Chapter Two Semiconductor device.pdf detail on Solar cells

About solar cells

1. CHAPTER TWO
Semiconductor for photovoltaic

2. P-N junction for solar cell
Recommended video for solar cell working
B. Rimal 2

3. PV systems are like any other electrical power generating systems, just the equipment used is
different than that used for conventional electromechanical generating systems.
B. Rimal 3

4. •Most Commonly used material for
construction of solar PV are shown.
Silicon, which is in group IV, is the most
commonly used semiconductor material. It
forms the basis for integrated circuit(IC)
chips and is the most mature technology.
Silicon based solar cells/ panels are most
widely used.
Semiconductor can either be of single
element, such as Si or Ge ,a compound,
such as GaAs, In P or CdTe, or an alloy,
such as SixGe(1-x) or AlxGa(1-x). As,
where x is the fraction of the particular
element and ranges from 0 to1.
B. Rimal 4

5. Si
•A typical silicon PV cell is composed
of a thin wafer consisting of an ultra-
thin layer of phosphorus-doped (N-
type) silicon on top of a thicker layer
of boron-doped (P-type) silicon.
•Silicon is abundant on the earth's
surface and therefore cheaper than
germanium. The PIV (Peak Inverse
Voltage) rating of silicon is much
higher than germanium and therefore
can withstand much higher
temperatures than germanium.

Fig: Electronic configuration of silicon
Fig: Structure of Si
B. Rimal 5

6. Valance band and Conduction band
Semiconductors are crystalline materials whose outer shell atomic levels
exhibit an energy band structure with an energy gap between valence and
conduction bands of the order of 1eV .
Electrons are
conducting, when
they are in
conduction band.
Holes are
conducting, when
they are in Valance
band

a conductor a semiconductor an insulator
B. Rimal 6

7. Si
Silicon is group IV element – with 4
electrons in their valence shell.
 When silicon atoms are brought together,
each atom forms covalent bond with 4
silicon atoms in a tetrahedron geometry
Adding impurities on Si means, adding P
type or N type material on Si.
Also known as extrinsic semiconductor.
-
Si Si Si
Si
Si Si
Si
Si
Si
Shared electrons
B. Rimal 7

8. Extrinsic Semiconductor, n-type Doping
Electron
-
Si Si Si
Si
Si Si
Si
Si
As
Extra
Valence band, E
v
E
g = 1.1 eV
Conducting band, E
c
E
d ~ 0.05 eV
 Doping silicon lattice with group V elements creates extra electrons in
the conduction band — negative charge carriers (n-type), As- donor.

 Doping concentration #/cm
3
(10
16
/cm
3
~ 1/million).
B. Rimal 8

9. Valence band, E
v
E
g = 1.1 eV
Conducting band, E
c
E
a ~ 0.05 eV
Electron
-
Si Si Si
Si
Si Si
Si
Si
B
Hole
 Doping silicon with group III elements creates empty holes in the
conduction band — positive charge carriers (p-type), B-(acceptor).
Extrinsic Semiconductor, p-type doping
B. Rimal 9

10. P-N junction
•To form a PN junction, a single semiconducting
crystal, are doped with impurities on both side.
•In figure left side is doped with acceptors
impurities or holes (P type) and right side is doped
with donor impurities or charge carriers (N type).
•A depletion layer is formed as shown. Depletion
layer act as a barrier to prevent recombination of
electron and hole. Hence there are no charge carrier
on depletion layer.
•Hence, In depletion layer there is no electron in
conduction band and holes in valance band.




B. Rimal 10

11. •The band gap is the minimum amount of
energy required for an electron to break
free of its bound state (valance band).
•When the band gap energy is met, the
electron is excited into a free
state(conduction band), and can therefore
participate in conduction.
•The band gap determines how much
energy is needed from the sun for
conduction, as well as how much energy is
generated.
•A hole is created where the electron was
formerly bound. This hole also participates
in conduction.

B. Rimal 11

12. •When photon falls on it and has sufficient energy
more than the band gap then some of the electron
on valance band absorbs the energy and use that
energy to jump at conduction band.
•When it happens in depletion layer, that electron
gets attracted to positive charge and holes to
negative charge (repelled by positive charge) as
shown.
•Hence these charge carrier are swept across due to
the field across depletion layer. By this process, a
lot of electron and holes get accumulated at the
terminal as shown. Then P type becomes positively
charged and N type terminal become negatively
charged.
•This effect is known as photovoltaic effect.

B. Rimal 12

13. •When load is connected across that
terminal, the electrons will flow
continuously and gets recombined
with holes. Due to photon, the electron
and holes are continuously formed and
moves through load causing a constant
and continuous current as shown.

•Hence, When light shines on a solar
cell, photovoltage is generated. The
generated voltage across the solar cell
drives the current in an external circuit
and, therefore, can deliver power.




B. Rimal 13

14. •Thus, Photovoltaic ( PV ) cells are made of almost entirely from
semiconductor silicon that has been processed into an extremely pure
crystalline material which absorbs the photons from sunlight.
•The photons hit the silicon atoms releasing electrons causing an electric
current to flow when the photoconductive cell is connected to an external
load.
•There are a variety of different measurements we can make to determine the
solar cell’s performance, such as its power output and its conversion
efficiency.
•The main electrical characteristics of a PV cell or module are summarized in
the relationship between the current and voltage produced on a typical solar
cell I-V characteristics curve.
•The intensity of the solar radiation (insolation) that hits the cell controls the
current ( I ), while the increases in the temperature of the solar cell reduces
its voltage ( V ).

B. Rimal 14

15. P-N junction diode revision
B. Rimal 15
Shockley Diode equation:
I
s is the reverse saturation current
……Ideal diode equation

16. Equivalent Circuit of Ideal solar cell
B. Rimal 16
A solar cell with out any losses (no resistance)
It can be represented as a current source (Iph i.e
photon generated current)in parallel with a diode.
I
ph is propertional with irradiance.
Current flowing out of solar cell can be represented as
I= I
ph –I
d
When there is no light present to generate any
current, the PV cell behaves like a diode. As the
intensity of incident light increases, current is
generated by the PV cell,

17. Equivalent circuit of practical solar cell
B. Rimal 17
In addition to ideal cell, practical solar cell has parasitic resistances.
Series resistances (Rse) are due to
resistance of contact, metal grid, and P-N
layer. (Small value)
Shunt resistance (Rsh) are due to leakage
current through P-N junction. (High value)
Current through PV cell I can be represented as
…..Equation of current
given by practical solar cell

18. Solar Cell I-V and P-V characteristics
The graph shows the current-
voltage ( I-V ) and Power-voltage
(P-V) characteristics of a typical
PV cell operating under normal
conditions.
Fig: I-V and P-V characteristics of solar cell
B. Rimal 18

19. With the solar cell open-circuited, that is not
connected to any load, the current will be at its
minimum (zero) and the voltage across the cell is
at its maximum, known as the solar cells open
circuit voltage, or Voc. At the other extreme,
when the solar cell is short circuited, that is the
positive and negative leads connected together,
the voltage across the cell is at its minimum
(zero) but the current flowing out of the cell
reaches its maximum, known as the solar
cells short circuit current, or Isc.
Then the span of the solar cell I-V characteristics curve ranges from the short circuit current
( Isc ) at zero output volts, to zero current at the full open circuit voltage ( Voc ). In other words,
the maximum voltage available from a cell is at open circuit, and the maximum current at
closed circuit. Of course, neither of these two conditions generates any electrical power, but
there must be a point somewhere in between were the solar cell generates maximum power.
B. Rimal 19

20. The power delivered by a single solar
cell or panel is the product of its output
current and voltage ( I*V) as shown.
There is one particular combination of
current and voltage for which the power
reaches its maximum value,
at Imp and Vmp. In other words, the
point at which the cell generates
maximum electrical power and this is
shown at the top right area of the green
rectangle. This is the ―maximum power
point‖ or MPP. Therefore the ideal
operation of a photovoltaic cell (or
panel) is defined to be at the maximum
power point.
B. Rimal 20

21. Some Parameters
V
OC = open-circuit voltage
Depends of temperature, material and very slightly with irradiance
It is logarithmically related to Isc.
I
SC = short-circuit current
Depends on solar irradiance level, area of solar cell and material
MPP = maximum power point = I
mp*V
mp


B. Rimal 21

22. B. Rimal 22
Hint For half sun, (Isc’)=Isc at full sun/2

23. Fill factor
The fill factor is the relationship between the
maximum power that the array can actually
provide under normal operating conditions and
the product of the open-circuit voltage multiplied
by the short-circuit current.

This fill factor value gives an idea of the quality of the array and the closer the fill factor is
to 1 (unity), the more power the array can provide. Typical values are between 0.7 and 0.8.
B. Rimal 23

24. Efficiency (η)
Efficiency is the ratio of the electrical power output P
out, compared to the solar power
input, P
in, into the PV cell. P
out can be taken to be P
MAX since the solar cell can be operated
up to its maximum power output to get the maximum efficiency.
The maximum efficiency (η
MAX) found from a light test is not only an indication of the
performance of the device under test, but, like all of the I-V parameters, can also be
affected by ambient conditions such as temperature and the intensity and spectrum of the
incident light. For this reason, it is recommended to test and compare PV cells using
similar lighting and temperature conditions.
The efficiency of a typical solar array is normally low at around 10-12%, depending on
the photovoltaic type (monocrystalline, polycrystalline, amorphous or thin film) of cell being
used.
B. Rimal 24
Here, Pin is taken as the product of the irradiance of
the incident light, measured in W/m
2
or in suns (1000
W/m
2
), with the surface area of the solar cell [m
2
].

25. 2/10/2022 25
Examples of PV Module Performance Data Under Standard Test Conditions
(1 kW/m
2
, AM 1.5, 25
◦
C cell Temperature)

26. Effect of Irradiance and Temperature
B. Rimal 26
Thus, Irradiance affect Current and Temperature affect voltage output of PV cell

27. Effect of Irradiance on I-V and P-V curve
B. Rimal 27
Where X is the
irradiance i.e 0.5 for
50% of full sun.
Voc= OC volatge at
full sun
Voc’=OC voltage at
X of full sun

28. Effect of temperature on I-V and P-V characteristics
B. Rimal 28
Like all other semiconductor devices, solar cells are sensitive to temperature.
Increasing the temperature reduces the band gap. In a solar cell, the parameter most
affected by an increase in temperature is the open-circuit voltage.

29. •The impact of increasing temperature are:
-Photovoltaics, perhaps surprisingly, therefore perform better on cold, clear
days than hot ones.
- For crystalline silicon cells, V
OC drops by about 0.37% for each degree
Celsius increase in temperature, I
SC increases by approximately 0.05%
and maximum power available decreases by 0.5%
-Given this significant shift in performance as cell temperature changes, it
should be quite apparent that temperature needs to be included in any
estimate of module performance.

B. Rimal 29

30. To help system designers account for changes in cell performance with
temperature, manufacturers often provide an indicator called the NOCT, which
stands for nominal operating cell temperature. The NOCT is cell
temperature in a module when ambient is 20
◦
C, solar irradiation is 0.8 kW/m
2
,
and wind speed is 1 m/s. To account for other ambient conditions, the
following expression is used:

B. Rimal 30
where T
cell is cell temperature (◦C), T
amb is
ambient temperature, and S is solar
insolation (kW/m
2
).

31. Estimate cell temperature, open-circuit voltage, and maximum power output
for the 150-W BP2150S module under conditions of 1-sun insolation and
ambient temperature 30°C. The module has a NOCT of 47◦C. Given that for
this module at the standard temperature of 25°C, V
OC = 42.8 V.
B. Rimal 31
Solution. Using S = 1 kW/m
2
, cell temperature is estimated to be;
From manufacturer data, for this module at the standard temperature of 25°C,
V
OC = 42.8 V. Since V
OC drops by 0.37%/
°
C, the new V
OC will be about;
With maximum power expected to drop about 0.5%/◦C, this 150-W module at
its maximum power point will deliver-

32. Thus
B. Rimal 32

33. Cell Module and Array
An individual cell produces only about 0.5 V, it
is a very small value for any application.


B. Rimal 33
The basic building block for PV
applications is a module consisting
of a number of pre-wired cells in
series, all encased in tough, weather-
resistant packages. A typical module
has 36 cells in series and is often
designated as a ―12-V module‖ even
though it is capable of delivering much
higher voltages than that.

34. Cell to Module
2/10/2022 TU,IOE, Pashchimancha Campus/BG 35
The packing density of solar cells in a PV module refers to the area of the module that is
covered with solar cells compared to that which is blank. The packing density affects the
output power of the module as well as its operating temperature The packing factor, β
c,
It is clear that β
c is less than unity. Its value is 1 only when the space is filled with the
rectangular cells.
Interconnection between cells, with wires,
introducing resistance
When photovoltaics are wired in series, they all
carry the same current, and at any given current
their voltages add as shown in the figure below.
We can use the general equation for solar cell to
find an overall module voltage V
module.

35. B. Rimal 36
Example Voltage and Current from a PV Module

A PV module is made up of 36 identical cells, all wired in series. With 1-sun insolation (1
kW/m
2
), each cell has short-circuit current I
SC =3.4 A and at 25
◦
C its reverse saturation
current is I
0 =6×10
−10
A. Parallel resistance R
P =6.6 Ω and series resistance R
S =0.005.
Find the voltage, current, and power delivered when the junction voltage of each cell is 0.50
V.

36. 2/10/2022 37
Solution
Using V
d =0.50 V along with the other data gives current:
So the voltage produced by the 36-cell
module,
Power delivered is,

37. Module to Array
•Multiple modules can be wired in series to increase voltage and in parallel to
increase current, the product of which is power. An important element in PV
system design is deciding how many modules should be connected in series
and how many in parallel to deliver whatever energy is needed. Such
combinations of modules are referred to as an array.
B. Rimal 38
N=8
V=??
I=??

38. Module in series
Modules can be wired in series to increase voltage, and in parallel to increase
current.
B. Rimal 39
Arrays are made up of some
combination of series and parallel
modules to increase power. For
modules in series, the I –V curves
are simply added along the voltage
axis. That is, at any given current
(which flows through each of the
modules), the total voltage is just
the sum of the individual module
voltages.

39. Module in Parallel
•For modules in parallel, the
same voltage is across each
module and the total current is
the sum of the currents. That is,
at any given voltage, the I –V
curve of the parallel
combination is just the sum of
the individual module currents
at that voltage.

B. Rimal 40

40. Series and Parallel Interconnection of Module
•When high power is needed, the array
will usually consist of a combination of
series and parallel modules for which
the total I –V curve is the sum of the
individual module I –V curves.

•There are two ways of wiring a
series/parallel combination of modules:
The series modules may be wired as
strings, and the strings wired in parallel
as in Fig. a, or the parallel modules
may be wired together first and those
units combined in series as in b.
B. Rimal 41

41. Effect of Shading
•Shading is the covering of the cells/module by
some object blocking the insolation.

•The output of a PV module can be reduced
dramatically when even a small portion of it is
shaded. Unless special efforts are made to
compensate for shade problems, even a single
shaded cell in a long string of cells can easily cut
output power by more than half.


B. Rimal 42
•External diodes, purposely added by the PV manufacturer or by the system designer,
can help preserve the performance of PV modules. The main purpose for such diodes
is to mitigate the impacts of shading on P-V I –V curves. Such diodes are usually
added in parallel with modules or blocks of cells within a module.

42. Lets consider an n-cell module with current I and output voltage V, and one
cell separated from the others (shown as the top cell, though it can be any cell
in the string).
The equivalent circuit of the top cell has been drawn using while the other
(n−1) cells in the string are shown as just a module with current I and output
voltage V
n−1.

B. Rimal 43
In Fig. a, all of the cells are in the sun
and since they are in series, the same
current I flows through each of them. In
Fig. b, however, the top cell is shaded
and its current source I
SC has been
reduced to zero. The voltage drop across
R
P as current flows through it causes the
diode to be reverse biased, so the diode
current is also (essentially) zero.

43. That means the entire current flowing through the module must
travel through both R
P and R
S in the shaded cell on its way to the
load. That means the top cell, instead of adding to the output
voltage, actually reduces it.
Consider the case when the bottom n−1 cells still have full sun
and still some how carry their original current I so they will still
produce their original voltage V
n−1. This means that the output
voltage of the entire module V
SH with one cell shaded will drop to



B. Rimal 44

With all n cells in the sun and carrying I, the output voltage was V so the voltage of the
bottom n−1 cells will be
Combining these two equations gives

44. B. Rimal 45
The drop in voltage ΔV at any given current I, caused by the shaded cell, is given by






Since the parallel resistance R
P is so much greater than the series resistance R
S,

At any given current, the I –V
curve for the module with one
shaded cell drops by ΔV. The
huge impact this can have is
illustrated in the figure.
Effect of shading one cell in an n-cell module
(at any given current, module voltage drops from V to V − ΔV)

45. B. Rimal 46
Example Impacts of Shading on a PV Module:
The 36-cell PV module has a parallel resistance per cell of R
p=6.6 Ω . In full sun and at
current I =2.14 A the output voltage was found there to be V = 19.41 V. If one cell is
shaded and this current somehow stays the same, then:
a. What would be the new module output voltage and power?
b. What would be the voltage drop across the shaded cell?
c. How much power would be dissipated in the shaded cell?

Solution:
a) The drop in module voltage is,



The new output voltage will be 19.41−14.66=4.75 V.
Power delivered by the module with one cell



For comparison, in full sun the module was producing 41.5 W.

46. B. Rimal 47
b. All of that 2.14 A of current goes through the parallel plus series resistance (0.005
Ω) of the shaded cell, so the drop across the shaded cell will be


(normally a cell in the sun will add about 0.5 V to the module; this shaded cell
subtracts over 14 V from the module).

c. The power dissipated in the shaded cell is voltage drop times current,


All of that power dissipated in the shaded cell is converted to heat, which can cause
a local hot spot that may permanently damage the plastic laminates enclosing the
cell.

47. B. Rimal 48
These results obtained for shading of one cell can be extended to develop I –V curves under
various conditions of shading. Figure shows curves for the example module one cell 50%
shaded, one cell completely shaded, and two cells completely shaded. The dashed vertical line
at 13 V is a typical operating voltage for a module charging a 12-V battery. The reduction in
charging current for even modest amounts of shading is severe. With just one cell shaded out
of 36 in the module, the power delivered to the battery is decreased by about two-thirds!

48. Mitigation of Shading Effect
1.Bypass Diode:
The voltage drop problem in shaded cells could be to corrected by adding a bypass
diode across each cell, as shown in figure. When a solar cell is in the sun, there is a
voltage rise across the cell so the bypass diode is cut off and no current flows through
it—it is as if the diode is not even there. When the solar cell is shaded, however, the
drop that would occur if the cell conducted any current would turn on the bypass
diode, diverting the current flow through that diode.
B. Rimal 49
The bypass diode, when it conducts,
drops about 0.6 V. So, the bypass diode
controls the voltage drop across the
shaded cell, limiting it to a relatively
modest 0.6 V instead of the rather large
drop that may occur without it.

49. The adjacent figure shows the
ability of bypass diodes to mitigate
shading when modules are charging
a 65 V battery.
(a)Normal (no shading) conditions
(b)Without bypass diodes, a
partially shaded module
constricts the current delivered
to the load
(c)With bypass diodes, current is
diverted around the shaded
module.

B. Rimal 51

50. I-V curve before and after Bypass Diode
B. Rimal 52

51. 2. Blocking Diode

B. Rimal 53
Mitigation of Shading Effect
When strings of modules are
wired in parallel, a similar
problem may arise when one of
the strings is not performing well.
Instead of supplying current to the
array, a malfunctioning or shaded
string can withdraw current from
the rest of the array.
By placing blocking diodes (also called isolation diodes) at the top of each
string as shown in the figure, the reverse current drawn by a shaded string can be
prevented.

52. Mitigation of Shading Effect
B. Rimal 54

53. B. Rimal 55
Testing of PV Module
Modules must be manufactured from specified
materials and components and subjected to
manufacturer’s quality assurance process. All samples
must be complete in every detail and supplied with the
manufacturer’s mounting/installation instructions.

What constitutes a ―good quality‖ module?
How ―reliable‖ it will be in the field?

IEC 61215 and IEC 61646 are the general standards for
testing a PV panel. Safety standards are governed by
61730.

54. B. Rimal 56
Diagnostic: Visual inspection, Hotspot.

Electrical: Insulation resistance, Wet leakage current

Performance: P
max at STC, Temperature coefficients, NOCT, P
max at low irradiance.

Thermal: Bypass diode test, Hotspot.

Irradiance: Outdoor exposure, UV exposure, Light soaking.

Environmental: Temperature cycles, Humidity freeze, Damp heat.

Mechanical: Mechanical load, Robustness of terminations, Hail impact.
Performance standards (IEC61215/61646)

55. B. Rimal 57
Safety standards IEC61730-1,2

Electrical hazards: Dielectric withstand, Ground continuity, Accessibility, Cut
susceptibility, Impulse voltage, Reverse current, Partial discharge.

Mechanical hazards: Module breakage.

Thermal hazards: Temperature test

Fire hazard: Fire resistance

56. B. Rimal 58
Modeling and Simulation
Commercial softwares are easily available for modeling PV systems.

Required Input Data
Weather data: irradiation, temperature, wind speed
Others: R
s, R
p

Output
I, V, P

Normally, single diode model (4-parameters model) is used to model a PV cell.

57. THANK YOU
B. Rimal 59