Theoretical estimation of optical absorption and photoluminescence in nanostructured silicon; an approach to improve efficiency in nanostructured solar cells

Renewable energy is growing in demand, and thus the the manufacture of solar cells and photovoltaic arrays has advanced dramatically in recent years. This is proved by the fact that the photovoltaic production has doubled every 2 years, increasing by an average of 48% each year since 2002. Covering the general overview of solar cell working, and its model, this thesis will start with the three generations of photovoltaic solar cell technology, and move to the motivation of dedicating research to nanostructured solar cell. For the current generation solar cells, among several factors, like photon capture, photon reflection, carrier generation by photons, carrier transport and collection, the efficiency also depends on the absorption of photons. The absorption coefficient,α, and its dependence on the wavelength, λ, is of major concern to improve the efficiency. Nano-silicon structures (quantum wells and quantum dots) have a unique advantage compared to bulk and thin film crystalline silicon that multiple direct and indirect band gaps can be realized by appropriate size control of the quantum wells. This enables multiple wavelength photons of the solar spectrum to be absorbed efficiently. There is limited research on the calculation of absorption coefficient in nano structures of silicon. We present a theoretical approach to calculate the absorption coefficient using quantum mechanical calculations on the interaction of photons with the electrons of the valence band. One model is that the oscillator strength of the direct optical transitions is enhanced by the quantumconfinement effect in Si nanocrystallites. These kinds of quantum wells can be realized in practice in porous silicon. The absorption coefficient shows a peak of 64638.2 cm-1 at = 343 nm at photon energy of ξ = 3.49 eV ( = 355.532 nm). I have shown that a large value of absorption coefficient α comparable to that of bulk silicon is possible in silicon QDs because of carrier confinement. Our results have shown that we can enhance the absorption coefficient by an order of 10, and at the same time a nearly constant absorption coefficient curve over the visible spectrum. The validity of plots is verified by the correlation with experimental photoluminescence plots. A very generic comparison for the efficiency of p-i-n junction solar cell is given for a cell incorporating QDs and sans QDs. The design and fabrication technique is discussed in brief. I have shown that by using QDs in the intrinsic region of a cell, we can improve the efficiency by a factor of 1.865 times. Thus for a solar cell of efficiency of 26% for first generation solar cell, we can improve the efficiency to nearly 48.5% on using QDs.

ASTAXANTHIN PRODUCTION FROM HAEMATOCOCCUS PLUVIALIS UNDER VARIOUS LIGHT INTENSITIES AND CARBON DIOXIDE CONCENTRATIONS

The microalga Haematococcus pluvialis was cultivated in MES-volvox medium at various light intensities and CO2 concentrations. It was found that CO2 concentrations of 10 and 15%, in combination with high irradiance at initial pH =6.7, accelerate astaxanthin accumulation in H. pluvialis cells but obstruct cell growth. The purpose of this research study was to devise a one-stage process consisting of the simultaneous cultivation of H. pluvialis and astaxanthin production using high light intensity and high CO2 concentration. This could be achieved at 200 µE/m2s and 15% CO2 in growth medium at initial pH = 4.3. Compared to the traditional two-stage H. pluvialis cultivation system, this one-step process can save up to 8-9 days of astaxanthin production time. The astaxanthin content in H. pluvialis cells induced with high light intensity only or with a combination of high light intensity and high CO2 concentration had comparable astaxanthin content; 94 and 97 mg/g dry biomass, respectively. However, it was extremely low in nitrate-free medium at high irradiance alone or combined with high CO2 concentration, with an average value of 4 mg/g dry biomass. Cell density was 40% less in cultures under discontinuous illumination compared to continuous illumination. This process could serve as a microalgal CO2 mitigation system after further understanding of the CO2 fixation ability of H. pluvialis has been gained.