We report a chalcopyrite thin film solar cell fabrication process established at NASA Glenn Research Center. The process was validated by fabricating a Al/ZnO:F/CdS/PVD-deposited chalcopyrite solar cell with an efficiency of 5.3 % under AM0 illumination. Further improvement was made by incorporating an intrinsic ZnO layer and Al/Ni top metal contact at Oberlin College, increasing the efficiency up to 6.7 %. Solar cells were also prepared with CuInS2 thin films deposited by aerosol-assisted chemical vapor deposition using a single-source precursor, (PPh3)2CuIn(SEt)4. The best solar cell parameters collected respectively from a series of the solar cells are Voc=412 mV, Isc=12.5 mA/cm , and FF=0.45.

Thin-film solar cells on flexible, lightweight, space-qualified substrates provide an attractive approach to fabricating solar arrays with high mass-specific power. A polycrystalline chalcopyrite absorber layer is among the new generation of photovoltaic device technologies for thin film solar cells. At NASA Glenn Research Center we have focused on the development of new single-source precursors (SSPs) for deposition of semiconducting chalcopyrite materials onto lightweight, flexible substrates. We describe the syntheses and thermal modulation of SSPs via molecular engineering. Copper indium disulfide and related thin-film materials were deposited via aerosol-assisted chemical vapor deposition using SSPs. Processing and post-processing parameters were varied in order to modify morphology, stoichiometry, crystallography, electrical properties, and optical properties to optimize device quality. Growth at atmospheric pressure in a horizontal hotwall reactor at 395 C yielded the best device films. Placing the susceptor closer to the evaporation zone and flowing a more precursor-rich carrier gas through the reactor yielded shinier-, smoother-, and denser-looking films. Growth of (112)-oriented films yielded more Cu-rich films with fewer secondary phases than growth of (204)/(220)-oriented films. Post-deposition sulfur-vapor annealing enhanced stoichiometry and crystallinity of the films. Photoluminescence studies revealed four major emission bands and a broad band associated with deep defects. The highest device efficiency for an aerosol-assisted chemical vapor deposited cell was one percent.

Copper indium disulfide thin films were deposited via aerosol-assisted chemical vapor deposition using single source precursors. Processing and post-processing parameters were varied in order to modify morphology, stoichiometry, crystallography, electrical properties, and optical properties in order to optimize device-quality material. Growth at atmospheric pressure in a horizontal hot-wall reactor at 395 C yielded best device films. Placing the susceptor closer to the evaporation zone and flowing a more precursor-rich carrier gas through the reactor yielded shinier, smoother, denser-looking films. Growth of (112)-oriented films yielded more Cu-rich films with fewer secondary phases than growth of (204)/(220)-oriented films. Post-deposition sulfur-vapor annealing enhanced stoichiometry and crystallinity of the films. Photoluminescence studies revealed four major emission bands (1.45, 1.43, 1.37, and 1.32 eV) and a broad band associated with deep defects. The highest device efficiency for an aerosol-assisted chemical vapor deposited cell was 1.03 percent.

For approximately ten years, the SPRAT conference series at NASA Glenn (formerly Lewis) Research Center has devoted a workshop to the topic of thin-film solar cell technology and its potential for space applications. While thin-film materials have been investigated for a number of years, including copper sulfide research at NASA in the 1960 s, there has been a re-birth of interest in this class of materials for space applications in the past dozen years or so. There are several reasons for this renaissance. An important contributor is efficiency improvements beyond ten percent. Another contributor is the increase in interest represented by funding opportunities by NASA and several agencies in the U.S. Department of Defense (Missile Defense Agency, Air Force, and DARPA). Finally, there have been several intriguing missions identified through various means, these include: station-keeping for high-altitude airships, space solar power, planetary surface power, and solar electric propulsion. To aid in leading the discussion for this workshop, a series of seven questions were posed. These are reproduced below as well as a summary of key points and conclusions from the workshop as well as an attendees list and results of an informal poll related to long-term potential of thin films for space.

Copper indium disulfide (CuInS2) films were deposited by aerosol-assisted chemical vapor deposition (AACVD) from a single-source precursor (SSP), (PPh3)2Cu(SEt)2In(SEt)2. Various deposition parameters were explored to understand how they affect the crystallography, stoichiometry, and morphology of the deposited films and the quality of fabricated solar cells. Parameters explored included the deposition temperature, location of substrate within CVD reactor, precursor concentration in toluene carrier solvent, and post-deposition annealing in a S-rich atmosphere. CuInS2 films have been fabricated into complete solar cells with the top-down composition of Al/ZnO:F/CdS/CuInS2/Mo/glass and the efficiency of 1.0% under simulated AM0 illumination.

The National Aeronautics and Space Administration (NASA) is interested in developing low-cost highly efficient solar cells on light-weight flexible substrates, which will ultimately lower the mass-specific power (W/kg) of the allowing extra payload for missions in space as well as cost reduction. In addition, thin film cells are anticipated to have greater resistance to radiation damage in space, prolonging their lifetime. The flexibility of the substrate has the added benefit of enabling roll-to-roll processing. The first major thin film solar was the solar cell - a heterojunction between p-type CuxS and n-type CdS. The research on CdS cells started in the late 1950s and the efficiency in the laboratory was up to about 10 % in the 1980s. Today, three different thin film materials are leading the field. They include amorphous Si, CdTe, and Cu(In,Ga)Se2 (CIGS). The best thin film solar efficiency of 19.2 % was recently set by CIGS on glass. Typical module efficiencies, however, remain below 15 %.

We report on our efforts to deposit transparent, conducting, zinc oxide films by reactive sputtering from metallic zinc and dopant (aluminum or indium) targets in an argon/oxygen gas mixture. Several methods have been used to include dopant atoms including: 1) placing pieces of Al over the Zn target, 2) sputtering from a Zn98Al02 alloy target, and 3) depostion of multilayer ZnO/In2O3 structures The best results were achieved using Method (1) with Al covering ~20% of the Zn target. Resulting films were highly transparent and had resistivities of 0.9 mΩ⋅cm. Method (2) was not successful. Method (3) showed promise, but appears to require thinner layers than we are able to currently achieve.

The thin-film solar cell program at NASA GRC is developing solar cell technologies for space applications which address two critical metrics: specific power (power per unit mass) and launch stowed volume. To be competitive for many space applications, an array using thin film solar cells must significantly increase specific power while reducing stowed volume when compared to the present baseline technology utilizing crystalline solar cells. The NASA GRC program is developing two approaches. Since the vast majority of the mass of a thin film solar cell is in the substrate, a thin film solar cell on a very lightweight flexible substrate (polymer or metal films) is being developed as the first approach. The second approach is the development of multijunction thin film solar cells. Total cell efficiency can be increased by stacking multiple cells having bandgaps tuned to convert the spectrum passing through the upper cells to the lower cells. Once developed, the two approaches will be merged to yield a multijunction, thin film solar cell on a very lightweight, flexible substrate. The ultimate utility of such solar cells in space require the development of monolithic interconnections, lightweight array structures, and ultra-lightweight support and deployment techniques.