Complete Report - University of New South Wales

In the Third Generation Strand, we are implementing strategy (a) by fabricating a tandem
cell based on silicon and its oxides, nitrides and carbides using reduced dimension silicon
nanostructures to engineer the band gap of an upper cell material. We are also tackling
strategy (b) by investigating the “Hot Carrier solar cell” in which carrier cooling is slowed such
that carriers can be extracted before thermalisation. This requires both an absorber with
slowed carrier cooling properties and collection of carriers over a limited range of energies,
such that cold carriers in the external contacts do not cool the hot carriers . Finally we are
investigating implementation of strategy (c) by up-conversion in a layer behind the Si cell. Rare
earth doped phosphors in the up-converter absorb below band gap photons and up-convert
two or more to above band gap photons which are then incident on the Si cell.
Hence all Third Generation approaches are based on tackling one or both of the “below band
gap” or “thermalisation” loss mechanisms mentioned above, as detailed below.
4.5.2 Si-Based Nanostructure tandem cells
Researchers:
Young Cho, Gavin Conibeer, Martin Green, Thipwan Fangsuwannarak, Chu-Wei Jiang, Dirk
König, Tom Puzzer, Giuseppe Scardera
Tandem cells are stacks of individual cells with different energy thresholds each absorbing
a different band of the solar spectrum, usually connected together in series. We aim to
engineer a new silicon-based material to form a top cell above a silicon cell, see Fig. 4.1.4.
This material is ‘engineered’ using a quantum dot nanostructure of silicon in a silicon based
dielectric matrix. The confi ned energy levels in the quantum dots will increase the lowest
absorption edge of the material compared to bulk silicon. If the quantum dot density is high
enough, the wavefunctions of the quantum dots will overlap to create true superlattice
minibands and increase the effective band gap of the material.
The main challenge for a nanostructure engineered material for a tandem cell is to achieve
suffi cient carrier mobility and hence a reasonable conductivity. For a nanostructure, this
generally requires formation of a superlattice with overlap of the wavefunction for adjacent
quantum wells or quantum dots; which in turn requires either close spacing between QWs or
QDs or low barrier height. Another requirement for a tandem cell element is the presence
of some form of junction for carrier separation. This can either be a grown or diffused p-n
junction or a p-i-n junction formed in the superlattice as the i-region.
As reported elsewhere [4.5.3, 4.5.4], such Si quantum dots have been fabricated by cosputtering
deposition of silicon rich layers interspersed with stoichiometric SiO 2 layers (an
application of the method of Zacharias et al. [4.5.5] to photovoltaics). On annealing at 1100°C
silicon precipitates from the super-saturated solid solution to form nanocrystals. For layers
of thickness less than about 4nm, the precipitation enters a regime of 2D diffusion in which
the dot size is accurately controlled by the layer thickness.
The increased strength of optical processes in confi ned structures due to localisation of
electrons and holes is highly advantageous for solar cell structures, as it means that only
very thin layers of these 100nm dots are required for strong absorption. With an appropriate
light-trapping scheme, thin 100nm fi lms of unconfi ned bulk Si to give high solar absorption.
Quantum dot devices of only 100nm thickness seem feasible given the increased optical
strength of the confi ned processes.
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