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.
The Si epitaxial films are grown on Si （100） substrates using pure Si2H6 as a gas source using ultrahigh vacuum chemical vapour deposition technology. The values of growth temperature Tg are 650 ℃, 700 ℃, 730 ℃, 750 ℃, and 800 ℃. Growth mode changes from island mode to step-flow mode with Tg increasing from 650 ℃ to 700℃. Rippled surface morphologies are observed at Tg = 700 ℃, 730 ℃, and 800℃, but disappear when Tg = 750℃. A model is presented to explain the formation and the disappearance of the ripples by considering the stability of the step-flow growth.
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.
The detailed balance method is used to study the potential of the intermediate band solar cell （IBSC）, which can improve the efficiency of the gi-based solar cell with a bandgap between 1.1 eV to 1.7 eV. It shows that a crystalline silicon solar ceil with an intermediate band located at 0.36 eV below the conduction band or above the valence band can reach a limiting efficiency of 54% at the maximum light concentration, improving greatly than 40.7% of the Shoekley-Queisser limit for the single junction Si solar cell. The simulation also shows that the limiting efficiency of the silicon-based solar cell increases as the bandgap increases from 1.1 eV to 1.7 eV, and the amorphous Si solar cell with a bandgap of 1.7 eV exhibits a radiative limiting efficiency of 62.47070, having a better potential.
A novel structure of silicon-riched nitride(SRN)/silicon-riched oxide(SRO) is proposed and prepared using RF reactive magnetron co-sputtering. High temperature annealing of SRN/SRO multilayers leads to formation of Si nanocrystals(NC) from isolating SRN and SRO layers simultaneously, which efficiently improves carrier transport ability compared to conventional SRN/Si_3N_4 counterpart. Micro-Raman scattering analysis reveals that SRN layer has dominating number of denser and smaller Si NCs, while SRO layer has relatively less, sparser and bigger Si NCs, as confirmed by high resolution transmission electron microscopy observation. The substitute SRO layers for Si_3N_4 counterparts significantly increase the amount of Si NCs as well as crystallization ratio in SRN layers; while the average Si NC size can be well controlled by the thickness of SRN layers and the content of N, and hence an obvious stronger absorption in UV region for the novel structure can be observed in absorption spectra. The I-V characteristics show that the current of hybrid SRN/SRO system increases up to 2 orders of magnitude at 1 V and even 5 orders of magnitude at 4 V compared to that of SRN/Si_3N_4 structure. Si NCs in Si Oylayers provide a transport pathway for adjacent Si NCs in Si Nxlayers. The obvious advantage in carrier transportation suggests that SRN/SRO hybrid system could be a promising structure and platform to build Si nanostructured solar cells.
Yeliao TaoJun ZhengYuhua ZuoChunlai XueBuwen ChengQiming Wang
Strain-compensated Ge/Si0.15Ge0.85 multiple quantum wells were grown on an Si0.1 Ge0.9 virtual substrate using ultrahigh vacuum chemical vapor deposition technology on an n＋-Si（001） substrate. Photoluminescence measurements were performed at room temperature, and the quantum confinement effect of the direct-bandgap transitions of a Ge quantum well was observed, which is in good agreement with the calculated results. The luminescence mechanism was discussed by recombination rate analysis and the temperature dependence of the luminescence spectrum.
Hu Wei-XuanCheng Bu-WenXue Chun-LaiZhang Guang-ZeSu Shao-JianZuo Yu-HuaWang Qi-Ming
