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University Staff:
Dr. Gavin Conibeer (Group Leader)
Dr. Richard Corkish
Prof. Martin Green
Research and Postdoctoral Fellows:
Dr. Robert Bardos
Dr. Patrick Campbell (Part Time)
Dr. Kylie R. Catchpole
Dr. Ximing Dai
Dr. Tammy Humphrey
Dr. Tom Puzzer (Part Time)
Dr. Bryce Richards, BSc
Dr. Thorsten Trupke
Research Associates:
Yidan Huang
The work of the Third Generation Strand has continued to build
on the work of the Third Generation Special Research Centre. The
principal objective is to significantly, rather than merely incrementally,
improve photovoltaic cell performance beyond that of present devices.
The term Third Generation derives from the progression
of photovoltaic technologies. First generation refers to high quality
and hence low defect single crystal photovoltaic devices these have
high efficiencies and are approaching the limiting efficiencies
for single band gap devices. However, such devices are labour and
energy intensive and are not likely to get lower than US$1/W (see
below). Second generation technology involves low cost and low energy
intensity growth techniques such as vapour deposition and electroplating.
Such processes can bring costs down to a little under US$0.50 but
because of the defects inherent in the lower quality processing
methods, have much reduced efficiencies compared to First Generation.
Efficiency and cost projections for first-, second-
and third-generation photovoltaic technology
(wafers, thin-films, and advanced thin-films, respectively).
Third Generation Approaches to Tackling
the Major Losses in PV Cells
Third Generation concepts are based on devices that can exceed
the theoretical solar conversion efficiency limit for a single
energy
threshold material. This was calculated in 1961 by Shockley and
Queisser as 31% under 1 sun illumination and 40.8% under maximal
concentration of sunlight (46,200 suns, which makes the latter
limit
more difficult to approach than the former).
Loss processes in a standard solar cell: (1) non-absorption
of below band gap photons; (2) lattice thermalisation loss;
(3) and (4) junction and contact voltage losses; (5) recombination
loss.
The routes to exceeding the Shockley-Queisser limit address the
below band gap and the thermalisation loss mechanisms in the above
figure. Means to tackling these have been variously quoted as falling
into three generic categories, namely: multiple energy threshold
devices; modification of the incident spectrum; and use of excess
thermal generation to enhance voltages or carrier collection. Specific
directions for experimental research on each of these were set for
the Third Generation SRC in 2002. These continue to be the most
promising routes to achieving the objectives of the Third Generation
Strand. The specific technologies chosen address these generic approaches
and respectively are:
Si Nanostructures
Bandgap engineering of silicon based material is a very promising
route towards third generation Si photovoltaic devices such as silicon
based tandem solar cells and energy selective contacts for hot carrier
solar cells. This project focuses on the fabrication and characterisation
of silicon nanostructures consisting of quantum well (QW) or quantum
dot (QD) superlattices to achieve such band gap control.
Up/Down Converters
Theoretical work in the Centre has shown that modification of
the solar spectrum can boost the efficiency of solar cells if the
bandwidth
of the light incident on the cell can be made narrower than the
solar spectrum. Luminescent materials are being investigated that
either absorb one high energy photon and emit more than one low
energy photon just above the band gap of the solar cell (down-converters)
OR that absorb more than one low energy photon below the band gap
of the cell and emit one photon just above the band gap (up-converters).
The important property for these devices is high quantum efficiency
meaning that they must be very radiatively efficient. Experimental
work is concentrating on Up converters as any conversion of low
energy light that is normally wasted can in principle increase
efficiency.
External quantum efficiencies of 1% have been measured in our work
for such materials. This 1% extra is small as yet but nonetheless
represents extra photons above the band gap and hence is promising.
Hot Carrier Cells
The Hot Carrier Cell tackles the major PV loss mechanism of thermalisation
of carriers. The underlying concept is to slow the rate of photoexcited
carrier cooling, caused by phonon interaction in the lattice, to
allow time for the carriers to be collected whilst they are still
“hot” thus enhancing the voltage of a cell. Work in
this area includes investigation of Selective
Energy Contacts and Hot Carrier
Absorbers.
Thermoelectric Cells
A spin-off of previous theoretical work has included the application
of the concept of energy-selective electron transport, used in hot
carrier solar cells, to thermoelectrics and thermionics. This has
led to the discovery of two types of equivalence that simplify the
description of this class of devices.
Additionally there are two other project areas being pursued
within the Strand:
Thermophotonics
Thermophotonics is an extension of the concept of Thermophotovoltaics
where photovoltaic conversion by a receiver cell of radiation from
an emitter, which could be heated by various sources. In a thermophotonic
system, a heated light-emitting diode radiates towards a PV cell
connected to a load.
Surface Plasmons
Surface plasmon resonances on metal nanoparticles can scatter incident
light into guided modes of a thin semiconductor layer. Applications
include novel light-trapping for very thin silicon films that are
too thin for conventional light-trapping structures.
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