Page 278 - A Comprehensive Guide to Solar Energy Systems

282 A COMPrehensIVe GUIDe TO sOlAr enerGy sysTeMs

rare earth based upconverter materials [5,18,25–28]. To date, high internal upconversion
quantum yield values at low irradiance suitable for photovoltaics have been reported only
for monocrystalline and multicrystalline materials [6]. Upconversion nanomaterials, un-
5
3
fortunately are typically a factor of 10 –10 less efficient than their bulk counter parts [29],
but have the advantage and possibility of being combined with concepts such as photon-
ics and plasmonics for enhancing their upconversion performance [30–35].
Organic upconverters utilized for addressing the sub-bandgap losses are based on
triplet–triplet annihilation (TTA). here, a sensitizer species absorbs light and crosses to a
triplet state, the energy of which is rapidly and efficiently transferred to a second species,
the emitter. The emitter triplet states are roughly half the energy of its first excited sin-
glet state, so when two such triplets, |T1〉|T1〉, combine to create a supramolecular singlet,
internal conversion to the |s1〉|s0〉 state occurs to yield subsequent upconverted fluores-
cence [36]. Though a quadratic process at low efficiency, once a certain triplet concentra-
tion is reached, the triplet decay is dominated by bimolecular reactions and the response
on illumination density changes to linear [37–40]. In comparison to lanthanide-based
upconverting materials, a higher absorptance can be achieved for thinner layers in or-
ganic upconverters, as the absorption transition is a singlet–singlet transition and not a
partially forbidden transition like within the lanthanide’s 4f shell. however, photostabil-
ity and chemical lifetime are major challenges for organic upconverter materials in the
context of their application in photovoltaics. several reviews on TTA-based upconversion
have been published, which discuss the available materials and underlying processes in
detail [28,41,42].
The theoretical quantum efficiency of upconversion process is limited to 50%. The up-
conversion layer is usually placed at the rear of a PV device to capture the transmitted
photons and thus it is possible to independently optimize the layer for enhanced device
performance. This requires firstly the PV device to be bifacial such that it efficiently trans-
mits the low-energy photons and secondly it efficiently utilizes the upconverted photons
emitted by the upconverter. For efficient utilization of external radiation that impinges on
the rear of the PV device, a rear reflector is placed behind the upconverter unit to enhance
the performance of both the solar cell and upconverter.
13.2.2 PV Devices With Upconverters

In 1983, saxena was the first to suggest the application of upconverters (terbium-doped
lanthanum fluoride and thulium-doped calcium tungstate materials) for PV devices. how-
ever, the actual measurements with the solar cells were not reported [43]. The following
subsections give a brief overview of the reported results on the upconversion-aided en-
hancement of Ir response of PV devices. In the GaAs, c-si as well as a-si solar cells, upcon-
verter is attached to the rear (in most cases) or front (proof-of-concept) of the solar cells.
Whereas in dye-sensitized solar cells (DssCs), organic solar cells, as well as perovskite so-
lar cells (PsCs), additional concepts for upconverters internally integrated into the device
have also been explored.