Challenges and solutions for high-efficiency quantum dot-based LEDs

Colloidal quantum dots (QDs) hold great promise as electrically excited emitters in light-emitting diodes (LEDs) for solid-state lighting and display applications, as highlighted recently by the demonstration of a red-emitting QD-LED with efficiency on par with that of commercialized organic LED technologies. In the past five years, important advances have been made in the synthesis of QD materials, the understanding of QD physics, and the integration of QDs into solid-state devices. Insights from this progress can be leveraged to develop a set of guidelines to direct QD-LED innovation. This article reviews the fundamental causes of inefficiency in QD-LEDs understood to date and proposes potential solutions. In particular, we emphasize the challenge in developing QD emitters that exhibit high luminescent quantum yields in the combined presence of charge carriers and electric fields that appear during traditional LED operation. To address this challenge, we suggest possible QD chemistries and active layer designs as well as novel device architectures and modes of QD-LED operation. These recommendations serve as examples of the type of innovations needed to drive development and commercialization of high-performance QD-LED

Automatic concept identification of software requirements in Turkish

Software requirements include description of the features for the target system and express the expectations of users. In the analysis phase, requirements are transformed into easy-to-understand conceptual models that facilitate communication between stakeholders. Although creating conceptual models using requirements is mostly implemented manually by analysts, the number of models that automate this process has increased recently. Most of the models and tools are developed to analyze requirements in English, and there is no study for agglutinative languages such as Turkish or Finnish. In this study, we propose an automatic concept identification model that transforms Turkish requirements into Unified Modeling Language class diagrams to ease the work of individuals on the software team and reduce the cost of software projects. The proposed work is based on natural language processing techniques and a new rule-set containing twenty-six rules is created to find object-oriented design elements from requirements. Since there is no publicly available dataset on the online repositories, we have created a well-defined dataset containing twenty software requirements in Turkish and have made it publicly available on GitHub to be used by other researchers. We also propose a novel evaluation model based on an analytical hierarchy process that considers the experts' views and calculate the performance of the overall system as 89%. We can state that this result is promising for future works in this domain

Study of field driven electroluminescence in colloidal quantum dot solids

Semiconductor nanocrystals, or quantum dots(QDs), promise to drive advances in electronic light generation. It was recently shown that long range transport of charge, which is typically required for electric excitation and which is inherently limited in nanosized materials, can be avoided by developing devices that operate through local, field-assisted generation of charge. We investigate such devices that consist of a thin film of CdSe/ZnS core-shell QD placed between two dielectric layers and that exhibit electroluminescence under pulsed, high field excitation. Using electrical and spectroscopic measurements, we are able to elucidate the dynamics of charge within the QD layer and determine that charge trapping and field-induced luminescence quenching are the main limitations of device performance.Massachusetts Institute of Technology. Energy Frontier Research Center for ExcitonicsUnited States. Dept. of Energy. Center for Excitonics (Award DE-SC0001088)National Science Foundation (U.S.

Research Update: Comparison of salt- and molecular-based iodine treatments of PbS nanocrystal solids for solar cells

Molecular- and salt-based chemical treatments are believed to passivate electronic trap states in nanocrystal-based semiconductors, which are considered promising for solar cells but suffer from high carrier recombination. Here, we compare the chemical, optical, and electronic properties of PbS nanocrystal-based solids treated with molecular iodine and tetrabutylammonium iodide. Surprisingly, both treatments increase—rather than decrease—the number density of trap states; however, the increase does not directly influence solar cell performance. We explain the origins of the observed impact on solar cell performance and the potential in using different chemical treatments to tune charge carrier dynamics in nanocrystal-solids.ISSN:2166-532

Research Update: Comparison of salt- and molecular-based iodine treatments of PbS nanocrystal solids for solar cells

Molecular- and salt-based chemical treatments are believed to passivate electronic trap states in nanocrystal-based semiconductors, which are considered promising for solar cells but suffer from high carrier recombination. Here, we compare the chemical, optical, and electronic properties of PbS nanocrystal-based solids treated with molecular iodine and tetrabutylammonium iodide. Surprisingly, both treatments increase—rather than decrease—the number density of trap states; however, the increase does not directly influence solar cell performance. We explain the origins of the observed impact on solar cell performance and the potential in using different chemical treatments to tune charge carrier dynamics in nanocrystal-solids

A quantitative model for charge carrier transport, trapping and recombination in nanocrystal-based solar cells

Improving devices incorporating solution-processed nanocrystal-based semiconductors requires a better understanding of charge transport in these complex, inorganic–organic materials. Here we perform a systematic study on PbS nanocrystal-based diodes using temperature-dependent current–voltage characterization and thermal admittance spectroscopy to develop a model for charge transport that is applicable to different nanocrystal-solids and device architectures. Our analysis confirms that charge transport occurs in states that derive from the quantum-confined electronic levels of the individual nanocrystals and is governed by diffusion-controlled trap-assisted recombination. The current is limited not by the Schottky effect, but by Fermi-level pinning because of trap states that is independent of the electrode–nanocrystal interface. Our model successfully explains the non-trivial trends in charge transport as a function of nanocrystal size and the origins of the trade-offs facing the optimization of nanocrystal-based solar cells. We use the insights from our charge transport model to formulate design guidelines for engineering higher-performance nanocrystal-based devices.ISSN:2041-172

Independent Composition and Size Control for Highly Luminescent Indium-Rich Silver Indium Selenide Nano crystals

ISSN:1936-0851ISSN:1936-086

In Situ Measurement and Control of the Fermi Level in Colloidal Nanocrystal Thin Films during Their Fabrication

ISSN:1948-718