Page 281 - A Comprehensive Guide to Solar Energy Systems

Chapter 13 • Upconversion and Downconversion Processes for Photovoltaics 285

13.2.2.3 Amorphous Silicon Solar Cells
Due to the higher-absorption threshold (around 730 nm), the potential gain due to up-
conversion for hydrogenated amorphous silicon (a-si:h) solar cells is even higher than for
crystalline silicon solar cells (absorption threshold around 1100 nm). Both lanthanum-
based upconverters and organic upconverters have been applied to a-si:h solar cells.
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In 2009 de Wild et al. first reported the application of β-nayF 4 :18%yb ,2%er embed-
ded in PMMA between a ZnO:Al 0.5% rear contact and a white paint rear reflector to a-si:h
solar cells [65]. An eQe UC of approximately 0.02% under laser illumination at 980 nm with
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2
3 W cm irradiance was reported (resulting in a normalized eQe UC of 0.7 × 10 cm W )
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[66]. In 2013, using a similar configuration with Gd 2 O 2 s:10% yb ,5%er upconverter, de
Wild et al. reported an eQe UC of 0.06% under laser illumination at 981 nm with 0.2 W cm
−2
irradiance (a higher normalized eQe UC of 0.003 cm W ) and a ∆j sC,UC of 0.1 mA cm un-
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−1
2
der broad band illumination of 20 suns concentration [67]. In 2012, Chen et al. reported
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the application of β-nayF 4 :25%yb ,1%ho microprisms to both a standard p–i–n a-si
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and an a-si/a-si tandem solar cell and observed that the relative enhancement of the cur-
rent is roughly the same for both the solar cells [68]. For the tandem solar cell, they report-
ed current enhancements equivalent to eQe UC of up to 0.007% under laser illumination
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−2
at 980 nm with 0.5 W cm irradiance (i.e., normalized eQe UC of 1.4 × 10 cm W ). By
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2
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replacing ho by er , an improved eQe UC of 0.015% under similar laser excitation was
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reported (i.e., normalized eQe UC of 2.88 × 10 cm W ) [69].
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2
In 2012, Cheng et al. first reported application of the organic TTA upconverting system
of PQ4PDnA/rubrene [nitroaminopalladiumtetrakis porphyrin (PQ4PdnA) as sensitizer
and rubrene as the emitter] to an a-si:h silicon solar cell. The cuvette with PQ4PDnA/
rubrene dissolved in toluene was optically coupled to the rear of the solar cells with an
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index-matching liquid. A ∆j sC,UC of 0.30 mA cm under irradiation with an equivalent con-
centration of 48 suns was reported [70]. shulze et al. increased the absorption and the out-
coupling of upconverted photons by adding 100 µm-diameter Ag-coated glass spheres to
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the cuvettes with PQ4PDnA/rubrene in toluene, resulting in ∆j sC,UC of 0.275 mA cm un-
der irradiation with an equivalent concentration of 19 suns [71]. The current was not mea-
sured directly, but calculated based on eQe UC measurements covering the relevant spec-
tral range. later in 2014, schulze and schmidt reported an ∆j sC,UC of 4.7 × 10 mA cm
−3
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under a very low irradiation of 1.4 suns, using the upconverter system of PQ4PDnA and
rubrene/9,10-bis-phenylethynylanthracene in cuvettes additionally filled with Ag-coated
glass spheres [28].
13.2.2.4 Dye-Sensitized Solar Cells
The sub-bandgap losses in DssCs strongly depend on the absorption threshold of the dye
species used. A relatively large number of studies have been published in literature re-
porting the use of ln -based upconverter materials to enhance the device performance
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of DssCs. however, many of these results must be considered with care as the enhance-
ments cannot be clearly attributed to upconversion. In 2017, liu et al. developed a com-
posite photoanode of nb 2 O 5 coated TiO 2 nWAs/UC-ey-TiO 2 nPs on flexible Ti mesh for