Yishan Wang

Pentacene-Based Photodetector in Visible Region With Vertical Field-Effect Transistor Configuration

Electrical and detection performances of pentacene-based photodetector with vertical field-effect transistor (VFET) configuration were investigated in full visible region. By comparing with planar FET-based photodetector ITO/ PMMA(520 nm)/Pentacene(35 nm)/Au, the VFET-based photodetector ITO/Pentacene(80 nm)/Al(15 nm)/Pentacene(80 nm)/ Au exhibits better performance. At an output current of ca. -8 × 10-7 A, the threshold voltage (Vth) was -0.61 V for the VFET-based device at VDS = -2 V, but Vth = -7.1 V for the planar one at VDS = -12 V. The performance of photodetectors depends on incident monochromatic light, and the VFET-based photodetector showed a maximum responsivity of 188 mA/W and a photosensitivity peak of 588 under 350-nm light, which were ~11.75 and 2.83 times as that of the planar one, respectively. Therefore, it provides an easy way to get the VFET-based organic photodetectors in full visible region with excellent photosensitivity, responsivity, and light selectivity, showing its promising application in all-organic image sensors working at low voltages.

Charge Carrier Conduction Mechanism in PbS Quantum Dot Solar Cells: Electrochemical Impedance Spectroscopy Study.

With its properties of bandgap tunability, low cost, and substrate compatibility, colloidal quantum dots (CQDs) are becoming promising materials for optoelectronic applications. Additionally, solution-processed organic, inorganic, and hybrid ligand-exchange technologies have been widely used in PbS CQDs solar cells, and currently the maximum certified power conversion efficiency of 9.9% has been reported by passivation treatment of molecular iodine. Presently, there are still some challenges, and the basic physical mechanism of charge carriers in CQDs-based solar cells is not clear. Electrochemical impedance spectroscopy is a monitoring technology for current by changing the frequency of applied alternating current voltage, and it provides an insight into its electrical properties that cannot be measured by direct current testing facilities. In this work, we used EIS to analyze the recombination resistance, carrier lifetime, capacitance, and conductivity of two typical PbS CQD solar cells Au/PbS-TBAl/ZnO/ITO and Au/PbS-EDT/PbS-TBAl/ZnO/ITO, in this way, to better understand the charge carriers conduction mechanism behind in PbS CQD solar cells, and it provides a guide to design high-performance quantum-dots solar cells.

Performance Enhancement of FET-Based Photodetector by Blending P3HT With PMMA

In this letter, we presented a solution-processed photodetector with a configuration of field-effect transistor (FET) ITO/poly(4-vinylphenol) (PVP)/poly(3-hexylthiophene) (P3HT): poly(methyl methacrylate) (PMMA)/Au in which PVP acts as a dielectric layer and different PMMA content (20 wt.% ~ N 60 wt.%) in P3HT as active layer. The best electrical property of the photodetector under no illumination was obtained with 20 wt.% PMMA content and the maximum ON-OFF current ratio and hole mobility of the as-prepared devices are 329 and 1.6 x 10-3 cm2/V · s, respectively. Under illumination with wavelengths varying from 350 to 650 nm, however, the 50 wt.% PMMA content device demonstrated highest performance, showing a maximum photoresponsivity of 166.45 mA/W under 65 μW/cm2 of 600-nm illumination. Atom force microscope (AFM) phase images of P3HT:PMMA film certify the phase separation between P3HT and PMMA, as well as the crystallinity improvement of P3HT film after blending PMMA. The performance of FET-based photodetector under illumination is discussed.

High performance solution-processed infrared photodetector based on PbSe quantum dots doped with low carrier mobility polymer poly(N-vinylcarbazole)

Colloidal quantum dots (CQDs) are promising materials for flexible electronics, light sensing and energy conversion. In particular, as a narrow bandgap semiconductor, lead selenide (PbSe) CQDs have attracted considerable interest due to their potential applications in infrared (IR) optoelectronics such as IR light-emitting diodes (LEDs), photodetectors and solar cells. Solution-processed photodetectors are more attractive owing to their flexible, large-scale and low-cost fabrication, and their performance depends greatly on the film quality and surface morphology. In this study, a high performance solution-processed infrared photodetector based on PbSe CQDs blended with low hole mobility polymer poly(N-vinylcarbazole) (PVK) is presented. In order to obtain a higher device performance, different volume ratios (K = VPVK/VPbSe) of PVK (20 mg ml−1 in chloroform) in PbSe CQDs (15 mg ml−1 in chlorobenzene) were investigated, and a maximum responsivity and specific detectivity of 2.93 A W−1 and 1.24 × 1012 jones, respectively, were obtained at VG = −20 V under 30 mW cm−2 980 nm laser illumination for field-effect transistor (FET)-based photodetector Au(S&D)/PbSe : PVK/PMMA/Al(G), in which PbSe : PVK nanocomposite with K = 1 : 2 acts as the active layer and poly (methyl methacrylate) (PMMA) as the dielectric layer. The reasons for the high device performance of PbSe : PVK nanocomposite as an active layer are discussed, in which PbSe nanoparticles were blended with low hole mobility polymer PVK but showed comparable detectivity as that blended with regioregular P3HT. Moreover, all these types of photodetectors are very stable for reverse fabrication using PMMA dielectric layer to shield the active layer from the environment and by inorganic ligand exchange treatment on the active layer.

High performance solution-processed infrared photodiode based on ternary PbSxSe1−x colloidal quantum dots

Semiconductor quantum dots (QDs) have been the subject for wide research studies owing to their quantum confinement effect. Photodetectors or photodiodes are recognized potential applications for QDs due to their high photosensitivity, solution processability and low cost of production. In this paper, a solution-processed near-infrared photodiode ITO/ZnO/PbSxSe1−x/Au, in which ternary PbSxSe1−x QDs act as the active layer and the ZnO interlayer acts as electron-transporting layer, was demonstrated. The photosensitive spectrum can be broadened by adjusting the molar fraction of ternary PbSxSe1−x QDs. The narrow band edge of absorption and photoluminescence exciton energy of PbSxSe1−x alloyed NCs were blue-shifted from the band edge of the same size PbSe QDs to the band edge of PbS QDs by controlling the S/(Se + S) molar ratio in the synthetic mixture. Efficient electron extraction was carried out by inserting a solution-processed ZnO interlayer between the indium-tin oxide (ITO) electrode and the active layer. Our experimental results show that the solution processing of the ZnO layer can lead to high-performance photodiodes by using photosensitized PbS0.4Se0.6 alloyed nanocrystals as the active layer. The effect of the thickness of the active layer on the device performance was briefly described and a maximum photoresponsivity and specific detectivity of 25.8 A/W and 1.30 × 1013 Jones, respectively, were obtained at a certain thickness under 100 μW cm−2 980 nm laser illumination. The devices are made stably by layer-by-layer ligand exchange treatment.