The Role of Different Wavelength-Shifting 3HF-Based Thin Films
on Solar Cells Efficiency
M. Buffa1,3, S. Carturan2,3, G. Maggioni2,3, A. Quaranta1,3, M. Tonezzer1, G. Della Mea1,3.
1
Dept of Materials Engineering and Industrial Technologies, University of Trento, Trento, Italy.
2
University of Padova, Padova, Italy.
3
INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy.
synthesized by mixing vinyl-terminated polydimethyl-codiphenyl siloxane (PMPS) with 22% of phenyl groups
(molar %) with hydride-terminated poly methylphenyl-comethylhydrosiloxane. 3HF was dissolved in the vinyl
terminated precursor resin prior to polymerization,
activated by a platinum catalyst. The doped polysiloxane
films were subsequently produced by casting the viscous
solution on a glass support (final thickness 60 μm).
Alternatively, 3HF doped polysiloxane samples were
obtained by dipping the bare siloxane obtained by casting
(1 mm thick) in 3HF containing acetone solution (0.5 g/L)
and leaving the sample standing therein for three days.
From the absorption spectrum, it can be deduced that 3HF
concentration within the siloxane is about 0.37 g/L.
UV/visible absorption measurements were performed in
the 250-800 nm range using a Jasco V-570 dual-beam
spectrophotometer. Emission and excitation spectra at
room temperature were collected by a Jasco FP-770
spectrofluorometer equipped with a 150-W xenon lamp.
The performance of the WLS was investigated using a
polycrystalline Si-solar cell (XGroup, Italy), a Hamamatsu
photodiode (S3204-05, Japan) and a Sun 2000 solar
simulator (Abet technology, USA).
Yield measurements on solar cells were performed by
using a solar simulator and placing the cell always at the
same distance and at the same position. A rheostat was
connected to the solar cell, a voltmeter was connected
parallel to the variable resistance and the amperometer was
connected in series with the resistance (figure 1).
INTRODUCTION
Several active photovoltaic materials show a poor
response to short-wavelength light, so a considerable
fraction of the incident photons are lost at the front surface.
This loss can be reduced by shifting photons into an energy
range where the cell has a higher spectral response. For a
typical silicon solar cell, photoluminescence would be
beneficial if wavelengths in the range 350-500 nm could be
shifted into the range 500–1000 nm. Luminescent
materials have been recently produced by using different
organic dyes or inorganic nanostructures in transparent
matrices in order to overcome the poor blue response of
solar cells: nevertheless the operational results are not
completely satisfactory and much more work has to be
done in this field.
In this work novel wavelength-shifting (WLS) materials
were prepared by dispersing 3-hydroxyflavone (3-HF) in
different transparent matrices. 3-hydroxyflavone (3-HF)
was chosen as the luminescent dye owing to its very
pronounced Stokes shift (Δλ ≅ 180 nm). In fact, upon
excitation at wavelengths lower than 400 nm, 3-HF emits
at wavelengths higher than 500 nm: this large Stokes shift
significantly improves the efficiency of the system
knocking down the re-absorption losses caused by the
overlap of the absorption and emission spectra.
EXPERIMENTAL
The experimental equipment used for the deposition of
the dye-containing parylene-C (PC) films was a PPCS
Labcoater LC300 (PPS, Germany). The procedure for the
synthesis of the 3HF embedded in parylene is described in
Ref. [1]. The films were deposited on two different
substrates: p-doped (100) silicon wafers lapped on both
faces (Bayville Chemical Co., USA) for fluorescence
measurements and glass slides (Vetrotecnica, Italy) for
optical absorption measurements.
The procedure for the synthesis of polyimide 6FDADAD (PI) is described in Ref. [2]. The flakes of produced
PI were dissolved in N-methyl pyrrolidone (NMP) with the
dye and then deposited on a glass slide via spin-coating
and/or drop casting. Spin-coated samples were 1.4 μm
thick, whereas the sample obtained by casting was 30 μm
thick.
The procedure for the synthesis of the polysiloxane
(PMPS) samples is described in Ref. [3]. The samples were
LNL Annual Report
Fig. 1. Absorption spectra of (a) PI-3HF (1 wt. %) cast from
NMP solution, (b) PMPS with 3HF (1 wt. %), (c) 3HF doped
parylene sample.
The parylene-based sample was directly deposited on a
solar cell to avoid optical matching problems. Polyimide
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Applied, General and Interdisciplinary Physics
and polysiloxane thin films were deposited on high-clarity
glass and coupled to the cell using polysiloxane resin,
whereas thicker PMPS samples doped by swelling samples
were used as self-supporting films.
%Y .I . =
(1)
where Pcell is the highest power obtained with the bare
solar cell, and PWLS is the highest power obtained with the
cell covered by the WLS. The experimental conditions
(temperature and position) are kept constant throughout the
testing run, results are displayed in table 1.
RESULTS
Samples are investigated with optical and electrical
measurements. Absorption of PI and PMPS cast samples
with 1 wt. % 3HF display similar absorption intensity,
despite the difference in thickness (30 μm for polyimide
film and 60 μm for polysiloxane). In all cases, the two
components (main peak at 345 nm, shoulder at 310 nm)
referable to 3HF are clearly visible, while the contribution
of the bare matrix to the absorption spectrum is negligible.
The parylene-based sample displays much lower
absorption intensity owing to the reduced thickness (1.93
μm) with respect to the other samples obtained by casting.
In figure 2 the excitation and emission spectra of PMPS
samples doped with 3HF by the two different routes
(dissolution and swelling) are shown. Curves in (a), (b) and
(b’) correspond to the sample obtained by mixing 1 wt. %
of 3HF directly with the vinyl terminated precursor resin,
while (c), (d) and (d’) curves correspond to the case of
3HF doped siloxane by swelling. The spectra (a), (b), (c)
and (d) were collected after few days from the preparation,
whereas (b’) and (d’) were gathered after three weeks. The
(b) curve represents the typical emission spectra of the
tautomeric 3HF form, whereas the curve (d) shows the
presence of an extra peak at about 420 nm, which results
from the fluorescence of the normal form of 3HF that after
three weeks the prevails on tautomeric form, detrimentally
affecting the WLS process efficiency.
Table 1. Performance results.
Sample
Si cell without film
Si cell+PC-3HF
Si photodiode (SiPH)
Si-PH + PI-3HF
spin
Si-PH+ PI-3HF
cast
(1) Si-PH+ PMPS3HF cast
After 3 weeks
(2) Si-PH+ PMPS3HF cast
Si-PH + PMPS3HF swell
Max Power (mW)
16.1 ± 0.2
16.5 ± 0.2
Y.I.%
-2.2 %
7.9 ± 0.1
--
8.1 ± 0.1
2.5 %
8.2 ± 0.1
3.7 %
8.3 ± 0.1
5.0 %
After 3 weeks
After 3 weeks
8.1 ± 0.1
2.5 %
8.0 ± 0.1
1.3 %
CONCLUSIONS
In this work WLS materials for the improvement of the
efficiency of solar cells have been produced by dispersing
3HF in different polymer networks.
3HF dispersed in PMPS resins suffers a decrease of the
luminescence yield owing to chemical interactions with the
polymer network. As produced PMPS cast films give rise
to an increase of the cell yield up to 5 %, which reduces to
2.5% after three weeks.
Good results are obtained by using thin films of
parylene-C with high 3HF concentration and a yield
increase as high as 2.2% was measured.
Polyimide samples, though doped with a lower 3HF
concentration, afford results comparable with parylene
based samples, giving a yield increase of 2.5%.
Both polyimide and parylene matrices do not interfere
with 3-hydroxyflavone tautomeric form, responsible of the
desired green emission. Further experimental work is now
in progress in order to characterize the samples
photostability.
Fig. 2. Excitation (a) and emission (b) spectra of 1 wt. % 3HF
doped (dissolution) PMPS based sample and excitation (c) and
emission (d) spectra of PMPS 3HF doped (swelling) sample.
Spectra with the pronunciation mark are taken after three weeks.
Many electrical measurements were performed before
and after the application of the WLS either to the solar cell
or to the photodiode. Power was calculated from the
Ohm’s law and the yield increase (%Y.I.) was calculated
with the following formula:
LNL Annual Report
PWLS − Pcell
× 100
Pcell
[1] M. Buffa, S. Carturan, G. Maggioni, M. Tonezzer, W.
Raniero, A. Quaranta,G. Della Mea, Proceedings of
European Solar Conference, Valencia 2010.
[2] S. Carturan et al., Sens. Actuators B, 137 (2009) 281.
[3] A. Quaranta et al., IEEE T-NS, 57 (2010) 891.
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Applied, General and Interdisciplinary Physics