Michael H. Jin

Thin-Film Solar Cells on Polymer Substrates for Space Power

Photovoltaic arrays have played a key role in power generation in space. The current technology will continue to evolve but is limited in the important mass specific power metric (MSP or power/weight ratio) because it is based on bulk crystal technology. Solar cells based on thin-film materials offer the promise of much higher MSP and much lower cost. However, for many space applications, a 20% or greater AM0 efficiency (eta) may be required. The leading thin-film materials, amorphous Si, CuInSe, and CdTe have seen significant advances in efficiency over the last decade but will not achieve the required efficiency in the near future. Several new technologies are herein described to maximize both device eta and MSP. We will discuss these technologies in the context of space exploration and commercialization. One novel approach involves the use of very lightweight polyimide substrates. We describe efforts to enable this advance including materials processing and device fabrication and characterization. Another approach involves stacking two cells on top of each other. These tandem devices more effectively utilize solar radiation by passing through non-absorbed longer wavelength light to a narrow-bandgap bottom cell material. Modeling of current devices in tandem format indicates that AM0 efficiencies near 20% can be achieved with potential for 25% in the near future. Several important technical issues need to be resolved to realize the benefits of lightweight technologies for solar arrays, such as: monolithic interconnects, lightweight array structures, and new ultra-light support and deployment mechanisms. Recent advances will be stressed.

Thin film CuInS/sub 2/ prepared by spray pyrolysis with single-source precursors

Both horizontal hot-wall and vertical cold-wall atmospheric chemical spray pyrolysis processes deposited near single-phase stoichiometric CuInS/sub 2/ thin films. Single-source precursors developed for ternary chalcopyrite materials were used for this study, and a new liquid phase single-source precursor was tested with a vertical cold-wall reactor. The depositions were carried out under an argon atmosphere, and the substrate temperature was kept at 400/spl deg/C. Columnar grain structure was obtained with vapor deposition, and the granular structure was obtained with (liquid) droplet deposition. Conductive films were deposited with planar electrical resistivities ranging from 1 to 30/spl Omega//spl middot/cm.

Post-deposition annealing of thin film CuInS/sub 2/ made from a single-source precursor

Analysis of the radiative recombination processes in CuInS/sub 2/ thin films deposited using a single-source precursor was made. The photoluminescence spectrum is dominated by donor-acceptor transitions and post-deposition annealing under sulfur gas quenched the transitions associated with sulfur vacancy indicating extra sulfur was incorporated into the films passivating the vacancies. In addition, a part of the broad emission band (between 1.24 eV and 1.32 eV) was lowered by S-annealing. Ar-annealing without sulfur also improved crystallinity of the films - X-ray diffraction showed diffraction peaks as sharp as those from S-annealing. However, the emission related to the deep defects was not reduced and the creation of sulfur vacancies increased the emission bands at 1.45 eV and 1.32 eV.

Characterization of deposition parameters in aerosol assisted chemical vapor deposition of CuInS/sub 2/ from a single-source precursor

Alloys of Cu(In:Ga)(S:Se)/sub 2/ have shown high potential as thin film photovoltaic absorbers due to their high absorption coefficients, near ideal band gaps, and good electrical properties. Efforts have been lead to create easily decomposing organometallic single-source precursors (SSP) to produce films at temperatures below 400/spl deg/C. Along with that, the SSP (PPh/sub 3/)/sub 2/Cu(SEt)/sub 2/In(SEt)/sub 2 /has been shown to deposit CuInS/sub 2/ films with good optical, morphological, and electrical properties via aerosol-assisted chemical vapor deposition (AACVD). Presented here are studies aimed to understand how certain deposition parameters can be used to optimize the AACVD process. Parameters included in this study are temperature of the deposition zone, substrate location within the reactor, and concentration of the SSP in solution. Deposition control has produced films with four distinct morphologies, varying in density, adhesion, smoothness, and color.