Haowei Wang

Bio: Haowei Wang is an academic researcher from Beijing Institute of Technology. The author has contributed to research in topics: Quantum dot & Specific detectivity. The author has an hindex of 11, co-authored 17 publications receiving 279 citations.

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

Abstract: 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.

Solution-Processed PbSe Colloidal Quantum Dot-Based Near-Infrared Photodetector

Abstract: A solution-processed near-infrared (NIR) photodetector based on PbSe colloidal quantum dots (CQDs) with a field-effect transistor (FET) configuration was presented. By blending PbSe CQDs into poly(3-hexylthiophene-2, 5-diyl) (P3HT) as active layer, the photosensitive spectrum of P3HT:PbSe nanocomposites extends into the NIR region. The responsivity and the specific detectivity of FET-based photodetector Au(gate)/PMMA (930nm)/P3HT:PbSe(55nm)/Au(source, drain) reached 500 A/W and $5.02 \times 10^{12}$ Jones, respectively, at $V_{\rm DS}=-40$ V and $V_{\rm G}=-40$ V with 40 mW/cm $^{2}$ of 980-nm laser illumination. It gets more stable due to its reverse fabrication using the dielectric layer to cover the active layer from environment air.

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

Abstract: 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

Abstract: 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.

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

Abstract: 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 photo-detector ITO/Pentacene(80 nm)/Al(15 nm)/Pentacene(80 nm)/ Au exhibits better performance. At an output current of ca. $-8\times 10^{-7}$ A, the threshold voltage ( $V_{\rm th}$ ) was −0.61 V for the VFET-based device at $V_{\mathrm {DS}} = -2$ V, but $V_{\mathrm {th}} = -7.1$ V for the planar one at $V_{\mathrm {DS}} = -12$ 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 $\sim 11.75$ 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.

Organic Photodiodes: The Future of Full Color Detection and Image Sensing

Abstract: Major growth in the image sensor market is largely as a result of the expansion of digital imaging into cameras, whether stand-alone or integrated within smart cellular phones or automotive vehicles. Applications in biomedicine, education, environmental monitoring, optical communications, pharmaceutics and machine vision are also driving the development of imaging technologies. Organic photodiodes (OPDs) are now being investigated for existing imaging technologies, as their properties make them interesting candidates for these applications. OPDs offer cheaper processing methods, devices that are light, flexible and compatible with large (or small) areas, and the ability to tune the photophysical and optoelectronic properties − both at a material and device level. Although the concept of OPDs has been around for some time, it is only relatively recently that significant progress has been made, with their performance now reaching the point that they are beginning to rival their inorganic counterparts in a number of performance criteria including the linear dynamic range, detectivity, and color selectivity. This review covers the progress made in the OPD field, describing their development as well as the challenges and opportunities.

Organic field-effect transistor-based flexible sensors

Abstract: Flexible electronic devices have attracted a great deal of attention in recent years due to their flexibility, reduced complexity and lightweight. Such devices can conformably attach themselves to any bendable surface and can possess diverse transduction mechanisms. Consequently, with continued emphasis on innovation and development, major technological breakthroughs have been achieved in this area. This review focuses on the advancements of using organic field-effect transistors (OFETs) in flexible electronic applications in the past 10 years. In addition, to the above mentioned features, OFETs have multiple advantages such as low-cost, readout integration, large-area coverage, and power efficiency, which yield synergy. To begin with, we have introduced organic semiconductors (OSCs), followed by their applications in various device configurations and their mechanisms. Later, the use of OFETs in flexible sensor applications is detailed with multiple examples. Special attention is paid to discussing the effects induced on physical parameters of OFETs with respect to variations in external stimuli. The final section provides an outlook on the mechanical aspects of OSCs, activation and revival processes of sensory layers, small area analysis, and pattern recognition techniques for electronic devices.

Colloidal quantum dots for optoelectronics

Abstract: This review is focused on new concepts and recent progress in the development of three major quantum dot (QD) based optoelectronic devices: photovoltaic cells, photodetectors and LEDs In each application, we discuss recent champion devices with a range of architectures and discuss in detail the chronological steps taken to produce significant improvements in efficiency We consider this relative to developments in colloidal quantum dots and their effects on these devices, covering alloyed, doped and core/shell QDs, quaternary Cu–Zn–In–S QDs, graphene and silicon QDs, and the wide range of highly promising NIR QDs The diverse range of novel device designs is examined, including all-quantum dot devices, ternary hybrid compounds, plasmonic enhancements, and nano-heterojunction architectures In addition, we analyse recent advances in charge transport layers, blocking layers, nanostructured photoanode fabrication and the importance of QD surface treatments Throughout, we emphasise the use of hybrid composite materials including combinations of QDs with metal oxides, plasmonic nanoparticles, graphene and others Finally, this review provides an analysis of prospects of these important selected quantum dot-based optoelectronic devices