·s), Vth ~ 0 V, negligible hysteresis, sharp sub-threshold swing (~300 mV/dec), and good bias temperature stress stability. When combined with polymer substrates such organic gate insulators yield truly flexible IGZO TFTs compatible with large-scale production methods.

CCDC 852483: Experimental Crystal Structure Determination

Related Article: H.Usta, Choongik Kim, Zhiming Wang, Shaofeng Lu, Hui Huang, A.Facchetti, T.Marks|2012|J.Mater.Chem.|22|4459|doi:10.1039/c1jm14713g

P-194L: Late-News Poster: Through-Breaking Organic TFT Materials for Active Matrix Display Backplane Application

In this paper, e will present a through-breaking result on new organic TFT materials for AMOLED display backplane application. We have achieved organic TFTs with mobility of ∼ 3 cm2/Vs, on/off ratio 107, good uniformity, and estimated bias stress lifetime >10,000hours. This offers a promising route to TFT backplanes for flexible/OLED displays.