Page 282 - A Comprehensive Guide to Solar Energy Systems

286 A COMPrehensIVe GUIDe TO sOlAr enerGy sysTeMs

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fully flexible DssC and studied the effects er -yb doping concentration on the upcon-
version luminescence intensity and corresponding photovoltaic performance. When the
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er and yb doping amounts were controlled at 2.0 er -1.0 yb , the PCe of the flexible
DssCs reached about 7.29%, which was enhanced about 52% for the DssCs of undoped
TiO 2 composites. The introduction of nb 2 O 5 coating further enhanced the PCe from 7.29
to 8.10% [72]. In another study, han et al. synthesized upconversion er, yb-CeO 2 hollow
spheres using carbonaceous spheres as removable template via hydrothermal method
for improving the efficiency of dye-sensitized solar cells (DssCs). PCe was enhanced by
27% due to the upconversion effect, scattering effect, and higher dye loading capacities of
the modified devices with er,yb-CeO 2 [73]. In 2016 Meng et al. mixed plasmon-enhanced
upconversion composite y 2 O 3 :er/Au@TiO 2 having three-dimensional star-like morphol-
ogy with submicron TiO 2 (200 nm) nanoparticles to form a multifunctional scattering
layer in TiO 2 -based DssCs. The system showed extended light-absorbing range to near-
infrared region and improved light-scattering ability, leading to an improved photovol-
taic performance of DssCs. With the optimum mixing ratio of 23%, a PCe of 8.62% was
attained as compared to 6.77% without the upconversion composite which corresponds
to an improvement of 27.6% [74]. In another study, luoshan et al. synthesized and incor-
porated novel multi-shell-coated and Au-nanoparticle decorated hexagonal submicron
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β-nayF 4 :yb , er @siO 2 @Au/@TiO 2 prisms into the photoanodes of DssCs. The optimal
PCe obtained in a typical DssC containing the modified photoelectrode was 7.79%, cor-
responding to 28.1% higher than that of the DssC with a pure TiO 2 photoanode [75]. later,
yu et al. designed up-conversion ybF 3 -ho/TiO 2 nanoheterostructures and explored them
as photoelectrode material to yield DssCs with enhanced performance. When integrating
the UC/TiO 2 nanoheterostructures into DssCs, an overall PCe of 8.0% was achieved—this
corresponded to 23% enhancement in the overall conversion efficiency [76]. liao et al.
on the other hand synthesized upconversion/semiconductor submicron hollow spheres
composed of inner na x GdF y O z :yb/er shell and outer TiO 2 shell by exploiting colloidal
carbon spheres as the scaffold. The hollow spheres were then incorporated into the TiO 2
nanoparticle film photoanode to yield DssCs with improved performance. Compared to
the PCe of 6.81% for the pristine device, the DssC assembled with the introduction of 8
wt% na x GdF y O z :yb/er@TiO 2 hollow spheres in the photoanode exhibited an optimal PCe
of 7.58%, corresponding to 11.31% performance enhancement [77]. Bai et al. utilized 1D
upconversion CeO 2 :er, yb nanofibers synthesized by electrospinning method as scatter-
ing layer on top of TiO 2 photoanode in DssC and observed an enhancement of 14% in the
PCe due to the upconversion and scattering effects [78]. In 2014 yuan et al. developed and
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explored hexagonal core–shell-structured β-naybF 4 :er (2%)/nayF 4 :nd (30%), as an nIr
energy relay material for DssCs. These UCnPs were shown to harvest light energy in mul-
tiple nIr regions, and subsequently convert the absorbed energy into visible light where
the DssCs strongly absorb. The nIr-insensitive DssCs show compelling photocurrent en-
hancement through binary upconversion under nIr light illumination either at 785 or
980 nm, substantiating efficient energy relay by these UCnPs. The overall conversion ef-
ficiency of the DssCs was improved from 4.95% to 5.24% with the introduction of UCnPs