Jizheng Wang

Low-cost fabrication of submicron all polymer field effect transistors

All polymer field effect transistors have been fabricated combining nanoimprint lithography and inkjet printing. Trenches with hydrophilic bottoms confined by hydrophobic walls with considerable height are patterned by nanoimprint lithography. Conducting polymer solutions were then delivered into these trench liquid containers by inkjet printing. Dried conducting polymer in nearby trenches forms source-drain electrodes with the channel length accurately defined by the gap in between the designed two trenches. Top-gate all polymer field effect transistors with submicron channel lengths were successfully realized by such low-cost process.

Suppression of short-channel effects in organic thin-film transistors

To improve the speed of organic thin-film transistor (TFT) circuits device architectures with submicrometer channel length are of interest. However, in conventional, submicrometer TFT structures the performance is degraded as a result of short-channel effects. Here we present an architecture for short-channel organic TFTs which is based on incorporating an insulating mesa structure in between source and drain electrodes. For submicrometer organic TFTs the mesa structure results in a significant enhancement of the on-off ratio and saturation characteristics. Device modeling shows that the mesa improves the ability of the gate electrode to modulate the carrier concentration in a submicrometer channel.

Poly(ethylene glycol) modified [60]fullerene as electron buffer layer for high-performance polymer solar cells

A poly(ethylene glycol) end caped fullerene derivative, PEGN-C60, is used as electron buffer layer (EBL) in polymer solar cells (PSCs). Three donor:acceptor blends, namely P3HT:PCBM, PBDTTT-C:PC70BM, and PBDTTT-C-T:PC70BM, are employed to test the behavior of PEGN-C60 as EBL. The optimized power conversion efficiencies of the three systems reach 3.84%, 6.22%, 7.45%, respectively, which are slightly higher than that of their corresponding devices with metal Ca as EBL and much higher than that of their devices without any EBLs. Our results indicate that exploring EBLs based on fullerene derivatives might be an efficient way in finding functional solution-processable EBLs for high-performance PSCs.

Photoluminescence of InAs quantum dots grown on GaAs surface

InAs quantum dots (QDs) grown on GaAs surface are investigated. The observed abnormal photoluminescence (PL) properties, including extremely sharp high-energy peaks, almost temperature-independent linewidth, and fast thermal quenching, are discussed in terms of the strong quantum confinement effects due to the absence of a cap layer and the lack of carrier redistribution channel caused by the small number of QDs capable of contributing to PL and the high-density surface defects

Efficiency-Enhanced Planar Perovskite Solar Cells via an Isopropanol/Ethanol Mixed Solvent Process

Solution processable perovskite solar cells traditionally employed isopropanol as the solvent of CH3NH3I in a two-step method. One of the largest issues of this technique is the uncontrollable morphology of the perovskite film. In this study, a homogeneous and dense PbI2 film was prepared by introducing DMSO as an additive into DMF and then reacting the mixture with CH3NH3I dissolved in an isopropanol/ethanol solvent to fabricate high-quality perovskite films. Results revealed that ethanol played a crucial role on morphology and components of perovskite films. When the ratio of isopropanol to ethanol was optimized, a power conversion efficiency of 15.76% was achieved, which was on average ∼50% higher than that of PSCs without DMSO and ethanol processing.

An efficient selenophene-containing conjugated copolymer for organic solar cells

A D–A polymer, PSeTPTI, was developed by copolymerizing a pentacyclic acceptor unit TPTI with a selenophene unit. PSeTPTI possesses a narrow optical bandgap, a low-lying HOMO energy level, and a high hole mobility of 0.26 cm2 V−1 s−1. PSeTPTI/PC71BM solar cells demonstrate a PCE of 6.04%, which is the highest efficiency for the conjugated copolymers using selenophene as the donor unit.

Transparent and transferrable organic optoelectronic devices based on WO3/Ag/WO3 electrodes

With a thin metal film inserted between two oxide layers, the WO3/Ag/WO3 (WAW) multilayer structure owns both high transmittance and high conductivity. By carefully optimizing Ag film thickness, WAW shows high average transmittance of 82.5% in 400–750 nm range and low sheet resistance of 20 Ω/sq. Employing such WAW electrodes, transparent organic photodetectors are fabricated, and with help of a polyacrylonitrile protective layer, the devices can be smartly separated and transferred onto other substrates while maintaining their performances well. Moreover, transferrable organic solar cells are also realized with such transparent WAW electrodes.

Bilayer PbS Quantum Dots for High‐Performance Photodetectors

Due to their wide tunable bandgaps, high absorption coefficients, easy solution processabilities, and high stabilities in air, lead sulfide (PbS) quantum dots (QDs) are increasingly regarded as promising material candidates for next-generation light, low-cost, and flexible photodetectors. Current single-layer PbS-QD photodetectors suffer from shortcomings of large dark currents, low on-off ratios, and slow light responses. Integration with metal nanoparticles, organics, and high-conducting graphene/nanotube to form hybrid PbS-QD devices are proved capable of enhancing photoresponsivity; but these approaches always bring in other problems that can severely hamper the improvement of the overall device performance. To overcome the hurdles current single-layer and hybrid PbS-QD photodetectors face, here a bilayer QD-only device is designed, which can be integrated on flexible polyimide substrate and significantly outperforms the conventional single-layer devices in response speed, detectivity, linear dynamic range, and signal-to-noise ratio, along with comparable responsivity. The results which are obtained here should be of great values in studying and designing advanced QD-based photodetectors for applications in future flexible optoelectronics.

Realization of nonvolatile organic memory device without using semiconductor

In our work, we realized bistable memory performance combining tunneling with trapping effect without using any semiconductor. In this insulator-metal system (ITO/PS(Polystyrene)/Ag/PI(Polyimide)/Al), we evaporated a discontinuous silver layer(about several nanometers) as traps between two insulator layers, which is sandwiched by two electrodes. For this kind of device we observed excellent bistable memory property with a large ratio of 103–104, long retention time of more than 104 s, good cycling performance and excellent reproducibility. Finally, according to the capacitance measurement, we found the tunneling process and trapping effect was the most possible processing mechanism.

Structural and optical characteristics of self-organized InAs quantum dots grown on GaAs (3 1 1)A substrates

Structural and optical investigations of InAs QDs grown on GaAs (3 1 1)A by molecular beam epitaxy (MBE) were reported. InAs/GaAs (3 1 1)A QDs with nonconventional, faceted, arrowhead-like shapes aligned in the [ - 2 3 3] direction have been disclosed by AFM image. Low defect and dislocation density on the QDs interfaces were indicated by the linear dependence of photoluminescence (PL) intensity on the excitation power. The fast red shift of PL energy and the monotonic decrease of FWHM with increasing temperature were observed and explained by carriers being thermally activated to the energy barrier produced by the wetting layer and then retrapped and recombined in energetically low-lying QDs states