Dennis Zill

Hindawi Publishing Corporation Advances in OptoElectronics Volume 2007, Article ID 69578, 11 pages doi:10.1155/2007/69578

Research Article Silicon Quantum Dots in a Dielectric Matrix for All-Silicon Tandem Solar Cells
Eun-Chel Cho,1 Martin A. Green,1 Gavin Conibeer,1 Dengyuan Song,1 Young-Hyun Cho,1 Giuseppe Scardera,1 Shujuan Huang,1 Sangwook Park,1 X. J. Hao,1 Yidan Huang,1 and Lap Van Dao21 ARC 2 ARC

Photovoltaics Centre of Excellence, University of New South Wales, Sydney 2052, NSW, Australia Centre of Excellence for Coherent X-Ray Science, Swinburne University of Technology, Hawthorn 3122, VIC, Australia

Received 1 April 2007; Accepted 26 June 2007 Recommended by Armin G. Aberle We report work progress on the growth of Si quantum dots in different matrices for futurephotovoltaic applications. The work reported here seeks to engineer a wide-bandgap silicon-based thin-film material by using quantum confinement in silicon quantum dots and to utilize this in complete thin-film silicon-based tandem cell, without the constraints of lattice matching, but which nonetheless gives an enhanced efficiency through the increased spectral collection efficiency. Coherent-sized quantumdots, dispersed in a matrix of silicon carbide, nitride, or oxide, were fabricated by precipitation of Si-rich material deposited by reactive sputtering or PECVD. Bandgap opening of Si QDs in nitride is more blue-shifted than that of Si QD in oxide, while clear evidence of quantum confinement in Si quantum dots in carbide was hard to obtain, probably due to many surface and defect states. The PLdecay shows that the lifetimes vary from 10 to 70 microseconds for diameter of 3.4 nm dot with increasing detection wavelength. Copyright © 2007 Eun-Chel Cho et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1.

INTRODUCTIONWafer-based first-generation photovoltaic technologies are presently booming, a situation likely to continue for the next decade. However, second-generation thin-film approaches also appear attractive because of their potential reduction in material costs. Third-generation approaches aim to reduce the manufacturing cost of thin-film second-generation technologies by increasing the efficiency throughusing multiple energy thresholds, whilst still using abundant and non-toxic materials. The effects of increasing efficiency and lowering the cost of thin-film processes can significantly leverage costs per peak watt (Wp) of generating capacity, perhaps to less than US $ 0.20/Wp [1]. Figure 1 shows main loss mechanisms in a standard pn junction solar cell. The most important loss mechanisms in singlep-n junction solar cells are the nonabsorption of i below-bandgap photons ( 1 in Figure 1) and thermalization of electron-hole pairs generated by absorption of shortwavelength photons, through electron (hole) relaxation to i the conduction (valence) band edge ( 2 in Figure 1). Other i i losses are junction loss ( 3 in Figure 1) and contact loss ( 4 in Figure 1), which can be arbitrarily small in anideal device. i Recombination loss ( 5 in Figure 1) can be reduced to purely radiative recombination again in an ideal device.

There have been proposed three families of approaches for tackling these first two major loss mechanisms [2]: (a) increasing the number of energy levels; (b) capturing hot carriers before thermalisation; and (c) multiple electron-hole pair generation per high energyphoton or generation of one higher energy-carrier pair with more than one low energy photon. An implementation of strategy (a), the tandem cells, is the only technology thus far to have been realized with efficiencies exceeding the Shockley-Queisser limit. The concept of the tandem solar cell is the use of several solar cells of different bandgaps stacked on top of each other, each absorbing photons...