6/21 研究科セミナー(Weizmann Inst. of Science: Prof. D. Cahen & National Cheng Kung Univ.: Prof. C. Kuo)

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KS-Ⅰ棟2階中講義室 14:00 - 15:15 (Prof. Cahen), 15:30 - 16:45 (Prof. Kuo)


Photovoltaic Solar Cells: What determines their Efficiency?

Professor David Cahen
Department of Materials & Interfaces, Weizmann Institute of Science (WIS)

While we know what are the solar cell efficiencies that we can hope to
achieve for perfect single crystalline inorganic single crystal-based
cells, as demonstrated by the recent Alta results for GaAs p/n cells,
applying the same criteria to other types of cells is pushing the
Shockley - Queisser model "beyond its own limits". So, while nothing
is wrong with the S-Q treatment, the question arises if, for the newer
cells, so-called 2nd and 3d generation ones, it tells the whole story.
In other words, are the differences between cell types, that are easily
gleaned from comparing S-Q - predicted, to actual performances, merely
a matter of insufficient efforts or do basic scientific bounds, beyond
those known today, exist. We argue that additional limits exist for
newer types of solar cells, including Organic PhotoVoltaics (OPV),
Dye-Sensitized Solar Cells and their siblings. Most strikingly, for
organic material-base cells, are the effects of disorder, static
disorder, expressed via tail states, and dynamic disorder, apparent
because of the importance of vibronic states. This then leads to a
challenge for hybrid solar cells, which by "virtue" of combining
organic with crystalline inorganic materials, introduce the extra
limits of organics. Can we find ways where the combination of desirable
characteristics overcomes the extra limits? Here it is likely that
focusing solely on the pure science will be insufficient and we need to
include factors related to ease, versatility and cost of fabrication,
to meet the challenge. We conclude by looking at some practical issues,
i.e., the power generated by PV today and what determines their price,
in terms of dollars and Joules
* Work done in part in collaboration with A. Kahn and with J. Bisquert. 



Ultra-High-Aspect-Ratio Nanomaterials for Optoelectronic Applications

Associate Professor Changshu Kuo
Laboratory of Nano-Constructed Materials, Department of Materials Science and Engineering, National Cheng Kung University

Nanomaterials with desired chemical or physical properties on their
amplified surface area are perfect candidates for optoelectronic
applications, such as photovoltaics, photocatalytics, and
electrochromics.  Particularly, the ultra-high-aspect-ratio (UHAR)
nanomaterials (fibers, wires, tubes, or rods) provide not only the
large amount of active surface area for efficient interfacial charge
exchanges, but also they construct the continuous one-dimensional
domains to ensure the material structures and the charge collections to
the electrodes.  Furthermore, these UHAR nanomaterials demonstrate the
light scattering effect as the incident light passing through the
nanostructured materials.  This light scattering effect results in the
prolonged light path-length, which significantly magnifies the light
absorption and the energy conversion efficiency.  Theoretical
enhancements show the second-order relationship with the thicknesses of
active layers, contrary to the linear relationship for the materials
without nanostructures.  In this presentation, UHAR semiconductor
nanomaterials from polymer-assisted electrospinning technique were
adapted for the investigations of the light scattering and light
propagation.  Core-shell UHAR nanomaterials were developed for various
electrochromic or heterojunction applications.  Simulation for the
light scattering in UHAR nanomaterials is also conducted in order to
achieve the optimized nanostructures and elevated energy conversion
efficiencies. 

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