Page 277 - A Comprehensive Guide to Solar Energy Systems

Chapter 13 • Upconversion and Downconversion Processes for Photovoltaics 281

reported the first theoretical study of upconversion in the context of photovoltaics and
showed that a system consisting conventional single-junction bifacial solar cells with a
single bandgap of 2 eV and an ideal upconverter at its rear can achieve a PCe of 47.6%
under nonconcentrated sunlight and 63.2% for concentrated sunlight [14]. later in 2012
Johnson and Conibeer reported that the theoretical limit for efficiency of a si solar cell
with bandgap of 1.2 eV and an ideal upconverted illuminated by nonconcentrated light
was 40%, as compared to efficiency of 33.25% for the solar cell alone [15]. In contrast to the
theoretical predictions, several reported experimental studies have shown, however, that
the expected efficiency increase in real devices are rather low, though significant enhance-
ments are possible with materials with high upconversion quantum yields [6]. The reasons
for such discrepancy and the ways to address these are discussed later in the chapter.

13.2.1 Upconversion Materials

Both lanthanide (ln)-based upconverters as well as organic upconverters have been ex-
plored in the literature to enhance the nIr response of PV devices. lanthanides belong to
the group of rare-earth (re) elements, along with yttrium and scandium. The lanthanides
include all the elements from lanthanum (with the atomic number 57 and an orbital con-
2
2
6
0
1
figuration 5s 5p 4f 5d 6s ) to lutetium (with the atomic number 71 and an orbital con-
2
14
1
6
2
figuration 5s 5p 4f 5d 6s ). Upconversion is observed in materials based on the trivalent
1
3+
2
lanthanide ions (ln ) (where 4f shell is partially filled and all 5d and 6s electrons are
removed, except in lanthanum with an empty 4f shell and lutetium with completely filled
3+
4f shell). ln ions exhibit unique optical properties due to many possible radiative tran-
sitions between the energy levels of the partially filled 4f shell, which are shielded by the
completely filled outer lying 5s and 5p shells. As a result, the optical transitions in the 4f
shell are only marginally affected by the surroundings and appear at nearly the same ener-
gies for different host materials [16]. Although the energetic position of an energy level is
mainly undisturbed by the surroundings, the precise nature of the energy levels (especially
the width and strength of the different transitions) is determined by the host material.
3+
The crystal field of the host material at the position of the ln results in a splitting of the
energy levels into crystal field components (the so-called stark levels) leading to an effec-
tively broader absorption spectrum. A broader absorption spectrum is very much desired
for applications in photovoltaics, as a larger fraction of the solar spectrum can be used
for upconversion and, consequently, be utilized by the solar cell. Also, the combination of
3+
the doping level of the ln in the host and the host crystal structure determines the aver-
3+
age distance d between the ln , which in turn has a strong impact on the upconversion
3+
performance. The host material of the ln also determines the likelihood of nonradiative
losses such as multiphonon relaxation (MPr). The transition probability for MPr depends
on the phonon energies of the host material and the energy gap between the considered
energy levels [17]. A large variety of different host materials with low phonon energies to
3+
suppress nonradiative losses have been proposed to be suitable host materials for ln
[5,16,18–25]. There are many reviews available that provide good overviews on available