Page 283 - A Comprehensive Guide to Solar Energy Systems

Chapter 13 • Upconversion and Downconversion Processes for Photovoltaics 287

under simulated AM 1.5 solar irradiation [79]. In another study, Zhou et al. synthesized
3+
3+
core–shell structured β-nayF 4 :er , yb @siO 2 upconversion nanoparticles with similar
diameters to TiO 2 nanoparticles and mixed them in TiO 2 photoelectrode, which elimi-
nated the size-dependent light scattering effect in the light harvesting process, leading
to an enhancement of 6% in the PCe (from 5.96% to 6.34%). In 2011, Xie et al. doped TiO 2
3+
3+
nanoparticles with er and yb ions, which were then directly incorporated for the elec-
3+
3+
trodes of the DssC. The er /yb acted not only as upconverter, but also as dopant, which
improved the electrical properties. Furthermore, the addition of upconverter also modi-
fied the scattering properties, as discussed in the later works on upconverters integrated
into the TiO 2 photoanode [80] or the rear reflector [81]. In 2012 yuan et al. intermixed col-
3+
3+
loidal nanocrystals of β-nayF 4 :20%yb ,2%er with the Z907 dye for DssC. Under mono-
−2
chromic excitation with 980 nm and 8 W cm , an eQe UC of 0.011% was observed, which is
−4
−1
2
equivalent to a normalized eQe UC of 0.132 × 10 cm W . Both fluorescence resonance
energy transfer and luminescence-mediated energy transfer were discussed as potential
routes for the migration of the energy from the upconverter to the dye [82]. In another re-
3+
port, in 2011 liu et al. attached y 3 Al 5 O 12 transparent ceramic co-doped with 3.0% yb and
3+
−2
0.5% er to the rear of a DssC, which showed a ∆j sC,UC of 0.2 mA cm under 980 nm laser
excitation of 500 mW power [83]. In 2013 Miao et al. applied microcrystalline yb 2 O 3 and
3+
3+
β-nayF 4 :15%yb ,3%er on top of DssC and observed under 980 nm laser illumination of
−2
∼474 W cm , eQe UC of only 0.029% for the device containing the former and 0.015% for
−7
−1
2
the device containing latter—resulting in normalized eQe UC values of 6.1 × 10 cm W
2
−1
−7
and 3.3 × 10 cm W , respectively [84]. In 2013 nattestad et al. first report the appli-
cation of organic upconverter in a DssC. A degassed solution of the PQ4PDnA/rubrene
upconverter system was placed in a cavity integrated between the DssC and rear reflec-
−3
−2
tor. For an illumination equivalent to 3 suns, ∆j sC,UC of 2.25 × 10 mA cm was observed
[85]. The same group in 2014 observed an improvement by using an upconverting system
PQ4PDnA with the hybrid emitter rubrene/9,10-bis-phenylethynylanthracene. An addi-
−2
−4
tional short-circuit current density of 4.05 × 10 mA cm under an extremely low irradia-
tion equivalent to 0.3 suns was observed [28].
13.2.2.5 Organic Solar Cells
In organic solar cells, sub-bandgap losses amount to more than 70% of the incident
photons and more than 50% of the incident power [6]. In 2011 Wang et al. used layer
of commercial upconversion phosphor based on a yttrium fluoride host doped with
ytterbium and erbium to P3hT:PCBM solar cell. An increase in photocurrent density
of ∼0.0135 mA cm due to upconversion was reported under 975 nm laser diode il-
−2
−2
lumination (25 mW cm ) [86]. later in 2012 the same group integrated upconversion
MoO 3 :yb /er layer as a buffer layer between the active layer and the electrode into a
3+
3+
P3hT:PCBM-based organic solar cell and reported an increase in photocurrent density
to upconversion under 975 nm laser diode illumination under different excitation inten-
sity [87]. In 2012 schulze et al. applied PQ4PDnA/rubrene-based upconverter materi-
als (as described for the a-si:h solar cells, in section 13.2.2.3) to two organic solar cells