The intermediate band （IB） solar cell is a promising third-generation solar cell that could possibly achieve very high efficiency above the Shockley-Queisser limit. One of the promising ways to synthesize IB material is to introduce heavily doped deep level impurities in conventional semiconductors. High-doped Ti with a concentration of 10^20 cm^-3- 10^21 cm^-3 in the p-type top Si layer of silicon-on-insulator （SOI） substrate is obtained by ion implantation and rapid thermal annealing （RTA）. Secondary ion mass spectrometry measurements confirm that the Ti concentration exceeds the theoretical Mott limit, the main requirement for the formation of an impurity intermediate band. Increased absorption is observed in the infrared （IR） region by Fourier transform infrared spectroscopy （FTIR） technology. By using a lateral p-i-n structure, an obvious infrared response in a range of 1100 nm 2000 nm is achieved in a heavily Ti-doped SOI substrate, suggesting that the improvement on IR photoresponse is a result of increased absorption in the IR. The experimental results indicate that heavily Ti-implanted Si can be used as a potential kind of intermediate-band photovoltaic material to utilize the infrared photons of the solar spectrum.
Well-aligned and closely-packed silicon nanopillar （SNP） arrays are fabricated by using a simple method with magnetron sputtering of Si on a porous anodic alumina （PAA） template at room temperature. The SNPs are formed by selective growth on the top of the PAA pore walls. The growth mechanism analysis indicates that the structure of the SNPs can be modulated by the pore spacing of the PAA and the sputtering process and is independent of the wall width of the PAA. Moreover, nanocrystals are identified by using transmission electron microscopy in the as-deposited SNP samples, which are related to the heat isolation structure of the SNPs. The Raman focus depth profile reveals a high crystallization ratio on the surface.
Light trapping plays an important role in improving the conversion efficiency of thin-film solar cells. The good wideband light trapping is achieved using our periodically truncated cone Si nanowire （NW） structures, and their inherent mechanism is analyzed and simulated by FDTD solution software. Ordered cylinder Si NW structure with initial size orS0 nm and length of 200 nm is grown by pattern transfer and selective epitaxial growth. Truncated cone Si NW array is then obtained by thermal oxidation treatment. Its mean reflection in the range of 300-900 nm is lowered to be 5% using 140 nm long truncated cone Si NW structure, compared with that of 20% using cylinder counterparts. It indicates that periodically truncated Si cone structures trap the light efficiently to enhance the light harvesting in a wide spectral range and have the potential application in highly efficient NW solar cells.
Nanocrystalline Ge （nc-Ge） single layers and nc-Ge/SiNx multilayers are prepared by laser annealing amorphous Ge （a-Ge） films and a-Ge/SiNx multilayers. The microstructures as well as the electrical properties of laser-crystallized samples are systematically studied by using various techniques. It is found that the optical band gap of nc-Ge film is reduced compared with its amorphous counterpart. The formed nc-Ge film is of p-type, and the dark conductivity is enhanced by 6 orders for an nc-Ge single layer and 4 orders for a multilayer. It is suggested that the carrier transport mechanism is dominant by the thermally activation process via the nanocrystal, which is different from the thermally annealed nc-Ge sample at an intermediate temperature. The carrier mobility of nc-Ge film can reach as high as about 39.4 cm2.V ^-1 .s^-1, which indicates their potential applications in future nano-devices.
