Page 286 - A Comprehensive Guide to Solar Energy Systems

290 A COMPrehensIVe GUIDe TO sOlAr enerGy sysTeMs

upconverter materials. Instead, they require the use of either implanted lanthanide ions or
the use of nanocrystalline or organic TTA-based upconverter materials.
13.2.3.2.2 SPECTRAL CONCENTRATION
The spectral range, in which upconverter efficiently converts incident photons to higher
energies, is quite narrow (typically <100 nm). The limited spectral range can be extended
by either co-doping or in combination with a sensitizer [95], or by using a second lumi-
nescent material that absorbs in a wide spectral range and emits in the absorption range
of the upconverter. The latter process called the spectral concentration has the advantage
of extending spectral range and enhancing the photon flux in the absorption range of the
upconverter, which then leads to a higher UCQy [96,97].

13.3 Downconversion
Downconversion (also known as “quantum cutting”) is the opposite process to upconver-
sion where one high-energy photon is “cut” into two lower-energy photons (see Fig. 13.3).
This process can reduce thermalization losses of hot charge carriers after the absorption
of a high-energy photon. If both lower-energy photons can be absorbed by the solar cell,
current doubling is achieved for the region of the solar spectrum that consists of photons
with energies exceeding 2 E g [98,99].
In 1957 Dexter first proposed the idea to obtain quantum yields >100% by creating
multiple photons through “cutting” a single photon into two lower-energy photons [100].
The mechanism he proposed involved the simultaneous energy transfer from a donor to
two acceptors, each accepting half the energy of the excited donor. In 1974 the first ex-
3+
perimental evidence for quantum yields >100% was reported for yF 3 :Pr . The mechanism
was not the one proposed by Dexter, but involved two sequential emission steps from the
3
3+ 1
1
1
high-energy s 0 level of Pr ( s 0 → I 6 followed by relaxation to the P 0 level and emission of
a second visible photon from P 0 ) [101,102]. later, quantum cutting via two sequential en-
3
3+
3+
ergy transfer steps in the Gd –eu couple was discovered and, based on the analogy with
the two-step energy transfer process leading to upconversion, it was called “downconver-
sion” [103]. The potential of downconversion for increasing the efficiency of solar cells was

FIGURE 13.3 (A) Schematic showing downconversion process (B) Frequently used configuration of integrating
downconverter layer at the rear of the solar cell for addressing transmission losses. The antireflection coating aids in
harvesting the upconverted photons.