Page 287 - A Comprehensive Guide to Solar Energy Systems

Chapter 13 • Upconversion and Downconversion Processes for Photovoltaics 291

realized soon afterwards [98]. The first experimental demonstration of downconversion
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for solar cells involved the Tb –yb couple where quantum cutting was achieved through
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cooperative energy transfer from Tb to two yb -ions, via the same mechanism that was
suggested by Dexter [104].
13.3.1 Downconversion Materials
Most of the early work on downconversion for solar cell applications is dominated by lan-
thanide ions due to their useful optical properties. recently, nanomaterials have also been
explored as potential downconverters. To maximize benefits obtained through downcon-
version, the host material and lanthanide ions must be carefully chosen accordingly with
the type and design of the solar cell. host materials must exhibit high transmittance, pho-
tostability, excitation energy, absorption strength, chemical and thermal stability, and low
scattering [105–107]. The formation of defects and traps within the host material results
in energy being absorbed inside the host instead of being transferred to the activator ion
[105,106]. Therefore a highly crystalline host is recommended. Main criteria to choose the
appropriate lanthanide ion are high emission lifetime and good chemical and electrical
stability [106]. A downconverter is usually placed on the top of a solar cell and one of the
drawbacks of this is that the layer often is highly reflective which causes radiative losses.
This can be avoided by an antireflective coating designed to reflect the downconverted
emission back into the solar cell [105]. Of the various lanthanide ions that have been ex-
plored as downconverters for solar cells, a good experimental demonstration involved a
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system of a Tb -yb couple in (y,yb)PO 4 :Tb [108]. Downconversion in Tb -yb is also
reported in GdAl 3 (BO 3 ) 4 [109], GdBO 3 [110], y 2 O 3 [111], CaF 2 nanocrystals [112,113], and
lanthanum borogermanate glass [114]. Good quantum yields desirables for solar cell ap-
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plications have also been obtained with Gd [115] and eu [116]. Other common down-
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conversion lanthanides materials useful for solar cell integration are Pr -yb co-doped
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Ky 3 F crystals [117], nd:srTiO thin films [118] and liGdF 4 :eu and liGdF 4 : er , Tb [119],
2+
among others [105,106]. nanophosphors such as an smPO 4 nPs-doped TiO 2 [120], eu -
2+
doped barium silicate (Ba 2 siO 4 :eu ) [121] have also been explored. recently, graphene
quantum dots (QDs) were explored as an option for downconversion in commercial solar
cells [122]. hybrid nanostructures, for example, colloidal Zns nanoparticles/si-nanotips
showed promising potential for efficient solar spectrum utilization in crystalline silicon
solar cells [123]. recently new kind of QDs based on inorganic perovskites have also been
explored [124].
13.3.2 PV Devices With Downconverters
since the pioneering work reported by Trupke and richards in 2002, different materials
such as oxides doped with lanthanide ions [125,126], silicon nanoparticles [127–129], and
quantum dots [130–133], among others have been explored as downconversion layers for
solar cells.