Photovoltaic Performance of Ultrasmall PbSe Quantum Dots

We investigated the effect of PbSe quantum dot size on the performance of Schottky solar cells made in an ITO/PEDOT/PbSe/aluminum structure, varying the PbSe nanoparticle diameter from 1 to 3 nm. In this highly confined regime, we find that the larger particle bandgap can lead to higher open-circuit voltages (~0.6 V), and thus an increase in overall efficiency compared to previously reported devices of this structure. To carry out this study, we modified existing synthesis methods to obtain ultrasmall PbSe nanocrystals with diameters as small as 1 nm, where the nanocrystal size is controlled by adjusting the growth temperature. As expected, we find that photocurrent decreases with size due to reduced absorption and increased recombination, but we also find that the open-circuit voltage begins to decrease for particles with diameters smaller than 2 nm, most likely due to reduced collection efficiency. Owing to this effect, we find peak performance for devices made with PbSe dots with a first exciton energy of ~1.6 eV (2.3 nm diameter), with a typical efficiency of 3.5%, and a champion device efficiency of 4.57%. Comparing the external quantum efficiency of our devices to an optical model reveals that the photocurrent is also strongly affected by the coherent interference in the thin film due to Fabry-Pérot cavity modes within the PbSe layer. Our results demonstrate that even in this simple device architecture, fine-tuning of the nanoparticle size can lead to substantial improvements in efficiency

Analytical and Numerical Study of Photocurrent Transients in Organic Polymer Solar Cells

This article is an attempt to provide a self consistent picture, including existence analysis and numerical solution algorithms, of the mathematical problems arising from modeling photocurrent transients in Organic-polymer Solar Cells (OSCs). The mathematical model for OSCs consists of a system of nonlinear diffusion-reaction partial differential equations (PDEs) with electrostatic convection, coupled to a kinetic ordinary differential equation (ODE). We propose a suitable reformulation of the model that allows us to prove the existence of a solution in both stationary and transient conditions and to better highlight the role of exciton dynamics in determining the device turn-on time. For the numerical treatment of the problem, we carry out a temporal semi-discretization using an implicit adaptive method, and the resulting sequence of differential subproblems is linearized using the Newton-Raphson method with inexact evaluation of the Jacobian. Then, we use exponentially fitted finite elements for the spatial discretization, and we carry out a thorough validation of the computational model by extensively investigating the impact of the model parameters on photocurrent transient times.Comment: 20 pages, 11 figure

Spatially controlled electrostatic doping in graphene p-i-n junction for hybrid silicon photodiode

Sufficiently large depletion region for photocarrier generation and separation is a key factor for two-dimensional material optoelectronic devices, but few device configurations has been explored for a deterministic control of a space charge region area in graphene with convincing scalability. Here we investigate a graphene-silicon p-i-n photodiode defined in a foundry processed planar photonic crystal waveguide structure, achieving visible - near-infrared, zero-bias and ultrafast photodetection. Graphene is electrically contacting to the wide intrinsic region of silicon and extended to the p an n doped region, functioning as the primary photocarrier conducting channel for electronic gain. Graphene significantly improves the device speed through ultrafast out-of-plane interfacial carrier transfer and the following in-plane built-in electric field assisted carrier collection. More than 50 dB converted signal-to-noise ratio at 40 GHz has been demonstrated under zero bias voltage, with quantum efficiency could be further amplified by hot carrier gain on graphene-i Si interface and avalanche process on graphene-doped Si interface. With the device architecture fully defined by nanomanufactured substrate, this study is the first demonstration of post-fabrication-free two-dimensional material active silicon photonic devices.Comment: NPJ 2D materials and applications (2018

Development of High-Performance Graphene-HgCdTe Detector Technology for Mid-Wave Infrared Applications

A high-performance graphene-based HgCdTe detector technology is being developed for sensing over the mid-wave infrared (MWIR) band for NASA Earth Science, defense, and commercial applications. This technology involves the integration of graphene into HgCdTe photodetectors that combines the best of both materials and allows for higher MWIR(2-5 m) detection performance compared to photodetectors using only HgCdTe material. The interfacial barrier between the HgCdTe-based absorber and the graphene layer reduces recombination of photogenerated carriers in the detector. The graphene layer also acts as high mobility channel that whisks away carriers before they recombine, further enhancing the detector performance. Likewise, HgCdTe has shown promise for the development of MWIR detectors with improvements in carrier mobility and lifetime. The room temperature operational capability of HgCdTe-based detectors and arrays can help minimize size, weight, power and cost for MWIR sensing applications such as remote sensing and earth observation, e.g., in smaller satellite platforms. The objective of this work is to demonstrate graphene-based HgCdTe room temperature MWIR detectors and arrays through modeling, material development, and device optimization. The primary driver for this technology development is the enablement of a scalable, low cost, low power, and small footprint infrared technology component that offers high performance, while opening doors for new earth observation measurement capabilities

Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published versio

The Boston University Photonics Center annual report 2016-2017

This repository item contains an annual report that summarizes activities of the Boston University Photonics Center in the 2016-2017 academic year. The report provides quantitative and descriptive information regarding photonics programs in education, interdisciplinary research, business innovation, and technology development. The Boston University Photonics Center (BUPC) is an interdisciplinary hub for education, research, scholarship, innovation, and technology development associated with practical uses of light.This has undoubtedly been the Photonics Center’s best year since I became Director 10 years ago. In the following pages, you will see highlights of the Center’s activities in the past year, including more than 100 notable scholarly publications in the leading journals in our field, and the attraction of more than 22 million dollars in new research grants/contracts. Last year I had the honor to lead an international search for the first recipient of the Moustakas Endowed Professorship in Optics and Photonics, in collaboration with ECE Department Chair Clem Karl. This professorship honors the Center’s most impactful scholar and one of the Center’s founding visionaries, Professor Theodore Moustakas. We are delighted to haveawarded this professorship to Professor Ji-Xin Cheng, who joined our faculty this year.The past year also marked the launch of Boston University’s Neurophotonics Center, which will be allied closely with the Photonics Center. Leading that Center will be a distinguished new faculty member, Professor David Boas. David and I are together leading a new Neurophotonics NSF Research Traineeship Program that will provide $3M to promote graduate traineeships in this emerging new field. We had a busy summer hosting NSF Sites for Research Experiences for Undergraduates, Research Experiences for Teachers, and the BU Student Satellite Program. As a community, we emphasized the theme of “Optics of Cancer Imaging” at our annual symposium, hosted by Darren Roblyer. We entered a five-year second phase of NSF funding in our Industry/University Collaborative Research Center on Biophotonic Sensors and Systems, which has become the centerpiece of our translational biophotonics program. That I/UCRC continues to focus on advancing the health care and medical device industries

Fabrication and Simulation of Perovskite Solar Cells

Since the dawning of the industrial revolution, the world has had a need for mass energy production. In the 1950s silicon solar panels were invented. Silicon solar panels have been the main source of solar energy production. They have set the standard for power conversion efficiency for subsequent generations of photovoltaic technology. Solar panels utilize light’s ability to generate an electron hole pair. By creating a PN Junction in the photovoltaic semiconductor, the electron and hole are directed in opposing layers of the solar panel generating the electric current. Second generation solar panels utilized different thin film materials to fabricate solar panels. Materials such as Cadmium Telluride, Copper Indium Gallium Selenide, and amorphous silicon. This technology is now seen commercially available around the world. In the research community a third generation of solar panel technology is being developed. Perovskites are an emerging third generation solar panel technology. Perovskites’ power conversion efficiency have increased from 3.8% to 24.2% over the span of a decade. Perovskite crystals have desirable optical properties such has a high absorption coefficient, long carrier diffusion length, and high photoluminescence. The most prominent types of perovskites for solar cell research are organic metal halide perovskites. These perovskites utilize the desirable properties of organic electronics. Electrochemical techniques such as additives, catalysts, excess of particular chemicals, and variations in antisolvents impact the electronic properties of the perovskite crystal. The perovskite is however on layer of the device. Solar cell devices incorporate multiple layers. The materials for the electron transport layer, hole transport material, and choice of metal electrode have an impact on device performance and the current voltage relationship. Current silicon photovoltaic devices are more expensive than conventional fossil fuel. Modeling perovskite solar cells in a simulated environment is critical for data analytics, real fabrication behavior projection, and quantum mechanics of the semiconductor device. Photovoltaic semiconductors are diodes which produce a current when exposed to light. The ideality factor is a parameter which tells how closely a semiconductor behaves to an ideal diode. In an ideal diode, the only mechanism for hole electron recombination is direct bimolecular recombination. Because there are multiple mechanisms of recombination, there are no real devices with a perfect ideality factor. The types of recombination occurring within a device can be inferred by its ideality factor. In this research. Analyzing fabricated perovskite solar cells using their ideality factor can indicate which type of recombination is dominant in the device. The interaction between the perovskite crystal and transport layers is of high interest as differentials in energy level bands can hinder overall power conversion efficiency and act as a site for nonradiative recombination loss. In addition, the use of Machine Learning (ML) to research and predict the opto-electronic properties of perovskite can greatly accelerate the development of this technology. ML techniques such as Linear Regression (LR), Support Vector Regression (SVR), and Artificial Neural Networks (ANNs) can greatly improve the chemical processing and manufacturing techniques. Such tools used to improve this technology have major impacts for the further proliferation of solar energy on a national scale. These tools can also be used to optimize power conversion efficiency of perovskites, This optimization is critical for commercial use of perovskite solar panel technology. Various electrochemical and fabrication strategies are currently being researched in order to optimize power conversion efficiency and minimize energy loss. There are current results which suggest the addition of particular ions in the perovskite crystal have a positive impact on the power conversion efficiency. The qualities of the cell such as crystallinity, defects, and grain size play important roles in the electrical properties of the cell. Along with the quality of the perovskite crystal, its interfacing with the transport layers plays a critical role in the operation of the device. In this thesis, perovskite solar cells are fabricated and simulated to research their optoelectronic properties. The optoelectronic behavior of simulated solar cells is manipulated to match that or cells. By researching this new optoelectronic material in a virtual environment, applicability and plausibility are demonstrated. This legitimizes the continued research of this third-generation solar panel material

Carrier Lifetime vs. Proton Radiation in Prototype III-V and II-VI Space-based Infrared Detectors

Researchers have spent over 50 years improving the performance of HgCdTe infrared (IR) detectors and it is currently the dominant technology in the field; however, further improvement may be limited due to devices reaching the intrinsic limits of their constituent materials. To further improve the state-of-the-art in space-based IR detection, alternative material systems are being considered. The focus of this work is testing the space-environment viability of innovative device structures, namely unipolar barriers with Type-II superlattice (T2SL) absorbers, made from the 6.1 Å family of III-V elements which are theoretically superior performers while being less costly. Sensitive IR photo-detection using III-V material systems has been demonstrated; however, overall performance to-date has been hindered by short minority carrier lifetimes attributed to high concentrations of Shockley-Read-Hall (SRH) recombination centers. This problem is exacerbated when these materials are exposed to charged particle vi irradiation, as is unavoidable for spacecraft electronics, due to displacement damage increasing the concentration of SRH defects. In this work, a measurement system was designed and constructed to directly measure the minority carrier recombination lifetimes of prototype IR detector structures at the wafer die level as functions of proton fluence and temperature, to include both HgCdTe and the new 6.1 Å T2SL nBn technology being considered. It is unique for two reasons: 1) it was designed to be portable which allows in-situ lifetime characterization vs. stepwise proton irradiation by deploying it to radiation sources across the country, and 2) through cryogenic cooling, it maintains samples at mission operating temperatures throughout entire irradiation experiments which enables post-radiation annealing studies. The typical radiation test found in literature is a single, large dose performed at room temperature. The conclusions in this dissertation are derived from analyses on data acquired from this measurement system at a monoenergetic proton source. Radiation tolerances of the minority carrier lifetime and post-radiation annealing effects are compared between HgCdTe photodiodes and 6.1 Å T2SL nBn detector structures, the effects of doping and other design parameters on the lifetime damage factors in III-V materials are investigated, and a damage factor vs. proton energy (NIEL) study was performed on III-V structures which allows spacecraft mission planners to extrapolate lifetime damage factors in these materials through any proton differential energy spectra of interest, i.e. satellite orbit

Optical TCAD on the Net: A tight-binding study of inter-band light transitions in self-assembled InAs/GaAs quantum dot photodetectors

A new capability of our well-known NEMO 3-D simulator (Ref. Klimeck et al., 2007 [10]) is introduced by carefully investigating the utility of III–V semiconductor quantum dots as infrared photodetectors at a wavelength of 1.2–1.5 μm. We not only present a detailed description of the simulation methodology coupled to the atomistic sp3d5s∗ tight-binding band model, but also validate the suggested methodology with a focus on a proof of principle on small GaAs quantum dots (QDs). Then, we move the simulation scope to optical properties of realistically sized dome-shaped InAs/GaAs QDs that are grown by self- assembly and typically contain a few million atoms. Performing numerical experiments with a variation in QD size, we not only show that the strength of ground state inter- band light transitions can be optimized via QD size-engineering, but also find that the hole ground state wavefunction serves as a control factor of transition strengths. Finally, we briefly introduce the web-based cyber infrastructure that is developed as a government- funded project to support online education and research via TCAD simulations. This work not only serves as a useful guideline to experimentalists for potential device designs and other modelers for the self-development of optical TCAD, but also provides a good chance to learn about the science gateway project ongoing in the Republic of Korea