This paper evaluates the enhancement of the up-conversion phenomena by combining rare-earth doped  phosphors with PbS quantum dots (QDs). We present results on the characterization of two different ways of  adhering the up-converter with this fluorescent material to bifacial solar cells: by dissolving the powder in a spin-on  oxide and by dissolving it in a silicone gel. The improvement in photocurrent detected for both the oxide and silicone  alternatives when including the QDs is 60% better than without them. It is shown that the absorption and emission  characteristics of the PbS QDs embedded in oxide and silicone can be tuned into to the desired spectral region.  Keywords: quantum dots, up-converter, solar cel

Cañizo Nadal, Carlos del

Luque López, Antonio

Pan, Aline Cristiane

Archivo Digital UPM

Improving up-conversion with PBS quantum dots

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

IMPROVING UP-CONVERSION WITH PBS QUANTUM DOTS 
 
 
A. C. Pana, C. del Cañizob and A. Luqueb 
aPontifícia UniversidadeCatólica do the Rio Grande do Sul – Faculdade de Física 
Av. Ipiranga, 6681 - Partenon - Porto Alegre/RS, 90619-900 – Brasi – email:aline.pan@pucrs.br 
bUniversidad Politécnica de Madrid – Instituto de Energía Solar 
ETSI Telecomunicación, Ciudad Universitaria, 28040 Madrid – Spain 
 
 
ABSTRACT: This paper evaluates the enhancement of the up-conversion phenomena by combining rare-earth doped 
phosphors with PbS quantum dots (QDs). We present results on the characterization of two different ways of 
adhering the up-converter with this fluorescent material to bifacial solar cells: by dissolving the powder in a spin-on 
oxide and by dissolving it in a silicone gel. The improvement in photocurrent detected for both the oxide and silicone 
alternatives when including the QDs is 60% better than without them. It is shown that the absorption and emission 
characteristics of the PbS QDs embedded in oxide and silicone can be tuned into to the desired spectral region. 
Keywords: quantum dots, up-converter, solar cell. 
 
 
 
1 INTRODUCTION 
 
The implementation and characterization of up-
converters (UCs) layers on the rear of bifacial silicon 
solar cells (BSSC) has been reported by several authors 
[1] [2]. Pan et al. [3] attached some commercial 
phosphors to the BSSC by dissolving them either in an 
spin-on oxide or in a silicone. 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 
strongly dependent on light intensity, and also because 
the wavelength range where 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 in a number of 
times for photovoltaic applications [4] [5]. The idea is to 
widen the IR light being used through a material that can 
absorb in a range of wavelengths where the UCs does not 
respond, and re-emit 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 pink very fine powder that seems to 
consist, according to EDX measurements, of ZnSO4 
doped with ytterbium (Yb) and a small fraction of erbium 
(Er).  
PbS QDs have appropriate absorption and emission 
properties for photovoltaic devices of this kind and are 
readily commercially available. There are several 
requirements on the QDs that have to be fulfilled for this 
purpose. Suyver et al. [6] reported that the diameter of 
the QDs should be below 30nm for to reduce light 
scattering, and for that reason 5.3 nm PbS QDs made by 
the company Evident Technology have been used. These 
QDs have large EQE and high indices of refraction 
compared to the phosphors, making them better matched 
for Si devices [7].The energy transfer will probably ocurr 
by radiative emission from the QDs followed by 
absorption by the UC phosphor. 
In this paper we combine the UCs with a PbS QDs in 
a BSSC and characterized the devices through Scanning 
Electron Microscopy (SEM), measurements photocurrent 
and photoluminescence. Two different ways of adhering 
the QDs to the BSCC and UCs have been implemented: 
by dissolving the QDs and power UCs in a spin-on oxide, 
and by dissolving it a silicone gel. 
 
2 APPLICATION OF THE UP-CONVERTERS AND 
QUANTUM DOTS IN BIFACIAL SILICON SOLAR 
CELLS 
 
 The process sequence previously developed at our 
institute [4] for to fabricate of the bifacial silicon solar 
cell, which is detailed in Figure 1 (a), has been 
implemented. A final step is added in which the two 
alternative deposition techniques for the attachment of 
the UC and QDs to the solar cell are used. The UCs are 
in powder form and the PbS QDs are in liquid form, so 
that a binding agent is needed, which at the same time 
should give a good optical coupling to the solar cell. Pan 
et al. [2] has been described details of these 
implementations. The schematic structure of the BSSC 
with UCs and QDs in both deposition techniques are 
shown in Figure 1 (b) and (c). 
 
 
 
Figure 1: . Schematic of the manufacturing process of 
the bifacial solar cells (a), showing the integration of the 
up-converter and quantum dots in the silicone gel (b) and 
spin-on oxide (c). 
 
 
25th European Photovoltaic Solar Energy Conference and Exhibition /
5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
57
3 CHARACTERIZATION OF UP-CONVERSION 
PERFORMANCE BY COMBINATION WITH 
QUANTUM DOTS 
 
 It is seen by SEM measurements (not shown here) 
that the introduction of the QDs in both cases does not 
alter the properties of the UC, which is good because it is 
needed that the UC and the QDs do not interfere. In 
addition, a smaller homogeneity has been observed in 
samples with QDs, that is to say, greater agglomeration of 
the active ions in the UC. This factor can also be 
favourable to the UC phenomenon, because it 
considerably increases the probabilities of energy transfer 
mechanisms [8] [9]. 
The increase in photocurrent detected for a BSSC 
with UCs and QDs is shown in Figure 2 for both 
implementation modes, the oxide (a) and the silicone (b). 
Details on the characterization setup can be found in the 
reference [10].  
 
(a) 
 
(b) 
 
Figure2: Increase in photocurrent versus wavelength for 
a bifacial silicon solar cell with UC embedded in the 
oxide (a) and in the silicone (b) with and without QDs. 
The dots correspond to the LED sources have been used, 
centered in two different wavelengths, and with a half 
band width of around 50 nm 
 
 The improvement in photocurrent detected for a 
BSSC with commercial UC and PbS-QDs is in all cases 
60% better than without them, demonstrating that the 
absorption and emission characteristics of the QDs 
embedded in oxide and silicone can be tuned into the 
desired spectral region. These experimental results are 
similar to those calculated by Löper et al. [11], where 
they calculated for commercial nanocrystals a fluorescent 
quantum efficiency of about 50. The photocurrent 
extracted from the BSSC is found under excitation from 
IR LED (1450 to 1550 nm), clearly coinciding with the 
material of the commercial UC used (4I11/24I15/2 Er3+ 
ions transition). 
To give insight into the phenomena taking place, the 
configuration of the Yb-Er UC system is represented in 
Figure 3 (a). The PL spectrum measured for the 
PTIR545/F-UC in silicone gel with and without QDs is 
also shown in Figure 3 (b). Laser excitation is at 514.5 
nm, but emission in the visible light wavelength 
(corresponding to transitions from the levels 4I9/2, 4F9/2, 
4H11/2 of the Er3+ ion directly to the ground state) cannot 
be detected. As the detector covers the NIR wavelength 
range, only transitions in various steps to lower levels 
(4I11/2 and 4I13/2) will leave a signature in the PL spectrum. 
These excited states decay to the next lower lying state or 
directly and radiatively to the ground state. Photons 
emitted by a transition process from 4I11/2 or higher 
excited levels back to the ground state have more energy 
than the bandgap for silicon. 
 
(a) 
 
(b) 
 
Figure3: Schematic configuration of the Yb3+-Er3+ UC 
system (a). Radiative absorption and emission are 
indicated by solid lines and non-radiative ones by dashed 
lines. The open circles show the initial state of the center. 
Photoluminescence spectra of a bifacial silicon solar cell 
with UC embedded in the silicone with and without QDs 
(b). 
 
Two NIR infrared emissions are observed in the PL 
spectrum. The 4I13/24I15/2 Er3+ ions transition (1540 nm) 
is clearly observed, and the increase expected from the 
implementation of QD is achieved. The difference 
observed in the emission spectrum of PL surely is due to 
the increase in the absorption of the rare earth ions by the 
introduction of the QDs. This transition can be possible 
either because of the resonance between the Er3+ (4I13/2) 
and PbS QDs, or more likely because QDs cause 
scattering of light with results in an increased coupling 
25th European Photovoltaic Solar Energy Conference and Exhibition /
5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
58
into the Er higher energy levels. On the other hand, the 
shape of this peak agrees with experiments reported by 
Auzel [12]. 
The transition from the level 4I11/2 back to the ground 
is not seen in the PL neither for the case with QDs nor for 
the case without them. But this can be explained because 
to induce the transition 4I15/24I11/2 for Er3+ ions two 
photons are needed, or a high energy transfer from the 
4I13/2 level, which seems not to be the case for our 
commercial phosphor. We have not a clear explanation 
for the peak at 1010 nm seen at the PL, which would 
need more investigation, but the fact is that the QD have 
no influence on it. 
Based on Figure 2 and Figure 3 (b), we conclude that 
the application of a layer with UC and QDs to a BSSC 
can enable such a solar cell to use sub-band gap light 
indirectly (λ>1100nm) that would otherwise not be 
absorbed by the silicon. The emission of the PbS-QDs 
coincides with the ground state absorption of the Er3+ 
(4I15/24I13/2). Consecutive absorption as well as energy 
transfer-based UC processes takes place from this first 
excited state to a second one [12]. However, the 
PTIR545/F-UC is not so appropriate for solar cells, 
because of the combination of Yb3+ and Er3+. The UC 
effect occurs because of the absorption of 980 nm-
photons in Yb3+, and after the de-excitation of this state, 
the energy is transferred to the Er3+, so that it competes 
with silicon for the NIR photons with wavelengths of 
between 900 and 1100 nm [13]. But the PTIR545/F-UC 
has additional absorption peaks as a result of the Er3+ 
(1509, 1522, 1547, 1552 and 1600 nm), and our 
measurements show that they can contribute to an 
enhancement of photocurrent. The small broadening of 
the excitation spectra as compared to other UCS used 
with BSSC [6] [14] can be to the result of the influence 
of the phonon energy of the different host materials. 
 
4 CONCLUSION 
 
This study reports on experimental efforts aimed at 
utilizing the unique optical properties of high-quality QD 
together with UC for solar cells applications. 
 Two effective procedures for incorporating PbS into 
a UC have been developed. We have demonstrated that 
the combination of an Yb/Er-doped phosphor with PbS-
QDs enhances the performance of the up-conversion 
phenomena. 
 
ACKNOWLEDGEMENTS 
This work has been carried out within the framework of 
the CrystalClear Integrated Project. The EC is gratefully 
acknowledged for financial support under Contract 
number SES6-CT-2003-502583. It has also been 
supported by the Brazilian Ministério de Educação e 
Ciência-CAPES through a scholarship, process 
BEX1876-03/1 and AEX2948/10-9, and financial 
professional support of the Pontifical University Catholic 
of the Rio Grande Do Sul – Brazil. 
 
 
 
REFERENCES 
 
[1] B. S. Richards, A. Shalav. IEEE Transactions on 
Electron Devices. Vol. 54, nº 10, (2007), 2679-2684. 
[2]] C. Strümpel et al. Conference Record of the 2006 
IEEE 4th World Conference. (2006), 87-90. 
[3] A. C. Pan, C. Del Cañizo, A. Luque. Materials 
Science and Engineering B-Solid. Vol. 159-160, (2009), 
212-215. 
[4] A Shalav. Ph. D Thesis, UNSW, Sydney, Australia, 
(2006) 
[5] C. Strümpel. Ph. D Thesis, UKON, Konstanz, 
Germany (2007) 
[6] J. F. Suyver et al. Optical Materials. Vol. 27, (2005), 
1111-1130 
[7] J. E. Reynolds, M. Locascio. Proceedings of the 
Materials Research Society (MRS) Fall Meeting, Boston,  
(2002), 31-36. 
[8] D. R. Gamelin, H. U. Güdel. Topics in Current 
Chemistry. Vol. 214, (2001), 1-56. 
[9] F. Auzel. Chemical Review. Vol. 104, (2004), 139-
173. 
[10] A. C. Pan et al. Solar Energy Materials and Solar. 
Cells. Accepted for publication (Online publication 
complete: 19-AUG-2010, DOI information: 
10.1016/j.solmat.2010.06.028). 
[11] P. Löper et al. Proceedings 22rd European 
Photovoltaic Solar Energy Conference, Milan, (2007), 
589-594. 
[12] F Auzel. Chemical Review. Vol. 104, (2004), 139-
173. 
[13] A. Shalav, B. Richards, M Green. Solar Energy 
Materials and Solar. Cells. Vol. 91, (2007), 829-842. 
[14] C. Strümpel et al. Solar Energy Materials and Solar. 
Cells. Vol. 91, (2007), 238-249. 
25th European Photovoltaic Solar Energy Conference and Exhibition /
5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain
59


http://oa.upm.es/8918/1/INVE_MEM_2010_83421.pdf

Improving up-conversion with PBS quantum dots

Abstract

Similar works

Full text

Available Versions

Archivo Digital UPM

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid