engleski

Nano-structured thin silicon for "third generation solar cells"

It is expected that by year 2100 the contribution of electric power from photovoltaic solar cells in total electricity production rich more than 30%. In order to achieve this goal, the price of solar electricity has to go down from today values that are 1-2 €/Wp towards few tents of euro. The solar radiation is dispersed on large area with approximately 1 kW/m2. For large energy production, solar cells should cover a large area which assumes corresponding material consumption. Simple estimation done by M.A. Green [1] show for crystalline silicon cells (90% of actual production) that the price for starting material is too high for achieving this goal. For thin films cells, the situation is better - the about two orders of magnitude lower material consumption comparing to crystalline lowers the price, while for fulfilling the expecting low price the efficiency should be much higher. Photovoltaic solar cells in the form of layers with thickness of ten to several hundred nano-meters that uses quantum confinement effects, seem to be a possible candidate for approach this goal. These solar cells are called "third generation" and the efficiency is expected be 4-5 times higher than actual values. The structural objects that could enable such high efficiency are nano-meter sized objects in the form of ordered 3D domens, quantum dots, QD, called also nano-crystals. In various concepts for active layers, QDs with one optical gap are immersed in host matrix with higher or lower optical gap that enables better use of solar spectrum in various processes. In design of passive elements, nano-meter sized metal particles immersed in glass or metal oxide matrix could improve efficiency of solar cells by surface plasmons generated in interaction of metal particles and photons that converts the spectrum of one wavelength towards this one where the conversion efficiency is maximal. Our efforts has been concentrated on thin Si films containing few nano-meter large crystals embedded in amorphous Si:H matrix. By changing the size of nano-crystals it is possible to change the distribution of electron and vibrational states in crystals. When combining this effect with volume contribution of nano-crystals, the effective optical gap can be varied. Furthermore, vibrational states in nano-crystals are discrete and life time of excited electron-hole par is orders of magnitude higher than in corresponding macro-crystals. If the sizes of nano-crystals remain in few nano-meter range, the optical transitions remain direct as in amorphous material, assuming absorption coefficient substantially larger than in C-Si for spectral range relevant for solar cells. The above mentioned properties opens the possibility for using this thin films in multilayer structures that correspond to high efficient (more than 30%) solar cells, if material can be deposited on large scale using conventional deposition techniques. Since the optical, electrical and vibrational properties of QD in thin films are not fully explained [2-4], among others due to substantial influence of matrix effect our work was partially devoted to find proper methods for structural characterization and in next step study the correlation between nano-structure and optical, electrical, vibrational and other properties. The second focus was formation of nano-crystalline-amorphous thin films by conventional plasma enhanced chemical vapour deposition. Our goals are developed procedure for production of nano-crystalline - amorphous Si thin films (nC-a-Si) on large scale (ၾ m2) and design of corresponding multilayer solar cells structures. In order to test production procedure, stability of nC-a-Si and multilayer structures the representative number of samples were produced on conventional industrial production line. The out door testing confirms that nano-crystalline-amorphous cells are more efficient and much more stabile than those that use standard amorphous material. Further development is underway. [1] M.A. Green, Third generation photovotaics: solar cells for 2020 and beyond, Physica E 14 (2002), 65-70) [2] Bukovski TJ., Simmons JH., Crit.Rev.Solid State Mat.Sci 27 (2002) 119 [3] Matthew C. Beard, Kelly P. Knutsen, Pingrong Yu, Joseph M. Luther, Qing Song, Wyatt K. Metzger, Randy J. Ellingson and Arthur J. Nozik , NanoLett. 7(8) 256-2512, 2007 [4] D.L.Williamson, Mater.Res.Soc.Symp.Proc. 377 (1995) 251

thin films solar cells; nano-crystalline Si

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