NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell) - 11.22.16

Overview | Description | Applications | Operations | Results | Publications | Imagery

ISS Science for Everyone

Science Objectives for Everyone
The International Space Station (ISS) relies on solar panels for electricity. The NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-Nanotube Solar Cell) investigation studies a new type of three-dimensional solar cell that absorbs sunlight more efficiently on Earth and in space. The investigation examines the solar cell response to the continually changing sun angles and the harsh environment of space.
Science Results for Everyone
Information Pending

The following content was provided by Jud Ready, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details

OpNom:

Principal Investigator(s)
Jud Ready, Ph.D., Georgia Institute of Technology, Atlanta, GA, United States

Co-Investigator(s)/Collaborator(s)
Information Pending

Developer(s)
Georgia Institute of Technology, Georgia Tech Research Institute, GA, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
National Laboratory (NL)

Research Benefits
Earth Benefits, Space Exploration, Scientific Discovery

ISS Expedition Duration
March 2016 - September 2016

Expeditions Assigned
47/48

Previous Missions
Information Pending

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Experiment Description

Research Overview

  • NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell) evaluates carbon nanotube (CNT) based textured photovoltaic (PV) cells using an earth abundant photoabsorber material, Copper zinc tin sulfide (CZTS), Cu2ZnSnS4, contrasted against the toxic and rare cadmium telluride, CdTe, in the external environment of space.
  • NanoRacks-NanoTube Solar Cell determines if CZTS can be used as a photoabsorber in a three-dimensional (3D) PV configuration, the optimum sequencing of copper, zinc, tin and sulfide layers with the CZTS ‘stack, if placement on the International Space Station (ISS) accelerates degradation of the CZTS and/or CNTs in the 3D PV configuration, and if the continuously-varying sun-angle upon the 3D PV at the ISS accelerates electrical characterization opportunities.

Description

Three-dimensional (3D) carbon nanotube (CNT) based photovoltaic (PV) devices (3DCNTPV) are a type of next generation solar cell pioneered by the Principal Investigator in 2005 and licensed commercially to Bloo Solar in 2011. They possess unique properties, which are advantageous to both terrestrial and space based applications. 3DCNTPV consist of large scale, periodic arrays of vertically aligned CNTs (VACNTs) over which thin film PV material is then deposited. VACNTs act as both a 3D ‘scaffolding’ to support the photoabsorber, as well as, a ‘wire’ that serves as the back contact to extract the photogenerated charge carriers.
 
3D PV cells have two main advantages over planar thin film PV devices. The first advantage is that while conventional devices are produced in a superstrate (top-down) configuration that mandates mechanical inflexibility due to the cover glass, 3DCNTPV are produced in a substrate (bottom-up) configuration. Substrate configuration cells can be produced atop metal foils to create lightweight, flexible, and cost efficient PV cells, which makes them ideal for both terrestrial and space applications.
 
Although all substrate configuration cells, planar or otherwise, have the previously mentioned advantages, 3D PVs have a significant advantage over planar cells. All planar solar cells have maximum power output when oriented normal to the solar flux and fall off as a cosine function at off-normal angles. In order to maximize the power output of an array of planar cells, machinery is used to orient the array and track the solar disc. However, this machinery is heavy, bulky, occasionally unreliable, and extremely expensive to produce, install and maintain, drastically increasing both a module's overall cost per watt-peak ($/Wp) and watt per kilogram (W/kg) figures of merit. The proposed 3D PV cells have been designed to produce more power at off-normal angles of incidence without this tracking machinery. Because of the 3D geometry provided by the VACNTs, these cells allow for multiple photon impingements when the solar flux is at off-normal angles of incidence. These multiple photon impingements lead to an increased number of interactions between a photon and the photoabsorber. An increased number of interactions leads to a greater likelihood of absorption and a resulting increase in photocurrent and power output as compared to similar planar cells. As the sun subtends all zenith angles, this increased power output yields an increased energy output of many times that of a planar cell throughout the day due to an effective integration of the ever-changing insolation values.
 
Going beyond the inherent novelty of the 3DCNTPV structure, is the choice of Copper zinc tin sulfide (CZTS),Cu2ZnSnS4,as the photoabsorber in the proposed work. Single-junction PV efficiencies are all generally converging near 15-20% and recent material improvements for CZTS have already increased laboratory cell efficiency above 11% within just a few years of serious study.
 
Thus, the limiting factor that emerges as the demand for PV cells reaches towards the terawatt scale, is not improved efficiency, since all single junction structures are approximately on-par with each other near 20%, but rather material cost and availability, and commercial scalability of device fabrication. The ideal PV material has the following characteristics:
  • Semiconducting properties: robust p-type and/or n-type conductivity
  • Strong optical absorption coefficient.
  • Direct bandgap of approximately 1.5 eV
  • Low-cost and abundant chemical elements
  • Low-cost material growth
  • Compatible with existing technologies, structures and materials.
CZTS is a material that possesses all of these traits. CZTS, like the more familiar copper indium gallium selenide (CIGS), is a quaternary semiconducting compound that belongs to a class of materials known as chalcogenides, which offer favorable optical and electronic properties that makes it well suited for use as a thin-film solar cell absorber layer. But, unlike CIGS (or other thin films such as cadmium telluride (CdTe)), CZTS is composed of abundant and non-toxic elements. The abundance of zinc and tin in earth’s crust is 1,500 times and 45 times greater than that of indium, respectively, and the price of indium is almost two orders of magnitude higher than that of zinc and tin. Concerns with the price and availability of indium in CIGS and tellurium in CdTe, as well as toxicity of cadmium have been a large motivator for the PV research community to search for alternative thin film solar cell materials, and CZTS has emerged within the past few years as an extremely attractive option.
 
Although lab-measured CZTS efficiencies (approximately 11%) have not yet matched commercial silicon cell values of 15-18%, the CZTS band gap of 1.5 eV and high absorption coefficient of 104 cm-1 are near theoretical optimums, and bode well for CZTS development. Furthermore, as thin-film absorbers, they naturally require less raw material to produce and due to their elemental abundance in the earth’s crust, which affords low acquisition cost, CZTS can quickly become financially competitive as PV cell efficiency rises only slightly.
 
With the NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-Nanotube Solar Cell), the Principal Investigator aims to explore two new technologies that individually challenge and shift existing research paradigms and when taken collectively are truly novel and will result in ground-breaking scientific discoveries. The two areas of innovation and significance are CZTS photoabsorbers, and light–trapping CNT-based PV cells. The Principal Investigator has direct access to all of the necessary equipment to manufacture and characterize these novel CZTS-based 3D PV cells in a terrestrial setting at Georgia Tech. Subsequent to this, the PV cells are mounted to the ISS, which provides both a continuous and rapidly changing insolation environment as well as an extremely aggressive physical environment too. The PV cells contain differing CNT array pitch/aspect ratio/morphology as well as differing CZTS sequences and deposition techniques. The cells are characterized both on the ground and in the space environment with critical data being captured on their temperature and current/voltage characteristics.

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Applications

Space Applications
The sun supplies an unlimited supply of energy, but solar cells have to be aligned very precisely to catch its light; when a solar cell is flat, the best configuration is with the sun directly overhead. To keep the sun directly above spacecraft, a mechanical joint moves the solar panels into the right positions, but this system is bulky and expensive. This investigation studies three-dimensional tubes that trap sunlight streaming from any direction, eliminating the need for mechanical arrays. The investigation uses carbon nanotubes, which are single-layer sheets of carbon atoms, and a thin film of copper, zinc, tin and silicon to absorb sunlight.

Earth Applications
Most solar panels on Earth are installed at a fixed angle, which is determined based on the sun’s typical position in the sky, instead of moving the panels to follow the sun throughout the day. But this means solar arrays capture only 70 percent of the energy that they otherwise would. This investigation studies a new three-dimensional solar cell that can trap sunlight coming from any direction, improving efficiency while eliminating the need to move the panels around. The investigation also studies a new material for absorbing sunlight, which is less toxic and less rare than the cadmium-telluride combination used in the past. Results from this investigation improve commercial development of the new technology, benefiting people on Earth.

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Operations

Operational Requirements and Protocols

The solar cells are hosted on the NanoRacks External Platform (NREP). The solar cells should be exposed to sunlight and the space environment for as long as possible, nominally six months or until the devices fail due to repeated thermal cycling or other space-environment impacts.

The solar cells are hosted on the NREP, which is mounted to the JEM Exposed Facility for a period of six months via the JEM airlock. The solar cells should be exposed to sunlight and the space environment for as long as possible, nominally six months or until the devices fail due to repeated thermal cycling or other space-environment impacts. Measurements of the power produced (current and voltage) as well as the temperature of the solar cell should be recorded as a function of insolation (sun angle with respect to solar cell). Data collection at a rate of approximately 1 Hz should be sufficient to assess the changing temperatures and powers produced by the solar cells.

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Decadal Survey Recommendations

Information Pending

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Results/More Information

Information Pending

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Related Websites
Georgia Tech EOSL

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Imagery

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Georgia Tech Research Team (Left to Right) Stephan Turano, Hunter Chan, Jud Ready, Kavin Manickaraj for the NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell). Image courtesy of Georgia Tech.

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image Principal Investigator for NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell), Jud Ready, with one of the solar cells. In the background is the plasma enhanced CVD tool used to grow the CNT arrays. Image courtesy of Georgia Tech.
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image Hunter Chan with one of the solar cells for NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell); numerous other cells (unpackaged) are on desk in front of him. Image courtesy of Georgia Tech.
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image PV solar cell for the NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell) investigation. Image courtesy of Georgia Tech.
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Kavin Manickaraj with one of the solar cells for NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell); numerous other cells (unpackaged) are on desk in front of him. Image courtesy of Georgia Tech.

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Stephan Turano showing an optical microscope image of one of the CNT array patterns on a solar cell for NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell). Actual cell is visible on microscope stage under the objective. Image courtesy of Georgia Tech.

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image Hunter Chan wirebonding photovoltaic cell to package for NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell). Image courtesy of Georgia Tech.
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image Wirebonding of photovoltaic cell to package for the NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell) completed by Hunter Chan. Image courtesy of Georgia Tech.
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image Hunter Chan wirebonding photovoltaic cell to package for NanoRacks-Earth Abundant Textured Thin Film Photovoltaics (NanoRacks-NanoTube Solar Cell). Image courtesy of Georgia Tech.
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