Page 279 - A Comprehensive Guide to Solar Energy Systems

Chapter 13 • Upconversion and Downconversion Processes for Photovoltaics 283

13.2.2.1 GaAs Solar Cells
In 1995, the first experimental demonstration of a proof-of-concept of upconversion en-
3+
hanced response for any PV system was reported by Gibart et al. Using a 100 µm-thick er -
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and yb -doped vitroceramic at the rear of ultrathin GaAs solar cells, an external quantum
efficiency due to upconversion (eQe UC ) of 2.5% under monochromatic laser excitation of
891 nm and ∼25 W cm irradiance was reported (corresponding to a normalized value of
−2
2
10 cm W ) [43]. since then considerable progress has been made in the field of applica-
−1
−3
tion of upconversion to PV devices.
13.2.2.2 Crystalline Silicon Solar Cells
The most frequently investigated materials for addressing the sub-bandgap losses of
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crystalline si solar cells are the ln -doped hexagonal sodium rare-earth tetrafluoride
(β-nareF 4 ) [44–47]. In addition to these, other alternatives such as ln doped rare-
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earth oxide (re 2 O 3 ), oxysulfide (re 2 O 2 s) [47–49], glasses, and glass ceramics have also
been explored and reported with the corresponding enhancement in solar cell response
[50,51].
In 2003 shalav et al. reported for the first time the application of microcrystalline β-
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nayF 4 :20%er upconverter to a bifacial crystalline silicon solar cell [52]. In 2005 the same
group used the upconverter mixed in an acrylic adhesive medium [53] and reported eQe UC
of the combined silicon solar cell upconverter system of 2.5 % under monochromatic exci-
−1
tation of 1523 nm at an irradiance ∼0.2 W cm (normalized eQe UC value < 0.125 cm W ).
−2
2
By improving the device and changing the matrix to white oil with rubberizer matrix, the
same group reported in 2007 an increased eQe UC of 3.4% at 1523 nm, however at increased
−2
irradiance of 2.4 W cm , resulting in a lower-normalized eQe UC of 0.014 cm W [54]. In
2
−1
−2
2010 Fischer et al. reported an eQe UC of 0.34% at a much lower irradiance of 1.09 W cm
3+
at 1523 nm for β-nayF 4 :20%er upconverter powder filled into a powder cell that was
attached to the rear of a silicon solar cell using an index-matching liquid. This corre-
sponds to a normalized eQe UC of 0.03 cm W [55]. Using an optimized bifacial silicon
2
−1
solar cell [56] on the other hand, an EQE UC of 1.69% was reported (normalized eQe UC of
3+
0.15 cm W ) and a further increase in eQe UC with increase in the er doping concen-
2
−1
tration to 25% as well as increasing the concentration of phosphor was reported in 2014
3+
[57–59]. For example, using 84.9 w/w% of β-nayF 4 :20%er embedded in perfluorocylobu-
tane (PFCB), an eQe UC as high as 5.72% was achieved under 1523 nm excitation with an
2
irradiance of 0.45 W cm , corresponding to a normalized eQe UC of 0.126 cm W [58].
−2
−1
3+
In 2013, Martin-rodriguez et al. explored microcrystalline Gd 2 O 2 s:10%er as a potential
alternative to β-nayF 4 :20%er [48]. Under monochromatic excitation at 1511 nm with an
3+
−2
irradiance of 0.50 W cm , an eQe UC of 4.09% was achieved for the microcrystalline ma-
terial filled in a powder cell and around 7.5% for the material embedded in PFCB with
a concentration of 84.9 w/w% [58]. In 2013 another potential alternative the monocrys-
talline Bay 2 F 8 :30%er with even higher eQe UC was reported by Boccolini et al. [16]. As
3+
proof-of-concept the upconverter was mounted in front of a conventional silicon solar