Birefringence Creation by Solar Light

A New Approach to the Development of Solar Cells with Azobenzene Materials

Pedro Farinha, Susana Sério, Paulo A. Ribeiro and Maria Raposo

CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, UNL, Campus de Caparica,

2829-516 Caparica, Portugal

Keywords: Birefringence, Solar Cell, Pazo, Layer-by-Layer, Azobenzene.

Abstract: The conversion of solar energy into electricity is one of the viable alternatives regarding energy demand and

sustainability. This work shows that novel energy storage devices can be developed by using materials

containing highly polarizable molecules as poly{1-(4-(3-carboxy-4-hydroxy-phenylazo)

benzenesulfonamido) -1,2-ethanediyl, sodium salt} (PAZO). In the present case, layer-by-layer (LbL) of

PAZO and poly(allylamine hydrochloride) (PAH) thin films were prepared and the creation and relaxation of

photoinduced birefringence were characterized. These results demonstrate that birefringence can be induced

in these films by visible light with a spectrum similar to solar light. Comparing the characteristics parameters

of birefringence creation and relaxation, it was seen that the writing birefringence characteristic time, with a

value of 1.4 hours, is quite slower than the obtained with laser beam but the relaxation characteristic time is

of the order of 7 days. In addition, the birefringence value is proportional to the amount of azo-groups in the

sample. These preliminary results allows us conclude that solar devices based in that principle can be studied,

namely, the conversion of the oriented dipoles stored energy in power.

1 INTRODUCTION

Exhaustion of fossil fuels reserves and greenhouse

gases suppression are two main issues to be solved

regarding energy demand and sustainability (Hill,

2012). As such, there is stress in spreading the use of

renewable sources, such as sun, wind and water,

particularly in what concerns to electrical energy

production. As often renewable energies sources are

not available when they are more needed it is also

fundamental to develop low cost efficient energy

storage systems. The conversion of solar energy into

electricity is one of the viable alternatives although it

is still somehow costly for large scale applications.

Sunlight hits the earth's surface intermittently and

with intensity unpredictable, i.e. the production of

electricity from solar radiation can change rapidly,

making it difficult to distribute.

Developments in nanotechnology, biotechnology,

and generally materials science may trigger new

solutions for efficient and cheaper solar energy

conversion-storage systems (Lewis, 2007). For

example the novel electrochemical capacitors, called

supercapacitors (Wang, 2014), are able to offer an

alternative to the most used lithium ions which are

known to lose their charging capacity with successive

charge-discharge cycles, thus, not meeting the high

speed requirements for high power systems. These

are believed to become the next power generation

storage devices, mainly due to high energy densities

stored, fast charge-discharge cycles and higher

service life (Hu,2014), (Wang,2011).

Another possibility to develop novel energy

storage devices is the use of materials containing

highly polarizable molecules, which can be easily

polarized by sunlight, as is the case of azobenzene.

These compounds are formed by two benzene rings

linked through two double bounded nitrogen atoms.

These molecules are characterized by an absorption

band, low energy in the visible region, and other, high

energy, in the ultraviolet region. Since in 1937,

Hartley discovered photoisomerization ability of

these molecules, or the ability to spatially rearrange

by light of adequate wavelength. This process can

give rise to a macroscopic birefringence in a bulky

material containing azobenzene molecules (Hartley,

1937).

This work addresses on the energy storage

capabilities of thin films of poly{1-(4-(3-carboxy-4-

hydroxy-phenylazo)benzenesulfonamido) -1,2-

Farinha, P., Sério, S., Ribeiro, P. and Raposo, M.

Birefringence Creation by Solar Light - A New Approach to the Development of Solar Cells with Azobenzene Materials.

DOI: 10.5220/0005843103650368

In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 367-370

ISBN: 978-989-758-174-8

367

ethanediyl, sodium salt} (PAZO), a main chain

azobenzene containing polyeletrolyte. Particularly

the optical birefringence response, induced by visible

light of layer-by-layer (LbL) of PAZO and

poly(allylamine hydrochloride) (PAH) thin films,

deposited on a substrate glass covered with a

semiconductor thin film of tin oxide doped with

fluorine (FTO) will be presented.

2 EXPERIMENTAL

2.1 Materials and Methods

The polyelectrolytes PAH and PAZO, supplied by

Sigma Aldrich, were used to prepare layer-by-layer

(LbL) films. The chemical structures of these

polyelectrolytes are shown in Figure 1. Solutions with

a concentration of 10

-2

M of PAH and PAZO were

obtained by dissolving the polyelectrolytes in Milli-Q

ultrapure water with resistivity of 18 MΩ·cm at 25ºC,

(Millipore). Both pH were maintained constant to 5

for PAH and 7 for PAZO. The PAH/PAZO LbL films

were prepared onto BK7 substrates with a layer of

FTO.The LbL film preparation procedure consisted

of alternated adsorption from solution of both

polyelectrolytes, namely, the substrate was:1)

immersed in the PAH solution, for 3 minutes; 2)

washed with ultrapure water; 3) immersed layer in the

PAZO solution, for 3 minutes; 4) washed with

ultrapure water. If the above procedure is repeated n

times, a film with n bilayers, referred as

(PAH/PAZO)n, is obtained. The immersion time in

the polyelectrolytes solutions of 3 minutes is a

compromise between optimized time for films

preparation and the time required to achieve 80% of

adsorbed amount per layer, taking into account the

adsorption kinetics described by (Ramsden, 1995)

and (Ferreira, 2007a). The maintenance of constant

adsorption times allows to control the thickness of the

films.

a) b)

Figure 1: Chemical structures of a) PAH and b) PAZO.

2.2 Birefringence Measurements

The samples were irradiated with a halogen lamp of

250 W and 24V to simulate the solar spectrum, after

this light to be polarized by a linear polarizer of

adjustable angle and focused with a convex lens. The

irradiated area was of 1cm

. The radiation power was

of 250 mW. This value was measured with a Heavy

Duty Light Meter, Extech Intruments.,

The photoinduced birefringence was measured

with a laser diode PASCO OS-8525A of 0.9 mW and

650 nm of wavelength. The sample was placed

between crossed polarizers, in what concerns to the

probe beam, with polarization set at 45º with respect

to the writing beam polarization. An optical chopper

modulates the probe beam and the birefringence

signal was measured with a photodetector, Newport

884-UV, through a lock-in amplifier, Princeton

Applied research, model 5101.

3 RESULTS AND DISCUSSION

In figure 2a) is shown the ultraviolet-visible

absorbance spectra of different number of bilayers of

PAH/PAZO LBL films. The absorbance peak centred

near 360 nm is associated to the π - π* chromophore

transition (Ferreira, 2013), and the absorbance at

maximum is seen to increase with the number of

bilayers indicating a linear film growth, see figure

2b). From the slope of that the fitted, one can calculate

the PAZO adsorbed amount per unit of area and per

bilayer which takes the value of 0.014±0.001 mg.m

-2

using the Beer-Lambert law and the PAZO absorption

coefficient, at 360 nm, ε

360

= 4.30±0.07 m

-1

(Ferreira, 2007b). The PAZO adsorbed amount for a

film of 30 bilayers is of 0.42±0.03 mg.m

-2

a high

value when compared with films prepared onto glass

(Timóteo, 2016).

The birefringence creation and relaxation kinetics

curves obtained for PAH/PAZO LbL films with are

shown in figure 3. The birefringence creation curves

correspond to the increase of transmitted light signal

until the polarized light be turned off while the

birefringence relaxation curves is obtained

immediately the light to be turned off and correspond

to decrease of the transmission signal.

he birefringence creation kinetic curves can be

analysed fitting the experimental data to a bi-

exponential function, containing two distinct

processes in accordance with literature, see (Ferreira,

2012) and references therein. The fast process is

normally assigned to the trans

−

cis

−

trans

photoisomerization processes contributing to the

birefringence, which depend on the free local volume

available and on interactions between chromophores

AOMat 2016 - Special Session on Advanced Optical Materials

368

Figure 2: a) Absorption spectra of PAH/PAZO LbL films

with distinct numbers of bilayers. b) Maximum absorbance

(363 nm) versus the number of bilayers in the PAH/PAZO

LbL film.

Figure 3: Birefringence creation and relaxation kinetics

curve at room temperature obtained in (PAH/PAZO)

LbL

films with polarized visible light.

and PAH. The slower process is attributed to the main

chain mobility, which relies on chain size and T

interactions between both polyelectrolytes. However,

in the present case, the birefringence

creation,



, is very slow and the experimental

data can be fitted only with a exponential function:





=



1−−









(1)

where 



is the pre-exponential factor that represent

the magnitude of the process and 



is the

characteristic time constant. In the present case, the

the characteristic time takes a value of 5180±20 s a

long time compared with the value of 1700±36 s

obtained in PAH/PAZO films prepared onto glass and

using as writing beam a 514 nm beamline of a

tuneable Ar+ laser (Monteiro-Timóteo, 2016). This

means that the incidence of light increase the

temperature of the sample hindering the creation of

birefringence. It is also observed that the induced

birefringence is proportional to the amount of azo-

groups present in the sample, i.e., of the number of

PAZO layers.

The obtained decay curve can be fitted with two

exponential Debye like processes, being the one with

short characteristic associated with dipole

disorientation and the one with longer characteristic

time the long-term relaxation related to disorientation

arising from the movement of polymer chains, as

follows:





=



−









+



−









(2)

where 



and 



are the pre-exponential factors for

the birefringence normalized intensity, 



and





are the characteristic time constants of the

processes. For the earliest moments of relaxation,

process the obtained characteristic time is a value of

1970±30 s while the second characteristic time is of

(0.63±0.03)x10

s. These values are in accordance

with the birefringence decay characteristic times of

PAZO (Ferreira, 2012). However, it should be

noticed that the second value correspond to a

characteristic time of 7.3 days. This value promises

the use of these materials as solar cells, however the

conversion of orientated dipoles energy in power

should be studied.

4 CONCLUSIONS

This work demonstrate that birefringence can be

induced in PAH/PAZO LbL films by irradiation with

visible light with a spectrum similar to solar light with

a characteristic time of 1.4 hours. Although the

writing birefringence characteristic time to be quite

slower that the obtained with a laser beam, the

300 400 500 600 700 800

0.0

0.5

1.0

1.5

10 bilayers

15 bilayers

20 bilayers

25 bilayers

30 bilayers

Absorbance

Wavelength (nm)

0 5 10 15 20 25 30

0.0

0.5

1.0

1.5

Absorbance at 363 nm

Number of Bilayers

0 5000 10000 15000 20000

0.00

0.05

0.10

0.15

Birefringence Signal (a.u.)

Time (s)

Birefringence Creation by Solar Light - A New Approach to the Development of Solar Cells with Azobenzene Materials

369

relaxation characteristic time is of the order of 7 days.

Moreover, it is also observed that the induced

birefringence is proportional to the amount of azo-

groups present in the sample. These achievements

guarantees that solar devices based in that principle

can be implemented and characterized.

ACKNOWLEDGEMENTS

This work was supported by the Portuguese research

Grant UID/FIS/00068/2013 through FCT-MEC, the

"Plurianual" financial contribution of "Fundação para

a Ciência e Tecnologia" (Portugal) and by the project

POCTI/FAT/47529/2002 (Portugal).

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