Dawen Li

Humidity effect on electrical performance of organic thin-film transistors

Humidity dependence of electrical performance of p-channel organic thin-film transistors (OTFTs) with various semiconductor compounds has been investigated. All devices showed decreased current output and mobility as the relative humidity (RH) was increased. The moisture sensitivity of the OTFT saturation current depends on the device geometry (bottom or top contact device) and channel length. The OTFT configuration with a short channel length and bottom contact was most affected by humidity compared to the top contact and larger channel length OTFT structures. The degradation of electrical performance under high RH is attributed to charge trapping at grain boundaries by polar water molecules reducing the rate of charge transport.

PS‐b‐P3HT Copolymers as P3HT/PCBM Interfacial Compatibilizers for High Efficiency Photovoltaics

A conducting diblock copolymer of PS-b-P3HT was added to serve as a compatibilizer in a P3HT/PCBM blend, which improved the power-conversion efficiency from 3.3% to 4.1% due to the enhanced crystallinity, morphology, interface interaction, and depth profile of PCBM.

Modelling of segmented high-performance thermoelectric generators with effects of thermal radiation, electrical and thermal contact resistances.

In this study, segmented thermoelectric generators (TEGs) have been simulated with various state-of-the-art TE materials spanning a wide temperature range, from 300 K up to 1000 K. The results reveal that by combining the current best p-type TE materials, BiSbTe, MgAgSb, K-doped PbTeS and SnSe with the strongest n-type TE materials, Cu-Doped BiTeSe, AgPbSbTe and SiGe to build segmented legs, TE modules could achieve efficiencies of up to 17.0% and 20.9% at ΔT = 500 K and ΔT = 700 K, respectively, and a high output power densities of over 2.1 Watt cm−2 at the temperature difference of 700 K. Moreover, we demonstrate that successful segmentation requires a smooth change of compatibility factor s from one end of the TEG leg to the other, even if s values of two ends differ by more than a factor of 2. The influence of the thermal radiation, electrical and thermal contact effects have also been studied. Although considered potentially detrimental to the TEG performance, these effects, if well-regulated, do not prevent segmentation of the current best TE materials from being a prospective way to construct high performance TEGs with greatly enhanced efficiency and output power density.

Switching phase separation mode by varying the hydrophobicity of polymer additives in solution-processed semiconducting small-molecule/polymer blends

Lateral and vertical phase separations play critical roles in the performance of the next-generation organic and hybrid electronic devices. A method is demonstrated here to switch between lateral and vertical phase separations in semiconducting 6,13-bis(triisopropylsilylethynyl) pentacene (TIPSE pentacene)/polymer blend films by simply varying the alkyl length of the polyacrylate polymer component. The phase separation modes depend on intermolecular interactions between small molecule TIPSE pentancene and polymer additives. The blend film with a dominant vertical phase separation exhibits a significant enhancement in average mobility and performance consistency of organic thin-film transistors.

Assembly and Organization of Poly(3-hexylthiophene) Brushes and Their Potential Use as Novel Anode Buffer Layers for Organic Photovoltaics

Buffer layers that control electrochemical reactions and physical interactions at electrode/film interfaces are key components of an organic photovoltaic cell. Here the structure and properties of layers of semi-rigid poly(3-hexylthiophene) (P3HT) chains tethered at a surface are investigated, and these functional systems are applied in an organic photovoltaic device. Areal density of P3HT chains is readily tuned through the choice of polymer molecular weight and annealing conditions, and insights from optical absorption spectroscopy and semiempirical quantum calculation methods suggest that tethering causes intrachain defects that affect co-facial π-stacking of brush chains. Because of their ability to modify oxide surfaces, P3HT brushes are utilized as an anode buffer layer in a P3HT-PCBM (phenyl-C₆₁-butyric acid methyl ester) bulk heterojunction device. Current-voltage characterization shows a significant enhancement in short circuit current, suggesting the potential of these novel nanostructured buffer layers to replace the PEDOT:PSS buffer layer typically applied in traditional P3HT-PCBM solar cells.

High-performance organic field-effect transistors with dielectric and active layers printed sequentially by ultrasonic spraying

High-performance organic field-effect transistors (OFETs) are reported with dielectric and active layers sequentially deposited by ultrasonic spray-printing. A cross-linkable insulator and a soluble small molecule semiconductor are developed which are both printable and highly robust. Using plastic with pre-patterned indium tin oxide gate contacts as required for display applications, two different layers are sequentially spray-printed: the semiconductor 6,13-bis(trisopropylsilylethynyl)pentacene (TIPS-PEN), and the insulator poly-4-vinylphenol (PVP). OFETs printed in ambient air with a bottom-gate/top-contact geometry are shown to achieve on/off ratios of >104 and mobilities up to 0.35 cm2 V−1 s−1. These rival the characteristics of the best solution-processable small molecule FETs fabricated by other processing methods such as drop casting and ink-jet printing.

Structural and magnetic properties of patterned perpendicular media with linearly graded anisotropy

L10-FePt thin films with linearly distributed anisotropy were fabricated by continuously varying substrate temperatures from 650 °C to 290 °C. An average ordering parameter S = 0.6 was obtained. The hysteresis loop has a clear bow-tie shape with low remanence and coercivity before patterning. The patterned nanopillar arrays exhibit an enhancement in both remanence and coercivity. Dynamic coercivity measurement shows that the nanopillar arrays with linearly graded anisotropy have a relatively small intrinsic coercivity and a relatively high value of the figure of merit. The switching field exhibits a narrow distribution, which indicates a strong coupling between soft and hard phases in nanopillar arrays with graded anisotropy.

Solution-grown small-molecule organic semiconductor with enhanced crystal alignment and areal coverage for organic thin film transistors

Drop casting of small-molecule organic semiconductors typically forms crystals with random orientation and poor areal coverage, which leads to significant performance variations of organic thin-film transistors (OTFTs). In this study, we utilize the controlled evaporative self-assembly (CESA) method combined with binary solvent system to control the crystal growth. A small-molecule organic semiconductor,2,5-Di-(2-ethylhexyl)-3,6-bis(5″-n-hexyl-2,2′,5′,2″]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione (SMDPPEH), is used as an example to demonstrate the effectiveness of our approach. By optimizing the double solvent ratios, well-aligned SMDPPEH crystals with significantly improved areal coverage were achieved. As a result, the SMDPPEH based OTFTs exhibit a mobility of 1.6 × 10−2 cm2/V s, which is the highest mobility from SMDPPEH ever reported.

Air-stable solution-processed n-channel organic thin film transistors with polymer-enhanced morphology

N,N′-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2) is an n-type semiconductor exhibiting high electron mobility and excellent air stability. However, the reported electron mobility based on spin-coated PDIF-CN2 film is much lower than the value of PDIF-CN2 single crystals made from vapor phase deposition, indicating significant room for mobility enhancement. In this study, various insulating polymers, including poly(vinyl alcohol), poly(methyl methacrylate) (PMMA), and poly(alpha-methylstyrene) (PαMS), are pre-coated on silicon substrate aiming to enhance the morphology of the PDIF-CN2 thin film, thereby improving the charge transport and air stability. Atomic force microscopy images reveal that with the pre-deposition of PαMS or PMMA polymers, the morphology of the PDIF-CN2 polycrystalline films is optimized in semiconducting crystal connectivity, domain size, and surface roughness, which leads to significant improvement of organic thin-film transistor (OTFT) performance. Particularly, an...

A polymorph of the 6,13-dichloropentacene organic semiconductor: crystal structure, semiconductor measurements and band structure calculations

Polymorphism is an important issue for the application of organic semiconductors in field-effect transistors due to the dependence of device performance on crystal packing. Recently an organic semiconductor, 6,13-dichloropentacene (DCP), was shown to give a field-effect mobility as high as 9 cm2 V−1 s−1 when crystallized in the form of microribbons obtained by physical vapor transport (PVT), in contrast with the much lower mobilities found in crystals grown from solution. In the present manuscript we show that the crystals grown in the vapor phase are a polymorph of the structure reported for the crystals grown from solution. Both polymorphs show a π-stacking motif but differ in the interplanar distances and the displacement of the molecules along the long axes of the molecules. The DCP PVT structure compares favorably to that of rubrene, but DCP shows a much lower mobility. Band structure calculations show that the rubrene crystal exhibits greater band dispersion than DCP even though the long-axis displacement in rubrene is much greater. We conclude that the transport properties cannot be inferred simply by the magnitude of the intrastack long-axis displacement, but requires a consideration of the relative location of the HOMO/LUMO coefficients of the neighboring molecules in the stack.