|Budget Amount *help
¥3,200,000 (Direct Cost : ¥3,200,000)
Fiscal Year 1998 : ¥1,000,000 (Direct Cost : ¥1,000,000)
Fiscal Year 1997 : ¥2,200,000 (Direct Cost : ¥2,200,000)
An attempt is made to develop a new class of epitaxial Si-based photovoltaic cells and relevant technologies by using multiple quantum well (QW) of a smaller bandgap material as an absorbing layer with the aid of advanced epitaxial techniques and band-gap engineering. The use of Si-lattice-matched, SiGe-alloy-based narrow-gap QWs allows absorption in the wavelength regime longer than 1 mum that is hardly covered by conventional Si-based solar cells (i.e., absorption edge lowering) without sacrificing a high photovoltage equivalent to that of Si.
The absorption edge lowering was confirmed by means of standard absorption, photoconductivity and photovoltage measurements, which indicates solar cell operation. The absorption edge of the QW occurred at longer wavelengths than that of a control Si sample, showing a characteristic 3/2 power-law dependence as a function of energy which is consistent with the dimensionality of holes (2-D) and electrons (3-D). Thermal excitation of carriers from t
he shallow QW to the barrier band edge was confirmed as an exponential increase of shortcircuit current with increasing temperature. QW absorption occurred at energies smaller than the Si bandgap and a photovoltage comparable to that of a Si solar cell (<approximately equal>O.4-O.6V) was obt
Carrier transport characteristics were studied in view of achieving a highly efficient photovolliaic effects. It was found that the location of QWs plays an important role in the efficiency of effective carrier blocking due to a QW potential. In fact, anomalies due to carrier redistribution were observed, i.e., dynamical backscattering, in the time-resolved luminescence of a QW blocker under longitudinal electric fields. The results indicates that carrier profiles in a QW photovoltaic cell are controllable by tuning the built-in electric field using intentional doping. The role of a built-in potential was separately studied by photocurrent measurements on a QW with an integrated SiGe-graded buffer. It was found that the photocurrent polarity switches abruptly as the wavelength is scanned in the near-infrared regime above the QW band-gap.
To enhance the solar cell capability, the Si/S i02 technique exploited for the realization of a Si-on-insulator QW (SOI-QW). The SOI geometry is expected to allow an enhancement of absorption due to the integrated mirror characteristics and therefore a more efficient solar cell. Spontaneous emission from the QW and normal incidence reflectance showed Fabry-Perot interference fringes, which indicates an optical cavity using QW and 501. Accordingly, absorption characteristics show resonant behavior and wavelength selectivity for prospective use as resonant photodetector. Less