The use of photovoltaic power has been instrumental in the human exploration and development of space. To continue to meet future photovoltaic space power requirements it will be necessary to move toward innovative device design and ultimately new material systems. In order to improve device efficiencies while reducing weight and maintaining structural integrity, we propose a next-generation approach to device design that involves the use of nanostructured materials in PV cells. In the near term this approach will allow us to improve on what are currently the best space solar cells available in terms of their efficiency and materials properties important for space utilization. In the future, the use of nanomaterials will allow us to develop viable thin film solar arrays for space and ultimately make these arrays out of lightweight, flexible, polymer-based materials.
The breakthrough in a nanomaterials approach to semiconductor device development centers around the fact that the electrical, optical, and even thermal properties of these materials can be controlled by changing the particle size. A groundbreaking theoretical study by A. Luque and A. Marti predicted that a PV cell with a single intermediate electronic band created by a layer of nanometer-sized semiconductor crystals (quantum dots) inserted into the i region of an ordinary p-i-n junction solar cell offers a conversion efficiency of 63.2%. A new study has recently extended the theory to two intermediate bands and calculated a limiting efficiency of 7l.7%. This is approximately a factor of two better than the SOA space solar cells.
This efficiency is theoretically comparable to a 36 junction multi-bandgap cell, (of course ignoring the difficulties experienced in designing a real multi-junction cell of lattice and current matching). In addition to efficiency enhancement, multiple quantum well photovoltaic devices recently demonstrated improved radiation tolerance and temperature coefficients as a result of their imbedded nanostructure. Finally, quantum dots and other nanomaterials have also recently been shown to provide dramatic improvement in performance of thin film photovoltaics and hybrid inorganic/organic conductive polymer based solar cells. Two types of nano-engineered devices are shown below:
Goals and Objectives
Our technical approach leverages many years of NASA research in nanomaterials development and over a decade of experience in multijunction cell growth. The Nanomaterials and Nanostructures for Space PV project capitalizes on a diverse team of senior researchers from the Photovoltaic and Space Environments Branch at Glenn Research Center, JPL, RIT, Penn State, and the Univ. of Houston with experience in multijunction and amorphous silicon cell growth, nanomaterials synthesis, and photovoltaic device development.
Our research objective is to develop new quantum dot materials and energy conversion structures that offer the potential for radical advances in space power generating capability by enabling up to 2X higher efficiency and/or up to 5X higher specific power (i.e., power per unit mass), better radiation tolerance and thermal behavior for space and ultimately lower total cost for future space solar arrays. We will accomplish these objectives by:
- Identifying suitable nanomaterials (matching the bandgaps, electron affinities, and compatibility of the various PV materials to the desired properties),
- Selecting a compatible structure commensurate with the ability to manufacture the nanomaterials (i.e. colloidal synthesis, laser ablation, or Stranski-Krastanow growth),
- Incorporating nanomaterials into a complete device structure,
- Characterizing the cell structure for intended H&RT space utilization and modular design.
We will investigate three material systems to take advantage of current research to yield near term improvements in the SOA. Results and lessons learned will be applied to systems currently at lower TRL levels with the goal of developing these systems into working devices in the next 6 – 8 years. The three systems are:
- Epitaxially grown III-V nanostructures (quantum dots and quantum wires) inserted into SOA multijunction space solar cells,
- Thin film amorphous silicon solar cells with silicon or chalcopyrite quantum dots, and
- Thin film, flexible polymeric solar cells incorporating quantum dots and/or carbon nanotubes.
In the next four years, we will focus on the use of epitaxially grown III-V nanostructures (quantum wires (QWs), and quantum dots (QDs)) to enhance the performance of the current SOA space solar cells. The use of these nanomaterials will allow us to bandgap engineer other device properties such as its temperature coefficients, radiation tolerance, and spectral response. We will no longer be constrained to the rigid materials properties imposed by the lattice matching approach of bulk semiconducting materials.
Furthermore, this approach is inherently portable to the current production methodologies associated with SOA space solar cells. A simple variation in processing parameters will allow the production of cells that can be optimized for near-Earth, Moon, Mars, or even more exotic applications such as robotic manufacture of PV devices using in-situ planetary resources.
Although the near-term benefits that can be achieved through the use of nanostructures in III-V technology are remarkable, there still remains the mass and mechanical restriction imposed on a crystalline technology. Extending the use of nanostructures and nanomaterials to thin film devices is the logical extension of the work proposed here: Thin film devices are extremely lightweight and flexible, with mass specific power on the order of 5X greater than crystalline technology.
Mass and materials cost savings are an extremely desirable goal of next-generation solar cells. Each cell type has applications in multiple areas of H&RT: low area, high efficiency, low temperature and radiation resistant III-V cells for robotic exploration and satellite communication systems; larger area, radiation resistant, low cost amorphous silicon or polymer cells for planetary or lunar surface power or interplanetary and power beaming applications.
Contact:
Sheila Bailey
Sheila.G.Bailey@nasa.gov
(216) 433-2228
