Showing papers by "Hoichang Yang published in 2016"

Efficient Visible Quasi-2D Perovskite Light-Emitting Diodes

Abstract: Efficient quasi-2D-structure perovskite light-emitting diodes (4.90 cd A(-1) ) are demonstrated by mixing a 3D-structured perovskite material (methyl ammonium lead bromide) and a 2D-structured perovskite material (phenylethyl ammonium lead bromide), which can be ascribed to better film uniformity, enhanced exciton confinement, and reduced trap density.

Li-Assisted Low-Temperature Phase Transitions in Solution-Processed Indium Oxide Films for High-Performance Thin Film Transistor.

Abstract: Lithium (Li)-assisted indium oxide (In2O3) thin films with ordered structures were prepared on solution-processed zirconium oxide (ZrO2) gate dielectrics by spin-casting and thermally annealing hydrated indium nitrate solutions with different Li nitrate loadings. It was found that the Li-assisted In precursor films on ZrO2 dielectrics could form crystalline structures even at processing temperatures (T) below 200 °C. Different In oxidation states were observed in the Li-doped films, and the development of such states was significantly affected by both temperature and the mol% of Li cations, [Li+]/([In3+] + [Li+]), in the precursor solutions. Upon annealing the Li-assisted precursor films below 200 °C, metastable indium hydroxide and/or indium oxyhydroxide phases were formed. These phases were subsequently transformed into crystalline In2O3 nanostructures after thermal dehydration and oxidation. Finally, an In2O3 film doped with 13.5 mol% Li+ and annealed at 250 °C for 1 h exhibited the highest electron mobility of 60 cm2 V−1 s−1 and an on/off current ratio above 108 when utilized in a thin film transistor.

Unified film patterning and annealing of an organic semiconductor with micro-grooved wet stamps

Abstract: A unified patterning and annealing approach was successfully demonstrated for 5,11-bis(triethylsilylethynyl)-anthradithiophene (TES-ADT) films spun-cast on polymer-treated SiO2 dielectrics. First, rubbery polydimethylsiloxane (μ-PDMS) stamps with microscale periodic grooves were swollen in 1,2-dichloroethane and then softly placed onto amorphous-like TES-ADT films. In this case, the film sides physically in contact with the wet stamps were quickly absorbed into the PDMS matrix while the non-contact area formed highly-ordered phases by the solvent-annealing effect. The resulting patterns of TES-ADT contained discernable crystallites, where the grain sizes drastically decreased and their shapes transformed from spherulites to optically featureless ones with a decreasing line width from 100 to 2.5 μm. Unlike ordinary systems containing spherulitic domains, the 2.5 μm line-confined TES-ADT patterns contained layer-stacked crystallites but an optically invisible grain boundary, yielding an unexpectedly high field-effect mobility of 2.60 cm2 V−1 s−1 in organic field-effect transistors (OFETs), with narrow deviations less than 8% (averaged from 42 devices). The results suggest that the well π-overlapped grains and their smooth connections are key factors to achieve high performance multi-array OFET applications.

Composition-dependent structural and electrical properties of p-type SnOx thin films prepared by reactive DC magnetron sputtering: effects of oxygen pressure and heat treatment

Abstract: The composition-dependent structural and electrical properties of SnOx films prepared by means of reactive DC sputtering at various oxygen partial pressures (PO) and post-heat treatment temperatures (TA) were investigated, toward these films' potential use in p-channel oxide thin-film transistors (TFTs). A SnOx film fabricated under the lowest studied PO of 4% and heat-treated at 210 °C consisted of dendritic phases and irregular protrusions of metallic Sn. The resulting p-channel SnOx thin-film transistors suffered from marginal field effect mobility (μFE) and low on/off current ratio (ION/OFF), suggesting that the imperfect phases caused by oxygen-deficient stoichiometry hinder hole carrier conduction and act as bulk trap states. The heterogeneous structures observed in SnOx films annealed at 210 °C could be eliminated by increasing PO during fabrication. The resulting TFTs based on p-type SnOx films prepared at the high PO of 8% showed high mobilities up to 2.8 cm2 V−1 s−1 and reasonable ION/OFF of approximately 103, demonstrating the critical role of these films' homogeneous ordered aggregates without any imperfect phases such as a dendritic phase or irregular protrusions of metallic Sn. Among TFTs based on the films fabricated under 8% PO, the μFE and ION/OFF performance metrics degraded with increasing TA from 210 to 300 °C, which was mainly related to the 2SnO → SnO2 + Sn disproportionation reaction.

A Physicochemical Approach Toward Extending Conjugation and the Ordering of Solution-Processable Semiconducting Polymers.

Abstract: Poly(3-hexylthiophene)s (P3HTs) were synthesized with a well-controlled molecular weight (Mw) and degree of regioregularity; additionally, π-conjugated P3HT structures in both solutions and films were systematically investigated. Conjugated P3HT phases in spin-cast films significantly changed from ordered nanorods, -fibrils, and -ribbons to less-ordered granules, depending on the conformation of the P3HT chains in solutions. The chain conformations could be physicochemically adjusted by modifying chain lengths (from 5 to 45 kDa), solvents, and ultrasonication. Highly extended conformations of the P3HT in ultrasound-treated solutions yielded longer degree of conjugation both the intra- and intermolecularly. When toluene was used as a marginal solvent, ultrasonicated 0.1 wt % 29 kDa P3HT solutions could be used to yield highly ordered aggregates in spin-cast films, including nanoribbons or nanosheets, with field-effect mobility (μFET) up to ∼0.1 cm2 V–1 s–1 being measured for organic field-effect transistor...

Temperature-Driven Phase Transition of a Fused Dithienobenzothiadiazole–Tetrathiophene Based Semiconducting Copolymer

Abstract: A high-molecular-weight donor–acceptor (D–A) copolymer, pDTfBT-4T, poly(5,8-di[3-octyldodecanthiopen-5-yl]dithieno[3′,2′:3,4:2″,3″:5,6]benzo[1,2-c][1,2,5]-thiadiazole-alt-2,2′-bithiophene), including alternating dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5]thiadiazole (DTfBT) and 3-octyldodecyltetrathiophene (4T) was synthesized by a Stille coupling polymerization. We found that pDTfBT-4T had a molecular number-average weight of 276 kg mol–1 and formed unexpectedly strong interchain aggregates in dilute solutions at room temperature, which was similar to those in as-spun thin solid films. pDTfBT-4T thin films were spun-cast from a warm dilute chlorobenzene solution on hydrophobic polymer-treated SiO2 dielectrics. Some were shortly annealed at various temperatures (T) for 10 min to improve π-overlapped structures as charge-carrier transport paths. The ordered domains of pDTfBT-4T in the annealed films could be tuned from one-dimensional nanorods to two-dimensional nanosheets with an increasing in T, whi...

Balancing Surface Hydrophobicity and Polarizability of Fluorinated Dielectrics for Organic Field‐Effect Transistors with Excellent Gate‐Bias Stability and Mobility

Abstract: It is demonstrated that treating dielectrics with fluorinated polymers, which have excellent hydrophobicity, chemical inertness, and the lowest polarizability, yields a semiconductor-compatible surface energy and excellent charge detrapping characteristics. Fluorocopolymers, polystyrene-random-poly(2,3,4,5,6-pentafluorostyrene) (PS-r-PPFS) copolymers with different 2,3,4,5,6-pentafluorostyrene (PFS) loadings, are synthesized to modify a SiO2 gate dielectric using radical polymerization. Surface energy (γ) of the copolymer-treated SiO2 dielectrics decreases from 40.7 to 24.0 mJ m−2 with increasing PFS mol% in the copolymer. Pentacene organic field-effect transistors (OFETs) show field-effect mobility (μFET) values ranging from 0.82 (for 0 mol% PFS) to 0.25 cm2 V−1 s−1 (for 100 mol% PFS). Enhancing the bias stress stability without affecting the μFET value is achieved via the introduction of a small mol% fluorocarbon segments onto the PS-r-PPFS backbone. 15 mol% PFS-loaded fluorocopolymer-coated SiO2 substrate yields a γ value of 35 mJ m−2, close to that (38 mJ m−2) of the lowest-γ crystal surface of pentacene, and the corresponding OFETs have μFET values up to 0.81 cm2 V−1 s−1 and excellent gate-bias stress stability in comparison to the rich fluorinated dielectric systems, which has degraded μFET values ranging from 0.2 to 0.4 cm2 V−1 s−1.

Control of consistent ordering in π-conjugated polymer films for organic field-effect transistor applications

Abstract: A novel conjugated copolymer, pDPP-(TV)2B-2DO, including donor (D) and acceptor (A) blocks of (TV)2B-2DO, 1,4-bis((E)-2-(thiophen-2-yl)vinyl)benzene with dodecyloxy moieties and diketopyrrolopyrrole (DPP) derivatives, respectively, was synthesized by Stille-coupling polymerization. The 60 kDa pDPP-(TV)2B-2DO showed good solubility in organic solvents and strong molecular interaction during film procedures. Semiconducting thin films were spun-cast on polymer-treated SiO2 from chloroform and annealed at temperatures ranging from 150 to 300 °C for 1 h. The electrical properties of these films in organic field-effect transistors (OFETs) were evaluated and correlated with π-conjugated ordering of the pDPP-(TV)2B-2DO, before and after annealing. Featureless aggregates of the face-on copolymer by the bulky B-2DO in the D block were dominant in the as-spun film and transformed into ordered rods or fibrils containing highly ordered edge-on polymers on the substrates after annealing. In terms of π-conjugated ordering, the as-spun and 250 °C-annealed pDPP-(TV)2B-2DO films contained highly-consistent chains with face-on and edge-on structures, respectively. The resulting OFETs yielded similar field-effect mobilities of 0.07 and 0.08 cm2 V−1 s−1, respectively, as well as on–off current ratios greater than 106 and a threshold voltage near 0 V. Therefore, the π-overlap of edge-on semiconducting polymers on gate dielectrics was not necessary to enhance the electrical performance of OFETs, but the high consistency in chain ordering was very important.