Page 280 - A Comprehensive Guide to Solar Energy Systems

284 A COMPrehensIVe GUIDe TO sOlAr enerGy sysTeMs

−2
cell, an eQe UC of 6.5% at a high irradiance of 8.5 W cm under illumination at 1557 nm
was reported [16]. later in 2015 by using optimized bifacial silicon solar cells and using the
3+
monocrystalline Bay 2 F 8 :30%er , a record value of eQe UC of 8.0% was reported at a higher
−2
irradiance of 0.45 W cm by Fischer et al. This value corresponds to normalized eQe UC
−1
2
value of 0.177 cm W [60]. Another group of upconversion materials that have been ap-
plied on top of the crystalline silicon solar cells are fluorozirconate glass with 9.1% er
3+
doping and fluoroindate glass with 2.25% er and 0.1% yb co-doping [61,62]. In 2009
3+
3+
henke et al. using fluorozirconate glass with 9.1% er doping, reported an eQe UC of 1.6%
3+
−2
achieved at 1540 nm, under a very high irradiance, ∼107 W cm , resulting in rather low
normalized value around 10 cm W [61].
−1
−9
2
later in 2013 hernandez-rodriguez et al. reported an eQe UC of 0.4% (1480 nm excita-
−2
tion, 67 W cm irradiance) corresponding to a normalized eQe UC of 6.0 × 10 cm W
−1
−5
2
for fluoroindate glass with 2.25% er and 0.1% yb co-doping [62]. Pellé et al. on the oth-
3+
3+
3+
er hand in 2011 reported eQe UC of 1.4% and 2.4% respectively for ZBlAn:4.7%er fluo-
−2
ride glass and disordered CaF 2 –yF 3 crystal, under 1540 nm excitation with 141 W cm
irradiance for upconverter material attached at the rear of a bifacial silicon solar cell
[63].
It is important to note that though for application in photovoltaics broadband illumina-
tion is more relevant, most of the early investigations as discussed above were carried out
under monochromatic excitation. In 2011, Goldschmidt et al. first reported experiments
applying broadband illumination to upconverter solar cell devices. Using 98 w/w% of β-
nayF 4 :20%er in zapon varnish, they showed an eQe UC of 0.81% averaged over the broad-
3+
band excitation region from 1460 nm to 1600 nm with a photon flux equivalent to 458 suns
(in the considered spectral region) [64]. For the assessment of materials under broadband
illumination, a more meaningful figure-of-merit is considered to be the additional short-
circuit current density ∆j sC,UC that can be generated due to upconversion of sub-band-
gap photons under illumination with the full solar spectrum onto the upconverting solar
cell device than the averaged eQe UC [63]. In 2014 Fischer et al. applied 75.7 w/w% of β-
nayF 4 :25%er embedded in the polymer PFCB with to an optimized bifacial silicon solar
3+
−2
−2
cell and reported a ∆j sC,UC of 3.4 mA cm and 13.3 mA cm for the equivalent solar concen-
tration levels of 78 suns and 207 suns. By increasing the concentration of β-nayF 4 :25%er
3+
embedded in PFCB to 84.9 w/w%, the additional current due to upconversion ∆j sC,UC was
reported to be increased to 8.1 mA cm at 70 suns concentration [58]. On the other hand,
−2
for 84.9 w/w% of Gd 2 O 2 s:10%er embedded in PFCB a ∆j sC,UC of 5.9 mA cm was reported
3+
−2
under 67 suns concentration, revealing the superior performance of the β-nayF 4 :25%er
3+
3
under broadband illumination compared to the Gd 2 O 2 s:10%er [47]. In 2015 Fischer et al.
also reported record enhancements in using monocrystalline Bay 2 F 8 :30%er to optimize
3+
bifacial silicon solar cells, under an equivalent solar concentration of 94 suns, an addi-
tional short-circuit current density due to upconversion of 17 mA cm was achieved. This
−2
corresponds to a record relative enhancement of 0.55% (40-fold increase compared to first
values published in 2011) in the additional short-circuit current density generated due to
upconversion [64].