Enhancement of up-conversion efficiency by combining rare earth-doped
phosphors with PbS quantum dots
A.C. Pan
a,
n
, C. del Can
˜
izo
a
,E.Ca
´
novas
a
, N.M. Santos
b
, J.P. Leit
~
ao
b
, A. Luque
a
a
Instituto de Energı
´
a Solar—Universidad Polite
´
cnica de Madrid, Ciudad Universitaria, Madrid 28040, Spain
b
Departamentento de Fı
´
sica e I3N—Universidade de Aveiro, Aveiro 3810-193, Portugal
article info
Available online 19 August 2010
Keywords:
Up-conversion
Quantum dots
Bifacial
Silicon
Solar cells
abstract
This paper aims to enhance the up-conversion phenomena observed in silicon solar cells by combining
a rare earth-doped phosphor with PbS quantum dots. Two different ways of adhering the up-converter
and the fluorescent material to a bifacial solar cell are implemented: dissolving the powder in a spin-on
oxide and by dissolving it in a silicone gel. Characterization is carried out through photocurrent and
photoluminescence measurements. The improvement in photocurrent detected by the combination of
the up-converter and the PbS quantum dots is 60% better than without them, demonstrating that the
absorption and emission characteristics of the quantum dots embedded both in the oxide or the silicone
can be tuned to the desired spectral region.
& 2010 Elsevier B.V. All rights reserved.
1. Introduction
Photon converters can enhance the performance of solar cells
as they have the ability to condition the solar spectrum, thus
suiting the semiconductor bandgap better. In the case of
up-conversion (UC), advantage can be taken of the transmitted
energy [1]. The implementation and characterization of
up-converters (UC) layers on the rear of bifacial silicon solar cells
(BSSC) has been reported by several authors [2–4]. Pan et al. [5]
attached some commercial phosphors to the BSSC by dissolving
them either in a spin-on oxide or a silicone. The performance was
characterized through external quantum efficiency (EQE)
measurements, demonstrating a gain in photocurrent in the IR
wavelength range. This gain is quite small, firstly because
response of the UC process is greatly dependent on light intensity,
and also because the wavelength range in which it takes place is
very narrow, corresponding to a small absorption range of the
rare-earth dopant. The use of photoluminescence materials to
enhance the UC phenomena has been suggested a number of
times for photovoltaic applications [6–8]. The idea is to widen the
IR light being used through a material that can absorb it in a range
of wavelengths where the UC does not respond, and re-emit it in
the wavelengths where it does respond.
The UC used in the experiments reported is called PTIR545/F,
made by the company Phosphor Technology. PTIR545/F is a very
fine pink powder that seems to consist, according to EDX
measurements of ZnSO
4
doped with ytterbium (Yb) and a small
fraction of erbium (Er). This commercial phosphor is typically sold
for applications in IR leds, printing inks, credit cards, etc. It can be
excited in the 1500 nm range and re-emits it in shorter
wavelengths, mainly in the 500 nm range.
PbS quantum dots (QDs) have appropriate absorption and
emission properties for combination with the UC and the BSSC [9],
and are readily commercially available. There are several require-
ments of the QDs that have to be fulfilled for this purpose. For
instance, Suyver et al. [10] reported that the diameter of the QDs
should be below 30 nm to reduce light scattering and for
that reason a 5.3 nm diameter PbS QDs made by the company
Evident Technology were selected and used in this work. These QDs
have large quantum efficiency and high indices of refraction
compared to the phosphors, which Si devices can take advantage
of [11]. The energy transfer will probably occur through radiative
emission from the QDs followed by absorption by the UC phosphor.
Fig. 1 details the normalized EQE as a function of wavelength
for the BSSC itself, the BSSC with PTIR545/F-UC, and the
absorption and emission of the Evident Technology PbS QDs.
While the EQE for the solar cell is significant in the range
350–1100 nm, the UC layer is able to extend it (although with a
very low response) in the 1488–1564 nm range. The PbS QDs have
absorption precisely in the range where neither the BSSC itself nor
the UC take advantage of the light (1200–1500 nm), and the
emission takes place in the range where the UC is active,
presenting a possible route to improve the UC efficiency.
In this paper we first present the characterization results for
the BSSC with UC, discussing the influence of the light power on
the UC efficiency. Second, we combine the UC with PbS QDs
characterize the approach through measurements of photocurrent
and photoluminescence (PL).
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/solmat
Solar Energy Materials & Solar Cells
0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2010.06.028
n
Corresponding author.
E-mail addresses: alinecpan@hotmail.com, aline@ies-def.upm.es (A.C. Pan).
Solar Energy Materials & Solar Cells 94 (2010) 1923–1926