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Critical Factors that Affecting Efficiency of Solar Cells

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A solar cell or photovoltaic cell is a device which generates electricity directly from visible light. However, their efficiency is fairly low. So, the solar cell costs expensive according to other energy resources products. Several factors affect solar cell efficiency. This paper presents the most important factors that affecting efficiency of solar cells. These effects are cell temperature, MPPT (maximum power point tracking) and energy conversion efficiency. The changing of these factors improves solar cell efficiency for more reliable applications.
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Smart Grid and Renewable Energy, 2010, 1, 47-50
doi:10.4236/sgre.2010.11007 Published Online May 2010 (http://www.SciRP.org/journal/sgre)
Copyright © 2010 SciRes. SGRE
Critical Factors that Affecting Efficiency of Solar
Cells
Furkan Dinçer, Mehmet Emin Meral
University of Yuzuncu Yil, Department of Electrical and Electronics Engineering, Van, Turkey.
Email: furkandincer@yyu.edu.tr, emeral@yyu.edu.tr
Received February 17
th
, 2010; revised May 7
th
, 2010; accepted May 8
th
, 2010.
ABSTRACT
A solar cell or photovoltaic cell is a device which generates electricity directly from visible light. However, their
efficiency is fairly low. So, the solar cell costs expensive according to other energy resources products. Several factors
affect solar cell efficiency. This paper presents the most important factors that affecting efficiency of solar cells. These
effects are cell temperature, MPPT (maximum power point tracking) and energy conversion efficiency. The changing of
these factors improves solar cell efficiency for more reliable applications.
Keywords: Solar Cell, Efficiency, Cell Factor, Cell Temperature
1. Introduction
Solar cells have seen remarkable improvements since the
first issue of the journal Solar Energy Materials in 1979.
The photovoltaic (PV) field has given rise to a global
industry capable of producing many gigawatts (GW) of
additional installed capacity per year [1].
The problems with energy supply and use are related
not only to global warming but also to such environmental
concerns as air pollution, acid precipitation, ozone deple-
tion, forest destruction, and radioactive substance emis-
sions. To prevent these effects, some potential solutions
have evolved including energy conservation through im-
proved energy efficiency, a reduction in fossil fuel use and
an increase in environmentally friendly energy supplies.
Among them, the power generation with solar cells sys-
tem has received great attention in research because it
appears to be one of the possible solutions to the envi-
ronmental problem [2].
Solar Energy is energy that comes from the sun. The
energy uses by solar cells that convert sunlight into direct
current electricity. Solar cells are composed of various
semi conducting materials. Semiconductors are materials,
which become electrically conductive when supplied with
light or heat, but which operate as insulators at low tem-
peratures.
When photons of light fall on the cell, they transfer their
energy to the charge carriers. The electric field across the
junction separates photo-generated positive charge carri-
ers (holes) from their negative counterpart (electrons). In
this way an electrical current is extracted once the circuit
is closed on an external load.
Several factors affect solar cell efficiency. This paper
examines the factors that affecting efficiency of solar cells
according to scientific literature. These factors are
changing of cell temperature, using the MPPT with solar
cell and energy conversion efficiency for solar cell.
2. Characterization of Solar Cells
It is quite generally defined as the emergence of an elec-
tric voltage between two electrodes attached to a solid or
liquid system upon shining light onto this system. Prac-
tically all photovoltaic devices incorporate a p-n junction
in a semiconductor across which the photovoltage is de-
veloped. These devices are also known as solar cells. A
cross-section through a typical solar cell is shown in Figure
1. The semiconductor material has to be able to absorb a
large part of the solar spectrum. Dependent on the absorp-
tion properties of the material the light is absorbed in a
region more or less close to the surface. When light quanta
are absorbed, electron hole pairs are generated and if their
recombination is prevented they can reach the junction
where they are separated by an electric field [3].
The photoelectric effect was first noted by a French
physicist, Edmund Bequerel, in 1839, who found that
certain materials would produce small amounts of electric
current when exposed to light [4,5]. The theory of the
solar cell is the solar effect of semiconductor material.
The solar effect is a phenomenon that the semiconductor
material absorbs the solar energy, and then the electron-hole
excitated by the photon separates and produces electro-
48 Critical Factors that Affecting Efficiency of Solar Cells
Copyright © 2010 SciRes. SGRE
Figure 1. A schematic of the layers of a typical PV cell [4]
Figure 2. The equivalent circuit of the solar cell [6]
motive force. The I-V characteristic of the solar cell
changes with the sunshine intensity
()
2
SW m and cell
temperature t (ºC), that is I = f (V, S, t). According to the
theory of electronics, when the load is pure resistance, the
actual equivalent circuit of the solar cell is as Figure 2 [6].
L
I
is current supplied by solar cell.
SS
L0
SH
q(V + IR ) V + IR
I = I - I exp - 1 -
AkT R






(1)
where [6],
d
I
, the junction current of the diode
1
S
d0
q(V + IR )
I
= I exp -
AkT






,
I, the load current
L
I
, the photovoltaic current,
0
I
, the reverse saturation current
q, electronic charge,
k, boltzmann constant,
T, absolute temperature, A, factor of the diode quality
S
R , series resistance,
SH
R , parallel resistance
Another important parameter is open circuit voltage
OC
V ;
ln( 1) ln( )
L
L
OC
00
I
I
kT kT
V
qI qI
 (2)
Figure 3. Typical I-V characteristic of a crystalline silicon
module with the variation of power [7]
Figure 3 shows an I-V characteristic together with the
power curve, to illustrate the position of the maximum
power point [7].
3. Solar Cells Efficiency Factors
3.1 Cell Temperature
As temperature increases, the band gap of the intrinsic
semiconductor shrinks, and the open circuit voltage (
OC
V )
decreases following the p-n junction voltage temperature
dependency of seen in the diode factor
q
kT
. Solar cells
therefore have a negative temperature coefficient of
OC
V
(β). Moreover, a lower output power results given the
same photocurrent because the charge carriers are liber-
ated at a lower potential. Using the convention introduced
with the Fill Factor calculation, a reduction in
OC
V results
in a smaller theoretical maximum power
max
P
SC
I
OC
V given the same short-circuit current
SC
I
[8].
As temperature increases, again the band gap of the
intrinsic semiconductor shrinks meaning more incident
energy is absorbed because a greater percentage of the
incident light has enough energy to raise charge carriers
from the valence band to the conduction band. A larger
photocurrent results; therefore, Isc increases for a given
insolation, and solar cells have a positive temperature
coefficient of
SC
I
(α) [8].
Figure 4 shows the I-V and P-V characteristics at the
constant illumination when the temperature changes [9].
Temperature effects are the result of an inherent charac-
teristic of crystalline silicon cell-based modules. They
tend to produce higher voltage as the temperature drops
and, conversely, to lose voltage in high temperatures. Any
solar panel or system derating calculation must include
adjustment for this temperature effect [10].
3.2 Energy Conversion Efficiency
A solar cell’s energy conversion efficiency (η, “eta”), is
Critical Factors that Affecting Efficiency of Solar Cells 49
Copyright © 2010 SciRes. SGRE
Figure 4. I-V and P-V characteristics of solar cell module [9]
the percentage of power converted (from absorbed light to
electrical energy) and collected, when a solar cell is con-
nected to an electrical circuit. This term is calculated
using the ratio of the maximum power point, P
m
, divided
by the input light irradiance (E, in W/m
2
) under standard
test conditions and the surface area of the solar cell (A
c
in
m
2
) [11].
m
c
P
η =
ExA
(3)
The efficiency of energy conversion is still low, thus
requiring large areas for sufficient insulation and raising
concern about unfavorable ratios of energies required for
cell production versus energy collected [12]. In order to
increase the energy conversion efficiency of the solar cell
by reducing the reflection of incident light, two methods
are widely used. One is reduction of the reflection of
incident light with an antireflection coating, and the other
is optical confinements of incident light with textured
surfaces. They showed that the transformation of the
wavelength of light could significantly enhance the spec-
tral sensitivity of a silicon photodiode from the deep UV
and through most of the visible region. [13].
The solar module has a different spectral response de-
pending on the kind of the module. Therefore, the change
of the spectral irradiance influences the solar power gen-
eration [14]. The solar spectrum can be approximated by a
black body of 5900 K which results in a very broad spec-
trum ranging from the ultraviolet to the near infrared. A
semiconductor, on the other hand can only convert pho-
tons with the energy of the band gap with good efficiency.
Photons with lower energy are not absorbed and those
with higher energy are reduced to gap energy by ther-
malization of the photo generated carriers. Therefore, the
curve of efficiency versus band gap goes through a
maximum as seen from Figure 5 [3].
3.3 Maximum Power Point Tracking
Currently, the electricity transformation efficiency of
the solar cells is very low that reach about 14%. The ef-
ficiency of solar cells should be improved with various
methods. One of them is maximum power point tracking
Figure 5. Dependency of the conversion efficiency on the
semiconductor band gap [3]
(MPPT) which is an important method. The MPPT oper-
ates with DC to DC high efficiency converter that presents
an optimal and suitable output power.
The resulting I-V characteristic is shown in Figure 5.
The photo generated current
L
I
is equal to the current
produced by the cell at short circuit (
0V ). The open
circuit Voltage
OC
V (when 0I ) can easily be obtained
as [15].
No power is generated under short or open circuit. The
maximum power P produced by the conversion device is
reached at a point on the characteristic. This is shown
graphically in Figure 6 where the position of the maxi-
mum power point represents the largest area of the rec-
tangle shown. One usually defines the fill-factor ff by
[15].
max m m
OC OC l
PVI
ff = =
VVI
(4)
where,
m
V and
m
I
are the voltage and current at the
maximum power point.
When the output voltage of the photovoltaic cell array
is very low, the output current changes little as the volt-
agechanges, so the photovoltaic cell array is similar to the
constant current source; when the Voltage is over a critical
value and keeps rising, the current will fall sharply, now
the photovoltaic cell array is similar to the constant volt-
Figure 6. The I-V characteristic of an ideal solar cell [15]
50 Critical Factors that Affecting Efficiency of Solar Cells
Copyright © 2010 SciRes. SGRE
age source. As the output voltage keeps rising, the output
power has a maximum power point. The function of the
maximum power tracker is to change the equivalent load
take by the photovoltaic cell array, and adjust the working
point of the photovoltaic cell array, in order that the
photovoltaic cell array can work on the maximum power
point when the temperature and radiant intensity are both
changing [6].
4. Conclusions
This paper examine factors that affecting efficiency of
solar cells. These are changing of cell temperature, using
the MPPT with solar cell and energy conversion effi-
ciency for solar cell.
Temperature effects are the result of an inherent char-
acteristic of solar cells. They tend to produce higher
voltage as the temperature drops and, conversely, to lose
voltage in high temperatures. The energy conversion
efficiency is increased by reducing the reflection of inci-
dent light. The function of the maximum power tracker is
to change the equivalent load take by the solar cell array,
and adjust the working point of the array, in order to im-
prove the efficiency.
Changing of these factors is very critical for solar cell
efficiency. The optimum factors make it possible to get
the great benefits of solar electricity at a much lower cost.
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This paper reviews the history, the present status and possible future developments of photovoltaic (PV) materials for terrestrial applications. After a brief history and introduction of the photovoltaic effect theoretical requirements for the optimal performance of materials for pn-junction solar cells are discussed. Most important are efficiency, long-term stability and, not to be neglected, lowest possible cost. Today the market is dominated by crystalline silicon in its multicrystalline and monocrystalline form. The physical and technical limitations of this material are discussed. Although crystalline silicon is not the optimal material from a solid state physics point of view it dominates the market and will continue to do this for the next 5–10 years. Because of its importance a considerable part of this review deals with materials aspects of crystalline silicon. For reasons of cost only multicrystalline silicon and monocrystalline Czochralski (Cz) crystals are used in practical cells. Light induced instability in this Cz-material has recently been investigated and ways to eliminate this effect have been devised. For future large scale production of crystalline silicon solar cells development of a special solar grade silicon appears necessary. Ribbon growth is a possibility to avoid the costly sawing process. A very vivid R&D area is thin-film crystalline silicon (about 5–30 μm active layer thickness) which would avoid the crystal growing and sawing processes. The problems arising for this material are: assuring adequate light absorption, assuring good crystal quality and purity of the films, and finding a substrate that fulfills all requirements. Three approaches have emerged: high-temperature, low-temperature and transfer technique. Genuine thin-film materials are characterized by a direct band structure which gives them very high light absorption. Therefore, these materials have a thickness of only one micron or less. The oldest such material is amorphous silicon which is the second most important material today. It is mainly used in consumer products but is on the verge to also penetrate the power market. Other strong contenders are chalcogenides like copper indium diselenide (CIS) and cadmium telluride. The interest has expanded from CuInSe2, to CuGaSe2, CuInS2 and their multinary alloys Cu(In,Ga)(S,Se)2. The two deposition techniques are either separate deposition of the components followed by annealing on one hand or coevaporation. Laboratory efficiencies for small area devices are approaching 19% and large area modules have reached 12%. Pilot production of CIS-modules has started in the US and Germany. Cadmium telluride solar cells also offer great promise. They have only slightly lower efficiency and are also at the start of production. In the future other materials and concepts can be expected to come into play. Some of these are: dye sensitized cells, organic solar cells and various concentrating systems including III/V-tandem cells. Theoretical materials that have not yet been realized are Auger generation material and intermediate metallic band material.
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A coating of fluorescent coloring agent (FCA) on the solar cells gives 30% increase in the energy conversion efficiency of the solar cell. This increase is attributable to the reduction of the reflection of incident light. The reflectances show low values at the excitation wavelengths, where the incident light is absorbed to excite the FCA. The fluorescence quantum yield for a dried FCA was much larger than that for FCAs dissolved in paint thinner.
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This study aims at developing an evaluation framework of strategic information systems (SIS) and evaluating the SIS planning and implementation by using a cognitive approach called the repertory grid technique. The findings are based on in-depth interviews ...
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Modern photovoltaics uses semiconductor solar cells with increasingly sophisticated structures. Silicon remains the most important material. This talk reviews some of the basic principles, especially concerning estimates of theoretical efficiency limits, improved photon utilization, contact optimization, and opportunities and technical problems arising for thin-film solar cells
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This study develops a high-performance stand-alone photovoltaic (PV) generation system. To make the PV generation system more flexible and expandable, the backstage power circuit is composed of a high step-up converter and a pulsewidth-modulation (PWM) inverter. In the dc-dc power conversion, the high step-up converter is introduced to improve the conversion efficiency in conventional boost converters to allow the parallel operation of low-voltage PV arrays, and to decouple and simplify the control design of the PWM inverter. Moreover, an adaptive total sliding-mode control system is designed for the voltage control of the PWM inverter to maintain a sinusoidal output voltage with lower total harmonic distortion and less variation under various output loads. In addition, an active sun tracking scheme without any light sensors is investigated to make the PV modules face the sun directly for capturing the maximum irradiation and promoting system efficiency. Experimental results are given to verify the validity and reliability of the high step-up converter, the PWM inverter control, and the active sun tracker for the high-performance stand-alone PV generation system.
Driving Performances of Solar Energy Powered Vehicle with MPTC
  • Y Suita
  • S Tadakuma
Y. Suita and S. Tadakuma, "Driving Performances of Solar Energy Powered Vehicle with MPTC," IEEE, 2006.