Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

ature were simulated for solar cell with standard area (100 cm 2 )
and thickness – 210 µm – Fig. 6.
The mechanism of current flow in internal structure of cell is
linked with temperature. Increase of temperature leads to linear
fall of open circuit voltage and almost linear growth of short circuit
current. Heating reduces the size of energy gap. The increase in
the short-circuit current is caused by rising the absorbed photon
current [5]. The heating has a destructive effect on the opencircuit
voltage. Finally, the efficiency of solar cell is reduced
accordingly. It is caused by reducing of conductivity. The rise of
temperature is also linked with high irradiation in areas with poor
ventilation. It is very desirable to employ a good air circulation
among PV systems.
Summary
The work referred to research on the factors which may contribute
to decrease the efficiency of solar cells. Progressive degradation
process for multi- and mono -crystalline solar cells was observed.
Optical measurements showed that effective reflectance is worse
for mc-Si than Cz-Si cells although they were coated antireflective
layer. It is caused generally by their heterogeneous structure. The
process of texturization in case of Cz-Si improves their optical parameters.
On the other hand it contributes to faster oxidation their
surfaces. The process of obsolescence was finally corroborated by
electrical measurements. The simulation of impact of heating showed,
that its high values cause decrease of solar cells efficiency.
References
[1] Swatowska B., A. Stańco, P. Panek: The causes of silicon solar cells
degradation. 35th International Microelectronics and Packaging IMAPS
– IEEE CPMT 21–24 September, Gdańsk–Sobieszewo, Poland, 2011.
[2] http://www.globalmarket.com/
[3] Swatowska B., T. Stapiński, J. Chojnacki: Parameters of silicon solar
cells in changeable external conditions. 23 International Conference of
IMAPS – CPMT IEEE 21–24 September, Pszczyna, Poland, 2009.
[4] Strehlke S., S. Bastide, J. Guillet, C. Lévy – Clément: Design of porous
silicon antireflection coatings for silicon solar cells. Materials
Science and Engineering B69 – 70 (2000) 81–86.
[5] Würfel P.: Physics of Solar Cells – From Principles to New Concepts.
WILEY – VCH Verlag GmbH & Co. KGaA, Weinheim, Germany 2005.
Low-ohmic contacts on the basis of silver nanopowder
dedicated to photovoltaic cells
Joanna Kalbarczyk, Marian Teodorczyk, Konrad Krzyżak, Grzegorz Gawlik,
Mateusz Jarosz, Anna Młożniak
Institute of Electronic Materials Technology, Warszawa
Obtaining satisfactory parameters of solar cell is largely dependent
on mastering the technique of producing contacts metal
– semiconductor. The m-s contact can be strictly mechanical
connection (e.g. trough contact electrode with semiconductor) as
well as inseparable, integral connection. The m-s contacts may
be regarded as ideal if they generate no additional resistance for
the current, do not react chemically with semiconductor material,
do not change characterictics while changing illumination,
temperature or value of applied electric filed and do not exhibit
rectifying effects. Such contact can be described with Ohm’s law
– its current-voltage characteristic is linear. It is obvious that ideal
contacts do not exist, but the aim of the technology is to approach
as close as possible to the imaginary ideal contact.
To eliminate serious mistakes while creating ohmic contacts it
is necessary to know the type of conduction. The material chosen
for contacts should create impurity centers that comply with semiconductor’s
type of conduction – it should contain donor impurities
in case of contact with n – type semiconductor and acceptor
impurities in case of p-type semiconductor.
Except specified electrical properties there are many other
physicochemical properties (such as good thermal conductivity,
compatibility of metal and semiconductor coefficient of heat expansion,
proper mechanical strength in different temperatures)
that should characterize a useful m-s contact [1].
In the following thesis, two methods of creating contacts will
be presented.
The first method is the cathode sputtering. The silicon base is
subjected to high vacuum ion bombardment in inert atmosphere,
then silver layer is deposited. The method provides obtaining layer
of high degree of purity.
The second method of making contacts is applying the paste
with silver nanopowder. The method is quite new and is an attempt
of confrontation already well-known and bringing satisfactory
effects in silver contact preparation procedure (sputtering)
and a new method which uses up-to-date nanomaterial that may
exhibit interesting properties for the technique of solar cells contact
preparation [2, 4].
82
Tabl. 1. Properties of nanosilver layers in Si substrates [4]
Sample
Polished side,
firing in 300 0 C
Mat side, firing
in 300 0 C
Polished side,
firing in 600 0 C
Mat side, firing
in 600 0 C
Resistance/□
[mΩ/□]
Layer
thickness [µm]
Resistivity
[Ω∙m×10 -8 ]
Up to now silver pastes used in screen printing technique have
been composed of silver powder (conductive phase), where grain
size varied between 1 and 3 µm, and glaze (auxiliary phase)
which has helped in the sintering process. The characteristic temperature
of sintering silver layers is 850 0 C. If the composition of
glaze has been appropriate – the temperature of sintering might
have been reduced to about 650 0 C. The temperature of firing
commercially used pastes (e.g. Du Pont) achieves about 750 0 C,
which is temperature of silicon wafer when temperature setting is
920 0 C [3–5].
Silver nanopowder paste represents a new generation of
thick-layer materials dedicated to screen printing. The conductive
phase is silver powder where grains size range from a few to over
a dozen nanometers. Methyl polimethacrylate has been used as
the matrix of nanocomposite [4, 5].
Nanomaterials often manifest new specific properties – physicochemical,
electrical, magnetic or catalytic. These new characteristics
arise from the quantum effects and high share of surface
atoms causing high reactivity. In consequence melting point
of nanoparticles can be much lower than microcrystalline solid.
Many other new phenomena may also occur in nanoscale [2-4].
Preliminary research in the field of silver nanopowder paste
contact preparation for silicon solar cells has brought to wellconducting
layer forming through firing in much lower tempera-
Adhesion
12,2 2 2,44 +
12,0 2 2,40 +
14,35 2 2,87 +
14,7 2 2,94 +
Elektronika 6/2012