Vikram L. Dalal - US grants

At this REU Site electrical engineering seniors will gain research experience by participating in projects in microelectronics and photonics. Specifically, they will work on continuing and new projects in a) advanced silicon processing for devices and integrated circuits, b) high-power and high-frequency bipolar transistors and other microwave devices, c) III-V compound quantum nanostructures, d) non-crystalline silicon structures for energy conversion, e) basic semiconductor material studies and f) ASIC design for high-frequency circuits and systems. They will work on these projects full time during the summer and part time during the academic year. Cooperation with Lincoln University, North Carolina A & T, and the State University of New York- New Palz, and institutions with a significant minority enrollment is planned. A microelectronics fabrication course each summer with teaching responsibilities shared by all four participating faculty is offered. Students will be able to transfer the course credits to their home institutions. A number of these research-trained and motivated students will continue into graduate studies and elsewhere; consequently, graduate students interested in microelectronics and photonics from the program participants will be actively recruited.

The Iowa State REU gives students research experience by participating in projects in microelectronics and photonics. These students work along side students supported by the Department of Education Undergraduate Research Opportunities for Minorities as well as the NSF Minority Scholars Program, and the NASA supported Iowa Space Grants Program. Specifically, these students work on continuing and new projects in a) advanced silicon processing for devices and integrated circuits, b) high power and high frequency bipolar transistors and other microwave devices, c) III-V compound quantum nanostructures, d) non-crystalline silicon structures for energy conversion, and e) basic semiconductor material studies. A microelectronics fabrication course is offered each summer of the program.

9906622
Dalal

Thin film electronic devices on plastic substrates are likely to become a very important class of new devices. Among the many applications of such devices are conformal flat panel displays, organic LED'S, very large, fault-tolerant memories for terrestrial and space applications, hybrid imagers and cameras based on combinations of amorphous and crystalline materials, strained-layer superlattice light emission devices in Group IV materials, chemical sensors, self-powered sensors, photovoltaic devices etc. To fabricate many of these devices, it is essential that the crystallinity, composition, electronic properties and stability of the materials and device interfaces be controlled in-situ during growth. To-date, various approaches, such as post-deposition laser recrystallization, have been used to fabricate devices on plastic films. But these approaches are not always feasible for large areas, nor do they easily allow mixed-phase [e.g. amorphous/crystalline] or superlattice devices to be made. Therefore, the critical research need for all these novel devices is the ability to deposit materials of high electronic quality, stability and controlled crystallinity directly on plastic substrates. In this exploratory proposal, we suggest using a well-controlled growth process, reactive plasma beam deposition, to deposit high quality Group IV materials, such as microcrystalline and polycrystalline Si, (Si,Ge) and (Si,C) directly on plastic substrates and then measuring their electronic properties. The deposition process uses a well controlled beam of reactive ions, such as H, produced using an ECR reactor, to control crystallinity, alloy composition and grain size during growth so that no post-processing laser crystallization is needed. The objective of the exploratory program is to develop the basic understanding and process control needed for depositing high quality, stable Group IV devices on plastic substrates in later years.

The program has a strong educational component, involving graduate students, post-Doctoral scientists, and undergraduate students.

The industrial impact of the program is likely to be significant. The devices mentioned are the cutting-edge technology for display industries, photovoltaic industries, sensors, new generation of imagers etc., and are likely to enjoy very high growth rates.
***

In recent years. new applications such as large area conformal displays, multi-color organic displays. fault-tolerant terra-byte memories, large area solar cells, conformal electronic sensors for biological applications etc. have demanded that active electronic devices be grown on plastic films. With the exception of organic LED's, almost all such devices use amorphous Si as the active materiaL However, a-Si material suffers from low carricr mobilities, and poor stability. Therefore, the ability to grow thin film electronic devices in cri stalline Si-based materials on plastic substrates may provide both better performance and greater stability in these applications. Such a development would also allow one to grow c-Si devices on polyimide layers, which are widely used as planarizing insulators in standard c-Si wafer-based device technology, and thereby allow three dimensional device architecture. The development of thin film crystalline devices would be very useful for efficient integration of organic LED's with driver circuits on a polymer substrate. It is the objective of this proposal to develop hi2h performance crystalline Si- based electronic devices on plastic substrates.
The proposed work is based on our recent success, under a NSF SGER grant, in being able to deposit high quality c-Si films on polyimide. and making proof-of-concept c-Si solar cells in them. We plan a comprehensive research project to improve the electronic properties of the film and to make novel TFT devices in thcse films. The project embraces the following tasks:
o Control of crystallinity of the films by using reactive ion beams of H, Cl and F during growth in a novel, well-controlled ECR reactor.
o Systematic investigation of structural properties by using Raman, TEM. SEM and x-ray diffraction.
o Systematic investigation of electronic properties by using DLTS, Hall measurements and capacitance spectroscopy of defect levels.
o Study of growth of stable field-oxides (gate insulators in MOS devices) by controlled plasma-oxidation with F and 0 and study of oxide/semiconductor interfaces, using capacitance and conductance techniques.
o Fabrication of horizontal TFT devices.
o Measurement of properties of these devices, including channel mobility and stability after charge pumping and charge injection.
o Fabrication of novel proottof. concept vertical TFT which may not be affected by grain boundaries.
The effort relies on using our past experience in material growth, material analysis, device fabrication and device design and analysis. The facilities exist to carry out this work.
A significant educational component is planned, including graduate student and undergraduate student involvement. The undergraduate students will be drawn from our on-going NSF-REU site program. A significant minority student research experience is also planned. Women students, particularly at the high school level, will also participate in the program. The program will lead to development of new courses in this area, thereby integrating research and education.
The project has a potentially significant impact on industry. We have planned a strong collaboration with a company, Micron technologies, which is expected to provide a cash grant for supporting additional graduate students in this research area.

This grant supports acquisition of a research grade multi-wavelength laser Raman spectrometer. The laser Raman spectrometer has two main features: (1) multiple laser wavelengths (400 to 800 nm, visible range) to complement the 1064 nm line of the FT-Raman spectrometer, and (2) microsampling. The multiple laser wavelengths is accomplished through a standard 633 nm Ne He laser. Notch filters will be used to get within 50-cm. exp. (-1) of the 413, 568, and 647 laser lines. To minimize the cost, the standard operating mode of spectrometer will be the microsampling microscope. For bulk samples, this provides the ability to spatially resolve the spectra to look for texture and variation in the sample. For microsamples, this provides the micron level resolution required. The microscope is equipped with an auto-focusing and xy-scan microscope stage that enables efficient collection of the spectra combined with the ability to perform automated xy-scanning of the surfaces of samples, greatly extending the power and flexibility of the spectrometer. Finally, a heating and freezing stage allows temperature dependent studies to be conducted from -196 to 600 degrees C. This enables examination of thermal stability of samples as well as the effect of temperature on the material structure. This laser Raman spectrometer will be heavily used in undergraduate and graduate teaching, as a core component in materials characterization courses that cover thermal and spectroscopic techniques of characterizing materials. In this way nearly every student in the MSE department will use these new instrument systems The new spectrometer will also be used for a summer NSF-REU site, where 12 undergraduate students from all over the country come to ISU to learn about semi-conducting materials and devices.

Raman spectroscopy combined with microsampling capability has been shown by many researchers to be a critical analytical tool in the study of bulk, surface, and micro samples. Its power, flexibility, and non-destructive nature lend itself to wide spread use for nearly all classes of materials. Recent advances in the development of highly efficient, tunable, reliable, compact, and cost effective lasers combined with high resolution and rugged grating monochromators and solid state CCD detectors has moved the use of laser Raman spectroscopy from the realm of highly skilled spectroscopists into routine use by practicing materials scientists and engineers. Raman spectroscopy is particularly well suited for use in materials research due to minimal sample preparation, wide flexibility of sampling conditions (low and high temperatures, low and high pressure, low and high magnetic field, etc.), generally very sharp well resolved lines for solid state samples, and its non-destructive nature. Combined with a microsampling Raman microscope, Raman spectroscopy can be used extremely effectively in the careful study of surface chemistry, structure, morphology, texture, and even stress.

0428220
Shinar

The objective of this proposal is to develop novel photoluminescence
(PL)-based sensors that are fully structurally integrated: The light
source, the sensing element, and the photodetector (PD) and associated
filter, are fabricated on transparent substrates and attached
back-to-back. The resulting sensors could therefore be extremely
compact, robust, selective, fast, autonomous, consume little power, and
inexpensive. The proposal focuses on sensing oxygen, a key tool in
medical, environmental, (bio)chemical, and food monitoring, and Bacillus
anthracis toxin (anthrax).

The intellectual merit. The central concept is the novel total
structural integration of the foregoing components. The light source is
an array of organic light-emitting device (OLED) pixels. The sensing
elements include porous films with an embedded dye, surface immobilized
species whose PL is selectively analyte-sensitive, or microfluidic
channels/wells with recognition elements in solution. The PD and filter
are multilayer thin films of hydrogenated nanocrystalline Si, SiGe,
and/or SiC. The geometry will be "back detection," i.e., the OLED and PD
pixels are fabricated on the same side of the substrate. The array of
long-pass filters and PD pixels is fabricated first, followed by the
OLEDs in the gaps between the PD pixels. The sensing element is
fabricated on a separate substrate and attached to the OLED/PD
substrate. In the complete device, the electronic circuitry (including
instruction receiver and data transmitter), readout, and battery will be
positioned "behind" the PD. Hence the whole device would be ~2.5"x5"x1",
far more compact and less costly than any sensors currently available.
The work results in a new platform for PL-based sensors, which can be
further developed to multianalyte sensor microarrays. Innovative
elements are (i) the complete integration of all the sensor components,
and (ii) the development of sensing elements utilizing microfluidic
architectures and films/surfaces tailored for specific recognition
molecules, which will enhance the sensitivity and shorten the response
time. Moreover, the OLEDs will be operated in a pulsed mode, which will
increase their lifetime and generate negligible heat, which is crucial
for heat-sensitive recognition elements and agents. Oxygen will be
monitored via the PL lifetime, thus eliminating the need for frequent
calibration.

Different approaches will be evaluated to generate robust sensors for
real-world applications. The oxygen sensor will be based on the
dynamical quenching of the PL of oxygen-sensitive dyes, initially with a
green OLED and Pt octaethyl porphyrin (PtOEP) dye. We will compare the
sensors with dyes embedded in solid films with dyes solutions. The
anthrax sensor will be based on the cleavage of certain peptides by
anthrax lethal factor. Labeled peptides will be synthesized at the
Protein Facility of Iowa State University (ISU), with a Forster
resonance energy transfer donor and acceptor on either side of the
cleavage site.

The broader impacts. The sensors for the two aforementioned agents will
be ideal for a broad range of applications in areas such as homeland
security, medical, environmental, biological, food/brewing, and
health/safety. Beyond these impacts, the devices define a new sensor
platform for chemical and biological agents, which could lead to
extremely compact and inexpensive multianalyte sensor microarrays. The
proposed work will serve as a basis for the development of this
platform. It will also expand the basic knowledge in
embedding/immobilizing recognition elements, sensor design, and sensor
engineering. It will also have a broad educational impact, promoting the
growth of the interdisciplinary biophysics program at ISU and training
students in condensed matter physics, electrical engineering,
biophysics, chemistry, and molecular biology. It will be integrated with
teaching by developing new experimental course modules for graduate
students. Significant participation of undergraduates, including
minorities and women, is planned.

The objective of this research is to acquire research instrumentation for doing research on photonic waveguides and photonic devices, integrated biosensors, sensors for detection of toxic chemicals, organic semiconductors and neuron regrowth. These devices demand state of the art alignment capability with both front and back alignments. Iowa State has active research programs in each of the above areas, but does not have back and front sub-micron mask aligner. In this proposal, we plan to acquire a Suss MA/BA6 mask aligner capable of sub-micron lithography and of simultaneous back and front alignment so that device can be made on both sides of a substrate and devices in the submicron range can be made. This aligner will complement our existing array of tools, thereby allowing for a significant increase in our research productivity across a large number of disciplines.
Intellectual merit and Broad Impact
The proposed instrument meets a critical need at the University. No other comparable instrument is available at Iowa State. The instrument complements the following tools we own: Alcatel Deep RIE, poly-Si CVD furnace, Standard Si fabrication tools, Metal evaporators, Nanocrystalline Si plasma deposition reactors, Organic LED reactors, Single cantilever AFM tool. The instrument will allow faculty, students and research staff from Chemical Engineering, Electrical Engineering, Materials Science and Engineering, Mechanical Engineering, Physics, Chemistry and Biology to conduct experiments in the fields of photonic bandgaps, directed neural regrowth, nanocrystalline Si devices, integrated chemical and biological sensors based on OLED's and GMR devices, thin film resonators and multi-cantilver AFM tools for combinatorial diagnosis of surfaces. The instrument will be housed at the Microelectronics Research Center (MRC), an inter-disciplinary center at Iowa State where all the complementary facilities exist and are available for use by all the Iowa State faculty, staff and students. A full-time scientist will be in charge of the instrument and will maintain it in addition to some of the other tools. A maintenance account will be set up to charge the users a use-fee to pay for maintenance.
There will be significant impact on education of students, both graduate and undergraduate. This instrument, along with the existing instruments will be used for future experimental modules in courses dealing with fabrication of semiconductor, photonic and MEMS materials and devices Approximately 40 graduate students in many disciplines who use MRC facilities, and about 15 undergraduates who do research at MRC as part of their senior design or independent research projects will benefit from the acquisition of this instrument. A significant number of women and minority students are expected to participate in research activities made possible by the acquisition of this instrument. The proposed research activity has potentially broad impacts in the fields of electronic and photonic devices, sensors and neural science and engineering.

The objective of the research is to develop high efficiency, stable solar cells on plastic substrates using nanocrystalline Si and its alloy materials. The approach is to use a novel device design to increase the performance of solar cells. The nanocrystalline Si and its alloy materials will be deposited using a low ion energy plasma process with controlled hydrogen dilution which can result in films with larger grain size and better photovoltaic devices. The novel device design will allow for more light collection by combining amorphous and crystalline devices in one cell structure. Graded interfaces will be used to optimize device performance. A systematic study of material properties as a function of plasma parameters, grain size and hydrogen dilution will be undertaken so as to optimize device performance. Significant research interaction with a company, Iowa Thin Film Technologies, is planned so as to be able to utilize the expertise of the company in depositing solar cells on plastic substrates and in optical enhancement technology.

The project will result in a significant enhancement of the technology for producing low cost solar cells on plastic substrates. Increasing the performance and stability of the solar cell will likely result in lowering the cost of solar energy conversion and in reducing global warming. The educational impact of the project is also significant, with a strong involvement of both undergraduate and graduate students in the project, including mentoring of minority students.

Photovoltaic energy conversion is an important industry for the future of the world?s energy system. The current production volume worldwide is over 2.5 GW/year, growing at over 40% per year. The current industry is based approximately 91% on the use of crystalline Si, and the growth of the industry has been hampered by the high and increasing cost of Si wafers.

Intellectual Merit:

This GOALI program focuses on developing low cost, high performance solar photovoltaic technology based on thin film Si deposited on polymer substrates, using a transformative device architecture.
This proposal addresses an alternative thin film silicon technology that can achieve a level of performance similar to that of thick crystalline silicon , while using only 1-2 micrometer thicknesses. The proposed work focuses on the investigation of novel device architectures including use of amorphous/crystalline superlattices, photonic bandgap structures and new material growth techniques to achieve higher performance in thin film silicon.

Broader Impact:

The program involves a strong collaboration with PowerFilm Solar, the leading U. S. company working on polymer based solar cells. The program will offer research and educational opportunities to both graduate and undergraduate students.

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

The objective of this project is to acquire a nanolithography instrument, which will significantly improve the research capabilities at Iowa State and the University of Iowa in the fields of microelectronics, photonics and biology. The nanolithography instrument is an e-Line system, which is capable of 20 nm accuracy and can be used to make nano patterns over large areas.

Intellectual merit: The instrument allows for precise patterning at the nanoscale of electronic, photonic and biological materials and devices. It will allow fabrication of photonic crystal structures for improving the efficiency of solar cells. The instrument will be used for defining nanochannels for ordered regrowth of neurons and stem cells and for defining channels that will be utilized for studying the influence of various pesticides on mobility of nematodes, which are a major corn pest. The instrument will also be used to produce the patterns needed for negative index materials for advanced photonic devices such as waveguides and light valves.

Broad impacts: The technological impact of the research will be significant in many varied fields such as solar energy conversion, stem cell development, photonic devices and more efficient lighting. There will be significant integration of research and teaching through development of new lab courses and introduction of new lab modules in existing courses. Women and minority students at the graduate and undergraduate levels will be involved in the various research projects and will also benefit from the new lab classes that will be developed.

The objective of this EAGER project is to rapidly increase the efficiency and stability of organic solar cells by combining them with an inorganic solar cell in a tandem or multiple junction arrangement and do the fundamental device research needed to get over 10% stable efficiency. Achieving higher efficiency and stability simultaneously is a critical need in the field of organic solar cells. The project is based on the very recent invention by the PI of a tandem junction comprising inorganic and organic solar cells which increased the conversion efficiency and the stability compared to the organic cell by itself.

Research Objectives and Approaches
The objective of this research is to combine thin film inorganic and organic solar cells for higher conversion efficiencies (>15%) and better stability. The approach is to utilize a multiple gap arrangement of inorganic and organic thin film cells, coupled with a novel photonic-plasmon structure.

Intellectual Merit:
1. A multiple-gap cell arrangement to harvest the solar spectrum.
2. An inorganic layer as the first, high-gap cell, to remove the damaging ultraviolet and blue photons.
3. The use of a novel periodic photonic-plasmon structure, designed using rigorous scattering-matrix simulation, to optimize photon absorption.
4. The use of organic materials with long wavelength absorption
5. The use of stable contact and hole and electron-transport materials
6. Unique measurement apparatus and analysis for degradation under varying intensities, light spectra and controlled atmosphere.
7. Utilizing easily scalable, roll-to-roll (R2R) nano-imprinting techniques for photonic-plasmonic structures.

Broader Impact:
The technology can be easily transferred to manufacturing using R2R techniques. The solar panels can be molded into the structures of buildings or onto windows. The project will educate graduate and undergraduate students, mentor a post-doctoral scientist, and recruit minority and women students. The research will be incorporated into a new PV course at Iowa State. The solar cells, and a demonstration movie, will be exhibited at the Des Moines Science Center. Simple kits to build organic cell devices will be developed for high school students. An industrial company, Lightwave Power, is the GOALI partner and they will develop manufacturing techniques.

PI: Dalal, Vikram / Schiff, Eric
Proposal Number: 1336134 / 1336147
Institution: Iowa State University / Syracuse University
Title: Collaborative Proposal: Fundamental Research on Physics of Instability of Organic Solar Cells

This project will to systematically study, identify, and potentially overcome the various physical phenomena in both materials and in devices that lead to degradation of organic solar cells when subjected to light. Organic photovoltaic (OPV) devices are an increasingly important photovoltaic (PV) energy conversion technology. Recent advances in efficiency to ~12% range in both single and tandem junction OPV solar cells are very encouraging for eventual commercial deployment. However, the devices are known to degrade rapidly when exposed to light, losing 20-30% of the initial efficiency within ~100 hours of illumination, even when encapsulated or kept in inert atmospheres. A major market for this technology is building-integrated products, since in principle, the OPV devices can be laminated onto existing window frames. For such commercial deployment, it is essential that the degradation be reduced significantly, to <10% range over the lifetime of the product, which is typically ~20 years for building products. Similarly, another major market segment, providing power for rural populations in developing countries, also requires relatively long life, even though they do not require the same power conversion efficiency as grid-connected central power in the U.S. This project will systematically investigate the changes in fundamental physical parameters such as optical absorption, hole mobility, deep state densities in both the absorber materials and at the hetero-junction interface when OPV materials and devices are subjected to illumination.

The PIs will study the evolution of defects using both electrical measurements such as capacitance-frequency at different temperatures, and structural measurements such as spin resonance. PIs will study the kinetics of defect evolution over time under varying intensities of light so as to establish kinetic laws that govern defect evolution. Then, the PIs will systematically explore the thermal annealing of these defects over time, thereby finding out activation energies for annealing. The PIs will correlate these kinetics and annealing energies to the structure, morphology and composition of the organic materials, and the specific technology used for fabricating the devices. A number of different materials such as P3HT and PCDTBT will be studied and the relationship between the various kinetic parameters to the nature of the bonding in the materials will be established. We will use these results to design and fabricate better polymers which are likely to be more stable while maintaining power conversion efficiency.

The broader impact consists of the industrial impact of the work, and in educating both graduate and undergraduate students in the field of OPV devices and materials in particular, and solar energy conversion devices in general. Significant attention will be paid to transfer the research results into education by including new lab sections in existing courses. Both women and under-represented minority group students are expected to play a significant role in the research. The results of the research will be broadly disseminated to scientists and engineers through publications, and by offering webinars through IEEE. Dissemination to the general public will be done by giving talks both in the U.S. and overseas.

PI: Dalal, Vikram L.
Proposal Number: 1332934
Institution: Iowa State University
Title: US-India workshop on organic photovoltaic materials and devices

This award is being co-funded by the Office of International and Integrated Activities. The primary objective of this proposal is to improve the science and technology of organic photovoltaic materials and devices through active collaboration between scientists in the U.S. and in India. A further objective is to further the education of U.S. graduate students in this field by providing opportunities for them to work collaboratively with their peers at Universities and research laboratories in India. This proposal is focused on starting such collaboration by holding two workshops in this field, one in the U.S., and one in India. The effort is to be jointly supported by U.S. NSF for U.S. scientists and by India's Indo-US Science and Technology Forum for the Indian scientists. This workshop will focus on the organic photovoltaic (OPV) devices that are an up and coming technology of significant interest for delivering low cost, distributed electric power for rural populations. In principle, OPV devices offer a very low cost pathway for generating electricity using energy from the sun. However, for economical generation of energy, both the conversion efficiency, and the stability of organic solar cells has to improve significantly. The current generation of OPV devices has reached energy efficiencies but degrade in a very short time. These are significant problems that need to be overcome to make the technology economically feasible, and this workshop will bring together some of the best people working in this field from both the U.S. and India to address the critical issues in this technology. It is expected that the workshops will lead to complementary and synergistic research projects in this field in both countries, thereby accelerating the development of the science and technology of OPV materials and devices. OPV technology offers a potential low-cost pathway for meeting some of the energy needs. Both India and the U.S. have a significant industrial base in the area of photovoltaic technology, and this joint effort will benefit industries in both countries. The impact on education of U.S. graduate students is likely to be significant. It is important that U.S. students in Engineering and Sciences experience the research and economic infrastructure in developing countries such as India, so that they can design the devices appropriate to India which is a vast potential market for U.S. companies.

Abstract:
Non-technical:
Solar photovoltaic (PV) energy conversion is an important technology for generating low-cost electricity to replace coal-generated power. It is also very important for providing electricity to half of the people in the world who currently lack grid-connected power. For economical generation of PV power, efficiency of solar cells is a very important consideration, since area-related costs such as encapsulation, wiring and structure decrease proportionately as the efficiency of the cell increases. A very recent development in this field has been the discovery that a new thin film material, hybrid organic-inorganic metal halide based perovskites, can be used to generate PV power with a conversion efficiency of ~20%. This new material has a larger bandgap than commonly used thin film materials such as copper-indium-gallium selenide (CIGS), and therefore, can be used as a higher bandgap first cell in a monolithic tandem solar cell arrangement with CIGS. Calculations predict that such a series-connected tandem cell structures can increase the efficiency of thin film CIGS solar cells to 30% from the current value of 20%, a 50% increase. This proposal is aimed towards fundamental material and device research to achieve such high efficiency tandem cell structures. It is hoped that such a structure will lead to further developments which result in a significant decrease in the cost of solar-electric power. A graduate student will participate in this project and new teaching materials will be incorporated into courses dealing with solar energy conversion.

Technical:

The proposal is focused on designing and fabricating a monolithic tandem cell structure with a high gap perovskite cell deposited on top of a high efficiency CIGS cell. The two cells are connected using novel tunnel junctions. Several new materials, processes and device structures need to be developed for the concept to work. Among these are:
-A perovskite material with a bandgap in the 1.7-1.8 eV range as opposed to the current 1.57 eV material. The higher bandgap is needed to make efficient tandem cells with a CIGS bottom cell. We will develop efficient Pb(I-Br) perovskites to increase the bandgap.
-A perovskite material which is physically stable at higher temperatures so that transparent contacts can be deposited on the device to allow light to be incident form the top of the cell. We will use novel organic precursors such as formamidinium iodide and urea hydroiodide to achieve thermally stable perovskites.
-A new vacuum process for depositing perovskites at higher temperatures which avoids the instability of the solution growth process.
-The use of inorganic heterojunction layers for electron and hole extraction , thus avoiding unstable organic heterojunction layers.
-ITO/ZnO tunnel junctions to connect the two cells.
-Innovative CIGS cells in1.1 to1.15 eV range with high efficiency achieved by using bandgap grading strategies along with larger grain growth and deliberate Na or K doping.
The project includes comprehensive material and device analysis tasks so as to understand the physics of the device.

Abstract:
Non-technical description of the project
The conversion of solar energy into electricity using photovoltaic (PV) technology is an important technology with significant potential for reducing global warming. Currently, the costs of the PV solar energy system are dominated by systems related costs, such as structural costs, field wiring and the costs of chemical encapsulant materials and glass. In order to significantly reduce the total costs of solar PV technology, the conversion efficiency needs to be increased from the current ~15-20% to >30%, because most of the systems costs reduce proportionately with an increase inefficiency. The only way to achieve such higher efficiencies is to use a tandem junction solar cell, comprising two materials, one with a high bandgap and one with a lower bandgap. In this project, we will investigate the fundamental properties of an inorganic material, CdSe, then make solar cells in this material, and then make proof-of-concept tandem junction solar cells by combining cells made in this material with cells made in Si. The project will involve the research training of graduate and undergraduate students, including women scientists. The advantage to society is a significant lowering of the cost of solar energy conversion technology, thereby accelerating the deployment of solar energy into the nation's grid. The project is a joint project between Iowa State University and Syracuse University.

Technical description of the project:
CdSe is an interesting new material with a bandgap (~1.72 eV) which is in the range needed to make tandem cells with Si. But not much is known about the electronic properties of this material, nor have any high efficiency cells been made in this material. A fundamental objective of this project is to study electronic properties of thin films of this material and then make proof-of-concept solar cells in the material. Among the properties to be studied will be doping concentration, mobility of electrons and holes, deep defect densities and recombination phenomena, minority carrier diffusion lengths, electron affinities and energies of band edges. Structural, electronic and optical measurement techniques will be used to measure the relevant properties. CdSe films will be deposited using multi-source evaporation techniques.
In addition to studying these fundamental electronic properties, we will make proof-of-concept devices using appropriate heterojunctions. In addition, we will make proof-of-concept tandem cells using tunnel junctions between CdSe and Si. The objective is to demonstrate that one can indeed make tandem junction cells using this new material system. Appropriate heavily doped contact layers will be used on both CdSe and c-Si cells to make efficient tunnel junctions to connect the two cells. The goal is to obtain an open-circuit voltage of ~1.5V in a tandem cell, and also a high fill factor, thereby demonstrating that the tandem junction concept works. The intellectual merit of the project lies in systematically investigating the potential of a stable, inorganic material system for making high efficiency solar cells.

Non-technical:

The key challenge in reducing the cost of solar power lies in increasing the conversion efficiency of solar panels. Tandem solar cells are made from sub-cells that respond to different wavelengths of light and could double the efficiency of solar panels. Their deployment, however, has been limited by high fabrication costs. Perovskite solar cells hold promise for use in low-cost tandem solar cells due to ease of processing and chemical tuning of their absorption. Most perovskite solar cells are made from hybrid organic-inorganic materials which decompose at temperatures above 100°C and degrade in the presence of moisture. The aim of this project is to develop a perovskite device that is thermally stable to 300?, and is relatively insensitive to moisture. The cell will be an all-inorganic structure, thereby potentially improving its stability against photo-induced degradation. The new device is likely to be useful for making tandem junction cells with silicon acting as the bottom cell in a tandem junction arrangement. The technology is likely to increase the efficiency of solar energy conversion and reduce its overall cost. The project will offer a significant opportunity for research to both graduate and undergraduate students, including those from underrepresented groups in STEM.

Technical:

This project will develop photovoltaic devices based on a new inorganic perovskite?a bromide based inorganic perovskite alloy, Cs(Pb,Sn)Br3. Bandgap engineering will enable the fabrication of an optimized tandem structure. The bandgap of the CsPbBr3 can be reduced from 2.3 eV to 1.9 eV by incorporating tin, with further reductions possible by mixing cesium and rubidium. Inorganic heterojunction layers will be used to fabricate the devices. The material will be deposited using vacuum deposition processes which are easily scalable. Detailed structural and electronic characterization of the material and the device will be undertaken to understand the photovoltaic performance of the device. The devices will be tested for their stability against moisture and heat and against photoinduced degradation.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

Non-technical:

The development of three-dimensional (3D) printing has allowed custom manufacturing of complex objects with millimeter resolution. Recent advances have enabled 3D patterning of the critical dimensions of materials and devices to the submicron scale. This project will acquire a Nanoscribe Photonic Professional GT2 system to be deployed at the Microelectronic Research Center of Iowa State University. The Nanoscribe is a computer-controlled pattern generator coupled with a 3D printer to produce accurate materials, structures, and devices with a minimum feature size five hundred times smaller than the diameter of a human hair. The Nanoscribe will allow researchers to conduct transformative research in diverse science and engineering areas, such as biomedicine, agriculture, energy, photonics, defense, and advanced manufacturing. This instrument will also promote the integration of research and teaching by using the Nanoscribe in multiple laboratory classes taught by the investigators. The instrument will enrich programs that develop the next-generation STEM workforce and help to broaden the participation of undergraduate and graduate students and postdoctoral researchers. Many undergraduates will participate in the research through REU projects and as part of their senior design projects. There will be a strong emphasis on recruiting women and students from underrepresented minorities in STEM. The education of students in STEM disciplines will be aided by hands-on demonstrations and by providing high school teachers the opportunity to use the instrument and conduct research.

Technical:

The Nanoscribe instrument will enable an array of new research projects in the area of micro-nano science and technology, including multimodal wearable sensors to monitor critical parameters in precision agriculture, portable exosome-based biomarker screening devices integrated with micro-optics, photonic crystals and CMOS imaging sensors, metamaterials and metadevices with functionalities attained through the exploitation of sub-wavelength scale structures to manipulate lights, biodegradable nerve regeneration structures for efficient peripheral nerve repairs, engineered cell microenvironment, soft matter-based electronics, bioinspired adhesives, microrobotic actuators, photovoltaic devices with printed high-aspect-ratio microgrooves, high-efficiency triboelectric energy harvesting materials, and high-performance X-ray collimator with nanogrooves. The instrument will promote multidisciplinary collaborations between engineers and scientists within the departments, across Iowa State University, and beyond the institution. The capabilities of the instrument will become apparent to undergraduate and graduate students and postdoc researchers in the areas of advanced manufacturing, internet-of-things sensors, energy harvesting materials and devices, biomedical devices and instruments, soft electronics, and condensed matter physics at Iowa State University. The instrument will help Iowa State University to become a regional hub for high-resolution 3D printing support to researchers in the State of Iowa.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.