Modeling DownConversion and DownShifting for Photovoltaic
Applications
Ahmed M. Gabr
1
, Jeffery F. Wheeldon
1
, Richard M. Beal
1
, Alex Walker
1
, Justin Sacks
2
, Rachel M.
Savidge
2
, Trevor J. Hall
1
, Rafael N. Kleiman
2
and Karin Hinzer
1
1
Center for Research in Photonics, University of Ottawa, 800 King Edward, K1N 6N5, Ottawa, ON, Canada
2
Department of Engineering Physics, McMaster University, 1280 Main Street West, L8S 4L7,
Hamilton, ON, Canada
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I. INTRODUCTION
The ideal maximum theoretical efficiency of a single
junction solar cell under standard, terrestrial solar irradiation
is 33% [1]. This is limited by subbandgap losses,
thermalization of carriers with energies greater than the
semiconductor bandgap, and radiative recombination losses.
For commercial grade silicon solar cells, efficiencies are
normally limited by nonradiative recombination losses, which
yield overall efficiencies closer to 1518%. Surface
recombination is an example of a nonradiative recombination
process that occurs across the interface between different
material layers due to the presence of a sufficiently high
density of trap states. This can be an important loss
mechanism for poorly passivated solar cells, which leads to a
significant rolloff in the spectral response at high photon
energies. Downconversion (DC) and luminescent down
shifting (LDS) are two methods being pursued in the
photovoltaic research community in order to mitigate
thermalization and/or surface recombination losses in single
junction solar cells [2–5]. It is envisaged that the application
of DC and LDS will be compatible with existing commercial
solar cell fabrication processes, thereby providing a cost
effective method to boost the efficiency of flatplate
photovoltaic devices.
DC, otherwise referred to as quantumcutting, is an optical
process whereby one highenergy photon is absorbed and
converted into two lowerenergy photons, as shown in Fig.
1(a). By tailoring the absorption and emission properties of a
DC layer, it should be possible to convert highenergy photons
into lowerenergy photons. These lowerenergy photons are
targeted to be more efficiently converted to charge carriers
within the solar cell due to a reduction in thermalization and
surface recombination losses at the lower energy. Down
conversion was first theoretically suggested by Dexter in the
1950’s [6] and was shown experimentally only in the 1970’s
using lanthanide ion praseodymium, Pr
3+
in an yttrium
fluoride, YF
3
host [7]. Trupke et al. have shown, using
detailed balance calculations, that a theoretical maximum
efficiency of 38.6% is achievable for a downconverter with
bandgap of 2.2 eV located on the front surface of a solar cell
with bandgap of 1.1 eV [2]. Trupke et al. assumed only
radiative recombination takes place in the device and a non
concentrated 6000 K black body spectrum.
LDS is also an optical process similar to DC with the
exception that one highenergy photon (blue) is absorbed and
converted into a single lowerenergy photon (red), as shown in
Fig. 1(b). In this case, the only effect is to ‘shift’ highenergy
photons into a more efficient region of the solar cell’s spectral
response which is typically at a lower energy. Losses due to
thermalization are not mitigated, but losses due to surface
recombination can be mitigated. The concept of coupling LDS
to solar cells that perform poorly in the UV region was first
introduced by Hovel et al [3]. Recent studies show the
potential of using LDS layers for solar cell performance
enhancement [5], [8–11].
978-1-4673-0066-7/12/$26.00 ©2011 IEEE 000048 978-1-4673-0066-7/12/$26.00 ©2011 IEEE 000048