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

Analysis of selected factors reducing the efficiency
of silicon solar cells
Agnieszka Stańco, AGH University of Science and Technology, Department of Electronics, Kraków
Nowadays the dominated method of receiving energy from natural
sources is photovoltaic (PV). Electric power generation from
sun is realized by solar cells closed in modules. Presently used
materials for the cells include mono – crystalline or multi – crystalline
silicon, amorphous silicon and other compounds based
on exemplary cadmium telluride, copper indium gallium selenide
or even polymers and organic combinations. The main factor describing
PV cells is its efficiency, which is determined by electrical
parameters. The possibility of long term using solar systems
requires to fulfill other conditions which have to limit the impact
of technology, temperature and lapse of time on correct working
of cells [1]. The producers state decreasing 90% of power solar
systems in about 12 years and 80% in 20 years [2]. The author of
this work call attention that it is very important for cell to be laminated
after manufacturing processes in order to secure them from
air and humidity. In addition, ambient conditions have also great
influence for working of solar cells. It is reported that increasing
temperature in conjunction with solar irradiation also decreases
efficiency of PV systems [3].
Experimental
The eight year-old mono and multi-crystalline silicon solar cells
(Cz-Si and mc-Si) of area 100 cm 2 were investigated. Morphology
of cells surface was studied by using of Scanning Microscopy
(NOVA NANO SEM 200, FEI Company). The measurements of
reflectance R(λ) were carried out by two beam spectrophotometer
UV-VIS-NIR Lambda 19 Perkin Elmer with integration sphere.
Electrical parameters such as: short circuit current I sc
, open
circuit voltage U oc
, fill factor FF and efficiency η were measured
by computer controlled global spectrum sun simulator I-V Curve
Tracer For Solar Cells Qualification. The simulation of temperature
impact on efficiency were realized by PC1D Version 5.4
application.
Results and Discussion
SEM studies reveal map of elements presence for surface mc-Si
with tagged electrical contact, where it is shown amplified amount
of silver (Ag) – Fig. 1.
We can see the presence of oxygen and titanium on the whole
surface of solar cell. It is normally because the silicon during
manufacturing process was coated by antireflective (ARC) layer
(TiO 2
). On the other hand the presence of oxygen on electrical
Fig. 2. SEM image of Ag contact structure
contacts is not desirable. The structure of contact is porous, which
favors the oxidizing process – Fig. 2. Finally, it leads to significant
decrease of current parameters and then efficiency.
The optical investigation were carried out on multicristalline
(ARC – a-Si:N:H) and moncrystalline (ARC – TiO 2
) cells with thickness
300 µm (eight – year old) and 210 µm (three – year old) and
confronted. The measurements of reflectance R(λ) allowed to calculate
effective reflectance R(λ) eff,
where N ph
(λ) constitute stream of
solar spectrum fotons AM1,5 in range of length of light (λ) [4].
The range of integration (400…1100 nm) results from having the
greatest solar cell photosensitive, and then quantum efficiency.
The parameter R(λ) eff
indicates to what degree a part of short
circuit current is lost as a result of light reflectance. The best value
of effective reflectance (5,1%) has Cz-Si with thickness 210 µm
– Fig. 4. It results from manufacturing process of mono-crystalline
solar cells, where first they are texturized and then coated with
antireflective layer. Texturized surface allows for better radiation
absorption in infrared.
80
Fig. 1. SEM image of mc-Si solar cell with
electrical contact – a) and map of elements
distribution
Fig. 3. Spectral reflectance dependence for silicon solar cells with
thickness 300 µm
Elektronika 6/2012