Complete Report - University of New South Wales

The parallel project with Toyota on high effi ciency thermoelectrics has made good progress,
with important developments in modelling and characterisation, which also link to the work on
Hot Carrier cells. In addition, a very signifi cant development is the successful proposal to the
Global Climate and Energy Program (GCEP) at Stanford University, for a project to develop the
Si nanostructure work to a prototype Si based tandem cell over the next three years.
4.5.1 Third Generation Photovoltaics
“Third generation” approaches aim to achieve high effi ciency for photovoltaic devices but
still use “thin fi lm” second generation deposition methods. The concept is to do this with only
a small increase in areal costs and hence reduce the cost per Watt peak [4.5.1]. Also, in
common with the silicon based second generation thin fi lm technologies, abundant and nontoxic
materials will be used. Thus these “third generation” technologies will be compatible with
large scale implementation of photovoltaics. The approach differs from “fi rst generation”
fabrication of high quality and hence low defect single crystal photovoltaic devices, which have
high effi ciencies, approaching the limiting effi ciencies for single band gap devices, but which
use energy and time intensive techniques. Third Generation aims to decrease costs to below
US$0.50/W, potentially to US$0.20/W or better, by dramatically increasing effi ciencies
but maintaining the economic and environmental cost advantages of thin fi lm deposition
techniques (see Figure 4.1.3). To achieve such effi ciency improvements, the targetted devices
aim to circumvent the Shockley-Queisser limit for single band gap cells that limits effi ciencies
to the “Present limit” indicated in Fig. 4.1.3 of between 31% to 41% (for one sun and maximum
concentration respectively). One approach multiple energy threshold devices such as the
tandem or multi-colour solar cell. We are investigating a number of approaches to achieve
such multiple energy threshold devices. [4.5.1, 4.5.2]
The two most important power loss mechanisms in single-bandgap cells arise from the inability
to absorb photons with energy less than the bandgap (1 in Figure 4.5.1) and thermalisation
of photon energy exceeding the bandgap, (2 in Figure 4.5.1). These two mechanisms alone
amount to the loss of about half of the incident solar energy in solar cell conversion to electricity.
Multiple threshold approaches can utilise some of this lost energy. Such approaches avoid
the Schockley-Queisser limit, by the exploitation of more than one energy level - in some form
– for which the limit does not apply. The limit which does apply is the thermodynamic limit
shown in Figure 4.1.3, of 67% to 86.8% (again depending on concentration).
There have been proposed three families of approaches for avoiding the Schockley-
Queisser limit [4.5.2]: (a) increasing the number of bandgaps; (b) capturing carriers before
thermalisation; and (c) multiple carrier pair generation per high energy photon or single carrier
pair generation with multiple low energy photons. Of these, tandem cells, an implementation
of strategy (a), are the only ones which have as yet been realised with effi ciencies exceeding
the Shockley-Queisser limit.
Figure 4.5.1: Loss processes in
a standard solar cell: (1) nonabsorption
of below band gap
photons; (2) lattice thermalisation
loss; (3) and (4) junction and contact
voltage losses; (5) recombination
loss.
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